Betty J. Baldwin

Near-infrared spectra of the Galilean satellites - Observations and compositional implications

Abstract We have obtained reflectivity spectra of the trailing and leading sides of all four Galilean satellites with circular variable filter wheel spectrometers operating in the 0.7- to 5.5-μm spectral interval. These observations were obtained at an altitude of 41,000 ft from the Kuiper Airborne Observatory. Features seen in these data include a 2.9-μm band present in the spectra of both sides of Callisto; the well-known 1.5-μm and 2.0-μm combination bands and the previously more poorly defined 3.1-μm fundamental of water ice observed in the spectra of both sides of Europa and Ganymede; and features centered at 1.35 ± 0.1, 2.55 ± 0.1, and 4.05 ± 0.05 μ m noted in the spectra of both sides of Io. In an effort to interpret these data, we have compared them with laboratory spectra as well as synthetic spectra constructed with a simple multiple-scattering theory. We attribute the 2.9-μm feature of Callistos spectra primarily to bound water, with the product of fractional abundance of bound water and mean grain radius in micrometers equaling approximately 3.5 × 10 −1 for both sides of the satellite. The fractional amounts of water ice cover on the trailing side of Ganymede, its leading side, and the leading side of Europa were found to be 50 ± 15, 65 ± 15, and 85% or greater, respectively. The bare ground areas on Ganymede have reflectivity properties in the 0.7- to 2.5-μm spectral region comparable to those of Callistos surface and also have significant quantities of bound water, as does Callisto. Interpretation of the spectrum for the trailing side of Europa is complicated by magnetospheric particle bombardment which causes a perceptible broadening of strong bands, but the ice cover on this side is probably comparable to that on the leading side. These irradiation effects may be responsible for much of the difference in the visual geometric albedos of the two sides of Europa. Minor, but significant, amounts of ferrous-bearing material (either ferrous salts or alkali feldspars but not olivines or pyroxenes) account for the 1.35-μm feature of Io. The two longer wavelength bands are most likely attributable to nitrate salts. Ferrous salts and nitrates can jointly also account for much of the spectral variation in Ios visible reflectivity, thereby eliminating the need to postulate large quantities of sulfur. The absence of noticeable features near 3-μm wavelength in Ios spectra leads to upper bounds of 10% on the fractional cover of water and ammonia ice and 10 −3 on the relative abundance of bound water and hydroxylated material on Io. The two sides of Io have similar compositions. We suggest that the systematic increase in fractional water ice cover from Callisto to Ganymede to Europa is bought about by variations in efficiencies of recoating the satellites surface by interior water brought to the surface, and by the deposition of extrinsic dust. The most important component of the latter is debris, derived from the outer irregular satellites of Jupiter, which impacts the Galilean satellites at relatively low velocities. Europa has the largest water ice cover because its crust is thinnest and thus the frequency of water recoating is the greatest, and because it is farthest from the sources of low-velocity dust. While models which depict Ios surface as consisting primarily of very fine-grained ice are no longer viable, we are unable to definitively distinguish between the salt assemblage and alkali feldspar models. The salt model can better account for Ios reflectivity spectrum from 0.3 to 5 μm, but the absence of appreciable quantities of bound water and hydroxylated material may not be readily understood within the context of that model.

Estimates of the Climatic Impact of Aerosols Produced by Space Shuttles, SST's, and other High Flying Aircraft

Abstract Aircraft and Space Shuttles flying through the stratosphere over the next several decades will add sulfuric acid and aluminum oxide particles, respectively, to this region of the atmosphere. To evaluate the effect of these additional aerosols on the global heat balance, we have performed solar and terrestrial radiative transfer calculations. The solar calculations employed an accurate numerical method for solving the multiple-scattering problem for unpolarized light to determine the dependence of the global (spherical) albedo on the optical depth perturbation Δτ. Correct allowance was made for absorption by gases. Using these results, and those obtained from calculations of the terrestrial thermal flux at the top of the atmosphere, we determined the resulting change in the mean surface temperature, ΔT, as a function of Δτ. In both calculations, we used the measured optical constants of the aerosol species. To apply these results to the problem of interest, we used engine exhaust properties of the...

The Venus ionosphere at grazing incidence of solar radiation - Transport of plasma to the night ionosphere

Abstract Using a quasi-two-dimensional model of the Venus ionosphere, we calculated the ion number densities and horizontal ion bulk velocities expected for a range of solar zenith angles near the terminator (80 to 100°), and compared them with data obtained from the Pioneer Venus Orbiter retarding potential analyzer. The calculated ion bulk velocity arises entirely from the solar EUV-induced plasma pressure gradient and has a magnitude consistent with observations; ionization by suprathermal electrons is neglected in those computations. We find that while photoionization is the dominant source of ionospheric plasma for solar zenith angles less than 92°, plasma transport from the dayside is the dominant plasma source for solar zenith angles greater than 95°. We also show that the main nightside plasma peak at approximately 140 km altitude is of the F2 type (i.e., is diffusion controlled). Its altitude and shape are thus quite insensitive to the altitude of the ion source.

A Determination of the Composition of the Venus Clouds from Aircraft Observations in the Near Infrared

Abstract We summarize the evidence showing that the first optical depth of the Venus cloud layer is composed of a water solution of sulfuric acid, including our earlier aircraft observations of Venus’ reflectivity in the 1–4 μm region obtained at a phase angle of 120° (Pollack et al.). Analyses of these aircraft results indicated that of all the proposed cloud candidates only a sulfuric acid solution with a concentration of 75% or more H2SO4, by weight was consistent with the observed 3 µm cloud feature. We present new aircraft observations of Venus obtained in the 1–4 µm region at a phase angle of 40° and in the 3–6 µm region at a phase angle of 136°. Comparing the two sets of observations in the 1–4 µm region, we find a striking phase effect: the reflectivity is much lower in the 3 µm region and there is a much more marked decline between 1.3 and 2.5 µm for the data obtained at the smaller phase angle. The observations made at the 40° phase angle are consistent with the theoretical behavior of a sulfuri...