Geology topics

William C. Feldman

Publications and source records attributed to William C. Feldman.

5 recordsLinked to original sources

USGS2013

Neutron absorption constraints on the composition of 4 Vesta

Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock-forming elements. From a circular, polar low-altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole-rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg-rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine-rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.

Meteoritics and Planetary Science↗

USGS2012

Elemental mapping by Dawn reveals exogenic H in Vesta's regolith

Using Dawn’s Gamma Ray and Neutron Detector, we tested models of Vesta’s evolution based on studies of howardite, eucrite, and diogenite (HED) meteorites. Global Fe/O and Fe/Si ratios are consistent with HED compositions. Neutron measurements confirm that a thick, diogenitic lower crust is exposed in the Rheasilvia basin, which is consistent with global magmatic differentiation. Vesta’s regolith contains substantial amounts of hydrogen. The highest hydrogen concentrations coincide with older, low-albedo regions near the equator, where water ice is unstable. The young, Rheasilvia basin contains the lowest concentrations. These observations are consistent with gradual accumulation of hydrogen by infall of carbonaceous chondrites—observed as clasts in some howardites—and subsequent removal or burial of this material by large impacts.

Science↗

USGS2008

A prelanding assessment of the ice table depth and ground ice characteristics in Martian permafrost at the Phoenix landing site

We review multiple estimates of the ice table depth at potential Phoenix landing sites and consider the possible state and distribution of subsurface ice. A two-layer model of ice-rich material overlain by ice-free material is consistent with both the observational and theoretical lines of evidence. Results indicate ground ice to be shallow and ubiquitous, 2-6 cm below the surface. Undulations in the ice table depth are expected because of the thermodynamic effects of rocks, slopes, and soil variations on the scale of the Phoenix Lander and within the digging area, which can be advantageous for analysis of both dry surficial soils and buried ice-rich materials. The ground ice at the ice table to be sampled by the Phoenix Lander is expected to be geologically young because of recent climate oscillations. However, estimates of the ratio of soil to ice in the ice-rich subsurface layer suggest that that the ice content exceeds the available pore space, which is difficult to reconcile with existing ground ice stability and dynamics models. These high concentrations of ice may be the result of either the burial of surface snow during times of higher obliquity, initially high-porosity soils, or the migration of water along thin films. Measurement of the D/H ratio within the ice at the ice table and of the soil-to-ice ratio, as well as imaging ice-soil textures, will help determine if the ice is indeed young and if the models of the effects of climate change on the ground ice are reasonable.

Journal of Geophysical Research E: Planets↗

USGS2007

Vertical distribution of hydrogen at high northern latitudes on Mars: The Mars Odyssey Neutron Spectrometer

Neutron leakage currents measured using the Mars Odyssey Neutron Spectrometer are used to develop a two‐layer model of the distribution of hydrogen (here parameterized as water‐equivalent hydrogen, WEH) at high northern latitudes. The WEH abundance in the upper layer, Wup, was found to range between 1% and about 5%. The maximum value of the apparent thickness, D, of this upper layer peaks at about 60° latitude, giving the appearance of zonal bands of enhanced D in both hemispheres. This maximum is consistent with an expected transition from WEH contained solely in hydrous minerals at lower latitudes, to WEH contained both in the forms of water ice and water of hydration at high latitudes. A strong anti‐correlation between the WEH concentration in the lower layer and apparent depth, D, at high latitudes is observed and may provide clues to the origin of these deposits.

Geophysical Research Letters↗

USGS2006

Refined thorium abundances for lunar red spots: Implications for evolved, nonmare volcanism on the Moon

We have used improved knowledge of the spatial distribution of thorium (Th) on the lunar surface, in conjunction with a forward modeling analysis of Lunar Prospector gamma ray data, to estimate the thorium abundances of lunar red spots. The results from this study can be combined with preexisting compositional and morphologic evidence to suggest that Hansteen Alpha, the Gruithuisen domes, and the Lassell massif are silicic, nonmare, volcanic constructs, similar in nature to terrestrial rhyolite domes. We propose that either silicate liquid immiscibility or, more likely, basaltic underplating could have produced lunar rhyolite domes. Thus the Lunar Prospector data presented in this study provide new information about the full range of volcanic and crustal processes that could have occurred on the Moon.

Journal of Geophysical Research E: Planets↗