52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2475.pdf

COMPARING THE JEZERO FLOOR UNIT AND THE CIRCUM-ISIDIS MAFIC CAP: MORPHOLOGY, STRATIGRAPHY, AND COMPOSITION. Carol B. Hundal1, John F. Mustard1, Jesse D. Tarnas1,2, Christopher H. Kremer1. 1Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, 02906. 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109. ([email protected]).

Introduction: An extensive, Mafic Capping unit shapes that can be interpreted as surface spectral signals overlies the stratigraphy of the circum-Isidis region on and varying degrees of structured noise. Factor Analysis [e.g., 1-5]. An apparently similar mafic geologic has been used on Mars in the past to identify mineral unit covers Jezero crater’s floor and a large fraction of components with both narrow [e.g., 15] and broad the rover landing ellipse [6]. As such, it absorption features [e.g., 16,17]. may be the subject of the mission's first groundbreaking Here, we analyze data from regions of interest in discoveries. MRO CRISM [18] data of the Mafic Capping, Olivine- Previous work has suggested that the Mafic Capping Rich, and Jezero Floor units. In addition to Factor and Jezero Floor units are genetically related due to Analysis, we use a number of techniques to reduce photogeologic similarities [5,7]. Here, we integrate structured noise––the methodology of which will be morphology, stratigraphy, and a compositional remote explained at the meeting. sensing investigation to further test this comparison. A Results: We find morphological, stratigraphic, and strong resemblance between these geologic formations compositional similarities and differences between the in our integrated dataset would support the hypothesis two geologic units. that the floor could be an explosive volcanic deposit. Morphology (Figure 1A): Both units are Additionally, it could mean the discoveries made on the characterized by a dark-toned, crater preserving surface Jezero floor may apply to the greater circum-Isidis with steep bounding scarps. Craters extend in size from region. The presence of dark-toned, mafic capping units less than 10 m up to ~300 m in diameter on both units. globally (e.g., , [8]) gives Each shows a similar degree of crater preservation from the Circum-Isidis Mafic Capping unit and this possible sharp crater rims to rounded, apparently degraded rims. connection to the Jezero floor added global significance. Crater cavities tend to be variably filled with aeolian Methods: Morphology/Stratigraphy: We analyzed sediments. morphology using images from the High Resolution Although quite similar on the sub-kilometer scale, the Imaging Science Experiment (HiRISE) [9] and the two units differ in their regional characteristics. The Context Camera (CTX) [10] aboard the Mars Mafic Capping unit forms scattered mesas on the order Reconnaissance Orbiter, as well as CTX mosaics [11]. of 1-5 km in diameter, while the Jezero floor is a Stratigraphic determinations were drawn from the contiguous unit on the order of 20-30 km across. literature, specifically [6,12-14]. However, this distinction could be due to differences in Spectra: Similar spectral signatures between these weathering and exposure. two units would imply common mineralogy and lend Stratigraphy (Figure 1B): The Mafic Capping and evidence to the hypothesis of a shared origin. To refine Jezero Floor units lie above the circum-Isidis Olivine- our analyses of potentially weak spectral components, Rich Unit (ORU). The cap is apparently in direct contact we used Factor Analysis. This statistical method with the ORU [5,14]. For the Jezero Floor, however, it calculates eigenvectors representative of independent is uncertain whether it lies stratigraphically above or variance in a dataset. The first eigenvector describes the below the delta unit [6]: does the floor directly overlie greatest spectral variance, while each successive one the olivine-rich unit, or is the delta in-between? For describes less. Low-order eigenvectors––those example, the floor near the delta is smooth, perhaps describing the most variance––tend to have spectral suggesting it was protected by a previously overlying

Figure 1: (A) Morphologic comparison of the pitted capping unit (HiRISE PSP_009718_2005) and the Jezero floor (HiRISE ESP_048908_1985). (B) Stratigraphic comparison (simplified). Both the pitted capping unit and the Jezero floor unit are above the olivine-rich unit. The position of the delta is debated [6].

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2475.pdf

unit, and there are no moat-like features between the 93, 2017. [4] Hundal, C. et al. LPSC 52, Abs. 1629, floor and delta as is seen at [19]. On the 2020. [5] Sun, V. & Stack, K. USGS SIM 3464, 2020. other hand, the floor also slopes upward in proximity to [6] Stack, M. et al. Space Sci Rev, 216:127, 2020. [7] the delta remnant, perhaps suggesting the floor was Sun, V. & Stack, K. LPSC 50 Abs. 2271, 2019. [8] emplaced after the delta [12]. McLennan, S. et al. Ann. Rev. of Earth and Pl. Sci., Several stratigraphic hypotheses from the literature 47(1):91–118, 2019. [9] McEwen, A. et al. JGR: and from new photogeologic mapping are synthesized , 112(E5), 2007. [10] Malin, M. et al. JGR: into possible endmember scenarios in [6]. These include Planets, 112(E5), 2007. [11] Dickson, J. et al. In LPS both stratigraphic possibilities, as well as the XLIX, Abstract #2480. 2018. [12] Goudge, T. et al. speculation that the Jezero floor and the underlying JGR:Planets, 120, 775-808, 2015. [13] Kremer, C. et al. olivine may be part of the same unit. The true In LPSC 50, Abstract #1656, 2019. [14] Kremer, C. et stratigraphic position of the delta has important al. Geol. 47(7):677-681, 2019. [15] J. D. Tarnas, et al. implications. Crater dating has been done for the Jezero GRL 771-782, 2019. [16] Bandfield, J. et al. JGR floor [18]; knowing how that age stratigraphically Planets 107(E11), 2002. [17] Glotch T. JGR 111, relates to other geologic units, particularly the delta unit, E12S06, 2006. [18] Murchie et al. (2006), JGR 114: E2. is hugely significant. [19] Ruff, S. 4th Early Mars Abstract #3076, 2017. [18] Composition (Figure 2): Both the floor and the cap Shahrzad, S. et al. GRL 46, 2408-2416, 2019. [19] produce eigenvectors with broad, crystal field Mandon, L. et al Icarus 336, 2020. [20] Robbins, S. et absorption shapes at one and two microns. We interpret al. Icarus, 211(2):1179 – 1203, 2011. [21] Bandfield, J. this to be consistent with a mix of high- and low- et al. Icarus, 222(1):188 – 199, 2013. calcium pyroxene. It should be noted that the southern portion of the Jezero Floor also has a second eigenvector consistent with reduced glass, similar to lunar glass, which is not present in the Mafic Capping unit. Apart from the above comparison, there are eigenvectors from both the cap and the Olivine-Rich Unit (ORU) which match each other closely (Figure 2). Implications for the Perseverance rover mission: We interpret these compositional and photogeological similarities to suggest the two geologic units are linked, either in terms of a formation mechanism or even a formation event/phase, corroborating the conclusions of [5,7]. Work by [4,7] showed the Mafic Capping unit is a clastic deposit, possibly of an ash-fall origin like the Olivine-Rich Unit, which it directly overlies [14, 19]. The similarities between the eigenvectors from the Olivine-Rich Unit and the cap unit may indicate a compositional transition in a sequence of explosive volcanism. We believe this is more likely than the presence of weathered material because this spectral signal is relatively uniform over both units instead of concentrated in craters, pits, and the edges of scarps. The instruments on the Perseverance rover could test this hypothesis, while a returned sample could provide invaluable information about the timing and the mechanisms of Mars' transition from explosive to effusive volcanism in the [20,21]. Acknowledgments: Many thanks to Tim Goudge, Michael Bramble, Ralph Milliken, and Jim for Figure 2: Comparison of eigenvectors calculated from insightful discussions. We used CTX mosaics produced CRISM observations of the Mafic Capping (pink, red, by the Bruce Murray Laboratory for Planetary orange, yellow), Jezero Floor (brown), and Olivine- Visualization. Rich Unit (ORU; blue). Library spectra (grey, black) References: [1] Tornabene, L. et al. JGR: Planets, show high- and low-calcium pyroxene. CRISM 113(E10), 2008. [2] Mustard, J. et al. JGR: Planets, observation IDs are noted in the figure. Areas without 114(E2), 2009. [3] Bramble, M. et al. Icarus, 293:66 – data are removed due to noise from atmospheric water.