Hala Hamadeh

Abdallah Almomani

Akhilesh Tyagi

Probabilistic Verification of Outsourced Computation Based on Novel Reversible PUFs

With the growing number of commercial cloud-computing services, there is a corresponding need to verify that such computations were performed correctly. In other words, after a weak client outsources computations to an untrusted cloud, it must be able to ensure the correctness of the results with less work than re-performing the computations. This is referred to as verifiable computation. In this paper we present a new probabilistic verifiable computation method based on a novel Reversible Physically Unclonable Function (PUF) and a binomial Bayesian Inference model. Our scheme links the outsourced software with the cloud-node hardware to provide a proof of the computational integrity and the resultant correctness of the results with high probability. The proposed Reversible SW-PUF is a two-way function capable of computing partial inputs given its outputs. Given the random output signature of a specific instruction in a specific basic block of the program, only the computing platform that originally computed the instruction can accurately regenerate the inputs of the instruction correct within a certain number of bits. To explore the feasibility of the proposed design, the Reversible SW-PUF was implemented in HSPICE using 45 nm technology. The probabilistic verifiable computation scheme was implemented in C++, and the Bayesian Inference model was utilized to estimate the probability of correctness of the results returned from the cloud service. Our proof-of-concept implementation of Reversible SW-PUF exhibits good uniqueness compared to other types of PUFs and exhibits perfect reliability and acceptable randomness. Finally, we demonstrate our verifiable computation approach on a matrix computation. We show that it enables faster verification than existing verification techniques.

Introduction

Verifiable computations (VC) have attracted enormous interest and attention with the recent growth in cloud computing. The concept of verifiable computation allows a lower-resource client to outsource the computation of a program to an untrusted cloud. With a proof provided by the cloud, the client can verify that the results produced are consistent with the program specification and that the computations were performed correctly. To be viable, the effort of performing the verification must be negligible compared to the actual computation. Three main solutions were proposed to support verifiable computation: VC based on Trusted Computing [START_REF] Chen | A protocol for property-based attestation[END_REF]. The main drawback of this approach was the assumption that the physical protections cannot be defeated. A second method, VC with a Non-Interactive Argument, is described in [START_REF] Parno | Pinocchio: Nearly practical verifiable computation[END_REF]. This approach is not practical because it relies on complex Probabilistically Checkable Proofs (PCPs) or fullyhomomorphic encryption (FHE). Finally, VC with Interactive Proofs [START_REF] Vu | A hybrid architecture for interactive verifiable computation[END_REF] has been propose. While this approach is often efficient, it applies to only a narrow class of computations.

In recent years, interest in physically-unclonable functions (PUFs) has evolved. PUFs have been deployed in different applications because of their ability to generate digital fingerprints of unique identities for a physical system. SW-PUF [START_REF] Hamadeh | Physical unclonable functions (pufs) entangled trusted computing base[END_REF] is a specific type of PUF that binds software execution to the exact hardware platform and produces unique signatures at various points in the softwares execution. The SW-PUF signature is a promising candidate for providing a proof that a specific computation was performed on a specific platform. By expanding the capabilities of a SW-PUF to include invertibility and commutativity, we achieve elements of verifiable computation. Invertibility is achieved by capturing a physical attribute such as time when an output bit settles using reversible functions. Reversibility is obtained with transmission gates.

The Reversible SW-PUF

The design of the Reversible SW-PUF is an extension of our previous work on the SW-PUF [START_REF] Hamadeh | Physical unclonable functions (pufs) entangled trusted computing base[END_REF]. As in the original SW-PUF, the ALU signatures of an instruction on the reversible SW-PUF are generated from an early sampling of the ALU results. However, in the reverse mode, the roles of inputs and outputs are reversed, and the early sampling is done on the original input end. Reversible SW-PUF has two modes: forward and reverse. The forward mode is similar to the SW-PUF where it generates a unique signature by capturing the delay variations of carry propagation in ripple-carry adders (which is a basic component in an ALU). The delay variation is caused by instruction input values and the silicon fabrication foundry variations. The reverse mode computes the partial inputs from the signature and the instruction output. Early sampling captures a subset of original input bits correctly in a platform specific manner, which itself is a platform specific secret. Only the computing platform that originally computed the instruction can regenerate the inputs of the instruction accurate within a certain number of bits. In this design, Reversible SW-PUF is implemented in reversible logic. Fredkin gate [START_REF] Fredkin | Conservative logic[END_REF] is used as the Boolean basis for conservative logic because it is universal.

Since Fredkin gates are based on transmutation gates, and TGs are slow compare to a regular gate, we propose to use two ALUs (fast-ALU, rev-ALU). The actual computation values consumed by the following program instructions occur at the fast-ALU. The rev-ALU is used only for verification.

Verifiable Computation Scheme

In this section an efficient Verifiable Computation Scheme based on Reversible SW-PUF is proposed. The proposed scheme fits with a probabilistic consistency guarantee. In this scheme, we are interested in estimating the probability of a cloud service to return a correct result for the outsourced function. The main idea is to bind the verification scheme to the cloud service hardware by entangling the computation with the SW-PUF. When the cloud computes the function, an instruction sequence for each instruction generates relevant attributes which are the two data inputs for the lth instruction -X l 0 , X l 1 , the instruction output Y l , and the PUF output P l . Effectively, the cloud node generates a signature (response) for each instruction (challenge) in the execution path. This entire sequence of challenge-response pairs will be returned to the client as a proof of computational consistency.

For the verification process, the client can verify the behavior of a program of variable granularities. Most straightforward granularity is to verify an individual instruction behavior. Pick a random challenge-response( C k , R k ) pair of an instruction I k to verify. The client needs to send the response part (the instruction output Y k , and the PUF output P k ) to the cloud node. The cloud instantiates the reversible SW-PUF to re-compute the challenge from the response (the data inputs of the instruction (X k 0 , X k 1 ). Only the cloud node that computed the original signature will be able to compute the inverse PUF, so that (X k 0 , X k 1 ) is consistent with the (X k 0 , X k 1 ) in the original computation's proof of consistency within a large number of bits. We assume that over all the clients and programs, the amount of data is too large to be archived by the cloud node preventing a look-up based response to the verification step.

Repeating this verification process for all the n instructions is not feasible because a large number of instructions could be executed during a program run. As we discuss later, an alternative approach to pick a subset of instructions is used to increase the confidence interval for the verification while maintaining an efficient verification.

Static program slices raise this granularity naturally. It is a technique for reducing a program to a minimal form that still retains the original program computation for a given variable at a chosen point. Merging the program slicing technique with our verification scheme leads to a more efficient Verifiable Computation. A program slice's input/output consistency can be established with the Reversible SW-PUF method. For a program slice, all of the instructions in its execution flow can be verified leading to a deterministic verification. The program slices can be extracted to maximize certain static properties. Figure 1 describes an example of a client that wants to run the program on a cloud server using the proposed protocol:

For choosing the slice set in our scheme, two elements are critical: the size of the slice, and the number of slices. Since small slices result in more efficient verification, we propose to use a selection method based on the super-node [START_REF] Zhang | Wcet-aware control flow checking with super-nodes for resource-constrained embedded systems[END_REF] algorithm to reduce the verification effort. However, certain types of program control flow graphs may not be amenable to small slices, and in such a case, Fig. 1. The proposed protocol different methods could be applied. As in any interactive proof system, increasing the number of slices will increase confidence in the computed results. To provide a desired probabilistic proof about the servers results, we propose to use a Bayesian Inference [START_REF] Dempster | A generalization of bayesian inference[END_REF] model to determine the appropriate number of static slices required.

Slices Selection: Given a program P that contains a set of instructions S, our goal is to find a subset of S called M such that M exhibits the same behavior as S with respect to one of the program outputs. Once we find M , while we want to generate static slices SS that go through M , the selection of M must be based in some randomized algorithm to prevent an adversary from producing the same M to cheat. For choosing M , we used the algorithm in [START_REF] Zhang | Wcet-aware control flow checking with super-nodes for resource-constrained embedded systems[END_REF], for selecting all the supernodes in P as our set of desired nodes. A super-node is formed from a strict dominator-post-dominator pair. A node X is defined as a dominator to a node Y if every path from the start node to Y goes through X. Similarly, a node X is defined as a post-dominator to a node Y if all paths to the exit node of the graph starting at Y go through X. The super-node method will reduce the proposed verification scheme overhead. Verifying at least one instruction from each super-node block will be sufficient to verify the entire slice.

Probabilistic Verification Algorithms: In this section, we propose use of a Bayesian inference on a binomial proportion method to verify the outsourced computation statistically. Bayesian inference is a statistical technique to update our subjective beliefs as new evidence or data becomes available. Our objective here is to characterize the probability density function for the outsourced computation correctness given that a set of slices were run correctly. In particular, we are interested in estimating confidence in verifying the correctness of the calculation results returned by an untrusted cloud server. Bayesian computation of probability distributions starts with a prior belief about a model parameter, then updates this distribution based on observed data to produce new posterior beliefs. The mathematical definition of the Bayesian method is as follows:

p(H|D) = p(D|H) × p(H) p(D) (1)
4 Evaluation of the Reversible SW-PUF

We evaluate 32-bit Reversible SW-PUF in HSPICE using predictive technology model. We studied three metrics: uniqueness, randomness, and reliability. Uniqueness: Uniqueness measures the capability to distinguish between different devices. Hamming distances (HD) between PUF responses are used to measure uniqueness. An ideal HD between any two PUF responses is 50% (16bit). To evaluate the uniqueness of the Reversible SW-PUF on the same ALU under different data inputs (Intra-chip ), we measured the average HD distribution between a pair of output data on the same device PUF instance with different set of input data. The uniqueness of the forward signature for the Reversible SW-PUF has been measured the same way as the regular SW-PUF [START_REF] Hamadeh | Physical unclonable functions (pufs) entangled trusted computing base[END_REF]. Figure 2 (A) shows the HD in Forward mode. For the reverse computation, both ALU inputs were measured on ten different PUF instances with identical output (response, which constitutes the input for a reversible PUF in reverse mode). The HD between each pair of different ALUs was calculated. Figure 2 (B) shows the HD in Reverse mode. and Technology (NIST) was used to evaluate the responses of the reversible SW-PUF. We have applied the NIST tests to 512-bit stream that was produced from the 16 PUF instances. Only two categories (rank and linear complexity), out of fifteen statistical tests, failed.

Reliability: Reliability measures robustness of a PUF in the presence of environmental variations. Temperature variations are the main factor that affect the stability of a PUF response. Figure 3 shows the reliability results for the responses of the Reversible SW-PUF for both the forward and backward computations. The reversible PUF is very stable under the temperature variation from -10°C to 65°C.

Case Study: Verification of Matrix Multiplication

We evaluate the proposed method thorough a matrix multiplication experiment, a widely-used example in Verifiable Computation Systems. We considered the following scenario: a client C needs to multiply two large scale matrices A(n × n)and B(n × n) using a cloud service S. However, since the client C does not completely trust the cloud S to return the correct results for multiplication, the client C could verify the results in many ways. A naive algorithm could replicate the multiplication using another cloud service and compare the results, but this method is expensive, e.g., multiplying n × n matrices execute O(n 3 ) time using the standard method. A faster check could use Freivalds' algorithm [START_REF] Freivalds | Fast probabilistic algorithms[END_REF], a probabilistic randomized algorithm that verifies matrix multiplication in O(kn 2 ) with a probability of failure less than 2 -k . Our approach improves Freivalds' algorithm by reducing the running time of the verification process by a factor of O(n). Finally, we compare the execution time of our approach with the Verifiable Computation method proposed in [START_REF] Zhang | New publicly verifiable computation for batch matrix multiplication[END_REF].

Experimental Setup We implemented a C++ tool to generate the random slices and perform the verification, and a LLVM compiler framework to compile the matrix multiplication program into LLVM Immediate Representation (IR). We used the Symbiotic 3 tool [START_REF] Chalupa | Symbiotic 3: New slicer and error-witness generation[END_REF] to obtain the backward static slice for the program. Symbiotic 3 linked with C++ code to generate the random slices in which the slicing criterion was one element of the output matrix. The number of slices was chosen based on the Bayesian Inference model. For simplicity, we assumed that client C challenges must completely match the server signatures, and any failure will result in rejection of the verification. Finally, a Pin tool was used to generate the desired instruction traces, while HSPICE was used to represent the Reversible SW-PUF to generate the signatures.

Performance evaluation

We performed the experiments for evaluating our scheme and present the computation time cost for each of its elements. The resultant time cost was obtained by averaging the outcomes of testing 10 different randomly generated inputs of the matrix multiplication code for matrix sizes ranging from 1000 to 7000. Table 1 shows a computational cost comparison between the server S (i.e. Matrix Multiplication, Reverse computations) and the client C (i.e. Slices Generation "the number of slices was picked to produce a probability of more than 0.97", Signatures Verification) sides. We evaluate the advantage of our scheme by comparing our experiment with the PVCBMM scheme proposed in [START_REF] Zhang | New publicly verifiable computation for batch matrix multiplication[END_REF]. Both of the experiments are performed on the same computer properties. However, we used the Strassen's algorithm [START_REF] Huss-Lederman | Implementation of strassen's algorithm for matrix multiplication[END_REF] to reduce the time required to multiply matrices. We studied seven dimensions size ranging from 1000 to 7000. As shown in Table 2, the experimental results reveal that our scheme is more efficient than the PVCBMM scheme.

Conclusions

We present reversible SW-PUF, a novel PUF design for computing partial inputs given a set of outputs. We implemented the reversible SW-PUF in HSPICE and established its desirable properties (uniqueness, randomness, and reliability). We then provided an efficient interactive verifiable computation scheme based on the proposed PUF and based on the Bayesian method. Our approach links outsourced computation with server cloud node hardware to provide proof of correctness of the results with high probability.

Fig. 2 .

 2 Fig. 2. Hamming distance distribution of reversible SW-PUF: (A) Forward mode; (B) Reverse mode

Fig. 3 .

 3 Fig. 3. The reliability of reversible SW-PUF against temperature variations.

Table 1 .

 1 Computation cost of proposed scheme for different problem size.

	Dimension	Verification at Client Side	Computations at Server Side
		Slices Generation Signatures Verification Matrix Multiplication Reverse computations
	n= 1000	10.025 ms	0.570 ms	0.201 s	0.008 s
	n= 2000	11.504 ms	0.684 ms	2.129 s	0.078 s
	n= 3000	12.753 ms	0.746 ms	6.372 s	0.183 s
	n= 4000	15.025 ms	0.866 ms	12.479 s	0.366 s
	n= 5000	16.875 ms	0.925 ms	20.692 s	0.675 s
	n= 6000	17.752 ms	0.990 ms	29.668 s	1.065 s
	n= 7000	20.057 ms	1.136 ms	44.050 s	1.523 s

Table 2 .

 2 Computation and Verification cost between two schemes.

	Dimension	The proposed scheme	PVCBMM scheme [11]
		Computations cost Verification cost Computations cost Verification cost
	n= 1000	0.201 s	0.018 s	1.75 s	6.94 s
	n= 2000	2.12 s	0.090 s	4.36 s	14.86 s
	n= 3000	6.37 s	0.19 s	8.35 s	32.26 s
	n= 4000	12.47 s	0.38 s	24.62 s	61.37 s
	n= 5000	20.69 s	0.69 s	36.31 s	85.03 s
	n= 6000	29.66 s	1.08 s	65.16 s	178.54 s
	n= 7000	44.05 s	1.54 s	105.28 s	193.86 s


J Aguzzi

M M Flexas

S Flo

C Lo Iacono

M Tangherlini

C Costa

S Marini

N Bahamon

S Martini

E Fanelli

R Danovaro

S Stefanni

L Thomsen

G Riccobene

M Hildebrandt

I Masmitja

J Del Rio

E B Clark

A Branch

P Weiss

A T Klesh

M P Schodlok

Exo-Ocean Exploration with Deep-Sea Sensor and Platform Technologies

Keywords:

One of Saturn's largest moons, Enceladus, possesses a vast extraterrestrial ocean (i.e., exo-ocean) that is increasingly becoming the hotspot of future research initiatives dedicated to the exploration of putative life. Here, a new bio-exploration concept design for Enceladus' exo-ocean is proposed, focusing on the putative presence of organisms across a wide range of sizes (i.e., from uni-to multicellular and animal-like), according to state-of-the-art sensor and robotic platforms, technologies used in terrestrial deep-sea research. In particular, we focus on combined direct and indirect life-detection capabilities, based on optoacoustic imaging and passive acoustics, as well as molecular approaches. Such biologically oriented sampling can be accompanied by concomitant geochemical and oceanographic measurements to provide data relevant to exo-ocean exploration and understanding. Finally, we describe how this multidisciplinary monitoring approach is currently enabled in terrestrial oceans through cabled (fixed) observatories and their related mobile multiparametric platforms (i.e., Autonomous Underwater and Remotely Operated Vehicles, as well as crawlers, rovers, and biomimetic robots) and how their modified design can be used for exo-ocean exploration.

Introduction

L iquid water is present in the form of vast extraterres- trial oceans (i.e., exo-oceans) on various icy moons of our solar system [START_REF] Nasem | An Astrobiology Science Strategy for the Search for Life in the Universe[END_REF][START_REF] Hendrix | The NASA roadmap to ocean worlds[END_REF][START_REF] Kamata | Pluto's ocean is capped and insulated by gas hydrates[END_REF]. Five icy moons have been confirmed as ocean worlds, namely, three satellites of Jupiter (Europa, Ganymede, and Callisto) and two of Saturn (Enceladus and Titan, the latter with an exo-ocean below a thick hydrocarbon layer; [START_REF] Iess | The tides of Titan[END_REF]. Another four are likely to host a subsurface ocean, such as Saturn's moon Dione, Neptune's icy moon Triton, and the dwarf planet Pluto. Moreover, the dwarf planet Ceres seems to have at least a subsurface sea [START_REF] Henin | Exploring the Ocean Worlds of Our Solar System[END_REF].

The primary conditions under which we could expect to find extant life in exo-oceans (although this hypothesis is still uncertain at this stage of scientific research) are the presence of energy sources that facilitate a non-equilibrium thermodynamic state of a marine-like medium containing abundant organic compounds [START_REF] Schwieterman | Exoplanet biosignatures: a review of remotely detectable signs of life[END_REF]).

Among the above-mentioned icy moons, Enceladus and Europa meet these conditions. Both host large water-based oceans in which geothermal activity is present and where life could be possible (Manga and Wang, 2007;[START_REF] Iess | The gravity field and interior structure of Enceladus[END_REF][START_REF] Deamer | Can life begin on Enceladus? A perspective from hydrothermal chemistry[END_REF][START_REF] Rovira-Navarro | Do tidally generated inertial waves heat the subsurface oceans of Europa and Enceladus?[END_REF]. In fact, relevant geothermal activity has been imaged on both moons by the Cassini-Huygens probe as well as by the Galileo and Hubble Space Telescope space missions [START_REF] Henin | Exploring the Ocean Worlds of Our Solar System[END_REF]. Notwithstanding, the presence of organic molecules as markers for more complex compounds (e.g., amino acids and nucleotides), dissolved into a salty marine medium, has been directly indicated by the Cassini-Huygens probe of Enceladus' plumes [START_REF] Mckay | Follow the plume: the habitability of Enceladus[END_REF][START_REF] Hsu | Ongoing hydrothermal activities within Enceladus[END_REF][START_REF] Kimura | Polymerization of building blocks of life on Europa and other icy moons[END_REF][START_REF] Mann | Inner workings: icy ocean worlds offer chances to find life[END_REF][START_REF] Postberg | Macromolecular organic compounds from the depths of Enceladus[END_REF]. This fact makes Enceladus the most promising site for extraterrestrial life exploration [START_REF] Postberg | Macromolecular organic compounds from the depths of Enceladus[END_REF].

Enceladus is 500 km in diameter with a gravity field of only 1.2% that of Earth (Manga and Wang, 2007). Its vast exo-ocean mechanically decouples the rocky core from the exterior ice shell [START_REF] Thomas | Enceladus's measured physical libration requires a global subsurface ocean[END_REF]. The water body is kept fluid by geothermal activity in combination with tidal warming through Saturn's tidal pull forces and by ice shell thickness variations, all likely contributing to abrupt changes in water column pressure [START_REF] Hussmann | Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-Neptunian objects[END_REF]Manga and Wang, 2007;[START_REF] Jansen | The turbulent circulation of a snowball Earth ocean[END_REF][START_REF] Saxena | Relevance of tidal heating on large TNOs[END_REF][START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF][START_REF] Neveu | Evolution of Saturn's midsized moons[END_REF]. Pressure changes result in active geysers, which eject water plumes into space, creating the phenomenon of cryovolcanism ( F1 c Fig. 1). Strong geothermal gradients and high pressure produce large fluxes of hot water, transported through the ice shell via cracks and crevasses [START_REF] Choblet | Powering prolonged hydrothermal activity inside Enceladus[END_REF]. Due to decompression shocks, water suddenly evaporates and freezes once it emerges into space, dropping back on the surface as snow [START_REF] Be ˇhounkova | Plume activity and tidal deformation on Enceladus influenced by faults and variable ice shell thickness[END_REF].

Exo-ocean salinity conditions on Enceladus seem to be similar to those on Earth [START_REF] Fifer | Abundance of gases in Enceladus' ocean is a potential fuel, if life is there to consume b AU5 it[END_REF], which would lead to a water density of approximately 1020 kg/m 3 , similar to that of terrestrial oceans [START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF]. However, the average depth is much higher, being approximately between 30 and 50 km [START_REF] Iess | The gravity field and interior structure of Enceladus[END_REF][START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF]. This would generate a total volume of around 40% of the mass of the moon itself (C ˇadek et al., 2016). Enceladus' ice shell has an average thickness of 20-30 km with reduced thickness at the South Pole [START_REF] Tobie | Enceladus's internal ocean and ice shell constrained from Cassini gravity, shape, and libration data[END_REF][START_REF] Lucchetti | Brittle ice shell thickness of Enceladus from fracture distribution analysis[END_REF][START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF].

Enceladus' exo-ocean seems to have been in a fluid state for a time span equivalent to that of the oceans on Earth [START_REF] Choblet | Powering prolonged hydrothermal activity inside Enceladus[END_REF][START_REF] Lunine | Ocean worlds exploration[END_REF][START_REF] Jia | Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures[END_REF], potentially allowing abiogenesis and evolution of unicellular and multicellular-like life-forms (Barge et al., 2017[START_REF] Barge | Redox and pH gradients drive amino acid synthesis in iron oxyhydroxide mineral systems[END_REF]. Geothermal activity seems to be a component that favored the emergence of primordial life on Earth, driving its evolution in the deep sea [START_REF] Baross | Submarine hydrothermal vents and associated gradient environments as sites for the origin and evolution of life[END_REF][START_REF] Burcar | RNA oligomerization in laboratory analogues of alkaline hydrothermal vent systems[END_REF]. Similarly, exo-ocean geothermal activity could favor the evolution of organisms with chemosynthetic metabolic pathways analogous to those documented in highly productive hydrothermal communities on Earth (e.g., [START_REF] Chyba | Life without photosynthesis[END_REF]; [START_REF] Barge | Experimentally testing hydrothermal vent origin of life on Enceladus and other icy/ ocean worlds[END_REF][START_REF] Seewald | Detecting molecular hydrogen on Enceladus[END_REF]. During the geological history, those exo-ocean hydrothermal vent systems could have evolved into biodiversity-rich environments with chemosynthetic autonomous communities of primary producers, grazers, predators, scavengers, and remineralizing organisms (e.g., [START_REF] Lelie `vre | Biodiversity and trophic ecology of hydrothermal vent fauna associated with tubeworm assemblages on the Juan de Fuca Ridge[END_REF].

Unfortunately, exploration for life in Enceladus' exoocean presents technological challenges of much higher complexity than the exploration of any location in the deep sea on Earth. Instrument payloads will likely have a constraining effect on their use for the exploration of Enceladus over the next decades, although the weight of their casing can be greatly reduced compared to ocean instrumentation on Earth due to the reduced gravity on Enceladus. Moreover, the penetration of a potentially large ice shell requires tools to carve or melt tunnels on the scale of kilometers, in order to open the passage for any marine-like exploring platforms [START_REF] Weiss | Study of a thermal drill head for the exploration of subsurface planetary ice layers[END_REF][START_REF] Flo ¨gel | A new deep-sea crawler system-MANSIO-VIATOR[END_REF]. Anyway, those technological efforts are already in place. Different projects such as the Enceladus Explorer (EnEx) and the Europa Explorer (EurEx) ( b AU1 Konstantinidis et al., 2013), as well as the Very-Deep Autonomous Laser-Powered Kilowatt-Class Yo-Yoing Robotic Ice Explorer (VALK-YRIE) are presently focusing on autonomous navigation and control of robotic systems on, and especially under, exoocean ice shells.

Objectives

Motivated by data indicating that Enceladus' exo-ocean may host complex organic life and given the time span of its existence as a fluid body [START_REF] Postberg | Macromolecular organic compounds from the depths of Enceladus[END_REF], we provide a perspective for implementing its environmental and lifeoriented exploration based on available deep-sea technologies. We first describe high-priority sensors that are currently in use for marine sciences, when aiming at the characterization of pelagic and benthic seascapes, whose environmental conditions may affect life itself. Then we focus on those sensors that allow the detection of multicellular life-forms (i.e., animal-like), which is, to date, primarily carried out by imaging systems. At the same time, we also describe complementary molecular methods for indirect traceability of life (that would also allow the capability of detection of unicellular life). Later, we illustrate the different marine monitoring platforms and their assemblage into high-tech networks that could be used as test beds for exo-ocean life-detecting technologies. Ultimately, we propose a forward-looking pathway for environmental exploration of exo-oceans based on adapted versions of previously described sensor and platform technologies.

Deep-Sea Sensors for Exo-Ocean Reckoning and Life Detection

Prokaryotic-like life (i.e., unicellular) could be (theoretically) inhabiting exo-oceans [START_REF] Merino | Living at the extremes: extremophiles and the limits of life in a planetary context[END_REF]. Traces of biological activity could then be detectable from tens of meters up to kilometers as has been accomplished on Earth's oceans with available deep-sea sensor technologies [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF].

At the same time, other sensors could be used to characterize exo-ocean seascapes, including circulation and bathymetry, as relevant ancillary information for describing ongoing oceanographic and geochemical processes, which may create conditions conducive to life itself. To date, a diversified group of environmental sensors are being used in a remote, synchronous, and long-lasting fashion at virtually any depth of benthic and pelagic oceanic realms [START_REF] Danovaro | Ecological indicators for an integrated global deep-ocean strategy[END_REF]. Below, we describe the most relevant sensors for acquiring oceanographic and geochemical information; then we move to those sensors, providing information on potential life activity ( T1 c

Table 1). It should be noticed that current marine sensors are far from being exo-ocean flight-ready in terms of mass, robustness, autonomy, reliability, and so on. Such a development would be an engineering effort, requiring important economic sustainment, the description of which is out of the scope of this work. Moreover, the environmental knowledge needed to use these sensors in situ in an exo-ocean is not yet currently available. Therefore, due to this limitation, we describe them assuming that Enceladus' water medium conditions are similar to those present in Earth's oceans [START_REF] Fifer | Abundance of gases in Enceladus' ocean is a potential fuel, if life is there to consume b AU5 it[END_REF][START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF]. In any case, such sensors are already conceived to currently operate in harsh deep-sea conditions [START_REF] Ramirez-Llodra | Deep, diverse and definitively different: unique attributes of the world's largest ecosystem[END_REF], including darkness, high pressure, extreme low or very high temperatures (e.g., 1-8°C at seabed and around 400°C close to hydrothermal vent emissions), and variable turbidity [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF].

Oceanographic and geochemical sensors

Enceladus' exo-ocean seascapes can be explored with different environmental sensors (Table 1). The concentration of floating particles as well as their size and organic or AST-2019-2129-ver9-Aguzzi_1P.3d 02/29/20 4:48am Page 4 inorganic composition can be measured by light absorbance sensors, including laser diffraction and Raman spectroscopy together with a wide range of related properties [START_REF] Boss | Optical techniques for remote and in situ characterization of particles pertinent to GEOTRACES[END_REF]. Floating particle size may be controlled by turbulence and by biogenic activity (e.g., marine snowlike aggregates; [START_REF] Turner | Zooplankton faecal pellets, marine snow, phytodetritus and the ocean's biological pump[END_REF], being the product of resuspension from deeper seafloors.

As salinity and carbon-dioxide contents of the exo-oceans seem to be similar to those on Earth [START_REF] Fifer | Abundance of gases in Enceladus' ocean is a potential fuel, if life is there to consume b AU5 it[END_REF][START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF], Conductivity-Temperature-Density (CTD) probes as well as oxygen and pH sensors could also be used as relevant markers for proteins and nucleic acids stability and to set the boundaries for metabolism existence as we know it on Earth (NASEM, 2018). Nitrate, phosphate, and even methane sensors could be used as well, since they efficiently operate in environments where marked fluctuations in those dissolved gases occur at different spatiotemporal scales (e.g., [START_REF] Thomsen | Ocean circulation promotes methane release from gas hydrate outcrops at the NEPTUNE Canada Barkley Canyon node[END_REF][START_REF] Doya | The seasonal use of smallscale space by benthic species in a transiently hypoxic area[END_REF]. In the case of Enceladus, measurements of dissolved methane may be of relevance in order to highlight the presence of biological chemosynthetic activity as it occurs on Mars [START_REF] Formisano | Detection of methane in the atmosphere of Mars[END_REF]. The occurrence of essential nutrients such as nitrates and phosphates may also provide relevant hints on the distribution and productivity of life into the exo-ocean itself, and when these data are coupled with those from currents (see next section), circulation effects on potential biological productivity can be modeled [START_REF] Olson | Oceanographic constraints on exoplanet life[END_REF].

Sonars

Enceladus' exo-ocean current regimes are presently unknown, and complex hydrodynamic seascapes may occur below the ice shell, within the water column, and near the rocky core (e.g., [START_REF] Rovira-Navarro | Do tidally generated inertial waves heat the subsurface oceans of Europa and Enceladus?[END_REF]. Scientific findings on Earth have shown that currents may deeply alter life existence, determining the concentration of life-limiting gases (e.g., oxygen minimum zones in oceans; [START_REF] Paulmier | Oxygen minimum zones (OMZs) in the modern ocean[END_REF] and nutrient dispersal [START_REF] Olson | Oceanographic constraints on exoplanet life[END_REF], thus conditioning the appearance of organisms as sessile or motile forms.

Active acoustic tools (e.g., acoustic Doppler current profilers, ADCP), commonly used in oceanographic studies for acquiring flow speed and direction data, could be deployed faced down, below the ice shell (Table 1) [START_REF] Fifer | Abundance of gases in Enceladus' ocean is a potential fuel, if life is there to consume b AU5 it[END_REF][START_REF] Hemingway | Enceladus's ice shell structure as a window on internal heat production[END_REF]. For example, Aquadopp models (Nortek1 ) working at 2000 kHz allow a maximum depth resolution of 6 km with an accuracy of 0.5 cm/s.

Multibeam echo sounders (MBES), based on the emission of multiple ultrasonic frequencies, are commonly used for the characterization of the seabed [START_REF] Lo Iacono | Seafloor characterization and backscatter variability of the Almerı ´a Margin (Alboran Sea, SW Mediterranean) based on highresolution acoustic data[END_REF][START_REF] Lurton | An Introduction to Underwater Acoustics: Principles and Application[END_REF][START_REF] Lecours | A review of marine geomorphometry, the quantitative study of the seafloor[END_REF], the analysis of the water column-seabed interface, and the identification of gas plumes [START_REF] Colbo | A review of oceanographic applications of water column data from multibeam echosounders[END_REF][START_REF] Innangi | High resolution 3-D shapes of fish schools: a new method to use the water column backscatter from hydrographic multibeam echo sounders[END_REF][START_REF] Zhao | Comprehensive detection of gas plumes from multibeam water column images with minimization of noise interferences[END_REF]. When the MBES are combined with navigation data of a moving platform (see next section), a complete characterization of Enceladus' rocky nucleus surface could be obtained (e.g., [START_REF] Wynn | Autonomous underwater vehicles (AUVs): their past, present and future contributions to the advancement of marine geoscience[END_REF]. In a similar way to Earth's findings, the effects of bioturbation as well as the presence of biogenic structures (actual or fossil) could be revealed by recurrent marks on the seabed surface or specific geomorphologies [START_REF] Baucon | Organism-substrate interactions and astrobiology: potential, models and methods[END_REF]. MBES systems could also be configured face-upward to scan the bottom of the ice shell, providing important information on its morphology and dynamics at the interface with the water (McPhail et al., 2009;Dutrieux et al., 2014aDutrieux et al., , 2014b)).

Multibeam echo sounders can also be used for the quantification of animal presence in large volumes of water via the analysis of acoustic backscatter returns, when an adequate assessment of signal-to-noise ratio can be made (e.g., [START_REF] Brisen ˜o-Avena | ZOOPS-O2: a broadband echosounder with coordinated stereo optical imaging for observing plankton in situ[END_REF]. Although preexisting knowledge on echo signature for acoustic signal crossreferencing is not yet available for exo-oceans and MBES cannot be used for the identification of any fauna, those sensors could be used to identify objects moving in the water column, thus contributing to the environmental characterization [START_REF] Dunlop | Detection and characterization of deep-sea benthopelagic animals from an autonomous underwater vehicle with a multibeam echosounder: a proof of concept and description of data-processing methods[END_REF].

Finally, acoustic tomography based on sound propagation could also be employed to measure temperature, currents, and internal tides among distant and time-keeping synchronized acoustic sources [START_REF] Munk | Ocean Acoustic Tomography, Cambridge Monographs on Mechanics[END_REF][START_REF] Finn | An acoustic tomography technique for concurrently observing the structure of the atmosphere and water bodies[END_REF]. Such technology could be used to derive large-scale information on exo-ocean circulation and geothermal activity. For example, acoustic tomography enabled the identification of localized convection ''chimneys'' in Greenland's deep sea (at 1800 m) that are caused by extreme surface winter cooling [START_REF] Wadhams | A deep convective chimney in the winter Greenland Sea[END_REF]. Similar techniques could be applied to detect exo-ocean geothermal fluxes.

Optical sensors

HD b AU2

imaging is widely used in ecological exploration of Earth's deep-sea, and current tools may be used to identify the presence of fauna with sessile or motile morphological designs on icy moons, although that possibility is to date still highly uncertain [START_REF] Newman | Universal EvoDevo?[END_REF]. Within this context, fast-developing deep-sea imaging technologies centered on high-definition photogrammetry, stereo, hyperspectral, miniaturized cameras and low-light vision are established tools that permit assessment of the presence and activity of organisms (e.g., [START_REF] Kokubun | Proximity of krill and salps in an Antarctic coastal ecosystem: evidence from penguin-mounted cameras[END_REF][START_REF] Bicknell | Camera technology for monitoring marine biodiversity and human impact[END_REF][START_REF] Corgnati | Looking inside the ocean: toward an autonomous imaging system for monitoring gelatinous zooplankton[END_REF], Marini et al., 2018a). These imaging assets could be adapted for the identification of exo-oceanic fauna in a broad range of sizes (i.e., equivalent to our prokaryotes, including bacterial mat formations, as well as micro-eukaryotes, micro-and meso-zooplankton, up to larger multicellular organisms). Those cameras require different levels of light intensity, which is a monitoring footprint whose biological effects are still under evaluation in deepsea contexts [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF].

Low-light imaging technologies

Different image-based technologies for life detection could also be used to avoid the exogenous light footprint. Such an imaging is capable of recording very low intensity emissions from organisms, as in the case of bioluminescence (see Table 1).

Environmental prerequisites that potentially favor bioluminescence existence in exo-oceans are light-deprivation and ecosystem stability as it occurs in Earth's deep sea. However, one should also consider the possibility that bioluminescence could be a non-existing phenomenon on Enceladus, even in the extreme case of having identified any life-form.

Bioluminescence is a ubiquitous phenomenon in environments that have been stable over large geological times on Earth (i.e., marine as compared to freshwater, where only a few bioluminescent species are known) [START_REF] Haddock | Bioluminescence in the sea[END_REF]. Bioluminescence evolved independently, being present in most of the major marine phyla [START_REF] Herring | Systematic distribution of bioluminescence in living organisms[END_REF][START_REF] Widder | Bioluminescence in the ocean: origins of biological, chemical, and ecological diversity[END_REF][START_REF] Martini | Quantification of bioluminescence from the surface to the deep sea demonstrates its predominance as an ecological trait[END_REF][START_REF] Martini | Distribution and quantification of bioluminescence as an ecological trait in the deep sea benthos[END_REF], as well as in some bacteria [START_REF] Martini | Bacteria as part of bioluminescence emission at the deep ANTARES station (North-Western Mediterranean Sea) during a one-year survey[END_REF]. Bioluminescence is produced by organisms for predation, defense, and intraspecific communication [START_REF] Haddock | Bioluminescence in the sea[END_REF], and organisms can emit it after mechanical stimulation at collisions [START_REF] Craig | Naturally occurring bioluminescence on the deep sea floor[END_REF].

Calibrated high-resolution measurements of mechanically stimulated bioluminescence are made by the Underwater Bioluminescence Assessment Tool (UBAT), similar to a Multipurpose Bioluminescence Bathyphotometer (MBBP; [START_REF] Herren | A multiplatform bathyphotometer for fine-scale, coastal bioluminescence research[END_REF]. Other systems use a stimulating grid mounted on oceanographic instruments, such as CTD, to obtain vertical pelagic profiling of bioluminescence via photomultiplying cameras (e.g., the Image Intensified Silicon Intensifier Target-ISIT; the Image Intensified Charge Couple Device for Deep-sea Research, ICDeep; e.g., [START_REF] Craig | Abundant bioluminescent sources of low-light intensity in the deep Mediterranean Sea and North Atlantic Ocean[END_REF]. Alternatively, other imaging systems have been developed, that is, the extreme low-light working LuSEApher camera with photon counting capability (e.g., [START_REF] Barbier | An ebCMOS camera system for extreme low light imaging[END_REF][START_REF] Dominjon | An ebCMOS camera system for marine bioluminescence observation: the Lu-SEApher prototype[END_REF] Other means for measuring the bioluminescence of organisms are provided by deep-sea neutrino telescopes [START_REF] Martini | Bacteria as part of bioluminescence emission at the deep ANTARES station (North-Western Mediterranean Sea) during a one-year survey[END_REF][START_REF] Aguzzi | Inertial bioluminescence rhythms at the Central Mediterranean KM3NeT deep-sea neutrino telescope[END_REF]. Their mooringlike towers cover the benthopelagic dimension and are primarily instrumented with thousands of photon-detecting sensors (i.e., photomultiplier tubes, PMTs) ( b F2 Fig. 2a-2c), capable of picturing the passage of neutrinos in the form of high-energy light. The main stimulation of organisms to emit light around those static structures comes on impact when swimming or as induced by turbulence behind them. The KM3NeT-Italia and ANTARES neutrino telescopes, off Capo Passero (Sicily, Western Ionian Sea) located at a depth of more than 2 km are an example of the threedimensionality of those infrastructures (reviewed by [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF]. Telescope moorings cover the benthopelagic dimension in the form of a cubic kilometer scale matrix of vertically extended, flexible strings which rise for hundreds of meters above the seabed [START_REF] Sapienza | High-energy neutrino astronomy[END_REF].

Acoustic imaging

Deep-sea video monitoring of fauna is being integrated with novel acoustic (multibeam, high-frequency) cameras into efficient optoacoustic packages [START_REF] Juanes | Visual and acoustic sensors for early detection of biological invasions: current uses and future potential[END_REF] that, with an appropriate design, could be used in the search for putative life in exo-oceans (see Table 1). The Dualfrequency Identification Sonar (DIDSON) and Adaptive Resolution Imaging Sonar (ARIS) can deliver threedimensional images of organisms and map seabed features in aphotic environments, depending on organism size and ambient turbidity conditions [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF]. That type of assets does not require light to identify and scale objects within the field of view. Unfortunately, the active emission of sound is also another monitoring footprint to be considered in unknown ecological contexts.

Passive acoustic monitoring

Passive acoustic monitoring (PAM) by listening hydrophones provides relevant information on biological activity based on specific sound markers (i.e., noise emission spectra) (see Table 1). In deep-sea areas on Earth, the types of organisms revealed by sound emissions cannot be identified with this sensor technology, unless we can associate their acoustic signaling with images (e.g., [START_REF] Archer | Glass sponge reef soundscapes[END_REF][START_REF] Mouy | Cataloging fish sounds in the wild using combined acoustic and video recordings[END_REF]. This condition cannot be met for exo-oceans where no previous environmental knowledge exists, but the broad characterization of soundscapes and their geological and hydrographic processes are of high value, when crossed with the multiparametric data collection proposed with geochemical and oceanographic sensors. For example, the use of this technique on Earth revealed the presence of gas bubbling beyond the reach of optoacoustic imaging technologies and provided information on the extension of the phenomenon [START_REF] Maksimov | Pressurized oceans and the eruption of liquid water on Europa and Enceladus[END_REF].

Molecular-based technologies

The detection of potential life in exo-oceans could seek environmental DNA/RNA forms (eDNA/eRNA-like) within a structural framework as known from Earth (e.g., the FISHbot initiative; Floyd, 2018) (see Table 1). Detection of nucleic acids may be measured via fluorescent dyes (indirect detection) or through nucleotide sequencing (direct detection). The former method relies on the binding of solubilized molecules with either double-or single-strand nucleic acids that interact with light at specific wavelengths. Fluorescence imaging devices (i.e., microscopy or spectrometry) are capable of detecting a wide range of dye molecules with high sensitivity, each of which shows preferential binding substrate [START_REF] Suseela | Far-red fluorescent probes for canonical and non-canonical nucleic acid structures: current progress and future implications[END_REF]. However, false positives may occur when fluorescence dye imaging is applied to environmental samples. Inefficient staining, nonspecific binding to sample components, and autofluorescence of mineral particles under light excitation often interfere with efficient DNA/RNA detection [START_REF] Li | Fluorescence microscopy for visualization of soil microorganisms: a review[END_REF].

Direct sequencing of eDNA/eRNA has become a cornerstone of future marine research [START_REF] Scholin | The quest to develop ecogenomic sensors: a 25-year history of the Environmental Sample Processor (ESP) as a case study[END_REF], and the next generation of Environmental Sample Processor (ESP) on board mobile robotic platforms (see below) is contributing toward this goal [START_REF] Zhang | Targeted sampling by autonomous underwater vehicles[END_REF]. A promising technology for in situ nucleic acid identification, not yet suited for the marine medium, is offered by nanopore devices (Oxford Nanopore Technologies). These devices have been successfully tested in the International Space Station [START_REF] Castro-Wallace | Nanopore DNA sequencing and genome assembly on the International Space Station[END_REF]. Presently, the Search for Extra-Terrestrial Genomes (SETG) program is aiming to detect free nucleic acids based on nanopore sequencing technology [START_REF] Carr | Towards in situ sequencing for life detection[END_REF].

Direct and indirect methods of detection of nucleic acids may be used for identifying environmental nucleoside al-ternative structures such as xeno-nucleic acids (eXNA; [START_REF] Cleaves | 227 Views of RNA: is RNA unique in its chemical isomer space?[END_REF]. Single-Walled Carbon NanoTubes (SWCNTs) could also be used for the detection of eXNA [START_REF] Gillen | Xeno nucleic acid nanosensors for enhanced stability against ion-induced perturbations[END_REF].

Other life-tracing technologies could be based on in situ mass spectrometry, which is being developed to target a wide range of organic and inorganic compounds dissolved in marine water by mobile robotic platforms (see below) (e.g., [START_REF] Wollschlager | In situ observations of biological and environmental parameters by means of optics development of nextgeneration ocean sensors with special focus on an integrating cavity approach[END_REF]. Additionally, the use of Lab-on-a-Chip (LOC) technologies should be advanced to facilitate miniaturized time-series measurements on those platforms [START_REF] Beaton | Lab-on-chip measurement of nitrate and nitrite for in situ analysis of natural waters[END_REF]. LOC could be used to trace metabolic products, based on the detection of free-circulating compounds through specifically designed markers, as suggested by Cassini-Huygens' recompiled information (Mathies et al., 2017).

Marine Platforms and Their Networks for Exo-Ocean Exploration

The development of fixed and autonomous mobile platforms is revolutionizing our ability to explore the deep-sea benthic and pelagic environments, acquiring information at a high resolution not achievable with vessels [START_REF] Wynn | Autonomous underwater vehicles (AUVs): their past, present and future contributions to the advancement of marine geoscience[END_REF][START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF]. A wide spectrum of oceanographic, geochemical, optic, and acoustic sensors can be installed on those platforms to explore the seafloor, the subseafloor, and the water column variability, including the potential presence of extant life (see Table 1).

Different power sources are currently used on Earthbased systems to sustain the functioning of those platforms. A continuous provision of energy can be given to fixed infrastructures by fiber optic cables or, if that is not possible, using in situ marine renewable energy resources such as water column turbines and vertical tidal oscillators, and even eolic mills and solar panels [START_REF] Favali | Seafloor Observatories: A New Vision of the Earth from the Abyss[END_REF]. In the case of exo-oceans, water turbines can be used if sufficiently strong and temporally sustained hydrodynamic forces exist. Space missions are currently using energy provision through radioactive decay [START_REF] Stone | SPINDLE: a 2-Stage nuclear-powered Cryobot for ocean world exploration[END_REF][START_REF] Cwik | An architecture for a nuclear powered cryobot to access the oceans of icy worlds[END_REF]) by a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) as the core of Radioisotope Power Systems (RPS) (NASA, 2011), as in the case of the Mars Science Laboratory [START_REF] Loren | The F1 Multi-Mission Radioisotope Thermoelectric Generator (MMRTG): a power subsystem enabler for the Mars Science Laboratory (MSL) mission[END_REF][START_REF] Holgate | Increasing the efficiency of the Multimission Radioisotope Thermoelectric Generator[END_REF]. A nuclear battery efficiently converting heat into electricity and generating electrical power in smaller increments could be used for a variety of space missions, from the vacuum of the space to exo-oceanic contexts. Such a solution may last up to decades, and it may be used in future autonomous long-lasting marine and exo-oceanic exploration missions.

Acoustic or light-based modem technologies [START_REF] Bai | Digital underwater communication with chaos[END_REF][START_REF] Han | Experimental demonstration of quasi-omni-directional transmitter for underwater wireless optical communication based on blue LED array and freeform lens[END_REF][START_REF] Shen | Single LED-based 46-m underwater wireless optical communication enabled by a multi-pixel photon counter with digital output[END_REF] are being developed for communication and hence inter-operability among mobile and fixed robotic platforms to increase their working autonomy (Del Rio et al., 2011, 2018;[START_REF] Dunbabin | Robots for environmental monitoring: significant advancements and applications[END_REF][START_REF] Wang | Design of optical-acoustic hybrid underwater wireless sensor network[END_REF]. Those intercommunication capabilities can be used for target tracking [START_REF] Masmitja | Optimal path shape for range-only underwater target localization using a Wave Glider[END_REF][START_REF] Masmitja | Range-only single-beacon tracking of underwater targets from an autonomous vehicle: from theory to practice[END_REF], as navigational aids (McPhail and Pebody, 2009), or locating docking stations [START_REF] Vallicrosa | Active range-only beacon localization for AUV homing[END_REF].

In this framework, this section describes the status of technological developments of marine robotic platforms with different levels of autonomy and mobility that can be adapted and used for exo-ocean exploration, either in standalone modes or coupled together into cooperative networks ( T2 c Table 2). It is important to note that network-based redundant and prolonged data collection is of relevance to highlight spatiotemporal variations in ecological processes [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF].

Cryobots

Enceladus exo-ocean exploration should be based on penetrating robots as melting probes. The NASA-funded cryobot named VALKYRIE, developed at Stone Aerospace (Texas, USA), consists of a laser beamed down a fiber optic cable as a heat source to melt the ice, and it was successfully tested in an Alaskan glacier [START_REF] Stone | Progress towards an optically powered cryobot[END_REF][START_REF] Stone | Laser-powered Cryobots and other methods for penetrating deep ice on ocean worlds[END_REF]. Insights on potential pitfalls and issues related to the penetration of ice shells could be gained from the results of similar trials carried out in Antarctic subglacial lakes (Siegert, 2018). In particular, Lake Vostok might be one of the best examples to be compared to the exploration of exooceans by being separated from the rest of Earth's atmosphere by a 4 km thick ice layer, which had to be drilled in order to access the liquid water [START_REF] Siegert | Antarctic subglacial lake exploration: first results and future plans[END_REF]. Unfortunately, in Lake Vostok the drill bit became damaged due to the thermal shock caused by contact with the lake water and produced an overspill of the drilling fluid (kerosene) that compromised the lake water analysis. The limits of the drilling technology used in Lake Vostok were overcome during the exploration of Lake Whillans, where a clean hot water drilling technology was used, making this lake the first successfully explored Antarctic subglacial lake [START_REF] Michaud | Solute sources and geochemical processes in Subglacial Lake Whillans, West Antarctica[END_REF]. Many drilling technologies are currently under investigation and under development for the exploration of the Antarctic [START_REF] Talalay | Thermal Ice Drilling Technology[END_REF] subglacial lakes and Solar System worlds [START_REF] Badescu | Outer Solar System[END_REF].

In relation to exo-oceans, similar melting probes could be conceived as actively ''driving'' through the ice, while taking and analyzing samples (e.g., [START_REF] Lucchetti | Brittle ice shell thickness of Enceladus from fracture distribution analysis[END_REF]. Active exploration could be performed while penetrating the ice, in order to detect remnants of life that were frozen in the ice when the shell ruptured. In particular, the VALKYRIE cryobot had an early on-board meltwater sampling system and an autonomous algorithm to command sampling [START_REF] Clark | An intelligent algorithm for autonomous scientific sampling with the VALK-YRIE cryobot[END_REF].

Observatories

Multiparametric seafloor observatories, receiving power and transferring data via telecommunication cables, are currently deployed on Earth's seabed [START_REF] Danovaro | A new international ecosystem-based strategy for the global deep ocean[END_REF]. These structures allow for highly integrated multiparametric environmental and biological data collection in benthic realms that can be extended to the pelagic ones through depth profiling yo-yo probes (i.e., performing cyclic water column ascent and descent; e.g., [START_REF] Fujii | Fine-scale monitoring of fish movements and multiple environmental parameters around a decommissioned offshore oil platform: a pilot study in the North Sea[END_REF][START_REF] Fanelli | NEREA, the Naples Ecological REsearch for Augmented observatories: towards an end-to-end transdisciplinary approach for the study of marine ecosystems[END_REF]. Such platforms are open windows of the continental margin, from coastal areas to abyssal plains, to remotely study in real time life activity and its responses to environmental changes [START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF]. For example, on Earth, compacted versions of these observatories have been successfully deployed in the deep sea close to hydrothermal vents, with cable-to-shore or in a stand-alone (i.e., moored) fashion, enabling a remote and long-lasting monitoring of biological components and en-vironmental variables at hydrothermal vent sites (e.g., [START_REF] Colac ¸o | MoMAR-D: a technological challenge to monitor the dynamics of the Lucky Strike vent ecosystem[END_REF][START_REF] Cuvelier | Biological and environmental rhythms in (dark) deep-sea hydrothermal ecosystems[END_REF]. On Enceladus, these types of platforms may provide long-lasting Eulerian measurements of the exo-oceanic proximal water mass characteristics, alerting scientists in the case of detection of any relevant transient object.

Such fixed platforms are used to further control docked mobile platforms (see next section and Table 2). Their operational value for exo-ocean exploration resides in the necessity to deploy fixed nodes below the ice shell, capable of releasing mobile platforms, providing communications capability and power energy, permitting sampling and exploration of a larger area, while yo-yo probes move cyclically from the bottom of the ice shell through the underlying water column.

Crawlers and rovers

Autonomous or tethered crawlers are mobile multiparametric platforms, moving on the seabed on caterpillars [START_REF] Flo ¨gel | A new deep-sea crawler system-MANSIO-VIATOR[END_REF]. They are used to expand the biological and environmental monitoring area around cabled nodes [START_REF] Aguzzi | Coastal observatories for monitoring of fish behavior and their responses to environmental changes[END_REF][START_REF] Thomsen | The oceanic biological pump: rapid carbon transfer to depth at continental margins during winter[END_REF]. Crawlers are known as Internet operated vehicles (IOVs) and have the advantage of great bandwidth with the onshore station, allowing real-time navigation capability and data collection/ transmission to land via interacting web interfaces [START_REF] Purser | Temporal and spatial benthic data collection via Internet operated Deep Sea Crawler[END_REF]. Deep-sea applications of crawler technology can be found in the study of cold seeps [START_REF] Chatzievangelou | High-frequency patterns in the abundance of benthic species near a cold-seep-an Internet operated vehicle application[END_REF][START_REF] Doya | Seasonal monitoring of deep-sea cold-seep benthic communities using an Internet operated vehicle (IOV)[END_REF] and the envisaged monitoring of ecological impacts at mining [START_REF] Chatzievangelou | Optimization of surveys with Internet Operated Deep-sea Crawlers, as an integrated tool for ocean cabled observatories: monitoring the benthic community of a methane hydrates site in Barkley Canyon (BC, Canada)[END_REF]. For Enceladus, crawler action may increase the monitoring radius around nodes providing larger monitoring coverage.

Presently, increasing autonomy in crawler missions and data collection is being implemented through technical solutions for full cable-independence via inductive powering (i.e., recharging is based on new depth-rated lithium batteries; [START_REF] Brandt | Cutting the umbilical: new technological perspectives in benthic deep-sea research[END_REF] and autonomous navigation [START_REF] Wehde | A flexible autonomous bottom resident infrastructure for bentho-pelagic monitoring[END_REF]. Rover (i.e., wheeled vehicle) technology is also being implemented as a nontethered alternative to crawlers, being operative through a vessel-deployable docking station [START_REF] Flo ¨gel | A new concept for high resolution benthic mapping and data acquisition: MANSIO-VIATOR[END_REF][START_REF] Wedler | ROBEX: components and methods for the planetary exploration demonstration mission[END_REF][START_REF] Flo ¨gel | A new deep-sea crawler system-MANSIO-VIATOR[END_REF]. For example, the benthic mobile physiology laboratory rover has been tested in the northeastern Pacific at 4000 m depth and 220 km west of the central California coast [START_REF] Mcgill | Initial deployments of the Rover, an autonomous bottom-transecting instrument platform for long-term measurements in deep benthic environments[END_REF].

Crawlers and rovers are of relevance for developing robotic operations practices during exo-ocean explorations, since both can move beneath the ice shells even in the presence of currents, waiting for commands from a distant control center. Reduced size/weight material and positively buoyant versions of those platforms could move upsidedown below the ice shell. Crawlers could even release floats and sink to the seafloor for upside operations. With the increase of their autonomous driving capability and multiparametric monitoring capacity, crawlers may be transformed in the future into a marine analog of the Mars Science Laboratory [START_REF] Loren | The F1 Multi-Mission Radioisotope Thermoelectric Generator (MMRTG): a power subsystem enabler for the Mars Science Laboratory (MSL) mission[END_REF][START_REF] Holgate | Increasing the efficiency of the Multimission Radioisotope Thermoelectric Generator[END_REF], but adaptable to Enceladus.

Platforms similar to the crawler, as the Buoyant Rover for Under-Ice Exploration (BRUIE; [START_REF] Berisford | Remote under-ice roving in Alaska with the Buoyant Rover for Under-Ice Exploration[END_REF] Operationally medium-ranged, since docked to cabled observatories, but under implementation for long-range autonomy, as sustained by new high-pressure rated lithium batteries.

AST-2019-2129-ver9-Aguzzi_1P.3d 02/29/20 4:48am Page 9

have already been tested in field deployments ( Jet Propulsion Laboratory, 2015a). These positively buoyant platforms have two wheels endowed with teeth, allowing them to adhere to the ice shell bottom surface with anchoring capability at greatly reduced energy costs. These platforms have odometry-navigation capabilities and four thrusters allowing yaw control as well as the viewing angle of the camera to be independent of the vehicle's motion. Control and data transfer can be either realized by a tether or with an acoustic modem. BRUIEs are an ideal option for the exploration and monitoring of areas around fixed observatories (see the next section).

Deep-sea autonomous underwater vehicles (AUVs)

Autonomous underwater vehicles are of high relevance for exo-ocean exploration due to their large versatility and autonomy. A relatively wide spectrum of sensor payloads can be installed on those platforms. AUVs' geophysical seafloor acoustic sensors can be programmed to autonomously map the seafloor and image the first meters of the subseafloor (e.g., MBES;[START_REF] Lo Iacono | Seafloor characterization and backscatter variability of the Almerı ´a Margin (Alboran Sea, SW Mediterranean) based on highresolution acoustic data[END_REF][START_REF] Lurton | An Introduction to Underwater Acoustics: Principles and Application[END_REF][START_REF] Lecours | A review of marine geomorphometry, the quantitative study of the seafloor[END_REF]. Such seafloor imaging can be coupled with other oceanographic and geochemical tools to explore and quantify water column variability [START_REF] Morris | A new method for ecological surveying of the abyss using autonomous underwater vehicle photography[END_REF][START_REF] Lecours | A review of marine geomorphometry, the quantitative study of the seafloor[END_REF]) (see Table 1).

Autonomous underwater vehicles are presently preprogrammed, unmanned, self-propelled vehicles that navigate for various distances possibly using dead-reckoning systems [START_REF] Paull | AUV navigation and localization: a review[END_REF]. Navigation systems are based on seafloor-relative velocity measurements through Doppler Velocity Log (DVL) instruments and Inertial Measurement Units (IMU). However, dead-reckoning systems need periodic adjustments in order to maintain an acceptable accuracy due to inherent errors and their accumulation over the time [START_REF] Masmitja | Optimal path shape for range-only underwater target localization using a Wave Glider[END_REF][START_REF] Masmitja | Range-only single-beacon tracking of underwater targets from an autonomous vehicle: from theory to practice[END_REF]. Usually, AUVs emerge on the sea surface to fix GPS positions or use a combination of ultra short baseline (USBL) acoustic communication or arrays of long baseline (LBL) acoustic beacons positioned on the seafloor. Operational constraints related to an ice shell covering an exo-ocean may limit traditional navigation methods and require other approaches such as range-only single-beacon navigation [START_REF] Masmitja | Range-only single-beacon tracking of underwater targets from an autonomous vehicle: from theory to practice[END_REF].

Autonomous underwater vehicles are being coupled with cryobots (see previous crawlers and rovers section) in projects such as Subglacial Polar Ice Navigation, Descent, and Lake Exploration (SPINDLE) or Sub-Ice Marine Planetary Analog Ecosystems (SIMPLE), both funded by NASA [START_REF] Stone | SPINDLE: a 2-Stage nuclear-powered Cryobot for ocean world exploration[END_REF]. A 20 km range hover-capable hybrid AUV, named Autonomous Rovers/airborne-radar Transects of the Environment beneath the McMurdo Ice Shelf (AR-TEMIS), developed at Stone Aerospace2 , is used to perform long-range surveying of the under-ice ocean. The hybrid AUV/ROV Nereid-Under Ice (NUI; Woods Hole Oceanographic Institution) has performed near-seafloor surveys under the ice pack in the Arctic Ocean ( Jakuba et al., 2018). Presently, AutoSub 3 performed the most successful underice-shelf exploration to date at the Pine Island Glacier (McPhail et al., 2009;[START_REF] Jenkins | Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat[END_REF]. While these large AUVs are not suited for exo-ocean exploration, they offer platforms on which to test new technologies and autonomous methods in an analogous environment on Earth.

Autonomous underwater vehicle autonomy is presently implemented through permanent docking at cabled observatories [START_REF] Wirtz | Design and test of a robust docking system for hovering AUVs[END_REF][START_REF] Hildebrandt | Combining cameras, magnetometers and machine-learning into a close-range localization system for docking and homing [accession number 17452565[END_REF]. Such docking capabilities, similar to stationary ROVs presently used by the deep-sea oil industry, will allow AUVs to perform depth-rated water column ascents or descents from beneath ice shell locations (e.g., ARTEMIS docking; [START_REF] Kimball | The ARTEMIS under-ice AUV docking system[END_REF]. A similar concept of remote control is represented by the Europa Underwater Probe ''Icefin.'' This AUV platform can be considered as an autonomous and remotely controlled small multiparametric probe designed to operate below the ice shell through a tether, and could also be used in exo-ocean exploration [START_REF] Spears | Underwater ice in Antarctica: the ICEFIN unmanned underwater vehicle development and deployment[END_REF].

Other innovative AUV approaches are based on novel emerging robotic technologies inspired by nature (i.e., biomimicking designs) and are of great relevance for space missions and for exo-ocean exploration. Fish-inspired solutions may be of some utility [START_REF] Menon | Biomimetic and robotics for space application: challenges and emerging technologies[END_REF] due to component miniaturization (low volume/weight), robustness, mission cooperative behavior (e.g., self-repair), and long-lasting autonomy (low-energy consumption) (e.g., see [START_REF] Bluman | Marsbee can a bee fly on Mars?[END_REF] and [START_REF] Funke | The search for signatures of life and habitability on planets and moons of our solar system[END_REF] for a conceptually analogous approach to the small cooperative units conceived for Mars land/atmospheric exploration). A new class of swimming robots (see Table 2) are currently being assembled with miniaturized sensor components and tested in coastal or shallow water areas [START_REF] Degnarian | Interoperable data management and instrument control experiences at OBSEA[END_REF]. In the near future, swarms of modular units (swarm-bot, or s-bot), showing some level of selfregrouping/self-repair capability and redundancy in data collection [START_REF] Hunt | The social animals that are inspiring new behaviors for robot swarms[END_REF], may prompt fine-tuned spatial coverage in Earth's deep-sea areas, to be later tested for space missions.

Drifting platforms

The Argo concept design3 could be implemented and adapted to explore exo-oceans. These multiparametric autonomous and freely drifting devices are being used to collect salinity and temperature in the water column down to 2000 m depth through consecutive cycles of ascent and descent [START_REF] Riser | Fifteen years of ocean observations with the global Argo array[END_REF]. When at surface, platforms transmit data via satellite before starting a new cycle. Presently, below ice-shelf observations have been successfully carried out with floats in a prolonged and autonomous fashion [START_REF] Dutrieux | Seaglider and float observations beneath Dotson ice shelf, West Antarctica[END_REF][START_REF] Lee | Sustained, autonomous observations beneath ice shelves[END_REF], also in challenging contexts of unpredictable hydrodynamic regimes [START_REF] Troesch | Autonomous control of marine floats in the presence of dynamic, uncertain ocean currents[END_REF].

In the past 10 years, Argo platforms have been implemented with new sensors and miniaturized analyzers such as fluorescence for chlorophyll-a and dissolved organic material, or oxygen, pH, and pCO 2 [START_REF] Riser | Fifteen years of ocean observations with the global Argo array[END_REF][START_REF] Stanev | Water intrusions and particle signatures in the Black Sea: a biogeochemical-Argo float investigation[END_REF]. Imaging devices are getting small enough to be integrated into an Argo structure (see Fig. 2) with hardware supports capable of running artificial intelligence-based computer vision for the detection of pelagic organisms (Marini et al., 2018a(Marini et al., , 2018b)). Those devices are being used to study a still evanescent life component of our oceans, which is represented by large aggregates (deepscattering layers) of bathymetrically displacing organisms, being hence of utility to scan large volumes of Enceladus' exo-ocean for similar purposes.

A Pathway for Exo-Ocean Exploration

A possible mission scenario can be globally drafted, according to previously presented sensor and platform technologies, following different steps described by previous authors (e.g., [START_REF] Cwik | An architecture for a nuclear powered cryobot to access the oceans of icy worlds[END_REF]. Our concept, summarized in b F3 Fig. 3, consists of three phases: Phase 1 is landing and platform delivery on the surface (only synthetically portrayed); Phase 2 is platform penetration (already treated in the section on cryobots above); and Phase 3 is platform dispersion below ice and the release of drifting assets.

Landing, platform delivery on surface, and data communication capability

Enceladus exploration scenarios are based on the presence of a fixed lander system that should arrive at a safe distance from active geysers and then should release cryobots that penetrate the ice shell [START_REF] Dachwald | IceMole: a maneuverable probe for clean in situ analysis and sampling of subsurface ice and subglacial aquatic ecosystems[END_REF][START_REF] Konstantinidis | A lander mission to probe subglacial water on Saturn's moon Enceladus for life[END_REF]. While descending, each cryobot should unroll a thin cable fixed to the lander and capable of transferring power and data. That cable should resist the mechanical stress of ice closing after its passage; such a stretch and compression reliance can be achieved by a specific coating (e.g., Kevlar).

The releasing platform should remain on the surface of the icy moon in order to transmit data back from the exoocean's moon to Earth. Such a platform should be responsible for all data communication and transmission, suffering different constraints which should be carefully taken into account because of the limited navigation autonomy of mobile platforms (as described in the following sections).

The latency associated with deep space communications (79 -8.3 min for Enceladus) and the communication dropout expected due to occlusion by orbital bodies (approximately 16.5 h for every 33 h on Enceladus) prevent real-time and continuous communication. The platform hosting the scientific instruments would autonomously prioritize objectives to maximize its efficiency while obeying resource and time constraints as well as completing mandatory activities, such as rendezvous for communication. To increase the effectiveness of the scientific operations, methods for semiautonomous and autonomous data collection would be designed and implemented for identifying regions of scientific interest [START_REF] Zhang | Using an autonomous underwater vehicle to track the thermocline based on peak-gradient detection[END_REF][START_REF] Zhang | Autonomous four-dimensional mapping and tracking of a coastal upwelling front by an autonomous underwater vehicle[END_REF][START_REF] Flexas | Autonomous sampling of ocean submesoscale fronts with ocean gliders and numerical model forecasting[END_REF], scientifically relevant features like hydrothermal vents [START_REF] Branch | Autonomous nested search for hydrothermal venting[END_REF], and select targets on which to perform observations [START_REF] Estlin | AEGIS automated science targeting for the MER Opportunity rover[END_REF][START_REF] Francis | AEGIS autonomous targeting for ChemCam on Mars Science Laboratory: deployment and results of initial science team use[END_REF]. A strategy for high-level human guidance is required to allow for refinement of autonomous behaviors based on analysis of data by scientists on Earth.

The large amount of data collected in situ cannot be entirely transmitted to Earth. Due to strict data communication constraints, it is mandatory to equip the observation platforms with software solutions able to identify and transmit only the relevant information collected. This problem happens also in deep-sea research, where solutions are provided by data science, pattern analysis, and artificial intelligence methodologies [START_REF] Skiena | The Data Science Design Manual[END_REF][START_REF] Aguzzi | New high-tech interactive and flexible networks for the future monitoring of deep-sea ecosystems[END_REF]. Simple computer vision algorithms can be executed on board platforms' imaging asset, to identify any subject different from the water or seabed itself [START_REF] Corgnati | Looking inside the ocean: toward an autonomous imaging system for monitoring gelatinous zooplankton[END_REF]Marini et al., 2018a). General approaches based on image enhancement, differencing, and background subtraction methods can be used to discard irrelevant information [START_REF] Moeslund | Introduction to Video and Image Processing[END_REF][START_REF] Peters | Foundations of Computer Vision[END_REF]; for example, in the case of water column, ice shell, or seabed surfaces, changes in patterns would be slower with respect to traveling objects. This information can be transmitted to Earth through periodic reports and analyzed by expert scientists. Specific algorithms could then be trained to recognize and classify relevant subjects (e.g., suspended particulate, living organisms). Then the updated algorithms could be sent back to the in situ platforms to improve their effectiveness.

Platform dispersion below ice and water mass reckoning

Once deployed in the exo-ocean, the cryobot should act as a fixed observatory equipped with a minimal set of scientific instruments for estimating the ocean's environmental conditions. According to a positive evaluation of those conditions, the cryobots should release the BRUIEs equipped with multiparametric sensors, which would start the exo-ocean observation of the surroundings of the fixed platform. These mobile units should be equipped with wireless intercommunication capability via acoustic or light-based modem technologies (see the previous section). Data download recharging and battery could be performed by inductive pinless connection (e.g., Fonn model by Wi-Sub4 ; [START_REF] Wehde | A flexible autonomous bottom resident infrastructure for bentho-pelagic monitoring[END_REF] among BRUIEs and/ or between the BRUIEs and the cryobot.

Those moving platforms could be endowed with highsensitivity cameras and acoustic imaging equipment (see Table 1), allowing for detection of organisms under extreme low light with a reduced footprint (i.e., potentially harmful light effects in aphotic environments). At the same time, photomultiplier tubes (PMTs) and passive acoustic technologies (PAM) should be included as well, to measure bioluminescence presence and characterize soundscapes in terms of biophony (as analogous to cetacean-like communication) and geophony (providing important data on background oceanographic and geological processes). Molecular sensors (e.g., nanopore sequencing technology) and mass spectrometry devices could complete the detection capability of any putative life, allowing organism traceability well beyond previously described sensor assets. Some of the BRUIEs could be endowed with moored lines replicating the above-described sensor asset, to be projected into the water mass in an initial reckoning phase at unknown hydrodynamic conditions (see Fig. 3). A Eulerian picturing of water masses could be carried out with sensors hosted in physically inert and depth/pressure-rated cases (e.g., glass spheres equivalent to those used for neutrino telescope assets; Fig. 2d,2e).

The release of drifting platforms

In a second stage, drifting platforms similar to Argo floats (i.e., Bio-Argo; [START_REF] Claustre | Bio-Optical Profiling Floats as New Observational Tools for Biogeochemical and Ecosystem Studies: Potential Synergies with Ocean Color Remote Sensing[END_REF] and even swarms of biomimicking robots capable of short-range swimming autonomy (see Table 2), could be released from moored projections of BRUIEs. A swarm of those walking or swimming platforms would allow local exploration of the water column or below the ice shell in areas where larger platforms could not arrive. These could deliver multidisciplinary redundant oceanographic, geochemical, and biological data within the shortest time span, to counteract any potential equipment failure under unknown oceanographic conditions (e.g., [START_REF] Gissinger | A magnetically driven equatorial jet in Europa's ocean[END_REF].

Once the local hydrodynamics have been characterized, BRUIE platforms may release also another group of larger cooperatively communicating mobile robotic autonomous units as AUVs (see Table 2). These AUVs would be required to expand the monitoring radius around each node. These should permit autonomous but coordinated sampling activity, constituting a locally flexible cooperative network, similar to what has been conceived for marine surveying (e.g., [START_REF] Thompson | Satellites to seafloor: toward fully autonomous ocean sampling[END_REF] and defense (e.g., Micro Unmanned Surface Vehicle Diving-USV from Aquabotix Technology Corporation).

A strategy for exo-ocean platform testing

The implementation and testing of exo-ocean lifedetection technologies could be performed in relevant deepsea environments on Earth, currently endowed with flexible monitoring benthopelagic networks of cabled fixed and mobile platforms. All these processes should involve marine scientists and aerospace engineers, who should be consulted for different mission stages of conceptualization, planning, and development. Having space agencies test their technologies at deep-sea monitoring networks will allow us to tie together the necessities for exploring remote ecosystems on Earth in order to explore extraterrestrial ones.

FIG. 2 .

 2 FIG. 2. Sensor devices hosted in depth-rated glass spheres. (A) Curled-up moored line of neutrino telescope spheres hosting PMTs, prior their deployment (the arrow indicates the element of the following plate, B). (B) Each single spherical unit where a set of PMTs is installed to face different angles. (C) Schematic representation of the extended moored line with all spherical units projected from the seabed, and whose PMTs are reading bioluminescence in all directions (color cones). (D) GUARD-One camera into a glass sphere connected to an Argo float (the arrow indicates the element of the following plate, E). (E) An enlargement of the glass sphere camera housing. Color images are available online.

FIG. 3 .

 3 FIG. 3. The implementation of the concept design for Enceladus exo-ocean exploration by a network of fixed and mobile cooperative platforms. (A) Landing and platform delivery on surface. (B) Platform penetration. (C) Platform dispersion below ice, water mass reckoning, and the releasing of drifting assets. Color images are available online.

Table 1 .

 1 Deep-Sea Multiparametric Sensors That Could Operate Synchronously for the General Exploration and Detection of Putative Life in Enceladus' Exo-Oceans

	FIG. 1. Enceladus is a small moon (di-
	ameter of about 500 km) that became a
	research hotspot when the space probe
	Cassini-Huygens discovered evidence of
	cryovolcanism, including exhalations into
	space by geysers in 2005. (A and B) Ima-
	ging of individual jets spurting ice mixed
	with vapor and trace organic compounds.
	(C and D) proposed mechanism generating
	observed geysers. Sources: DCode by
	Discovery;	https://www.youtube.com/
	watch?v=MjOpZrYLE1U;	NASA/JPL/
	Space Science Institute; https://www.jpl
	.nasa.gov/news/news.php?feature=3382.
	Color images are available online.

Table 2 .

 2 , Multiparametric Platforms with Different Operative Ranges and Autonomy, to be Potentially Adapted for Exo-ocean Exploration

	8

https://www.nortekgroup.com/products/aquadopp-6000-m EXO-OCEAN EXPLORATION WITH DEEP-SEA TECHNOLOGIES

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Acknowledgments

This work was developed within the framework of the Tecnoterra (ICM-CSIC/UPC) and the following project activities: ARIM (Autonomous Robotic sea-floor Infrastructure for benthopelagic Monitoring; MartTERA ERA-Net Cofound; PIs: J.A., S.F., and L.T.), ARCHES (Autonomous Robotic Networks to Help Modern Societies; German Helmholtz Association; PI: S.F.), RESBIO (TEC2017-87861-R; Ministerio de Ciencia, Innovacio ´n y Universidades; PIs: J.d.R., J.A.). M.M.F.'s work was partially funded by the National Aeronautics and Space Administration through grant number NNX15AG42G. A special thanks is also due to Dr. R. Sforza and Dr. G. Flati for their suggestions at writing.

A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration

Author Disclosure Statement

The authors declare no competing financial interests. 2016) are not found in the reference list. Please reconcile. AU2: Please spell out HD. AU3: The following are not found in the text: [START_REF] Aguzzi | Challenges to assessment of benthic populations and biodiversity as a result of rhythmic behaviour: video solutions from cabled observatories[END_REF], [START_REF] Godø | Marine Ecosystem Acoustics (MEA): quantifying processes in the sea at the spatio-temporal scales on which they occur[END_REF], [START_REF] Liu | Suave: swarm underwater autonomous vehicle localization[END_REF], [START_REF] Teodoro | The Enceladus ionizing radiation environment: implications for biomolecules[END_REF], [START_REF] Trask | Near real-time data recovery from oceanographic moorings[END_REF]. Please reconcile. AU4: Update reference, if available. AU5: Please add the abstract number. AU6: Please also define AWAC and HP in the table footnote.