ABSTRACT

A significant fraction of the Earth's prebiotic volatile inventory may have been delivered by asteroidal and cometary impacts during the period of heavy bombardment. The realization that comets are particularly rich in organic material seemed to strengthen this suggestion. Previous modeling studies, however, indicated that most organics would be entirely destroyed in large comet and asteroid impacts. The availability of new kinetic parameters for the thermal degradation of amino acids in the solid phase made it possible to readdress this question.
We present the results of new high-resolution hydrocode simulations of asteroid and comet impact coupled with recent experimental data for amino acid pyrolysis in the solid phase. Differences due to impact velocity as well as projectile material have been investigated. Effects of angle of impact were also addresses. The results suggest that some amino acids would survive the shock heating of large (kilometer-radius) cometary impacts. At the time of the origin of life on Earth, the steady-state oceanic concentration of certain amino acids (like aspartic and glutamic acid) delivered by comets could have equaled or substantially exceeded concentrations due to Miller-Urey synthesis in a CO₂-rich atmosphere. Furthermore, in the unlikely case of a grazing impact (impact angle of about 5° from the horizontal), an amount of some amino acids comparable to that due to the background steady-state production or delivery would be delivered to the early Earth.

COLOR FIGURES (To download GIF files click on the figures)

Temperature map of (a) peak shock temperature and (b) postshock (after 4 seconds) temperature inside the projectile for a 1 km radius comet impacting at 20 km/s on the Earth.