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Project Progress

After nearly ten years of operation, the NASA MER rover Opportunity continues to explore the surface of Mars. Andrew Knoll and John Grotzinger continue to serve on the MER science team, taking part in mission planning and the interpretation of data telemetered back from Mars. In recent progress, Opportunity has documented the oldest materials yet encountered on the mission, a volcanic and impact related stratigraphy exposed within the inner rim of Endeavour crater that predates the sulfate-rich sandstones explored by Opportunity for some eight years. Geochemical analyses of these older rocks indicate the presence of clay minerals, in turn documenting limited chemical alteration of rock substrates by postdepositional fluid flow. Alteration zones and more diffuse patterns of alteration indicate sustained fluid flow under conditions of varying redox potential, providing evidence for what may have been the most habitable environments yet documented by Opportunity. A paper on this exploration is currently in revision (Arvidson et al., submitted). Knoll also participated in a second project to characterize and interpret Martian sandstones encountered by the Spirit rover (Cabrol et al., submitted).

In related research, Knoll, Milliken and Summons are analyzing shales from well preserved Proterozoic basins on Earth, with three goals in mind. First, they are gathering unprecedented data on the clay mineral content of ancient sedimentary successions on Earth. Second, they are analyzing organic carbon content of the same rocks to understand the relationship between biomarker preservation and clay mineral content. And third, all samples are being analyzed by remote sensing instruments comparable to those that have flown or are contemplated for mars missions, the idea being to prove ground truth for remote sensing inferences.

At Brown University the Milliken team has measured over 100 rock powders that represent a wide range in age and depositional environment. These include Mesoproterozoic shales from the McArthur and Roper Groups, Neoproterozoic shales from Spitsbergen and clay samples from the Duoshantuo Fm. in China, and younger Eocene samples from the Green River Fm., to name a few. Reflectance measurements of these rock powders reveal a wide range in the strengths of absorption features attributed to organic material. These features are found primarily near 3.4 µm, where they can overlap with broad absorptions due to water and narrower features associated with carbonate minerals. The primary minerals we have identified in these samples are clays (primarily illite and saponite), carbonates (dolomite and siderite), and Feoxides (hematite). These minerals were identified by modeling the absorption features observed in the reflectance spectra from ~1 – 5 µm and comparing the fits to library spectra of relatively pure minerals. Initial results show that organic absorption features are relatively easy to detect in samples dominated by clay minerals and/or Feoxides, where the latter become relatively transparent at longer wavelengths. However, the level of water content in the clays and/or the bulk sample affects the degree to which weak organic features can be identified. In addition, the overlap between carbonate features near ~3.4 µm and organic features in this wavelength region make unambiguous detection of organics in carbonate bearing samples difficult.

Based on these initial results our current work is focused on several aspects. First, we are beginning to independently characterize the total organic content (TOC) of select samples to understand the detection limits of organic material using reflectance spectroscopy. Second, we are developing algorithms to see whether or not organics in carbonatebearing sediments/rocks can be uniquely identified and, if so, assess the detection limits for these cases. Finally, we are selecting a subset of samples to study in greater detail using high resolution techniques for imaging and chemical analysis (e.g., SEMEDS, EMP, confocal Raman, etc.). These methods will help us to determine how organic material is physically preserved in the samples and whether or not certain types of clay or carbonate have stronger associations with organic material preservation. Our goal is to characterize the relationships between organic material, mineral assemblages, and assess the utility of reflectance spectroscopy as a rapid, remote, nondestructive technique for detecting potential organic signatures in ancient rocks, including core samples.

After a little more than a year on Mars, the Mars Science Laboratory Curiosity rover has discovered fine grained sedimentary rocks, which are inferred to represent a habitable lake environment, whose chemistry indicates the presence of more neutral to alkaline conditions than the environments previous rovers explored. The science team has also determined the age of a Martian rock, found evidence the planet could have sustained microbial life, and taken radiation readings on the surface, which will help NASA create better models regarding the radiation environment for future astronauts. While exploring Yellowknife Bay (an area just to the east of where the rover landed), Curiosity drilled into a rock nicknamed “Cumberland”, which has age estimates at 4.21 +/- 0.35 billion years old, inferred from a radiometric method for dating Earth rocks that measures the decay of an isotope of potassium as it changes into argon (Farley et al., 2013). Curiosity also detected higher amounts of organics in the Cumberland sample, although it can’t be ruled out that the organics could have been brought from Earth (Ming et al. 2013).

The first drill area, nicknamed “John Klein”, had evidence for a Martian environment favorable for microbial life long ago. The clay-rich lake bed was not too acidic or salty, with the key chemical elements for life. Recent results show the chemical elements in the rock indicate the particles were carried from the rim of Gale Crater (upstream) and most of the chemical weathering happened after deposition (McLennan et al., 2013). This habitable environment, marked by stream, lakes and groundwater networks, likely persisted for millions of years. These results highlight the biological viability of fluviallacustrine environments in the post Noachian history of Mars. (Grotzinger et al., 2013).

A key to improved understanding of the stratigraphic and spatial distribution of prospective target rocks in Gale Crater lies in having reliable maps based on satellite and surface imagery and spectrocopy. Sumner chairs the Gale Mapping Working Group, which is producing geomorphic and geologic maps of the landing area and lower slopes of Mt. Sharp in Gale Crater. This map is being used for long term planning of science campaigns for Curiosity as well as to put observations into a regional context.

Preservation of organic materials on Earth is most often associated with rapid mineralization of deposits. Working at Caltech, Kirsten Siebach has recognized a set of large scale boxwork structures in a sedimentary layer on Mount Sharp, Gale Crater’s central mound, that are indicative of extensive groundwater cementation and represent a possibly habitable environment where organic molecules may have been preserved. Mapping of the structures is used to constrain a minimum volume of water at this site, which is recommended for Curiosity’s future traverse.