Evolution of exoplanet atmospheres: The Mars example of geological sources and sinks in addition to solar/stellar stripping of gas

Introduction:  Understanding exoplanet habitability and its evolution requires understanding the relative importance of the different sinks for atmospheric volatiles.  While a lot of attention is paid to stripping of atmospheric gas by stellar EUV and stellar winds, geological and geochemical processes also can have a major affect volatiles.  This is demonstrated with the example of volatile sources and sinks for Mars.  Mars has undergone significant stripping of gas to space, but also has lost gas and had gas supplied by multiple other processes.  We focus here on CO₂ and H₂O as the most-relevant volatiles for climate and habitability. 

Martian CO₂: CO₂ has been the most important greenhouse gas for Mars, although other gases may be important as well. 

Supply of CO₂ to the atmosphere and near-surface region involves early catastrophic outgassing and release from the mantle through volcanism or as gas dissolved in water released to the surface.

Loss of an early, thicker CO₂ atmosphere to get to today’s 6-mbar atmosphere was driven by the following processes:

- Impact ejection to space during the tail end of planetary accretion and heavy bombardment.

- Stripping to space by the early EUV and the solar wind and solar storms, all of which were more intense than today.

- Storage in the polar ice caps, which is thought to contain the equivalent of ~6 mbar of CO₂ today.

- Storage as H₂O-CO₂ clathrate hydrate, possibly present in the polar caps or high-latitude ground ice.

- Formation of near-surface carbonate minerals that can store CO₂ in mineral form.

- Sequestration in the form of gas adsorbed onto regolith grains, which depends on the composition and thickness of the regolith.

- Deep-crustal carbonates, revealed where impacts or tectonic processes have exposed them.

Together, these sinks can account for a minimum of 1.5 bars of CO₂ from an early atmosphere, with roughly half of the CO₂ having gone to sinks other than loss to space.

Martian H₂O:  Evidence points to Mars having had much more water at its surface early in its history.  The figure shows the range of potential processes that control the evolution of Martian H₂O.

Supply of H₂O to the atmosphere and near-surface region results from early catastrophic release as a result of planetary formation, differentiation, early crustal formation, and release from late-accreted material up until ~3.7 b.y.a.  In addition, water has been supplied by volcanism through time, and has been stored in the crust and released by catastrophic flooding.

Loss of Martian water was driven by the following processes:

- Sequestration in the polar caps and high-latitude ground ice, thought to be capable of exchanging with today’s atmosphere on various timescales.

- Loss to space, via photodissociation and escape of the atomic H and O and of molecular H₂.

- Incorporation into minerals as water of hydration, oxidation, or adsorbed gas, with the minerals being identified and mapped today.

- Potential buried water ice, from an early ocean if the water did not evaporate back into the atmosphere.

- Free liquid water or ice sequestered in the crust or megaregolith, based on water released in the catastrophic floods that drained only a tenth of the crust.

These sinks can account for removal of ~400-2000 m H₂O (global equivalent layer), with loss to space accounting for ~100-500 m.  For comparison, today’s atmosphere holds the equivalent of about 10 micrometers of water if it all were condensed onto the surface.

Discussion:  Clearly, evolution of Mars’ climate and habitability depends on more than just stripping of gas to space by the host star.  For Mars, the geological and geochemical processes of removing gas are important and may have dominated the total volatile loss; these sinks also can store gas and later release it back into the atmosphere.  (Additional processes may operate on exoplanets that have not operated on Mars, e.g., involving plate tectonics.)  There’s no reason to think that exoplanets will have the same relative importance of each process as does Mars.  However, processes intrinsic to a planet have to be considered as a key part of volatile and climate evolution. Even with the three very different examples of Earth, Venus, and Mars, we cannot predict the geologically driven behavior of volatiles on a rocky planet; clearly, however, these processes could dominate the evolution of exoplanet atmospheres and habitability