Impact Erosion

Over the last few decades, it has become clear that impacts of asteroids, comets and other Solar System bodies played a fundamental role in the evolution of terrestrial planets and their atmospheres. As discussed in Section 2.1.1, impacts are a primary mechanism of planetary accretion and are responsible for the delivery of water and organic matter to young planetary bodies. Large impactors may have also inhibited the formation of life in the early history of planetary formation.

Thus, the impact of a planetesimal can erode part of an existing atmosphere or it can add volatiles to it. The balance on delivery and loss from an atmosphere depends on the composition of the impactor and on the mass of the growing planet [27, 33]. When Venus and Earth attained their present masses and escape velocities, impact erosion became very inefficient, but Mars with its smaller mass was rather vulnerable and still would it be if the impact population had not essentially died out. Impact studies on Mars show that the planet could have lost a CO2 atmosphere between 5 and 10 bars due to impact erosion over the first Ga [33]. Once an atmosphere had been eroded, it could have been resublimed by comets [34].

There is also evidence that the Martian water after the end of the heavy bombardment may have been enriched with cometary sources, after the majority of the initial water was lost by diffusion-limited hydrodynamic escape, because the average value of about 2.3 times the TSW ratio measured in the Shergottites [12, 35, Chap. 1 by Baker et al.] fits well the observed D/H ratio in comets Halley [36], Hyakutake [37] and Hale-Bopp [38]. These rates are all about twice the value for terrestrial seawater, but are consistent with rates in "hot cores" of molecular clouds. It is believed that ion-molecule reactions in dense molecular clouds at temperatures close to 35 K can produce these D/H enrichments.

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