Multiple volcanic deposits, both pyroclastic and effusive, have been identified on the surface of Mercury. The modeling of volcanic processes on Mercury, particularly with respect to the amount and composition of volcanic volatiles, is hindered by a lack of existing experiments performed at or near Mercurian conditions. Most notable is Mercury’s extremely reducing nature (3–8 log units below the iron-wüstite buffer), which is well beyond the range of fO2s for which existing thermodynamic models are calibrated. The high carrying capacity of sulfur compared to carbon or hydrogen in reduced magmas, combined with remarkably high S concentrations in Mercurian surface materials, has led to the assumption that S is an important driver of volcanic activity on the planet. However, evidence for a primary graphite floatation crust as well as graphite present within Mercury’s regolith provide a mechanism for C-rich gas production via magma-graphite smelting reaction. Smelting, in which graphite is oxidized to CO and CO2 gas and melt oxide species are reduced to metal (e.g., Cgraphite + FeOmelt = COgas + Fe0metal), would also serve to remove O from the silicate melt, consistent with the production of a remarkably reduced surface environment. We carried out experiments to emulate conditions for graphite-induced smelting of three Mercurian magma compositions at high temperatures (ramped from ambient to 1195–1390 °C) and low pressures (8–10 mbar). The compositions of resultant gases were measured in situ via an evolved gas analyzer, and solid run products were analyzed by electron microprobe. Degassing vapor was always dominated by C-O-H species, and S degassing was not detected in any experiments. No significant C releases were measured in experiments using transition metal oxide-free starting silicate compositions, suggesting that transition metal reduction may be required to oxidize graphite to gas. Experiments that produced vapor formed Fe-Si metal alloy blebs, which were always in contact with residual graphite, strongly supporting metal and gas production via smelting between graphite and melt. Our results indicate that CO and CO2 are likely the most dominant volcanic degassers (and thus drivers of explosive volcanism) on Mercury, and that S degassing plays only a subordinate role, contrary to what has been hitherto assumed.

All material that is collected from Mars (gases, dust, rock, regolith) will need to be carefully handled, stored, and analyzed following Earth return to minimize the alteration or contamination that could occur, and to maximize the scientific information that can be extracted from the samples, now and into the future. A Sample Receiving Facility (SRF) would be where the Earth Entry System is opened, and the sample tubes opened and processed after they land on Earth. The Mars Sample Return (MSR) Science Planning Group Phase 2 (MSPG2) was tasked with identifying the steps that encompass the curation activities that would happen within an MSR SRF and any anticipated curation-related requirements. To make the samples accessible for scientific investigation, a series of observations and preliminary analytical measurements would need to be completed to produce a sample catalog for the scientific community. The sample catalog would provide data to make informed requests for samples for scientific investigations and for the approval of allocations of appropriate samples to satisfy these requests. The catalog would include data and information generated during all phases of activity, including data derived from the landed Mars 2020 mission, during sample retrieval and transport to Earth, and upon receipt within the SRF, as well as through the initial sample characterization process, sterilization- and time-sensitive and science investigations. The Initial sample characterization process can be divided into three phases, with increasing complexity and invasiveness: Pre-Basic Characterization (Pre-BC), Basic Characterization (BC), and Preliminary Examination (PE). A significant portion of the Curation Focus Group’s efforts was determining which analyzes and thus instrumentation would be required to produce the sample catalog and how and when certain instrumentation should be used. The goal is to provide enough information for the PIs to request material for their studies but to avoid doing targeted scientific research better left to peer-reviewed competitive processes. Disclaimer: The decision to implement Mars Sample Return will not be finalized until NASA’s completion of the National Environmental Policy Act (NEPA) process. This document is being made available for planning and information purposes only.