Robert Richard Howell

The dark side of Iapetus

Abstract This paper presents new photometric and spectrophotometric observations of the dark (leading) hemisphere of Saturns satellite Iapetus. Spectrophotometry from 0.3–1.0 um (May 1979) shows the dark hemisphere to be very red, similar to a few asteroids and the Moon, but with no spectral features attributable to olivine or pyroxene. Near-infrared spectrophometry in the regions 1.4–2.5 um (May 1981) and 3.0–3.8 um (February 1981) reveals water ice absorption bands, probably resulting from the polar caps intruding onto the dark hemisphere. The reflectance of Iapetus is unlike that of carbonaceous chondrites or C-type asteroids and most closely resembles the reflectance (and low albedo) of carbonaceous (organic) residue from the Murchison C2 carbonaceous chondrite. The Murchison material has the same red slope and a probable spectral feature near 0.6 um seen in Iapetus data. Three hypotheses for the formation of the dark hemisphere are discussed in light of the observational data. The favored hypothesis is that debris from Phoebe or other unknown outer satellites of Saturn impacts the dark hemisphere of Iapetus as Poynting-Robertson drag causes the debris to spiral toward Saturn. The high-velocity impacts preferentially remove ice from the satellites surface, causing enrichment of included carbonaceous material intrinsic to Iapetus. The reflectance of Phoebe itself is significantly different from that of Iapetus, suggesting that relatively little Phoebe debris lies on the dark hemisphere. There remains the possibility that the impacting debris originates from another body of composition similar to the Murchison residue and that this material is exposed on the surface of Iapetus.

Sulfur dioxide on Io: Spatial distribution and physical state

Abstract Observations of the 4-μm SO 2 band on Jupiters satellite Io and laboratory measurements of SO 2 frost are presented. The observations confirm the existence of a large longitudinal variation in band strength but show no evidence of temporal changes. Comparison of the band position and shape in Ios spectrum with those in the laboratory frosts suggests that the bulk of the absorption on Io is due to frost, not adsorbed gas. The derived SO 2 coverage is large enough to require that SO 2 be present in most terrain types on Io and not just in the white plains unit. To reconcile the infrared observations that indicate large amounts of SO 2 with the ultraviolet observations of Voyager and IUE that show little, the SO 2 must be mixed intimately with the sulfur (or other material) so that at each wavelength the darker component dominates the spectrum.

High-resolution infrared spectroscopy of Io and possible surface materials

Abstract We have obtained new spectra of Io in the 3.5- to 4.2- and 4.5- to 5.4-μm regions with a resolution (λ/Δλ) of roughly 500. The Io spectra cover a range of longitudes, and times from 1983 through 1985. Laboratory spectra of various materials have also been obtained. In this wavelength region are located several strong bands of SO 2 , known to be a major component of Ios surface, as well as the bands of several potential surface materials. In the Io spectra we identify several new features attributable to SO 2 , and have obtained strengths of the 33 S, 34 S, and 18 O isotopic bands, which appear normal. We place limits on the amounts of Na 2 SO 3 , and Na 2 SO 4 present. Finally, we use the data to place limits on the SO 2 gas abundance in Ios atmosphere.

Restored methane band images of Uranus and Neptune

Abstract Charge-coupled device images of Uranus and Neptune taken in the 8900-A absorption band of methane are presented. The images have been digitally processed by means of nonlinear deconvolution techniques to partially remove the effects of atmospheric seeing. The restored Uranus images show strong limb brightening consistent with previous observations and theoretical models of the planets atmosphere. The computer-processed images of Neptune show discreted cloud features similar to those reported previously by B. A. Smith, H. J. Reitsema and S. M. Larson (1979 Bull. Amer. Astron. Soc. 11 , 570). A time series of the restored Neptune images shows a continuous variation which may be due to the planets rotation.

Sulfur Dioxide Ice on IO

The 4-μm band system in the spectrum of Io, as well as the ultraviolet absorptions, are caused by SO2 ice or frost distributed over a major fraction of the satellite. The spectral contribution of any adsorbed gas component of the surface cannot be discriminated from the ice absorption band with the data now available. The 4-μm band is strongest on the leading hemisphere of Io and weakest on the trailing. No temporal changes are seen in the six-year interval in which the infrared data have been studied. Individual spectral features attributed to SO2 with various combinations of the 32S, 34S, 16O, and 18O isotopes are seen in the 4-μm region.