No abstract

Thermal models for the north polar region of Mercury, calculated from topographic measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. MESSENGER measurements of near-infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface, and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface. We propose that the dark surface layer is a sublimation lag deposit that may be rich in impact-derived organic material.

Abstract. Jupiter's icy satellites Europa, Ganymede, and Callisto are subject to energy fluxes from electrons, photons, and magnetospheric plasma ion bombardment. As water ice and CO2 ice are thought to be present on their surfaces, the radiolysis of these materials over time should produce more complicated CHO-containing molecules. These may include CH3OH, H2CO , CH2CO , C302, HCOOH, CH3COOH, H2CO3, HCOOCH, (CH3)2CO , CH3CH2OH , HOCH2CH2OH , polymeric C302, and polymeric H2CO. The water fragmentation products OH, 0 2, HO2, H202, and 03 should also be produced. The molecules formed should be detectable with ground-based intruments because of their many active infrared bands. Another product produced is CO, which will have a high vapor pressure over a Galilean satellite surface at typical temperatures. The vapor pressure of CO at the nighttime temperature of 70 K could be as high as 150 mbar. Ganymede's unique dipolar magnetic field should induce more chemistry in its polar regions due to the focusing of radiation to higher latitudes. The observed lack of leading/ trailing asymmetry in its SO2 absorption correlates with this redirection of plasma ions toward the poles. The observance of O atom emissions at high latitudes by Hall et al.[1998] is also consistent with this picture. The ratio of plasma energies directed to the poles to those directed to the equatorial regions is ---4.

Summary. Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH 3 , and uniquely NH 3 , in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H20) = 0.01-Q.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4 )/Q(H2 0) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.

No abstract