Figures

top

Introduction

Mars has recently emerged as a natural laboratory for studying the acquisition and processing of volatile elements on the terrestrial planets. Because undifferentiated building blocks of terrestrial planets were likely volatile-bearing (Schaefer and Fegley, 2017

Schaefer, L., Fegley Jr., B. (2017) Redox States of Initial Atmospheres Outgassed on Rocky Planets and Planetesimals. The Astrophysical Journal 843, 120. https://doi.org/10.3847/1538-4357/aa784f

) and Martian accretion likely occurred in the presence of the solar nebula (Dauphas and Pourmand, 2011

Dauphas, N., Pourmand, A. (2011) Hf-W-Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature 473, 489–U227. https://doi.org/10.1038/nature10077

), questions about different volatile sources and formation processes for the primordial Martian atmosphere can now be addressed (Saito and Kuramoto, 2018

Saito, H., Kuramoto, K. (2018) Formation of a hybrid-type proto-atmosphere on Mars accreting in the solar nebula. Monthly Notices of the Royal Astronomical Society 475, 1274–1287. https://doi.org/10.1093/mnras/stx3176

; Péron and Mukhopadhyay, 2022

Péron, S., Mukhopadhyay, S. (2022) Krypton in the Chassigny meteorite shows Mars accreted chondritic volatiles before nebular gases. Science 377, 320–324. https://doi.org/10.1126/science.abk1175

). High precision isotopic measurements display a dichotomy in the sources of Martian volatiles. Volatiles dissolved in melts derived from the Martian mantle are observed to have an isotopic composition akin to chondrites for hydrogen, nitrogen, krypton and xenon (Usui, 2019

Usui, T. (2019) Hydrogen reservoirs in Mars as revealed by martian meteorites, Volatiles in the Martian Crust. Elsevier, 71–88. https://doi.org/10.1016/B978-0-12-804191-8.00004-0

; Péron and Mukhopadhyay, 2022

Péron, S., Mukhopadhyay, S. (2022) Krypton in the Chassigny meteorite shows Mars accreted chondritic volatiles before nebular gases. Science 377, 320–324. https://doi.org/10.1126/science.abk1175

; Deligny et al., 2023

Deligny, C., Füri, E., Deloule, E., Peslier, A., Faure, F., Marrocchi, Y. (2023) Origin of nitrogen on Mars: First in situ N isotope analyses of martian meteorites. Geochimica et Cosmochimica Acta 344, 134–145. https://doi.org/10.1016/j.gca.2023.01.017

) whereas the isotopic composition of krypton and xenon in the Martian atmosphere – which do not currently experience escape – record a solar-like source (Pepin, 1991

Pepin, R.O. (1991) On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Icarus 92, 2–79. https://doi.org/10.1016/0019-1035(91)90036-S

; Conrad et al., 2016

Conrad, P.G., Malespin, C.A., Franz, H.B., Pepin, R.O., Trainer, M.G., Schwenzer, S.P., Atreya, S., Freissinet, C., Jones, J., Manning, H. (2016) In situ measurement of atmospheric krypton and xenon on Mars with Mars Science Laboratory. Earth and Planetary Science Letters 454, 1–9. https://doi.org/10.1016/j.epsl.2016.08.028

). These observations prompt questions about the relationship between Martian interior and atmospheric volatiles.

The volatile dichotomy between a chondrite-like mantle and a solar-like atmosphere has recently been interpreted to mean that the Martian atmosphere cannot be the result of magma ocean outgassing and must be the result of nebular capture (Péron and Mukhopadhyay, 2022

Péron, S., Mukhopadhyay, S. (2022) Krypton in the Chassigny meteorite shows Mars accreted chondritic volatiles before nebular gases. Science 377, 320–324. https://doi.org/10.1126/science.abk1175

). There is, however, empirical evidence that silicate Mars experienced outgassing during the lifetime of the short-lived and volatile radionuclide ¹²⁹I (τ_1/2 = 16 Myr), resulting in the observed ¹²⁹Xe depletion in Martian interior reservoirs (Marty and Marti, 2002

Marty, B., Marti, K. (2002) Signatures of early differentiation of Mars. Earth and Planetary Science Letters 196, 251–263. https://doi.org/10.1016/S0012-821X(01)00612-4

). Early selective removal of volatile elements from silicate Mars – for example via outgassing from a molten state (Elkins-Tanton, 2008

Elkins-Tanton, L.T. (2008) Linked magma ocean solidification and atmospheric growth for Earth and Mars. Earth and Planetary Science Letters 271, 181–191. https://doi.org/10.1016/j.epsl.2008.03.062

) – points towards transport of chondritic volatiles into the primordial atmosphere. Although this episode of primordial outgassing is empirically supported, it has yet to be reconciled with the observed dichotomy between the Martian interior and atmospheric volatiles (Péron and Mukhopadhyay, 2022

Péron, S., Mukhopadhyay, S. (2022) Krypton in the Chassigny meteorite shows Mars accreted chondritic volatiles before nebular gases. Science 377, 320–324. https://doi.org/10.1126/science.abk1175

). Here, we consider hybrid accretionary atmospheres (Saito and Kuramoto, 2018

Saito, H., Kuramoto, K. (2018) Formation of a hybrid-type proto-atmosphere on Mars accreting in the solar nebula. Monthly Notices of the Royal Astronomical Society 475, 1274–1287. https://doi.org/10.1093/mnras/stx3176

) in which an outgassed high molecular weight gas is mixed into the distended low molecular weight nebular atmosphere. We show that such mixing can reconcile primordial outgassing of chondritic volatiles with nebular capture of solar-like gases into the Martian atmosphere, that mixing before dissipation of the solar nebula strongly enhances the mass of nebular captured gas, and that comparison of hybrid-source compositions with that of the present day atmosphere can yield new insights into the history of Martian atmospheric evolution. As an example, the hybrid origin model implies that most Martian argon loss occurred primordially rather than during subsequent geologic epochs as commonly assumed. We show that a hybrid initial composition is a viable initial condition for understanding Martian atmospheric history.

top

Model and Results

Enhancement of nebular capture via mixing. We calculate structures for dusty (Eq. S-4) captured Martian atmospheres present during accretion in hydrostatic equilibrium and thermal steady state with the solar nebula. We consider atmospheres both with and without mixing of an outgassed high mean molecular weight layer (see Supplementary Information for discussion of the energetics of mixing via thermal convection). Atmospheres without a high molecular weight component are called “pure” and are presented as a reference. The atmospheres are convective at depth and radiative at altitude, and are assumed to blend into the solar nebula at the Hill radius at ≈320 Mars radii. Because the mass (and heat capacity) of these atmospheres is relatively small, a heat source other than secular cooling is needed to calculate quasi-static structures. An absolute lower limit on the heat flow derives from long-lived radioactive decay and is equivalent to accretion rates ≈10⁻⁴ Mars masses/Myr (Erkaev et al., 2014

Erkaev, N., Lammer, H., Elkins-Tanton, L., Stökl, A., Odert, P., Marcq, E., Dorfi, E., Kislyakova, K., Kulikov, Y.N., Leitzinger, M. (2014) Escape of the martian protoatmosphere and initial water inventory. Planetary and Space Science 98, 106–119. https://doi.org/10.1016/j.pss.2013.09.008

). More likely heat flows relevant to the first few million years of Martian history derive from ongoing planetesimal accretion – which also delivers high molecular weight volatiles – and/or ²⁶Al decay. We consider heat flows at the base of the atmosphere equivalent to planetesimal accretion rates of 0.01–1 Mars masses/Myr (Eq. S-1), covering the range from energetic accretion consistent with large-scale melting (Dauphas and Pourmand, 2011

Dauphas, N., Pourmand, A. (2011) Hf-W-Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature 473, 489–U227. https://doi.org/10.1038/nature10077

) down to reduced heat flows unable to maintain a magma ocean (Saito and Kuramoto, 2018

Saito, H., Kuramoto, K. (2018) Formation of a hybrid-type proto-atmosphere on Mars accreting in the solar nebula. Monthly Notices of the Royal Astronomical Society 475, 1274–1287. https://doi.org/10.1093/mnras/stx3176

) and more consistent with sweep-up of planetesimals during the waning stages of accretion. For heat flows equivalent to accretion rates of 0.01–1 Mars masses/Myr, the mass (M_H) of a “pure” nebular captured atmosphere is ≈0.2–3.1 x 10¹⁸ kg and equivalent to ≈0.05–0.8 bars of H₂ at the Mars surface (Fig. 1). Lower accretion rates produce cooler, denser atmospheres that are more massive, a behaviour summarised with the maxim: “to cool is to accrete” (Lee and Chiang, 2015

Lee, E.J., Chiang, E. (2015) To cool is to accrete: analytic scalings for nebular accretion of planetary atmospheres. The Astrophysical Journal 811, 41. https://doi.org/10.1088/0004-637X/811/1/41

).

Full size image

Mixing of the nebular atmosphere with an outgassed high molecular weight gas – for which there is sufficient energy (Eq. S-13) – sharply enhances the mass of the captured gas inventory. To illustrate the magnitude of this effect, we calculate the structure of hybrid-source atmospheres in which the outgassed component (μ_CO2 = 44 amu with an inventory size characterised by M_CO2) and the nebular component (μ_H = 2.4 amu with an inventory size characterised by M_H) are assumed to be fully-mixed into a homogenous atmosphere (see equations S-1 to S-3 and S-7 to S-12 in the Supplementary Information). To find solutions, we take the mean molecular weight of the mixture as a free parameter, varying it across a range (μ = 6.6–19 amu) that allows sampling of the solution space. When a well-mixed hybrid atmosphere hydrostatically equilibrates with the solar nebula and achieves thermal steady state, the mass of the captured inventory (M_H) strongly depends on the inventory of the high molecular weight gas (M_CO2) with which it is mixed (see Gas-assisted capture; Fig. 1). For planetesimal accretion rates of 0.01–1 Mars masses/Myr and heavy gas inventories (M_CO2 = 3.9–390 x 10¹⁹ kg) equivalent to 10–1000 bars of CO₂ at the planetary surface (Elkins-Tanton, 2008

Elkins-Tanton, L.T. (2008) Linked magma ocean solidification and atmospheric growth for Earth and Mars. Earth and Planetary Science Letters 271, 181–191. https://doi.org/10.1016/j.epsl.2008.03.062

), the captured nebular inventory (M_H) is in the range of 1.2–120 x 10¹⁹ kg, equivalent to ≈3–300 bars of H₂ at the surface (Fig. 1). The strong enhancement of nebular capture via mixing an outgassed component is mainly due to an increase in mean molecular weight, which increases the gravitational coupling between the atmosphere and planet and decreases the atmospheric scale height, which – like cooling – causes contraction and an increase in the hydrogen density of the lower atmosphere where most of the atmospheric mass resides. This behaviour can be summarised with another maxim: “to mix is to accrete.”

The dependence of the nebular captured inventory (M_H) on the outgassed inventory (M_CO2) in fully-mixed hybrid-source atmospheres (Fig. 1, and Eqs. S-11 and S-12) sets upper limits on the contribution of the nebular component to the Martian atmosphere. A measure of the relative contribution of captured and outgassed inventories is the mean molecular weight of the mixture, which only varies by a factor of ∼three for the full range of conditions that we consider (Fig. 1). Although the mass of the outgassed component is dominated by carbon species (e.g., CO₂), other outgassed volatiles (H, N, and noble gases) were also present and the elemental abundances and isotopic composition of the hybrid mixture can be used – in comparison with observed abundances – to further constrain the nebular contribution to Martian volatiles. Whereas the physics of nebular capture into fully-mixed hybrid atmospheres yields upper limits on the nebular contribution, lower limits can be derived from the cosmochemistry of chondritic-nebular gas mixtures, as we show in the following section.

Chondritic-nebular gas mixtures. To describe cosmochemical consequences of mixing a nebular component into an outgassed atmosphere, we calculate two-component mixtures including major volatiles (H, C, N) and noble gases (Ne, Ar, Kr, Xe) with chondritic and nebular end members. We neglect He because – like He in Earth’s atmosphere – the lifetime of this noble gas with respect to escape from the Martian atmosphere is extremely short relative to geological time scales and its present day abundance simply reflects a balance between recent supply and loss (Krasnopolsky et al., 1994

Krasnopolsky, V., Bowyer, S., Chakrabarti, S., Gladstone, G., Mcdonald, J. (1994) First measurement of helium on Mars: Implications for the problem of radiogenic gases on the terrestrial planets. Icarus 109, 337–351. https://doi.org/10.1006/icar.1994.1098

). For the chondritic end member, we adopt the 55 % H chondrite, 45 % EH chondrite model for Mars (Sanloup et al., 1999

Sanloup, C., Jambon, A., Gillet, P. (1999) A simple chondritic model of Mars. Physics of the Earth and Planetary Interiors 112, 43–54. https://doi.org/10.1016/S0031-9201(98)00175-7

). For the purposes of the mixing calculations, we include all chondritic volatiles (interior and outgassed), although a substantial fraction of chondritic H (or “water”) is expected to remain sequestered in the interior (Sim et al., 2024

Sim, S.J., Hirschmann, M.M., Hier‐Majumder, S. (2024) Volatile and trace element storage in a crystallizing martian magma ocean. Journal of Geophysical Research: Planets 129, e2024JE008346. https://doi.org/10.1029/2024JE008346

). The assumption of complete outgassing may be more accurate for C, N, and the noble gases. With end member compositions specified, the composition of the mixture can be described with one parameter, the relative contribution of the two components.

The compositional characteristics of a nebular-chondritic mixture can be calculated for comparison with the observed atmospheric abundances. For “Gas-assisted capture” (Fig. 1), representing fully-mixed hybrid-sourced atmospheres in hydrostatic equilibrium with the solar nebula, the nebular contribution to the total volatile budget of Mars, counting atoms, is ≈46–77 %, depending on the accretion rate and outgassed volatile inventory. The total volatile budget is dominated by H and He from the nebular component and H and C from the chondritic component. For these relative proportions of the nebular component to the hybrid mixture, the neon (>99 %), argon (>99 %) and krypton (>90 %) inventories are dominated by the nebular component, whereas carbon (<1 %) and nitrogen (<2 %) inventories experience negligible nebular additions and continue to be dominated by the outgassed component (Fig. 2). Hydrogen and xenon are intermediate cases in which the inventories in the mixture are derived from comparable contributions from the two sources. In summary, nebular capture via complete mixing into a hybrid-source atmosphere produces Martian Ne, Ar and Kr with solar heritage, C and N with chondritic heritage, and H and Xe with mixed heritage.

Full size image

A hybrid primordial mixture is consistent with the observed isotopic composition of the Martian atmosphere. Mass-selective loss has fractionated stable Ar isotopes (³⁶Ar from ³⁸Ar) but a solar-like source is a viable starting composition for atmospheric argon (Atreya et al., 2013

Atreya, S.K., Trainer, M.G., Franz, H.B., Wong, M.H., Manning, H.L., Malespin, C.A., Mahaffy, P.R., Conrad, P.G., Brunner, A.E., Leshin, L.A. (2013) Primordial argon isotope fractionation in the atmosphere of Mars measured by the SAM instrument on Curiosity and implications for atmospheric loss. Geophysical Research Letters 40, 5605–5609. https://doi.org/10.1002/2013GL057763

). Krypton in the atmosphere is distinct from chondrites but nearly indistinguishable from solar (Pepin, 1991

Pepin, R.O. (1991) On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Icarus 92, 2–79. https://doi.org/10.1016/0019-1035(91)90036-S

; Conrad et al., 2016

Conrad, P.G., Malespin, C.A., Franz, H.B., Pepin, R.O., Trainer, M.G., Schwenzer, S.P., Atreya, S., Freissinet, C., Jones, J., Manning, H. (2016) In situ measurement of atmospheric krypton and xenon on Mars with Mars Science Laboratory. Earth and Planetary Science Letters 454, 1–9. https://doi.org/10.1016/j.epsl.2016.08.028

). Atmospheric xenon can be modelled either as mass-fractionated solar or mass-fractionated chondritic gas (Swindle, 2002

Swindle, T.D. (2002) Martian noble gases. Reviews in Mineralogy and Geochemistry 47, 171–190. https://doi.org/10.2138/rmg.2002.47.6

). Although the physics of nebular capture into fully-mixed hybrid atmospheres sets upper limits on the nebular contribution (<46–77 %), partial mixing could yield a lower nebular contribution. The requirement that Martian Kr be nearly indistinguishable from solar but distinct from chondrites constrains the nebular contribution to the total Martian volatile budget to >10 %, counting atoms (Fig. 2). Next, we consider the consequences of primordial hybrid mixtures for inferring Martian atmospheric history.

Elemental abundances elucidate escape processes. Of all the major volatiles (H, C, N) and noble gases (Ne, Ar, Kr, Xe) we consider, krypton is most nearly isotopically unfractionated in Mars’s atmosphere relative to its apparent source, the solar nebula. Accordingly, to gain insight into the nature of evolutionary processes, we consider elemental abundances normalised to krypton and relative to solar abundances. Hybrid-source elemental abundances have some affinity to modern Mars (Fig. 3), with important differences. Relative to hybrid-source mixtures, the modern Martian atmosphere is depleted in H, C, N, Ne, Ar, and Xe, each of which is also enriched in the heavy isotopes in the Martian atmosphere (Bogard et al., 2001

Bogard, D., Clayton, R., Marti, K., Owen, T., Turner, G. (2001) Martian volatiles: Isotopic composition, origin, and evolution. Chronology and Evolution of Mars: Proceedings of an ISSI Workshop, 10–14 April 2000, Bern, Switzerland. Springer. https://doi.org/10.1007/978-94-017-1035-0_17

). Such coupled elemental and isotopic fractionation suggests the viability of a hybrid-source mixture as a precursor to the modern Martian atmosphere, the two being linked via compositional evolutionary processes, among which mass-selective losses to space looms large.

Full size image

We consider elemental (³⁶Ar/⁸⁴Kr) and isotopic (³⁶Ar/³⁸Ar) fractionation accompanying argon loss. Argon is suitable for examining ancient processes because the inventory of atmospheric ³⁶Ar is primordial meaning it cannot be accounted for by volcanic outgassing over time (Jakosky and Treiman, 2023

Jakosky, B.M., Treiman, A.H. (2023) Mars volatile inventory and outgassing history. Icarus 402, 115627. https://doi.org/10.1016/j.icarus.2023.115627

). The non-radiogenic Ar/Kr ratio in the modern Martian atmosphere is lower than that of a hybrid mixture by a factor of ≈50 (Fig. 3) whereas the ³⁶Ar/³⁸Ar is lower than plausible sources by ≈25 % (Atreya et al., 2013

Atreya, S.K., Trainer, M.G., Franz, H.B., Wong, M.H., Manning, H.L., Malespin, C.A., Mahaffy, P.R., Conrad, P.G., Brunner, A.E., Leshin, L.A. (2013) Primordial argon isotope fractionation in the atmosphere of Mars measured by the SAM instrument on Curiosity and implications for atmospheric loss. Geophysical Research Letters 40, 5605–5609. https://doi.org/10.1002/2013GL057763

). A loss process that strongly separates Ar from Kr but weakly discriminates ³⁶Ar from ³⁸Ar is indicated. We consider an episode of extreme ultraviolet (EUV) powered hydrodynamic escape in which an outflow of H₂ and CO₂ entrains trace gases via frequent collisions (Zahnle et al., 1990

Zahnle, K., Kasting, J.F., Pollack, J.B. (1990) Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape. Icarus 84, 502–527. https://doi.org/10.1016/0019-1035(90)90050-J

). Entrainment involves all trace gases up to a maximum molecular mass whose value depends on the strength of the flow (See Eq. S-16 for details). Results reveal the existence of a hydrodynamic outflow sufficiently strong to reproduce the chemical (Ar/Kr) and isotopic (³⁶Ar/³⁸Ar) fractionation observed in the Martian atmosphere starting from a hybrid mixture while remaining sufficiently weak to allow Kr to be retained and its solar isotopic heritage to be preserved (Fig. 4). Of course, mass-selective argon loss from Mars via other mechanisms (e.g., solar wind sputtering) occurs and is ongoing (Jakosky et al., 2017

Jakosky, B.M., Slipski, M., Benna, M., Mahaffy, P., Elrod, M., Yelle, R., Stone, S., Alsaeed, N. (2017) Mars’ atmospheric history derived from upper-atmosphere measurements of ³⁸Ar/³⁶Ar. Science 355, 1408–1410. https://doi.org/10.1126/science.aai7721

). The hybrid mixture model provides a framework for assessing the relative importance of various mechanisms of atmospheric loss over the entire history of the planet.

Full size image

top

Discussion

The origin of the Martian hydrosphere. A significant feature of the Martian volatile record is that the surface hydrosphere inferred by geomorphology – like the atmospheric ³⁶Ar reservoir – cannot be generated via volcanic outgassing over time (Jakosky and Treiman, 2023

Jakosky, B.M., Treiman, A.H. (2023) Mars volatile inventory and outgassing history. Icarus 402, 115627. https://doi.org/10.1016/j.icarus.2023.115627

). The hydrosphere was apparently placed on the Martian surface early in planetary history. Independent evidence for the existence of a Martian surface hydrosphere in the first 100 Myr comes from an excess of ¹²⁹Xe in the atmosphere from the decay of water soluble and short-lived ¹²⁹I (Musselwhite et al., 1991

Musselwhite, D.S., Drake, M.J., Swindle, T.D. (1991) Early outgassing of Mars supported by differential water solubility of iodine and xenon. Nature 352, 697–699. https://doi.org/10.1038/352697a0

). The hybrid origin model suggests a new mechanism for the formation of a Martian hydrosphere, similar to the generic scenario recently explored in Kimura and Ikoma (2020)

Kimura, T., Ikoma, M. (2020) Formation of aqua planets with water of nebular origin: effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets. Monthly Notices of the Royal Astronomical Society 496, 3755–3766. https://doi.org/10.1093/mnras/staa1778

. Although we have considered the nebular and chondritic gases to be chemically inert, the nebular gas can react with outgassed oxides to produce new water (H₂ + CO₂ →H₂O + CO). If the CO thus produced escapes as CO, there is a net gain of water at the Martian surface. Both the strong D/H enrichment of the early hydrosphere (Greenwood et al., 2008

Greenwood, J.P., Itoh, S., Sakamoto, N., Vicenzi, E.P., Yurimoto, H. (2008) Hydrogen isotope evidence for loss of water from Mars through time. Geophysical Research Letters 35. https://doi.org/10.1029/2007GL032721

) and the anomalous oxygen recorded in ∼4.43 billion year old zircons (Nemchin et al., 2014

Nemchin, A., Humayun, M., Whitehouse, M., Hewins, R., Lorand, J., Kennedy, A., Grange, M., Zanda, B., Fieni, C., Deldicque, D. (2014) Record of the ancient martian hydrosphere and atmosphere preserved in zircon from a martian meteorite. Nature Geoscience 7, 638–642. https://doi.org/10.1038/ngeo2231

) may result from isotopic exchange between a hydrosphere and an escaping H₂-dominated atmosphere (Pahlevan et al., 2022

Pahlevan, K., Schaefer, L., Elkins-Tanton, L.T., Desch, S.J., Buseck, P.R. (2022) A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars. Earth and Planetary Science Letters 595, 117772. https://doi.org/10.1016/j.epsl.2022.117772

; Zahnle and Kasting, 2023

Zahnle, K.J., Kasting, J.F. (2023) Elemental and isotopic fractionation as fossils of water escape from Venus. Geochimica et Cosmochimica Acta 361, 228–244. https://doi.org/10.1016/j.gca.2023.09.023

).

Cometary contribution to the inner Solar System. The low C/³⁶Ar and N/³⁶Ar of the Martian atmosphere relative to chondrites (Fig. 3) has previously been attributed to a possible contribution from Ar-rich comets (Marty et al., 2016

Marty, B., Avice, G., Sano, Y., Altwegg, K., Balsiger, H., Hässig, M., Morbidelli, A., Mousis, O., Rubin, M. (2016) Origins of volatile elements (H, C, N, noble gases) on Earth and Mars in light of recent results from the ROSETTA cometary mission. Earth and Planetary Science Letters 441, 91–102. https://doi.org/10.1016/j.epsl.2016.02.031

). However, a cometary origin introduces some problems even as it solves others. Results from the Rosetta mission allowed the identification of cometary xenon as a likely source for terrestrial xenon, in particular the long hypothesised component called U-Xe that is apparent in the atmosphere of Earth but not Mars (Marty et al., 2017

Marty, B., Altwegg, K., Balsiger, H., Bar-Nun, A., Bekaert, D.V., et al. (2017) Xenon isotopes in 67P/Churyumov-Gerasimenko show that comets contributed to Earth’s atmosphere. Science 356, 1069–1072. https://doi.org/10.1126/science.aal3496

). Assuming comet 67P/Churyumov-Gerasimenko – which Rosetta sampled – is representative of the cometary reservoir, the question arises as to why Martian atmospheric xenon does not record the cometary signature. The resolution to this dilemma may be the retention of volatiles during impacts onto Earth and Mars. Cometary impacts onto terrestrial planets are high velocity events sufficiently energetic to vapourise both icy and silicate components, producing vapour plumes. The fate of vapour plumes (retained or lost) depends on the ratio of impact to escape velocity, such that cometary vapour plumes on Earth tend to be gravitationally retained whereas those on Mars tend to disperse (Zahnle, 1993

Zahnle, K.J. (1993) Xenological constraints on the impact erosion of the early Martian atmosphere. Journal of Geophysical Research: Planets 98, 10899–10913. https://doi.org/10.1029/92JE02941

). More work is needed to better understand the role of various escape processes in sculpting the volatile inventory of the terrestrial planets.

top

Acknowledgements

The authors acknowledge discussions with Sujoy Mukhopadhyay, comments on an early draft from Alessandro Morbidelli and James Lyons, and extensive reviews by Kevin Zahnle and an anonymous referee, which improved the manuscript. KP acknowledges support from NASA’s Solar System Workings Program (80NSSC21K1833).

Editor: Helen Williams

top

References

top

Supplementary Information

The Supplementary Information includes:

• Enhancement of Nebular Capture
• Energetics, Mechanisms, and Extent of Mixing
• Fractionation via Hydrodynamic Escape
• Magnitude of Impact Erosion
• Supplementary Tables S-1 and S-2
• Data Availability Statement
• Supplementary Information References

Download the Supplementary Information (PDF)