William M. Grundy

The steep red spectrum of 1992 AD: An asteroid covered with organic material?

Abstract A spectrum of the newly discovered asteroid 1992 AD (now numbered 5145) was obtained by us with our CCD camera and spectrometer 1992 February 01.23. The reflection spectrum of 1992 AD displays a very steep and constant red slope between 0.5 and 1.0 μm and exhibits no absorption nor emission features. The red slope is steeper than that of any presently known Solar System object. The reflectivity ratio between 1.0 and 0.55 μm is a factor of 3.5 (1.36 magnitudes), or using a slight extrapolation, a factor of 4.90 (1.72 magnitudes) for the wavelength octave 1.0 to 0.5 μm. The steep red slope is difficult to match with conventional silicate or meteoritic materials. While allotropes of sulfur may give a partial match, the best match is provided by the steep red spectra of mixtures of tholins , the residues left after subjecting organic molecules to an energetic radiation environment.

A new spectrum of Triton near the time of the Voyager encounter

Abstract We present a new spectrum of Triton from 5200 to 10,000 A in which the 8900-A methane ice absorption band can be clearly seen and accurately measured. The data were obtained during the summer of 1989, just before the Voyager II spacecraft encountered the Neptune system. By combining data from the spacecraft with our telescopic data, we have shown that the absorption is entirely caused by CH 4 ice on Tritons surface. We have modeled Tritons spectrum with a simple Hapke-type model which allowed us to set a 20-μm lower limit on the mean grain size of methane ice on Triton. We feel that the true grain size is probably somewhat larger, of the order 100 μm. Our model has led us to believe that methane ice is widely distributed on the surface of Tritons southern hemisphere. If CH 4 condenses together with N 2 , the CH 4 must be significantly more concentrated relative to N 2 in the surface ice than it is in Tritons atmosphere.

The absorption coefficient of the liquid N2 2.15-μm band and application to triton

Abstract We have measured the temperature dependence of the absorption coefficient and integrated absorption of the liquid nitrogen 2.15-μm (4647 cm −1 ) 2-0 collision-induced band. The integrated absorption of the liquid is observed to be smaller than that of the gas, and decreases slightly with decreasing temperature. The band gets sharper with decreasing temperature with a half-width proportional to the square root of the temperature. If this temperature-dependent behavior is extrapolated to the nominal surface temperature of Triton, a new estimate of the grain size of N 2 ice on Tritons south polar cap can be made. We employ Hapke scattering theory with previously published IR spectra of Triton to estimate a mean nitrogen grain size of between 0.7 and 3 cm. This results is consistent with results from grain growth rate calculations.

Pluto is the new Mars

Data from NASAs New Horizons encounter with Pluto in July 2015 revealed an astoundingly complex world. The surface seen on the encounter hemisphere ranged in age from ancient to recent. A vast craterless plain of slowly convecting solid nitrogen resides in a deep primordial impact basin, reminiscent of young enigmatic deposits in Mars Hellas basin. Like Mars, regions of Pluto are dominated by valleys, though the Pluto valleys are thought to be carved by nitrogen glaciers. Pluto has fretted terrain and halo craters. Pluto is cut by tectonics of several different ages. Like Mars, vast tracts on Pluto are mantled by dust and volatiles. Just as on Mars, Pluto has landscapes that systematically vary with latitude due to past and present seasonal (and mega-seasonal) effects on two major volatiles. On Mars, those volatiles are H2O and CO2; on Pluto they are CH4 and N2. Like Mars, some landscapes on Pluto defy easy explanation. In the Plutonian arctic there is a region of large (approx. 40 km across) deep (approx. 3-4 km) pits that probably could not be formed by sublimation, or any other single process, alone. Equally bizarre is the Bladed terrain, which is composed of fields of often roughly aligned blade-like ridges covering the flanks and crests of broad regional swells. Topping the unexpected are two large mounds approximately150 km across, approx. 5-6 km high, with great central depressions at their summits. The central depressions are almost as deep as the mounds are tall. These mounds have many of the characteristics of volcanic mountains seen on Mars and elsewhere in the inner solar system. Hypotheses for the formation of these Plutonian mounds so far all have challenges, principally revolving around the need for H2O ice to support their relief and the difficulty imagining mechanisms that would mobilize H2O. From the perspective of one year after the encounter, our appreciation of the extent of Plutos diversity and complexity is quite reminiscent of the perspective the science community had of Mars, with similar quality data sets, soon after the early reconnaissance of that planet in the late 1960s and early 70s. So certainly in this sense, Pluto is the new Mars.