Duane O. Muhleman

Mapping Mars water vapor with the Very Large Array

Abstract We observed Mars with the Very Large Array (VLA) in the spectral line mode on December 3/4 and 6/7 of 1990. Operating at a wavelength of 1.35 cm, we were able to map the 22-GHz water emission line around the atmospheric limb of Mars. The variation of the emission around the limb and the measured lineshapes yield information on the diurnal, latitudinal, and vertical distributions of water vapor in the Mars atmosphere. The VLA limb spectra yield a global average water column of 3.0 ± 0.8 pr μ m, which is significantly lower than that returned by the 1988 IR reflectance observations of Rizk et al. (1991) for the same Mars season ( L S = 340−350°, early northern spring). The Rizk et al. measurement of 10 pr μm is roughly twice that returned by Viking MAWD observations for this driest of Mars seasons (Farmer et al. 1977), whereas our 1.35-cm emission determination is roughly one half that of the Viking MAWD determination. This implies interannual variations in global water vapor of the same order as the seasonal variations observed by Viking. The latitudinal variation measured by this VLA experiment indicates a maximum water vapor abundance of 4.5 ± 1.0 pr μ m (75 ppmv for an altitude-independent mixing ratio) at equatorial latitudes, decreasing to 2.9 ± 0.7 pr μ m (50 ppmv) at midlatitudes. The polar (60–90°NS) water mixing ratio fell below 20 ppmv at altitudes of 15–50 km. We do not find significant diurnal (5 AM vs 5 PM) variation of water vapor at low latitudes. However, it does appear that the water column at mid latitudes (40–60°NS) is ∼30% smaller for the morning versus the evening limb. An analysis of the pressure-broadened 22-GHz lineshape indicates that water vapor was well mixed to an altitude of ≥45 km, although a vertical mixing gradient as large as 50% over the 0 to 50-km altitude range would fall within the measurement uncertainties.

Sublimation from icy jets as a probe of the interstellar volatile content of comets

Comets are some of the most primitive bodies left over from the Solar Systems early history. They may preserve both interstellar material and material from the proto-solar nebula, and so studies of their volatile components can provide clues about the evolution of gases and ices, as a collapsing molecular cloud transforms into a mature planetary system,. Previous observations of emission from rotational transitions in molecules have averaged over large areas of the inner coma, and therefore include both molecules that sublimed from the nucleus and those that result from subsequent chemical processes in the coma. Here we present high-resolution observations of emission from the molecules HNC, DCN and HDO associated with comet Hale–Bopp. Our data reveal arc-like structures—icy jets—offset from (but close to) the nucleus. The measured abundance ratios on 1–3″ scales are substantially different from those on larger scales, and cannot be accounted for by models of chemical processes in the coma,,; they are, however, similar to the values observed in the cores of dense interstellar clouds and young stellar objects. We therefore propose that sublimation from millimetre-sized icy grains ejected from the nucleus provides access to relatively unaltered volatiles. The D/H ratios inferred from our data suggest that, by mass, Hale–Bopp (and by inference the outer regions of the early solar nebula) consists of ⩾15–40% of largely unprocessed interstellar material.