45th Lunar and Planetary Science Conference (2014) 1296.pdf

DOMINICI CRATER WALL HOLLOWS: POTENTIAL SPECTRAL EVIDENCE FOR SULFIDE MINERALOGY ON MERCURY. Faith Vilas1, Deborah L. Domingue1, Jörn Helbert2, Mario D'Amore2, Noam R. Izenberg3, Rachel L. Klima3, Karen R. Stockstill-Cahill4, and James W. Head5. 1Planetary Science Institute, 1700 E. Fort Lowell Rd., Suite 106, Tucson, AZ 85719, USA, [email protected]; 2DLR, Rutherfordstrasse 2, Berlin, Germany; 3The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA; 4Proxemy Research, Lay- tonsville MD, 20882, USA; 5Department of Geological Sciences, Brown University, Providence, RI 02912, USA.

Reflectance spectrophotometry acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is governed by the spectral resolution of, and spectral range across, data sets obtained with the Mercury Dual Imaging System (MDIS) and Mercury Atmospheric and Sur- face Composition Spectrometer (MASCS) instru- ments. Global and regional studies of Mercury's re- flectance properties show variations in spectral charac- teristics and tie them to surface geological processes [1, 2] and elemental compositions derived from the MESSENGER Gamma-Ray and Neutron Spectrome- ter (GRNS) and X-Ray Spectrometer (XRS) [3, 4]. We have undertaken a search for spectral evidence of surface mineralogy related to specific geologic fea- tures. Mercury’s "hollows" are shallow, irregular, rim- less, flat-floored depressions with high-reflectance interiors and halos [5,6,7], are fresh in appearance, and have reflectance that increases less steeply with in- Figure 1. NAC high-resolution image creasing wavelength than the general spectral reflec- (CN0253965560M_RA_4) of Dominici crater (1.4°N, tance of Mercury. Hollows are generally associated 323.5°E; ~20 km diameter ). The hollows on the south with crater walls, floors, and peaks and represent one wall of the crater have the highest reflectance. of the distinctive classes of features discovered on Mercury's surface [5]. Early during the spacecraft’s first Earth year in orbit, MESSENGER imaged the crater Dominici (1.4°N, 323.5°E) using both the nar- row-angle camera (NAC) and wide-angle camera (WAC) on MDIS. Figure 1 shows a high-resolution image of Dominici crater obtained with the NAC. Two areas of hollows are apparent, in the center and on the south wall of the crater. The hollows located on the south crater wall have brighter halos and well- defined depressions compared with the hollows in the center of Dominici. This difference suggests that these two sets of hollows have undergone different histories of environmental processing [8,9]. We report here on the results of the examination of the south crater wall/rim photometry. A color mosaic of the areas we sampled near Titian and Dominici craters in the Homer basin region is Figure 2. Color mosaic of regions sampled near Titian shown in Fig. 2. The MDIS WAC images provide and Dominici craters. The red, green, and blue chan- photometry across the visible/near-infrared spectral nels are the reflectance at 996 nm, 749 nm, and 433 range, although they lack the spatial resolution pro- nm, respectively, derived from MDIS WAC filters. vided by the NAC. Seeking to isolate the hollows The regions sampled include intercrater plains (yellow spatially in the WAC 8-filter color-image sets, we outline), interior crater smooth plains (Titian, green), identified a range of latitude and longitude coordinates bright crater ejecta (blue), interior hollows (Dominici, that outline and define this hollows region. We applied red), and brighter crater wall hollows (Dominici, pur- these constraints to WAC 8-filter color-image sets ple). 45th Lunar and Planetary Science Conference (2014) 1296.pdf

containing Dominici crater with spatial coverage at least 6 pixels in areal extent, and we hand-picked the pixels from each set to sample the hollows on the floor and southern wall/rim of Dominici in addition to sam- pling regions within the local ejecta field. The reflec- tance data (expressed as I/F, where I is light reflected from Mercury's surface and F is incident sunlight) have been corrected for global geometric effects. We identify an absorption feature in two color sets of the bright wall hollows of Dominici crater ranging in wavelength from near 480 to 630 nm, centered near 559 nm (Fig. 3). Figure 4 compares the I/F for the

Dominici bright wall hollows with three nearby areas Figure 3. Scaled spectral reflectance (R = 1.0 at 559 external to the crater, within one of the color-image nm) for two WAC color sets (D filter images sets examined. We confirm that the lower-spatial- CW0210935614D_IF_4 (red), CW0210936146D_IF_4 resolution global color mosaic also shows an absorp- (blue) from each of the two sets). The spectral cover- tion feature for the bright wall/rim hollows. We are unable to confirm the feature with existing MASCS age of the potential feature estimated to be near 559 spectra as the MASCS observational footprints en- nm is marked by the green horizontal bar beginning at compass enough surrounding darker terrain to dimin- a lower wavelength of 470 nm. ish any potential absorption feature. Targeted MASCS spectral transects focused on the Dominici wall/rim hollows, and for which inclusion of surround- ing terrain within the instrument footprint is mini- mized, have been scheduled. On first look, this absorption feature is consistent with laboratory spectra of heated sulfide powders, such as oldhamite (CaS), as analogs for Mercury's surface material [10,11]. The spectral range is consis- tent with the feature present in spectra for CaS, which has been identified as a potential sulfur-bearing phase for Mercury on the basis of the correlation between the elemental abundances of sulfur and calcium as ob- served by the XRS [3,4] . The laboratory absorption Figure 4. I/F for one WAC color set (D filter image feature for heated CaS is shallower, however, than the CW0210935614D_IF_4) comparing (a) brighter absorption observed in the MDIS color data. One pos- Dominici crater wall hollows (red), (b) bright crater sible explanation for this difference in band depth is a ejecta (blue), (c) Titian interior crater smooth plains difference in particle size between the material on (green), and (d) intercrater plains (yellow). Mercury and the laboratory sample. Generation of sulfides as a result of impact melt formation on Mer- cury has been extensively modeled [12,13]. As a result References: [1] Denevi B. W. et al. (2013) JGR Plan- of this modeling, CaS is predicted to be the major ets, 118, 1–17. [2] Izenberg N. R. et al. (2014) Icarus lithophile sulfide on Mercury [13]. Collectively, these 228, 364-374. [3] Nittler L. R. et al. (2011) Science, lines of investigation support the identification of CaS 333, 1847–1850. [4] Weider S. Z. et al. (2012) JGR, as part of the bright wall/rim hollows material. 117, E00L05. [5] Blewett D. T, et al. (2011) Science, 333, 1856–1859. [6] Blewett D. T. et al. (2013) JGR Planets, 118, 1013-1032. [7] Thomas R. J. et al. (2014), Icarus, 229, 221–235. [8] Hawke B. R. and Head J.W. (1977) in Impact and Explosion Cratering (D. J. Roddy et al., Eds.) pp. 815-841. [9] Beach M. J. et al. (2012) LPS, 43, abstract 1335. [10] Helbert J. et al. (2013) EPSL, 369-370 233-238. [11[ Burbine T. H. et al. (2002), MPS, 37, 1233–1244. [12] Vaughan W. M. et al. (2012) LPS, 43, abstract 1187. [13] Vaughan W. M. et al. (2013) LPS, 44, abstract 2013.