Northern Xanthe Terra: Update on the Reference Site for the Exomars2022 Landing Site in Oxia Planum
Authors: Frueh Thomas , Hauber Ernst , Adeli Solmaz , Tirsch Daniela , Nass Andrea 
Published: July 2021

Introduction:&#160;ESA&#8217;s ExoMars rover will land in the&#160;Oxia&#160;Planum&#160;(OP)&#160;region [1,2], which&#160;was chosen for its ancient age, evidence for the sustained presence of water, the presence of layered deposits, and the potential for biosignature preservation [2]. While some of the geological characteristics have been studied in detail [3-5], other aspects are less well understood.&#160;In particular, it&#160;is unclear if the situation in&#160;OP&#160;is a unique one, or if the geologic setting is representative&#160;for Mars&#160;on a regional, or even&#160;a&#160;global scale. Specifically, the stratigraphic relationship of the geologic units&#160;in&#160;OP&#160;to the nearby ancient impact basin,&#160;Chryse&#160;Planitia, requires further study. For example, a&#160;&#187;bathtub&#171;-like ring of Fe/Mg-rich phyllosilicates around&#160;Chryse&#160;[6] may indicate that the phyllosilicates at&#160;OP&#160;reflect aqueous processes that operated in a specific context at basin-scale.&#160;&#160;To test the hypothesis that&#160;OP&#160;is representative for a circum-Chryse&#160;geologic setting, we selected&#160;a site in northern&#160;Xanthe&#160;Terra&#160;(XT)&#160;(~9-13.5&#176;N/315-318.5&#176;E; Fig. 1) to investigate key geologic features and compare them to&#160;OP. This site displays several characteristics&#160;similar to&#160;OP, making it a suitable reference site: The presence of phyllosilicates [7], the proximity to fluvial features [8-10], and the abundance of remnant buttes that are indicative of widespread erosional processes [11]. Here we present preliminary results of our mapping and geologic analysis.&#160;Preliminary&#160;Results&#160;and Discussion:&#160;The study area is approximately 225&#160;&#215;&#160;190 km&#160;in size.&#160;The&#160;general&#160;elevation&#160;increases in height&#160;from&#160;north to south&#160;from&#160;approximately&#160;-2,960&#160;m to&#160;-1,800&#160;m.&#160;Both at&#160;OP&#160;and&#160;XT, the&#160;lowland-highland boundary is expressed&#160;as&#160;a gradual slope rather than&#160;a sharp topographic&#160;step.&#160;Figure 1: (A) THEMIS nighttime IR and (B) HRSC image mosaics of the study site in Xanthe Terra.In the northwestern study&#160;area, the&#160;Hypanis&#160;fan&#160;represents&#160;the arguably&#160;largest sedimentary&#160;fan&#160;deposit&#160;on Mars&#160;[8-10].&#160;These deposits&#160;are finely&#160;layered&#160;and&#160;appear dark&#160;in THEMIS&#160;nighttime&#160;IR images&#160;(Fig. 1).&#160;Sedimentary&#160;fan&#160;deposits, while having a smaller extent,&#160;are&#160;also present in&#160;OP&#160;at the termination&#160;of the&#160;Coogoon&#160;Valles&#160;fluvial&#160;system&#160;[5,12,13]. Both the&#160;sedimentary&#160;deposits in&#160;XT&#160;[8-10]&#160;and&#160;OP&#160;[13]&#160;are thought to&#160;have a fluvial origin.&#160;Phyllosilicates are widespread in&#160;OP&#160;[4-6] and are the main target of the ExoMars rover.&#160;Apart from their spectral signature, they are also characterized by a bright THEMIS nighttime IR signature and a polygonised texture at HiRISE scale. A global-scale search for chemical alteration signatures revealed the presence of Fe/Mg phyllosilicates in northern&#160;XT,&#160;marginal to&#160;Chryse&#160;[4]. As in&#160;OP, the phyllosilicate-bearing&#160;plains&#160;appear bright in THEMIS nighttime&#160;IR&#160;images (Fig.&#8239;1).&#160;At least two different units are present in this region. The most widespread is a light and layered unit, which is partly eroded, exposing a darker material underneath (Fig. 2). NW &#8211; SE orientated small dunes and periodic bedrock ridges, a type of aeolian erosional landforms which is ubiquitous in the clay-bearing plains of OP&#160;[14], are present on top of the darker unit. HiRISE coverage of both units shows distinct polygonised textures. Several inverted craters are present within this two units. The material of these inverted craters differs from both units, and, therefore, represents the remnants of at least one further&#160;eroded unit. The phyllosilicate-bearing plains are overlain by a widespread unit, which seems to be present in most of the northern study area.Figure 2: (A) HRSC mosaic of the phyllosilicate-bearing plains. The erosional contact is shown in (B). (C) and (D) are HiRISE images of the lighter and darker unit, respectively.The rims of&#160;ancient&#160;filled craters&#160;are&#160;outlined by rings of&#160;mounds.&#160;The&#160;diameters&#160;of these mounds&#160;range from&#160;tens of meters to several kilometers.&#160;Their&#160;appearance&#160;is variable and&#160;resembles&#160;mesas, rounded&#160;knobs/buttes,&#160;and &#8220;bread-crust-like features&#8221;&#160;(Fig. 3).&#160;These mounds&#160;are increasing in height and abundance towards north, i.e. towards&#160;Chryse. A similar trend of height and abundance&#160;was also observed in&#160;OP&#160;[15].&#160;However, the&#160;height of these mounds varies&#160;individually. Rounded&#160;buttes, for example,&#160;can be higher than mesas in their&#160;immediate&#160;neighborhood.&#160;Some&#160;mounds&#160;show a fine&#160;layering,&#160;and, in some cases,&#160;mounds&#160;stand on top of a&#160;finely layered&#160;basis.&#160;The mounds&#160;appear dark in THEMIS nighttime&#160;infrared images&#160;(Fig. 1).&#160;Similar mounds&#160;are present&#160;along the eastern&#160;highland boundary&#160;of&#160;Chryse [11].&#160;The composition&#160;of the&#160;mounds&#160;and their&#160;erosional&#160;history are not yet clear. The low thermal inertia and the fine layering of&#160;some of&#160;the&#160;mounds,&#160;however,&#160;favor a sedimentary material.&#160;The&#160;timing of mound formation&#160;relative&#160;to other&#160;geological units is&#160;also&#160;still unknown since&#160;stratigraphic relationships&#160;between these units&#160;still seem conflicting.&#160;Figure 3: (A) Mapped mounds in the study area. Rounded buttes are shown in green and mesas in orange. (B) CTX and HiRISE images of the remnant mound types.Preliminary Conclusions:&#160;Northern&#160;XT&#160;and&#160;OP&#160;share&#160;various geological&#160;characteristics&#160;(e.g., sedimentary fans, phyllosilicates,&#160;remnant mounds)&#160;and a comparable location at the lowland-highland boundary at&#160;Chryse&#160;Planitia.&#160;These similarities&#160;enable a&#160;comparison between both&#160;sites and a testing of hypotheses related to the&#160;ExoMars&#160;rover&#160;landing&#160;site.&#160;References:&#160;&#160;[1]&#8239;Vago, J. L. et al. (2015)&#160;Sol. Syst. Res.,&#160;49,&#160;538&#8211;542. [2]&#8239;Bridges, J. C. et al. (2018),&#160;LPS XLIX, Abstract #2177. [3]&#8239;Vago, J. L. et al. (2017)&#160;Astrobiol., 17,&#160;471&#8211;510. [4]&#8239;Carter, J. et al., (2013)&#160;J.&#160;Geophys. Res.,&#160;118, 831&#8211;858. [5]&#160;Quantin-Nataf, C. et al. 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