A. D. Rogers

Morphological, structural, and spectral characteristics of amorphous iron sulfates

Current or past brine hydrologic activity on Mars may provide suitable conditions for the formation of amorphous ferric sulfates. Once formed, these phases would likely be stable under current Martian conditions, particularly at low- to mid-latitudes. Therefore, we consider amorphous iron sulfates (AIS) as possible components of Martian surface materials. Laboratory AIS were created through multiple synthesis routes and characterized with total X-ray scattering, thermogravimetric analysis, scanning electron microscopy, visible/near-infrared (VNIR), thermal infrared (TIR), and Mössbauer techniques. We synthesized amorphous ferric sulfates (Fe(III)2(SO4)3 · ~ 6-8H2O) from sulfate-saturated fluids via vacuum dehydration or exposure to low relative humidity (<11%). Amorphous ferrous sulfate (Fe(II)SO4 · ~1H2O) was synthesized via vacuum dehydration of melanterite. All AIS lack structural order beyond 11 Å. The short-range (<5 Å) structural characteristics of amorphous ferric sulfates resemble all crystalline reference compounds; structural characteristics for the amorphous ferrous sulfate are similar to but distinct from both rozenite and szomolnokite. VNIR and TIR spectral data for all AIS display broad, muted features consistent with structural disorder and are spectrally distinct from all crystalline sulfates considered for comparison. Mössbauer spectra are also distinct from crystalline phase spectra available for comparison. AIS should be distinguishable from crystalline sulfates based on the position of their Fe-related absorptions in the visible range and their spectral characteristics in the TIR. In the NIR, bands associated with hydration at ~1.4 and 1.9 μm are significantly broadened, which greatly reduces their detectability in soil mixtures. AIS may contribute to the amorphous fraction of soils measured by the Curiosity rover.

Thermal and near-infrared analyses of central peaks of Martian impact craters: Evidence for a heterogeneous Martian crust

Central peaks of impact craters contain materials exhumed from depth, and therefore, investigation of these materials provides clues to subsurface geology and mineralogy. A global spectral survey of central peaks of Martian impact craters between 10 and 200 km diameter was completed using Mars Odyssey Thermal Emission Imaging System (THEMIS) data. Twenty-six central peaks with distinctive spectral signatures from surrounding plains were identified and characterized with thermal infrared and visible/near-infrared data. The distribution of spectrally distinct central peaks (SDCPs) shows some degree of regional clustering, with most craters found in western Noachis Terra, Tyrrhena Terra, within the northern rim of Hellas Basin, and fewer in the northern lowlands. With the exception of four craters in western Noachis Terra, SDCPs contain only one spectrally distinct unit at THEMIS resolution (100 m/pixel). The maximum number of spectrally distinct units observed was three, in Jones and Ostrov craters. The western Noachis Terra SDCPs may expose crustal stratigraphies of multiple igneous compositions or impact materials from Argyre. In the highlands, most SDCP units are consistent with enrichments in olivine or pyroxene relative to surrounding plains, suggesting olivine and pyroxene basaltic lithologies; few are olivine and pyroxene poor. No spatial trend in spectrally derived compositions of SDCPs was observed. Three SDCPs contain THEMIS signatures consistent with high abundances of phyllosilicates, which may contain the most phyllosilicate-rich lithologies found in central peak-associated materials globally.

Quantitative compositional analysis of sedimentary materials using thermal emission spectroscopy: 2. Application to compacted fine‐grained mineral mixtures and assessment of applicability of partial least squares methods

Fine-grained sedimentary deposits on planetary surfaces require quantitative assessment of mineral abundances in order to better understand the environments in which they formed. One way that planetary surface mineralogy is commonly assessed is through thermal emission (~6-50 µm) spectroscopy. To that end, we characterized the TIR spectral properties of compacted, very fine-grained mineral mixtures of oligoclase, augite, calcite, montmorillonite and gypsum. Non-negative linear least squares minimization (NNLS) is used to assess the linearity of spectral combination. A partial least squares (PLS) method is also applied to emission spectra of fine-grained synthetic mixtures and natural mudstones to assess its applicability to fine-grained rocks. The NNLS modeled abundances for all five minerals investigated are within ±10% of the known abundances for 39% of the mixtures, showing the relationships between known and modeled abundance follow non-linear curves. The poor performance of NNLS is due to photon transmission through small grains over portions of the wavelength range and multiple reflections in the volume. The PLS method was able to accurately recover the known abundances (to within +/-10%) for 78-90% of synthetic mixtures and for 85% of the mudstone samples chosen for this study. The excellent agreement between known and modeled abundances is likely due to high absorption coefficients over portions of the thermal infrared (TIR) spectral range, and thus combinations are linear over portions of the range. PLS can be used to recover abundances from very fine-grained rocks from TIR measurements, and could potentially be applied to landed or orbital TIR observations.

Thermal emission spectroscopy of microcrystalline sedimentary phases: Effects of natural surface roughness on spectral feature shape

Distinguishing betweenmicrocrystalline andmacrocrystalline mineral phases can help constrain the conditions under which those minerals formed or the degree of postdepositional alteration. This study demonstrates the effects of crystal size and surface roughness on thermal infrared emission spectra of micro and macrocrystalline phases of the two most common minerals on Earth, quartz and calcite. Given the characteristic depositional and environmental conditions under which microcrystalline minerals form, and the recent observations of high-silica deposits on Mars, it is important to understand how these unique materials can be identified using remote infrared spectroscopy techniques. We find that (a) microcrystalline minerals exhibit naturally rough surfaces compared to their macrocrystalline counterparts at the 10μm scale; and that (b) this roughness causes distinct spectral differences within the Reststrahlen bands of each mineral. These spectral differences occur for surfaces that are rough on the wavelength scale, where the absorption coefficient (k) is large. Specifically, the wavelength positions of the Reststrahlen features for microcrystalline phases are narrowed and shifted compared to macrocrystalline counterparts. The spectral shape differences are small enough that the composition of the material is still recognizable, but large enough such that a roughness effect could be detected. Petrographic and topographic analyses of microcrystalline samples suggest a relationship between crystal size and surface roughness. Together, these observations suggest it may be possible to make general inferences about microcrystallinity from the thermal infrared spectral character of samples, which could aid in reconstructions of sedimentary rock diagenesis where corresponding petrographic or microimaging is not available.