Eruptive style on Mercury, constraints from spectral analysis

Introduction 

The explosive volcanism of a planet is a key process for understanding its thermal and geochemical evolution and for assessing its volatile budget over time. The MESSENGER (MErcury’s Surface, Space ENvironment GEochemistry and Ranging) spacecraft highlighted products of Mercury's explosive activity. Pit landforms are interpreted to be endogenic pyroclastic vents on the basis of their irregular and often elongated morphology and their lack of raised rim [1]. These endogenic pits are often surrounded by high albedo deposits with diffuse borders, named faculae. They are interpreted as pyroclastic deposits formed by the fragmentation and ejection of magma particles from the central volcanic vent. Faculae exhibit redder spectral slope and stronger downturn of reflectance in the ultraviolet (UV) than their background terrains [2]. Here, we present a detailed spectral analysis of 26 faculae using all the available data of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard MESSENGER. In order to constrain the eruptive style and the pyroclast composition, we investigated the spectral variability between the faculae. 

Material and method

We used the MASCS Derived Data Record (DDR) data products available on the Planetary Data System (PDS). These data have been radiometrically and photometrically calibrated by the MESSENGER team. Additional processing, using the method developed by [3], is applied to the data to obtain a continuous spectrum from 300 to 1450 nm. The MASCS observations are filtered by instrument temperature and incidence angle. The data carried out in the highest temperature regime of the instrument (temperature exceeding 40°C) are discarded. Observations made at incidence angles greater than 75° have also been removed in order to limit variations due to photometry at high phase angles. With the aim to highlight spectral properties of faculae, we computed spectral parameters: the UV-downturn [2], the reflectance at 750 nm (R750) and the slope in the visible (VIS-slope) [4]. The parameters UV-downturn and VIS-slope are normalized by the Mercury’s reference spectrum [5], thus the VIS-slope and UV-downturn are respectively equal to 1 and 3.0 (reevaluated at 3.1 by Besse et al., 2020) for the average surface of Mercury. 

The faculae with the highest number of observations, spatial coverage and spatial distribution of observations have been selected for this study [4]. The size of 26 of them has been determined with MASCS data allowing optimal spectral analysis [4]. As in [6], the spectral properties of the faculae are taken at the midpoint between the limit of the vent and edges of the facula. This allows to minimize the issue due to the intrinsic variability across the faculae (spectral parameters decrease from the limit of the vent to the edge of the facula) [3,6]. 

Results and discussion

The 26 faculae are located over a latitudinal range between 60°S and 50°N and a longitudinal range between -165°E and 160°E (Fig. 1). About three quarters of them are located within impact craters.

Figure 1: Location of the faculae analyzed here. The color code is the crater host diameter. The white faculae are not located within impact craters. 

Spectral analysis revealed that faculae located within the same impact crater have close values of spectral parameters (Fig. 2). This finding may be explained by 1) same magmatic source between the nearby faculae or 2) large contributions of country rocks in the pyroclastic material. The first hypothesis has already been observed in lunar pyroclastic deposits within the Lavoisier impact crater which share similar spectral features indicating similar mineralogical compositions. The second hypothesis which suggests a low fraction of melt in rising magmas, is supported by the high volatile content needed to emplace. In addition, topographic data and modelling suggest that faculae are very thin blankets, which implies a low volume of material consistent with the hypothesis that the faculae are mainly composed of country rocks. 

Nathair facula, which is the largest pyroclastic deposit on Mercury, appears as an endmember in the visible, with the highest values of R750 and VIS-slope, at the midpoint of the facula (Fig. 2) . The high reflectance of the Nathair facula in the visible may be due to finer grain size [7]. High dispersal and high degree of fragmentation of the pyroclasts are representative of phreatomagmatic eruptive style on Earth. The phreatomagmatic-like process occurs when hot-magma interacts with a volatile-rich surficial or subsurface volatile layer. The proximity of the Nathair facula to a carbon-enriched layer could explain this phreatomagmatic-like event.

Figure 2: Parameter spaces at the midpoint of the faculae in the visible and ultraviolet domains from [4]. The black dashed lines represent the Mercury mean spectrum (Izenberg et al., 2014). The error bars correspond to the standard deviation of the MASCS observations in each facula. The color code represents the host crater diameter (km). a) UV-downturn versus R750 at the midpoint of the facula. b) UV-Downturn versus VIS-slope at the midpoint of the facula.

Conclusion 

Spectral analysis of 26 faculae at the surface of Mercury revealed that the hermean pyroclastic deposits may consist of a large fraction of country rock compared to juvenile magma. This hypothesis suggests that rising magmas are mainly composed of gas. The physical and spectral characteristics of the Nathair facula suggest emplacement by a phreatomagmatic-like event.

Future observations of the BepiColombo/ESA-JAXA mission will help to test these hypotheses about Mercury's explosive activity. Combined observations from BELA, SIMBIOS-SYS, MERTIS, MGNS and MIXS could constrain the volume of the faculae, the grain size (and thus the degree of fragmentation) and the composition of the pyroclasts (e.g. percentage of volcanic glass, mineralogical composition) in order to better determine the eruptive dynamics of explosive activity on Mercury. 

References 

[1] Thomas, R. et al., (2014), J. Geophys. Res. Planets 119. [2] Goudge, T. A. et al., (2014), J. Geophys. Res. Planets, 119. [3] Besse, S. et al., (2015), J. Geophys. Res. Planets, 120. [4] Barraud, O. et al (submitted). Icarus. [5] Izenberg, N.R. et al. (2014). Icarus, 228. [6] Besse, S. et al., (2020), J. Geophys. Res. Planets, 125 [7] Crown, D.A. and Pieters, C.M. (1987). Icarus, 72.