Oleg Abramov

Evaluating an impact origin for Mercury's high‐magnesium region

During its four years in orbit around Mercury, the MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft’s X-Ray Spectrometer revealed a large geochemical terrane in the northern hemisphere that hosts the highest Mg/Si, S/Si, Ca/Si, and Fe/Si and lowest Al/Si ratios on the planet. Correlations with low topography, thin crust, and a sharp northern topographic boundary led to the proposal that this high-Mg region (HMR) is the remnant of an ancient, highly degraded impact basin. Here we use a numerical modeling approach to explore the feasibility of this hypothesis and evaluate the results against multiple mission-wide datasets and resulting maps from MESSENGER. We find that a ~3000-km diameter impact basin easily exhumes Mg-rich mantle material but that the amount of subsequent modification required to hide basin structure is incompatible with the strength of the geochemical anomaly, which is also present in maps of Gamma Ray and Neutron Spectrometer data. Consequently, the high-Mg region is more likely to be the product of high-temperature volcanism sourced from a chemically heterogeneous mantle than the remains of a large impact event. (178 words)

Io's hot spots in the near-infrared detected by LEISA during the New Horizons flyby

The New Horizons spacecraft flew past Jupiter and its moons in February and March 2007. The flyby provided one of the most comprehensive inventories of Ios active plumes and hot spots yet taken, including the large 350 km high eruption of Tvashtar. Among the suite of instruments active during the flyby was the Linear Etalon Infrared Spectral Array (LEISA), a near-infrared imaging spectrometer covering the spectral range 1.25 to 2.5 µm. We have identified 37 distinct hot spots on Io in the nine LEISA spectral image cubes taken during the flyby. We describe the thermal emissions from these volcanoes and fit single-component blackbody curves to the hot spot spectra to derive eruption temperatures, areas, and power output for the hot spots with sufficient signal-to-noise. Of these, 11 hot spots were seen by LEISA more than once, and East Girru showed short-term variability over a few days, also seen by other New Horizons instruments. This work presents a comprehensive look at the global distribution of Ios volcanism at the time of the flyby. From these measurements, we estimate the global power output of high-temperature (>550 K) volcanism on Io to be ~8 TW. This work provides the first short-wavelength near-infrared survey with global coverage at all longitudes on the nightside of Io without sunlight contamination at these wavelengths. A major conclusion from this study is that 90% of all the volcanoes observed in the New Horizons LEISA near-infrared data in 2007 were also observed during the Galileo epoch, suggesting these are all long-lived hot spots.

Thermal effects of late accretion to the crust and mantle of Mercury

Abstract Impact bombardment on Mercury in the solar systems late accretion phase (ca. 4.4–3.8 Ga) caused considerable mechanical, chemical and thermal reworking of its silicate reservoirs (crust and mantle). Depending on the frequency, size and velocity of such impactors, effects included regional- and global-scale crustal melting, and thermal perturbations of the mercurian mantle. We use a 3D transient heating model to test the effects of two bombardment scenarios on early (pre-Tolstojan) Mercurys mantle and crust. Results show that rare impacts by the largest (≳100 km diameter) bodies deliver sufficient heat to the shallow mercurian mantle producing high-temperature ultra-magnesian (komatiitic s.s.) melts. Impact heating leading to effusive (flood) volcanism can account for the eponymous “high-magnesium region” (HMR) observed during the MErcury Surface, Space Environment, GEochemistry Ranging (MESSENGER) mission. We find that late accretion to Mercury induced volumetrically significant crustal melting (≤58 vol.%), mantle heating and melt production, which, combined with extensive resurfacing (≤100%), also explains why its oldest cratering record was effectively erased, consistent with crater-counting statistics.