The European Space Agency and Roscosmos' ExoMars rover mission, which is planned to land in the Oxia Planum region, will be dedicated to exobiology studies at the surface and subsurface of Mars. Oxia Planum is a clay-bearing site that has preserved evidence of long-term interaction with water during the Noachian era. Fe/ Mg-rich phyllosilicates have previously been shown to occur extensively throughout the landing area. Here, we analyze data from the High Resolution Imaging Science Experiment (HiRISE) and from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instruments onboard NASA's Mars Reconnaissance Orbiter and the Colour and Stereo Surface Imaging System (CaSSIS) onboard ESA's Trace Gas Orbiter to characterize, at a high spatial resolution, the morphological and spectral variability of Oxia Planum's surface deposits. Two main types of bedrocks are identified within the clay-bearing, fractured unit observed throughout the landing site: (1) an orange type in HiRISE correlated with the strongest detections of secondary minerals (dominated by Fe/Mg-rich clay minerals) with, in some locations, an additional spectral absorption near 2.5 mm, suggesting the mixture with an additional mineral, plausibly carbonate or another type of clay mineral; (2) a more bluish bedrock associated with weaker detections of secondary minerals, which exhibits at certain locations a *1 mm broad absorption feature consistent with olivine. Coanalysis of the same terrains with the recently acquired CaSSIS images confirms the variability in the color and spectral properties of the fractured unit. Of interest for the ExoMars mission, both types of bedrocks are extensively outcropping in the Oxia Planum region, and the one corresponding to the most intense spectral signals of clay minerals (the primary scientific target) is well exposed within the landing area, including near its center.

No abstract

No abstract

<p><strong>Introduction:</strong></p><p>In an effort to aid the characterisation of Oxia Planum, selected as the Rosalind Franklin rover’s landing site partly due to its extensive Noachian-era clay deposits [1, 2], a comparison of fractured terrains at Oxia and Gale Crater along with an analysis of Colour and Stereo Surface Imaging System (CaSSIS) [3] imagery are currently underway.</p><p>An analysis of fractured terrains is a useful tool for determining the history and material properties of Oxia, as the form a fracture network takes varies depending both on the mechanisms which generated it as well as the materials within which the fracturing occurred [4-6]. Comparisons between fractured terrains across Oxia, as well as with those at Gale crater due to the ground truth provided by the Curiosity rover, are being made. This is done in an effort to predict material properties and the fracture’s formation mechanisms, along with determining how fractures across Oxia relate to one another.</p><p>CaSSIS is a high-resolution (4.5 m/pixel) 4-band VNIR imager with the ability to take stereo images in a single pass of a target. From a study carried out using CaSSIS, along with co-analysis from CRISM and HiRISE colour imagery, two spectrally and morphologically distinct subunits of the Oxia clay unit [7] were identified. These were a lower member showing metre-scale fracturing and spectral signatures indicative of Fe/Mg-rich clay minerals, and an upper member showing decametre scale fracturing with Fe/Mg-rich clay mineral/olivine signatures. To expand upon the mapping carried out using HiRISE colour and CRISM data, which was limited by data coverage [7], CaSSIS and HiRISE RED i.e. greyscale imagery, was used to identify these sub-units. This was done to aid future planning of rover traverses to high priority surface targets.</p><p> </p><p><strong>Methods:</strong></p><p><em>Fracture Analysis.</em> Our fracture analysis involves tracing out a given fracture network in HiRISE imagery using ArcGIS, then using a tool developed at the Open University (as seen in [8]) to measure metrics of the fractures or the polygons formed by them, such as the angle between intersecting fractures, polygon area, etc. These were then mapped out for comparison using Kernel Density Estimation (KDE) diagrams, and compared statistically via two-sample Kolmogorov-Smirnov tests.</p><p><em>CaSSIS mapping.</em> Radiometrically and geometrically corrected images are initially band ratioed and combined into an RGB image, allowing CaSSIS images to distinguish between ferrous and ferric minerals [9]. This is important given that the lower clay unit has a ferric component potentially due to the presence of hematite/ferric oxides [2], in contrast to the ferrous component of the upper member due to containing olivine [7], making the two members distinguishable with CaSSIS.</p><p>The band ratios (BR’s) used were NIR/PAN, PAN/BLU and PAN/NIR, with CaSSIS’s RED channel replacing its NIR depending on which was available from a given CaSSIS image [10]. These are sensitive to ferric and ferrous minerals for the former two and latter one respectively. Dark Subtraction [11] was then applied to minimize the effects of dust-derived atmospheric scatter. These images were used in conjunction with assessment of fracture length using HiRISE imagery to map out the two members. It has previously been identified [7, 12] that high dust opacity at the time of imaging likely skews the apparent ferric content of the image. This has been noted and is addressed via repeat imaging of such affected areas over Oxia Planum.</p><p> </p><p><strong>Results: </strong></p><p><em>Fracture Analysis.</em> The Gale sites which have comparable metric distributions to those at Oxia are the NE section of Yellowknife Bay’s Sheepbed member [13], which has similarities to several sites seen in the centre of the Oxia landing ellipses, and a site at Vera Rubin ridge similar to an area adjacent to Coogoon Vallis in the SE of Oxia Planum and another abutting the capping unit in the centre of the landing ellipses. Both of these are at the 2-sigma level of certainty for the majority of the metrics. See Figure 1 for examples of the KDE graphs used.</p><p><em>CaSSIS mapping.</em> Figure 2 shows the current extent of the mapping within the 1-sigma landing ellipses at Oxia. There are fewer upper member exposures in the area mapped in fig.2, in line with what has been identified previously [7].</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.b5a087c033fe59306992951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=b9994cff1998dcd5ad66275f281ae606&ct=x&pn=gnp.elif" alt=""></p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.eebb3ed033fe52606992951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=b04f07a821cf306b93101cbb1eb37349&ct=x&pn=gnp.elif" alt=""></p><p> </p><p><strong><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.e100fbf033fe51906992951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=99a3a39709689d0cd187016f7ae02fc2&ct=x&pn=gnp.elif" alt=""></strong></p><p> </p><p><strong>Discussion: </strong></p><p><em>Fracture Analysis.</em> While the results from this study show that there are similarities between some of the fractured terrains examined at Oxia Planum and Gale Crater, what the shared properties of these sites are that leads to these similarities is still being considered. Possibilities include possessing a similar grain size, or sharing the same formation mechanism such as hydraulic fracturing [1].  </p><p><em>CaSSIS mapping.</em> Mapping is ongoing for the 1-sigma landing ellipses, with the intention for it to cover the 3-sigma ellipses ultimately. That the map utilises distinctly lower resolution colour imagery of CaSSIS in conjunction with the 0.25 m/pixel HiRISE imagery for observation of fracturing raises the issue of misidentification of the two units where they are in close proximity. Pan-sharpening is being investigated as a solution to this; currently work is ongoing to remove artifacts within the generated products prior to their use in future mapping efforts.</p><p> </p><p><strong>References: </strong>[1] Quantin-Nataf C. et al. (submitted in Astrobiology), [2] Carter et al. (in prep), [3] Thomas et al. (2017) SSR 212, [4] Tang C-S. at. al. (2011) Geoderma, 166(1):111-118, [5] Plummer P. S. et. al. (1981) JSP, 51:1147-1156, [6] Goehring L. et. al. (2010) SM, 6:3562-3567, [7] Mandon L. et al. (submitted in Astrobiology), [8], Brooker L. M. et al. (2018) Icarus 302:386-406 [9] Tornabene L. L. et al. (2018) SSR, 214, [10] Tornabene L. L. et al. (2019) LPSC 50, Abstract #2678, [11] Chavez et al., (1988), RSE, 24.3: 459-479, [12] Parkes Bowen A. et al. (2020) BPSC #2, [13] Grotzinger J. P. et al. (2014) Science, 343:6169</p>

<p>Current knowledge of the clay unit at Oxia Planum, the Rosalind Franklin rovers landing site, is based in large part on spectroscopy data from the OMEGA and CRISM instruments. While these instruments have proved useful for creating a broad map of this unit, along with identifying candidates for the clay making up the unit, their usefulness is limited by their spatial resolution. Mapping at Oxia has primarily been carried out using 1200-300m/pixel OMEGA or 200-100m/pixel CRISM data and, even accounting for the intermittent 18m/pixel CRISM hyperspectral data available, existing clay maps are insufficient for the purposes of rover traverse planning.</p><p>Images from the Colour and Stereo Surface Imaging System<sup>1</sup> (CaSSIS), which has a resolution of 4m/pixel, can improve upon this. Work done by members of the CaSSIS science team identified certain CaSSIS band ratios which can aid in identifying the presence of ferric/ferrous minerals<sup>2</sup>. In a more recent study CRISM, HiRISE colour and CaSSIS data were used to identify that at least two spectrally and morphologically distinct subunits make up the Oxia clay unit<sup>3</sup>. These sub units are divided into a lower and upper member. The lower member appears orange in CaSSIS/HiRISE VNIR images, shows extensive metre-scale fracturing and possesses CRISM spectral signatures consistent with the presence of a Fe/Mg-rich clay mineral. The upper member, blue in CaSSIS/HiRISE VNIR images, shows metre-decametre scale fracturing along with CRISM spectral signatures consistent with a mix of a Fe/Mg-rich clay mineral and olivine.</p><p>This work demonstrates that ferric detections within CaSSIS band ratios correlate well with CRISM, and that the lower clay member appears to have a higher ferric content than the upper member. Given this a new, higher resolution clay map is being created using CaSSIS band ratios in conjunction with HiRISE greyscale imagery to observe fracture size. This map, currently being constructed over the 1-sigma landing ellipses, delineates between the two subunits well in addition to revealing those areas where the two subunits are too intermixed to reliably differentiate at CaSSIS’s resolution. Given that CaSSIS has higher resolution in comparison to the CRISM/OMEGA instruments, that it can differentiate between the clay sub-units, and that it provides higher landing site coverage compared to CRISM hyperspectral data, means this map will provide a significant improvement over what is currently available for the sites clay unit.</p><p><em>References;</em> 1; Thomas N. et al. (2017). "The Colour and Stereo Surface Imaging System (CaSSIS) for the ExoMars Trace Gas Orbiter." Space Science Reviews 212(3-4): 1897-1944. 2; Tornabene L. L. et al. (2017). "Image Simulation and Assessment of the Colour and Spatial Capabilities of the Colour and Stereo Surface Imaging System (CaSSIS) on the ExoMars Trace Gas Orbiter." Space Science Reviews 214(1). 3; Mandon L. et al. (in review). "Spectral Diversity and Stratigraphy of the Clay-Bearing Unit at the Exomars 2020 Landing Site Oxia Planum." Astrobiology</p><p><em>Acknowledgement;</em> CaSSIS is a project of the University of Bern, with instrument hardware development supported by INAF/Astronomical Observatory of Padova (ASI-INAF agreement n.2020-17-HH.0), and the Space Research Center (CBK) in Warsaw.</p>