Isotopic Fractionation during Hydrodynamic Hydrogen Escape from Ancient Mars
Abstract
Many of the elements in Mars's atmosphere are enriched in heavy isotopes. The implication is that these elements were affected by escape to space. The most challenging to account for are carbon and xenon, as neither currently escapes. Xenon is challenging because it is the heaviest gas in the atmosphere, and still more challenging because krypton - a lighter noble gas - is not just unfractionated but almost exactly solar in composition. Carbon is challenging because (i) it has not been shown to escape today, and (ii) it is difficult to separate from CO or CO2, yet the reported fractionation of 4.4% between 13C and 12C exceeds the fractionation seen in oxygen. Hydrodynamic hydrogen escape from ancient Mars can provide a common, isotopically-fractionating path for carbon and xenon to escape that does not apply to Kr and can act as strongly on carbon as on oxygen.
Xenon is a tracer of hydrogen escape. Xenon escape is possible, whilst Kr escape is not, because Xe alone of the noble gases is more easily ionized than hydrogen. The mechanism is the same as in traditional hydrodynamic hydrogen escape, but the collisional interaction is the Coulomb force between Xe+ and H+ ions, which acts at a distance and hence provides strong coupling. Cassata (EPSL 479, 322 (2017), LPSC 2018) reports that Xe isotopes were as fractionated in ALH84001 (4.1 Ga) as Xe is today. Transient, impact-generated H2-rich atmospheres are highly probable on early Mars. Hydrogen is a favored product of thermochemistry in volatile-rich asteroids and comets and also in shock-heated martian crustal materials if they contained water. Xenon escape indicates that such atmospheres existed. Carbon fractionation occurs when CO or CO2 molecules escape in the hydrogen wind. Mars's gravity is small enough that, when EUV radiation is within the range expected of the young Sun before 4.1 Ga, hydrogen escape, thermal conduction, and radiative cooling are insufficient to balance EUV heating. Under these circumstances, atoms and molecules heavier than hydrogen are entrained with the flow. The heavy molecules increase the mass of the wind, and scale heights of radiatively active molecules are inflated. The net effect is that the wind settles into a different kind of energy-limited equilibrium, in which increasing EUV radiation is balanced by increasing the flux of entrained heavy molecules.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P23B3479Z
- Keywords:
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- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6207 Comparative planetology;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6296 Extra-solar planets;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5405 Atmospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS