INFLUX OF COMETARY VOLATILES TO PLANETARY MOONS - THE ATMOSPHERES OF 1000 POSSIBLE TITANS

We use a Monte Carlo model to simulate impact histories of possible Ti tans, Callistos, and Ganymedes. Comets create or erode satellite atmos pheres, depending on their mass and velocity distributions: faster and bigger comets remove atmophiles; slower or smaller comets supply them . Mass distributions and the minimum total mass of comets passing thro ugh the Saturn system were derived from the crater records of Rhea and Iapetus. These were then scaled to give a minimum impact history for Titan. From this cometary population, of 1000 initially airless Titans 16% acquired atmospheres larger than Titan's present atmosphere (9 x 10(21) g), and more than half accumulated atmospheres larger than 10(2 1) g. In contrast to the work of Zahnle et al. (1992), we find that, i n most trials, Callisto acquires comet-based atmospheres. Atmospheres acquired by Callisto and, especially, Ganymede are sensitive to assump tions regarding energy partitioning into the ejecta plume. If we assum e that only the normal velocity component heats the plume, the majorit y of Ganymedes and half of the Callistos accreted atmospheres smaller than 10(20) g. If all the impactor's velocity heats the plume, Callist o's most likely atmosphere is 10(17) g and Ganymede's is negligible. T he true cometary flux was most likely larger than that derived from cr ater records, which raises the probability that Titan, Ganymede, and C allisto acquired substantial atmospheres. However, other loss processe s (e.g., sputtering by ions swept up by the planetary magnetic field, solar UV photolysis of hydrocarbons) are potentially capable of elimin ating small atmospheres over the age of the solar system. The dark mat erial on Callisto's surface may be a remnant of an earlier, now vanish ed atmosphere.