# The Quest For Water

Yves Marrocchi, Pierre Beck

► To cite this version:

Yves Marrocchi, Pierre Beck. The Quest For Water. Elements, 2022, 18 (3), pp.149-153. 10.2138/glements.18.3.149. hal-04243645

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# The Quest For Water

Yves Marrocchi<fnref n="1" /> and Pierre Beck<fnref n="2" />

## ABSTRACT

Water played a key role in shaping the Solar System—from the formation of early solids to the processes of planetary and **moon** formation. The presence of water in molecular clouds influences the initial abundance and distribution of water in the circumsolar disk, which, in turn, affected the water budget of the terrestrial planets and, therefore, their geological activity and habitability. On Earth, surficial and deep-water cycles have largely governed the planet's geodynamical and geochemical evolution. This issue focuses on the past and present distribution of water within the Solar System and how this important molecule affects astrophysical and geological processes.

**KEYWORDS:** water; planetary bodies; density; circumsolar disk; meteorites

## WATER: THE GAME-CHANGER MOLECULE

Water is a tiny molecule (~3 Å), present in massive amounts within the Solar System, that accounts for ~50% by mass of all condensable species in a gas of solar composition (Lodders 2003). Water had a fundamental influence on the structure of the circumsolar disk that surrounded the Sun 4.56 billion years ago, and the mechanisms by which early solids formed during its evolution (FIG. 1). Water represents a significant fraction of the building blocks of planetesimals (i.e., 0.2–10 wt.%, Vacher et al. 2020) and, therefore, participated in the processes of planetary formation. In addition to being fundamental to the development of life on Earth, this peculiar molecule drives large-scale planetary differentiation processes, magma evolution and volcanism, and the development of atmospheres (Gaillard et al. 2021). Water is thus the crucial molecule that establishes and controls the limits of planetary habitability, i.e., the ability of planetary bodies to develop and sustain life.

Within the Solar System, the ability of Earth to support water in its three forms—vapor, liquid, and solid—makes it a unique planet (FIG. 1). Throughout Earth's history, the “visible” water has

<footnote n="1">¹ Centre de Recherches Pétrographiques et Géochimiques CRPG-CNRS
54501 Vandœuvre-lès-Nancy, France
E-mail: yvesm@crpg.cnrs-nancy.fr</footnote>

<footnote n="2">² Univ. Grenoble Alpes CNRS, IPAG
38000 Grenoble, France
E-mail: pierre.beck@univ-grenoble-alpes.fr</footnote>
been distributed among distinct reservoirs: the oceans, the cryosphere (i.e., ice sheets and glaciers), the atmosphere, and terrestrial storage; the latter corresponding to surface and root-zone soil moisture, groundwater, rivers, and lakes, as well as water stored in vegetation. If the oceans represent the main reservoir, with an estimated amount of water of ~1.4 × 10²¹ kg, an additional 0.15 ocean masses is stored in other surficial reservoirs (Hirschmann 2006). This visible water is characterized by constant motion, which is referred to as the water or hydrological cycle (FIG. 1). Solar-driven evaporated surficial waters (i.e., oceans, lakes, rivers, streams, land surface and plants) are transferred into the atmosphere under the action of wind, where they experience condensation to form clouds and water droplets, as well as ice crystals (FIG. 1). Hence, atmospheric water comes back to the Earth's surface through precipitation (i.e., rain or snow), replenishing the earthbound parts of the surficial water cycle and percolating downward to form groundwater, which slowly moves through rocks and surface materials to end up in streams, rivers, and lakes, with some water going back directly to the oceans (FIG. 1). The volume of surficial water and its evolution throughout Earth's history are thus key parameters that control Earth's climate and habitability.

The question of how much water there is on Earth is not trivial to answer, as surficial water represents only a fraction of Earth's total water budget. Although it was discovered in recent decades that a significant quantity of water is stored in the Earth's mantle (which is also the case for other rocky planetary bodies), its precise estimation is still the subject of very active debate. This difficulty lies in the fact that—contrary to refractory elements—volatile elements are lost and fractionated during magma degassing, making the estimation of water abundance in different mantle source regions difficult. Depending on the mantle-derived rocks considered (either those emitted along mid-ocean ridges or at oceanic islands), between 0.1 to 2.5 times the mass of surficial reservoirs could be stored in the mantle (Hirschmann 2006). Another estimate based on the noble gas systematics of the Earth's mantle suggests that the bulk Earth water content could correspond to 10 ± 5 ocean masses (Marty 2012). Even by considering the largest estimate of water stored in the mantle, the Earth appears surprisingly dry, as water represents less than 1% of our planet's mass. This peculiar characteristic is thus a key constraint for understanding the origin and evolution of Earth's water.

The amount of water on Earth, and its transfer between the different terrestrial reservoirs, plays a key role in controlling the geological processes and behavior of the solid Earth (FIG. 1). Deep-Earth water is mainly incorporated as point defects in the silicate minerals of the mantle. The addition of H into nominally anhydrous minerals profoundly changes their physical properties (i.e., mineral strength, electrical conductivity, diffusion rates of other constituent ions, seismic properties). Water also has a significant influence on mantle rheology and viscosity, as well as the solidus of the mantle and, therefore, the melting regime. Taken together,
water greatly affects mantle convection and plate tectonics, which in return influence the influx
of water to the mantle through the subduction of wet oceanic lithosphere and back to the
surface via mid-ocean ridge, arc, and hotspot volcanism. The ubiquitous presence of water
makes this molecule fundamental for both astrophysical, cosmochemical, and geological
processes, and highlights the importance of understanding its evolution in time and space
throughout the 4.56 billion year history of the Solar System.

WATER IN THE SOLAR SYSTEM: WHERE, HOW, AND HOW MUCH?

Given its significance in planetary and biological evolution, water has been searched for and
found throughout the Solar System. Quantifying its abundance and distribution is not easy
business, as only a few samples from planetary bodies are available on Earth compared to the
stunning diversity of worlds in our Solar System. However, water can be detected using infrared
remote sensing techniques at the surface of planetary objects (FIG. 2), because the strong dipole
moment of the O-H bond leads to particularly strong absorption in the infrared spectral range.
Water, as well as hydrated or hydroxylated minerals, can be identified and distinguished by
analyzing reflected sunlight based on specific absorption features around 1.4, 1.9, 2.2-2.3, and 3
µm (FIG. 2). It can also be observed remotely from the rotational modes of water molecules that
can be probed by radio telescopes and, depending on the abundance of water in the line of sight,
its isotopic composition may be determined (hydrogen having two stable isotopes: hydrogen H
and deuterium D, expressed as the D/H ratio). Water D/H ratios can also be accessed by in situ
mass spectrometry measurements onboard orbiters or landers.

Depending on particle density, water ice grains under high vacuum will sublimate at
temperatures over 100–150 K. This limit, known as the water snowline (FIG. 1), was effective
during the circumsolar disk evolution where its location was a direct function of the stellar
luminosity, accretion rate, and gas density. Although its position is generally expected to drift
inward during disk evolution (from ~ 3 to 1 AU; astronomical unit AU = Earth-Sun distance =
150 × 10⁶ km). The position of the snowline (and its evolution over time) is a fundamental
question, because it controls the amount of water available in the asteroidal- and planetary-
accretion regions, a characteristic that, in turn, controls the geodynamic evolution of asteroidal
and planetary bodies. The snowline is located near Jupiter's orbit under current Solar System
conditions, but specific local conditions at the surface of planetary bodies could disturb this
limit and impact the water distribution. Water ice is thus dominantly unstable today at the
surfaces of the (atmosphere-free) Moon and Mercury, but has been detected at locations
permanently shaded throughout their whole revolution around the Sun. In addition to water ice,
-OH-related absorption bands have been also detected at the surface of the Moon at high
latitude with a suggested abundance of the order of 100s µg·g⁻¹ of water. Both signatures have
been interpreted to reflect an exogenous origin, either from micrometeorites, asteroidal and/or cometary impacts, or by the impact-triggered transformation of solar wind-implanted hydrogen into water molecules.

The density and temperature of the Venusian atmosphere preclude the presence of water-ice deposits on the surface, as well as the presence of most hydrous minerals (but not all). There has been speculation about the amount of water hosted in the interior of Venus, but further evidence through dedicated space missions is needed. In the case of Mars, water has been identified all over the planet, whether in the form of ice, hydrated minerals, and possibly brines (FIG. 2). While the presence of clay minerals in ancient Martian terranes offers a window into past water cycles, there is also an active surface-atmosphere water cycle today. Two massive permanent ice caps have been identified in the north and south poles of the red planet, which provide enough water to fill the Mediterranean Sea. The redistribution of these ice masses in response to Mars' orbital evolution led to "Quaternary" geological processes identified by their morphological fingerprints.

The asteroid belt hosts a menagerie of objects that probably formed in very different locations in the Solar System. To first-order, there is a dichotomy between darker and brighter objects that echoes the dichotomy found in meteorites between ordinary and carbonaceous chondrites (DeMeo and Carry 2014). The silicate-rich asteroids (S-type) do not show strong evidence of hydration on their surface, in line with the general dryness of ordinary chondrites to which they are associated (typically 0.1 wt.% H₂O; Vacher et al. 2020). On the contrary, C-type asteroids, which make up roughly half of the mass of the main belt, often show strong evidence of phyllosilicate minerals based on observations of a deep 3-µm band (Usui et al. 2019; FIG. 3). The strength of the feature is much higher than has been observed on the Moon, for instance, and suggests the presence of a significant amount of equivalent water (>5 wt.%) on their surfaces. The meteorite record of aqueous alteration and phyllosilicate production is presented in Vacher and Fujiya (2022 this issue).

The observation that some main-belt asteroids are active, i.e. that they eject a significant amount of dust, is suggestive that, together with OH-bearing minerals, a significant fraction of water ice is present in asteroids, which blurs the boundary with comets. The outer part of the asteroid belt, as well as Jupiter's Trojan asteroids, is dominated by particular types of objects, the P- and D-types, which share strong optical similarities with comets, and can be envisioned as dormant comets (Vernazza and Beck 2017). However, unlike C-type asteroids, they generally do not show strong evidence for water ice or hydrated minerals at their surface, although water is probably a major constituent, as indicated by their density and potential D-type-derived
meteorites. Their thermal evolution was possibly cold enough to preclude the melting of water and production of phyllosilicate at least on their surface.

Density is the strongest constraint we have today to infer the abundance of water in the interior of small bodies. Accurate density estimates are limited to only a tiny portion of the overall population, but suggest different bulk compositions for S-, C-, and to some extent D-type asteroids (FIG. 4; Vernazza et al. 2021). The density of 100-km-sized, S-type asteroids is in the range 3000–4000 kg/m³, which is in agreement with measurements on ordinary chondrites found on Earth, and implies a low fraction of water in their interior (FIG. 4). By comparison, the density of C-type asteroids is significantly lower (~1000–2000 kg/m³ for 100-km objects; FIG. 4), which suggests that they are composed of an important fraction of low-density material (presumably water ice and/or hydrated minerals; FIG. 3). Density estimates for D-type asteroids are rare, but the recently obtained value for 87-Sylvia (1330 ± 70 kg/m³) is slightly lower than that estimated for C-types of similar size (FIG. 4; Vernazza et al. 2021). Estimating the bulk water content of asteroids is not straightforward, as it requires constraints or hypotheses on their porosity. Interestingly, density and diameter are correlated for a given class of asteroid, which is interpreted by an increase of porosity in smaller objects, thus allowing the bulk composition of asteroids to be estimated. For instance, the 2060 ± 200 kg/m³ density of the 420-km-diameter C-type asteroid 10-Hygeia could be explained by a volume mixture of 20% porosity, 55% rock/metal, and 25% water ice.

Comets are, by definition, objects emitting dust as a consequence of volatile sublimation at their surface. Overall, water is the major volatile species within comets, but other volatile species, such as CO or CO₂, may play a role in their activity. While the water loss rate from a comet can be determined from the ground, the determination of the water/dust ratio is more complex and requires accurate modeling of the light scattered by the dust particles. Detailed observations in the 3-µm region of the nucleus of comet 67P Churyumov Gerasimenko revealed the general absence of water ice on the surface, and that hydrogen is instead present in surface material in the form of ammonium salt and aliphatic organics (FIG. 3). If the surface seems “dry”, the low density of the few cometary nuclei observed by spacecraft (~500–2000 kg/m³; Fig. 4) points toward a significant fraction of interior volatiles. Estimates based on the Rosetta mission are in the range of 10–50 vol% water (Davidsson et al. 2016; Fulle et al. 2019) and an equivalent un- compacted density of approximately 1800–1900 kg/m³ (McKinnon et al. 2017).

Optical observations of trans-Neptunian objects (TNOs) and centaurs (objects on unstable orbits between Jupiter and Neptune) have revealed the presence of water ice on their surfaces (Barucci et al. 2008, see also Charon in FIG. 2), but not systematically. When present at the surface, water ice appears to be in a crystalline form, rather than amorphous, which is at odds
with a very low-temperature origin (<50 K, i.e., directly inherited from the protosolar nebula) and suggests that it may have been thermally processed. The largest known TNOs have well-constrained densities that are lower than C-type asteroids of similar size (except for 1-Ceres). In the case of the Pluto-Charon system, the determined values are of 1850 and 1700 kg/m³, respectively, which translates to 30-40 wt.% of water ice based on a two-component model with a rocky component (McKinnon et al. 2017). It should be noted that one of the limitations of the two-component models is that it neglects the presence of carbonaceous compounds, which appear to represent a major constituent of comets, and could then constitute a significant fraction of TNOs.

Looking at the overall picture, if water is present at least locally at the surface of all types of Solar System objects, density estimates suggest that the fraction of volatiles, including water, is higher in objects distant from the Sun. It appears that the fraction of water is homogeneous among C- and D-type asteroids, comets, and TNOs (within an order of magnitude to be conservative), while the inner Solar System is depleted in water compared to these objects. Understanding this global picture requires a determination of the astrophysical origin of water (and thus what constituents were incorporated into the Solar System) and how and where planetary bodies accreted.

## ORIGIN, AMOUNT, AND DISTRIBUTION OF WATER WITHIN THE EARLY SOLAR SYSTEM

The current understanding of the astrophysical origin and pathways of water toward planetary systems is presented in the article by Ceccarelli and Dun in this issue. In the cold and diffuse environment of the interstellar medium (ISM), water may dominantly exist in the form of water-ice grains (FIG. 1). In the diffuse ISM, the presence of water ice has only been observed within density-increased molecular clouds where water molecules form through reactions between O and H atoms in the gas phase and/or at the surface of interstellar dust via catalyzed processes (see FIG. 2 in Ceccarelli and Du in this issue). The latter implies an intimate association of water and refractory grains (silicates or carbon-rich grains) at the onset of star formation. Whatever the processes involved, fundamental questions are related to the amount and origin of water present in the starting material from which the Solar System formed (before disk evolution processing) and how its distribution evolved over the 4567-million-year lifetime of the disk. Several approaches can be followed involving more or less straightforward methods. For instance, calculating how elements with solar abundances partition within silicates, carbon-rich grains, sulfides, CO, and H₂O allows a water/dust ratio of ~1 to be estimated (Min et al. 2011). Depending on the amount of carbon incorporated into carbonaceous dust, the dust density estimates range from 1450 to 1800 kg/m³. Interestingly, these values are in the range estimated
for Pluton and Charon and suggest that their bulk composition may correspond to that of primordial Solar System building blocks (although other large TNOs, like Eris or Haumea, have higher density). In addition, the inferred water/dust content seems much higher than that estimated for comet 67P and for carbonaceous chondrites. The latter show solar abundances for most elements except for the most volatile, which are characterized by water/rock ratios ranging from 0.01 to 0.4 (Marrocchi et al. 2018). However, such estimations correspond to final values, and a significant fraction of the initial water may have been degassed during secondary alteration processes of carbonaceous asteroids (Vacher and Fujiya 2022 this issue).

In theory, a more direct approach would be to measure the abundance and H and O isotopic compositions of water trapped in meteorites at both bulk and in situ scales, as they bear invaluable information on the source and abundance of water accreted by asteroids (Deloule and Robert 1995; Alexander et al. 2012). This proves difficult in practice, however, as immediately after reaching the Earth's surface, meteorites are exposed to terrestrial moisture and precipitation that could significantly affect their initial water contents and isotopic characteristics (Vacher et al. 2020). Furthermore, secondary minerals formed through fluid circulation within meteoritic parent bodies are intimately mixed with H-bearing organic compounds, thus impeding the accurate estimation of hydrogen isotopic compositions retained in hydrated minerals (Piani et al. 2018). However, recent studies have overcome these difficulties by developing new analytical procedures, which allow the estimation of the water concentrations and isotopic compositions to be precisely determined in a large set of meteorites (Vacher et al. 2020; Piani et al. 2018). If they confirm that the meteorites formed in the outer Solar System are water-rich (McCubbin and Barnes 2019), it also appears that meteorites formed in the inner Solar System are characterized by higher water abundances than previously thought (Vacher et al. 2020). This led to a paradigm shift where the inner Solar System could no longer be considered as completely dry and volatile-poor; these characteristics having a strong influence on the accretion processes within the disk (Lichtenberg et al. 2021), as well as on the redox states of the processes that controlled the evolution of the terrestrial planets (Gaillard et al. 2021). From an isotopic perspective, it appears that the asteroidal water budget corresponds to a mixture between (i) local water formed via sublimation/recondensation events during the evolution of the disk and (ii) inherited deuterium-enriched interstellar ices from the molecular clouds unaffected by disk processes (Alexander et al. 2012; Piani et al. 2018). This could result from protracted interactions with the molecular cloud all along with the disk history (Piani et al. 2021). Interestingly, this model would naturally explain the not-so-dry characteristics of the inner Solar System, as interstellar ices could thus episodically seed the inner Solar System over several million years, especially when chondrule production processes have stopped and the temperature has dropped below the water-ice sublimation temperature (Piani et al. 2021).
Taken together, this implies that the water distribution within the early Solar System resulted
from complex processes involving local production and molecular cloud legacy, and that the
asteroidal water budget depends on both their location and timing of accretion within the disk.

This issue of *Elements* describes several aspects that may have acted in setting the current distribution of water throughout the Solar System. It begins by presenting how water was produced in the molecular cloud from which the Solar System was formed, and how water molecules may have been distributed in the solar protoplanetary disk (Ceccarelli and Du 2022 this issue). At that point, water is present in the vapor phase or as water-ice grains associated with tiny refractory grains. The next step is a big one: growing planetesimals, and then terrestrial planets. The mechanism from which planetesimals can grow and how the dynamical evolution of the early Solar System could have been key in delivering water in the early Solar System is presented in Izidoro and Piani (2022 this issue). Overall, a major constraint on the possible sources of water is the D/H ratio. Peslier and De Sanctis (2022 this issue) present and discuss the variation of water abundance and isotopic composition among various types of inner Solar System materials. We also focus on two different situations where liquid occurred or occurs today in the Solar System. Vacher and Fujiya (2022 this issue) present how carbonaceous chondrites record hydrothermal reactions that occurred just a few million years after the Solar System formed. Finally, many large Solar System objects are now recognized as past or present “ocean worlds”, and are a major focus of current Solar System exploration. This is presented in Castillo-Rogez and Kalousová (2022 this issue).

Acknowledgments: The authors deeply thank the Elements editorial team for the invitation to guest edit this issue. They also thank Romain Tartèse for his careful review of this article. PB and YM acknowledge support from the Programme National de Planétologie and the Centre National d'Etude Spatiale. PB acknowledges support from the European Research Council (ERC) under the grant SOLARYS ERC-CoG2017-771691.
<figure><img src="image_2.png" /><figcaption>**FIGURE 1:** Schematic representation of astrophysical and terrestrial environments where water plays a key role. Within the circumsolar disk, water-ice grains could have been formed locally or have been inherited from the molecular cloud from which the Sun formed. The distribution of water-ice grains within the disk depends on the location relative to the snowline, a limit that corresponds to the distance from the Sun at which the temperature is low enough for water to condense (~150–170 K). Depending on their accretion location, planetary bodies thus can incorporate a significant amount of water that will influence their geodynamic evolution and water cycle, such as on Earth.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**FIGURE 2:** Examples of the detection of water ice and hydrated minerals based on infrared reflectance spectroscopy on several Solar System objects. A menagerie of clay minerals has been identified on Mars based on diagnostic absorption related to metal-OH vibration harmonic and combinations, as well as hydrated phases in the form of sulfates (the example of a mono-hydrated Mg-sulphate is shown), zeolite, or opals. The signature of water ice is also shown as observed on the north polar cap of Mars together with observations of ice on Europa and Charon (McCord et al. 1998; Grundy et al. 2016). Changes in the shape of the water-ice absorption among these three objects can be seen around 1.6 µm, which are related to temperature. Mars spectra are from the CRISM database.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**FIGURE 3:** Comparison of the reflectance spectra of selected asteroids and a cometary nucleus. The 3-µm region displayed here is particularly sensitive to the presence of water and hydroxylated minerals, because the fundamental OH stretching is encountered in this spectral range. Such observations reveal the presence of phyllosilicates on several dark C-type asteroids (ex: (2) Pallas; Rivkin et al. 2015; Usui et al. 2019). In a few cases only (but including the largest asteroid / minor planet (1) Ceres; De Sanctis et al. (2015)), phyllosilicates appear to bear ammonia in their interlayer space, as evidenced from the presence of an absorption band around 3.06 µm. The asteroid (24) Themis, with an orbit in the outer main belt, shows a different spectrum in the 3-µm region compared to a phyllosilicate-rich object; a spectrum that was initially attributed to the presence of water ice on its surface. Observations of the nucleus of comet 67P by the Rosetta mission revealed the absence of phyllosilicate, the scarcity of water</figcaption></figure>
ice on the surface, and the presence of a peculiar signature in the 3-µm region. This spectral signature has been attributed to a mixture of ammonium salts and organic compounds, and is reminiscent of the spectra of (24) Themis.

<figure><img src="image_5.png" /></figure>

FIGURE 4: Density of various types of Solar System objects as a function of diameter (McKinnon et al. 2017; Vernazza et al. 2021). Due to the possible presence of porosity, asteroids, centaurs, and trans-Neptunian objects seem to present density-diameter trends. Still a general decrease of density is present with heliocentric distance that can be explained by an increasing fraction of water ice. The C-type panel (center) also shows the density of D-type asteroid (87) Sylvia (light green) and dwarf planet (1) Ceres (yellow).
Alexander CMO'D and 5 coauthors (2012) The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets. Science 337: 721-723, doi: 10.1126/science.1223474

Barucci MA, Brown ME, Emery JP, Merlin F (2008) Composition and surface properties of transneptunian objects and centaurs. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The Solar System Beyond Neptune. University of Arizona Press, Tucson, pp 143-160

Castillo-Rogez JC, Kalousová K (2022) Ocean worlds in our Solar System. Elements 1: XXX-XXX

Ceccarelli C, Du F (2022) We drink good 4.5-billion-year-old water. Elements 18: XXX-XXX

Deloule E, Robert F (1995) Interstellar water in meteorites? Geochimica et Cosmochimica Acta 59: 4695-4706, doi: 10.1016/0016-7037(95)00313-4

DeMeo FE, Carry B (2014) Solar System evolution from compositional mapping of the asteroid belt. Nature 505: 629-634, doi: 10.1038/nature12908

De Sanctis MC and 28 coauthors (2015) Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres. Nature 528: 241-244, doi: 10.1038/nature16172

Fulle M and 8 coauthors (2019). The refractory-to-ice mass ratio in comets. Monthly Notices of the Royal Astronomical Society 482: 3326-3340, doi: 10.1093/mnras/sty2926

Gaillard F and 8 coauthors (2021). The diverse planetary ingassing/outgassing paths produced over billions of years of magmatic activity. Space Science Reviews 217: 22, doi: 10.1007/s11214-021-00802-1

Grundy WM and 34 coauthors (2016) Surface compositions across Pluto and Charon. Science 351: aad9189, doi: 10.1126/science.aad918

Hirschmann MM (2006) Water, melting, and the deep earth H₂O cycle. Annual Review of Earth Planetary Sciences 34: 629-653, doi: 10.1146/annurev.earth.34.031405.125211

Izidoro A, Piani L (2022) Origin of water in the terrestrial planets: insights from meteorite data and planet formation models. Elements 18: XXX-XXX

Lichtenberg T, Drażkowska J, Schönbächler M, Golabek GJ, Hands TO (2021) Bifurcation of planetary building blocks during Solar System formation. Science 371: 365-370, doi: 10.1126/science.abb3091

Lodders K (2003) Solar system abundances and condensation temperatures of the elements. The Astrophysical Journal 591: 1220-1247, doi: 10.1086/375492

Marrocchi Y, Bekaert DV, Piani L (2018) Origin and abundance of water in carbonaceous asteroids. Earth and Planetary Science Letters 482: 23-32, doi: 10.1016/j.epsl.2017.10.060

Marty B (2012) The origins and concentrations of water, carbon, nitrogen and noble gases on Earth. Earth and Planetary Science Letters 313-314: 56-66, doi: 10.1016/j.epsl.2011.10.040

McCord TB and 12 coauthors (1998). Salts on Europa's surface detected by Galileo's near infrared mapping spectrometer. Science 280: 1242-1245, doi: 10.1126/science.280.5367.124

McCubbin FM, Barnes JJ (2019) Origin and abundances of H₂O in the terrestrial planets, Moon, and asteroids. Earth and Planetary Science Letters 526: 115771, doi: 10.1016/j.epsl.2019.115771
McKinnon WB and 13 coauthors (2017). Origin of the Pluto-Charon system: constraints from the New Horizons flyby. Icarus 287: 2-11, doi: [10.1016/j.icarus.2016.11.019](https://doi.org/10.1016/j.icarus.2016.11.019)

Min M, Dullemond CP, Kama M, Dominik, C (2011) The thermal structure and the location of the snow line in the protosolar nebula: axisymmetric models with full 3-D radiative transfer. Icarus 212: 416-426, doi: [10.1016/j.icarus.2010.12.002](https://doi.org/10.1016/j.icarus.2010.12.002)

Peslier AH, De Sanctis MC (2022) Water in differentiated planets, the Moon and asteroids. Elements 18: XXX-XXX

Piani L, Yurimoto H, Remusat L (2018). A dual origin for water in carbonaceous asteroids revealed by CM chondrites. Nature Astronomy 2: 317-323, doi: [10.1038/s41550-018-0413-4](https://doi.org/10.1038/s41550-018-0413-4)

Piani L, Marrocchi Y, Vacher LG, Yurimoto H, Bizzarro M (2021) Origin of hydrogen isotopic variations in chondritic water and organics. Earth and Planetary Science Letters 567: 117008, doi: [10.1016/j.epsl.2021.117008](https://doi.org/10.1016/j.epsl.2021.117008)

Rivkin AS and 6 coauthors (2015) Astronomical observations of volatiles on asteroids. In: Michel P, DeMeo FE, Bottke WF (eds) Asteroids IV. University of Arizona Press, Tucson, pp 65-87, doi: [10.2458/azu_uapress_9780816532131-ch004h](https://doi.org/10.2458/azu_uapress_9780816532131-ch004h)

Usui F, Hasegawa S, Ootsubo T, Onaka T (2019) AKARI/IRC near-infrared asteroid spectroscopic survey: AcuA-spec. Publications of the Astronomical Society of Japan 71: 1-41, doi: [10.1093/pasj/psy125](https://doi.org/10.1093/pasj/psy125)

Vacher LG and 6 coauthors (2020) Hydrogen in chondrites: influence of parent body alteration and atmospheric contamination on primordial components. Geochimica et Cosmochimica Acta 281: 53-66, doi: [10.1016/j.gca.2020.05.007](https://doi.org/10.1016/j.gca.2020.05.007)

Vacher LG, Fujiya W (2022) Recent advances in our understanding of water and aqueous activity in chondrites. Elements 18: XXX-XXX

Vernazza P, Beck P (2017) Composition of Solar System small bodies. In: Elkins-Tanton LT, Weiss BP (eds) Planetesimals, Cambridge University Press, Cambridge, pp 269-297, doi: [10.1017/9781316339794](https://doi.org/10.1017/9781316339794)

Vernazza P and 66 coauthors (2021). VLT/SPHERE imaging survey of the largest main-belt asteroids: final results and synthesis. Astronomy & Astrophysics 654: A56, doi: [10.1051/0004-6361/202141781](https://doi.org/10.1051/0004-6361/202141781)