Surface Ices and the Atmospheric Composition of Pluto

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.

Dark matter in the outer solar system

There are now a large number of small bodies in the outer solar system that are known to be covered with dark material. Attempts to identify that material have been thwarted by the absence of discrete absorption features in the reflection spectra of these planetesimals. An absorption at 2.2 micrometers that appeared to be present in several objects has not been confirmed by new observations. Three absorptions in the spectrum of the unusually red planetesimal 5145 Pholus are well-established, but their identity remains a mystery.

Infrared spectroscopy of organics of planetological interest at low temperatures

In the context of prebiotic chemistry in space, some of the outer planetary objects display H, C, N and O rich chemistry similar to the one in the biosphere of Earth. Of particular interest are Saturn's moon, Titan; Neptune's moon, Triton; and Pluto where extreme cold conditions prevail. Identifications of chemical species on these objects (surfaces and atmospheres) is essential to a better understanding of the radiation induced chemical reactions occurring thereon. There have been several ground based observations of these planetary objects in the infrared windows from 1 to 2.5 micrometers. Voyager also provided spectra in the thermal infrared (6 to 50 micrometers) region. Interpretation of these data require laboratory infrared spectra of relevant species under the temperature conditions appropriate to these objects. The results of some of these studies carried out in our laboratory and elsewhere and their impact on the analyses of the observed data will be summarized.

The search for other planets: clues from the solar system

Studies of element abundances and values of D/H in the atmospheres of the outer planets and Titan support a two-step model for the formation of these bodies. This model suggests that the dimensions of Uranus provide a good index for the sensitivity required to detect planets around other stars. The high proportion of N2 on the surfaces of Pluto and Triton indicates that this gas was the dominant reservoir of nitrogen in the early solar nebula. It should also be abundant on pristine comets. There is evidence that some of these comets may well have brought a large store of volatiles to the inner planets, while others were falling into the sun. In other systems, icy planetesimals falling into stars should reveal themselves through high values of D/H.

Evidence for a Low Surface Temperature on Pluto from Millimeter-Wave Thermal Emission Measurements

Thermal continuum emission from the Pluto-Charon system has been detected at wavelents of 800 and 1300 micrometers, and significant upper limits have been obtained at 450 and 1100 micrometers. After the subtraction of emission from Charon, the deduced surface temperature of much of Pluto is between 30 and 44 kein, probably near 35 to 37 kelvin. This range is significantly cooler than what radiative equilibrium models have suged and cooler than the surface temperature derived by the Infrared Astronomy Satellite. The low temperature indicates that methane cannot be present at the microbar pressure levels indicated by the 1988 stellar occultation measurements and that the methane features in Pluto's spectrum are from solid, not gas-phase, absorptions. This result is evidence that Pluto's atmosphere is dominated by nitrogen or carbon monoxide rather than methane.

Spectroscopic Determination of the Phase Composition and Temperature of Nitrogen Ice on Triton

Laboratory spectra of the first overtone band (2.1480 micrometers, 4655.4 reciprocal centimeters) of solid nitrogen show additional structure at 2.1618 micrometers (4625.8 reciprocal centimeters) over a limited temperature range. The spectrum of Neptune's satellite Triton shows the nitrogen overtone band as well as the temperature-sensitive component. The temperature dependence of this band may be used in conjunction with ground-based observations of Triton as an independent means of determining the temperature of surface deposits of nitrogen ice. The surface temperature of Triton is found to be 38.0(+2.0)(-1.0) K, in agreement with previous temperature estimates and measurements. There is no spectral evidenceforthe presence of alpha-nitrogen on Triton's surface, indicating thatthere is less than 10 percent carbon monoxide in solid solution with the nitrogen on the surface.