Imaging spectroscopy of geological samples and outcrops: Novel insights from microns to meters

Tracing Carbonate Formation, Serpentinization, and Biological Materials With Micro-/Meso-Scale Infrared Imaging Spectroscopy in a Mars Analog System, Samail Ophiolite, Oman

Visible-shortwave infrared (VSWIR) imaging spectrometers map composition remotely with spatial context, typically at many meters-scale from orbital and airborne data. Here, we evaluate VSWIR imaging spectroscopy capabilities at centimeters to sub-millimeter scale at the Samail Ophiolite, Oman, where mafic and ultramafic lithologies and their alteration products, including serpentine and carbonates, are exposed in a semi-arid environment, analogous to similar mineral associations observed from Mars orbit that will be explored by the Mars-2020 rover. At outcrop and hand specimen scales, VSWIR spectroscopy (a) identifies cross-cutting veins of calcite, dolomite, magnesite, serpentine, and chlorite that record pathways and time-order of multiple alteration events of changing fluid composition; (b) detects small-scale, partially altered remnant pyroxenes and localized epidote and prehnite that indicate protolith composition and temperatures and pressures of multiple generations of faulting and alteration, respectively; and (c) discriminates between spectrally similar carbonate and serpentine phases and carbonate solid solutions. In natural magnesite veins, minor amounts of ferrous iron can appear similar to olivine's strong 1-μm absorption, though no olivine is present. We also find that mineral identification for carbonate and serpentine in mixtures with each other is strongly scale- and texture-dependent; ∼40 area% dolomite in mm-scale veins at one serpentinite outcrop and ∼18 area% serpentine in a calcite-rich travertine outcrop are not discriminated until spatial scales of <∼1-2 cm/pixel. We found biological materials, for example bacterial mats versus vascular plants, are differentiated using wavelengths <1 μm while shortwave infrared wavelengths >1 μm are required to identify most organic materials and distinguish most mineral phases.

Hydrothermal Alteration of the Ocean Crust and Patterns in Mineralization With Depth as Measured by Micro-Imaging Infrared Spectroscopy

Processes for formation, cooling, and altering Earth's ocean crust are not yet completely understood due to challenges in access and sampling. Here, we use contiguous micro-imaging infrared spectroscopy to develop complete-core maps of mineral occurrence and investigate spatial patterns in the hydrothermal alteration of 1.2 km of oceanic crust recovered from Oman Drilling Project Holes GT1A, GT2A, and GT3A drilled in the Samail Ophiolite, Oman. The imaging spectrometer shortwave infrared sensor measured reflectance of light at wavelengths 1.0-2.6 μm at 250-260 μm/pixel, resulting in >1 billion independent measurements. We map distributions of nine key primary and secondary minerals/mineral groups-clinopyroxene, amphibole, calcite, chlorite, epidote, gypsum, kaolinite/montmorillonite, prehnite, and zeolite-and find differences in their spatial occurrences and pervasiveness. Accuracy of spectral mapping of occurrence is 68%-100%, established using X-ray diffraction measurements from the core description. The sheeted dikes and gabbros of upper oceanic crust Hole GT3A show more pervasive alteration and alteration dominated by chlorite, amphibole, and epidote. The foliated/layered gabbros of GT2A from intermediate crustal depths have similarly widespread chlorite but more zeolite and little amphibole and epidote. The layered gabbros of the lower oceanic crust (GT1A) have remnant pyroxene and 2X less chlorite, but alteration is extensive within and surrounding major fault zones with widespread occurrences of amphibole. The results indicate greater distribution of higher temperature alteration minerals in the upper oceanic crust relative to deeper gabbros and highlight the importance of fault zones in hydrothermal convection in the lower ocean crust.

Thermal metamorphism of CM chondrites: A dehydroxylation-based peak-temperature thermometer and implications for sample return from asteroids Ryugu and Bennu

The target bodies of C-complex asteroid sample return missions are carbonaceous chondrite-like near-Earth asteroids (NEAs), chosen for the abundance and scientific importance of their organic compounds and "hydrous" (including hydroxylated) minerals, such as serpentine-group phyllosilicates. Science objectives include returning samples of pristine carbonaceous regolith from asteroids for study of the nature, history, and distribution of its constituent minerals, organic material, and other volatiles. Heating after the natural aqueous alteration that formed the abundant phyllosilicates in CM and similar carbonaceous chondrites dehydroxylated them and altered or decomposed other volumetrically minor constituents (e.g., carbonates, sulfides, organic molecules; Tonui et al. 2003, 2014). We propose a peak-temperature thermometer based on dehydroxylation as measured by analytical totals from electron probe microanalysis (EPMA) of matrices in a number of heated and aqueously altered (but not further heated) CM chondrites. Some CM lithologies in Maribo and Sutter's Mill do not exhibit the matrix dehydroxylation expected for surface temperatures expected from insolation of meteoroids with their known orbital perihelia. This suggests that insolated-heated meteoroid surfaces were lost by ablation during passage through Earth's atmosphere, and that insolation-heated material is more likely to be encountered among returned asteroid regolith samples than in meteorites. More generally, several published lines of evidence suggest that episodic heating of some CM material, most likely by impacts, continued intermittently and locally up to billions of years after assembly and early heating of ancestral CM chondrite bodies. Mission spectroscopic measures of hydration can be used to estimate the extent of dehydroxylation, and the new dehydroxylation thermometer can be used directly to select fragments of returned samples most likely to contain less thermally altered inventories of primitive organic molecules.

Assessing Laser Cleaning of a Limestone Monument by Fiber Optics Reflectance Spectroscopy (FORS) and Visible and Near-Infrared (VNIR) Hyperspectral Imaging (HSI)

Fiber optics reflectance spectroscopy (FORS) and visible and near-infrared (VNIR) hyperspectral imaging (HSI) were applied to assess and control the laser cleaning process of a deeply darkened limestone surface collected from the historic entrance gate of Castello Svevo, Bari, Italy. Both techniques enabled us to verify the different degree of removal of a thick deposit of black crust from the surface of the walls. Results obtained were in good agreement with those of previous studies of the elemental composition achieved by application of laser-induced breakdown spectroscopy (LIBS). Coupling FORS and VNIR-HSI provided important information on the optimal conditions to evaluate the conservation status and determine the more appropriate level of cleaning restoration, thus avoiding over- and/or under-cleaning. Imaging spectroscopy was used to obtain maps of areas featuring the same or different spectral characteristics, so to achieve a sufficient removal of unwanted layers, without modifying the surface underneath, and to increase the efficiency of traditional cleaning techniques. The performance of the combined non-invasive approach used in this work shows promise for further applications to other types of rocks and highlights the potential for in situ assessment of the laser cleaning process based on reflectance spectroscopy.

Miniature high-speed, low-pulse-energy picosecond Raman spectrometer for identification of minerals and organics in planetary science

The motivation behind time-resolved Raman spectroscopy for planetary surface exploration is (1) to provide comprehensive identification of minerals (nearly all rock-forming minerals and weathering products) and many organics of prime importance including fossilized carbonaceous materials; (2) to do so ensuring that it is possible to characterize even the most sensitive materials that would be altered by current state-of-the-art pulsed lasers (e.g., dark minerals, organics). These goals are accomplished here using a lightweight, high-speed (MHz) pulsed (<100ps) Raman spectrometer based on a high-speed microchip laser combined with a single photon avalanche diode detector array. Using a Mars analog sample set and an automated grid sampling technique, we demonstrate consistent identification of major minerals and kerogen detection at ∼≥1% by volume, without losses typically associated with the two biggest problems: fluorescence interference and sample damage. Despite improvements, we find that time-resolved Raman spectroscopy is still limited by the availability of a suitable laser and detector. As technology advances and such devices become available, we expect that this technique will hold an important place in Raman spectroscopy for both commercial and planetary science applications. We also discuss the utility of Raman point mapping for planetary science (e.g., in comparison with other common techniques like infrared reflectance spectroscopy) and conclude that the choice of technique must be planetary mission-specific; one must consider whether incurring the time to map single microscopic points is worthwhile, and how many points would be sufficient to gain the required information to characterize the surface.

Exploring, Mapping, and Data Management Integration of Habitable Environments in Astrobiology

New approaches to blending geoscience, planetary science, microbiology-geobiology/ecology, geoinformatics and cyberinfrastructure technology disciplines in a holistic effort can be transformative to astrobiology explorations. Over the last two decades, overwhelming orbital evidence has confirmed the abundance of authigenic (in situ, formed in place) minerals on Mars. On Earth, environments where authigenic minerals form provide a substrate for the preservation of microbial life. Similarly, extraterrestrial life is likely to be preserved where crustal minerals can record and preserve the biochemical mechanisms (i.e., biosignatures). The search for astrobiological evidence on Mars has focused on identifying past or present habitable environments - places that could support some semblance of life. Thus, authigenic minerals represent a promising habitable environment where extraterrestrial life could be recorded and potentially preserved over geologic time scales. Astrobiology research necessarily takes place over vastly different scales; from molecules to viruses and microbes to those of satellites and solar system exploration, but the differing scales of analyses are rarely connected quantitatively. The mismatch between the scales of these observations- from the macro- satellite mineralogical observations to the micro- microbial observations- limits the applicability of our astrobiological understanding as we search for records of life beyond Earth. Each-scale observation requires knowledge of the geologic context and the environmental parameters important for assessing habitability. Exploration efforts to search for extraterrestrial life should attempt to quantify both the geospatial context and the temporal/spatial relationships between microbial abundance and diversity within authigenic minerals at multiple scales, while assimilating resolutions from satellite observations to field measurements to microscopic analyses. Statistical measures, computer vision, and the geospatial synergy of Geographic Information Systems (GIS), can allow analyses of objective data-driven methods to locate, map, and predict where the "sweet spots" of habitable environments occur at multiple scales. This approach of science information architecture or an "Astrobiology Information System" can provide the necessary maps to guide researchers to discoveries via testing, visualizing, documenting, and collaborating on significant data relationships that will advance explorations for evidence of life in our solar system and beyond.

Full-spectrum denoising of high-SNR hyperspectral images

The high spectral redundancy of hyper/ultraspectral Earth-observation satellite imaging raises three challenges: (a) to design accurate noise estimation methods, (b) to denoise images with very high signal-to-noise ratio (SNR), and (c) to secure unbiased denoising. We solve (a) by a new noise estimation, (b) by a novel Bayesian algorithm exploiting spectral redundancy and spectral clustering, and (c) by accurate measurements of the interchannel correlation after denoising. We demonstrate the effectiveness of our method on two ultraspectral Earth imagers, IASI and IASI-NG, one flying and the other in project, and sketch the major resolution gain of future instruments entailed by such unbiased denoising.

Microterracettes in Sabkha Oum Dba (Western Sahara, Morocco): Physical and Biological Interactions in the Formation of a Surface Micromorphology

Small-scale terracing (microterracettes) is a surface geomorphic feature that recurs under a range of environmental settings, such as those existing in high to low temperature geothermal springs and evaporitic environments, through the single or combined action of physicochemical agents and microbiological processes. Such morphology can also be observed in a confined sector of the Sabkha Oum Dba, which is an inland sabkha of the Western Sahara (Morocco), where field and laboratory investigations revealed that they primarily depend on the accumulation of naviculoid diatoms. Through their biofilm production ability, these benthic diatoms are able to stabilize surface morphologies and make organic alveolar frameworks where the precipitation of low Mg calcite occurs in areas subjected to active oxygenic photosynthesis. Because microterracettes arise in a specific set of environmental conditions, they have environmental significance and, thanks to a high fossilization potential due to mineral precipitation, they can be an effective source of biomorphological and chemical evidence for life. The relationship with aqueous environments, considered to be widespread on Mars especially during a period of intense hydrologic activity as in the late Noachian and Hesperian periods, make the understanding of surficial processes useful (such as the formation of microterracettes) whose formation is frequent in terrestrial analogues for martian environments, such as ephemeral saline continental lakes (sabkhas) and related to the products of bacterial and eukaryotic life, as in the case of biofilms, in search for similar life forms beyond Earth.

Raman spectra of Martian glass analogues: A tool to approximate their chemical composition

Raman spectrometers will form a key component of the analytical suite of future planetary rovers intended to investigate geological processes on Mars. In order to expand the applicability of these spectrometers and use them as analytical tools for the investigation of silicate glasses, a database correlating Raman spectra to glass composition is crucial. Here we investigate the effect of the chemical composition of reduced silicate glasses on their Raman spectra. A range of compositions was generated in a diffusion experiment between two distinct, iron-rich end-members (a basalt and a peralkaline rhyolite), which are representative of the anticipated compositions of Martian rocks. Our results show that for silica-poor (depolymerized) compositions the band intensity increases dramatically in the regions between 550-780 cm^-1 and 820-980 cm^-1. On the other hand, Raman spectra regions between 250-550 cm^-1 and 1000-1250 cm^-1 are well developed in silica-rich (highly polymerized) systems. Further, spectral intensity increases at ~965 cm^-1 related to the high iron content of these glasses (~7-17 wt % of FeO_tot). Based on the acquired Raman spectra and an ideal mixing equation between the two end-members we present an empirical parameterization that enables the estimation of the chemical compositions of silicate glasses within this range. The model is validated using external samples for which chemical composition and Raman spectra were characterized independently. Applications of this model range from microanalysis of dry and hydrous silicate glasses (e.g., melt inclusions) to in situ field investigations and studies under extreme conditions such as extraterrestrial (i.e., Mars) and submarine volcanic environments.