Emanuel Granero Fernandez

Jean-Stèphane Condoret

Vincent Gerbaud
email: vincent.gerbaud@ensiacet.fr

Yaocihuatl Medina Gonzalez
email: yaocihuatl.medinagonzalez@ensiacet.fr

Molecular Dynamics Simulations of Gas-Expanded Liquids

Keywords: molecular dynamics, alkyl acetates, viscosity, density

Gas-expanded Liquids (GXL's) are liquids whose volume is expanded by a pressurized gas. The incorporation of the gaseous compound and the expansion of the liquid phase lead to changes in solvation and transport properties of the new expanded phase.

This work aims at developing a Molecular Dynamics (MD) computational study for liquid methyl and ethyl acetates when expanded by CO 2 at pressures up to 120 bars and at temperatures between 298 and 393 K. Especially, density and viscosity of the liquid expanded phase were calculated by MD simulations. Good agreement with existing literature data has been obtained ascertained the validity of the method

Introduction

With the perspective of finding alternative benign media for different applications, solvent engineering represents an interesting alternative to propose innovative solutions to industrial problems. Gas-expanded liquids (GXLs), in particular CO 2 -expanded liquids (CXLs), which are liquids whose volume is expanded by pressurized CO 2 , represent a very promising strategy to perform solvent engineering.

A number of properties, including solvation properties, can be tuned by the addition of CO 2 to a liquid solvent [START_REF] Jessop | Gas-expanded liquids[END_REF] allowing the use of CXLs as "solubility switchers". In many cases, solutes that are soluble in CO 2 -expanded solvents will precipitate when CO 2 is removed and vice versa. For example, the addition of CO 2 to a biphasic system of a fluorinated solvent and an organic solvent causes the phases to become miscible [START_REF] West | CO2-Induced Miscibility of Fluorous and Organic Solvents for Recycling Homogeneous Catalysts[END_REF]. Opposite effect is seen in cases of water-soluble organics and an aqueous phase, where the addition of CO 2 forces the phases to split [START_REF] Lu | Tunable Solvents for Homogeneous Catalyst Recycle[END_REF]. Both systems may have applications in catalyst recycle and/or product recovery. Knowledge of phase equilibrium properties but also of transport properties and in particular viscosity, is evidently important to design processes exploiting the peculiar behavior of these two-phase systems.

Moreover, high-pressure experimental determinations of density and viscosity of systems involving hazardous compounds and/or at harsh conditions of pressure and temperature are usually problematic. As an alternative, numerical experiments with the help of molecular simulation, like Molecular Dynamics (MD) have gained interest in recent years as the performance of computational tools is continuously improving. In particular, MD simulations have gained enormous progress in terms of 1) simulation methods, 2) understanding of complex phenomena, and 3) ability to model extremely complex systems. MD simulations accuracy is highly dependent on the proper representation of the interaction forces acting on the molecules. This depends on the choice of a suitably parametrized force field and on a statistically representative sampling of the states of the system. Previous works on CO 2 -expanded acetates [START_REF] Smith | Volumetric behavior of ethyl acetate, ethyl octanoate […] in the presence of supercritical CO2[END_REF][START_REF] Aida | Analysis of volume expansion mechanism of CO2-acetate systems at 40 °C[END_REF] describe some of these system properties, such as the Vapor-Liquid Equilibrium curves and the expanded phase density. However, no data were found about the viscosity of the alkyl acetates/CO 2 expanded phase, probably due to the difficulty of high-pressure expanded phase determinations.

Hence, this work rather to present a new methodology of properties calculation, intends to emphasize an alternative application for MD simulations, as it is to numerically predict equilibria and transport properties of CO 2 -expanded acetates. The MD method and its accuracy were first evaluated for other CO 2 -expanded solvents before predicting the densities and the viscosities of systems involving methyl and ethyl acetate. Indeed, Acetone (C 3 H 6 O) and Decane (C 10 H 22 ) where chosen for this task for two main reasons: The availability of CO 2 -expanded phase viscosity data and the presence of hydrocarbon chain in decane and the carbonyl group in acetone, both groups found in alkyl acetates. It is worth to note that accurate results for CO 2 -expanded acetates systems will open up the possibility to study more complex CO 2 -expanded systems, for which no experimental data exist.

Method

Theory background

The fluctuation-dissipation theorem explains how transport coefficients associated to irreversible processes can be described using reversible microscopic fluctuations [START_REF] Kubo | The fluctuation-dissipation theorem[END_REF]. The Green-Kubo [START_REF] Green | Markoff Random Processes and the Statistical Mechanics of Time-Dependent Phenomena. II. Irreversible Processes in Fluids[END_REF][START_REF] Kubo | Statistical-Mechanical Theory of Irreversible Processes. I. General Theory and Simple Applications to Magnetic and Conduction Problems[END_REF] relations give the exact mathematical expression for transport coefficients in terms of integrals of timecorrelation functions.

In the standard Green-Kubo method shear viscosity can be calculated by integrating the autocorrelation function of the stress tensor over time (1):

(1)

Where P xy (t)P xy (0) is the correlation function of the xy component of the stress tensor, V is the volume of the simulation box, k B is the Boltzmann constant, T is the absolute temperature and t is the time.

Simulation details

The required liquid phase composition data at equilibrium was extracted from literature and from our own determinations. MD computations were carried out using the Materials Processes and Simulations (MAPS) Suite from Scienomics and in the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code from Sandia National Laboratories. The "Amber_Cornell_Extended" force field was selected (Table 1). The geometries for all the molecules were optimized at the HF/6-31G+** level by using DFT quantum calculations. The partial charges were then fitted using two separate methods. The first one used the Electrostatic Potential method (ESP) on the optimized geometries at the HF/6-31G+** level. The second one used the AM1-BBC method, which includes the optimization of the initial geometry at the AM1 level, and after the calculation of the Mulliken charges, the BCC bond correction was applied (Semi empirical Quantum Mechanics). The fitted charges are displayed in Table 2.

After repeated tests, a number of molecules per box and a finite time of equilibration, production and integration were defined. The simulated boxes were set with periodic boundaries in all directions containing a total of 1000 molecules in the Isobaric-Isothermal ensemble. The simulations were performed as follows: NPT equilibration and density production runs of 1,200,000 steps. Afterwards, NVT equilibration runs for 400,000 steps were followed by NVE viscosity production runs for 1,200,000 steps. All runs were performed with a timestep of 1 fs. The L-J and Coulombic cut-off was set to 8 Å with a switching distance of r =12 Å. The simulations were started with randomly assigned velocities, the total momentum of the system was set to zero and Newton's equations of motion were solved with the standard Velocity-Verlet algorithm.

Calculations were performed with the EOS super-calculator of CALMIP. [START_REF] Cullick | Densities and Viscosities of Mixtures of Carbon Dioxide and n-Decane from 310 to 403 K and 7 to 30 MPa[END_REF].

Results and Discussion

In Figures 1 c) andd) two distinct zones can be observed. Firstly, the density increases monotonically as the CO 2 solubilizes. Then, after a certain point (x CO2 ≈ 0.7), higher CO 2 molar fraction produces a drastic volume increment and the consequent decrease in density. Error bars were calculated as the Standard Deviation between the average values every 10.000 steps. This behavior has been explained by an increase in free volume observed in some CXLs [START_REF] Li | Solvation and Solvatochromism in CO2-Expanded Liquids. 1[END_REF], which would be responsible for their fluidity and solvation properties. Indeed, the initial solvent intermolecular spaces can first host the CO 2 to some extent, producing an imperceptible increment in volume. Then, after a certain threshold in CO 2 , its solubilisation induces a rapid volume expansion. At this point, the model lacks of accuracy with increasing CO 2 concentration because of its molecular quadrupole, and the error in calculated values against literature increases.

Viscosity

Calculated viscosities are shown in Figure 2 as function of CO 2 molar fraction at different temperatures and pressures up to 120 bars. Literature data is plotted when available. Figure 2 shows a) CO 2 +Acetone and b) CO 2 +Decane systems for a single T with both charges sets; and c) CO 2 +MA and d) CO 2 +EA systems viscosity at different temperatures as a function of x CO2 . For these two systems, no literature data was found concerning viscosity. Standard deviation for all systems falls under 5 E-10 due to integration over time smoothing. The observed viscosities behavior coherent with previously reported CO 2 -expanded systems [START_REF] Li | Solvation and Solvatochromism in CO2-Expanded Liquids. 1[END_REF]. In these systems, the viscosity decreases as a quasi linear function of CO 2 mole fraction. This general behavior is explained by the decrease of electrostatic forces between the solvent molecules due to the interposing CO 2 molecules, clustering around carbonyl groups [START_REF] Aida | Analysis of volume expansion mechanism of CO2-acetate systems at 40 °C[END_REF]. No significant differences can be appreciated between ESP and AM1 charges, showing that the Coulombic energies have a smaller impact on pressure fluctuation than L-J parameters.

Conclusion

MD simulations and Green-Kubo stress correlation function were used to predict densities and viscosities of CO 2 -expanded ethyl and methyl acetates systems. The technique was also assessed as relatively accurate by comparing simulated densities and viscosities of CO 2 -expanded acetone and decane systems to literature data. This simple approach using existing MD potentials and DFT charges calculation can be used now to obtain other properties of other promising CO 2 -expanded fluids.

Figure 1 :

 1 Figure1: CO 2 -expanded density (g/cm 3 ) vs CO 2 molar fraction for all studied systems. Literature refs: (a)[START_REF] Sih | Viscosity measurements on saturated gas expanded liquid systems-Acetone and carbon dioxide[END_REF]; (b)[START_REF] Cullick | Densities and Viscosities of Mixtures of Carbon Dioxide and n-Decane from 310 to 403 K and 7 to 30 MPa[END_REF].

Figure 2 :

 2 Figure 2: Viscosity (cP) vs CO 2 molar fraction of CO 2 -expanded acetone, decane, MA and EA. Refs a) (Sih and Foster 2008) b)[START_REF] Cullick | Densities and Viscosities of Mixtures of Carbon Dioxide and n-Decane from 310 to 403 K and 7 to 30 MPa[END_REF],

Table 1 :

 1 Amber_Cornell_Extended Force field bond and pairwise potentials

	Bond	Angle	Improper	Dihedral (hybrid)
	VdW : L-J 12-6		Coulombic

Table 2 :

 2 Fitted atomic center charges

	Atom	AM1	ESP	Atom	AM1	ESP
	Acetone			Decane
	C	-0,2041	-0,522162	C	-0,21	-0,326006
	C1	0,5631	0,811667	C1	-0,159	0,213133
	C2	-0,2041	-0,512053	C2	-0,158	0,05664
	O	-0,5311	-0,580332	C3	-0,158	-0,085466
	H	0,063033	0,141684	C4	-0,158	0,11713
	H1	0,063033	0,131853	C5	-0,158	0,12006
	H2	0,063033	0,131853	C6	-0,158	-0,087939
	H3	0,063033	0,138492	C7	-0,158	0,057575
	H4	0,063033	0,129499	C8	-0,159	0,213154
	H5	0,063033	0,129499	C9	-0,21	-0,324668
				H	0,072	0,068357
	Methyl Acetate (MA)			H1	0,072	0,068324
	C	-0,1501	-0,664509	H2	0,072	0,075387
	C1	0,6291	0,915207	H3	0,078	-0,040085
	O	-0,4449	-0,433813	H4	0,078	-0,040075
	C2	0,1267	0,0011	H5	0,079	-0,018183
	O1	-0,541	-0,594817	H6	0,079	-0,018193
	H	0,0757	0,180868	H7	0,079	-0,001635
	H1	0,0757	0,178806	H8	0,079	-0,001689
	H2	0,0757	0,17922	H9	0,079	-0,034637
	H3	0,0617	0,084701	H10	0,079	-0,034645
	H4	0,0447	0,076499	H11	0,079	-0,035151
	H5	0,0447	0,076736	H12	0,079	-0,035188
				H13	0,079	-0,001125
	Ethyl Acetate (EA)			H14	0,079	-0,001192
	C	-0,1501	-0,603183	H15	0,079	-0,018348
	C1	0,6311	0,910216	H16	0,079	-0,018373
	O	-0,4469	-0,51646	H17	0,078	-0,040213
	C2	0,1384	0,356665	H18	0,078	-0,040178
	C3	-0,1001	-0,34453	H19	0,072	0,075719
	O1	-0,544	-0,613058	H20	0,072	0,068759
	H	0,0517	0,011202	H21	0,072	0,06875
	H1	0,0517	0,011164
	H2	0,0757	0,16283
	H3	0,0757	0,167183
	H4	0,0757	0,167011
	H5	0,0467	0,078193
	H6	0,0477	0,106401
	H7	0,0477	0,106364

Acknowledgment

This work has been carried out with help of the CALMIP supercomputer facility (project n° P1535). Authors deeply thank CALMIP for the hours allocated to this project.


Lyubov A Ryabova

Christophe Robaglia

Christian Meyer
email: christian.meyer@inra.fr

Keywords: Abiotic stresses, abscisic acid, auxin, biotic stresses, mRNA translation, nutrient signaling, rapamycin, S6 kinase, SnRK1 kinase, sugars, TOR kinase

Target of Rapamycin kinase: central regulatory hub for plant growth and metabolism

The adaptation of plants to their environment requires tight regulation of metabolism and growth processes through central and highly connected signalling pathways. The signalling cascade involving the evolutionarily conserved Target of Rapamycin (TOR) represents just such a central regulatory hub, and research on this protein kinase in plants has progressed significantly during the past decade. TOR is now firmly established as a central player in plant responses to the availability of nutrients such as sugars, stresses including those from pathogens, and hormones. Moreover plant-specific targets and regulators have recently been identified. The reviews in this special issue explore the various facets of regulation exerted by this fascinating kinase as well as its potential for crop improvement.

Eukaryotic cells and organisms need to adjust basic processes such as cell division, growth and metabolism to the available resources and external conditions. These are sensed either directly, as in unicellular eukaryotes, or indirectly, for example through hormonal or nutrient signals, as occurs in multicellular organisms. Central to this regulation is the conserved kinase Target of Rapamycin (TOR), which has been shown to be a critical component of sensing mechanisms.

The discovery of TOR is a perfect example of the serendipitous nature of research. It all started with the identification of a molecule produced by Streptomyces hygroscopicus, a bacterium isolated in the 1970s in a soil sample from the remote and mysterious Easter Island, known as Rapa Nui in Polynesian. This compound was thus named rapamycin (Vezina et al., 1975). Rapamycin was found to inhibit cell proliferation but the mechanism of action was unknown. It was only in the 1990s that Michael Hall's group in Basel identified mutations in yeast which conferred resistance to rapamycin. This led to the seminal discovery of TOR, inhibition of which by rapamycin leads to arrested growth [START_REF] Heitman | Targets for cell cycle arrest by the immunosuppressant rapamycin in yeast[END_REF][START_REF] Montané | TOR inhibitors: from mammalian outcomes to pharmacogenetics in plant and algae[END_REF]. Michael Hall was later awarded the 2017 Lasker prize for medical research for this paramount discovery.

TOR was subsequently identified in humans, where it was given the name mTOR (mammalian or mechanistic TOR), in various animals including flies and worms [START_REF] Saxton | mTOR signaling in growth, metabolism, and disease[END_REF][START_REF] Mossmann | mTOR signalling and cellular metabolism are mutual determinants in cancer[END_REF], flowering plants [START_REF] Menand | Expression and disruption of the Arabidopsis TOR (target of rapamycin) gene[END_REF] and algae (Perez-Perez et al., 2017). In all eukaryotes, TOR is a very large (around 250 kDa) serine/threonine kinase belonging to the phosphatidylinositol 3-kinase-related kinase (PIKK) family. Indeed, although TOR is a protein kinase, its catalytic domain is unconventional and resembles that of PI lipid kinases. Other members of this family include the conserved DNA damage checkpoint ataxia-telangiectasia mutated (ATM) and ATMrelated (ATR) kinases, which are also present in plants.

In yeast and animals, where TOR has been studied extensively, various upstream regulators and downstream effectors have been discovered, forming a complex and highly connected signalling pathway [START_REF] Blenis | TOR, the gateway to cellular metabolism, cell growth, and disease[END_REF]. In these organisms ScTOR and mTOR play a major role in the regulation of cell growth and metabolism through the control of gene transcription, cell trafficking, insulin responses as well as protein synthesis and degradation. Therefore, TOR is involved in many human diseases including cancer and diabetes. Thorough biochemical work showed that TOR belongs to two very large complexes, TORC1 and TORC2, containing common and specific companion proteins [START_REF] Wullschleger | TOR signaling in growth and metabolism[END_REF][START_REF] Saxton | mTOR signaling in growth, metabolism, and disease[END_REF][START_REF] Mossmann | mTOR signalling and cellular metabolism are mutual determinants in cancer[END_REF]. Structural models for TOR complexes are now available [START_REF] Aylett | Architecture of human mTOR complex 1[END_REF][START_REF] Karuppasamy | Cryo-EM structure of Saccharomyces cerevisiae target of rapamycin complex 2[END_REF]. In plants only the TORC1 complex has been described so far which, together with TOR, comprises the evolutionarily conserved LST8 and RAPTOR proteins (Box 1).

TOR and the control of plant growth

As in yeast and animals, TOR has been implicated in controlling plant growth and cell division [START_REF] Caldana | The magic 'hammer' of TOR: the multiple faces of a single pathway in the metabolic regulation of plant growth and development[END_REF] and prominent inducers of TOR activity have been shown to be sucrose and glucose [START_REF] Xiong | Rapamycin and glucose-target of rapamycin (TOR) protein signaling in plants[END_REF]Dobrenel et al., 2016a;[START_REF] Shi | TOR signaling in plants: conservation and innovation[END_REF]. Indeed, a strong sugar-TOR growth-controlling axis has emerged from several studies in both roots and shoots [START_REF] Xiong | Glucose-TOR signalling reprograms the transcriptome and activates meristems[END_REF][START_REF] Pfeiffer | Integration of light and metabolic signals for stem cell activation at the shoot apical meristem[END_REF][START_REF] Wu | Integration of nutrient, energy, light, and hormone signalling via TOR in plants[END_REF]. Basically, sugars like sucrose and glucose strongly up-regulate TOR activity by a largely unknown mechanism and TOR activates meristems by inducing E2F and Wuschel transcription factors in root and shoot apical meristems, respectively [START_REF] Xiong | Glucose-TOR signalling reprograms the transcriptome and activates meristems[END_REF][START_REF] Pfeiffer | Integration of light and metabolic signals for stem cell activation at the shoot apical meristem[END_REF]. Nutrient starvation or defects in nutrient assimilation also affect TOR activity. For example, a decrease in sulfur assimilation due to mutations in the sulfite reductase gene diminishes TOR activity and growth through a reduction is sugar accumulation [START_REF] Dong | Sulfur availability regulates plant growth via glucose-TOR signaling[END_REF]. The review by Ahmad [START_REF] Leiber | The TOR pathway modulates the structure of cell walls in Arabidopsis[END_REF], and a new suppressor of lrx1 (leucine-rich repeat extensin 1), a mutation which affects cell wall architecture and is compensated by TOR inhibition, has been identified and found to encode isopropylmalate synthase 1, a protein involved in leucine biosynthesis [START_REF] Schaufelberger | Mutations in the Arabidopsis ROL17/isopropylmalate synthase 1 locus alter amino acid content, modify the TOR network, and suppress the root hair cell development mutant lrx1[END_REF].

Reduction of TOR activity by starvation or other signals increases lifespan for several eukaryotes [START_REF] Wang | Reciprocal regulation of the TOR Kinase and ABA receptor balances plant growth and stress response[END_REF]. Indeed, inhibition of TOR with rapamycin is currently the only known treatment that increases lifespan in all organisms that have been studied. In plants, however, there is a more complex relationship between TOR activity and aging [START_REF] Quilichini | Live long and praise TOR: a role for TOR signaling in every stage of plant life[END_REF].

TORC1: master regulator of stress and adaptive responses

In plants, data suggests that activity of the nutrient-and energy-induced TOR kinase is balanced in an antagonistic and probably reciprocal way with one of the starvation-induced kinases of the SnRK (Snf1-related kinase) 1 and 2 families (Box 1; Dobrenel et al., 2016a;Baena-Gonzalez and Hanson, 2017;[START_REF] Jamsheer | Evolution of TOR-SnRK dynamics in green plants and its integration with phytohormone signaling networks[END_REF][START_REF] Margalha | SnRK1 and TOR: modulating growth-defense trade-offs in plant stress responses[END_REF]. SnRK1 has been shown to phosphorylate the RAPTOR component of the TORC1 complex and to inactivate TOR [START_REF] Nukarinen | Quantitative phosphoproteomics reveals the role of the AMPK plant ortholog SnRK1 as a metabolic master regulator under energy deprivation[END_REF]; this function is evolutionarily conserved, since the animal orthologue of SnRK1, the AMP-activated kinase, performs the same post-translational modification to inactivate TOR in response to energy and carbon starvation [START_REF] Gwinn | AMPK phosphorylation of raptor mediates a metabolic checkpoint[END_REF]. Interestingly, changes in subcellular localization of SnRK1 could be involved in inactivating TOR in different compartments [START_REF] Blanco | Dual and dynamic intracellular localization of Arabidopsis thaliana SnRK1.1[END_REF]. SnRK2s have also recently been shown to phosphorylate RAPTOR and to inactivate TOR in response to ABA [START_REF] Wang | Reciprocal regulation of the TOR Kinase and ABA receptor balances plant growth and stress response[END_REF]. Consistently the same report showed that TOR is able to repress ABA signalling in normal, non-stressed conditions by phosphorylating and inactivating the ABA PYL receptor and that there is a close and antagonistic relationship between TOR and ABA signalling (Box 1; [START_REF] Deprost | The Arabidopsis TOR kinase links plant growth, yield, stress resistance and mRNA translation[END_REF][START_REF] Punzo | TIP41 network analysis and mutant phenotypes predict interactions between the TOR and ABA pathways[END_REF]Forzani et al., 2019, Preprint;[START_REF] Wu | Integration of nutrient, energy, light, and hormone signalling via TOR in plants[END_REF]. Furthermore, the YAK1 Box 1. The basics of plant TOR

The conserved TOR (Target of Rapamycin) protein kinase acts in TORC1 (TOR complex 1) with LST8 and RAPTOR protein partners. This complex is found in all eukaryotes and is an important molecular element connecting nutrient, hormonal and stress signals to metabolism, growth and hormonal responses. TORC1 can be seen as a switch turning on anabolic and growth processes when conditions are favourable and inhibiting catabolism and nutrient recycling by autophagy. TORC1 controls mRNA translation globally but also the translation of specific mRNAs such as those encoding proteins needed for stress or hormone responses. Mounting evidence suggest that TOR and SnRK1/2 (Snf1related kinases) act in an opposing way and it has been shown that SnRKs inhibit TOR activity by phosphorylating the RAPTOR protein.

In recent years, many targets of TOR have been identified in plants and algae including the ribosomal protein S6 kinase, the PP2A phosphatase partner TAP46, several components of the translation and cell division machinery, and also the PYL ABA receptors.

HEAT, HEAT [Huntingtin, elongation factor 3 (EF3), protein phosphatase 2A (PP2A), TOR1] repeat domain involved in protein-protein interactions; FRB, FKBP12-rapamycin binding domain; KD, kinase domain.

kinase, which is related to ABA signalling [START_REF] Kim | Arabidopsis YAK1 regulates abscisic acid response and drought resistance[END_REF], has been shown to suppress the effects of TOR inhibition (Box 1; [START_REF] Barrada | A TOR-YAK1 signaling axis controls cell cycle, meristem activity and plant growth in Arabidopsis[END_REF]Forzani et al., 2019, Preprint).

TOR, as a central regulatory module, has connections with many plant hormones apart from ABA [START_REF] Schepetilnikov | Recent discoveries on the role of TOR (Target of Rapamycin) signaling in translation in plants[END_REF][START_REF] Jamsheer | Evolution of TOR-SnRK dynamics in green plants and its integration with phytohormone signaling networks[END_REF]. For example, brassinosteroids partly suppress the defects in hypocotyl elongation observed in TOR-deficient lines [START_REF] Zhang | TOR signaling promotes accumulation of BZR1 to balance growth with carbon availability in arabidopsis[END_REF] and the ROP2 small Rho protein participates in the auxin-induced stimulation of TOR activity [START_REF] Li | Differential TOR activation and cell proliferation in[END_REF][START_REF] Schepetilnikov | GTPase ROP2 binds and promotes activation of target of rapamycin, TOR, in response to auxin[END_REF]. Light is also an efficient inducer of TOR and recently the stimulation of auxin-ROP2-TOR signalling and mRNA translation observed in the light has been shown to be connected with COP1 (constitutive morphogenesis 1) [START_REF] Cai | COP1 integrates light signals to ROP2 for cell cycle activation[END_REF][START_REF] Chen | TOR and RPS6 transmit light signals to enhance protein translation in deetiolating[END_REF][START_REF] Ahmad | Cell cycle control by the target of Rapamycin signalling pathway[END_REF].

Involvement of TOR in the trade-off between growth and defence

Given the central role of the TOR pathway in regulating gene expression, protein synthesis and metabolic adjustments, it could be questioned whether it is also wired to defence responses or targeted during pathogen infection. Growth and defence responses are known to be antagonistic and TOR inhibition was found to prevent growth and activate the salicylic acid-dependent defence pathway in Arabidopsis [START_REF] Moreau | Mutations in the Arabidopsis homolog of LST8/GβL, a partner of the target of Rapamycin kinase, impair plant growth, flowering, and metabolic adaptation to long days[END_REF][START_REF] Dong | Expression profiling and functional analysis reveals that TOR is a key player in regulating photosynthesis and phytohormone signaling pathways in Arabidopsis[END_REF]. [START_REF] Meteignier | Translatome analysis of an NB-LRR immune response identifies important contributors to plant immunity in Arabidopsis[END_REF] found a significant subset of genes translationally regulated by both TOR silencing and activation of the immune response through conditional expression of the Pseudomonas syringae avrRPM1 elicitor. Consistently, TOR-silenced plants display enhanced resistance to P. syringae, leading to the hypothesis that active TOR negatively regulates immunity at the translational level.

In another study, the awr5 effector of Ralstonia solanacearum when expressed in yeast produced similar effects to rapamycin [START_REF] Popa | The effector AWR5 from the plant pathogen Ralstonia solanacearum is an inhibitor of the TOR signalling pathway[END_REF]. TOR inhibition by AZD8055 was also found to block the growth of Xanthomonas citri in Citrus spp. [START_REF] Soprano | Regulation of tRNA biogenesis in plants and its link to plant growth and response to pathogens[END_REF]. The X. citri effector PthA4 is an interactor of Maf1, a conserved target of TOR, and repressor of Pol III and growth. Maf1 phosphorylation levels decreased upon TOR knockdown and increased following knockdown of the catalytic subunits of PPP4 and PP2A [START_REF] Ahn | Characterization of Maf1 in Arabidopsis: function under stress conditions and regulation by the TOR signaling pathway[END_REF].

The role of TOR in the trade-off between growth and immunity was further substantiated in rice; genetic experiments involving overexpression or silencing of OsTOR or OsRAPTOR, combined with rapamycin treatments, showed that TOR negatively regulates several defence-related WRKY and MYB transcription factors, JA and SA signalling and resistance to various pathogens such as Xanthomonas oryzaea [START_REF] Vleesschauwer | Target of rapamycin signaling orchestrates growth-defense trade-offs in plants[END_REF]. TOR down-regulation or raptor mutation were also reported to reduce susceptibility to Fusarium graminearum in an Arabidopsis infection model [START_REF] Aznar | TOR signaling downregulation increases resistance to the cereal killer Fusarium graminearum[END_REF]. In this work the TOR inhibitor PP242 was also found to reduce leaf infection by Fusarium, although a direct role of the TOR inhibitor on the eukaryotic pathogen itself is probable and cannot be ruled out. Indeed, beside its emerging role in the trade-off between growth and immunity in plant hosts, an active TOR pathway is also involved in the virulence of several eukaryotic pathogens [START_REF] Shertz | Exploiting and subverting Tor signaling in the pathogenesis of fungi, parasites, and viruses[END_REF] Several links between TOR and pathogen infection emerged from virus studies [START_REF] Margalha | SnRK1 and TOR: modulating growth-defense trade-offs in plant stress responses[END_REF]. One of the first discoveries was the direct interaction between TOR and the Cauliflower mosaic virus transactivator protein (TAV) which promotes reinitiation of translation along the CaMV multicistronic mRNA [START_REF] Schepetilnikov | Viral factor TAV recruits TOR/S6K1 signalling to activate reinitiation after long ORF translation[END_REF]. The role of TOR was also investigated in the case of potyviruses, the largest group of plant RNA viruses. It was found that TOR-silenced or AZD8055-treated plants were resistant to Watermelon Mosaic Virus (WMV). Remarkably, AZD8055 treatment was able to eliminate the virus from infected plants [START_REF] Ouibrahim | Potyviruses differ in their requirement for TOR signalling[END_REF]. More recently, repression of TOR activity by Tombusvirus infection was observed in plants and yeast, and TOR pathway inhibition or genetic repression was shown to reduce virus replication [START_REF] Inaba | Tombusvirus RNA replication depends on the TOR pathway in yeast and plants[END_REF]. It was further hypothesized that Tombusviruses may recruit the glycolytic pathway, which is regulated by TOR, to furnish energy for virus replication and that TOR repression is part of the plant defence response.

TOR controls mRNA translation

Stress negatively affects plant growth and development, triggering transcriptional, translational and metabolic reprogramming [START_REF] Bechtold | Molecular mechanisms controlling plant growth during abiotic stress[END_REF][START_REF] Margalha | SnRK1 and TOR: modulating growth-defense trade-offs in plant stress responses[END_REF]. Indeed, cells can increase or decrease the production of proteins to accommodate changes in response to stress (Bailey-Serres and Juntawong, 2012; [START_REF] Schepetilnikov | Recent discoveries on the role of TOR (Target of Rapamycin) signaling in translation in plants[END_REF].

TOR-related pathways and molecular mechanisms that mediate the impact of stress signals on translation are largely unknown. Low energy stress can drastically decrease global protein synthesis rates, manifested by low levels of active ribosomes (polysomes), but this is reversible in conditions of energy supply [START_REF] Tomé | The low energy signaling network[END_REF].

Cold treatment can reduce global polysome levels in rice [START_REF] Park | Posttranscriptional control of photosynthetic mRNA decay under stress conditions requires 3' and 5' untranslated regions and correlates with differential polysome association in rice[END_REF], while in Arabidopsis they are not significantly affected. Whether TOR is involved in cold stress tolerance in Arabidopsis, as was suggested for AtGCN1 [START_REF] Wang | The inhibition of protein translation mediated by AtGCN1 is essential for cold tolerance in Arabidopsis thaliana[END_REF], would require further research. However, the authors suggest that decreased translation levels in conditions of TOR inhibition might increase the ability of Arabidopsis seedlings to survive under cold stress [START_REF] Wang | The inhibition of protein translation mediated by AtGCN1 is essential for cold tolerance in Arabidopsis thaliana[END_REF].

The most critical TOR upstream input comes from light and photosynthesis; suppression of TOR activity negatively affects light-dependent plant growth [START_REF] Chen | TOR and RPS6 transmit light signals to enhance protein translation in deetiolating[END_REF]. Although there is a tight link between light, TOR activity, RPS6 (ribosomal protein S6) phosphorylation and global protein synthesis efficiency [START_REF] Piques | Ribosome and transcript copy numbers, polysome occupancy and enzyme dynamics in Arabidopsis[END_REF][START_REF] Floris | Posttranscriptional control of light-harvesting genes expression under light stress[END_REF][START_REF] Enganti | Phosphorylation of ribosomal protein RPS6 integrates light signals and circadian clock signals[END_REF], mechanisms used by TOR to impact global protein synthesis have not yet been properly elucidated.

Recent work has identified an MRF (Ma3 Domain-Containing Translation Regulatory Factor) family of proteins that function in translation, especially under energy-deficient conditions [START_REF] Lee | MRF family genes are involved in translation control, especially under energy-deficient conditions, and their expression and functions are modulated by the TOR signaling pathway[END_REF]. Strikingly, the human homologue of the MRFs, hPDCD4, inhibits translation initiation by binding eIF4A and preventing its interaction with eIF4G and, thus, formation of the functional eIF4F complex [START_REF] Loh | Structural basis for translational inhibition by the tumour suppressor Pdcd4[END_REF]. In addition, similarly to hPDCD4, which is phosphorylated by S6K1/TOR, MRF1 phosphorylation was regulated positively by S6K1/TOR [START_REF] Lee | MRF family genes are involved in translation control, especially under energy-deficient conditions, and their expression and functions are modulated by the TOR signaling pathway[END_REF].

One efficient way of rapidly altering protein production in response to internal and external stimuli is to regulate mRNA translation selectively. In eukaryotes, TOR is implicated in translational regulation of a specific set of TOP (5'-terminal oligopyrimidine tract)-containing mRNAs via phosphorylation of an La RNA binding protein (LARP1) [START_REF] Tcherkezian | Proteomic analysis of cap-dependent translation identifies LARP1 as a key regulator of 5'TOP mRNA translation[END_REF]. In plants, heat stress can dramatically induce an mRNA degradation process that is dependent on Arabidopsis LARPs (La-related Proteins) [START_REF] Deragon | The role of LARP1 in translation and beyond[END_REF]. Although LARP1 was identified as a TOR/S6K1 downstream target in Chlamydomonas reinhardtii and Arabidopsis [START_REF] Werth | Investigating the effect of target of rapamycin kinase inhibition on the Chlamydomonas reinhardtii phosphoproteome: from known homologs to new targets[END_REF][START_REF] Van Leene | Integration of phosphoproteomics and interactomics elucidates substrates of the target of rapamycin kinase in plants[END_REF], its role in translation of TOP-containing mRNAs (Dobrenel et al., 2016b) remains to be identified.

Active TOR impacts translation of a specific set of mRNAs that harbour upstream open reading frames (uORFs) within their leader regions [START_REF] Schepetilnikov | TOR and S6K1 promote translation reinitiation of uORF-containing mRNAs via phosphorylation of eIF3h[END_REF]. Moreover, subunit h of eukaryotic translation initiation factor 3 (eIF3h) was identified as a new TOR/S6K1 target, and its phosphorylation site mapped to Ser178 [START_REF] Schepetilnikov | TOR and S6K1 promote translation reinitiation of uORF-containing mRNAs via phosphorylation of eIF3h[END_REF]. The authors proposed that TOR, when activated, promotes phosphorylation of eIF3h and maintains its active phosphorylation status in Arabidopsis to ensure efficient reinitiation events [START_REF] Schepetilnikov | Recent discoveries on the role of TOR (Target of Rapamycin) signaling in translation in plants[END_REF]. However, other players in auxin signalling, such as phospholipase D zeta2 (PLD; required for auxin responses) [START_REF] Li | Arabidopsis PLDzeta2 regulates vesicle trafficking and is required for auxin response[END_REF], which is controlled by auxin, might also participate in TOR activation either directly or via production of phosphatidic acid (PA).

The growing wealth of phosphoproteomic data reveals hundreds of proteins with quantitative changes in their phosphorylation status in response to TOR inhibition in C. reinhardtii [START_REF] Roustan | Quantitative phosphoproteomic and system-level analysis of TOR inhibition unravel distinct organellar acclimation in Chlamydomonas reinhardtii[END_REF][START_REF] Werth | Investigating the effect of target of rapamycin kinase inhibition on the Chlamydomonas reinhardtii phosphoproteome: from known homologs to new targets[END_REF] and Arabidopsis [START_REF] Van Leene | Integration of phosphoproteomics and interactomics elucidates substrates of the target of rapamycin kinase in plants[END_REF]. Among these TOR signalling-related targets are homologues of an elongation factor 2 kinase (EEF2K), components of the eIF2B complex, eiF4E-binding protein, LARP1 and CTC-interacting domain 4 (CID4). RPS6 Ser240 was clearly identified as a robust and conserved TOR/S6K1-dependent phosphosite in Arabidopsis and Chlamydomonas 40S ribosomal subunit (Dobrenel et al., 2016b;[START_REF] Enganti | Phosphorylation of ribosomal protein RPS6 integrates light signals and circadian clock signals[END_REF][START_REF] Werth | Investigating the effect of target of rapamycin kinase inhibition on the Chlamydomonas reinhardtii phosphoproteome: from known homologs to new targets[END_REF]. Consistently, an increase in RPS6 phosphorylation was also observed after loss of SnRK1 activity [START_REF] Nukarinen | Quantitative phosphoproteomics reveals the role of the AMPK plant ortholog SnRK1 as a metabolic master regulator under energy deprivation[END_REF]. However, the role of this evolutionarily conserved TOR-dependent phosphorylation event in the control of mRNA translation is not very well defined and will need further research.

Conclusion and future directions

It is now clear that, as in other eukaryotes, the plant TOR signalling pathway integrates information about external conditions, either directly or through hormonal signals, to mount the necessary physiological, molecular and developmental responses. In recent years the plant TOR field has been blooming. Compared to animals and yeasts, the TOR pathway may be even more critical and integrate specific functions in algae and multicellular plants, which cannot escape from adverse environmental conditions or nutrient scarcity.

A better understanding of the role and regulation of the TOR kinase is still needed but recent phosphoproteomic and interatomic resources will be a great help for deciphering this fascinating regulatory hub [START_REF] Van Leene | Integration of phosphoproteomics and interactomics elucidates substrates of the target of rapamycin kinase in plants[END_REF]. This should open novel routes for increasing crop yield through stimulation of growth, improved pathogen defence and manipulation of metabolite partitioning.

  available online free of all access charges (see https://academic.oup.com/jxb/pages/openaccess for further details) Downloaded from https://academic.oup.com/jxb/article/70/8/2211/5460007 by Inst Biologie Moleculaire user on 08 December 2020 et al. (2019) presents further detail on regulation of the plant cell cycle by E2F and other conserved complexes in relation to TOR activity. TOR also influences the cell wall

Acknowledgments

Reviews and research articles in this special issue were developed following the EMBO workshop 'TOR signaling in photosynthetic organisms' held in Alsace, France in 2018. Journal of Experimental Botany contributed to funding for this meeting. The authors were funded by the DecoraTOR ANR grant (ANR14-CE19-007). CM is also funded by a French State grant (LabEx Saclay Plant Sciences-SPS, ANR-10-LABX-0040-SPS).


Tatsuaki Okada
email: okada@planeta.sci.isas.jaxa.jp

Tetsuya Fukuhara

Satoshi Tanaka

Makoto Taguchi

Takehiko Arai

Hiroki Senshu

Naoya Sakatani

Yuri Shimaki

Hirohide Demura

Yoshiko Ogawa

Kentaro Suko

Tomohiko Sekiguchi

Toru Kouyama

Jun Takita

Tsuneo Matsunaga

Takeshi Imamura

Takehiko Wada

Sunao Hasegawa

Jörn Helbert

Thomas G Müller

Axel Hagermann

Jens Biele

Matthias Grott

Maximilian Hamm

Marco Delbo

Naru Hirata

Naoyuki Hirata

Yukio Yamamoto

Seiji Sugita

Noriyuki Namiki

Kohei Kitazato

Masahiko Arakawa

Shogo Tachibana

Hitoshi Ikeda

Masateru Ishiguro

Koji Wada

Chikatoshi Honda

Rie Honda

Yoshiaki Ishihara

Koji Matsumoto

Moe Matsuoka

Tatsuhiro Michikami

Akira Miura

Tomokatsu Morota

Hirotomo Noda

Rina Noguchi

Kazunori Ogawa

Kei Shirai

Eri Tatsumi

Hikaru Yabuta

Yasuhiro Yokota

Manabu Yamada

Masanao Abe

Masahiko Hayakawa

Takahiro Iwata

Masanobu Ozaki

Hajime Yano

Satoshi Hosoda

Osamu Mori

Hirotaka Sawada

Takanobu Shimada

Hiroshi Takeuchi

Ryudo Tsukizaki

Atsushi Fujii

Chikako Hirose

Shota Kikuchi

Yuya Mimasu

Naoko Ogawa

Tadateru Takahashi

Yuto Takei

Tomohiro Yamaguchi

Kent Yoshikawa

Fuyuto Terui

Takanao Saiki

Satoru Nakazawa

Makoto Yoshikawa

Seiichiro Watanabe

Yuichi Tsuda

Highly porous nature of a primitive asteroid revealed by thermal imaging

Carbonaceous (C-type) asteroids 1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites 2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known, because carbonaceous chondrite meteoroids tend not to survive entry to Earth's atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu taken by the thermal infrared imager (TIR) 4 onboard the spacecraft Hayabusa2 5 , indicating that the asteroid's boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m -2 s -0.5 K -1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites 6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirmed the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect 7,8 . We also observed colder boulders in the close-up thermal images, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites 6 . These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity 9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This low-porosity asteroid may link cosmic fluffy dust to dense celestial bodies 10 .

C-type asteroids are the most abundant taxonomic class in the outer main belt and display reflectance spectra with weak or no features and with flat or very moderate slopes 2,3 . Ground-based and space-based remote observations of Ryugu [START_REF] Müller | Hayabusa-2 mission target asteroid 162173 Ryugu (1999 JU3): searching for the object's spin-axis orientation[END_REF] have characterized the asteroid's taxonomy as C-type, its rotation period to be 7.63 h, its diameter to be about 0.85 km, and its global thermal inertia to be 150-300 tiu, interpreting its surface as being covered with millimetre-scale granules.

After the spacecraft's arrival at the Home Position, which is 20 km earthward from Ryugu, Hayabusa2 globally observed the asteroid [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-a spinning top-shaped rubble pile[END_REF][START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF][START_REF] Kitazato | Surface composition of asteroid 162173 Ryugu as observed by the Hayabusa2 NIRS3 instrument[END_REF] , confirming its size, rotation rate and albedo to be within the predicted ranges, but finding its structure to be double-top-shaped, with perpendicularly retrograde rotation [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-a spinning top-shaped rubble pile[END_REF] , and subdued absorptions at around the 0.7 μm and 3 μm bands, indicating minerals less hydrated or dehydrated in the past [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF][START_REF] Kitazato | Surface composition of asteroid 162173 Ryugu as observed by the Hayabusa2 NIRS3 instrument[END_REF] . Ryugu has a bulk density [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-a spinning top-shaped rubble pile[END_REF] of 1,190 ± 20 kg m -3 and its surface is evenly covered with boulders [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF] , without the kind of smooth terrains found on asteroid 25143 Itokawa [START_REF] Fujiwara | The rubble-pile asteroid Itokawa as observed by Hayabusa[END_REF] . These facts indicate that Ryugu is a rubble-pile asteroid with bulk porosity of 50% or higher, assuming the asteroid is composed of CI (about 2,420 kg m -3 ) or CM (about 2,960 kg m -3 ) carbonaceous chondrites 6 .

Thermal imaging was conducted to investigate the surface physical state, especially in terms of particle size, porosity, boulder abundance and surface roughness, derived from the thermal inertia, which is remotely sensed without physical contact-even for porous or loosely bound surfaces. The surfaces of the Moon, Mars and large satellites are predominantly covered with regolith, which is formed by hypervelocity meteoritic impacts. However, the surface of small bodies remains poorly known [START_REF] Housen | Ejecta from impact craters[END_REF] because most impact ejecta could escape owing to the low gravity, depending on the physical properties of the surface materials; their surface physical state therefore needs in-depth investigation.

We obtained a set of high-resolution global thermal infrared images of asteroid Ryugu on 30 June 2018, at about 18 m per pixel or about 50 pixels across the disk of Ryugu, covering its rotation in steps of 6°. Temperatures of the observed hemisphere ranged from 300 K to 370 K for the solar distance of 0.987 astronomical units (au), at the typical solar phase angle of 18.5°, and a rough surface of porous materials was preliminarily indicated from the peak temperatures and flat diurnal profiles [START_REF] Rozitis | Directional characteristics of thermal-infrared beaming from atmosphereless planetary surfaces-a new thermophysical model[END_REF][START_REF] Shimaki | Surface roughness and thermal inertia of asteroid Ryugu inferred from TIR on Hayabusa2[END_REF] .

Higher-resolution one-rotation thermal image sets were taken at an altitude of 5 km, with spatial resolution of about 4.5 m per pixel at the solar distance of 1.057 au, during the Mid-Altitude Observation Campaign on 1 August 2018. Global thermal images of Ryugu at 90° steps show the brightness temperature distribution on the asteroid (Fig. 1), from 270 K (black) to 360 K (white). The colder regions, such as the shadows, are black, whereas the hottest points at the base of boulders are white, owing to self-heating or possibly the existence of fine-grained sediments. Geologic features are seen including the largest boulder Otohime Saxum (around 160 m in width) at the south pole, the largest crater, Urashima (about 270 m in diameter), the ridge around the equator, and other boulders, craters and grabens. Surprisingly, the boulders show similar temperatures to their surroundings (before the arrival of Hyabusa2 the boulders were expected to be denser and observed as cold spots).

A colour-scale brightness-temperature image of Ryugu plotted on the asteroid shape model (SHAPE_SFM_200k_v20180804 in AiGIS) is compared with simulated images [START_REF] Takita | Feasibility and accuracy of thermophysical estimation of asteroid 162173 Ryugu (1999 JU3) from the Hayabusa2 Thermal Infrared Imager[END_REF] for eight uniform thermal inertia models from 50 tiu to 1,000 tiu (Fig. 2). Although none of them matches the observations perfectly, the apparent global thermal inertia of Ryugu seems likely to be 300 ± 100 tiu for both large boulders and their surrounding surfaces. It is also valid for the largest boulder Otohime Saxum, whose surface is apparently flat [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF] .

Diurnal temperature profiles of six different sites on Ryugu are shown in Fig. 3 and Extended Data Table 1, including a large flat boulder at the south pole, a medium-sized boulder in the equatorial region, and the surrounding sites at low-and mid-latitudes in both northern and southern hemispheres. These profiles are compared to the calculation [START_REF] Takita | Feasibility and accuracy of thermophysical estimation of asteroid 162173 Ryugu (1999 JU3) from the Hayabusa2 Thermal Infrared Imager[END_REF] for Q2 Q3 Q4 Q5 uniform thermal inertias. An extremely high surface roughness [START_REF] Rozitis | Directional characteristics of thermal-infrared beaming from atmosphereless planetary surfaces-a new thermophysical model[END_REF][START_REF] Shimaki | Surface roughness and thermal inertia of asteroid Ryugu inferred from TIR on Hayabusa2[END_REF] might explain these flat diurnal profiles, when observed from a relatively small solar phase angle of about 19°. In such a rough surface model, it becomes colder at the nadir point, where not only the sunlit (hotter) but also the shadowed (colder) surfaces are observed, whereas it becomes hotter at the earlier and later local times, where the sunlit surfaces are selectively observed but most of the shadowed areas are invisible from the spacecraft. The diurnal temperature profile could be explained by a global surface thermal inertia of 327 ± 127 tiu with a root-mean-square surface roughness of 0.4 ± 0.05 (ref. [START_REF] Shimaki | Surface roughness and thermal inertia of asteroid Ryugu inferred from TIR on Hayabusa2[END_REF] ). Although a two-layer surface model also explains the shift of peak temperature and its delay from the local noon, it never matches the flat diurnal temperature profiles [START_REF] Biele | Effects of dust layers on thermal emission from airless bodies[END_REF] . Similar flat diurnal temperature profiles have been observed on the surface of comets 19 103P/Heartley-2 and 9P/Tempel-1, suggesting that these cometary surfaces might be covered with a rough, granular or rugged, probably highly porous material.

After September 2018, Hayabusa2 started descent operations for sampling touchdowns, as well as for the release of MINERVA-II twin rovers [START_REF] Yoshimitsu | MINERVA-II rovers developed for Hayabusa-2 mission[END_REF] and the MASCOT lander [START_REF] Ho | MASCOT-the Mobile Asteroid Surface Scout onboard the Hayabusa2 mission[END_REF] . During each descent operation, close-up thermal images were taken by TIR down to altitudes below 60 m for the lander release operations and to below 20 m for the samplings and rehearsals. Thermal images of these local sites were taken at a resolution of a few centimetres per pixel.

Close-up thermal images were taken during the touchdown rehearsal TD1-R1-A near the sampling site on 15 October 2018 at 1.28 au from the Sun. These images indicate that the surrounding surface is not covered with soil or granules but mainly by boulders (Fig. 4a,c). The surface features seen in the close-up images are evenly distributed around the asteroid, causing similar temperature profiles between large boulders and their surroundings. Most boulders have typical temperatures of 300-310 K, corresponding to highly porous materials with a thermal inertia of 200-300 tiu, as predicted by global thermal images.

In the close-up images we observed a few boulders below 280 K (Fig. 4b,d), corresponding to 600-1,000 tiu (a typical value for (lines or dotted lines) with uniform thermal inertias from 20 tiu to 800 tiu for each site labelled 'b' to 'g' in panel a, respectively. L3b is the level-3b product, which is brightness temperature plotted on the asteroid shape model. The polygon IDs, the positions in longitude and latitude, and site information for sites 'b' to 'g' are shown in Extended Data Table 1. carbonaceous chondrites 6 ). The surface of Ryugu is dominated by highly porous boulders, except for some dense boulders similar to carbonaceous chondrite meteorites. This is consistent with the images taken by the camera MASCam [START_REF] Jaumann | In-situ investigation of asteroid (162173) Ryugu by the Mobile Asteroid Surface Scout (MASCOT) camera (MASCam)[END_REF] on MASCOT showing that a large majority of boulders have cauliflower-like crumbly surfaces, while a minority have flat surfaces.

The average thermal inertia is consistent with that of 282 -35 +93 tiu derived from in situ measurements of a single boulder using the radiometer MARA [START_REF] Grott | Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu[END_REF] on MASCOT, and is also consistent with terrestrial and space remote observations [START_REF] Müller | Hayabusa-2 mission target asteroid 162173 Ryugu (1999 JU3): searching for the object's spin-axis orientation[END_REF] . The boulder observed by MARA seems to be typical for boulders distributed globally on Ryugu. Such highly porous boulders should have a microporosity ϕ μ of 30-50% under vacuum conditions [START_REF] Grott | Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu[END_REF][START_REF] Opeil | Stony meteorite thermal properties and their relationship with meteorite chemical, and physical states[END_REF][START_REF] Okada | Thermal inertia of surface materials of solar system small bodies and its dependence on porosity[END_REF] .

Accounting for the asteroid's bulk density, Ryugu must have a bulk porosity ϕ b of 50-60%, assuming it consists of materials like those of CI or CM chondrites, respectively [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-a spinning top-shaped rubble pile[END_REF] . The macroporosity ϕ m caused by voids between rocks, defined as

ϕ m = 1 -(1 -ϕ b )/(1 -ϕ μ )
, should be below 20% for a ϕ μ of 50%, or 30-40% for a ϕ μ of 30%, and so we conclude that Ryugu is a loosely packed rubble pile asteroid made of porous materials formed under microgravity conditions. A trend for small asteroids to have lower thermal inertia than typical carbonaceous chondrites 26 might be caused not by fine regolith (as predicted) but by porous boulders. A similarly low thermal inertia of about 300 tiu was also reported for the B-type asteroid 101955 Bennu explored by the OSIRIS-REx mission [START_REF] Dellagiustina | Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis[END_REF] .

These facts might constrain the formation history of Ryugu proposed in the previous study [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF] . Ryugu could be formed from fragments of a parent body that experienced a low degree of consolidation of originally porous materials. This work provides evidence that C-type asteroids may have a less consolidated nature, formed from fluffy dust or pebbles in the early Solar System. This discovery also implies that large asteroids in the main belt with high porosity [START_REF] Britt | Asteroid density, porosity, and structure[END_REF] could have formed in a similar way; such asteroids are very common among the C-type asteroids. Moreover, those less consolidated materials might have low mechanical strength, causing breakup of C-type meteoroids (fragments of asteroids) during atmospheric entry. This might be why their spectra did not match perfectly with meteorite samples on the Earth [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF][START_REF] Kitazato | Surface composition of asteroid 162173 Ryugu as observed by the Hayabusa2 NIRS3 instrument[END_REF] . Dense boulders might be formed within a consolidated inner core of the parent body, or might have an exogenic origin (they might be surviving fragments of meteoritic impacts). These C-type asteroids might share their highly porous nature with planetesimals that formed from fluffy dust 10 in the early Solar System and could have strongly affected planetary formation processes such as cratering and collisional fragmentation by attenuating shock propagation [START_REF] Britt | Asteroid density, porosity, and structure[END_REF] . The possibility still cannot be ruled out that Ryugu's low thermal inertia and low density arise from surface materials different from carbonaceous chondrites, such as the organicrich material discovered on comet 67P/Churymov-Gerasimenko [START_REF] Bardyn | Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta[END_REF] . This question will, however, be resolved upon sample return.

Online content

Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-020-2102-6.

Article

Methods

Hayabusa2 mission and its proximity observation phases

Hayabusa2 5 is the second asteroid sample return mission organized by the Japan Aerospace Exploration Agency. The spacecraft is technically inherited from the Hayabusa mission [START_REF] Kawaguchi | The MUSES-C mission for the sample and return-its technology development status and readiness[END_REF] , which explored the S-type asteroid 25143 Itokawa [START_REF] Fujiwara | The rubble-pile asteroid Itokawa as observed by Hayabusa[END_REF]

Asteroid reference model before arrival

Before the arrival of Hayabusa2 at Ryugu, it seemed likely that the surface of the asteroid (as now illustrated in Fig. 1 at http://www.hayabusa2. jaxa.jp/en/topics/20190328e_illustration/) was covered with regolith dominated by sands and gravels formed by impact ejecta, and not dominated by huge boulders but by aqueously altered substances with impact craters, as envisioned in Fig. 3 at http://www.hayabusa2.jaxa. jp/en/topics/20190328e_illustration/). The Hayabusa2 landing site selection data preparation team constructed a Ryugu reference model called 'Ryugoid' to perform the landing site selection process. An example of the thermal image of Ryugoid is shown in Extended Data Fig. 1, where boulders were identified as 'cold spots', assuming a thermal inertia for dense boulders of 1,600 tiu, and granular surroundings of 300 tiu as indicated by the averaged thermal inertia derived from ground observations [START_REF] Müller | Hayabusa-2 mission target asteroid 162173 Ryugu (1999 JU3): searching for the object's spin-axis orientation[END_REF] . Thermal inertia, denoted as (ρκc p ) 0.5 expresses the surface's thermophysical properties, where ρ is the bulk density, κ is the thermal conductivity, and c p is the specific heat capacity.

TIR instrument and temperature control

Thermal radiometry has been among the frequently used methods [START_REF] Kieffer | Preliminary report on infrared radiometric measurements from the Mariner 9 spacecraft[END_REF][START_REF] Chase | Mariner 10 infrared radiometer results: temperatures and thermal properties of the surface of Mercury[END_REF][START_REF] Kieffer | Thermal and albedo mapping of Mars during Viking primary mission[END_REF][START_REF] Christensen | Mars Global Surveyor Thermal Emission Spectrometer experiment: investigation description and surface science results[END_REF][START_REF] Paige | The Lunar Reconnaissance Orbiter Diviner lunar radiometer experiment[END_REF] in planetary missions to investigate the physical state of planetary surfaces and is often conducted along the track of an orbiting spacecraft. In the Hayabusa2 mission, however, a two-dimensional imager was used because the spacecraft remains at a distant stationary position owing to the low gravity of the small body.

TIR is a thermal imager based on a two-dimensional microbolometer array of 344 × 260 pixels (328 × 248 effective pixels). TIR observes the wavelength range 8-12 μm, for a field of view of 16.7° by 12.7°, corresponding to the spatial resolution of 0.88 mrad per pixel. Its design is the same as the Longwave Infrared Camera [START_REF] Fukuhara | LIR: longwave infrared camera onboard the Venus orbiter Akatsuki[END_REF] on the Akatsuki Venus Climate Orbiter except for the thermal design. TIR consists of the sensor and data handling unit (TIR-S) and the power supply unit (TIR-AE), mounted on the inside of the +Y panel, as well as the corn-shaped sunshade, mounted on the outside of the -Z panel. Its data processing, such as the integration of multiple images, the dark-frame subtraction, and the data compression was conducted using the Digital Electronics (DE) component on Hayabusa2. The optics of TIR-S is made of a germanium lens, pointed towards the -Z direction, like the other optical instruments. TIR-S is mounted on a dedicated panel that is thermally isolated from the surrounding environment, and whose temperature is controlled within a narrow range by the spacecraft heater control electronics. Hence, the temperature of TIR-S is kept stable even during the descent operations. The detector temperature is controlled with a Peltier thermoelectric device at 313 K (40 °C) within 0.019 K (equivalent to a least significant bit (LSB) of digital data). Temperatures are accurately monitored within 0.002 K at the sensor package T pkg , the case (the adaptor of the 8-12 μm filter) T case , the shutter T sht , and the lens cover T lens . The package, case and lens temperatures are kept stable within ±0.1 K even during the descent operations, while the shutter temperature increases by 1 K at the lowest altitude where the input of thermal radiation from the asteroid is maximal, warming the shutter. This effect is limited and in any case compensated because all the thermal images of TIR are processed after subtraction of the shutter-closed dark-frame image from the shutter-open image.

TIR calibration

Pre-flight calibration of TIR was carried out using three types of calibration targets: a collimator, a cavity blackbody source and blackbody plates. We used the same apparatus prepared for the calibration of the Longwave Infrared Camera onboard the Japanese Venus explorer Akatsuki [START_REF] Fukuhara | Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera onboard Akatsuki[END_REF] . It allows us to control the temperature of a blackbody plate from -40 °C to +50 °C in a vacuum chamber. Temperatures at the asteroid surface in the daytime are estimated to be up to 100 °C. Therefore, we set up another experimental apparatus using an oil bath to control the temperature of the blackbody plate from +50 °C to +125 °C. These apparatuses enable us to investigate the response of all the pixels of TIR. The measured emissivity of the blackbody plates is 0.925.

The temperature inside the instrument affects the output of TIR. During the calibration test, we corrected the data using the four monitoring temperatures. We found that the shutter temperature is especially effective. Linear relations are seen for each temperature of the calibration target. We found that a better linear relationship appears after the package and case temperature correction:

( )

D D T T ′ = -6.125 - ( 1) case pkg
where D and D′ are the original and calibrated digital numbers for each pixel. This calibration corresponds to temperature correction of the gain of the analog-to-digital converter. After the case and package temperature correction, we carried out shutter temperature correction using the equation:

D D T T ″ = ′ -6.158( - ) (2) 0 s ht
where D′′ is the calibrated digital number, T sht is the shutter temperature, T 0 = 28 °C is the standard temperature, following the method in ref. [START_REF] Fukuhara | Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera onboard Akatsuki[END_REF] . After the calibration using the instrument temperatures, we made a look-up table (LUT) to convert the digital number to the radiance of the target. The blackbody temperature is converted to effective radiance, based on the method [START_REF] Arai | Thermal imaging performance of TIR onboard the Hayabusa2 spacecraft[END_REF] :

∫ I T εB λ T R λ λ ( ) = ( , ) ( )d (3) 0 ∞
where ε is the emissivity (equal to 0.925 for the black body plate), λ is wavelength, R(λ) is the response function of TIR 4 , and B(λ,T) is the spectral radiance given by the Planck function. By linear least-squares fitting of output digital number and radiance, we defined the slope a i,j and intercept b i,j for all pixels (i, j) as:

D a I T b ″ = ( ) + (4) i j i j i j , , ,
A critical problem for accurate calibration of infrared imagers and thermometers is the size-of-source effect [START_REF] Saunders | Dealing with the size-of-source effect in the calibration of direct-reading radiation thermometer[END_REF] (SSE). SSE occurs owing to scattering and diffraction of thermal radiation within the optical component. We evaluate the SSE of TIR using a collimator and a cavity blackbody, whose temperatures were controlled from +20 °C to +125 °C.

The measurements of these targets were carried out in the atmosphere owing to the constraints of the apparatus. We found SSE for TIR during the calibration (see Extended Data Fig. 2); the output values become relatively low for the small image of the different calibration targets (collimator < cavity blackbody < blackbody plate). The position of the targets in the image was changed to check the SSE on the sensitivity of each pixels.

For the correction of SSE, we focused on the effective diameter of the targets, which is defined as D N = 2 /π , where N is the number of pixels of the observed target. The effective diameters of the collimator and cavity blackbody are about 60 and 125 pixels, respectively. The effective diameters of asteroid Ryugu are around 55 and 180 pixels for the Home Position and Mid-altitude observations, respectively. The effective diameters of 322 pixels are set both for a close-up image where the TIR image is fully covered with the asteroid surface and for calibration data of the blackbody plate. The SSE for continuous observations during a descent operation for gravity measurement was detemined on 6 and 7 August 2018. Averaged digital number (DN) values of Ryugu increased linearly with increasing the effective diameter, but a rapid jump in DN value occurred with an effective diameter larger than 300 pixels. The LUT applied to the asteroid image is built based on the effective diameter D of imaged Ryugu. In the range D ≤ 300 pixels, LUT is created by linear interpolation and extrapolation of the LUTs for collimator and cavity blackbody. Note that the data for the blackbody plate is not fitted to this linear extrapolation owing to this higher effective diameter regime (300 pixels < D ≤ 322 pixels). Therefore, we used another linear function derived from the LUT data for blackbody plate and extrapolated values at D = 300 pixels from the lower effective diameter regime (see Extended Data Fig. 3). Using the LUT derived from the effective diameter of the asteroid, the DN values are converted to the radiance values for each pixel using equation (4). The derived radiant flux images are converted to brightness temperature images from equation (3), assuming an emissivity of 1 independent of the wavelength.

A history of Ryugu formation

The history of Ryugu formation in the previous study [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes[END_REF] , mainly using the results of the optical navigation camera, proposed three scenarios with the different types of parent bodies before catastrophic disruption. In scenario 1 it is an aqueously altered parent body dehydrated by internal heating; in scenario 2 it is an aqueously altered parent body dehydrated by impact heating; and in scenario 3 it is an incipiently altered parent body. The results of TIR observations constrain or update the possible scenarios of Ryugu's formation, suggesting that scenario 3 seems to be most suitable, as shown below, although further in-depth study including the analysis of returned samples is needed for a final determination.

A scenario is shown mainly using the results of TIR in Extended Data Fig. 4. (1) Fluffy dust particles containing minerals, water and organics accumulated to form planetesimals in the early Solar System.

(2) Planetesimals remained porous owing to a low degree of consolidation under low-gravity conditions, although they might have experienced impact compaction, fragmentation and re-accretion processes.

(3) The parent body of Ryugu remained porous throughout most of its volume, but its innermost interior might be more consolidated and thermally altered to form a dense core consisting of typical carbonaceous meteorites, if the lithostatic pressure reached the level of its bulk modulus of 50 MPa or higher. A gradual increase of the degree of consolidation might be expected, rather than a clear boundary of the possible inner core. C-type asteroid 253 Mathilde [START_REF] Veverka | NEAR Encounter with Asteroid 253 Mathilde: Overview[END_REF] , which has a very low spin rate, a low density of 1,300 ± 300 kg m -3 , and a number of extremely large craters of almost half the size of its diameter, may be an example of a porous, loosely bound rubble-pile asteroid. (4) Intense impact fragmentation by catastrophic disruption occurred to expose the interior materials. (5) Some of the impact fragments accreted to form Ryugu. Most of its surface consists of porous materials that have undergone only a low degree of consolidation, including large boulders. Some dense boulders might be from the innermost part of the parent body. Alternatively, a dense small asteroid might have impacted the porous parent body to form Ryugu by re-accretion of fragments, where the dense rocks could be from the dense impactor. (6) A rubble-pile asteroid that formed from mostly porous fragments has been reshaped to become a double-top-shape [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-a spinning top-shaped rubble pile[END_REF] owing to a change in spin rate by the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect [START_REF] Rubincam | Radiative spin-up and spin-down of small asteroids[END_REF]

  images of Ryugu taken at 5 km altitude during the Mid-Altitude Observation Campaign. a-d, Thermal images were taken on 1 August 2018 at the solar distance of 1.06 au and the Sun-Probe-Earth angle of 19.0°: a, for the longitude (Lon) of 0°; b, for the longitude of 90°; c, for the longitude of 180°; and d, for the longitude of 270°. Relatively higher temperatures in the southern hemisphere of the asteroid were due to the summer season. Geologic features are clearly identified such as Otohime Saxum (the largest boulder near the south pole, 160 m width) (a), Catafo Saxum (b), Cendrillon crater (c), Momotaro crater (d), Kibidango crater (e), Urashima crater (the largest crater, 270 m diameter) (f), Ejima Saxum (g), Kintaro crater (h), Tokoyo Fossa (i), Brabo crater (j), Kolobok crater (k) and Ryujin Dorsum (the equatorial ridge) (l).

Fig. 2 |Fig. 3 |

 23 Fig. 2 | Comparison of a temperature plot on the three-dimensional shape model with calculated images for thermal inertia of 50-1,000 tiu. a, A temperature plot in kelvin on the asteroid Ryugu shape model (SHAPE_ SFM_200k_v20180804), using the thermal image taken by TIR at 18:16:32 Coordinated Universal Time (utc) on 1 August 2018 during the Mid-Altitude

Fig. 4 |

 4 Fig. 4 | Cold spots discovered in the close-up thermal images. a, A close-up thermal image taken during the TD1-R1-A campaign on 15 October 2018, at the solar distance of 1.28 au and the Sun-Probe-Earth angle of 10.6°, at 13:34:44 UTC from an altitude of 78.8 m. b, The temperature profile along the yellow line in panel a, showing the 'cold spot' boulder. c, A close-up thermal

  and resurfaced by impact cratering and boulder movement. Extended Data Fig. 2 | Temperature calibration of TIR using multiple apparatuses. DN, digital number. a, An example of the TIR pixel response of a centre pixel at (164, 124) obtained by the ground-based calibration tests. The purple dots and line show the data for the blackbody plate and their linear regression. The blue and green dots show the data for the cavity blackbody and the collimator, respectively. b-d, Thermal images from TIR using the different apparatuses for the 100 °C target: b, the blackbody plate (total area coverage), c, the cavity blackbody (simulation to Mid-Altitude, 5 km), and d, the collimator (simulation to Home Position). Extended Data Fig. 3 | Effective diameter dependence of the LUT for TIR calibration. This figures show the values of the slope (a) and the intercept (b) at the centre pixel at (164, 124) in terms of the effective diameter D of the target. For D ≤ 300 pixels, the red lines are derived from the data of the collimator and the cavity blackbody sources. For 300 pixels <D ≤ 322 pixels, the green lines are derived from the blackbody (BB) plate source. Extended Data Fig. 4 | A formation scenario of Ryugu from a porous parent body. (1) Formation began with fluffy dust in the solar nebula. (2) Porous planetesimals were formed by accretion of dust or pebbles. (3) The parent body of Ryugu might have remained porous owing to a low degree of consolidation. A clear boundary of the inner core is illustrated but a gradual increase of consolidation by depth might be expected. (4) Impact fragmentation of the parent body occurred. Some large fragments are the boulders on Ryugu. (5) Part of fragments re-accreted to form Ryugu, with porous boulders and sediments on the surface, and some dense boulders originating from the inner core. (6) Re-shaping caused by a change in rotation rate to form a double-topshape.

Acknowledgements

We thank all the members of the Hayabusa2 Project and supporting staff for their technical assistance and scientific discussions, and especially S. Matsuura of Kwansei Gakuin University the use of the cavity blackbody and collimator to calibrate TIR. This research is supported in part by the JSPS KAKENHI (numbers JP26287108, JP17H06459, JP17K05639, JP19H01951 and JP19K03958), and the JSPS Core-to-Core programme "International Network of Planetary Sciences". M.D. acknowledges support from the French space agency CNES. T.G.M. received funding from the European Union's Horizon 2020 Research and Innovation Programme, under grant agreement number 687378, as part of the project "Small Bodies Near and Far" (SBNAF). A.H. was supported by STFC grant ST/ S001271/1.

Code availability

The display tool for the TIR thermal images is AiGIS, developed by author Naru Hirata of the University of Aizu, which is available at https:// arcspace.jp/aigis/. The basic code of the asteroid thermal model that supports the calculation of thermal images using several thermal inertia values was constructed by ref. [START_REF] Takita | Feasibility and accuracy of thermophysical estimation of asteroid 162173 Ryugu (1999 JU3) from the Hayabusa2 Thermal Infrared Imager[END_REF] , and is applicable to any asteroid shape model even of more than one million nodes and capable of calculating the self-heating effect between nodes facing each other and the shadowing effect of insolation by geological features. This code is accessible from the corresponding author upon reasonable request.

Data availability

The source data of the TIR global thermal infrared images used for Fig. 1a-d are prepared from hyb2_tir_20180801_142608_l2.fit for Fig. 1a, from hyb2_tir_20180801_162120_l2.fit for Fig. 1b, from hyb2_tir_20180801_181632_l2.fit for Fig. 1c, and from hyb2_ tir_20180801_201144_l2.fit for Fig. 1d. The source data of the TIR close-up thermal images used for Fig. 4a, c are prepared from hyb2_tir_20181015_133444_l2.fit, and hyb2_tir_20181015_134420_ l2.fit, respectively. A TIR radiance image plotted on the Ryugu shape model (SHAPE_SFM_200k_v20180804) in Fig. 2 is from hyb2_ tir_20180801_181632_l3b.txt. The maximum temperature plot in Fig. 3a and the plots of Fig. 3b-g are obtained from the datasets of hyb2_tir_20180801_150432~215056_l3b.txt. All the data related to this manuscript, including the raw thermal images (L1), the temperatureconverted thermal images (L2), and the temperature plots on the shape model (L3) of TIR and the corresponding ancillary data for the SPICE tool are archived in the DARTS database (darts.isas.jaxa.jp/planet/ project/hayabusa2/), and will be archived in the PDS4 within one year after the end of its nominal mission, according to the Hayabusa2 science data policy.

Author contributions T.O. led TIR development and experiments, including interpretations of TIR data. TIR development and calibrations: T.O., T.F., S. Tanaka, M.T., T.A., H. Senshu, N.S., H.D., Y.O., T. Sekiguchi, T.K., J.T., T.M., T. Imamura, T.W., S. Hasegawa, J.H., T.G.M. and A.H. TIR data acquisitions and reductions: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., H.D., Y.O., K. Suko and T.K. Thermophysical modelling and discussions: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., M.D., J.B., M.G. and M.H. Shape modelling contributions: Naru Hirata, Naoyuki Hirata, Y. Yamamoto, K.M. and A.M. Landing site selection discussions: K.W., C. Honda, R.H., Y.I., K.M., M.M., T.M., A.M., T.M., H.N., R.N., K.O., K. Shirai, E.T., H. Yabuta, Y. Yokota, H. Yano and M. Yamada. Science operations of spacecraft: M. Abe, M. Hayakawa, T. Iwata, M.O., H. Yano, S. Hosoda, O.M., H. Sawada, T. Shimada, H.T., R.T., A.F., C. Hirose, S.K., Y.M., N.O., G.O., T.T., Y. Takei, T.Y., K.Y., F.T., T. Saiki, S.N., M. Yoshikawa, S.W. and Y. Tsuda; Project administration: S.S., N.N., K.K., T.O., M. Arakawa, S. Tachibana, H.I., M.I., S. Tanaka, F.T., T. Saiki, S.N., M. Yoshikawa, S.W., and Y. Tsuda, Interpretation and writing contribution: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., H.D., J.H., J.B., M.G., M.D., T.G.M., A.H., E.T., T.M. and S.S. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information is available for this paper at https://doi.org/10.1038/s41586-020-2102-6. Correspondence and requests for materials should be addressed to T.O. Reprints and permissions information is available at http://www.nature.com/reprints. The following queries have arisen during the editing of your manuscript. Please answer by making the requisite corrections directly in the e.proofing tool rather than marking them up on the PDF. This will ensure that your corrections are incorporated accurately and that your paper is published as quickly as possible.

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