SHOCK PRODUCTION OF ORGANICS DURING COMETARY IMPACT

Comets may have been a major source of volatile elements and organic material on the early Earth. Because the terrestrial planets formed primarily from planetesimals originating within the inner, hotter regions of the solar nebula, they are low in volatile elements. Several workers have recently suggested that a considerable amount of water could have been accreted by the early Earth in the h a l stages of formation due to the interception of comets scattered from the vicinity of Neptune and Uranus (Chyba, 1987; Ip and Fernandez, 1988). An ocean of water could have been accreted between 4.5 and 3.8 Gyr ago if comets comprised more than 10 % of the mass of the impacting population (Chyba, 1987). For the N2/H20 ratios observed in comets, this input could also account for the complete inventory of atmospheric nitrogen. Since the comets are also rich in organic matter (see, e.g., Clark et d., 1987; Kissel and Krueger, 1987) an influx of this amount of water represents an potentially enormous influx of organic material. This cometary organic material could have played a role in the origin of life on Earth, as first suggested by Orb (1961). However, i t is doubtful that organic material within a cometary body impacting the Earth could survive. The typical impact velocities are of the order of 20 km s-l. The energy per unit mass contained in this kinetic energy is enough to completely pyrolize organic material. Even encounter