The low-iron, reduced surface of Mercury as seen in spectral reflectance by MESSENGER

Izenberg, N. R.; Klima, R. L.; Murchie, S. L.; Blewett, D. T.; Holsclaw, Gregory M.; McClintock, W. E.; Malaret, E.; Mauceri, Calogero; Vilas, F.; Sprague, A. L.; Helbert, J.; Domingue, D. L.; Head, J. W.; Goudge, T. A.; Solomon, Sean C.; Hibbitts, C. A.; Dyar, M. D.

doi:10.1016/j.icarus.2013.10.023

Cited by 88 publications

(157 citation statements)

References 57 publications

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“…We calculated spectral slope (VISr, the ratio of reflectance at 430 nm to that at 750 nm) by division of one image by the other. This measure of visible spectral slope is comparable with that used for VISr in MASCS data by Izenberg et al (2014Izenberg et al ( , 2015 (415 nm / 750 nm) (Figure 1). We coregistered higher-resolution NAC images with the WAC data, and used these to identify regions that correspond to the specific unit types listed in Table 2 (Figure 3).…”

Section: Spectral Analysissupporting

confidence: 88%

“…Analyses have therefore relied on spectral reflectance data at low spectral but moderately high spatial resolution from the Mercury Dual Imaging System (MDIS) (11 bands across 433 -1010 nm, down to < 100 m/pixel) [e.g., Blewett et al, 2011Blewett et al, , 2013Vilas et al, 2016] and at high spectral but low spatial resolution from the Visible and Infrared Spectrograph (VIRS) component of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) (5-nm resolution across 300 -1450 nm, footprint size ≥ 0.1 km cross-track and 3 km along-track) [Izenberg et al, 2015]. However, the relatively featureless character of Mercury's surface spectra presents an obstacle to direct determination of composition: absorption bands used to determine mineralogy on other planetary surfaces and in terrestrial laboratory experiments are weakly developed even in fresh material due to a low abundance of ferrous iron [Hapke et al, 1975;Vilas, 1985;Warell, 2003;Izenberg et al, 2014], and are further weakened by rapid optical maturation in the high-energy space environment at Mercury [Lucey and Riner, 2011;Riner and Lucey, 2012].…”

Section: Introductionmentioning

confidence: 99%

“…This hypothesis has recently gained further support from observations of anomalously high exospheric calcium above the extensively-hollowed Tyagaraja crater [Bennett et al, 2016]. Laboratory experiments with magnesium, calcium and manganese sulfides do indicate that sulfides can be volatilized at the daytime temperatures experienced at Mercury's surface, and that extreme thermal cycling on the planet's surface could account for the lack of characteristic absorption bands [Helbert et al, , 2013], but have not provided a spectral match to BCFDs, LRM, or indeed any surface unit on Mercury [Blewett et al, 2013;Izenberg et al, 2014].…”

Section: Introductionmentioning

confidence: 99%

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Mercury's low-reflectance material: Constraints from hollows

Thomas

Hynek

Rothery

et al. 2016

Icarus

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Highlights We investigate the composition of Mercury's deep volatile-rich low-reflectance layer. Reflectance spectra of lag on sublimation hollow floors within it provide constraints. Analysis supports volatile (Mg,Ca) sulfides and non-volatile graphite within LRM. Unusually low reflectance material, within which depressions known as hollows appear to be actively forming by sublimation, is a major component of Mercury's surface geology. The observation that this material is exhumed from depth by large impacts has the intriguing implication that the planet's lower crust or upper mantle contains a significant volatile-rich, lowreflectance layer, the composition of which will be key for developing our understanding of Mercury's geochemical evolution and bulk composition. Hollows provide a means by which the composition of both the volatile and non-volatile components of the low-reflectance material (LRM) can be constrained, as they result from the loss of the volatile component, and any remaining lag can be expected to be formed of the non-volatile component. However, previous work has approached this by investigating the spectral character of hollows as a whole, including that of bright deposits surrounding the hollows, a unit of uncertain character. Here we use high-resolution multispectral images, obtained as the MESSENGER spacecraft approached Mercury at lower altitudes in the latter part of its mission, to investigate reflectance spectra of inactive hollow floors where sublimation appears to have ceased, and compare this to those of the bright surrounding products and the parent material. This analysis reveals that the final lag after hollow-formation has a flatter spectral slope than that of any other unit on the planet and reflectance approaching that of more space-weathered parent material. This indicates firstly that the volatile material lost has a steeper spectral slope and higher reflectance than the parent material, consistent with (Ca,Mg) sulfides, and secondly, that the low-reflectance component of LRM is non-volatile and may be graphite.

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Section: Spectral Analysissupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Mercury's low-reflectance material: Constraints from hollows

Thomas

Hynek

Rothery

et al. 2016

Icarus

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“…High sulfur concentrations in Mercurian lavas may result either from high sulfur solubility in magmas (Zolotov et al, 2013) and/or from transport of sulfide droplets from the mantle source regions to the surface of the planet (Malavergne et al, 2014). The first hypothesis is consistent with the absence of spectral evidence for sulfide minerals in surface rocks (McClintock et al, 2008;Izenberg et al, 2014) while the second could explain the correlations between S and Ca-Mg observed in Mercurian lavas . Understanding the origin of high sulfur concentrations at the surface of Mercury is important to better constrain the structure of the planet and the distribution of sulfur amongst the different reservoirs (mantle, core and crust), the mechanisms of explosive volcanism (Kerber et al, 2009;Thomas et al, 2014a;Weider et al, 2016), and the formation of the hollows (sub-kilometer scale shallow depressions surrounded by bright deposits) which may have formed during sublimation of volatiles (Blewett et al, 2013;Thomas et al, 2014b).…”

Section: Introductionmentioning

confidence: 77%

“…This implies that their high sulfur content is related to high sulfur solubility in the mafic magma, which probably explains the absence of spectral evidence for a sulfide phase at the Mercurian surface (McClintock et al, 2008;Izenberg et al, 2014).…”

Section: Transport Of Sulfur From the Mantle To The Mercurian Surfacementioning

confidence: 99%

Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury

Namur

Charlier

Holtz

et al. 2016

Earth and Planetary Science Letters

118

160

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Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the speciation and distribution of sulfur on Mercury, we performed high temperature (1200-1750 • C), lowto high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S 2− and forms complexes with Fe 2+ , Mg 2+ and Ca 2+ . The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from <1 wt.% S at IW-2 to >10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity ( f O 2 ), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW-5.4 ± 0.4. We also calculate that the mantle of Mercury contains 7-11 wt.% S and that the metallic core of the planet has little sulfur (<1.5 wt.% S). The external part of the Mercurian core is likely to be made up of a thin (<90 km) FeS layer.

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

et al. 2014

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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.

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The low-iron, reduced surface of Mercury as seen in spectral reflectance by MESSENGER

Cited by 88 publications

References 57 publications

Mercury's low-reflectance material: Constraints from hollows

Mercury's low-reflectance material: Constraints from hollows

Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury

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

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