Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

Abstract

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Spectroscopic properties of explosive volcanism within the Caloris basin with MESSENGER observations

“…In fact, the UV spectral properties of the pyroclastic deposits are clustered and separated from Mercury's background. This has been already shown in previous studies [ Goudge et al , ; Izenberg et al , ] for different pyroclastic deposits. However, it is important to note that all the deposits analyzed in this study and within the same basin are separated from the average surface, supporting the fact that they should indeed be pyroclastic in origin.…”

“…The figure compiles the candidates from Kerber et al [] and Thomas et al []. The red dots correspond to candidates identified by Thomas et al [], the yellow dots candidates identified by Kerber et al [, ] only, and the blue dot is the candidate analyzed by Goudge et al [] that has not been identified by Thomas et al []. Candidates 13, 21, and 38, although marked in red, have been also analyzed by Goudge et al [].…”

Phase-ratio images of the surface of Mercury: Evidence for differences in sub-resolution texture

“…Since then more than 50 examples have been cataloged (Kerber et al, 2011;Goudge et al, 2014). The deposits are generally higher in reflectance than their surroundings and have an overall spectral slope in the visible to NIR that is steeper (''redder'') than that of the average spectrum for Mercury's surface Murchie et al, 2008;Blewett et al, 2009).…”

“…Of particular interest for Mercury are hollows (Blewett et al, , 2013Thomas et al, 2014), pyroclastic deposits (Head et al, 2008;Kerber et al, 2011;Goudge et al, 2014), and a prominent flow of dark impact melt (Klima et al, 2011;D'Incecco et al, 2013). A brief description of the features considered in this report is given in the next section.…”

Global Distribution and Spectral Properties of Low‐Reflectance Material on Mercury

“…Prior to MESSENGER, Mercury's giant core and apparently low ferrous iron surface suggested that materials that accreted and then differentiated to form Mercury were extremely reduced, with an oxygen fugacity 3-5 orders of magnitude below that of the Moon (e.g., McCubbin et al, 2012;Zolotov, 2011). MESSENGER imaging revealed landforms for whose formation volatiles were critical, including pyroclastic vents (e.g., Goudge et al, 2014;Kerber et al, 2011) and hollows ; the latter are extremely fresh features formed by scarp retreat, probably due to loss of volatiles (Blewett et al, , 2016Thomas et al, 2014a). MESSENGER imaging revealed landforms for whose formation volatiles were critical, including pyroclastic vents (e.g., Goudge et al, 2014;Kerber et al, 2011) and hollows ; the latter are extremely fresh features formed by scarp retreat, probably due to loss of volatiles (Blewett et al, , 2016Thomas et al, 2014a).…”