A. M. Ollila

Martian Fluvial Conglomerates at Gale Crater

Going to Mars The Mars Science Laboratory spacecraft containing the Curiosity rover, was launched from Earth in November 2011 and arrived at Gale crater on Mars in August 2012. Zeitlin et al. (p. 1080) report measurements of the energetic particle radiation environment inside the spacecraft during its cruise to Mars, confirming the hazard likely to be posed by this radiation to astronauts on a future potential trip to Mars. Williams et al. (p. 1068, see the Perspective by Jerolmack) report the detection of sedimentary conglomerates (pebbles mixed with sand and turned to rock) at Gale crater. The rounding of the rocks suggests abrasion of the pebbles as they were transported by flowing water several kilometers or more from their source. Observations from the Curiosity rover of rounded pebbles in sedimentary rocks confirm ancient water flows on Mars. [Also see Perspective by Jerolmack] Observations by the Mars Science Laboratory Mast Camera (Mastcam) in Gale crater reveal isolated outcrops of cemented pebbles (2 to 40 millimeters in diameter) and sand grains with textures typical of fluvial sedimentary conglomerates. Rounded pebbles in the conglomerates indicate substantial fluvial abrasion. ChemCam emission spectra at one outcrop show a predominantly feldspathic composition, consistent with minimal aqueous alteration of sediments. Sediment was mobilized in ancient water flows that likely exceeded the threshold conditions (depth 0.03 to 0.9 meter, average velocity 0.20 to 0.75 meter per second) required to transport the pebbles. Climate conditions at the time sediment was transported must have differed substantially from the cold, hyper-arid modern environment to permit aqueous flows across several kilometers.

Elemental Geochemistry of Sedimentary Rocks at Yellowknife Bay, Gale Crater, Mars.

Sedimentary rocks examined by the Curiosity rover at Yellowknife Bay, Mars, were derived from sources that evolved from an approximately average martian crustal composition to one influenced by alkaline basalts. No evidence of chemical weathering is preserved, indicating arid, possibly cold, paleoclimates and rapid erosion and deposition. The absence of predicted geochemical variations indicates that magnetite and phyllosilicates formed by diagenesis under low-temperature, circumneutral pH, rock-dominated aqueous conditions. Analyses of diagenetic features (including concretions, raised ridges, and fractures) at high spatial resolution indicate that they are composed of iron- and halogen-rich components, magnesium-iron-chlorine–rich components, and hydrated calcium sulfates, respectively. Composition of a cross-cutting dike-like feature is consistent with sedimentary intrusion. The geochemistry of these sedimentary rocks provides further evidence for diverse depositional and diagenetic sedimentary environments during the early history of Mars.

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

The ChemCam instrument, which provides insight into martian soil chemistry at the submillimeter scale, identified two principal soil types along the Curiosity rover traverse: a fine-grained mafic type and a locally derived, coarse-grained felsic type. The mafic soil component is representative of widespread martian soils and is similar in composition to the martian dust. It possesses a ubiquitous hydrogen signature in ChemCam spectra, corresponding to the hydration of the amorphous phases found in the soil by the CheMin instrument. This hydration likely accounts for an important fraction of the global hydration of the surface seen by previous orbital measurements. ChemCam analyses did not reveal any significant exchange of water vapor between the regolith and the atmosphere. These observations provide constraints on the nature of the amorphous phases and their hydration.

Calcium sulfate veins characterized by ChemCam/Curiosity at Gale crater, Mars

The Curiosity rover has analyzed abundant light-toned fracture-fill material within the Yellowknife Bay sedimentary deposits. The ChemCam instrument, coupled with Mastcam and ChemCam/Remote Micro Imager images, was able to demonstrate that these fracture fills consist of calcium sulfate veins, many of which appear to be hydrated at a level expected for gypsum and bassanite. Anhydrite is locally present and is found in a location characterized by a nodular texture. An intricate assemblage of veins crosses the sediments, which were likely formed by precipitation from fluids circulating through fractures. The presence of veins throughout the entire similar to 5 m thick Yellowknife Bay sediments suggests that this process occurred well after sedimentation and cementation/lithification of those sediments. The sulfur-rich fluids may have originated in previously precipitated sulfate-rich layers, either before the deposition of the Sheepbed mudstones or from unrelated units such as the sulfates at the base of Mount Sharp. The occurrence of these veins after the episodes of deposition of fluvial sediments at the surface suggests persistent aqueous activity in relatively nonacidic conditions.

Igneous mineralogy at Bradbury Rise: The first ChemCam campaign at Gale crater

Textural and compositional analyses using Chemistry Camera (ChemCam) remote microimager and laser-induced breakdown spectroscopy (LIBS) have been performed on five float rocks and coarse gravels along the first 100 m of the Curiosity traverse at Bradbury Rise. ChemCam, the first LIBS instrument sent to another planet, offers the opportunity to assess mineralogic diversity at grain-size scales (~ 100 µm) and, from this, lithologic diversity. Depth profiling indicates that targets are relatively free of surface coatings. One type of igneous rock is volcanic and includes both aphanitic (Coronation) and porphyritic (Mara) samples. The porphyritic sample shows dark grains that are likely pyroxene megacrysts in a fine-grained mesostasis containing andesine needles. Both types have magnesium-poor basaltic compositions and in this respect are similar to the evolved Jake Matijevic rock analyzed further along the Curiosity traverse both with Alpha-Particle X-ray Spectrometer and ChemCam instruments. The second rock type encountered is a coarse-grained intrusive rock (Thor Lake) showing equigranular texture with millimeter size crystals of feldspars and Fe-Ti oxides. Such a rock is not unique at Gale as the surrounding coarse gravels (such as Beaulieu) and the conglomerate Link are dominated by feldspathic (andesine-bytownite) clasts. Finally, alkali feldspar compositions associated with a silica polymorph have been analyzed in fractured filling material of Preble rock and in Stark, a putative pumice or an impact melt. These observations document magmatic diversity at Gale and describe the first fragments of feldspar-rich lithologies (possibly an anorthosite) that may be ancient crust transported from the crater rim and now forming float rocks, coarse gravel, or conglomerate clasts.

Calibrating the ChemCam laser-induced breakdown spectroscopy instrument for carbonate minerals on Mars

The ChemCam instrument suite onboard the NASA Mars Science Laboratory rover includes the first laser-induced breakdown spectroscopy (LIBS) instrument for extraterrestrial applications. Here we examine carbonate minerals in a simulated martian environment to better understand the LIBS signature of these materials on Mars. Both chemical composition and rock type are determined using multivariate analysis techniques. Composition is confirmed using scanning electron microscopy. Our results show that ChemCam can recognize and differentiate between different types of carbonate materials on Mars.

Trace element geochemistry (Li, Ba, Sr, and Rb) using Curiosity's ChemCam: Early results for Gale crater from Bradbury Landing Site to Rocknest

The ChemCam instrument package on the Mars rover, Curiosity, provides new capabilities to probe the abundances of certain trace elements in the rocks and soils on Mars using the laser-induced breakdown spectroscopy technique. We focus on detecting and quantifying Li, Ba, Rb, and Sr in targets analyzed during the first 100 sols, from Bradbury Landing Site to Rocknest. Univariate peak area models and multivariate partial least squares models are presented. Li, detected for the first time directly on Mars, is generally low ( 100 ppm and >1000 ppm, respectively. These analysis locations tend to have high Si and alkali abundances, consistent with a feldspar composition. Together, these trace element observations provide possible evidence of magma differentiation and aqueous alteration.

Chemistry of fracture‐filling raised ridges in Yellowknife Bay, Gale Crater: Window into past aqueous activity and habitability on Mars

The ChemCam instrument package on the Curiosity rover was used to characterize distinctive raised ridges in the Sheepbed mudstone, Yellowknife Bay formation, Gale Crater. The multilayered, fracture-filling ridges are more resistant to erosion than the Sheepbed mudstone rock in which they occur. The bulk average composition of the raised ridges is enriched in MgO by 1.2-1.7 times (average of 8.3-11.4 wt %; single-shot maximum of 17.0 wt %) over that of the mudstone. Al2O3 is anticorrelated with MgO, while Li is somewhat enriched where MgO is highest. Some ridges show a variation in composition with different layers on a submillimeter scale. In particular, the McGrath target shows similar high-MgO resistant outer layers and a low-MgO, less resistant inner layer. This is consistent with the interpretation that the raised ridges are isopachous fracture-filling cements with a stratigraphy that likely reveals changes in fluid composition or depositional conditions over time. Overall, the average composition of the raised ridges is close to that of a Mg- and Fe-rich smectite, or saponite, which may also be the main clay mineral constituent of the host mudstone. These analyses provide evidence of diagenesis and aqueous activity in the early postdepositional history of the Yellowknife Bay formation, consistent with a low salinity to brackish fluid at near-neutral or slightly alkaline pH. The fluids that circulated through the fractures likely interacted with the Sheepbed mudstone and (or) other stratigraphically adjacent rock units of basaltic composition and leached Mg from them preferentially.

First detection of fluorine on Mars: Implications for Gale Crater's geochemistry

Volatiles and especially halogens (F and Cl) have been recognized as important species in the genesis and melting of planetary magmas. Data from the Chemical Camera instrument on board the Mars Science Laboratory rover Curiosity now provide the first in situ analyses of fluorine at the surface of Mars. Two principal F-bearing mineral assemblages are identified. The first is associated with high aluminum and low calcium contents, in which the F-bearing phase is an aluminosilicate. It is found in conglomerates and may indicate petrologically evolved sources. This is the first time that such a petrologic environment is found on Mars. The second is represented by samples that have high calcium contents, in which the main F-bearing minerals are likely to be fluorapatites and/or fluorites. Fluorapatites are found in some sandstone and may be detrital, while fluorites are also found in the conglomerates, possibly indicating low-T alteration processes.

High manganese concentrations in rocks at Gale crater, Mars

The surface of Mars has long been considered a relatively oxidizing environment, an idea supported by the abundance of ferric iron phases observed there. However, compared to iron, manganese is sensitive only to high redox potential oxidants, and when concentrated in rocks, it provides a more specific redox indicator of aqueous environments. Observations from the ChemCam instrument on the Curiosity rover indicate abundances of manganese in and on some rock targets that are 1–2 orders of magnitude higher than previously observed on Mars, suggesting the presence of an as-yet unidentified manganese-rich phase. These results show that the Martian surface has at some point in time hosted much more highly oxidizing conditions than has previously been recognized.