Impact delivery of prebiotic organic matter to planetary surfaces

Organic compounds, liquid water, and a source of energy are necessary requirements for life as we know it. Few places in the solar system appear to satisfy these requirements. Besides Earth, Mars and Europa may have provided at some point during their history the most promising environments for the origin of life. Here we address the role of impacts as a mechanism for the delivery of organic compounds to Earth, Mars, Europa, and the Moon through high-resolution hydrocode simulations. The results suggest that on the Earth some amino acids (such as aspartic acid and glutamic acid) could survive large cometary impacts at the percent level, enough to equal or exceed concentrations due to Miller-Urey synthesis in a CO₂-rich atmosphere. In particular, a grazing impact could have delivered to early Earth amounts of certain amino acids comparable to the background steady-state production. Substantial survival of some amino acids occurs in cometary impacts for Mars as well. Analogous to the situation on Earth, asteroid impacts on Mars do not seem to result in signi.cant survival, even if lower impact velocities increase the survival of amino acids. In cometary impacts, however, the increased amino acid survival in part counteracts the effect of the lower Martian escape velocity (5 km/s for Mars versus 11.2 km/s for the Earth) that causes some projectile material to escape Mars gravity and be lost to space. Projectile escape becomes dominant in grazing impacts, which are thus not a significant source of organics on Mars. Projectile escape is dominant on Europa (escape velocity of 2 km/s); as a result, cometary impacts provide a negligible contribution to Europa's prebiotic organic inventory. However, as models of the circum-Jovian nebula suggest that Europa might have formed largely bereft of some biogenic elements, cometary impacts could be the primary source of some of Europa's biogenic elements. Finally, although subject to an impact history similar to that of the Earth and Mars, impact delivery on the Moon is limited by the lower gravity field, just as with Europa. This drastically limits the amount of organic molecules that can be successfully delivered intact to the lunar surface.