Titan: some new results

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 Titans, Callistos, and Ganymedes. Comets create or erode satellite atmospheres, 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 through 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(21) g. In contrasts to the work of Zahnle et al. (1992), we find that, in most trials, Callisto acquires comet-based atmospheres. Atmospheres acquired by Callisto and, especially, Ganymede are sensitive to assumptions regarding energy partitioning into the ejecta plume. If we assume that only the normal velocity component heats the plume, the majority of Ganymedes and half of the Callistos accreted atmospheres smaller than 10(20) g. If all the impactor's velocity heats the plume, Callisto's most likely atmosphere is 10(17) g and Ganymede's is negligible. The true cometary flux was most likely larger than that derived from crater records, which raises the probability that Titan, Ganymede, and Callisto acquired substantial atmospheres. However, other loss processes (e.g., sputtering by ions swept up by the planetary magnetic field, solar UV photolysis of hydrocarbons) are potentially capable of eliminating small atmospheres over the age of the solar system. The dark material on Callisto's surface may be a remnant of an earlier, now vanished atmosphere.

Titan's surface and troposphere, investigated with ground-based, near-infrared observations

New observations of Titan's near-infrared spectrum (4000-5000 cm-1) combined with points taken from Fink and Larson's (1979) spectrum (4000-12500 cm-1) provide information on Titan's haze, possible clouds, surface albedo, and atmospheric abundance of H2. In the near-infrared, the main features in Titan's spectrum result from absorption of solar radiation by CH4. The strength of this absorption varies considerably with wavelength, allowing us to probe various atmospheric levels down to the surface itself by choosing specific wavelengths for analysis. At 4715 cm-1, the pressure-induced S(1) fundamental band of H2 lies in the wings of CH4 bands. Based on current values for the CH4 line parameters, Titan's spectrum can be best interpreted with a volume mixing ratio of H2 between 0.5 and 1.0%. Our observations suggest the existence of an optically thin CH4 cloud layer. The optical depths that we derive for Titan's haze and clouds are small enough to allow us to sense the surface of Titan at 4900, 6250, and 7700 cm-1. The most plausible interpretation of the albedos determined at these wavenumbers suggests a surface dominated by "dirty" water ice. A global ethane ocean is not compatible with these albedos.