Oxia Planum: The Landing Site for the ExoMars "Rosalind Franklin" Rover Mission: Geological Context and Prelanding Interpretation

Abstract

The European Space Agency (ESA) and Roscosmos ExoMars mission will launch the "Rosalind Franklin" rover in 2022 for a landing on Mars in 2023.The goals of the mission are to search for signs of past and present life on Mars, investigate the water/geochemical environment as a function of depth in the shallow subsurface, and characterize the surface environment. To meet these scientific objectives while minimizing the risk for landing, a 5-year-long landing site selection process was conducted by ESA, during which eight candidate sites were down selected to one: Oxia Planum. Oxia Planum is a 200 km-wide low-relief terrain characterized by hydrous clay-bearing bedrock units located at the southwest margin of Arabia Terra. This region exhibits Noachian-aged terrains. We show in this study that the selected landing site has recorded at least two distinct aqueous environments, both of which occurred during the Noachian: (1) a first phase that led to the deposition and alteration of ∼100 m of layered clay-rich deposits and (2) a second phase of a fluviodeltaic system that postdates the widespread clay-rich layered unit. Rounded isolated buttes that overlie the clay-bearing unit may also be related to aqueous processes. Our study also details the formation of an unaltered mafic-rich dark resistant unit likely of Amazonian age that caps the other units and possibly originated from volcanism. Oxia Planum shows evidence for intense erosion from morphology (inverted features) and crater statistics. Due to these erosional processes, two types of Noachian sedimentary rocks are currently exposed. We also expect rocks at the surface to have been exposed to cosmic bombardment only recently, minimizing organic matter damage.

Keywords: ExoMars; Landing site; Mars; Oxia Planum..

FIG. 1.

(A) Global map darkening the places noncompliant with elevation, thermal inertia, and latitude constraints. Hesperian is in orange units and Noachian is in red (Tanaka et al., 2014). (B) Enlarged map focused on Chryse and Isidis regions of Mars darkening the places noncompliant with elevation, thermal inertia, and latitude constraints and highlighting the hydrous mineral detection from Carter et al. (2013).

FIG. 2.

Location of Oxia Planum area in the region between Ares Vallis and Mawrth Vallis. (A) Regional geological map from the work of Tanaka et al. (2014). (B) Topographic context of MOLA data (Smith et al., 2001). MOLA, Mars Orbiter Laser Altimeter.

FIG. 3.

Hydrous mineral map from OMEGA and CRISM MSP data (Carter et al., 2016) over MOLA elevation map. The hydrous minerals bearing unit is widespread and drapes the current topography from −2600 m of elevation to −3100 m where ExoMars 2020 ellipses are located. The dark ellipses are the 3-sigma probability of the two ellipse endmembers of the launch period: LPO and LPC. The dotted circle highlights a putative ancient circular depression. CRISM, Compact Reconnaissance Imaging Spectrometer for Mars; OMEGA, Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité; LPC, launch period closing; LPO, launch period opening; MSP, multispectral survey.

FIG. 4.

Geological map of the ExoMars ellipses area. The red ellipses are the three-sigma LPO and LPC ellipses.

FIG. 5.

(A) Location of the impact craters used in (B) for the crater-based age determination, (B) cumulative crater density superimposed on the clay-bearing unit in an incremental diagram performed by the Craterstats2 software (Michael et al., 2012). In the crater size distribution of craters >1 km, only the craters >5 km follow an isochron. Only these craters >5 km have been used to assess the age of the clay-bearing unit. The size distribution of the crater <5 km has a lower slope than any isochron (e.g., some isochrons are plotted in light gray), indicating continuous resurfacing processes (see section “Erosional history of Oxia Planum”). We can note that some of the craters are partly filled (marked in pink) by the dark resistant unit described latter in section “Dark resistant unit (Adru).” For these filled craters, we carefully checked that the clay-bearing unit is exposed in the ramparts of the filled craters attesting that the craters were emplaced in the clay-bearing rocks.

FIG. 6.

Layers of the Noachian clay-rich formation. (A) Location of the subfigures B, C, D, E, F and Fig. 7 and location of the 2.6 km crater with unaltered ejecta discussed in the article. The background is THEMIS daytime mosaic overlapped by the hydrated mineral mapping detailed in the work of Carter et al. (2016). (B) Rampart of an impact crater exposing ∼100 m of layers (HiRISE image ESP_044811_1985). (C) Layers exposed on the rampart of a 2 km impact crater. At least 50 m of layers are exposed here (HiRISE image ESP_043268_1980). (D) Possible cross-stratifications exposed in impact crater walls (HiRISE image PSP_003195_1985). (E) Layers exposed in the wall of a 1.3 km impact crater (HiRISE image ESP_051905_1990). (F) layers exposed in the wall of a 1.1 km impact crater (HiRISE image ESP_050705_1985). HiRISE, High Resolution Imaging Science Experiment; THEMIS, Thermal Emission Imaging System.

FIG. 7.

High-resolution morphologies and color variations of the fractured clay unit, as seen in HiRISE color data of ESP_045378_1980. (A) Closeup on the transition from upper and bluish unit and the reddish lower unit with the location of the topographic profile displayed in B and the two closeup (C, D). (B) Topographic profile from CTX DTM and interpretative cross-section. (C) Closeup on the bluish upper unit with decametric fractures. (D) Closeup on the reddish lower unit with metric-scale fractures. CTX, Context Camera; DTM, Digital Terrain Model.

FIG. 8.

Catchment area and valley networks converging to Oxia Planum. The valley networks as depressions or as inverted channels are mapped as blue lines. The LPO and LPC ellipses of Oxia Planum are mapped in red. The impact craters with preserved ejectas are underlined as dotted dark lines. The valley network system is largely overlapped by these nondegraded craters.

FIG. 9.

(A) Map of the craters identified as younger than Coogoon Valles fluvial system used in (B) for the age estimation. The three largest craters postdating the main fluvial activity are mapped as hatched with their ejecta. (B) Cumulative crater size distribution returning an age of 3.86 Ga with the Craterstats2 software (Michael et al., 2012). The size distribution of craters >3.2 km (marked in blue) follows an isochron and was used to assess the age of the valley system. If we include all craters >3.2 km, the age returned is 3.86 Ga, while the age is 3.83 Ga if the six largest ones are excluded (as they are not perfectly aligned on the 3.86 isochron).

FIG. 10.

Overview of the delta fan: (A) CTX context where in blue the fan material has been mapped based on its thermal inertia signature. The arrow denotes the lineation observed on the top of the fan. The pink circle highlights the overlap of lineations of different orientations. The subfigure 11A is located. (B) THEMIS Thermal inertia map and location of the subfigures 11B–E. The red arrow points out the low thermal inertia fan at the outlet of another valley system than Coogoon Valles at similar elevation to the main delta fan.

FIG. 11.

HiRISE views of the delta fan: (A) CTX closeup of lineations (part of CTX image G02_019084_1991_XI_19N024W). Red arrows point toward the lineations. (B) HiRISE closeup of a finger-type termination where we observe that the fan is lying on the top of the fractured unit (part of HiRISE ESP_019084_1980). The red arrows denote the retreating scarps. (C, D) HiRISE closeup of embedded impact craters within the fan in respective part of HiRISE ESP_019084_1980 for (C) and HiRISE ESP_039721_1980 for (D). Red arrows denote the fan layers affected by the impact craters, attesting that the craters postdate a part of the fan activity and the blue arrows denote the fan material that filled the crater, attesting that the fan activity lasts after the craters emplacement. The direction of the blue arrow agrees with the putative flow sense. (E) HiRISE closeup of the layers exposed on the scarps of the fan (part of HiRISE ESP_039721_1980).

FIG. 12.

Main water levels recorded in Oxia Planum (background is MOLA topography): (A) the different delta fans around −2960 m of elevation denote first water level. The upper delta implies a higher water level at around −2940 m, while the secondary delta implies a lower water level at around −3010 m. In such case, the bathymetry would be of the order of 240 m for the deepest point of the basin, and the standing body of water would be open to the northern plains according to the current topography. (B) A second main water level is recorded at the outlet of the valleys downstream the delta fans at around −3120 m. Some valleys imply an even lower water level, indicating a restricted standing body of water of 80 m for the deepest part. According to the current topography, the basin would be enclosed and separated from Chryse Planitia. (C) Current dry conditions.

FIG. 13.

Rounded buttes distribution. (A) Location of rounded buttes mapped in black. The red ellipses are LPO and LPC landing ellipses. The background is a mosaic of CTX DTM. The square denotes the location of Figure 14. (B) HiRISE closeup of a rounded butte (ESP_051905_1990). (C) THEMIS thermal inertia map of the rounded butte displayed in (B). The range of thermal inertia is from 0 to 800 TIU. The rounded butte has a clear low thermal inertia signature that cannot be explained by recent loose material since the lowest thermal inertia is observed at the top of the butte. (D) HiRISE closeup of a rounded butte showing an impressive exposure of layers (ESP_037070_1985). (E) HiRISE closeup of a rounded butte displaying inverted ridges marked as red arrows (ESP_044679_1985). (F) HiRISE closeup of a rounded butte lying on a crater rim (ESP_51206_1985). (G) HiRISE closeup of a rounded butte displaying inverted ridges marked as red arrow (ESP_037070_1985).

FIG. 14.

Rounded buttes stratigraphy. (A) Global topographic view of a 15 km crater filled by the dark resistant unit. The background is a mosaic of CTX images with CTX DTM topography displayed in transparency. The red arrows locate the rounded buttes observed on the rim of impact craters. The two red squares denote the location of the subfigure (B, C). (B) HiRISE closeup of the rounded buttes linked to the 15 km crater (ESP_044811_1985). The rounded butte 1 is on the floor of the impact crater, while the rounded butte 2 is on the rim of the crater. They both are surrounded by the dark resistant unit, suggesting that the dark resistant unit embedded the rounded buttes. This observation implies that the rounded buttes predate the dark resistant unit's emplacement and where already as remnant buttes by that time. (C) HiRISE closeup of the rim of the 15 km crater (ESP_44811_1985). CTX DTM topography is superimposed in transparency. We note the light-toned layers exposed in the inner wall of the impact crater.

FIG. 15.

The mantling unit. (A) HiRISE view of the transition between the dark resistant unit, the mantling unit, and the clay-bearing unit (ESP_019084_1980). The mantling unit is covering the clay-bearing unit but has never been observed at the top of the dark resistant unit. (B) HiRISE closeup of the mantling unit right at the feet of the delta fan (ESP_019084_1980). The mantling unit is often observed around and downstream the main delta fan. (C) HiRISE closeup of the flank exposure of the mantling unit (ESP_019084_1980). Layers seem to be present. The unit is indurated and ∼5 m thick as measured here on a HiRISE DTM. (D) HiRISE closeup of the mantling unit in the center part of the landing ellipse where the clay-rich unit is more exposed. There, the mantling unit is less extensive and is only observed as remnants in local lows such as impact craters or as inverted impact craters.

FIG. 16.

The dark resistant unit: (A) CTX image mosaic superimposed by CTX DTM mosaic showing that the dark resistant unit is in topographic lows. Black squares locate the subfigure (B, E). (B) HiRISE closeup of the border of the dark resistant unit where the retreating scarp allows the subjacent clay-bearing unit to be freshly exposed (PSP_009735_1985), (C) Crater counts corresponding to the pinkish craters mapped in (A). Incremental crater size distribution with an age model displayed corresponding to Hartmann (2005) isochrones and Ivanov (2001) impact rate model. The crater size distribution observed follows the isochrons slope with best age model of 2.57 Ga. The counted area is 83.75 km² where 413 craters have been mapped. (D) Topographic profile from X and Y points displayed in (E). Elevation has been extracted from CTX DTM mosaic. The dark resistant unit is here ∼15 m thick. (E) HiRISE closeup of an impact crater filled by the dark resistant capping unit as inverse morphology (ESP_051206_1985).

FIG. 17.

Crater obliteration history. (A) global crater count on the clay-bearing unit. Light red corresponds to the hydrated mineral mapping from the work of Carter et al. (2016). The craters >1 km (in red) have been assessed over the entire Oxia Planum area at THEMIS image scale. Four hundred two craters have been mapped over an area of 28,626 km². (B) Crater counts done at higher spatial resolution on CTX images on a part of well-exposed clay-bearing unit. Two hundred fifty-six craters <1 km have been mapped on a surface of 216 km². (C) In red is the entire crater size distribution of crater from 100 m to 30 km of diameter from the crater count presented in A and B. In blue is the modeled crater size distribution according to the crater obliteration model presented in D that best fits the data. The methodology of the modeling and best fitting is presented in the work of Quantin-Nataf et al. (2019). (D) The crater obliteration evolution of the best fit model of crater size distribution. Like Mars globally, Oxia Planum has undergone a decrease of crater obliteration rate with time from a rate as large as 8 m/Ma in Noachian. The total amount of crater obliteration is in the order of 900 m. It is distributed as 650 m between 4 and 3 Ga and 240 m over the last 3 Ga.

FIG. 18.

Proposed scenario of the evolution of Oxia Planum. (A) In middle Noachian, the layered clay-rich unit forms. (B) Much later than the clay-rich layered formation but still during Noachian, a fluviodeltaic system occurs depositing the currently observed main delta fan. Extended deposits corresponding to the remnant buttes observed today may have formed during this period. (C) Later the water level decreases allowing erosion of the main delta fan and eroding valleys downstream the main delta fan as well as all other deposits. (D) During Hesperian time and early Amazonian times, a large amount of erosion occurred leading to remnant rounded buttes. (E) A widespread unit emplaced (the dark resistant unit) possibly by volcanism, like lava flow process. (F) After the early Amazonian, erosion continues leading to the erosion of the dark resistant unit allowing for the exposure of the eroded landscape at their stage of early Amazonian times.