Water Photochemistry

The important role of H₂O as a source of odd hydrogen on Mars should now be clear and Water photochemistry also regulates the escape of hydrogen. The water vapor concentrations observed in the Martian atmosphere are strongly dependent on the local temperature and therefore, on altitude, latitude, and time of day. Most atmospheric H₂O is probably confined near the surface by vapor pressure limitations, where it is destroyed by photodissociation and by reaction with energetic O (¹D) from ozone photolysis:

O (¹D) + H₂O ® OH + OH

The odd hydrogen produced by these processes is rapidly cycled through H, OH, and HO₂. The odd hydrogen sinks are:

H + HO₂ ® H₂O + O

® H₂ + O₂

HO₂ + HO₂ ® H₂O₂

HO₂ + OH ® H₂O + O₂

The hydrogen peroxide produced here may either freeze out in the polar regions or photodissociate to reform odd hydrogen. The spectroscopic observations from the probe to Mars suggest that the O₂ abundance exceed the CO abundance on Mars; the O₂ generated by water photochemistry is the only possible source of this additional oxygen. In a dry CO₂ atmosphere O₂ abundance to be one half that of CO.

Frost on the Martian Surface

Reaction cycles that generate hydrogen gas, however, raise the question of atmospheric escape from Mars. Molecular hydrogen is generated by reaction of H with HO₂, both of which are products of H₂O photodissociation in the lower atmosphere. Some H₂ is removed by O (¹D) + H₂ ® OH + H near the ground, leading next to water vapor, and also, in the ionosphere, by

CO₂⁺ + H₂ ® COOH⁺ + H

COOH⁺ + e^- ® CO₂ + H

H₂ + hv ® H + H

Photolysis of molecular hydrogen requires rare photons with l < 850 Å, which penetrate poorly through carbon dioxide. Thus almost all H₂ photolysis occurs at altitudes above about 80 km, close to the top of the atmosphere.

In a steady state, the outward flux of hydrogen must be balanced by the production of H₂ from H₂O. This production rate is hard to estimate because of poorly known rate constants and the unknown catalytic effects of the ground and atmospheric aerosols on several important reactions. The total amount of water vapor that would be destroyed on Mars if the present escape flux of H atoms were sustained over the lifetime of the planet is about one thousandth of the water in the oceans on Earth today. This small water loss will certainly not have noticeably depleted Martian water resources. The other product of water photolysis, O₂, does present a problem. With the observed escape rate of H, photolysis will produce the observed O₂ amount in just 10⁵ years.

Similarly, the calculated oxygen atom escape rate is in remarkable agreement with the production rate of O from H₂O photodissociation inferred from the present H escape rate. This balance is no coincidence, but is in fact expected from the buffering effect of molecular hydrogen. It these escape rates have remained constant over geologic time, comparison of the total water lost by this mechanism with the total CO₂ present in the atmosphere can be made. The inferred H₂O: CO₂ ratio in the outgassing of Mars is about 45:1. Comparison of the water content of Earth’s oceans with the CO₂ in sedimentary carbonates on Earth actually yields a similar ratio. However, there are a number of cosmochemical reasons for expecting the H₂O: CO₂ ratio on Mars to considerably exceed that on Earth. It so, there must exit H₂O removal mechanisms much more potent than atmospheric escape.

The escape of H₂, operating together with the mechanisms for removing O₂ from the atmosphere, results in net loss of H₂O. This loss rate is limited by the average H₂O photolysis rate, 10⁹ molecules cm^-2 s ^–1. Solar wind sweeping may also be a contribution to escape from the Martian atmosphere. Ions are produced by photo-ionization and by charge exchange with solar wind protons near the planetary limb. These ions, which include O₂⁺, CO₂⁺, O⁺, N₂⁺, NO⁺, and CO⁺, can be swept up by the electric field of the impinging solar wind. Significant escape rate of CO₂ ( 5 ´ 10⁵ molecules cm^-2 s ^–1 ) and N₂ ( 2 ´ 10⁵ molecules cm^-2 s ^–1 ) may result.

Explosive Blow-away

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