Adam Parkes Bowen, University of Leicester, Email: [email protected]

Using band ratioed CaSSIS imagery and analysis of fracture morphology to characterise Oxia Planum’s clay-bearing unit

Adam Parkes Bowen1, Lucia Mandon2, John Bridges1, Cathy Quantin-Nataf2, Livio Tornabene3, James Page1, Jemima Briggs1, Nicolas Thomas4, and Gabriele Cremonese5

• Ahead of the Rosalind Franklin rover’s arrival, characterisation of its landing site Oxia Planum is essential • Due to limits in the resolution (300m/pixel) of OMEGA and coverage of the highest resolution (18m/pixel) CRISM data over Oxia Planum, improvements in the clay maps currently available of the site can be made and would be of use for future rover traverse planning • Following work identifying that the clay unit consists of at least two subunits, and that band-ratioed CaSSIS imagery can distinguish between them, a map of these subunits’ is currently being constructed using CaSSIS. Given CaSSIS’s resolution of 4.5m/pixel, this would significantly increase the resolution of clay maps available at Oxia • Alongside this, fracture networks at Oxia Planum and Gale crater are being studied • This is done as depending on the formation mechanism, the surrounding environment and various properties of the host material, fracture networks can vary greatly. By comparing those fractured units that have been observed at Oxia Planum with those characterised by Curiosity at Gale Crater, similarities or differences between these fracture networks can be used to gauge Oxia’s history and its material properties

1Space Research Centre, University of Leicester, UK, 2Université de Lyon 1, France, 3CPSX, Western University, Canada, 4Universität Bern, Switzerland, 5Astronomical Observatory of Padova-INAF, Italy CaSSIS: relating ferric and ferrous material to clay detections • From work carried out by members of the CaSSIS team1 it has been shown that certain CaSSIS band ratio’s (NIR/BLU, PAN/BLU, PAN/NIR composite shown centre left) can be used to detect and distinguish between ferric and ferrous (yellow and blue in figure) materials, band ratioing being the process where one image band is divided by another on a pixel by pixel basis • Using CaSSIS, with co-analysis from CRISM and HiRISE colour imagery, two clay subunits were found to exist at Oxia Planum2. These are a lower lying “orange” member rich in Fe/Mg clays (either Vermiculite or Saponite) possessing metre-scale fracturing, and an over lying “blue” member showing signatures of both the Fe/Mg clay and Olivine possessing decametre-scale fracturing. • This work found that ferric detections by CaSSIS correlated well with CRISM clay detections (compare bottom-left figure and a CRISM spectral map in the centre) and that the two subunits were differentiable using this imagery (see comparison of the band ratio values normalised to band ratio 1 bottom-right, with the member values taken from the CaSSIS image MY34_005664)

4.5 MY34_005012 FRT00009a16_07_Fe 4 Band ratioed /Mg Phyllosilicates 3.5 Orange 3 member 2.5

Normalised 2 1.5 1 Blue 0.5 member

Band Ratio Band Ratio 0 2.5 km 2.5 km 1 2 3 Band Ratio

2 Adam Parkes Bowen CaSSIS: mapping of the two subunits

• Following this discovery band ratioed CaSSIS imagery, Landing envelope in conjunction with HiRISE RED data to observe Opening ellipse fracture size, is currently being used to map out these Closing ellipse two sub-units. The CaSSIS images are Dark Subtracted to minimise the effects of atmospheric scatter prior to being band ratioed • The map as it stands is shown, set on the HiRISE RED images used to observe fracturing and with a THEMIS day IR background. The latest 2022 1σ landing ellipses at the opening and closing of the launch window, and the wider 1σ landing envelope, are shown • As can be seen, the NW of Rosalind Franklin’s closing landing ellipse is dominated by the orange subunit, with small areas of the blue unit being present primarily adjacent to a capping unit exposure which can be seen in the north of the image • Expansion to the rest of the 1σ landing ellipse is ongoing currently, which will be followed by expansion 3 km of the map out to the 3σ landing ellipse

3 Adam Parkes Bowen Fracture analysis: mapping of the networks and visualising data

• The various properties of a fracture network are related to a mix of factors. For example, length and area of fracture polygons are partly related to grain size, with finer grained material being prone to fracturing3, and the depth of the fractured area, where the thicker the fractured material is the larger and more widely spaced the fractures formed are3. Intersection angle meanwhile can be an indicator of fracture formation mechanism and a measure of how evolved the fracture network is4 • By mapping fractures at Oxia Planum and comparing them to networks within the well characterised surfaces at Gale Crater, we can use the ground truth from the latter to make predictions about the former. The mapping was carried out at a 50 x 50 m scale, with all fractures visible being mapped out • Following this Kernel Density Estimation (KDE) diagrams were used to visualise and compare the data. KDE diagrams are similar to histograms however, rather than the data points being sorted into discrete bins, each is represented as a kernel function such as a Gaussian centred on the data point. These functions are summed at each point to create the KDE function for the distribution, see the accompanying figure5 for an example of this • Additionally, 2-sample Kolmogorov-Smirnov tests were used to determine the probability that each data set could have originated from the same population, with a 2-sigma level of dissimilarity being used as the cut off point where the fracture networks were considered to come from differing populations

4 Adam Parkes Bowen Fracture analysis: comparison of Oxia Planum and Gale Crater

• From the primary comparison between those sites at Gale Crater and at Oxia Planum, the majority of the Gale sites showed no consistent similarities to the Oxia sites across the three metrics utilised • Two sites did however; one network in the Sheepbed member at Yellowknife Bay showed similarities to both a network within the Oxia clay unit (site 8), as well as a network in an olivine-rich unit (site 9). Neither of these sites were themselves similar to each other. The other exception was in Vera Rubin ridge, comparing with two networks within sites 4 and 7 in the Oxia clay unit. In contrast to the other comparison, these Oxia networks were similar to each other • Comparing between the networks at Oxia Planum itself there appear to be two groupings. Two networks in the clay unit, sites 7/8, located near the large capping units form a grouping distinct to the others seen in the SE of the landing site, while site 1 in the clay unit and site 9, directly adjacent to a large delta fan, also form their own distinct group

Yellowknife Bay Vera Rubin Ridge Oxia Planum fracture sites

40 m 25 m 15 km

5 Adam Parkes Bowen Fracture analysis: conclusions

• Currently the results from this analysis are still being considered. The two networks at Gale which are consistent across all metrics to those at Oxia are both within mudstones, and are both the least dust covered/well-preserved of the sites looked at. The Oxia sites they are comparable with are limited, with these networks not being confined to a specific area or to what is nominally the same unit. This would suggest a more complicated history of Oxia Planum’s clay unit than CRISM and OMEGA maps alone would indicate; in what particular way is still to be determined • Of the comparisons between the Oxia networks, Oxia clay (OC) 7/8 possess metric distributions dissimilar to the other networks, with the exception of OC 4 which has consistent similarities to OC 7. The networks could have formed under significantly different conditions to those in the SE of the landing ellipse, or perhaps were better preserved due to being more recently uncovered by erosion of the adjacent capping unit. Given the similarity with OC 4 however these suggestions maybe too simplistic a conclusion; further work needs to be done to help determine this • The two networks directly adjacent to the delta fan, OC 1 and the olivine-rich unit, both have very similar metric distributions and are distinct to those seen elsewhere at the site. Given their location and strong differences in comparison to the other networks, it is likely that the event which formed these fractures was linked to or influenced by the emplacement of the delta and may have occurred at a different time

Acknowledgments and References: 1; 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 Sci Rev 212, 2; Mandon, L. et. al. (in review). "Stratigraphy and spectral diversity of the clay-bearing unit at the ExoMars 2020 landing site Oxia Planum" Astrobiology, 3; Kindle, E. M., 1917, "Some Factors Affecting the Development of Mud-Cracks“, The Journal of Geology 25(2): 135-144., 4; Goehring, L., et al., 2010, "Evolution of mud-crack patterns during repeated drying cycles." Soft Matter 6(15): 3562., 5; Wicklin R., 2016, Kernel density estimate (KDE) and component densities, PNG, SAS Blogs, viewed 3rd September 2020, https://blogs.sas.com/content/iml/2016/07/27/visualize-kernel-density-estimate.html

6 Adam Parkes Bowen