William Kent Wells

The sodium and potassium atmosphere of the moon and its interaction with the surface

Abstract Observations of lunar atmospheric sodium and potassium from May 1988 to July 1991 are reported and analyzed. Densities at 80° north and south are less than equatorial ones by a factor of 2–3. For our observations, which do not reach above 800 km from the limb, the apparent scale heights for the intensity are 119–611 km for Na, and 85 and 154 km for K; most of these are much larger than would be expected for atoms thermalized to the surface temperature. However, the intensity drops off with increasing radius at a much greater rate than would be observed for an atmosphere that is mostly escaping. We interpret our data using both single- and two-component analyses. We amplify an earlier suggestion that source atoms are quickly redistributed into thermal and suprathermal populations by “competing release mechanisms” acting at the surface. The suprathermal distributions are produced by solar radiation releasing atoms adsorbed on the surface ( photodesorption ). We present reasons why the energy distribution seems to mimic a Maxwellian. The competing release mechanisms explain an obvious trend of decreasing apparent scale heights toward the subsolar point, where the density in the lowest 100 km appears to be dominated by thermally desorbed atoms. The suprathermal component is expected to appear at greater altitudes, but the early subsolar data do not extend high enough to reveal it. Six of the data sets are tentatively resolved into thermal and suprathermal components. The variation with latitude is naturally explained if a larger fraction of the atoms at large solar zenith angles are adsorbed to the surface, rather than being visible in the atmosphere. Migration to the dark side may also play a role. It is shown that at most a very small fraction of the observed atoms below a few hundred km altitude can be on escape trajectories. We apply these ideas to the budget of atomic oxygen. We suggest that the inventory of oxygen atoms is greatly reduced because they stick to the surface with high efficiency similar to that of the alkalis, and subsequently recombine with each other or with partially reduced oxides of such atoms as Mg, Fe, Al, and Si.

Martian water vapor, 1988-1995

We report new measurements of Martian atmospheric water vapor for the period 1991–1995 and discuss implications of these and earlier measurements from 1988 to 1989. Our measurements indicate abundances (precipitable micrometers (pr μm)) that show some departures from those of the Viking Mars atmospheric water detector (MAWD) experiment and other ground-based measurement programs. Variation of water abundance within Martian season is sometimes as large as a factor of 3 from one year to the next. However, the seasonal shifts and variations between hemispheres show the same trends as observed by MAWD. Column abundances of water vapor varied from barely detectable, <1 (at Ls 320–340) to 36.4 pr μm (Ls 100) at high northern latitudes. Strong latitude variations were observed for all Ls seasons, with late spring and summers wet in both hemispheres. Northern latitudes are up to 5 times wetter than southern latitudes. Equatorial regions (30°S–30°N) show a rather stable abundance of atmospheric water varying between 2 and 20 pr μm, while much larger variations are observed at high latitudes. Southern atmospheric water drops below 10 pr μm rapidly in early autumn and is below our measurement threshold by late autumn. Strong diurnal variations show lowest water column abundance near the evening terminator.

The structure of Io's corona

A spatial profile of the distribution of sodium in Ios corona has been constructed using measurements obtained during satellite mutual eclipses. The data reveal a fairly symmetric corona whose density field falls steeply from the surface out of 6 Ios radius and more slowly outside. An upper limit of 700 km is placed on the exobase altitude, but the observations do not constrain the surface density. Several theorical models adequately match some traits of the corona, but none satisfies all the observations. No strong upstream/downstream asymmetry of the corona is observed, so it is unlikely that the corona is primarily generated by the impact of corotating ions in the trailing hemisphere.

Meridional Martian water abundance profiles during the 1988–1989 season

Abstract In this paper we report on measurements of the latitudinal distribution of atmospheric water vapor column abundance that were collected on 4 nights during spring and summer in the Martian southern hemisphere before and after the 1988 opposition. The profiles for early southern spring and southern autumnal equinox agree with those measured during the same season in 1977 by the Mars Atmospheric Water Detector (MAWD) on the Viking Orbiters, but the profiles during southern mid- and late summer show twice as much water in the southern hemisphere and planetwide as did the Viking MAWD. An equatorial water column abundance maximum was a relatively constant feature of the profiles acquired and 4 × 10 14 g was observed to appear and disappear at latitudes south of −30°. Based on the lack of global dust storms observed during the 1988–1989 spring and summer compared to the 1977 Viking measurements during the same period, the water abundance measurements reported in this paper represent observations in a relatively clear atmosphere.

Eclipse Measurements of Io's Sodium Atmosphere

The satellites of Jupiter eclipsed each other in 1985, and these events allowed an unusual measurement of the sodium in Ios extended atmosphere. Europa was used as a mirror to look back through the Io atmosphere at the sun. The measured column abundances suggest that the atmosphere is collisionally thin above 700 kilometers and may be collisionally thin to the surface. The sodium radial profile above 700 kilometers resembles a 1500 kelvin exosphere with a surface density near 2 x 104 sodium atoms per cubic centimeter, but a complete explanation of the dynamics requires a more complex nonthermal model: the calculated loss rates suggest that the atmosphere is being replaced on a time scale of hours.

Thermal infrared lightcurves of the impact of comet Shoemaker-Levy 9 fragment R

The impact of fragment R was observed at thermal infrared wavelengths of 7.85, 10.3 and 12.2 µm from the NASA/Infrared Telescope Facility on July 21 UT, using the MIRAC2 mid-infrared array camera. Thermal emission at the three wavelengths was sampled sequentially using a 1.8% circular variable filter, with an average time interval of 17 seconds between observations at different wavelengths. Continuous imaging of Jupiter in this mode began at 5:08 UT and extended to 5:55 UT. We present calibrated lightcurves for the three wavelengths. Clear evidence for enhanced emission from the impact region first appears at 5:41 UT, with the peak in emission at all three wavelengths occurring ∼3.5 minutes later. The information content of the data is presented in terms of plots of the product of emissivity times angular size versus source temperature for each wavelength. Assuming that the peak in the lightcurves is due to rotation of the hot impact site into view from Earth, we estimate a diameter of ∼1900 km for the source emitting area at 5:44:30 UT and estimate a lower limit on the source temperature at this time of ∼1350 K. This lower limit drops to 800 K if the diameter of the emitting region was actually a factor of two larger.