Deborah L. Domingue

The Evolution of Mercury’s Crust: A Global Perspective from MESSENGER

MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planets orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planets surface. Now, after MESSENGER, only 10% of Mercurys surface remains to be imaged up close. Denevi et al. (p. 613) use this near-global data to look at the mechanisms that shaped Mercurys crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercurys surface is dotted with impact craters. Watters et al. (p. 618) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercurys contraction as its interior cooled over time. Mercurys exosphere and magnetosphere were also observed (see the Perspective by Glassmeier). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercurys exosphere, which may shed light on the processes that create and maintain the exosphere. Data from the Mariner 10 and MESSENGER flybys imply that a substantial fraction of Mercury’s surface is volcanic in origin. Mapping the distribution and extent of major terrain types on a planet’s surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury’s crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically. A low-reflectance, relatively blue component affects at least 15% of the surface and is concentrated in crater and basin ejecta. Its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron- and titanium-bearing oxides.

Detection of ozone on Ganymede

An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of ozone (O3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 × 1016 per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O2). An estimated number density ratio of [O3]/[O2] ≈ 10−4 to 10−3 requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O2-O3 “atmosphere” is trapped in Ganymedes surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O3 band.

Reflectance and Color Variations on Mercury: Regolith Processes and Compositional Heterogeneity

Multispectral images of Mercury obtained by the MESSENGER spacecraft reveal that its surface has an overall relatively low reflectance with three large-scale units identified on the basis of reflectance and slope (0.4 to 1.0 micrometer). A higher-reflectance, relatively red material occurs as a distinct class of smooth plains that were likely emplaced volcanically; a lower-reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals, possibly more common at depth. A spectrally intermediate terrain probably forms most of the upper crust. Three other spectrally distinct but spatially restricted units include fresh crater ejecta less affected by space weathering than other surface materials; high-reflectance deposits seen in some crater floors; and moderately high-reflectance, relatively reddish material associated with rimless depressions.

Geology of the caloris basin, mercury: A view from MESSENGER

The Caloris basin, the youngest known large impact basin on Mercury, is revealed in MESSENGER images to be modified by volcanism and deformation in a manner distinct from that of lunar impact basins. The morphology and spatial distribution of basin materials themselves closely match lunar counterparts. Evidence for a volcanic origin of the basins interior plains includes embayed craters on the basin floor and diffuse deposits surrounding rimless depressions interpreted to be of pyroclastic origin. Unlike lunar maria, the volcanic plains in Caloris are higher in albedo than surrounding basin materials and lack spectral evidence for ferrous iron-bearing silicates. Tectonic landforms, contractional wrinkle ridges and extensional troughs, have distributions and age relations different from their counterparts in and around lunar basins, indicating a different stress history.

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros.

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 × 13 × 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of ‘ejecta blocks’. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a ‘pond’. Properties of the ‘ponds’ are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.

Spectroscopic Observations of Mercury's Surface Reflectance During MESSENGER's First Mercury Flyby

During MESSENGERs first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercurys surface.

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

In-flight performance of MESSENGER's Mercury Dual Imaging System

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched in August 2004 and planned for insertion into orbit around Mercury in 2011, has already completed two flybys of the innermost planet. The Mercury Dual Imaging System (MDIS) acquired nearly 2500 images from the first two flybys and viewed portions of Mercurys surface not viewed by Mariner 10 in 1974-1975. Mercurys proximity to the Sun and its slow rotation present challenges to the thermal design for a camera on an orbital mission around Mercury. In addition, strict limitations on spacecraft pointing and the highly elliptical orbit create challenges in attaining coverage at desired geometries and relatively uniform spatial resolution. The instrument designed to meet these challenges consists of dual imagers, a monochrome narrow-angle camera (NAC) with a 1.5° field of view (FOV) and a multispectral wide-angle camera (WAC) with a 10.5° FOV, co-aligned on a pivoting platform. The focal-plane electronics of each camera are identical and use a 1024×1024 charge-coupled device detector. The cameras are passively cooled but use diode heat pipes and phase-change-material thermal reservoirs to maintain the thermal configuration during the hot portions of the orbit. Here we present an overview of the instrument design and how the design meets its technical challenges. We also review results from the first two flybys, discuss the quality of MDIS data from the initial periods of data acquisition and how that compares with requirements, and summarize how in-flight tests are being used to improve the quality of the instrument calibration.

Disk-resolved photometric analysis of European terrains

Abstract Using Hapkes photometric model we examined the photometric textures of a representative subset of the geologic units on Europa defined by Lucchitta and Soderblom (1982, Satellites of Jupiter , pp. 521–555, University of Arizona Press, Tucson). The individual units studied possess the same small mean photometric slope angle as that found by Domingue et al. (1991, Icarus 90, 30–42) for the entire disk. With one major exception, the particles composing the surfaces of most of the units have similar scattering properties. However, the single-particle scattering functions of the two bright plains regions examined are very different from each other and from the other units. If the bright plains units are geologically younger than the other units, an aging process that affects the single-particle phase functions and albedos is inferred. In our previous study we concluded that the disk-integrated photometric properties of Europa could not be explained adequately by ion bombardment. However, a new study on the effects of ion irradiation on the optical properties of materials (Sack et al. 1991, J. Geophys. Res. E 96(2), 17535–17539) renders this conclusion invalid.

Radiative transfer models for light scattering from planetary surfaces

New, accurate numerical solutions of the radiative transfer equation are compared with the Hapke [1981, 1984, 1986] analytic approximation, which is widely used in planetary data analyses. The numerical solutions use the Ambartsumian invariance principle as do the well-known Chandrasekhar [1960] H function solutions. The invariance principle has been reexpressed in a form which allows high order-accurate numerical integrations without any required interpolations. The new numerical solutions reproduce the Chandrasekhar H function solutions for Legendre phase functions but also allow single-scattering phase functions of arbitrary form. Accurate numerical solutions of the radiative transfer equation for Henyey-Greenstein phase functions reveal that the errors in the Hapke model for estimating bidirectional reflectance values range from <2% rms for dark surfaces like the Moon to <7% rms for bright surfaces such as Europa. Further comparisons demonstrate that Hapke model fitting procedures estimate single particle scattering albedo values to within <3% for both dark and bright surfaces.