Nancy L. Chabot

Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER

MESSENGER observations of Mercury’s high northern latitudes reveal a contiguous area of volcanic smooth plains covering more than ~6% of the surface that were emplaced in a flood lava mode, consistent with average crustal compositions broadly similar to terrestrial komatiites. MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury’s high northern latitudes and occupies more than 6% of the planet’s surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury’s post–heavy bombardment era.

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.

Core formation in the Earth and Moon: new experimental constraints from V, Cr, and Mn

Abstract The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to the concentrations of these elements in chondritic meteorites. The similar depletions have been used as evidence that the Moon inherited its mantle from the Earth after a giant impact event. We have conducted liquid metal–liquid silicate partitioning experiments for V, Cr, and Mn from 3 to 14 GPa and 1723 to 2573 K to understand the behavior of these elements during planetary core formation. Our experiments have included systematic studies of the effects of temperature, silicate composition, metallic S-content, metallic C-content, and pressure. Temperature has a significant effect on the partitioning of V, Cr, Mn, with all three elements increasing their partitioning into the metallic liquid with increasing temperature. In contrast, pressure is not observed to affect the partitioning behavior. The experimental results show the partitioning of Cr and Mn are hardly dependent on the silicate composition, whereas V partitions more strongly into depolymerized silicate melts. The addition of either S or C to the metallic liquid causes increased metal–silicate partition coefficients for all three elements. Parameterizing and applying the experimental data, we find that the Earth’s mantle depletions of V, Cr, and possibly Mn can be explained by core formation in a high-temperature magma ocean under oxygen fugacity conditions about two log units below the iron–wustite buffer, though the depletion of Mn may be due entirely to its volatility. However, more oxidizing conditions proposed in recent core formation models for the Earth cannot account for any of the depletions. Additionally, because we observe no pressure effect on the partitioning behavior, the data do not require the mantle of the Moon to be derived from the Earth’s mantle, although this is not ruled out. All that is required to create depletions of V, Cr, and Mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation. Such conditions could have existed on the Moon-forming impactor.

Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity

Rimless shallow depressions on Mercury may still be forming by outgassing, volcanism, sublimation, or space weathering. High-resolution images of Mercury’s surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference that Mercury’s interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system’s innermost planet.

Potassium solubility in metal: the effects of composition at 15 kbar and 1900°C on partitioning between iron alloys and silicate melts

Abstract To determine the role of radioactive heating as an energy source in planetary cores, the solubility of K in metal has been examined experimentally. All experiments were conducted at 15 kbar and 1900°C and involved K partitioning between Fe alloys and silicate melts. Experiments conducted with different concentrations of S in the metallic liquid indicate that S increases the solubility of K in metal. Unlike S, the presence of C in the metallic liquid does not increase K solubility in metal to a level detectable with the electron microprobe. The silicate composition significantly affects the solubility of K in S-rich metal, with the metal/silicate partition coefficient for K increasing by nearly two orders of magnitude with increasing depolymerization of the silicate melt. Using an appropriate silicate composition for the early, differentiating Earth and assuming that S is a significant light element in the core, the metal/silicate partition coefficient for K is 6×10 −3 at 15 kbar and 1900°C. Such a partitioning value, if representative of the behavior of K at core formation conditions, suggests the presence of less than 1 ppm K in the Earths core with a present-day heat generation of 10 10 W, which is 2–3 orders of magnitude lower than estimates of the power necessary to drive the Earths geodynamo. Other thermodynamic variables, namely pressure, temperature, and oxygen fugacity may also affect the solubility of K in metal.

Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft

Images acquired by NASAs MESSENGER spacecraft have revealed the morphology of frozen volatiles in Mercurys permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits. The images show extensive, spatially continuous regions with distinctive reflectance properties. A site within Prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance, indicating that the surficial ice was emplaced after formation of the underlying craters. In areas where water ice is inferred to be present but covered by a thin layer of dark, organic-rich volatile material, regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercurys poles are geologically young, relative to the time scale for lateral mixing by impacts, and either are restored at the surface through an ongoing process or were delivered to the planet recently.

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.

Comparison of areas in shadow from imaging and altimetry in the north polar region of Mercury and implications for polar ice deposits

Earth-based radar observations and results from the MESSENGER mission have provided strong evidence that permanently shadowed regions near Mercurys poles host deposits of water ice. MESSENGERs complete orbital image and topographic datasets enable Mercurys surface to be observed and modeled under an extensive range of illumination conditions. The shadowed regions of Mercurys north polar region from 65°N to 90°N were mapped by analyzing Mercury Dual Imaging System (MDIS) images and by modeling illumination with Mercury Laser Altimeter (MLA) topographic data. The two independent methods produced strong agreement in identifying shadowed areas. All large radar-bright deposits, those hosted within impact craters ≥6 km in diameter, collocate with regions of shadow identified by both methods. However, only ∼46% of the persistently shadowed areas determined from images and ∼43% of the permanently shadowed areas derived from altimetry host radar-bright materials. Some sizable regions of shadow that do not host radar-bright deposits experience thermal conditions similar to those that do. The shadowed craters that lack radar-bright materials show a relation with longitude that is not related to the thermal environment, suggesting that the Earth-based radar observations of these locations may have been limited by viewing geometry, but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice.

Imaging Mercury's polar deposits during MESSENGER's low‐altitude campaign

Images obtained during MESSENGERs low-altitude campaign in the final year of the mission provide the highest-spatial-resolution views of Mercurys polar deposits. Images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign. All of these regions of permanent shadow were found to have low-reflectance surfaces with well-defined boundaries. Additionally, brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions, supporting the conclusion that multiple volatile organic compounds are contained in Mercurys polar deposits, in addition to water ice. A recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to Mercury. Either scenario is consistent with the distinctive reflectance properties and well-defined boundaries of Mercurys polar deposits and the presence of volatiles in all available cold traps.

Analysis of MESSENGER high-resolution images of Mercury's hollows and implications for hollow formation

High resolution images from MESSENGER provide morphological information on the nature and origin of Mercurys hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low-reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales <20 m indicate that the average depth of hollows is 24 ± 16 m. We propose that hollows cease to increase in depth when a volatile-depleted lag deposit becomes sufficiently thick to protect the underlying surface. The difficulty of developing a lag on steep topography may account for the common occurrence of hollows on crater central peaks and walls. Disruption of the lag, e.g., by secondary cratering, could restart growth of hollows in a location that had been dormant. Extremely high-resolution images (~3 m/pixel) show that the edges of hollows are straight, as expected if the margins formed by scarp retreat. These highest-resolution images reveal no superposed impact craters, implying that hollows are very young. The width of hollows within rayed crater Balanchine suggests that the maximum time for lateral growth by 1 cm is ~10,000 yr. A process other than entrainment of dust by gases evolved in a steady-state sublimation-like process is likely required to explain the high-reflectance haloes that surround many hollows.