¹Megan D. Mouser,¹Nicholas Dygert,²Brendan A. Anzures,¹Nadine L. Grambling,³Rostislav Hrubiak,^4,5Yoshio Kono,³Guoyin Shen,²Stephen W. Parman
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2021JE006946]
¹Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA
²Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
³HPCAT, X-ray Science Division, Argonne National Laboratory, Argonne, IL, USA
⁴Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL, USA
⁵Now at Geodynamics Research Center, Ehime University, Matsuyama, Japan
Published by arrangement with John Wiley & Sons

Mercury has a compositionally diverse surface that was produced by different periods of igneous activity suggesting heterogeneous mantle sources. Understanding the structure of Mercury’s mantle formed during the planet’s magma ocean stage could help in developing a petrologic model for Mercury, and thus, understanding its dynamic history in the context of crustal petrogenesis. We present results of falling sphere viscometry experiments on late-stage Mercurian magma ocean analogue compositions conducted at the Advanced Photon Source, beamline 16-BM-B, Argonne National Laboratory. Owing to the presence of sulfur on the surface of Mercury, two compositions were tested, one with sulfur and one without. The liquids have viscosities of 0.6–3.9 (sulfur-bearing; 2.6–6.2 GPa) and 0.6–10.9 Pa·s (sulfur-free; 3.2–4.5 GPa) at temperatures of 1600–2000°C. We present new viscosity models that enable extrapolation beyond the experimental conditions and evaluate grain growth and the potential for crystal entrainment in a cooling, convecting magma ocean. We consider scenarios with and without a graphite flotation crust, suggesting endmember outcomes for Mercury’s mantle structure. With a graphite flotation crust, crystallization of the mantle would be fractional with negatively buoyant minerals sinking to form a stratified cumulate pile according to the crystallization sequence. Without a flotation crust, crystals may remain entrained in the convecting liquid during solidification, producing a homogeneous mantle. In the context of these endmember models, the surface could result from dynamical stirring or mixing of a mantle that was initially heterogeneous, or potentially from different extents of melting of a homogeneous mantle.

¹M. Lo,^1,2G. La Spina,¹K. H. Joy,¹M. Polacci,¹M. Burton
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2021JE006939]
¹Department of Earth and Environmental Sciences, University of Manchester, Manchester, UK
²Istituto Nazionale di Geofisica e Vulcanologia Sezione di, Catania, Sicilia, Italy
Published by arrangement with John Wiley & Sons

The Moon is not volcanically active at present, therefore, we rely on data from lunar samples, remote sensing, and numerical modeling to understand past lunar volcanism. The role of different volatile species in propelling lunar magma ascent and eruption remains unclear. We adapt a terrestrial magma ascent model for lunar magma ascent, considering different compositions of picritic magmas and various abundances of H₂, H₂O, and CO (measured and estimated) for these magmas. We also conduct a sensitivity analysis to investigate the relationship between selected input parameters (pre-eruptive pressure, temperature, conduit radius, and volatile content) and given outputs (exit gas volume fraction, velocity, pressure, and mass eruption rate). We find that, for the model simulations containing H₂O and CO, CO was more significant than H₂O in driving lunar magma ascent, for the range of volatile contents considered here. For the simulations containing H₂ and CO, H₂ had a similar or slightly greater control than CO on magma ascent dynamics. Our results showed that initial H₂ and CO content has a strong control on exit velocity and pressure, two factors that strongly influence the formation of an eruption plume, pyroclast ejection, and overall deposit morphology. Our results highlight the importance of (a) quantifying and determining the origin of CO, and (b) understanding the abundance of different H-species present within the lunar mantle. Quantifying the role of volatiles in driving lunar volcanism provides an important link between the interior volatile content of the Moon and the formation of volcanic deposits on the lunar surface.

¹Sylvain Piqueux,¹Tuan H. Vu,¹Jonathan Bapst,²Laurence A. J. Garvie,¹Mathieu Choukroun,³Christopher S. Edwards
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2021JE007003]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
²Center for Meteorite Studies, Arizona State University, Tempe, AZ, USA
³Department of Astronomy and Planetary Sciences, Northern Arizona University, Flagstaff, AZ, USA
Published by arrangement with John Wiley & Sons

Specific heat capacity Cp(T) is an intrinsic regolith property controlling planetary surface temperatures along with the albedo, density, and thermal conductivity. Cp(T) depends on material composition and temperature. Generally, modelers assume a fixed specific heat capacity value, or a standard temperature dependence derived from lunar basalts, mainly because of limited composition-specific data at low temperatures relevant to planetary surfaces. In addition, Cp(T) only appears to vary by a small factor across various materials, in contrast with the bulk regolith thermal conductivity, which ranges over ∼3–4 orders of magnitude as a function of the regolith physical state (grain size, cementation, sintering, etc.). For these reasons, the impact of the basaltic assumption on modeled surface temperature is often considered unimportant although this assumption is not particularly well constrained. In this paper, we present specific heat capacity measurements and parameterizations from ∼90 to ∼290 K of 28 meteorites including those possibly originating from Mars and Vesta, and covering a wide range of planetary surface compositions. Planetary surface temperatures calculated using composition-specific Cp(T) are within urn:x-wiley:21699097:media:jgre21756:jgre21756-math-00012 K of model runs assuming a basaltic composition. This urn:x-wiley:21699097:media:jgre21756:jgre21756-math-00022 K range approaches or exceeds typical instrumental noise or other sources of modeling uncertainties. These results suggest that a basaltic assumption for Cp(T) is generally adequate for the thermal characterization of a wide range of planetary surfaces, but possibly inadequate when looking at leveraging subtle trends to constrain subsurface layering, roughness, or seasonal/diurnal volatile transfer.