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Project Progress

Understanding Mineralogy of Acidic Environments on Earth as Analogs for Mars

The goals of this ongoing project are to characterize the clay and sulfate minerals in acidic environments, understand the relationship of these minerals to organic and microfossil preservation, and provide a basis for linking field, laboratory and airborne remote sensing observations.

Team members Ralph Milliken, Andy Knoll and Brown University graduate student Hannah Kaplan conducted fieldwork at Rio Tinto, Spain in May 2014. This was an interdisciplinary effort that partnered with colleagues David Fernández Remolar and Ricardo Amils at the Centro de Astrobiología (CAB) in Madrid, Spain. Under the guidance of our CAB colleagues the field team was able to visit a number of locations along the Rio Tinto system that span a range in age, aqueous geochemistry, and mineralogy. Team members Ralph Milliken and Hannah Kaplan used airborne remote sensing data (visible-near infrared reflectance data acquired by the HyMap system) to guide the field analysis, focusing on regions in HyMap data that exhibited spectral signatures similar to rock units on Mars. The HyMap data span a similar wavelength range to what is acquired by the CRISM spectrometer on the Mars Reconnaissance Orbiter. Therefore, the HyMap and field measurements allow us to understand how outcrop-scale mineralogy relates to airborne or orbital-scale reflectance data.

XRD and IR spectroscopic analyses in Ralph Milliken’s lab allowed us to verify which mineral combinations in the Rio Tinto samples led to specific spectral signatures. The team found that the relative proportions of iron sulfates versus iron oxides varied between the deposits of different ages and that these variations were partly observable in the airborne data. Of particular interest were a variety of samples that contained mixtures of detrital clay (illite/muscovite) and authigenic Fe-sulfate (jarosite). These materials exhibit spectral signatures that are very similar to enigmatic Mars CRISM spectra for deposits within and near Valles Marineris. Integrating our field, lab, and airborne observations with Mars CRISM data indicates that the enigmatic spectral features observed on Mars may reflect mixtures of detrital clays and authigenic jarosite. Therefore, mapping these spectral variations on Mars can provide insight into sediment transport and clay mineral stability associated with acidic aqueous systems on Mars. Our NAI team and CAB collaborators will present this work at the upcoming 46th Lunar and Planetary Science Conference (Kaplan et al., 2015).

Remote Methods for Organics Detection on Mars
Ralph Milliken, Hannah Kaplan and Andy Knoll continued to explore the relationship between clay mineralogy and organic detection and preservation. Under the guidance of team member Milliken, Hannah Kaplan has continued to explore how near- and mid-infrared reflectance spectroscopy can be used as a rapid and non-destructive technique for the detection of organics in sedimentary rocks. The Milliken and Knoll research groups focused on characterizing the clay mineralogy and inorganic and organic carbon content of a large suite of shales, primarily Proterozoic in age. The Milliken group has demonstrated that fine-grained sediments with as low as 0.08 wt.% organics can be definitively detected using reflectance spectroscopy, and that ultimate detection limits are likely much lower. In addition, our past year’s analysis of natural samples has shown that there are clear relationships between organic absorption strength in reflectance spectra and organic content. This indicates the potential for reflectance spectroscopy to be used as a quantitative tool for assessing total organic content.

Hannah Kaplan is now carrying out detailed laboratory experiments to understand how factors such as sample albedo, water content, and mineralogy affect organic detection and quantification. This is being done through a series of well-controlled experiments that rely on creating synthetic mixtures of various clays and solid organics. In this manner we can isolate different variables (e.g., clay mineral type, organic type, albedo and water content), which will allow us to place tighter constraints on detection limits. This ongoing work also includes X-ray diffraction, IR reflectance, confocal Raman, elemental analysis, and organic carbon measurements to fully characterize our suite of terrestrial shales. Together, these measurements continue to provide an improved understanding of the link between clay mineralogy, organic preservation, and how we might detect the presence of organics using rapid and non-destructive methods.

This work has demonstrated the viability of reflectance spectroscopy as a tool for remote detection of organics in complex, natural rocks. Advantages of reflectance spectroscopy include a stronger signal from more intense scattering of light at standoff distances, lack of fluorescence, and the ability to acquire spatial and spectral information in a single measurement without rasterizing (that is, the ability to use hyperspectral imaging). The latter is particularly important when considering applications such as rapid drill core or outcrop scanning, both of which will be of interest for future planetary missions. However, in addition to those applications we can extend these methods to meteorites such as carbonaceous chondrites. Milliken’s research group has recently begun these types of anlayses, first focusing on characterizing the spectral response of hydrated minerals in CM meteorites. Over the coming year we expect to apply the techniques developed by team member Kaplan to our spectral measurements of a wide range of carbonaceous chondrites, providing new information on the distribution of organics in these meteorites.

Exploration by the Curiosity Rover
After two and a half years on Mars, the Mars Science Laboratory Curiosity rover has characterized an 80 m thick stratigraphic section of fluvial and lacustrine rocks laterally across 8 km. John Grotzinger, Dawn Sumner and other team members have interpreted these rocks as showing lateral facies variations that demonstrate Gale Crater hosted a substantial fluvial-lacustrine system. In the past year, Curiosity crossed from Aeolis Palus to the lower most slopes of Aeolis Mons (informally known as Mt. Sharp). Observations across this boundary suggest the fluival outcrops that dominated the traverse across Aeolis Palus transition lateral into lacustrine facies that comprise the basal unit of Mt. Sharp. Fluvial and lacustrine facies are interbedded over a few hundred meters, and sedimentary structures are consistent with a deltaic transition between the two. The thickness of the fluvial facies (>50 m) and the expected thickness of the lacustrine facies based on orbital mapping (>200 m) suggest that these rocks represent a long-lived (millions of years) environment dominated by rivers and lakes in Gale Crater.

The chemistry of the conglomerates, sandstones, and mudstones demonstrate that at least some of the water was neutral pH and low salinity, e.g. were habitable environments. In addition, there are diverse diagenetic features that demonstrate temporally extended water-rock reactions during burial. Studies by Grotzinger, Miliken, Sumner and other team members on the rocks at the base of Mt. Sharp suggest that there were at least two, likely three, episodes of diagenesis. These include an early phase consisting of cementation and concretion formation, which likely included the in situ reaction of detrital minerals, such as olivine, to clay and possibly other minerals. Ralph Milliken has used observations by the ChemCam instrument to create a chemostratigraphic section at Pahrump, demonstrating the existence of clear chemical changes with stratigraphic position. These chemical variations may indicate changes in sediment source region, varying proportions of detrital and authigenic minerals, or open system weathering.

A later fluid, rich in sulfate and calcium, flowed through fractures in the bedrock and deposited calcium sulfates that vary in hydration state. These sulfate fractures cross cut other diagenetic features, demonstrating their late origin. In addition, CheMin XRD measurements of the rocks sampled so far in the base of Mt. Sharp indicate the presence of hematite, magnetite, and possibly jarosite. These minerals indicate oxidizing and acidic water chemistry, which are not consistent with some of the other diagenetic minerals, e.g. clay minerals. In addition, the presence of oxidized iron phases suggests that at least some of these rocks provide a poor taphonomic window for organic preservation. The team is designing additional experiments to understand the complicated diagenetic history of these rocks.

In addition, Ralph Milliken’s work has demonstrated that orbital visible-near infrared reflectance (CRISM) spectra of strata equivalent to Pahrump Hills indicate the presence of hematite and one or more hydrated phases. The hematite has now been confirmed in situ by CheMin, but the nature of the spectrally detected hydrous component(s) remain ambiguous. Phases that may explain these orbital signatures include hydrated salts (e.g., jarosite, gypsum), clay minerals, and/or hydrated silica, some of which have also been observed by Curiosity. Alternatively, the observed metal-OH and H₂O absorptions may be associated with amorphous components. Intriguingly, the CRISM spectral features for Pahrump Hills bear some resemblance to spectra of clay and jarosite-bearing samples that our team has collected at Rio Tinto, suggesting the mineral assemblages at Rio Tinto may be useful analogs for understanding certain processes in lower Mt. Sharp.

Milliken’s research group has mapped the orbital spectral units in detail and continues to integrate laboratory data and Curiosity results to understand the origin of these spectral signatures. The ongoing exploration of Pahrump by Curiosity has allowed team member Milliken to more clearly link in situ rover-scale observations to regional scale observations seen from orbit. This has allowed us to put the Curiosity observations in a much broader context and extrapolate our geologic knowledge to regions beyond the rover traverse path. As such, this ongoing integration of rover and orbital data will aid in identifying areas that may have superior taphonomic windows for organic preservation.

In addition, Dawn Sumner and her graduate students (Frances Rivera-Hernandez and Tyler Mackey) started to develop facies models for sedimentation in perennially ice-covered lakes. Such models are necessary to distinguish between open lakes and those covered in ice, which can imply significantly different climatic conditions. Given the evidence for flowing water early in martian history and the later dominance of ice, perennially ice-covered lakes must have been present for some interval on Mars. Developing criteria to identify perennial ice cover will aid in determining if any of the lacustrine deposits observed by Curiosity show evidence of ice cover, and thus help constrain climate models.

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. Opportunity continues to document and characterize the oldest strata encountered thus far on the mission, some of which are shown to contain clay minerals based on orbital observations. These units predate the sulfate-rich sandstones that have dominated Opportunity’s exploration for the past eleven years and provide new insight into water-rock interaction on early Mars. Continued observations by Opportunity and Curiosity have shown evidence for the presence of manganese oxides, providing important constraints on local redox processes. The presence of these phases in rocks at both landing sites, and the purported ancient (Late Noachian – Early Hesperian) age of these strata, indicates it may be possible to correlate observations from these disparate locations to provide insight into global-scale redox conditions on ancient Mars.

A study panel co-chaired by Roger Summons and Alex Sessions produced a report that examined issues pertaining to anthropogenic contamination of spacecraft and laid down guidelines to help in planning the Mars2020 mission. This study by the 2014 Organic Contamination Panel comprising R.E. Summons and A.L. Sessions (co-chairs), A.C. Allwood, H.A. Barton, D.W. Beaty, B. Blakkolb, J. Canham, B.C. Clark, J.P. Dworkin, Y. Lin, R. Mathies, S.M. Milkovich, and A. Steele was commissioned by Michael Meyer and Lisa May of NASA HQ. Besides being formally submitted to NASA as a report, this study was recently published as a news and views article in the journal Astrobiology.