Mawrth Vallis contains one of the largest exposures of phyllosilicates on Mars. Nontronite, montmorillonite, kaolinite, and hydrated silica have been identified throughout the region using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). In addition, saponite has been identified in one observation within a crater. These individual minerals are identified and distinguished by features at 1.38–1.42, ∼1.91, and 2.17–2.41 μ m. There are two main phyllosilicate units in the Mawrth Vallis region. The lowermost unit is nontronite bearing, unconformably overlain by an Al‐phyllosilicate unit containing montmorillonite plus hydrated silica, with a thin layer of kaolinite plus hydrated silica at the top of the unit. These two units are draped by a spectrally unremarkable capping unit. Smectites generally form in neutral to alkaline environments, while kaolinite and hydrated silica typically form in slightly acidic conditions; thus, the observed phyllosilicates may reflect a change in aqueous chemistry. Spectra retrieved near the boundary between the nontronite and Al‐phyllosilicate units exhibit a strong positive slope from 1 to 2 μ m, likely from a ferrous component within the rock. This ferrous component indicates either rapid deposition in an oxidizing environment or reducing conditions. Formation of each of the phyllosilicate minerals identified requires liquid water, thus indicating a regional wet period in the Noachian when these units formed. The two main phyllosilicate units may be extensive layers of altered volcanic ash. Other potential formational processes include sediment deposition into a marine or lacustrine basin or pedogenesis.
Introduction: Spectral analysis of the Libya Montes/Isidis Planitia region using OMEGA spectra noted outcrops of olivine [1]. Observations by CRISM revealed a range of minerals in Libya Montes including olivine, pyroxene, and phyllosilicate minerals [2, 3]. Our spectral analyses of CRISM images in Libya Montes have identified carbonates as well [here and in 4]. We have also prepared laboratory mixtures of phyllosilicate-carbonate, carbonate-olivine, phyllosilicateolivine, and ternary samples and measured their spectra in order to better understand the Libya Montes spectra containing these compounds. Libya Montes is located on the southern rim of the Isidis Basin. Both the Isidis Basin and the mountains of Libya Montes formed in response to a large impact 4.6 – 3.5 Ga [5]. The rocks of Libya Montes are therefore very old and this site was chosen as the study area because it may hold clues for early Martian history. Different mineralogical units are exposed in Libya Montes due to the influences of wind, water, and smaller impact craters. We discuss these mineralogical units and their relationships. Methods: CRISM acquires spectral images ~10 km wide from 0.4 – 4.0 μm in 544 wavelengths with two detectors, one in the visible and one in the infrared, to produce images of 18 meter per pixel [6]. CRISM images were then processed to remove noise [7]. Mineral indicator maps were generated using band parameters selected to highlight specific mineral types [8] in order to identify likely minerals. Results and Discussion: Phyllosilicates, carbonates, olivine, and pyroxene were identified in CRISM images of Libya Montes in this study (e.g. Fig. 1).
Martian aqueous mineral deposits have been examined and characterized using data acquired during Mars Reconnaissance Orbiter's (MRO) primary science phase, including Compact Reconnaissance Imaging Spectrometer for Mars hyperspectral images covering the 0.4–3.9 μ m wavelength range, coordinated with higher–spatial resolution HiRISE and Context Imager images. MRO's new high‐resolution measurements, combined with earlier data from Thermal Emission Spectrometer; Thermal Emission Imaging System; and Observatoire pour la Minéralogie, L'Eau, les Glaces et l'Activitié on Mars Express, indicate that aqueous minerals are both diverse and widespread on the Martian surface. The aqueous minerals occur in 9–10 classes of deposits characterized by distinct mineral assemblages, morphologies, and geologic settings. Phyllosilicates occur in several settings: in compositionally layered blankets hundreds of meters thick, superposed on eroded Noachian terrains; in lower layers of intracrater depositional fans; in layers with potential chlorides in sediments on intercrater plains; and as thousands of deep exposures in craters and escarpments. Carbonate‐bearing rocks form a thin unit surrounding the Isidis basin. Hydrated silica occurs with hydrated sulfates in thin stratified deposits surrounding Valles Marineris. Hydrated sulfates also occur together with crystalline ferric minerals in thick, layered deposits in Terra Meridiani and in Valles Marineris and together with kaolinite in deposits that partially infill some highland craters. In this paper we describe each of the classes of deposits, review hypotheses for their origins, identify new questions posed by existing measurements, and consider their implications for ancient habitable environments. On the basis of current data, two to five classes of Noachian‐aged deposits containing phyllosilicates and carbonates may have formed in aqueous environments with pH and water activities suitable for life.
Observations by the Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars in the Mawrth Vallis region show several phyllosilicate species, indicating a wide range of past aqueous activity. Iron/magnesium (Fe/Mg)–smectite is observed in light-toned outcrops that probably formed via aqueous alteration of basalt of the ancient cratered terrain. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that may have formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry. A spectral feature attributed to an Fe 2+ phase is present in many locations in the Mawrth Vallis region at the transition from Fe/Mg-smectite to aluminum/silicon (Al/Si)–rich units. Fe 2+ -bearing materials in terrestrial sediments are typically associated with microorganisms or changes in pH or cations and could be explained here by hydrothermal activity. The stratigraphy of Fe/Mg-smectite overlain by a ferrous phase, hydrated silica, and then Al-phyllosilicates implies a complex aqueous history.
