Roger J. Phillips

The Crust of the Moon as Seen by GRAIL

The Holy GRAIL? The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moons gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668, published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moons upper crust while fracturing it extensively. Wieczorek et al. (p. 671, published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675, published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moons history. The Moons gravity field shows that the lunar crust is less dense and more porous than was thought. High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moons highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

The MESSENGER mission to Mercury: Scientific objectives and implementation

Abstract Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercurys anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercurys geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercurys core, and the processes controlling volatile species in Mercurys polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercurys exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.

Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution

From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere (T_e) required to support the observed topographic load since the time of loading. We convert T_e to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass-sheet (small-amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite-amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary “shape”) in lieu of “topography” referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived T_e values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

Gravity Field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission

The Holy GRAIL? The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moons gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668, published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moons upper crust while fracturing it extensively. Wieczorek et al. (p. 671, published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675, published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moons history. The Moons gravity field reveals that impacts have homogenized the density of the crust and fractured it extensively. Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.

Gravity Field and Internal Structure of Mercury from MESSENGER

Mercury Inside and Out The MESSENGER spacecraft orbiting Mercury has been in a ∼12-hour eccentric, near-polar orbit since 18 March 2011 (see the Perspective by McKinnon). Smith et al. (p. 214, published online 21 March) present the most recent determination of Mercurys gravity field, based on radio tracking of the MESSENGER spacecraft between 18 March and 23 August 2011. The results point to an interior structure that differs from those of the other terrestrial planets: the density of the planets solid outer shell suggests the existence of a deep reservoir of high-density material, possibly an Fe-S layer. Zuber et al. (p. 217, published online 21 March) used data obtained by the MESSENGER laser altimeter through to 24 October 2011 to build a topographic map of Mercurys northern hemisphere. The map shows less variation in elevation, compared with Mars or the Moon, and its features add to the body of evidence that Mercury has sustained geophysical activity for much of its history. Mercury’s outer solid shell is denser than expected, suggesting a deep reservoir of high-density material, possibly iron-sulfide. Radio tracking of the MESSENGER spacecraft has provided a model of Mercury’s gravity field. In the northern hemisphere, several large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury’s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia C/MR2 = 0.353 ± 0.017, where M and R are Mercury’s mass and radius, and a ratio of the moment of inertia of Mercury’s solid outer shell to that of the planet of Cm/C = 0.452 ± 0.035. A model for Mercury’s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core.

Meridiani Planum and the global hydrology of Mars

The Opportunity Mars Exploration Rover found evidence for groundwater activity in the Meridiani Planum region of Mars in the form of aeolian and fluvial sediments composed of sulphate-rich grains. These sediments appear to have experienced diagenetic modification in the presence of a fluctuating water table. In addition to the extensive secondary aqueous alteration, the primary grains themselves probably derive from earlier playa evaporites. Little is known, however, about the hydrologic processes responsible for this environmental history—particularly how such extensive evaporite deposits formed in the absence of a topographic basin. Here we investigate the origin of these deposits, in the context of the global hydrology of early Mars, using numerical simulations, and demonstrate that Meridiani is one of the few regions of currently exposed ancient crust predicted to have experienced significant groundwater upwelling and evaporation. The global groundwater flow would have been driven primarily by precipitation-induced recharge and evaporative loss, with the formation of the Tharsis volcanic rise possibly playing a role through the burial of aquifers and induced global deformation. These results suggest that the deposits formed as a result of sustained groundwater upwelling and evaporation, rather than ponding within an enclosed basin. The evaporite formation coincided with a transition to more arid conditions that increased the relative impact of a deep-seated, global-scale hydrology on the surface evolution.

Evidence for extensive denudation of the Martian highlands

High-resolution topographic data from the Mars Orbiter laser altimeter reveal evidence for widespread denudation from Margaritifer Sinus to northern Arabia Terra, an area of ∼1 × 107 km2. A major resurfacing event is indicated by: (1) a heavily degraded landscape hosting numerous inliers, (2) truncation or absence of valley networks, and (3) a break in slope at the edge of early to middle Noachian plateau materials. Geomorphic mapping was completed on the type locale of denudation (0°–30°S, 0°–30°W). Superposition relations and crater counts for geomorphic units indicate that large-scale resurfacing took place in the late Noachian, eroding the Martian highlands and resulting in transportation and deposition of ∼4.5 × 106 km3 of sediment in the northern plains. This is equivalent to a 120-m-thick uniform layer of sediment on the surface of Mars north of 30°N. Geomorphic mapping and crater counts limit the timing of denudation to the late Noachian, an interval of 350–500 m.y. Using this limit, we estimate a minimum rate of denudation of 2.0 μm/yr, comparable to denudation of typical slopes in a temperate maritime climate on Earth. The morphology, rate of denudation, and extensive nature of upland degradation suggest that precipitation-fed surface runoff is the most likely geomorphic agent capable of such a process, indicative of a warm, wet Mars during the late Noachian Epoch.

Return to Mercury: a global perspective on MESSENGER's first Mercury flyby.

In January 2008, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft became the first probe to fly past the planet Mercury in 33 years. The encounter revealed that Mercury is a dynamic system; its liquid iron-rich outer core is coupled through a dominantly dipolar magnetic field to the surface, exosphere, and magnetosphere, all of which interact with the solar wind. MESSENGER images confirm that lobate scarps are the dominant tectonic landform and record global contraction associated with cooling of the planet. The history of contraction can be related to the history of volcanism and cratering, and the total contractional strain is at least one-third greater than inferred from Mariner 10 images. On the basis of measurements of thermal neutrons made during the flyby, the average abundance of iron in Mercurys surface material is less than 6% by weight.

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

Mercury Inside and Out The MESSENGER spacecraft orbiting Mercury has been in a ∼12-hour eccentric, near-polar orbit since 18 March 2011 (see the Perspective by McKinnon). Smith et al. (p. 214, published online 21 March) present the most recent determination of Mercurys gravity field, based on radio tracking of the MESSENGER spacecraft between 18 March and 23 August 2011. The results point to an interior structure that differs from those of the other terrestrial planets: the density of the planets solid outer shell suggests the existence of a deep reservoir of high-density material, possibly an Fe-S layer. Zuber et al. (p. 217, published online 21 March) used data obtained by the MESSENGER laser altimeter through to 24 October 2011 to build a topographic map of Mercurys northern hemisphere. The map shows less variation in elevation, compared with Mars or the Moon, and its features add to the body of evidence that Mercury has sustained geophysical activity for much of its history. Mercury’s topography indicates sustained geophysical activity for most of the planet’s geological history. Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury’s topography occurred after the earliest phases of the planet’s geological history.