Spectral Properties and Physical Extent of Pyroclastic Deposits on Mercury: Variability Within Selected Deposits and Implications for Explosive Volcanism

Abstract

Explosive volcanism on the surface of planet Mercury is visible through the pyroclastic deposits that surround morphologic features often identified as the vent. Those deposits are known as faculae. The understanding of explosive volcanism provides important information on Mercury's geological, thermal, and volcanic history. Observations by the MESSENGER spacecraft are used to analyze in detail the spectral properties of 14 selected faculae with the aim of understanding their chemical and physical properties. Scientific observations obtained by the MASCS instrument are particularly suitable for this task, although their observational and geometrical constraints limit definitive conclusions. Nonetheless, spectral properties in the visible, ultraviolet and near-infrared indicate that the selected faculae are probably larger than visible in images solely. Spectral parameters provide a means to isolate Mercury's pyroclastic deposits with respect to Mercury's average spectral behavior. The similar spectral behavior of the visible, ultraviolet and near-infrared domains suggests that the amount of mixing of pyroclastic materials with the underlying material, the differences in grain sizes between and inside the faculae, and the presence of opaque/mineral phases, could play significant roles in the spectral properties observed. Observations by the BepiColombo mission in nadir configuration covering a large range of phase angles will be highly complementary to the MESSENGER observations. Plain Language SummaryThe presence of volcanism on Mercury has been confirmed from observations taken by the NASA MESSENGER mission in 2011. As on Earth, various styles of volcanism have been detected; explosive volcanism which involves a low volume of lava and high volume of gas, and effusive volcanism, which is richer in lava and poorer in gas. Using observations in the near-infrared and visible spectral domain, this analysis aims at better characterizing the physical and chemical properties of the deposits resulting from explosive volcanism. In this manuscript, it is shown that the scale of deposits resulting from explosive volcanism has been underestimated, which cascades to a potential underestimation of the quantity of gas present in the interior of Mercury through its history. Additionally, it is shown that the amount of mixing of pyroclastic materials with the underlying material, the differences in grain sizes between and inside the faculae, and the presence of opaque/mineral phases, could play significant roles in the spectral properties observed in near infrared and visible spectral domain. Unfortunately, the limitations in the measurements from MESSENGER complicate the exploration of physical and chemical properties. These issues will be better explored with the BepiColombo mission, the next mission to explore the surface of Mercury.

Near‐Ultraviolet to Near‐Infrared Spectral Properties of Hollows on Mercury: Implications for Origin and Formation Process

“…Besse et al. (2015, 2020) demonstrated that the NIR channel could be analyzed with reasonable confidence in the study of pyroclastic deposits.…”

“…By definition, slopes and curvature are equal to 1 for the average surface of Mercury. The UV-downturn parameter, previously estimated to 3.0 for Mercury's background using Izenberg et al's (2014) reference spectra, was re-evaluated as 3.1 by Besse et al (2020). (Izenberg et al, 2014).…”

“…Moreover, spectral properties of faculae were studied by Goudge et al (2014) and Besse et al (2015) using the MASCS spectrometer. Besse et al (2020) determined the diameter of 14 faculae, and 12 of them ( Figure 4 and Table S3) have instrumental conditions (i.e., temperatures, see Section 2.1) similar to those determined suitable in this analysis, and are thus used for comparison to hollows.…”

Rationale for BepiColombo Studies of Mercury’s Surface and Composition

“…Considerations of recharge apply to the site of magmagenesis if the volatiles arrive with the magma, or to some layer in the crust (or even just the regolith) if the ascending magma gains its volatiles from those units during approach to the surface. Analysis of faculae using MESSENGER's non-imaging spectrometer (MASCS) demonstated the value of spectral parameters in defining the limits of these diffuse-edged features and confirms that they are larger than was apparent when first identified on MES-SENGER fly-by images (Besse et al 2020). High-resolution MASCS data are limited to discrete profiles (usually oriented close to north-south), so VIHI imaging spectroscopy is likely to be valuable in better constraining facula dimensions.…”

“…5 map), which before it received its formal name was usually known as NE Rachmaninoff. This facula is at least 140 km in radius (Besse et al 2020), making it the largest recognised pyroclastic deposit on Mercury. Nathair Facula was overflown by MES-SENGER during a targeted 'staring mode' at a time when X-ray fluorescence was strongly increased because of a powerful solar flare.…”