Remote detection of a lunar granitic batholith at Compton-Belkovich.

Granites are nearly absent in the Solar System outside of Earth. Achieving granitic compositions in magmatic systems requires multi-stage melting and fractionation, which also increases the concentration of radiogenic elements1. Abundant water and plate tectonics facilitate these processes on Earth, aiding in remelting. Although these drivers are absent on the Moon, small granite samples have been found, but details of their origin and the scale of systems they represent are unknown2. Here we report microwave-wavelength measurements of an anomalously hot geothermal source that is best explained by the presence of an approximately 50-kilometre-diameter granitic system below the thorium-rich farside feature known as Compton-Belkovich. Passive microwave radiometry is sensitive to the integrated thermal gradient to several wavelengths depth. The 3-37-gigahertz antenna temperatures of the Chang'e-1 and Chang'e-2 microwave instruments allow us to measure a peak heat flux of about 180 milliwatts per square metre, which is about 20 times higher than that of the average lunar highlands3,4. The surprising magnitude and geographic extent of this feature imply an Earth-like, evolved granitic system larger than believed possible on the Moon, especially outside of the Procellarum region5. Furthermore, these methods are generalizable: similar uses of passive radiometric data could vastly expand our knowledge of geothermal processes on the Moon and other planetary bodies.

New Illumination and Temperature Constraints of Mercury's Volatile Polar Deposits.

Images from the Mercury Dual Imaging System (MDIS) aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission reveal low-reflectance polar deposits that are interpreted to be lag deposits of organic-rich, volatile material. Interpretation of these highest-resolution images of Mercury's polar deposits has been limited by the available topography models, so local high-resolution (125 m pixel-1) digital elevation models (DEMs) were made using a combination of data from the Mercury Laser Altimeter (MLA) and from shape-from-shading techniques using MDIS images. Local DEMs were made for eight of Mercury's north polar craters; these DEMs were then used to create high-resolution simulated image, illumination, and thermal models. The simulated images reveal that the pixel brightness variations imaged within Mercury's low-reflectance deposits are consistent with scattered light reflecting off of topography and do not need to be explained by volatile compositional differences as previously suggested. The illumination and thermal models show that these low-reflectance polar deposits extend beyond the permanently shadowed region, more than 1.0 km in some locations, and correspond to a maximum surface temperature of greater than 250 K but less than 350 K. The low-reflectance boundaries of all eight polar deposits studied here show a close correspondence with the surface stability boundary of coronene (C24H12). While coronene should only be viewed as a proxy for the myriad volatile compounds that may exist in Mercury's polar deposits, coronene's surface stability boundary supports the idea that Mercury's low-reflectance polar deposits are composed of macromolecular organic compounds, consistent with the hypotheses of exogenous transport and in situ production.

Evidence for surface water ice in the lunar polar regions using reflectance measurements from the Lunar Orbiter Laser Altimeter and temperature measurements from the Diviner Lunar Radiometer Experiment.

We find that the reflectance of the lunar surface within 5 ° of latitude of the South Pole increases rapidly with decreasing temperature, near ~110K, behavior consistent with the presence of surface water iceThe North polar region does not show this behavior, nor do South polar surfaces at latitudes more than 5° from the pole. This South pole reflectance anomaly persists when analysis is limited to surfaces with slopes less than 10° to eliminate false detection due to the brightening effect of mass wasting, and also when the very bright south polar crater Shackleton is excluded from the analysis. We also find that south polar regions of permanent shadow that have been reported to be generally brighter at 1064 nm do not show anomalous reflectance when their annual maximum surface temperatures are too high to preserve water ice. This distinction is not observed at the North Pole. The reflectance excursion on surfaces with maximum temperatures below 110K is superimposed on a general trend of increasing reflectance with decreasing maximum temperature that is present throughout the polar regions in the north and south; we attribute this trend to a temperature or illumination-dependent space weathering effect (e.g. Hemingway et al. 2015). We also find a sudden increase in reflectance with decreasing temperature superimposed on the general trend at 200K and possibly at 300K. This may indicate the presence of other volatiles such as sulfur or organics. We identified and mapped surfaces with reflectances so high as to be unlikely to be part of an ice-free population. In this south we find a similar distribution found by Hayne et al. 2015 based on UV properties. In the north a cluster of pixels near that pole may represent a limited frost exposure.

Planetary science. Low-altitude magnetic field measurements by MESSENGER reveal Mercury's ancient crustal field.

Magnetized rocks can record the history of the magnetic field of a planet, a key constraint for understanding its evolution. From orbital vector magnetic field measurements of Mercury taken by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft at altitudes below 150 kilometers, we have detected remanent magnetization in Mercury's crust. We infer a lower bound on the average age of magnetization of 3.7 to 3.9 billion years. Our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in Mercury's history. Ancient field strengths that range from those similar to Mercury's present dipole field to Earth-like values are consistent with the magnetic field observations and with the low iron content of Mercury's crust inferred from MESSENGER elemental composition data.

Thermal stability of volatiles in the north polar region of Mercury.

Thermal models for the north polar region of Mercury, calculated from topographic measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. MESSENGER measurements of near-infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface, and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface. We propose that the dark surface layer is a sublimation lag deposit that may be rich in impact-derived organic material.

Diviner Lunar Radiometer observations of the LCROSS impact.

The Lunar Reconnaissance Orbiter (LRO) Diviner instrument detected a thermal emission signature 90 seconds after the Lunar Crater Observation and Sensing Satellite (LCROSS) Centaur impact and on two subsequent orbits. The impact heated a region of 30 to 200 square meters to at least 950 kelvin, providing a sustained heat source for the sublimation of up to ~300 kilograms of water ice during the 4 minutes of LCROSS post-impact observations. Diviner visible observations constrain the mass of the sunlit ejecta column to be ~10(-6) to 10(-5) kilograms per square meter, which is consistent with LCROSS estimates used to derive the relative abundance of the ice within the regolith.

Diviner Lunar Radiometer observations of cold traps in the Moon's south polar region.

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.