domain: sde chim.anal stat

text:
Binta Diémé

Marie Lagrée

Martin Leremboure

Mounir Traïkia

Cyril C Jousse

Anne-Marie Delort

Boris Eyheraguibel

Untargeted

Untargeted metabolomics approach to study polymers biodegradation by Rhodococcus Rhodochrous using LC-HRMS

Untargeted metabolomics approach to study polymers biodegradation by Rhodococcus Rhodochrous using LC-HRMS Background Many environmental studies have identified contaminations of marine, soil, freshwater and atmospheric ecosystems by (micro) plastics. Some studies have highlighted microorganism enable of using plastics polymers as a carbon source. However, the metabolic pathways involved in polymer biodegradation are largely unknown. In order to evaluate these processes, we used a metabolomics approach. A bacterial strain Rhodococcus Rhodochrous was grown with or without different carbon sources: glucose (GLU) or oleic acid (OA) or polyethylene (PE) or no carbon source (Minimum Medium, MM).

Objective: highlight the metabolic pathways involved in the biodegradation of PE Did the bacteria able to degradate Polyethylene ?1 H-NMR analysis highlighted a decrease in the amount of PE in the medium after 1 week of incubation. The amount of protein in the extracellular medium is greater in the PE condition. We presume the bacteria produce enzymes able to cut polymer chains of PE. Multivariate statistical analysis showed a separation of samples according to the conditions of culture. These results suggest differences in metabolic profiles depending on the source of carbon.

Results

Oleic

Conclusions

The different extraction protocols allowed us to detect a bigger part of the metabolome. The peaks detected are mainly more intense in the PE group compared to the other groups. The annotated peaks are mostly amino acids and dipeptides and are generally in higher amounts in the PE group compared to AO, GLU groups. This is in agreement with the other results and shows an important bacterial activity in the PE group. These metabolites are involved in the metabolism of Glutathione, the urea cycle and in many pathways of amino acids metabolism.

Binta Diémé 1 , Marie Lagrée 1 , Martin Leremboure2 , Mounir Traïkia 1,2 , Cyril Jousse 1,2 , Anne Marie Delort 1,2 , Boris Eyheraguibel 2

week of incubation Harvesting supernatant 1 H-NMR analysis Harvesting pellets LC/MS analysis

  acid

	Without Carbon
	source
	Polyethylene
	Glucose
	After 1 week of incubation 1 At T0 Measures of Rhodococcus Rhodochrous activities Samples Mean PE Mean OA	Concentrations of proteins (µg/ml) 81.85 36.57
	Mean GLU	38.05
	Analysis by 1 H-NMR of supernatants of Rhodococcus Rhodochrous
	incubated with polymers of PE

Concentrations of proteins in media after one week of incubation with

PE or OA or GLU Statistical analysis on data from negative ionization mode Putative annotations of metabolites with in house database

Université Clermont-Auvergne, Unité de Nutrition Humaine, UMR 1019, Plateforme d'Exploration du Métabolisme, Clermont-Ferrand, France

2 Université Clermont-Auvergne, Institut de Chimie, UMR6296, Clermont-Ferrand, France


R Toor

R Denoyel

L Liggieri

M Schmitt

M Antoni

Influence of surfactant concentration on spontaneous emulsification kinetics

Keywords: Spontaneous emulsification, water/oil interfaces, Span 80, CTAB, SDS, water transport

The kinetics of spontaneous emulsification is investigated on aqueous pendant drops in paraffin oil. Optical microscopy in transmission mode is used for high-spatial-resolution image recording. The influence of a lipophilic surfactant (Span 80) and two water-soluble surfactants (CTAB and SDS) is investigated. As time runs, the drops interface turns opaque due to the formation of microstructures associated with spontaneous emulsification. The time evolution of this phenomenon is shown to depend upon temperature and surfactant concentration which leads to an overall shrinkage due to gradual water uptake and transport into paraffin oil. Spontaneous emulsification kinetics depends upon the chemical composition. Higher concentration of Span 80 and CTAB (resp. SDS) are shown to promote (resp. hinder) water transport. This work provides new insights into the understanding of spontaneous emulsification when combining the properties of non-ionic and ionic surfactants.

Introduction

Emulsions have for long demonstrated their important potential for technological developments and industrial applications in various fields such as oil recovery , pharmaceutics [START_REF] Ahnfelt | Lipiodol-Based Emulsions Used for Transarterial Chemoembolization and Drug Delivery: Effects of Composition on Stability and Product Quality[END_REF][START_REF] Laredj-Bourezg | Pickering Emulsions Stabilized by Biodegradable Block Copolymer Micelles for Controlled Topical Drug Delivery[END_REF] , cosmetics [START_REF] Gilbert | Stretching Properties of Xanthan, Carob, Modified Guar and Celluloses in Cosmetic Emulsions[END_REF][START_REF] Bouchemal | Nano-Emulsion Formulation Using Spontaneous Emulsification: Solvent, Oil and Surfactant Optimisation[END_REF] , paint and food industry [START_REF] Piorkowski | Emulsions: Recent Developments in Formulation, Production, and Applications[END_REF][START_REF] Lim | Stabilization of Orange Oil-in-Water Emulsions: A New Role for Ester Gum as an Ostwald Ripening Inhibitor[END_REF] . Common emulsions (or macroemulsions) consist of at least two immiscible liquids and are known to be thermodynamically unstable systems. They involve ageing mechanisms driven by buoyancy, drop/drop interactions, Ostw 7 ald ripening 8 . Kinetic stability over long time scales can however be achieved when adding surfactants and/or particles in the solutions. But for both thermodynamic and kinetic aspects, interfacial properties are paramount as they mostly govern all mass transfer mechanisms [START_REF] Ravera | Emulsification and Emulsion Stability: The Role of the Interfacial Properties[END_REF] .

Emulsions can be generated by a variety of high or low energy methods [START_REF] Lignel | Water-in-Oil Droplet Formation in a Flow-Focusing Microsystem Using Pressure-and Flow Rate-Driven Pumps[END_REF][START_REF] Mcclements | Food-Grade Nanoemulsions: Formulation, Fabrication, Properties, Performance, Biological Fate, and Potential Toxicity[END_REF][START_REF] Leong | Minimising Oil Droplet Size Using Ultrasonic Emulsification[END_REF] . High energy approaches involve the application of mechanical agitation with homogenizers, microfluidizers, ultrasonication [START_REF] Lignel | Water-in-Oil Droplet Formation in a Flow-Focusing Microsystem Using Pressure-and Flow Rate-Driven Pumps[END_REF][START_REF] Mcclements | Food-Grade Nanoemulsions: Formulation, Fabrication, Properties, Performance, Biological Fate, and Potential Toxicity[END_REF] while low energy ones only depend on the physicochemical processes in play [START_REF] Solans | Nano-Emulsions: Formation by Low-Energy Methods[END_REF][START_REF] Solans | Spontaneous Emulsification[END_REF][START_REF] Zhao | The Stability of Three Different Citrus Oil-in-Water Emulsions Fabricated by Spontaneous Emulsification[END_REF] . Low energy methods, such as spontaneous emulsification, are less invasive, making them very attractive to industry because of their obvious economic and societal benefits. They are of particular importance in the field of formulation where temperature control is often required in emulsification protocols.

Spontaneous emulsification (SE) results in the dispersion of one phase into another without external energy input [START_REF] Solans | Nano-Emulsions: Formation by Low-Energy Methods[END_REF] . It has been investigated extensively in a large variety of applications and emulsion types [START_REF] Zhao | The Stability of Three Different Citrus Oil-in-Water Emulsions Fabricated by Spontaneous Emulsification[END_REF][START_REF] Miller | Spontaneous Emulsification Produced by Diffusion -A Review[END_REF][START_REF] Li | Advances of Spontaneous Emulsification and Its Important Applications in Enhanced Oil Recovery Process[END_REF][START_REF] Riehm | Dispersion of Oil into Water Using Lecithin-Tween 80 Blends: The Role of Spontaneous Emulsification[END_REF][START_REF] Bahtz | Decoupling of Mass Transport Mechanisms in the Stagewise Swelling of Multiple Emulsions[END_REF][START_REF] Wen | Effects of Osmotic Pressure on Water Transport in W1/O/W2 Emulsions[END_REF][START_REF] Silva | Spontaneous Emulsification of Water in Oil at Appreciable Interfacial Tensions[END_REF] since its discovery more than one century ago [START_REF] Gad | von Der Blutrache[END_REF] . In spontaneous emulsifying systems, microstructures form, and their properties can be monitored by the chemicals that are used. They can for example behave as micro-reactors or provide efficient sol-gel synthesis routes for surfactant template-assisted mesostructured materials such as solid or hollow spheres [START_REF] Andersson | Combined Emulsion and Solvent Evaporation (ESE) Synthesis Route to Well-Ordered Mesoporous Materials[END_REF] , thins films [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF] or high surface area tubules [START_REF] Peng | Fabrication of Titania Tubules with High Surface Area and Well-Developed Mesostructural Walls by Surfactant-Mediated Templating Method[END_REF] . Oil-in-water nano-emulsions can also be produced via SE [START_REF] Solè | Study of Nano-Emulsion Formation by Dilution of Microemulsions[END_REF] and offer important applications in drug delivery techniques [START_REF] Saberi | Fabrication of Vitamin E-Enriched Nanoemulsions: Factors Affecting Particle Size Using Spontaneous Emulsification[END_REF][START_REF] Saberi | Effect of Salts on Formation and Stability of Vitamin E-Enriched Mini-Emulsions Produced by Spontaneous Emulsification[END_REF][START_REF] Guttoff | Formation of Vitamin D Nanoemulsion-Based Delivery Systems by Spontaneous Emulsification : Factors Affecting Particle Size and Stability[END_REF][START_REF] Date | Self-Nanoemulsifying Drug Delivery Systems: Formulation Insights, Applications and Advances[END_REF] .

Recent work focusing on pendant water drops in paraffin oil/Span 80 solutions revealed SE kinetics and evidenced the spontaneous occurrence of microstructures along the water/oil interface with typical morphology determined by the concentration in Span 80 [START_REF] Schmitt | Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces[END_REF] . Water transport from the drops into paraffin oil by reverse micelles was evidenced and shown to depend upon Span 80 concentration. When low (high), transport kinetics is slow (fast) and micrometer sized droplets (brush-like microstructures) are formed. As time runs, they multiply, swell and finally invade the whole interface. Water is therefore gradually removed from the drops leading to an overall decrease of their volume. Similar findings were already reported in literature [START_REF] Bahtz | Decoupling of Mass Transport Mechanisms in the Stagewise Swelling of Multiple Emulsions[END_REF][START_REF] Bahtz | Quantification of Spontaneous W/O Emulsification and Its Impact on the Swelling Kinetics of Multiple W/O/W Emulsions[END_REF] . SE studies with dodecane/Span 80 solutions have for example evidenced the generation of two dimensional colloidal structures made of spherical water droplets [START_REF] González-Ochoa | Spontaneous Two-Dimensional Spherical Colloidal Structures[END_REF] . Spontaneous formation of water droplets was also observed with Span 80 for W/O/W (water/oil/water) emulsions [START_REF] Wen | Effects of Surfactants on Water Transport in W1/O/W2 Emulsions[END_REF] .

The dynamic processes causing SE are still a matter for discussion. Two main ideas are proposed in the literature for their understanding: Mechanical breakup or chemical instability. Mechanical breakup can be caused by either interfacial turbulence or vanishing interfacial tension [START_REF] Dabros | Emulsification through Area Contraction[END_REF] while chemical instability can be promoted by bilayer collapse due to pressure gradients [START_REF] Shahidzadeh | Dynamics of Spontaneous Emulsification for Fabrication of Oil in Water Emulsions[END_REF][START_REF] Shahidzadeh | A New Mechanism of Spontaneous Emulsification: Relation to Surfactant Properties[END_REF] , local inversion of surfactant supersaturated regions [START_REF] Greiner | Spontaneous Formation of a Water-Continuous Emulsion from a w/o Microemulsion[END_REF][START_REF] Ruschak | Spontaneous Emulsification in Ternary Systems with Mass Transfer[END_REF] or chemical reactivity [START_REF] Nishimi | Spontaneous Emulsification Produced by Chemical Reactions[END_REF] . When mechanical breakup is the driving phenomenon, low surfactant concentrations are sufficient to evidence SE whereas, if chemical instability prevails, high surfactant concentrations are required and swelling of reverse micelles caused by chemical potential gradients is the driving mechanism.

The present study investigates SE in a system involving aqueous pendant drops suspended in paraffin oil with the aim of addressing the effect of composition changes. Non-ionic (Span 80, sorbitan monooleate), anionic (SDS, sodium dodecyl sulfate) and cationic (CTAB, cetyltrimethylammonium bromide) surfactants are considered. Synergetic effects of CTAB and SDS have been investigated for micro and nano-emulsion stability [START_REF] Xin | Influence of CTAB and SDS on the Properties of Oil-in-Water Nano-Emulsion with Paraffin and Span 20/Tween 20[END_REF][START_REF] Zdrali | The Interfacial Structure of Nanoand Micron-Sized Oil and Water Droplets Stabilized with SDS and Span80[END_REF] . However, to our knowledge, never for systems experiencing SE. Span 80 will be used in all the experiments as SE has not been observed in its absence. Following this introductive section, section 2 will present the materials and methods employed. Section 3 concentrates on how SE kinetics is modified when changing Span 80 concentration. This section completes a previous investigation by taking fully into account a delay in SE attributed to the formation of surfactant monolayers [START_REF] Schmitt | Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces[END_REF] . Section 4 focuses on the combined effect of Span 80 and CTAB on one hand and Span 80 and SDS on the other. The possible underlying mechanisms responsible for SE changes in the presence of ionic surfactants is discussed. In the last section, conclusions and some perspectives are finally proposed.

Materials and methods

Materials

Span 80 was purchased from Fluka (catalog n. 09659). The HLB of 4.3 is reported for this surfactant, which is therefore soluble in non-polar solvents and often used to stabilize water-in-oil emulsions. Its CMC is 0.43 mM (0.18 -0.26 g/L) at 20 °C in paraffin oil [START_REF] Santini | Interfacial Rheology of Span 80 Adsorbed Layers at Paraffin Oil-Water Interface and Correlation with the Corresponding Emulsion Properties[END_REF] . It consists ofa mixture of esters in the following fractions: sorbitan dioleate (36.5 %), Sorbitan triooleate (24.8 %), Sorbitan monooleate (19.1 %), Isosorbide monooleate (12.9 %) and Sorbitan tetraoleate (6.7 %). The presence of few unidentified components, which probably correspond to residues of the starting materials used for synthesis such as oleic acid and sorbitol were also identified [START_REF] Abou-Nemeh | Some Aspects of Emulsion Instability on Using Sorbitan Monooleate (Span 80) as a Surfactant in Liquid Emulsion Membranes[END_REF][START_REF] Abou-Nemeh | Sorbitan Monooleate (Span 80) Decomposition during Membrane Ageing. A Kinetic Study[END_REF] . Hexadecyltrimethylammonium bromide (CTAB), pure to 99 %, was obtained from Sigma Aldrich (catalog n. 52365). CTAB is a cationic surfactant (HLB=10) with CMC of 1.0 mM (0.36 g/L) at 20 °C in water [START_REF] Van Os | Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants[END_REF] . Sodium dodecyl sulfate (SDS) was purchased 99% pure from Sigma-Aldrich (catalog n. L3771). Its HLB is about 40 and its CMC is 8 mM (2.3 g/L) at 20 °C in water. Paraffin oil is provided by Sigma Aldrich (catalog n. 76235) and has density of 0.83 g/ml. All chemicals were used as supplied without any further purification. Water used for all the experiments was obtained from a Millipore Direct-Q water purification system (resistivity > 18,2 MΩ•cm, TOC < 5 ppb).

Sample preparation

The organic phase used here is paraffin oil (PO) loaded with Span 80. Fresh stock solutions are systematically used and prepared starting with a small amount of PO in a 100 mL volumetric flask. A given mass of Span 80, controlled by weighing of this flask, is added to reach the target stock solution concentration (10 g/L). The flask is filled up to the gauge line and the solution is homogenized by magnetic stirring at 600 rpm for 15 min at room temperature. Concentration of Span 80 is decreased by dilution down to 1 g/L. Concentrations are therefore always above CMC. The two ionic surfactants, CTAB and SDS, were used separately. Stock solutions for the aqueous phases are prepared by weighing and dissolution in water (total volume 50 mL) under magnetic stirring at 800 rpm for 30 min. The lower concentration solutions are then prepared by cascade dilutions of the stock solutions. For CTAB solutions, the studied concentration ranges between 0.01 g/L and 0.1 g/L (below CMC) while for SDS concentration is varied from 0.01 g/L to 1 g/L (also below CMC). The reason for using such moderate CTAB and SDS concentrations (when compared to the one of Span 80) results from the necessity to prevent interfacial tension to become too small. High SDS or CTAB concentrations make indeed pendant drop configurations unstable. They can deform, flow downward due to buoyancy and develop an unstable neck that eventually brings the drops to detach from the needle tip (see hereafter).

Experimental apparatus

Pendant drops (about 0.4 μL) of water are created at the end of a hydrophobic capillary needle. A Hamilton micro syringe (62 RN SYR, 2.5 μL) fitted with the needle is employed. Its tip is immersed in a 4 mL quartz cuvette previously filled with the chosen PO/Span 80 solution and covered with a stopper provided with a small hole to guide the needle and produce pendant drops centered in the field-ofview of a microscope. Typical images are shown in Figure 1. This sample cell is in a thermostatcontrolled environmental chamber that can be moved with micrometric screws for the fine adjustment of centering. Temperature is controlled with a Peltier facility and a circulating bath granting temperature adjustments in the range -40°C to 100°C within ±0.5°C. The environmental chamber sits on an optical bench to limit the influence of uncontrolled external perturbations.

Optical microscopy and image analysis

The optical microscope is equipped with a 1024×1024-pixel CCD camera (Mikrotron, MC1310) encoded on 255 grey shades, allowing the observation of a magnified image corresponding to a field of view (FOV) of 1×1 mm 2 , with up to 500 Hz acquisition frame rate. The optical resolution of the microscope is therefore about 1 μm 2 per pixel. Light source is a LED producing a collimated light beam and a homogeneous background. The acquisition of images is started immediately after producing drops to capture early SE kinetics. Acquisition frame rate was set from 1 image every 5 seconds to about 1 image every 72 seconds depending on the experimental requirements. Sequences of several hundred images are produced by each experiment and post-processed using the Image-J Package with dedicated macros. Each image is binarized so as to be encoded on only two grey shades, white or black. A contrastlike function is then used to follow darkening/brightening of the images 31 :

f(𝑡) = !(#)%! !"# !(&)%! !"# (Eq. 1)
where N(t) (resp. N(0)) is the number of pixels with grey shades above 128 at time t (resp. initially).

Nmin is the minimum value reached by N(t) in the processed image sequence. With this definition, f(t)=1 initially and f(t)=0 when the brightness of in the FOV is minimal. A decreasing (resp. increasing) trend of f(t) is therefore the signature of an overall increasing (resp. decreasing) darkening of the images. f(t) is a quantity obtained by post processing of the images. It is measured over the complete FOV (1 mm 2 ) and depends upon both the overall size of the pendant drop and the transparency of the water/paraffin oil interface. It is simple to define and has the great advantage of allowing straightforward quantitative measurements. For example, a shrinking of the size of the drop makes FOV brighter and therefore leads to an increase of its value. When considering light transmitted across the water/paraffin oil interface (ie. across the drop, see Figure 1), intensity depends upon the structure of the interface and upon the density of the scattering sites, here water microstructures, that the incident light beam meets along its optical path.

The greater this density and the size of the microstructures, the more the transmitted intensity will be attenuated and therefore the smaller f(t) will be. Still, the definition of f(t), though practical, is limiting. It does not allow for example to discriminate the characteristic time associated with the densification kinetics of the microstructures on the interfaces from that associated with their swelling kinetics. It does not help either to probe the structure in depth of the water/paraffin oil interfaces. More precise techniques should be implemented, such as high-resolution tomographic microscopy, to achieve this level of description.

3. Results

Spontaneous emulsification at the water/paraffin oil interface

Figure 1 illustrates the evolution of a pendant drop for [Span 80] = 10 g/L at 30 °C. As the drop is under background illumination, its central area turns bright as expected from normal incidence. Initially, the interface between both liquids is smooth and the central area displays uniform illumination as shown in the magnified inset of Figure 1(a). As time runs, SE starts, and microstructures spontaneously develop with increasing crowding (Figure 1(b) and(c)). They are located at the interface, mostly consist of micrometer sized vesicles and act as scattering sites making the central area of the drop increasingly darker 31 . The formation of similar aggregate structures has recently been reported, but for a hydrophilic nonionic surfactant [START_REF] Hu | High Internal Phase Emulsions Stabilized by Adsorbed Sucrose Stearate Molecules and Dispersed Vesicles[END_REF] . This evolution modifies the overall contrast of the images that is easy to track and quantify with f(t) (Eq. ( 1)). Microstructures do not appear instantly and, depending upon Span 80 concentration, it may take minutes to hours for them to show up. It is therefore hypothesized that they are the result of the swelling of smaller objects produced at earlier time scales. Note that samples with Span 80 concentrations below CMC do not show such unusual behavior when investigated over periods of several days [START_REF] Silva | Spontaneous Emulsification of Water in Oil at Appreciable Interfacial Tensions[END_REF] . In these last conditions, images keep similar to that of Figure 1(a) and show no significant change. Back to the results illustrated in Figure 1. Up to t=1400 s (Figure 1 (c)), the drop shows a progressive darkening, yet maintaining its initial size and axi-symmetry. Microstructures are produced, grow with time and gradually invade the whole interface as can be seen by a detailed inspection of the zooms of Figure 1(b) and (c). They flocculate and arrange in a compact network until densely packed. Cohesion forces remain strong enough for buoyancy or mechanical stress generated by swelling/flocculation to be small and microstructures to remain attached to the interface. Over longer periods of time, microstructures still develop but also start to detach. They behave as mobile objects dispersed in PO and transport water apart from the pendant drop similarly to what has been observed for capsules [START_REF] Nishimi | Spontaneous Emulsification Produced by Chemical Reactions[END_REF][START_REF] Fessi | Nanocapsule Formation by Interfacial Polymer Deposition Following Solvent Displacement[END_REF][START_REF] Quintanar-Guerrero | A Mechanistic Study of the Formation of Polymer Nanoparticles by the Emulsification-Diffusion Technique[END_REF] . A new evolution regime sets then in where the initial drop axi-symmetry is lost as the drop undergoes shrinkage and deformation progressively. Deformation results here from non-isotropic interfacial stress generated by the progressive modification of the water/PO interface into a complex layer that looks very similar to a thin polymerized membrane [START_REF] Wiese | Phase Transitions and Critical Phenomena[END_REF] . As this phenomenon appears randomly on the interface, it produces drop shapes that can hardly be controlled. Eventually, drops appear as an opaque vesicle and water transport stops. Drops turn into vesicles consisting in a loose membrane of bounded microstructures through which water transport is no more possible (cf. Figure 1(d)).

Contrast function f(t) is employed to make the above discussion quantitative [START_REF] Schmitt | Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces[END_REF] . Figure 2(a) reports its evolution for three repetitions of the experiment illustrated in Figure 1. A decreasing trend is first observed. It vanishes when SE is sufficiently developed for the drops to prevent normal transmitted light. The curves of all three experiments overlap for t < 2500 s and therefore demonstrate good reproducibility. Drops keep axi-symmetric within this time interval. At longer time scales, f(t) increases as the shrinking and deformation of drops become measurable. Curves do not overlap anymore due to non-reproducible behavior resulting from the strong non-linearities in the microstructures interactions.

The average behavior of the three experiments, <f(t)>, is plotted in Figure 2(b) for t < 2000 s. Three different SE regimes indicated as R0, R1 and R2 are identified with characteristic times t0, t1 and t2 defined by: <f(t0)> = 0.95, <f(t1)> = 0.5 and <f(t2)> = 0.05. R0, R1, and R2 are typical of the behavior of the drops for the Span 80 concentration range investigated in this work (1 to 10 g/L) and provide important insights into SE typical time scales. R0 corresponds to the initial lag stage of SE where no microstructures are observed under optical microscopy. This regime, that was not investigated in our previous work [START_REF] Schmitt | Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces[END_REF] , takes place within time t <t0 ≈ 1000 s for [Span 80]=10 g/L and corresponds to the time needed for Span 80 molecules to diffuse from the PO solution onto the drops interface. R0 is therefore an early-stage evolution period where molecular processes are creating the conditions for SE to be triggered. For the same Span 80 concentration, previous studies reported equilibration times of only few tens of second for the interfacial tension to achieve its value (about 5 mN/m). This is much shorter than the values of t0 measured in this work and suggests that the mechanisms driving SE are already in action well before this time [START_REF] Shahidzadeh | A New Mechanism of Spontaneous Emulsification: Relation to Surfactant Properties[END_REF] . Optical microscopy is obviously not appropriate to catch molecular phenomena in action as R0 takes place and this explains why <f(t)> remains almost unchanged. Complex time-dependent processes modifying the structure of the Span 80 adsorption layer, without having significant effects on its interfacial tension, cannot be excluded here [START_REF] Ravera | Modelling of Dilational Visco-Elasticity of Adsorbed Layers with Multiple Kinetic Processes[END_REF][START_REF] Ravera | Influence of Surface Processes on the Dilational Visco-Elasticity of Surfactant Solutions[END_REF] . Large surfactant concentrations as investigated here could moreover foster thick self-assemblies of complexes of water and oil which could, in turn, trigger SE [START_REF] González-Ochoa | Spontaneous Two-Dimensional Spherical Colloidal Structures[END_REF][START_REF] Pautot | Spontaneous Formation of Lipid Structures at Oil/Water/Lipid Interfaces[END_REF] . In the second regime, R1, drops turn dark due to increasing backlight scattering by ongoing SE creating a sharp decrease of <f(t)> when t > t0. Microstructures increase in number and size while remaining attached onto the drop interface and densely packing. In the experimental conditions of Figure 2, t1≈1300 s and R1 lasts up to time t2≈1600 s. Beyond this time, regime R2 starts and drops turn completely opaque making the optical investigations no longer of interest. Direct visual observation however still indicates a rich phenomenology where microstructures continuously detach from the interface, creating free areas where new ones pop up. Simultaneously, the ones remaining attached to the interface continue to rearrange themselves in a way to generate layers with increasing compactness and roughness 31 . To describe the role of Span 80 on SE kinetics, experiments were performed over a range of concentrations spanning from 1 g/L to 10 g/L. Figure 3 shows the behavior of t0, t1 and t2 as a function of [Span 80]. Each measurement point is the average of at least three experiments. All three times decrease monotonously with increasing [Span 80] indicating that SE kinetics becomes faster. Changing [Span 80] from 1 g/L to 10 g/L leads for instance, to a decrease in t1 from about 11000 s down to 1400 s. Full lines in Figure 3 result from a fitting of the data with a power law given in Eq. 2 (c0=1g/L for homogeneity) and presented here to guide eyes. Fitting parameters tref and b are given in Table 1 and all indicate reasonable error ranges. As b ≈ -1, the characteristic times can be considered approximately as inversely proportional to the Span 80 concentration. Surprisingly, despite the complexity of the Table 1 : Fitting parameters : tref and b (cf. Eq.2 and curves in Figure 3). The system investigated in this work is simple since it consists only of a pendant water drop, oil, and a lipophilic surfactant. But one still open question here is the understanding of the instability creating SE. As Span 80 concentration is much larger than CMC, it causes the formation of reverse micelles. Surfactant in the vicinity of the water/PO interface is therefore in large excess making bulk diffusion less effective in the overall kinetics of surfactant layer formation. Span 80 being a non-single component, all its constituents are expected, in principle, to contribute to SE processes and may involve surfactant/cosurfactant phenomena. To our knowledge, neither the ternary diagram of water/SPAN80/PO mixtures nor microscopic modelling of this system is available in the literature. Several mechanisms can be hypothesized to explain the very first step of SE. For instance, interfacial turbulence or a drastic decrease, or even a negative, interfacial tension resulting from Span 80 supersaturation. Diffusion and stranding may also come into play [START_REF] Abou-Nemeh | Some Aspects of Emulsion Instability on Using Sorbitan Monooleate (Span 80) as a Surfactant in Liquid Emulsion Membranes[END_REF][START_REF] Abou-Nemeh | Sorbitan Monooleate (Span 80) Decomposition during Membrane Ageing. A Kinetic Study[END_REF] . Another possible process relies on the formation of thick nonuniform multilamellar interfacial layers entrapping water films. Driven by chemical potential gradients, the latter could be fed by the drop water reservoir, swell until turning unstable most likely by Plateau-Rayleigh instability, and finally break off into microscopic objects [START_REF] González-Ochoa | Spontaneous Two-Dimensional Spherical Colloidal Structures[END_REF][START_REF] Pautot | Spontaneous Formation of Lipid Structures at Oil/Water/Lipid Interfaces[END_REF] . All these mechanisms, if effective in SE, probably do coexist in a complex interplay as suggested by scanning electron microscopy analysis that revealed microstructures hierarchies consisting of protruding water vesicles at the water/oil interface 31 . Their shape depends on the Span 80 concentration and consists of either spheres or fibers depending on the Span 80 concentration. They grow in size and number, arranging themselves in a compact network while remaining attached to the interface. This is illustrated in the insets of Figure 1b and Figure 1c. It is also important to note here that no microstructure coalescence was evidenced. It is observed, instead, that small microstructures grow at the expense of the pendant drops, suggesting that Ostwald ripening clearly cannot be invoked to explain SE. The power spectra extracted from Figure 1b and Figure 1c (not shown here) indicate the presence of a peak corresponding to microstructures of characteristic size of 25+/-5 µm. This size is constant throughout the R1 regime. Thus, the footprint of the microstructures on the drop interface remains nearly constant once they are in contact. The decrease in <f(t)> in R1 (Figure 2(b)) can therefore be interpreted as a consequence of a growing complexity of the microstructures in the direction perpendicular to the drop interface [START_REF] Shahidzadeh | Dynamics of Spontaneous Emulsification for Fabrication of Oil in Water Emulsions[END_REF] .

Observations being done in transmission mode, optical distortions created by the curvature of the pendant drop and the proximity of the microstructures when densely packed should be accounted for here. <f(t)> is moreover an average quantity. It gives consequently an integrated information on the whole drop and optical path of the incident beam and therefore does not provide any relevant information on the structure of the interface at the microscopic scale. It would have been interesting here access to such a description while SE develops and in particular to observe the phenomena occurring on both sides of the interface [START_REF] Shahidzadeh | Dynamics of Spontaneous Emulsification for Fabrication of Oil in Water Emulsions[END_REF] . Unfortunately, the transmission visualization technique used here does not allow such measurements. Preliminary tests on flat interfaces show that confocal microscopy may provide answers to this question. Span 80 has indeed detectable fluorescence. But addressing SE by this technique still requires important efforts. To have a sufficient magnification, the working distance must indeed be small (typically 2 mm) and this condition may produce experimental artifacts due to the proximity of solid surfaces. These measurements are still in progress and should allow, if successful, an in-situ study of the phenomena involved in SE.

The effect of temperature can be observed on comparing the time values at 30 °C in this work to those at 40 °C in our previous work [START_REF] Schmitt | Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces[END_REF] . For instance, Figure 3 shows that for [Span 80] = 10 g/L, t1 ≈1300 s at 30 °C and it decreases to t1 =700 s at 40 °C in the previous work [START_REF] Schmitt | Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces[END_REF] . SE becomes therefore faster at higher temperature. Viscosity of both paraffin oil and water decrease on increasing the temperature [START_REF] Fujiu | Influence of Temperature on Production of Water-in-Oil Emulsions by Microchannel Emulsification[END_REF] . Temperature changes also modify relative solubility of surfactants, interfacial tensions, and bulk viscosity [START_REF] Kovalchuk | Effect of Temperature on the Dynamic Properties of Mixed Surfactant Adsorbed Layers at the Water/Hexane Interface under Low-Gravity Conditions[END_REF] . Faster SE is a straightforward consequence of enhanced Brownian motion of Span 80 that eases relaxation to local equilibrium and therefore naturally accelerates the overall SE kinetics. This trend is similar to the one described in foaming ability of surfactants when temperature is modified 56 .

Influence of CTAB and SDS

The three regimes illustrated in Figure 2 still show up when CTAB or SDS is added to the aqueous phase and the three characteristic times t0, t1 and t2 all show trends similar to the ones of Figure 3 when their concentration is changed (see Figure 4). This is the reason why forthcoming discussion will be limited to the description behavior of t1 only. Maximal CTAB (resp. SDS) concentration is 0.08 g/L (resp. 0.3 g/L) here for stability purposes. For larger values interfacial tension becomes indeed too small, pendant drops get elongated due to buoyancy and detach from the capillary tip after few seconds. Note that both values do depend upon Span 80 in the concentration range investigated in this study. CTAB and SDS concentrations being smaller than their CMC, the pendant drop/PO interface is not compactly covered. As already discussed in the previous section for Span 80, SE kinetics depends on how fast layers form, compete, and eventually get reorganized. The comparatively fast stability loss illustrated in Figure S3suggests that the formation of CTAB (or SDS) and Span 80 layers are most likely successive processes. . This is essentially explained by electrostatic interactions weakening. In the absence of surfactant, water-oil interfaces are negatively charged because of hydroxide ions adsorption [START_REF] Marinova | Charging of Oil-Water Interfaces Due to Spontaneous Adsorption of Hydroxyl Ions[END_REF][START_REF] Vácha | The Orientation and Charge of Water at the Hydrophobic Oil Droplet-Water Interface[END_REF][START_REF] Beattie | The Pristine Oil/Water Interface: Surfactant-Free Hydroxide-Charged Emulsions[END_REF][START_REF] Ma | Impact of Spontaneously Adsorbed Hydroxide Ions on Emulsification via Solvent Shifting[END_REF] . Being cationic, CTAB reduces zeta potential and, as local electrostatic interactions are weakened, adsorption energy barrier is lowered which promotes Span 80 adsorption. A similar behavior of CTAB is known to modify nano-emulsions stability [START_REF] Xin | Influence of CTAB and SDS on the Properties of Oil-in-Water Nano-Emulsion with Paraffin and Span 20/Tween 20[END_REF] . The trend of Figure 5 turns consistent with this loss of stability and suggest that nanostructures formed at the very beginning of SE become less stable for increasing CTAB concentration. CTAB and Span 80 therefore act in a way to displace the CTAB/Span 80/hydroxyl groups adsorption/co-adsorption quasi-equilibrium in the sense to favor Span 80 only, making zeta-potential even smaller and the overall water transfer faster. The above discussion invoked only surface charge effects. Other contributions, like hydrophobic interactions or steric hindrance, were assumed to be negligible [START_REF] Xin | Influence of CTAB and SDS on the Properties of Oil-in-Water Nano-Emulsion with Paraffin and Span 20/Tween 20[END_REF] . One way to verify this hypothesis is to investigate interfacial charge modifications with SDS. SDS being anionic, zeta-potential becomes now more negative when increasing its concentration, and in contrast to CTAB, SDS is expected to yield a slowing down of SE. Figure 6 illustrates the evolution of t1 when the water of the drops contains CTAB only (a) and SDS only (b). It shows a decreasing (resp. increasing) linear trend for CTAB (resp. SDS). Both figures also demonstrate stronger sensitivity to CTAB and SDS for small Span 80 concentrations. This is consistent with the fact that CTAB adsorption results in a better neutralization of the water/PO interface charge, in respect to SDS adsorption, that is also reflected by a different stability of the corresponding emulsions. The results are also consistent with previous studies that investigated and reported the synergistic interactions between nonionic and cationic surfactants [START_REF] Roque | Stability and Characterization Studies of Span 80 Niosomes Modified with CTAB in the Presence of NaCl. Colloids Surfaces A Physicochem[END_REF][START_REF] Wiertel-Pochopien | Synergistic Effect of Binary Surfactant Mixtures in Two-Phase and Three-Phase Systems[END_REF][START_REF] Zhang | Adsorption of Mixed Cationic-Nonionic Surfactant and Its Effect on Bentonite Structure[END_REF] . For instance, the adsorption of CTAB was enhanced due to the synergies between CTAB and TX-100 (Triton X-100, non-ionic surfactant) [START_REF] Hou | Wettability Alteration of an Oil-Wet Sandstone Surface by Synergistic Adsorption/Desorption of Cationic/Nonionic Surfactant Mixtures[END_REF] . A similar behavior was also reported for the surfactant mixture of DDAB (didodecyldimethylammonium bromide, cationic surfactant) and Igepal CO-890 (polyoxyethylene-40 nonylphenyl ether, non-ionic surfactant) in oil-in-water (O/W) emulsion [START_REF] Banerjee | Effect of Electrolytes on Solution and Interfacial Behaviors of Double Chain Cationic-Nonionic Surfactant Mixtures for Hydrophobic Surface Wetting and Oil/Water Emulsion Stability Applications[END_REF] . In fact, while CTAB and SDS are good emulsifiers, it turns that below CMC, CTAB is a less efficient O/W emulsion stabilizer [START_REF] Zhu | Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant[END_REF][START_REF] Zhu | Stimuli-Responsive Biomass Cellulose Particles Being Able to Reversibly Self-Assemble at Fluid Interface[END_REF] than SDS that provides very stable emulsions already at concentrations well below the CMC 68 . This work provides new experimental insights into the complexity of spontaneous emulsification. Aqueous pendant drops suspended in a paraffin oil solution in the presence of non-ionic (Span 80) and ionic (CTAB and SDS) surfactants are studied. It is demonstrated that spontaneous emulsification generates microstructures at the water paraffin oil interface. For a given composition, three specific kinetic regimes mediated by the Span 80 concentration are identified. When adding CTAB (resp. SDS), SE kinetics turns faster (resp. slower). This change is explained by the polarity of water molecules and the modification in water/paraffin oil zeta potential associated with the ionic surfactants. CTAB tends in fact to reduce the zeta-potential and therefore facilitates the migration of water molecules while the SDS reinforces zeta-potential. Electrostatic interaction being strengthened, water transport is hindered, producing a delay in the occurrence of spontaneous emulsification. This work brings new insights and should help the understanding into possible mechanisms involved in spontaneous emulsification when ionic and non-ionic surfactants are in competition.

For Table of Contents Use Only

Figure 1 .

 1 Figure 1. Left: Pictures (1 x 1 mm 2 ) of a pendant drop for [Span 80] = 10 g/L at 30 °C and at different times. (a) t = 0 s, (b) t = 1100 s and (c) t = 1400 s and (d) t = 10000 s. Right: Zoom in the selected region of interest (red square in right images).

Figure 2 .

 2 Figure 2. (a): Evolution of f(t) for [Span 80] = 10 g/L at 30°C up to t=15000 s for three identical experiments. Evolution becomes irreproducible once the drop undergoes shrinking (t > 2500 s). (b): Evolution of averaged fitted contrast function <f(t)> for the three experiments shown in (a) and time sequences associated to regimes R0, R1 and R2.

  involved (see below), the global kinetics of the SE follows a rather simple law when changing composition. t = t !"# #

Figure 3 .

 3 Figure 3. t0, t1 and t2 as a function of [Span 80] at 30°C. Measurement points are the result of an average of at least three replica experiments. Errors bars are smaller than marker size when not visible.

Figure 4 .

 4 Figure 4. Evolution of <f(t)> for [Span 80] = 4 g/L at T=40°C. (a) [CTAB] = 0.06 g/L. (b) [SDS] = 0.3 g/L.

Figure 5

 5 Figure 5 displays the evolution of <f(t)> for [Span 80] = 10 g/L and for varying [CTAB]. The decaying trend is similar to that of Figure 2 but yet appears earlier for increasing [CTAB]. This is essentially explained by electrostatic interactions weakening. In the absence of surfactant, water-oil interfaces are negatively charged because of hydroxide ions adsorption[START_REF] Marinova | Charging of Oil-Water Interfaces Due to Spontaneous Adsorption of Hydroxyl Ions[END_REF][START_REF] Vácha | The Orientation and Charge of Water at the Hydrophobic Oil Droplet-Water Interface[END_REF][START_REF] Beattie | The Pristine Oil/Water Interface: Surfactant-Free Hydroxide-Charged Emulsions[END_REF][START_REF] Ma | Impact of Spontaneously Adsorbed Hydroxide Ions on Emulsification via Solvent Shifting[END_REF] . Being cationic, CTAB reduces zeta potential and, as local electrostatic interactions are weakened, adsorption energy barrier is lowered which promotes Span 80 adsorption. A similar behavior of CTAB is known to modify nano-emulsions stability[START_REF] Xin | Influence of CTAB and SDS on the Properties of Oil-in-Water Nano-Emulsion with Paraffin and Span 20/Tween 20[END_REF] . The trend of Figure5turns consistent with this loss of stability and suggest that nanostructures formed at the very beginning of SE become less stable for increasing CTAB concentration. CTAB and Span 80 therefore act in a way to displace the CTAB/Span 80/hydroxyl groups adsorption/co-adsorption quasi-equilibrium in the sense to favor Span 80 only, making zeta-potential even smaller and the overall water transfer faster.

Figure 5 .

 5 Figure 5. Evolution of <f(t)> as a function of [CTAB] for [Span 80] = 10 g/L at T=30°C. ¢ [CTAB]=0 g/L, 0.02 g/L, p 0.04 g/L,  0.06 g/L and £ 0.08 g/L.

Figure 6 .

 6 Figure 6. t1 as a function of [CTAB] (a) and [SDS] (b) for three values of [Span 80] (¢ 4 g/L, 8 g/L and p 10 g/L) at T=40 °C. Error bars are smaller than markers when not visible.

Acknowledgements

The authors acknowledge the financial support of CNES -Centre National des Etudes Spatialeswithin the project 'Stabilité des Emulsions -Mesures et modélisation' and ESA -European Space Agency, within the project 'EDDI-Emulsion Dynamics and Droplet Interfaces -EDDI', ESA Contract n. 4000128643/19/NL/PG and M. Yvan CECERES from MADIREL for important technical support.


Thierry Fouchet

Raphaël Moreno

Emmanuel Lellouch

Franck Montmessin

Marco Giuranna

Vittorio Formisano

Interferometric millimeter observations of water vapor on Mars

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Interferometric millimeter observations of water vapor on Mars

Abstract

We present interferometric mapping of the 225.9-GHz HDO and 203.4-GHz H 2 18 O lines on Mars obtained with the IRAM Plateau de Bure facility (PdBI). The observations were performed during martian year 28 (MY28), at L s = 320.3 • for the HDO line, and at L s = 324.3 • for the H 2 18 O line. The HDO line is detected at the eastern (morning) and western (evening) limbs of the northern hemisphere, corresponding to a water column density in the range 3-6 pr.-µm(Fig. 1). The H 2 18 O line is not detected, which is compatible with the column densities derived from the HDO line (Fig. 2). Quasi-simultaneous far infrared measurements obtained by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft confirm our PdBI results, yielding a 5 ± 1 pr.-µm meridionally constant water column abundance (Fig 3

T

  . Fouchet (1,4,5), R. Moreno (1), E. Lellouch (1), F. Montmessin (2), M. Giuranna (3) and V. Formisano (3) (1) Observatoire de Paris, LESIA, Meudon, France (thierry.fouchet@obspm.fr), (2) LATMOS, IPSL, Guyancourt, France, (3) IFSI-INAF, Roma, Italy, (4) Université Pierre et Marie Curie, UMR8109, Paris, France, (5)Institut Universitaire de France, Paris, France

  ). Such a low water abundance during the southern mid-autumn of MY28 does not correspond to the standard martian climatology as observed during the previous years. It was however already retrieved from near-infrared observations performed by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter spacecraft [Smith, M. D., Wolff, M. J., Clancy, R. T., Murchie, S. L. 2009. CRISM observations of water vapor and carbon monoxide. J. Geophys. Res. 114, doi:10.1029/2008JE003288]. Our observations thus confirm that the planet-encircling dust storm that occured during MY28 significantly affected the martian water cycle. Our observations also demonstrate the usefulness of interferometric submillimeter observations to survey the martian water cycle from groundbased facilities.

Figure 1 :Figure 2 :

 12 Figure 1: Spectral map in the HDO line at 225.897 GHz (black histogram lines), compared with synthetic models. At beam positions where the HDO line is detected the best-fit model is displayed in red along with the best-fit water column density. At beam positions where the HDO line is not detected a synthetic spectrum calculated for 5 pr-µm is displayed in green. RA, Dec offsets (in ) are indicated for each beam position. The surface, equator, central meridian and terminator of Mars are indicated along with the sub-Earth and subsolar points, and the synthetized beam.

Figure 3 :

 3 Figure 3: Left panel: Surface temperature (black diamond) and atmospheric temperature at 100 Pa (red triangles) retrieved from PFS spectra obtained during orbit #4791. Right panel: Water column abudance retrieved from the same dataset.


Timothy H Mcconnochie
email: tmcconnochie@spacescience.org

Thierry Fouchet

Franck Montmessin

P Beck

B Chide

R Francis

Olivier Gasnault

Jérémie Lasue

C Legett

Mark T Lemmon

Modes T H Mcconnochie

S Maurice

R T Newell

C E Newman

D M Venhaus

R C Wiens

M J Wolff

Mars Atmospheric Science with SuperCam's Visible and Near-Infrared Passive Spectroscopy Modes

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Introduction:

The Mars 2020 "Perseverance" rover's SuperCam instrument suite [1, 2 ,3, 4] provides a wide variety of active and passive remote sensing techniques [5,6,7,8] including passive visible & nearinfrared ("VISIR") spectroscopy [9]. Here we present our plans to use the VISIR technique for atmospheric science by observing solar radiation scattered by the Martian sky or by one of SuperCam's rover-mounted calibration targets [4]. See, e.g., [8], which describes the SuperCam microphone, for non-VISIR SuperCam atmospheric science. The technique of using VISIR-like passive-mode spectra of the sky, known as "passive sky", has previously been demonstrated with ChemCam on the Mars Science Laboratory (MSL) rover [10]. SuperCam provides a superset of the ChemCam capabilities used in [10], and in particular it adds a nearinfrared 1300-2600 nm spectral range that includes important gas and aerosol absorption and scattering features.

This presentation will focus on the objectives and methods of SuperCam VISIR atmospheric science. We do not expect that observations targeted for VISIR atmospheric science will have occurred prior to this year's conference. However, data acquired for other purposes may provide enough instrument performance information to yield preliminary validation of predicted atmospheric science capabilities, and we will present such validation results if available.

Objectives: O2 column abundance. O2 (like H2O) is a key player in Martian photochemical cycles, and its chemistry is crucial to the long-term stability of CO2 [e.g. 11]. It had been universally expected to have such a long chemical lifetime [e.g. 12, 13, 14] that it behaves like an inert trace gas on timescales relevant to atmospheric circulations and annual cycles. However, in-situ atmospheric sampling by the Sample Analysis at Mars (SAM) Quadrupole Mass Spectrometer (QMS) on the MSL rover showed unexpected seasonal and interannual variability in the O2 mixing ratio [15]. The magnitude of the unexpected variability was too large to be explained by any purely atmospheric source or sink because CO2 photolysis is known to be too slow and water vapor abundances are too small [15]. This lead the authors [15] to suggest source and sinks in surface soils and to note that perchlorates represent a sufficiently large reservoir of oxygen atoms, albeit one lacking a known mechanism for sufficiently rapid exchange with the atmosphere.

Recent numerical modeling of perchlorate chemistry implying the release of oxygen from perchlorates in the combined presence of silicates, water vapor, and UV radiation [16] in Mars-like conditions suggests a potential way forward for the O2 variability problem. Our objectives for O2 measurements are therefore to confirm the surprising results of [15] at a second site, and to test potential correlations between excess O2 variability and local or global water vapor abundance as well as local or global aerosol opacity (which modulates surface UV flux.) The desired precision for O2 measurements is ± 50 ppm in column-averaged volume mixing ratio, comparable to the typical precision in [15].

CO column abundance. In contrast to O2, CO has been observed [17] to follow the expected seasonal cycle of a chemically inert non-condensable trace gas. To resolve the expected seasonal cycle of CO, our target measurement precision is ± 100 ppm.

Water vapor column abundance. Local water vapor measurements are particularly valuable for assessing exchanges of water vapor between the atmosphere and surface materials. Our primary objective for water vapor is therefore to routinely sample the daytime water vapor column, for direct comparison with nighttime mixing ratio measurements made by the Mars 2020 Mars Environmental Dynamics Analyzer (MEDA) humidity sensor [18], but also to help connect in-situ measurements by MEDA with measurements from orbit. We will target a precision of ± 0.5 precipitable µm, comparable to ChemCam [10].

Aerosol particle sizes and composition. Both MEDA [18] and Mastcam-Z [19] on the Mars 2020 payload provide direct measurements of column aerosol opacity in addition to sky brightness measurements that can substantially constrain particle sizes. SuperCam will contribute to constraining particle sizes and will, with its broad spectral range and near-infrared coverage, be uniquely well-suited to constraining the relative contributions of water ice and dust to column opacity, and to distinguishing the dust aerosol particle size distribution from that of water ice.

Relationship to Mars 2020 mission objectives. Because both O2 and H2O cycle into and out of Martian soil, that soil is currently or was in the recent past in some kind of equilibrium with these atmospheric volatiles. Thus the atmospheric H2O and O2 cycles are an important part of the geochemical and geological context for near-surface samples and the Jezero field site. Aerosols, too, provide relevant geological context because some portion of surface-accumulated aerosols will inevitably be present in returned samples. In addition, evaluation of airborne dust hazards is an important precursor to human exploration, and aerosols along with water vapor and carbon monoxide are important for validating global atmospheric models. These species are important for model validation because they are tracers of dynamical processes, and because surface-atmosphere exchange of dust and volatiles are important modeled physical processes.

Measurement approach: The SuperCam spectrometers used for VISIR mode are: a ChemCam-heritage reflection spectrometer covering 385-465 nm ("violet") with < 0.2 nm res. [3]; an intensified transmission spectrometer covering 536-853 nm with 0.3 -0.7 nm res. [3]; an acousto-optic-tunablefilter (AOTF) -based IR spectrometer on the rover mast covering 1300 -2600 nm with 20 -30 cm -1 res. [2,9].

Primary "passive sky" observing strategy. Our primary observing strategy is the same approach used for MSL ChemCam "passive sky" observations [10]: we point SuperCam at two different elevation angles in the sky that yield two different path lengths through the gas absorptions of interest. Ratioing instrument signals from the two pointing positions eliminates solar spectrum and instrument response uncertainties that are ~100x and ~10x larger than signals of interest for the transmission and AOTF IR spectrometers, respectively. For the intensified transmission spectrometer, instrument response will fluctuate significantly with instrument temperature, hence we must ratio different atmospheric pointings acquired within minutes of each other. To obtain the sensitivity objectives for O2 and H2O vapor, we estimate ~15 minutes of observation time will be required including slew time and integration with all three spectrometers. (We estimate 5 -10 additional required minutes if the CO objective is included.)

Single-point observing strategy. AOTF instrument response is expected to be sufficiently repeatable over periods of tens of days, allowing a relative calibration for single pointings spanning these time periods. Therefore, between primary passive sky observations we will sample H2O and aerosols with single pointings directed at the white SuperCam calibration target, which serves to scatter direct solar flux toward the optics.

Aerosol-only observing strategy. If we drop the requirement for the extremely high precision needed to measure gas absorption (which is S/N ~ 3,000 for the transmission spectrometer and ~1,000 for the IR spectrometer), we can better constrain aerosol properties by observing at significantly more than two pointing positions in a reasonable amount of time; the improved constraint comes from sampling numerous points on the aerosol scattering phase function.

Background subtraction. The AOTF IR spectrometer is designed to acquire a dark current measurement with every integration. For the violet and transmission spectrometers a portion of spectral acquisitions measure dark current by acquiring nonilluminated detector rows, and by leaving the intensifier effectively switched off, respectively. Small remaining background residuals, due to dark current nonuniformity and intensifier background, are likely negligible (~0.5 DN per acquired spectrum), but we will measure them with occasional calibrations acquired at night with the rover in total darkness.

Radiative transfer modeling and retrieval approach: We will use the same discrete ordinates multiple scattering radiative transfer model (with correlated-k gas absorptions) used by [10] to model ChemCam "passive sky" observations and retrieve gas and aerosol abundances. The planned retrieval will proceed by using downhill-simplex optimization to solve for dust and ice particle size, ice opacity contribution, plus one dust and one ice vertical profile parameter, simultaneously. The vertical profile parameters are controlled by the depth of CO2 absorption bands near ~790 nm and ~2000 nm. We have shown

[10] that these CO2-band driven parameterizations remove the influence of aerosol scattering uncertainties on gas absorption band models.

1367.pdf 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

The non-CO2 gas absorption bands are fitted individually with a grid-search approach.

References: [1] Wiens R. G. et al. (2021), this meeting. [2] Maurice S. et al. (2020) SSR, in press. [3] Wiens R.C. et al. (2021) SSR 217, 4. [4] Manrique J.-A. et al. (2020) SSR 216, 138. [5] Beyssac O. et al. (2021), this meeting. [6] Ollila A.M. et al. (2018) LPSC 49, 2786. [7] Anderson R. B. et al. (2021), this meeting. [8] Chide B. et al. (2021), this meeting. [9] Fouchet T. et al. (2021), this meeting. [10] McConnochie T. H et al. (2018), Icarus 307, 294. [11] Atreya S. K. and Gu Z. G. (1994), JGR 99, 13133. [12] Atreya S. K. and Gu Z. G. (1995), Adv. Space Res, 16, #6, 57. [13] Clancy R. T. and Nair H. (1996), JGR 101, 12785. [14] Lefèvre F. and Krasnopolsky V. (2017), in The Atmosphere and Climate of Mars, Cambridge U. Press, 374. [15] Trainer M. G. et al. (2019), JGR 124, 3000. [16] Escamilla-Roa E. et al. (2020) Applied Surface Science, 512, 145634. [17] Smith M. D. et al. (2018), Icarus 301, 117. [18] Manfredi et al. (2020) SSR, in review. [19] Bell et al. (2020), SSR, in press.