Geology topics

W. M. Calvin

Publications and source records attributed to W. M. Calvin.

22 records · Page 2Linked to original sources

USGS1996

O2 on ganymede: Spectral characteristics and plasma formation mechanisms

Weak absorption features in the visible reflectance spectrum of Jupiter's satellite Ganymede have been correlated to those observed in the spectrum of molecular oxygen. We examine the spectral characteristics of these absorption features in all phases of O 2 and conclude that the molecular oxygen is most likely present at densities similar to the liquid or solid γ-phase. The contribution of O 2 to spectral features observed on Ganymede in the near-infrared wavelength region affects the previous estimates of photon pathlength in ice. The concentration of the visible absorption features on the trailing hemisphere of Ganymede suggests an origin due to bombardment by magnetospheric ions. We derive an approximate O 2 formation rate from this mechanism and consider the state of O 2 within the surface.

Geophysical Research Letters↗

USGS1996

Detection and monitoring of H2O and CO2 ice clouds on Mars

We have developed an observational scheme for the detection and discrimination of Mars atmospheric H 2 O and CO 2 clouds using ground-based instruments in the near infrared. We report the results of our cloud detection and characterization study using Mars near IR images obtained during the 1990 and 1993 oppositions. We focused on specific wavelengths that have the potential, based on previous laboratory studies of H 2 O and CO 2 ices, of yielding the greatest degree of cloud detectability and compositional discriminability. We have detected and mapped absorption features at some of these wavelengths in both the northern and southern polar regions of Mars. Compositional information on the nature of these absorption features was derived from comparisons with laboratory ice spectra and with a simplified radiative transfer model of a CO 2 ice cloud overlying a bright surface. Our results indicate that both H 2 O and CO 2 ices can be detected and distinguished in the polar hood clouds. The region near 3.00 μm is most useful for the detection of water ice clouds because there is a strong H 2 O ice absorption at this wavelength but only a weak CO 2 ice band. The region near 3.33 μm is most useful for the detection of CO 2 ice clouds because there is a strong, relatively narrow CO 2 ice band at this wavelength but only broad “continuum” H 2 O ice absorption. Weaker features near 2.30 μm could arise from CO 2 ice at coarse grain sizes, or surface/dust minerals. Narrow features near 2.00 μm, which could potentially be very diagnostic of CO 2 ice clouds, suffer from contamination by Mars atmospheric CO 2 absorptions and are difficult to interpret because of the rather poor knowledge of surface elevation at high latitudes. These results indicate that future ground-based, Earth-orbital, and spacecraft studies over a more extended span of the seasonal cycle should yield substantial information on the style and timing of volatile transport on Mars, as well as a more detailed understanding of the role of CO 2 condensation in the polar heat budget.

Journal of Geophysical Research E: Planets↗

USGS1993

Spectral Distinctions between the Leading and Trailing Hemispheres of Callisto: New Observations

An analysis of recent telescopic observations of Callisto results in new insights regarding spectral variations from the leading to the trailing hemisphere of Callisto. Examination of data in the wavelength range from 2.0 to 2.5 ??m indicates that previous suggestions of spectral differences are most likely the result of experimental uncertainty or error. Slight variations in the slope of this wavelength range are consistent with larger ice grain sizes on the trailing hemisphere. The new observations confirm the presence of an absorption feature centered on 3.4 ??m in the spectrum of the leading hemisphere. Theoretical spectral modeling indicates this feature is caused by small amounts of fine-grained water ice. Finally, an absorption feature near 3.1 ??m is indicated but cannot be confirmed due to the strong variation in the spectrum of water ice in this region. If this feature is real, rather than an artifact of the reflectance modeling, it is similar in location and bandwidth to a feature seen in the spectrum of Ceres, attributed to NH4-bearing clays. ?? 1993 Academic Press. All rights reserved.

Icarus↗

USGS1992

Evidence for ammonium-bearing minerals on Ceres

Spectra obtained from recent telescopic observation of 1-Ceres and laboratory measurements and theoretical calculations of three component mixtures of Ceres analog material suggest that an ammoniated phyllosilicate is present on the surface of the asteroid, rather than H 2 O frost as had been previously reported. The presence of an ammoniated phyllosilicate, most likely ammoniated saponite, on the surface of Ceres implies that secondary temperatures could not have exceeded 400 kelvin.

Science↗