INVESTIGATING the FLOOR of PALEOLAKE JEZERO by WAY of GUSEV CRATER. S. W. Ruff1, 1Arizona State University, School of Earth

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Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3076.pdf

INVESTIGATING THE FLOOR OF PALEOLAKE JEZERO BY WAY OF GUSEV CRATER. S. W. Ruff1, 1Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287-6305, [email protected].

Introduction: Gusev crater was selected as the sized Gusev kipuka. An onlapping relationship is a landing site for the Spirit rover because of features possible explanation, but the lack of a prominent moat indicative of an ancient lake, including the ~900 km among any of these mounds implies that none has ex- long Ma’adim Vallis channel system entering the perienced erosion sufficient to open a gap between the crater, and the landforms at its entry point originally mound sediments and the embaying volcanic unit. This thought to represent an eroded delta [e.g., 1]. On the could arise if the rate of erosion of the volcanic unit floor of Gusev, Spirit encountered olivine-rich basaltic either keeps pace with or outpaces that of the mound rubble, subsequently recognized as the remnants of sediments. Similarly, the contact between the VF and low viscosity lava flows that spread across the floor the main delta escarpment shows no prominent moat ~3.65 billion years ago [2]. An earlier geologic histCTX ory along its entire length, perhaps for the same reason. is preserved in higher-standing terrains that were embayed by the lava flows, including the Columbia Hills, Columbia Hills which represent a kipuka or island of older terrain fully Moat Onlap A’ encircled by lava. The carbonate-rich outcrops there A are perhaps the remnants of an evaporating lake in Gusev crater [3]. A A’ Jezero crater is the leading candidate site for the Mars 2020 rover mission because of evidence that it once hosted a lake, including an eroded fan deposit that is confidently interpreted as a fluvial delta [4].

Although the evidence for a lake is robust, much of the Kipukas floor of Jezero displays basaltic mineralogy in terrain Fig. 2a interpreted as a volcanic capping unit [5; 6]. This unit shares similarities with the basaltic floor of Gusev Lobate crater, but comparisons between the two have not been margin explored previously. Here I present observations from the two craters that highlight the similarities and dif-CTX 5 km ferences of their respective volcanic floor units, with implications for the stratigraphy and geologic history Moat of Jezero crater. Onlap Observations: Lobate margins and kipukas are evident on the floor of both craters (Fig. 1), consistent B B’ with an interpretation of the volcanic floor unit (VF; Fig. 2b [6]) in Jezero resulting from flowing lava. The kipukas

at both sites include examples where the original high- Kipukas er-standing terrain is still present; is eroded below B grade; or has been removed altogether. Where still B’ Lobate present, such kipukas can show both an onlapping rela- margin tionship with the embaying lava flows and a marginal depression or moat, the result of erosion of the higher- standing terrain after embayment. Such characteristics are displayed by the Columbia Hills, offering ground truth for comparisons elsewhere. A small kipuka ~12 km from the Columbia Hills 5 km displays a prominent moat fully encircling higher- standing terrain of varying morphology . This kipuka is comparable in size to mounds on the floor of Jezero Figure 1. Comparison of common features on the floor previously mapped as outliers of the delta deposits and of Gusev crater (top) and Jezero crater (bottom) shown described as “kipuka-like” [5] (Fig. 2). However, they in CTX mosaics. Insets show HiRISE DTM-based elevation profiles across a kipuka in both craters. lack a moat feature akin to that of the comparably- Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3076.pdf

It is noteworthy that the only examples of kipukas at the same rate or more slowly than VF in order to in Jezero that display a prominent moat are associated maintain the apparent topographic relationship. with material described as the light-toned floor unit Conclusions: The volcanic floor unit of Jezero (LTF), which is highly fractured and shows spectral crater displays features analogous to those on the floor evidence for carbonate [6]. So the presence of promi- of Gusev that resulted from emplacement of low vis- nent moats among kipukas of LTF and not the delta cosity basaltic lava flows. Unequivocal examples of deposits may indicate that the former is more easily kipukas in both craters attest to a history of emplace- eroded. ment and erosion of various rock units prior to embayment by lava. The embayment of the delta deposits by lava flows in Jezero is equivocal, demonstrating the need for further investigation. References: [1] Cabrol, N. A., et al. (2003), J. Geophys. Res., 108, E12, doi:10.1029/2002JE002026, [2] Greeley, R., et al. (2005), J. Geophys. Res., 110, E05008, 10.1029/2005JE002401. [3] Ruff, S. W., et al. (2014), Geology, 42, 4, 359-362, 10.1130/G35508.1. [4] Goudge, T. A., et al. (2017), Earth Planet. Sci. Lett., 458, 357-365, 10.1016/j.epsl.2016.10.056. [5] Schon, S. C., et al. (2012), Planet. Space Sci., 67, 28-45, 10.1016/j.pss.2012.02.003. [6] Goudge, T. A., et al. (2015), J. Geophys. Res., 775-808, 10.1002/2014JE004782.

Figure 2. Comparison of a small kipuka in Gusev (A) and candidate kipukas of similar size in Jezero (B). The mounds in B lack a prominent moat like in A. An Alternative Hypothesis: The lack of any prominent moats among the delta deposits contrasts with the style of embayment of LTF in Jezero as well as rock units in Gusev. This raises the possibility that the delta deposits are not actually embayed by VF but instead were deposited on top of it. In this scenario, the contacts between VF and delta deposits are erosional; the delta deposits are being stripped off of VF. This obviates the requirement that the delta sediments erode