Carl Bernard Pilcher

Thermal migration of water on the Galilean satellites

Abstract We have modeled the thermal migration of water on the Galilean satellites under the assumption of ballistic molecular trajectories. We find that water migrating owing to solar radiation on an ice-covered satellite will build up in the temperate latitudes, in general not reaching the poles. As much as 50 m of ice may have been lost by this process from the equatorial regions of Europa over the age of the solar system. The disappearance of patches of ice—for instance, the bright rays surrounding some impact craters—from the equatorial regions of Ganymede and Callisto may approach a value (the irreversible evaporation rate) three orders of magnitude larger than the net equatorial loss rate for ice-covered Europa. The presence of water ice pole caps on Ganymede extending to the latitudes at which thermal migration becomes important suggests that some process distributed an extensive, thin covering of water on the satellite, and that the equatorial regions were subsequently cleared by the thermal process.

Images of Jupiter's Sulfur Ring

Images of the ring of singly ionized sulfur encircling Jupiter obtained on two successive nights in April 1979 show that the ring characteristics may change dramatically in ∼ 24 hours. On the first night the ring was narrow and confined to the magnetic equator inside Ios orbit. On the second it was confined symmetrically about the centrifugal symmetry surface and showed considerable radial structure, including a fan extending to Ios orbit. Many of the differences in the ring on the two nights can be explained in terms of differences in sulfur plasma temperature.

Detection of Singly Ionized Oxygen Around Jupiter

Forbidden emission from singly ionized oxygen at wavelengths of 3726 and 3729 angstroms has been detected in the inner Jovian magnetosphere. The emission is present between ∼4 and ∼7 to 8 Jovian radii from the planet and appears concentrated in the magnetic equator. The line intensity ratio indicates the same plasma characteristics as those derived from observations of forbidden sulfur emission.

Magnetic longitude variations in the IO torus

Abstract We review the evidence for both systematic variations in the optical properties of the Io torus with magnetic longitude and for plasma corotational lag. We summarize the results of a recent paper in which we showed that observed longitudinal variations in [SII] emission and contemporaneous measurements indicating little longitudinal variation in total charge density can be explained in terms of local (i.e., longitudinally confined) plasma sources and effects of corotational lag. We present here new measurements of both [SII] and [SIII] emissions as a function of magnetic longitude that indicate local plasma production, but no significant departure of the bulk plasma from corotation. The data suggest, however, that the longitudes of plasma formation drift in the sense of subcorotation.

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.

The stability of water in Io

Abstract A variety of processes have been examined to determine their impact on water loss from Io and the formation of an anhydrous surface. Thermal escape, photolysis, and gas-phase charged particle interactions are shown to be unimportant in this regard. Recent laboratory experiments have shown that charged-particle sputtering is likely to be an effective mechanism for the removal of water ice from Ios surface. Vaporization of ice by meteoroid impacts may also be significant. The overall sputtering rate appears to be sufficiently high that the formation of a substantial regolith due to meteoroid bombardment will be prevented. However, meteoroid bombardment is probably capable of maintaining a thin (− 500 μ m overturned surface layer from which all free water has been removed by sputtering. Alternatively, a thick anhydrous surface layer may have formed on Io as the result of primordial heating. The survival of such a layer to the present implies the absence of subsequent water evolution onto the surface of the satellite.

Polar cap formation on Ganymede

It is argued that Ganymedes polar caps are the remnants of a more extensive covering of water ice that formed during a period in which the satellite was geologically active. It is inferred that the initial thickness of this covering was a significant fraction of the gardening depth since the covering formed. This suggests an initial thickness of at least a few meters over heavily cratered regions such as the south polar grooved terrain. The absence of similar polar caps on Callisto apparently reflects the absence of comparable geologic activity in the history of this satellite.

Jovian sodium emission from Region C2

Observations of emission from sodium presumed to have been ejected from Io in the orbital plane of the Gallilean satellites and extending outward to at least 35 Jupiter radii are reported. Spectra of the sodium D line were observed to have an intensity of 20 to 30 R. Mechanisms for the escape of sodium from Io after sputtering from the surface are: (1) ejection from Io at velocities greater than Jupiter escape velocity, (2) ejection at velocities corresponding to orbits with apoapses of 35 Jupiter radii, and (3) ionization followed by magnetic field sweeping and recombination. The relative contributions of each mechanism are estimated and used to predict the sodium emission intensity at 35 Jupiter radii to be 7 to 8 R, a value considered to be in reasonable agreement with observations. Observations of emission line width and the correlation of intensity with the orbital phase of Io would provide further means of evaluating the proposed mechanisms of sodium ejection.

Methane band limb-brightening on Uranus

Area scanner measurements of the brightness distributions of Uranus at 6300 and 7250 A are presented. At the former wavelength, which is characteristic of continuum radiation, the observed limb-darkening is consistent with the results of Stratoscope observations. At the latter wavelength, in the center of a strong methane absorption band, the planet shows substantial limb-brightening as has been reported by other investigators. The limb-brightening and geometric albedo of Uranus in this methane band can be explained in terms of a clear atmospheric model in which the methane partial pressure above the saturation level is equal to its equilibrium vapor pressure. This conclusion is relatively insensitive to the methane mixing ratio below the saturation level. The presence of a high-altitude, conservatively, isotropically scattering haze of optical thickness 0.1, such as that previously proposed as a possible cause of the 7250-A limb-brightening, would produce a geometric albedo at this wavelength that is substantially larger than the value observed for Uranus.

Limb-darkening scans of Jupiter

Abstract An area scanning photometer has been used to obtain photometrically calibrated limb-darkening scans of Jupiter at four wavelengths: 6190, 6300, 7250, and 8200 A. The first and third of these correspond to methane absorptions and the second and fourth to continuum regions near the 4-0 and 3-0 H2 quadrupole bands, respectively. Single-scattering albedos have been calculated for several areas on the planet at all four wavelengths assuming a semi-infinite, homogeneous, isotropically scattering atmosphere. The values obtained at the wavelengths of the quadrupole bands range from 0.98 over the NEB to ≳ 0.99 over the NTrZ and the bright band in the southern hemisphere. The single-scattering albedo values are used to show that the 5μm-emitting equatorial regions of the planet may be relatively clear and the tropical regions relatively cloudy.