Higher Martian Atmospheric Temperatures at All Altitudes Increase the D/H Fractionation Factor and Water Loss

Abstract

The surface of Mars is marked with ample evidence of its wetter past. Today, water on Mars exists only in the polar caps, subsurface ice, and atmosphere, but geomorphological and geochemical evidence points to significant alteration of the surface by liquid water. The presence of compounds like jarosite and hematite indicate past pooling and evaporation (Klingelhöfer et al., 2004; Squyres et al., 2004), while substantial evidence of hydrated silicates supports the theory that ancient river deltas, lake beds, catastrophic flood channels, and dendritic valley networks were formed by water (Ehlmann & Edwards, 2014; M. H. Carr & Head, 2010, and references therein). Because the contemporary Martian climate cannot support liquid water on the surface, Mars must have once had a thicker and warmer atmosphere. The Mars science community generally agrees that the atmosphere has escaped over time, with a significant amount in the form of thermal escape of H, in which a fraction of H atoms are hot enough that their velocity exceeds the escape velocity. Because H is primarily found in water on Mars, this has effectively desiccated the planet (Jakosky et al., 2018).

Isotopic fractionation of water and its photolytic products in the atmosphere of Mars

“…The 18 O/ 16 O is found to be much less variable than the D/H ratio. The value representative of the present-day atmospheric ratio is found to be 1.14 ± 0.08 VSMOW, which is in line with values reported in previous studies 3,11 .…”

“…Estimations of the size of the ancient water reservoir on Mars from its isotope composition depend not only on the present-day atmospheric ratio, but also on the net escape fractionation factor, f . This factor relates the efficiency of escape of D with respect to H, including all fractionation processes occurring along the physical and chemical pathways by which the initial H 2 O and HDO molecules are decomposed and ultimately escape to space by either thermal or non-thermal means [15][16][17][18] .…”

“…We investigate the vertical distribution of the isotope composition (D/H and 18 O/ 16 O) of water vapour in the Martian atmosphere using solar occultation measurements made by the mid-infrared (MIR) channel of the Atmospheric Chemistry Suite 28 (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). ACS MIR is an echelle cross dispersion spectrometer dedicated to solar occultation measurements in a spectral range between 2.2 and 4.4 µm.…”

“…Similarly, as for HDO, H 18 2 O molecules are expected to have a different UV absorption cross-section than H 16 2 O, which can impact the isotope composition of the dissociation products of water vapour and the efficiency of 18 O to reach the upper atmosphere 51 . We estimate that, while the dissociation products are expected to be about 2-2.5 times depleted in D/H with respect to the ratio in water vapour in the lower atmosphere, this factor would just be 1.025 for the 18 O/ 16 O ratio (R p ( 18 O/ 16 O) ∼ 0.975), revealing a much higher efficiency of 18 O to be transferred to the dissociation products with respect to D. This might explain the dichotomy between the large enrichment in the heavy isotope of the D/H ratio (∼5 VSMOW) as opposed to the mild enrichment in the 18 O/ 16 O ratio (∼1.14 VSMOW): while 18 O is efficiently transferred to the photolysis products of water vapour, HDO molecules are less efficiently photolysed, decreasing the D/H ratio of the lighter species that ultimately escape to space. Therefore, as H and O escape from the Martian atmosphere, they will more efficiently enrich the D/H ratio than the 18 O/ 16 O ratio.…”

Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust

“…We do not include fractionation associated with degassing or its redox sensitivity because these are negligible compared with the large range of potential D/H compositions of the volcanic gas inferred from meteorites (5). Atmospheric escape causes D/H of the exchangeable reservoir to fractionate toward heavier values, which we modeled through stepwise Rayleigh distillation, a common isotopic reservoir modeling technique, at each 10-million-year time step with an a escape of 0.002 to 0.32 (28)(29)(30). The fractionation factor between smectite, the most common hydrated mineral found on Mars, and water [a smectiteÀH2O ¼ 0:95 ( 5)] is used in the stepwise Rayleigh distillation model as a first-order approximation of fractionation through crustal hydration (table S3) (5); we found that this fractionation is minor compared with that caused by atmospheric escape.…”