Bruce G. Bills

Diviner lunar radiometer observations of cold traps in the moon's south polar region

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.

Eruptions arising from tidally controlled periodic openings of rifts on Enceladus.

In 2005, plumes were detected near the south polar region of Enceladus, a small icy satellite of Saturn. Observations of the south pole revealed large rifts in the crust, informally called ‘tiger stripes’, which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions. Models of the ultimate interior source for the eruptions are under consideration. Other models of an expanding plume require eruptions from discrete sources, as well as less voluminous eruptions from a more extended source, to match the observations. No physical mechanism that matches the observations has been identified to control these eruptions. Here we report a mechanism in which temporal variations in tidal stress open and close the tiger-stripe rifts, governing the timing of eruptions. During each orbit, every portion of each tiger stripe rift spends about half the time in tension, which allows the rift to open, exposing volatiles, and allowing eruptions. In a complementary process, periodic shear stress along the rifts also generates heat along their lengths, which has the capacity to enhance eruptions. Plume activity is expected to vary periodically, affecting the injection of material into Saturn’s E ring and its formation, evolution and structure. Moreover, the stresses controlling eruptions imply that Enceladus’ icy shell behaves as a thin elastic layer, perhaps only a few tens of kilometres thick.

A partially differentiated interior for (1) Ceres deduced from its gravity field and shape

Remote observations of the asteroid (1) Ceres from ground- and space-based telescopes have provided its approximate density and shape, leading to a range of models for the interior of Ceres, from homogeneous to fully differentiated. A previously missing parameter that can place a strong constraint on the interior of Ceres is its moment of inertia, which requires the measurement of its gravitational variation together with either precession rate or a validated assumption of hydrostatic equilibrium. However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected. Here we report gravity and shape measurements of Ceres obtained from the Dawn spacecraft, showing that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37. These data show that Ceres is a partially differentiated body, with a rocky core overlaid by a volatile-rich shell, as predicted in some studies. Furthermore, we show that the gravity signal is strongly suppressed compared to that predicted by the topographic variation. This indicates that Ceres is isostatically compensated, such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta, this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient. To further investigate the interior structure, we assume a two-layer model for the interior of Ceres with a core density of 2,460–2,900 kilograms per cubic metre (that is, composed of CI and CM chondrites), which yields an outer-shell thickness of 70–190 kilometres. The density of this outer shell is 1,680–1,950 kilograms per cubic metre, indicating a mixture of volatiles and denser materials such as silicates and salts. Although the gravity and shape data confirm that the interior of Ceres evolved thermally, its partially differentiated interior indicates an evolution more complex than has been envisioned for mid-sized (less than 1,000 kilometres across) ice-rich rocky bodies.

A harmonic analysis of lunar topography

Abstract A global lunar topographic map has been derived from existing Earth-based and orbital observations supplemented in areas without data by a linear autocovariance predictor. Of 2592 bins, each 5° square, 1380 (64.7% by area) contain at least one measurement. A spherical harmonic analysis to degree 12 yields a mean radius of 1737.53 ± 0.03 km (formal standard error) and an offset of the center of figure of 1.98±0.06 km toward (19±2)°S, (194±1)°E. A Bouguer gravity map, derived from a 12-degree free-air gravity model and the present topography data, is presented for an elevation of 100 km above the mean surface. It is confirmed that the low-degree gravity harmonics are determined primarily by surface height variations and only secondarily by lateral density variations.

Lunar orbital evolution: A synthesis of recent results

The present rate of tidal dissipation in the Earth-Moon system is known to be anomalously high, in the sense that the implied age of the lunar orbit is only 1.5×109 years, though other evidence suggests an age closer to 4×109 years. To assess how long the anomalous dissipation has persisted, we use published estimates of lunar orbital configurations derived from (a) fine grained sediments containing tidal laminations and (b) numerical ocean models averaged over varying ocean geometries. The implied histories of the lunar semimajor axis are surprisingly consistent over the past 109 years. The ocean models imply, on average, reduced dissipation in the past because of a spatial mismatch between tidal forcing and oceanic normal modes of higher frequencies. Webbs ocean model suggests that the “anomalous” oceanic dissipation began about 109 years ago and has been increasing since then.

Isostatic rebound, active faulting, and potential geomorphic effects in the Lake Lahontan basin, Nevada and California

The high shoreline of the late Pleistocene (Sehoo) lake in the Lahontan basin is used as a passive strain marker to delineate the magnitude and character of regional deformation since 13 ka. The elevations of 170 high shoreline sites document that the once horizontal (equipotential) shoreline, which traverses almost 4° of latitude and 3° of longitude, is now deflected vertically about 22 m. Most of the deformation is attributed to isostatic rebound, but a small down-to-the-north regional tilting also appears to contribute to the overall deformation pattern. Active faults locally offset the high shoreline, but cannot explain the regional upwarping attributed to isostatic rebound since 13 ka. Preliminary models of the rebound yield an upper mantle viscosity of 10 18 Pa s that implies a Maxwell relaxation time of about 300 yr. The rapid Earth response, coupled with the rapid fall in lake level at the end of Pleistocene time, may have acted to divert some of the major rivers flowing into the basin from one terminal subbasin to another. The regional deformation caused by the rebound may also have acted to control the present location of Honey Lake. These shoreline data, therefore, support the potential for a link be

Viscosity structure of the crust and upper mantle in western Nevada from isostatic rebound patterns of the late Pleistocene Lake Lahontan high shoreline

Received 13 July 2005; revised 26 October 2006; accepted 31 January 2007; published 8 June 2007. [1] Large lakes can both produce and record significant crustal deformation. We present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake Lahontan, in western Nevada, using a layered Maxwell viscoelastic model. The inferred viscosity structure depends on loading history. We use three variants of a well-documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure. A simple two-layer model, with an elastic plate over an inviscid half-space, fits the observed elevation pattern quite well, with a residual variance of 32% of the data variance. Using multilayered, finite viscosity models, the residual variance is reduced to 20% of the data variance, which is very near to the noise level. In the higher-resolution models, the viscosity is below 10 18 Pa s over the depth range from 80 to 160 km. The minimum viscosity is very similar to the value that has been seen in the eastern Great Basin, from similar analyses of Lake Bonneville shorelines, but the lowviscosity zone is thinner beneath Bonneville. Making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations. Additionally, we find that small variations in proposed loading models can result in presumably spurious density inversions, and suggest that this modeling approach provides a test for loading histories.

Geophysical implications of the long‐wavelength topography of the Saturnian satellites

[1] We use limb profiles to quantify the long‐wavelength topography of the Saturnian satellites. The degree 2 shapes of Mimas, Enceladus, and Tethys are not consistent with hydrostatic equilibrium. We derive 2‐D topographic maps out to spherical harmonic degree 8. There is a good correlation with topography derived from stereo techniques. If uncompensated, topography at degree 3 and higher is large enough to be detectable during close spacecraft flybys. If not properly accounted for, this topography may bias estimates of a satellite’s degree 2 gravity coefficients (which are used to determine the moment of inertia). We also derive a one‐dimensional variance spectrum (a measure of how roughness varies with wavelength) for each body. The short‐wavelength spectral slope is − 2t o−2.5, similar to silicate bodies. However, unlike the terrestrial planets, each satellite spectrum shows a reduction in slope at longer wavelengths. If this break in slope is due to a transition from flexural to isostatic support, the globally averaged elastic thickness Te of each satellite may be derived. We obtain Te values of ≥5 km, 1.5–5 km, ≈5 km, and ≥5 km for Tethys, Dione, Rhea, and Iapetus, respectively. For Europa, we obtain Te ≈ 1.5 km. These estimates are generally consistent with estimates made using other techniques. For Enceladus, intermediate wavelengths imply Te ≥ 0.5 km, but the variance spectrum at wavelengths greater than 150 km is probably influenced by long‐ wavelength processes such as convection or shell thickness variations. Impact cratering may also play a role in determining the variance spectra of some bodies. Citation: Nimmo, F., B. G. Bills, and P. C. Thomas (2011), Geophysical implications of the long‐wavelength topography of the Saturnian satellites, J. Geophys. Res., 116, E11001, doi:10.1029/2011JE003835.

Venus gravity - Analysis of Beta Regio

Radio tracking data acquired over Beta Regio were analyzed to obtain a surface mass distribution from which a detailed vertical gravity field was derived. In addition, a corresponding vertical gravity field was evaluated solely from the topography of the Beta region. A comparison of these two maps confirms the strong correlation between gravity and topography which was previously seen in line-of-sight gravity maps. It also demonstrates that the observed gravity is a significant fraction of that predicted from the topography alone. The effective depth of complete isostatic compensation for the Beta region is estimated to be 330 km, which is somewhat deeper than that found for other areas of Venus.

Obliquity‐oblateness feedback: Are climatically sensitive values of obliquity dynamically unstable?

A new model is presented for feedback between rotational and climatic variations, operative on time scales of 104–107 years. Due to the combined effect of planetary perturbations to the Earths orbit plane and luni-solar torques on the oblate figure of the Earth, the obliquity varies by ∼1° on a 4·104 year time scale. Associated changes in the seasonal and latitudinal pattern of incident solar radiation cause major glaciations. Mass transport from the oceans to the polar ice sheets during these glaciations can change the gravitational oblateness of the Earth by amounts approaching 1%. As the rate of spin axis precession is directly proportional to the oblateness, the climatically forced mass transport can by dynamically significant. A simple parameterization of us coupled orbital-rotational-climatic system suggests that there is a strong tendency for the system to evolve away from climatically sensitive values of the obliquity. This may explain the mid-Pleistocene transition from an obliquity dominated regime to the present regime in which most climatic variability is concentrated at longer (105 year) periods.