ArticlePDF Available

Mounds in Oxia Planum: The Burial and Exhumation of the ExoMars Rover Landing Site

Authors:

Abstract and Figures

Oxia Planum, the planned landing site of the ExoMars “Rosalind Franklin” rover, is a low relief clay‐bearing plain, of which approximately 1% is covered by 396 upstanding isolated landforms (“mounds”). The mounds are continuous with a circum‐Chryse mound population representing the remnants of a regionally significant Noachian‐aged deposit. This detailed study suggests that the Oxia Planum mounds are also erosional remnants of this deposit, with little evidence to suggest they are constructional landforms such as sedimentary volcanoes. We calculate that up to 130 m of mound‐forming material has been removed from the landing site through erosion. The mound‐forming layer lies unconformably on the clay‐bearing plains with the upper surface severely truncated by significant erosion resulting in the topography we see today. The mounds themselves comprise at least three members, distinct in color and texture, separated stratigraphically by further unconformities. Calculated minimum erosion/deposition rates of the removed mound material are comparable to previous Noachian estimates, suggesting a more erosive (and probably therefore warmer and/or wetter) environment than today. The clay‐bearing materials which remain buried directly under the mounds have been continually protected from the Martian environment since the Noachian, and are likely to represent some of the most pristine clay‐rich materials in the landing site. By inference, the plains directly adjacent to the mounds are most likely to have been exposed for less time than areas further from the mounds. These are therefore amongst the most likely locations where Rosalind Franklin could sample recently exposed materials and hence detect biosignatures.
Observations of different component mound members throughout the landing site and wider Oxia Planum area; (a) High Resolution Imaging Science Experiment (HiRISE) infrared‐red‐blue (IRB) [ESP_057747_1985] showing a cluster of mounds around a larger, dissected mound (gullies shown by dotted lines) north of Germania Lacus, where the rough, yellow upper member (UM) covers a blue‐white toned lower member (LM), which we suggest is uneroded sections of blue member of the clay‐bearing unit (CBU); (b) exposed layering (red dotted lines) within the LM; (c) Color and Stereo Surface Imaging System (CaSSIS) [MY36_016481_161_0] showing a cluster of mounds southwest of the study area showing two mounds with the UM overlying middle member (MM) on the blue member of the CBU, isolated patches of MM (white arrows), and a potential unconformity shown by possible MM (due to similar color and texture) infilling craters within the blue member of the CBU; (d) CaSSIS [MY36_018354_017_0] showing patches of MM which overlie blue CBU, with the larger example showing blue‐white colors with curvilinear fractures, and smaller, more eroded examples being more yellow in color; (e) HiRISE IRB [ESP_037070_1985] showing a 54 m‐tall mound with layered UM overlying a ∼6 m thick orange layer at its base. Inset shows bright‐toned meter‐scale boulders demarcating the base of layers within the UM; and (f) HiRISE IRB [ESP_051905_1990] showing draping meter‐ to decameter‐scale layers (white arrows) in the flank of a pale‐toned mound. North is up, unless stated otherwise.
… 
Mounds in Oxia Planum, showing stratigraphic and tectonic relationships: (a) rounded mound on the rim of a ∼2 km diameter crater (Sardinia Lacus) in the clay‐bearing unit (CBU), which has been later infilled by dark resistant unit (DRU) (High Resolution Imaging Science Experiment [HiRISE], ESP_047501_1985); (b) 3D 2x exaggerated view of mound showing the mound‐CBU boundary (see inlay), which is elevated ∼20 m above the topographic mound boundary (HiRISE, PSP_009880_1985); (c) mound with upstanding linear features (red) showing a ∼60° intersection angle at the mound center (HiRISE, ESP_048358_1985); (d) a mound showing similar features to that in panel (c), where upstanding linear to curvilinear features truncate a mound at a ∼60° intersection angle (HiRISE, ESP_037070_1985); (e) intra‐crater and crater‐rim mound on the rim of Malino Crater, where the CBU has been exposed through the backstepping of DRU and/or mound material (HiRISE, ESP_062191_1985); (f) intra‐crater mound in Malino Crater, where the onlap of the DRU onto the mound flank (red dashed line) is clearly visible on fractured the lower member (LM) or CBU, with an additional DRU layer in yellow (ESP_062191_1985); (g) mound with two members: a thin blue‐toned middle member (MM) that underlies both the DRU, and an upper member (UM) which is yellow‐white (Color and Stereo Surface Imaging System [CaSSIS], MY35_007623_019_0); and (h) orange and blue‐toned members of the CBU overlain by mounds, shorter mounds (black arrow), and the bases of tall mounds (red arrow) are white‐blue, suggesting the presence of the LM or MM, and upper sections (UM) are yellow‐white (white arrow; CaSSIS, MY35_008742_019_0). North is up, unless stated otherwise.
… 
This content is subject to copyright. Terms and conditions apply.
1. Introduction
Oxia Planum is the planned landing site of ESA's ExoMars “Rosalind Franklin” rover (Vago etal.,2017). This
low-relief plain is situated on the hemispheric dichotomy boundary in the transitional terrain (classified as
HNCc1—Early Hesperian to Late Noachian in Tanaka et al.(2005)) between the ancient, rugged highlands
of Arabia Terra and the younger, smoother lowlands of Chryse Planitia. Rosalind Franklin will investigate the
surface and subsurface geologic and geochemical environments of Oxia Planum, with a primary objective to
search for evidence of extinct life through the identification of potential habitable paleoenvironments and biosig-
natures (Vago etal.,2017). Oxia Planum contains a diversity of Noachian-aged geologic features and geochemical
characteristics which make it a suitable candidate for astrobiological exploration, including extensive Fe/Mg-rich
clay-bearing plains indicative of widespread aqueous alteration (e.g., Brossier etal.,2022; Carter et al.,2016;
Gary-Bicas & Rogers,2021; Mandon et al., 2021; Parkes Bowen et al.,2022; Quantin-Nataf etal., 2021), a
hydrated silica-bearing sedimentary fan at the terminus of the Coogoon Valles network suggesting the presence
Abstract Oxia Planum, the planned landing site of the ExoMars “Rosalind Franklin” rover, is a low relief
clay-bearing plain, of which approximately 1% is covered by 396 upstanding isolated landforms (“mounds”).
The mounds are continuous with a circum-Chryse mound population representing the remnants of a regionally
significant Noachian-aged deposit. This detailed study suggests that the Oxia Planum mounds are also
erosional remnants of this deposit, with little evidence to suggest they are constructional landforms such as
sedimentary volcanoes. We calculate that up to 130m of mound-forming material has been removed from the
landing site through erosion. The mound-forming layer lies unconformably on the clay-bearing plains with the
upper surface severely truncated by significant erosion resulting in the topography we see today. The mounds
themselves comprise at least three members, distinct in color and texture, separated stratigraphically by further
unconformities. Calculated minimum erosion/deposition rates of the removed mound material are comparable
to previous Noachian estimates, suggesting a more erosive (and probably therefore warmer and/or wetter)
environment than today. The clay-bearing materials which remain buried directly under the mounds have been
continually protected from the Martian environment since the Noachian, and are likely to represent some of the
most pristine clay-rich materials in the landing site. By inference, the plains directly adjacent to the mounds are
most likely to have been exposed for less time than areas further from the mounds. These are therefore amongst
the most likely locations where Rosalind Franklin could sample recently exposed materials and hence detect
biosignatures.
Plain Language Summary The Oxia Planum region of Mars is the landing site of ESA's ExoMars
Rosalind Franklin” rover, which will search for evidence of past life in the ancient rocks of the area. In the
landing site there are hundreds of sub-kilometer-scale “mounds” that are likely to have been part of an extensive
layer which covered the region in the distant past. To understand more about the mounds, we examined
their geological features, calculated the volume of eroded material, and observed their relationships to other
important features within the landing site. We find: (a) On average, the layer was up to 70% thinner in Oxia
Planum than elsewhere in the region. (b) There are gaps in the geological record below, within, and above the
mounds indicating periods of erosion. (c) The layer was eroded at a much faster rate in the past than in the
present, consistent with a warmer and wetter ancient Martian environment. Areas around the mounds were
probably exposed relatively recently, and are therefore likely to have been protected from the harsh Martian
environment for longer than other areas. Consequently, these areas may be amongst the best places for the rover
to search for evidence of past life.
MCNEIL ETAL.
© 2022. The Authors.
This is an open access article under
the terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.
Mounds in Oxia Planum: The Burial and Exhumation of the
ExoMars Rover Landing Site
Joseph D. McNeil1 , Peter Fawdon1 , Matthew R. Balme1 , Angela L. Coe2 , and Nicolas Thomas3
1School of Physical Sciences, The Open University, Milton Keynes, UK, 2School of Earth, Environment and Ecosystem
Sciences, The Open University, Milton Keynes, UK, 3Physikalisches Institut, University of Bern, Bern, Switzerland
Key Points:
Mounds in Oxia Planum represent
eroded remnants of a circum-Chryse
deposit that was up to 130m thick in
the ExoMars rover landing site
The mounds unconformably overlie
the clay-bearing unit (CBU) and are in
turn unconformably onlapped by the
dark resistant unit
The plains immediately adjacent to
the mounds are the most recently
exposed parts of the astrobiologically
important CBU
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
J. D. McNeil,
joe.mcneil@open.ac.uk
Citation:
McNeil, J. D., Fawdon, P., Balme, M.
R., Coe, A. L., & Thomas, N. (2022).
Mounds in Oxia Planum: The burial and
exhumation of the ExoMars rover landing
site. Journal of Geophysical Research:
Planets, 127, e2022JE007246. https://doi.
org/10.1029/2022JE007246
Received 24 MAR 2022
Accepted 18 OCT 2022
Author Contributions:
Conceptualization: Joseph D. McNeil,
Peter Fawdon, Matthew R. Balme, Angela
L. Coe
Data curation: Joseph D. McNeil, Peter
Fawdon, Nicolas Thomas
Formal analysis: Joseph D. McNeil
Funding acquisition: Matthew R. Balme
Investigation: Joseph D. McNeil
Methodology: Joseph D. McNeil
Project Administration: Joseph D.
McNeil, Peter Fawdon, Matthew R.
Balme, Angela L. Coe
Resources: Peter Fawdon
Software: Joseph D. McNeil
10.1029/2022JE007246
RESEARCH ARTICLE
1 of 17
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
2 of 17
of a standing body of water (e.g., Fawdon etal.,2022; Molina etal.,2017; Quantin-Nataf etal.,2021), and a
series of landforms revealing a complex depositional and erosional history (Quantin-Nataf etal.,2021; Roberts
etal.,2021).
The widely exposed clay-bearing plains will be the focus of Rosalind Franklin's biosignature detection and
surface/subsurface aqueous environment investigations. The clay-bearing plains are bright-toned, highly frac-
tured, low-relief regions that are consistent with detections of phyllosilicates by Observatoire pour la Minéral-
ogie, l'Eau, les Glaces et l'Activité (Bibring et al., 2004) and absorption spectra in hyperspectral Compact
Reconnaissance Imaging Spectrometer for Mars (CRISM; Murchie et al., 2007) data which correspond to
Fe/Mg-rich phyllosilicates such as vermiculite and/or smectite clays (Carter etal.,2016; Mandon etal., 2021;
Parkes Bowen etal.,2022). The clay-bearing plains contain at least two distinct geologic members (Mandon
etal.,2021). These are a lower, thicker member which exhibits orange tones in High Resolution Imaging Science
Experiment (HiRISE; McEwen etal.,2007) images and contains closely spaced meter-scale fractures, and over-
lying this, an upper, thinner member which exhibits blue-white tones in HiRISE and contains more widely spaced
decameter-scale fractures (Parkes Bowen etal.,2022; Quantin-Nataf etal.,2021).
Overlying the clay-bearing plains is a population of kilometer to sub-kilometer-scale mounds (Figure1), which
resemble terrestrial erosional features such as buttes (e.g., Monument Valley, Utah/Arizona), mesas (e.g., Grand
Mesa, Colorado), inselbergs (e.g., Uluru, Australia), and hills (McNeil etal.,2021a; Quantin-Nataf etal.,2021).
These mounds are topographically prominent features which represent three-dimensional exposures of mate-
rial accessible for study by Rosalind Franklin. Their relatively high albedo in Context Camera (CTX; Malin
etal.,2007) and white-yellow tones in Color and Stereo Surface Imaging System (Thomas etal.,2017) Near-IR,
Panchromatic, Blue-Green products (NPB, see Section2) contrast with the orange-blue tones of the surrounding
clay-bearing plains in HiRISE and CaSSIS (Parkes Bowen etal.,2022; Quantin-Nataf etal.,2021).
The mounds are part of a regional population of similar positive-relief isolated landforms that occur around
the highland margin of Chryse Planitia (Figure1a). This wider population of over 14,000km-scale mounds
is interpreted as the eroded remnants of a layer, up to 500m thick, that superposed the circum-Chryse region
during the Noachian and may be similar in age and composition to the Mawrth Vallis phyllosilicates (McNeil
etal.,2021a). Prior to this study, it was unclear how the smaller Oxia Planum mounds relate to the circum-Chryse
layer-forming population. The smaller size of the mounds in Oxia Planum means that a higher resolution exami-
nation is required to understand their geology, and to determine whether these are landforms which are part of the
original circum-Chryse mound-forming layer (and are therefore erosional in nature), or whether they are individ-
ual landforms formed through geographically isolated processes (and are therefore primarily constructional). The
mounds in Oxia Planum overlie the clay-bearing plains (McNeil etal.,2021a; Quantin-Nataf etal.,2021), and
could therefore record depositional processes which occurred after the deposition or emplacement of the plains.
Furthermore, because the mounds could be erosional remnants, they may provide information about subsequent
erosional processes in Oxia Planum. Previous models have suggested that up to 900m of material has been
removed from the landing site (Quantin-Nataf etal.,2021), but how much belonged to individual mounds or the
hypothetical mound-forming layer compared to the clay-bearing plains or other stratigraphic units at the landing
site, is unknown.
In this paper, we: (a) Explore the morphology, morphometry, and stratigraphic relationships of mounds in Oxia
Planum; (b) Assess whether the mounds originated as constructional landforms in their current isolated locations,
or if they are erosional features which were once part of previously more laterally extensive deposits; and (c)
Calculate the hypothetical thickness and volume of the putative mound-forming layer to assess the quantity and
timing of overburden removal from the landing site and the feasibility of this hypothesis. From this, we discuss
the implications for the geology of the mounds, the depositional and erosional history of the landing site, and the
ramifications for the scientific objectives of the “Rosalind Franklin” rover.
2. Data and Methods
We have used the informal geographical names given to features and regions of the landing site by the ExoMars
Rosalind Franklin rover team (see Fawdon etal.,2021a). Using ArcGIS Pro software, we created a Geographical
Information Systems (GIS) project for the study area and included all available remote sensing data. Mounds
(upstanding, bright-toned, topographically prominent features) were identified using a combination of visible
Supervision: Peter Fawdon, Matthew R.
Balme, Angela L. Coe
Validation: Joseph D. McNeil, Peter
Fawdon, Matthew R. Balme
Visualization: Joseph D. McNeil, Angela
L. Coe
Writing – original draft: Joseph D.
McNeil
Writing – review & editing: Joseph
D. McNeil, Peter Fawdon, Matthew R.
Balme, Angela L. Coe
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
3 of 17
(CaSSIS, HiRISE and CTX) and topographic (CTX and HiRISE) data in the ∼8,700km
2 study area (Figure1).
Geologic and stratigraphic observations of mounds were primarily made using CaSSIS NPB (Near-Infrared,
850 nm, Panchromatic, 650 nm, and Blue–Green, 475 nm; Thomas et al., 2017) and HiRISE RED and
infrared-red-blue (IRB); McEwen etal.,2007) data. The lower mound boundaries (where the mound-forming
material contacts the plains-forming material) were identified in a 6m/pixel CTX mosaic (Fawdon etal.,2021a)
and manually digitized as polygons. Equidistant buffers of 50m were generated around each mound to represent
the surface the mounds are sitting on. This buffer size was selected as it is large enough to capture a represent-
ative portion of the mound-adjacent topography from the DTM, and small enough that it reduces the likelihood
that other topographically prominent features (e.g., other mounds, craters) could be unintentionally captured in
the buffer. Elevation data for all mounds were derived from CTX Digital Terrain Models (DTMs) with a spatial
resolution of 20m/pixel and an expected vertical precision of between 1.34 and 6.16m (mean 3.19m; Fawdon
etal.,2021a). Using the ArcGIS Pro zonal statistics tool, the maximum elevation of each mound (Zapex) was
extracted from the DTM. The median elevation of the topography within the surrounding buffer (Zbase) was
also extracted to capture the elevation of the surrounding plains. Mound heights were calculated by subtracting
Zbase from Zapex. These data were encapsulated as a feature class in the GIS, with points generated at the geographic
center of each mound that contained the geographic and morphometric information required for further analysis.
To determine whether the Oxia Planum mounds formed part of the larger circum-Chryse mound-forming deposit,
we analyzed the volumes and surfaces of this hypothetical layer using the mound morphometric data from this
study and from McNeil etal. (2021a). From the elevation data, three interpolated surfaces were created using
the 3D Analyst tools in ArcGIS Pro (Figure2). A minimum upper bounding surface (MUBS) was generated
from Zapex using the Natural Neighbor tool (which uses the heights and distances between points, weighted by
proportional overlaps of constructed Voronoi diagrams, e.g., Sibson(1981)) to generate a surface. The MUBS
is the minimum elevation of the upper surface of the layer from which the mounds originally eroded. The lower
bounding surface (LBS) was generated from Zbase also using the Natural Neighbor tool, and this represents a
Figure 1. Mounds in the Oxia Planum region, showing: (a) the location of mounds in the study area relative to ∼14,000
mounds (white) in the circum-Chryse region; background of Mars Orbiter Laser Altimeter (Smith etal.,2001) and High
Resolution Stereo Camera (Jaumann etal.,2007) hillshade, TV: Tiu Valles, AV: Ares Valles, and MV: Mawrth Vallis, (b) 2x
exaggerated 3D High Resolution Imaging Science Experiment image of a typical mound (ESP_039299_1985), showing its
prominence over the surrounding clay-bearing unit and dark resistant unit, and (c) Context Camera digital terrain model of
396 mounds (white) and the 1-sigma (green) and 3-sigma (yellow) landing ellipses, GL: Germania Lacus, KC: Kilkhampton
Crater. The panels in Figures3 and4 are labeled in cyan.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
4 of 17
generalization of the paleosurface that the mound layer was deposited onto. We used the heights of all mesas
and tiered mounds (assumed to be the tallest and most complete sections of the mound-forming deposit) east of
Ares Vallis from the database of circum-Chryse mounds in McNeil etal. (2021b) to generate a Chryse Planitia
upper bounding surface (CPUBS) with the Inverse Distance Weighted surface interpolation tool. The Inverse
Distance Weighted tool was required instead of the Natural Neighbor tool to achieve a surface that could be
extrapolated to the landing site. The CutFill tool in ArcGIS Pro was used to estimate the volumes bounded by
the three interpolated surfaces and the present-day surface described by the CTX DTM (Figure2b). The volume
between the undulating present-day surface and the MUBS is a minimum estimate of the amount of inter-mound
material removed from Oxia Planum, and the volume between the present-day surface and the CPUBS is an esti-
mate of the total material that could have been removed assuming the Oxia Planum and Circum-Chryse mounds
were originally the same height. The volumes between the LBS and the MUBS, and the LBS and CPUBS, are
the minimum amount of removed mound material, and the best estimate of the total removed mound material,
respectively. As the present-day surface is below the LBS over significant parts of Oxia Planum, subtracting
the present-day surface from the LBS where the mounds are not present yields an estimate of the amount of
material eroded from below the base of the mounds. This eroded material may have been mound material and/or
clay-bearing unit (CBU).
Figure 2. Schematic transects across Chryse Planitia and Oxia Planum, showing (a) surfaces generated from mound
elevation data (Chryse Planitia Upper Bounding Surface, Minimum Upper Bounding Surface, and Lower Bounding Surface),
the relationships between the mounds, clay-bearing unit and dark resistant unit and (b) close-up of a mound in Oxia Planum
showing the interplay between surfaces and resultant calculated volumes. Dashed lines indicate interpolated surfaces; circles
indicate known elevation points used in surface construction. Most Oxia Planum mounds are classified as hills in McNeil
etal. (2021a) owing to their rounded tops.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
5 of 17
3. Results and Interpretation
3.1. Population Distribution
We have identified 396 mounds within the 8,700km
2 Oxia Planum Rosalind
Franklin landing site study area. There are 9 mounds (2.3% of the population)
within the 2022 1-sigma landing ellipses (Figure1), and 26 mounds (7.6% of
the population) within 3km of these ellipses. There are 83 individuals (21%)
within or partially within the 3-sigma landing ellipse (Figure1), and a total
of 129 (32.6%) examples within 3km of these ellipses. The mounds cover
a total of 0.56% of the study area, 0.16% of the 1-sigma landing ellipse, and
0.88% of the 3-sigma landing ellipse. Assuming that Rosalind Franklin lands
somewhere in the 1-sigma ellipse, it will be an average of ∼2.1km from a
mound, and the furthest it will be from a mound is ∼6.0km. This increases to
∼3.7km (mean) and ∼18.6km (maximum) for the 3-sigma ellipses, but this
is highly dependent on exactly where the rover lands in the 3-sigma ellipse; in
the northern half of this ellipse the maximum rover-mound distance reduces
to 8.1km. Mounds occur throughout the study area but are more common
at lower elevations (Figures S1 and S2 in Supporting InformationS1). As a
result, they are also more abundant toward the north and northwest of the landing site (Figure1 and Figure S1 in
Supporting InformationS1), where they appear to become continuous with the mound population seen across the
circum-Chryse region. The morphometric and elevation data for the mounds are summarized in Table1.
3.2. Mound Geology
The mounds are bright-toned and texturally smooth in CTX. In CaSSIS NPB data, they are conspicuous
white-yellow features (likely red in true color) that contrast the darker blue and orange tones of the underlying
CBU (e.g., Figure3a). Generally, the tallest parts of mounds appear relatively more white-yellow than the bases,
which appear to be more white-blue in HiRISE IRB data. This difference in coloration is consistent in CaSSIS
NPB images taken at different times of day, suggesting it is not an effect of illumination. Most mounds are smooth
at the meter scale and highly rounded. From orbital data, we have identified three different sub-units (members)
associated with mounds in the landing site. The mounds vary considerably in their form, surface texture, and
color; some mounds exhibit all three members, whereas, in most others, only one or two of the members may be
present. As a whole, the mounds are less homogenous than the larger circum-Chryse population, which contain
much thicker, stratigraphically continuous deposits of clays (McNeil etal., 2021a).
The lower mound member is bright white and blue-toned in CaSSIS NPB and HiRISE IRB data, contains
decameter-scale fractures, and sometimes exhibits layering (Figure3b). This member is exposed in the flanks of
mounds where stratigraphically younger material has been eroded away (Figures3a and3b). It is not commonly
observed, suggesting that it either does not exist in many mounds, or does exist and is obscured by overlying
members. The lower member shares many similarities with the blue member of the CBU including the observa-
tion that it is at the same stratigraphic level, contains decameter-scale fractures, has a bluish tone, and does not
commonly form prominent topographies (Mandon etal.,2021). Therefore, the lower member may not be true
“mound material” but could be prominences of uneroded (or relatively less eroded than the surrounding material)
upper sections of the blue CBU (Figure4). Within the mounds, this member might therefore simply reflect differ-
ential erosion of the CBU, where the upper part of the mound has protected the CBU beneath it.
Where it occurs, the middle mound member is always stratigraphically below the upper member (Figure4). We
do not observe the middle member unambiguously in contact with the lower member within any mound, but it
always overlies the blue member of the CBU (Figures3c and 3d), thus we place it stratigraphically above the
lower member. The middle member is not present in all mounds, although we cannot be its true abundance as it
is thin (no more than a few meters in thickness), and may be covered by loose material at the bases of mounds
in many cases. In CaSSIS NPB, larger exposures are blue in tone, transitioning into yellow tones at the edges
and in smaller, more eroded examples (Figure3d). Where exposed, it is bright and sometimes fractured at the
meter-decameter scale in HiRISE (Figures3 and4), and is similar to the underlying lower member/CBU. The
boundary between this member and the blue member of the CBU (and by extension, the lower member) is
Number of mounds 396
Mean height 18.4m
Maximum height 157m
Mean area 0.12km
2
Maximum area 2.83km
2
Minimum elevation at mound base −3,176m
Maximum elevation at mound base −2,771m
Mean elevation at mound base −3,058m
Elevation range 406m
Mean aspect ratio 0.68
Mean orientation 91.2°
Table 1
Summary Oxia Planum Mound Morphometric and Elevation Data
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
6 of 17
Figure 3. Observations of different component mound members throughout the landing site and wider Oxia Planum area; (a)
High Resolution Imaging Science Experiment (HiRISE) infrared-red-blue (IRB) [ESP_057747_1985] showing a cluster of
mounds around a larger, dissected mound (gullies shown by dotted lines) north of Germania Lacus, where the rough, yellow
upper member (UM) covers a blue-white toned lower member (LM), which we suggest is uneroded sections of blue member
of the clay-bearing unit (CBU); (b) exposed layering (red dotted lines) within the LM; (c) Color and Stereo Surface Imaging
System (CaSSIS) [MY36_016481_161_0] showing a cluster of mounds southwest of the study area showing two mounds
with the UM overlying middle member (MM) on the blue member of the CBU, isolated patches of MM (white arrows), and
a potential unconformity shown by possible MM (due to similar color and texture) infilling craters within the blue member
of the CBU; (d) CaSSIS [MY36_018354_017_0] showing patches of MM which overlie blue CBU, with the larger example
showing blue-white colors with curvilinear fractures, and smaller, more eroded examples being more yellow in color; (e)
HiRISE IRB [ESP_037070_1985] showing a 54m-tall mound with layered UM overlying a ∼6m thick orange layer at its
base. Inset shows bright-toned meter-scale boulders demarcating the base of layers within the UM; and (f) HiRISE IRB
[ESP_051905_1990] showing draping meter- to decameter-scale layers (white arrows) in the flank of a pale-toned mound.
North is up, unless stated otherwise.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
7 of 17
interpreted to be unconformable, as we observe material interpreted as the middle member in a crater within the
blue CBU (Figure3c); it is, for this reason, we have placed it stratigraphically above the lower member (Figure4).
Some isolated examples of this member appear to have been fractured and completely separated from the main
mound (Figure3d). The middle member also underlies patches of the dark resistant unit (DRU), for example, at
Germania Lacus (Figure4f), suggesting the top of the middle member may represent a paleosurface upon which
the upper mound unit and DRU were deposited.
The upper mound member is the stratigraphically highest and by implication the youngest mound-forming unit.
It has been divided into upper members (part a and part b), which share some visual characteristics and occupy
approximately the same stratigraphic level in the mound sequence. The upper member is invariably bright
yellow-orange in CaSSIS data, and its texture varies between smooth (in the case of upper member [part a];
Figures3c, 3e and3f) and rough (upper member [part b]; Figure5c). There is no clear example of upper member
(part a) and upper member (part b) being in direct contact in a single mound, so it is unclear exactly how they are
related to each other, and they may be laterally equivalent (Figure4). Upper member (part b) usually has a much
greater thickness than the underlying units and therefore often forms almost the entire topographic extent of the
mounds (Figure3e). Where the lower member and middle member are absent, the upper member is observed to
directly contact the CBU (Figures3 and5). The upper member occurs on the tops, flanks and bases of mounds
(e.g., Figure3b) and also shares a flat boundary with the CBU (e.g., Figure3e) suggesting that it mantled a
paleosurface upon deposition. In HiRISE, the upper member (part b) is layered at the meter- and decameter
scale, with all layers being approximately horizontal. Fifteen mounds in this study exhibit clear layering within
the upper member at HiRISE scale, with layers often being picked out by boulders (Figure3e). A mound in the
east of the landing site shows rounded, bright-toned, meter-scale boulders defining a basal horizon in otherwise
thinner layers which occur in repetitive, ∼3m-thick packages within upper member (part b; Figure3e). This
could be explained by recurring depositional events capable of depositing meter-scale boulders, followed by a
drop in energy, and/or a change in the source of the deposit. The bright-toned boulders could be eroded material
Figure 4. Generalized stratigraphic column of the mound and inter-mound areas showing the temporal relationship between
observed morphostratigraphic landing site units, and the mound members (this study), including the inferred hiatuses between
them and the interpolated surfaces of Figure2. This diagram does not show the physical relationships between units and
members, and is derived from multiple observations of members and contacts; orange ovals and connectors denote panels in
Figures3 and5 where the best examples of members and their stratigraphic relationships can be seen.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
8 of 17
Figure 5. Mounds in Oxia Planum, showing stratigraphic and tectonic relationships: (a) rounded mound on the rim of
a ∼2km diameter crater (Sardinia Lacus) in the clay-bearing unit (CBU), which has been later infilled by dark resistant
unit (DRU) (High Resolution Imaging Science Experiment [HiRISE], ESP_047501_1985); (b) 3D 2x exaggerated view of
mound showing the mound-CBU boundary (see inlay), which is elevated ∼20m above the topographic mound boundary
(HiRISE, PSP_009880_1985); (c) mound with upstanding linear features (red) showing a ∼60° intersection angle at the
mound center (HiRISE, ESP_048358_1985); (d) a mound showing similar features to that in panel (c), where upstanding
linear to curvilinear features truncate a mound at a ∼60° intersection angle (HiRISE, ESP_037070_1985); (e) intra-crater
and crater-rim mound on the rim of Malino Crater, where the CBU has been exposed through the backstepping of DRU and/
or mound material (HiRISE, ESP_062191_1985); (f) intra-crater mound in Malino Crater, where the onlap of the DRU onto
the mound flank (red dashed line) is clearly visible on fractured the lower member (LM) or CBU, with an additional DRU
layer in yellow (ESP_062191_1985); (g) mound with two members: a thin blue-toned middle member (MM) that underlies
both the DRU, and an upper member (UM) which is yellow-white (Color and Stereo Surface Imaging System [CaSSIS],
MY35_007623_019_0); and (h) orange and blue-toned members of the CBU overlain by mounds, shorter mounds (black
arrow), and the bases of tall mounds (red arrow) are white-blue, suggesting the presence of the LM or MM, and upper
sections (UM) are yellow-white (white arrow; CaSSIS, MY35_008742_019_0). North is up, unless stated otherwise.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
9 of 17
transported from higher topographies in the Oxia basin, or they could be an erosional feature of the outcrop.The
upper member does not have the same polygonal fractures seen in other members, but instead sometimes contains
bright, positive-relief linear and curvilinear ridges (Figures5c and5d). These ridges occur primarily on mound
surfaces but are also observed to extend into the underlying CBU. One mound (Figure5c) contains individual
upstanding linear features and a triangular section on the northern flank which is bounded by upstanding linear
ridges (Figures5c and 5d). A set of upstanding linear features intersect the mound at a 60° angle at its apex,
resembling conjugate fractures. Another mound in the east of the study area (Figure5d) exhibits similar curvi-
linear features. We interpret these features to be surface expressions of erosion-resistant material such as miner-
alized fractures of sedimentary or igneous origin, that have reinforced the bulk mound structure against erosion
(e.g., De Toffoli etal.,2019; Okubo & McEwen,2007).
3.3. Mound Stratigraphy
3.3.1. Relationship Between Mounds and CBU
The contact between the upper member and CBU is either horizontal or gently dipping throughout the study
area, suggesting that the upper member was deposited onto the paleotopography. Given that the upper member
also directly overlies both the lower member and the middle member, it is likely that a paleolandscape contain-
ing elements of eroded lower member, middle member and CBU was present at the time of the upper member's
deposition (Figure4). Mounds located on the rims of highly eroded, ejecta-free craters that occur within the
CBU demonstrate that substantial erosion and time occurred between the impact events (and therefore the CBU
formation itself) and the deposition of the mound layer. Malino crater (Figure1) contains crater-rim mounds and
two intra-crater mounds (Figures5e and5f) at similar elevations, suggesting that the mound layer was deposited
after both crater infill, and erosion of the crater rim and ejecta. It is unclear how much time elapsed between the
CBU formation and emplacement of the mound layer.
Another mound at 18.277°N, 24.523°W shows a horizontal contact ∼20m above the topographic base of the
mound (Figure5b). We interpret this contact as the boundary between the CBU (lower member in this case as it
is part of a mound) and the middle member, as it is at a similar elevation relative to the outcrops of the middle
member on the west of Germania Lacus (Figure5g). Below the contact (but still on the topographic mound flank),
approximately 20m of lower member material is exposed. Above the contact, the upper member forms most of
the topography of this mound. As such, this is the only landform in the study where all three mound-forming units
are unambiguously present.
3.3.2. Relationship Between Mounds and Dark Resistant Unit
The DRU is a dark-toned, rugged unit which occurs throughout the Oxia Planum study area in topographic lows,
and as remnant, upstanding mesas. There is no consensus on the origin of the DRU, with both sedimentary and
igneous origins proposed (Gary-Bicas & Rogers,2021; Quantin-Nataf etal.,2021). Its age is also debated; one
estimate is early to mid-Amazonian or older (>2.6Ga, Quantin-Nataf etal.,2021), and others estimate Early
Hesperian (∼3.7Ga, Ivanov etal.,2020; Uthus,2020).
The DRU onlaps onto a mound in Malino crater, showing that it unconformably overlies the mounds (Figure5f).
Around another intra-crater mound in Malino crater, about 100–200m of backstepping has occurred (Figure5e),
however, it is not clear whether this is a result of erosion of the DRU, or the mound itself. The erosion in these
areas has revealed polygonal fractures in the underlying crater-fill unit, similar to those seen in the CBU.
These relationships show both that the DRU is younger than the mounds, and that the DRU was emplaced
after the mounds had been eroded. No evidence has yet been found for hydrous minerals in the DRU (Carter
etal.,2016; Quantin-Nataf etal.,2021), whereas several mounds throughout the Chryse region show evidence of
hydrous minerals (McNeil etal., 2021a), suggesting that a change in environmental conditions occurred between
the emplacement of the mound members and DRU. The lack of CRISM data in the area means that the DRU
could contain hydrous minerals, but is not detected due to poor coverage.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
10 of 17
3.4. Volumetric Calculations of Putative Mound Layer Thickness
To test whether the Oxia Planum mounds could have formed part of the circum-Chryse mound-forming deposit,
we calculated the total volume of eroded material which must have been removed to leave the upstanding mounds.
In this section, we first assume that the mounds were part of the circum-Chryse layer in order to calculate
removed volumes, and then discuss whether this is a realistic hypothesis in Section4. Given that the digitized
mound boundaries are the point at which the mound-forming material contacts the material that forms the CBU
(lower member in some mounds), the boundaries approximate the stratigraphic position of the middle member
(which is extremely thin), and the LBS approximates the paleosurface upon which the middle member rests.
The surface generated by interpolating the mound apexes in the study area (MUBS, Figure2 and Figure S3 in
Supporting InformationS1) reveals a minimum total thickness of eroded mound material in the Oxia Planum
study area of 41.4m, and therefore a minimum volume of 208±32km
3 (Table2) would have had to be removed
if the mounds formed from a previously contiguous layer (or layers). This can be subdivided into the minimum
eroded depth and volume of the mound-forming layer above the LBS (21.6m, 124±37km
3), and the eroded
depth and volume of the material below the interpolated mound contact or LBS (19.8m, 84±27km
3). This
eroded material is assumed to have been either CBU or lower mound material.
Using the surface generated from the heights of the Chryse Planitia mesas (CPUBS, Figure2 and Figure S2 in
Supporting InformationS1), the estimated total thickness of eroded material in the study area is over three times
higher than MUBS at 129m, with a volume of 680km
3. This can be subdivided into the eroded thicknesses and
volumes of the mound-forming layer (111.6m, 605km
3), and the amount of material from below the interpo-
lated lower mound contact or LBS (17.6m, 75km
3). The CPUBS intersects the topography in Oxia Planum at
an elevation of approximately −2,800m, equivalent to the upper elevation of the Mawrth Plateau deposits to
the northeast. Above the CPUBS, which represents the inferred top of the original mound forming layer across
both Chryse Planitia and Oxia Planum, there should theoretically be fewer or no mounds found; our observations
support this, with only four low-relief mounds having bases above the −2,800m contour. The total volume of
mounds in the study area is approximately 1.3km
3, which is ∼1% of the MUBS volume estimate for the study
area and 0.21% of the CPUBS volume estimate for the study area.
Based on stratigraphic relationships observed in the wider circum-Chryse mound population, the mounds are
likely to have been deposited, indurated, and eroded within approximately 200Myr, between ∼4.0Ga (the crater
retention age of the underlying CBU; Quantin-Nataf etal.,2021, and the time at which the regional Mawrth Vallis
plateau was mostly formed; Loizeau etal.,2012) and ∼3.8Ga (the crater retention age of the top of the Mawrth
Vallis clay-bearing stratigraphy—and therefore also the circum-Chryse clay-rich deposit—which is correlatable
to the youngest parts of mounds in Chryse Planitia). Assuming the Oxia Planum mounds once formed a contig-
uous part of this circum-Chryse mound-forming deposit and that they are therefore of a similar age, we can
estimate the rate of erosion for this deposit in Oxia Planum.
It is unclear what proportion of this time was devoted to deposition versus erosion, or whether the mounds in
Oxia Planum were eroded over the same timescales as the larger mounds in Chryse Planitia, so the following
calculations of erosion rates are minimum estimates. The minimum thickness of the removed material in Oxia
Planum was 41.4m, and the estimated mean thickness was ∼129m (Table2). Assuming rapid initial deposition
at 4.0Ga, erosion of these thicknesses at a constant rate for 200Myr
−1 equates to a minimum steady-state erosion
rate of 0.21–0.65mMyr
−1. Conversely, minimum deposition rates are the same, assuming rapid final erosion.
Mean thickness (m) Volume (km
3)
Minimum removed overburden (MUBS—CTX) 41.4 208
Estimate of total removed overburden (CPUBS—CTX) 129 680
Minimum removed mound material (MUBS—LBS) 21.6 124
Estimate of total removed mound material (CPUBS—LBS) 112 605
Material removed below lower mound boundary (LBS—CTX) 19.8 84
Table 2
Summary of Eroded Thicknesses and Volumes of the Mound-Forming Layer in the Oxia Planum Study Area
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
11 of 17
4. Discussion
4.1. Stratigraphic Implications
The main mound-forming layer (upper member) and the DRU at Germania Lacus both directly overlie the middle
member which in turn overlies the lower member/CBU. We interpret the boundary between the middle member
and upper member to represent a paleosurface which existed at the time of deposition of both the upper member
and the DRU. The middle member is extremely thin, and its presence underneath highly eroded remnants of
both the upper member and DRU (Figure5g) could suggest that it remained relatively unchanged in the time
between the deposition of the upper member and DRU. By extension, this could imply that DRU is more ancient
than previously thought and could be closer in age to the early Hesperian estimated by Ivanov etal.(2020) and
Uthus(2020) than the early Amazonian lower limit in Quantin-Nataf etal.(2021). Therefore, it is possible that
the DRU is genetically related to the extensive dark plains material which occurs throughout Chryse Planitia
(∼3.6Ga, Loizeau etal.,2012; Tanaka etal.,2005) and embays mounds (McNeil etal.,2021a), as postulated in
Quantin-Nataf etal.(2021).
4.2. Constructional Isolated Features, or Erosional Layer-Derived Landforms?
This section discusses whether the mounds are constructional (their morphologies formed in isolation from each
other through the progressive build-up of material) or if they are erosional (their morphologies formed through
erosion of a previously more extensive layer or layers).
The strongest evidence that supports mound formation through erosional processes, and that the mounds were
once more extensive than they are today, comes from the structure and stratigraphy of the mounds:
1. Mounds in the Oxia Planum population are all composed of the same internal stratigraphy; the upper, middle,
and lower members occur in the same succession and are of similar relative thicknesses where they are present
(Figure3).
2. The lower member and the middle member are composed of similar material (in both color and texture) to the
CBU (Figures3c and3d) that surrounds the mound landforms. Because the only difference between “plains”
and “mounds” is the slope of the rock exposure, this strongly suggests that the lower sections of mounds are
less eroded parts of the CBU (Figures5b and5h), which have been eroded relative to the mounds.
3. The upper member of the mound stratigraphy that forms steeply dipping and rounded flanks are consistently
covered in material that mostly obscures other elements of the internal mound structure (Figure3b) on the
flanks. This is most consistent with scree slopes suggesting ongoing erosion. Whilst this overall form is not
inconsistent with constructional landforms, if the origin was constructional then the covering material would
need to be actively replaced by that constructional process, to maintain their shape over geological time.
These observations show that the mounds comprise multiple distinct layered members, some of which appear to
be separated by unconformities and thus record substantial amounts of geologic time. This stratigraphic consist-
ency and erosional environment support the interpretation that the layers were previously continuous between
the mounds. Furthermore, this evidence reduces the likelihood that they are constructional in nature, as any
constructional process would have had to independently reproduce the same members at consistent thicknesses
at different points in space.
Outside the study area (geographically focused by our science questions) the population of mounds is continu-
ous with those in Chryse Planitia. The origin of mounds in Oxia Planum must be consistent with the rest of this
population because:
1. There is no sudden change in the tone, brightness, size or shape characteristics between mounds in Oxia and
those nearby in the circum-Chryse region; there is an overall gradual increase in size to the north, consistent
with a layer of increasing thickness and/or increased erosion to the north.
2. The top of the extrapolated circum-Chryse upper bounding surface forms a slope of low gradient which over-
lays the entire Oxia Planum population. This is what we would expect if the Oxia Planum mounds represented
smaller examples of the circum-Chryse mound deposit described in McNeil etal. (2021a).
3. Clusters of partially dissected mounds (Figure3a) illustrate an evolutionary sequence from larger plateaus
through clusters of mounds to increasingly isolated features (e.g., Figure5h).
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
12 of 17
Additional contextual evidence precludes the survival of small constructional landforms over geological time: the
mounds are in a region with multitudes of other erosional landforms such as inverted channels (Davis etal.,2022;
Fawdon etal.,2021a,2022), inverted craters (Quantin-Nataf etal.,2021; Roberts etal.,2021), Periodic Bedrock
Ridges (Favaro etal.,2021; Silvestro etal.,2021), and the DRU (Fawdon etal.,2021a; Quantin-Nataf etal.,2021).
These, along with the low surface dust index (Ruff & Christensen,2002) and population of aeolian bedforms
indicative of transport over a long period of time (Favaro etal.,2021), are indicative of an erosive environ-
ment in Oxia Planum. This is consistent with an erosional origin, and inconsistent with the survival of isolated
constructional landforms, which, given their relationship to the DRU, must have predated the substantial amount
of erosion that has affected that unit (Figure5b); consequently it is not plausible that they would have simultane-
ously predated this erosive environment and have been substantially unaffected by it.
In addition to the evidence consistent with an erosional origin, there is a lack of evidence to support common
processes that construct isolated, mound-like landforms. There is no geomorphologic evidence to indicate that
these mounds are volcanic in origin; they do not look like other small-scale volcanic features from Mars, either
igneous (Brož & Hauber,2012,2013; Brož et al., 2014) or sedimentary (Brož etal.,2019,2022; Oehler &
Allen,2010) in origin, and we see no evidence for features typical of these processes such as summit craters,
conical morphologies, or igneous mineralogy. There are no instances associated with flow-like landforms on
their flanks and we do not observe any regionally contextual volcanic architecture of comparable age or apparent
preservation.
The mounds could also be hydrothermal in origin. However, we do not observe any evidence of vents or bright
outcrops (Skok etal.,2010) or the context of a long-lived volcanic system (e.g., Fawdon etal.,2015), nor do we
see mineral assemblages associated with hydrothermalism (e.g., serpentine, chlorite) in hyperspectral CRISM
data in the Oxia Planum region (Mandon etal.,2021; Skok etal.,2010). Some mounds contain what appear to be
linear resistant ridges (Figures5c and5d) which could be interpreted as dykes of intrusive material, but given the
lack of contextual volcanic architecture, these are more simply interpreted as mineralized fractures—commonly
seen in Martian terrains (Caswell & Milliken,2017; Okubo & McEwen,2007)—which are exposed in more
eroded parts of the mounds. Although this doesn't support a constructional origin, it does mean that hydrothermal
mineralization could have been a significant factor in preserving the mounds. Furthermore, the lack of supporting
mineralogy may be attributed to a combination of dust coverage, the small area of bedrock exposed on mound
surfaces, and a lack of quality data coverage.
Either a hydrothermal and a volcanic/sedimentary volcanic origin for the mounds would require a prolonged and
regionally extensive thermal source, which could be provided from a mantle plume or the residual energy from a
prior large impact. The former is unlikely, given the lack of obvious large-scale volcanic landforms in the region,
however the latter is superficially more plausible. Chryse Planitia is comparable in size (diameter: ∼1,500km)
to other large impact basins on Mars, such as Hellas (diameter: ∼2,000km), which are likely to have produced
hydrothermal systems at the rims and central peak that may have been sustained for ∼10Myr after the impact
events (Abramov & Kring,2005). Oxia Planum is located at the edge of the Chryse basin near its proposed rim
(e.g., Pan etal.,2019), so there is circumstantial evidence the region may have been subjected to hydrothermal
activity in the tens of millions of years after the impact. However, it is unclear if this is stratigraphically consistent
with the timing of deposition and/or alteration of the CBU in Oxia Planum and the wider circum-Chryse clays
(Brossier etal.,2022; Carter etal.,2015; Mandon etal.,2021).
On balance, we suggest there is substantially more evidence, requiring fewer special circumstances, that the
mounds formed from erosional processes eroding one or more layers of pre-existing material. We find little to
no direct or contextual evidence to support constructional processes, and whilst there is no irrefutable evidence
to rule out a constructional hypothesis the observations are commensurate with the current mound morphology
being primarily erosional in nature.
4.3. Wider Implications
Mounds in the Oxia Planum region are smaller (in height and diameter) than most of the mounds in the wider
circum-Chryse region. It is unclear whether this is primarily because the mound-forming layer was thinner in Oxia
Planum, or if more erosion occurred here, or a combination of the two. Given that CPUBS predicts a thinner layer
in Oxia Planum, and that this surface was constructed using mesas (thought to be the most complete sections of
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
13 of 17
the mound-forming stratigraphy; McNeil etal., 2021a), we favor the hypothesis that the mound-forming layer was
thinner in Oxia Planum, or that the mounds represent three-dimensional exposures of the top of the clay-bearing
succession in this region of Chryse Planitia. If the former is true, the thinning of the mound-forming layer
toward the basin margin implies constraints on its formation mechanism. Thinner deposits at basin margins are a
common feature of sedimentary basins on Earth, where the accumulated sediment thickness decreases toward the
basin margin as accommodation space decreases. Furthermore, meter-scale layering in the Oxia Planum mounds
could be consistent with deposition through sedimentary or ashfall processes whose thickness across a basin is
usually influenced by accommodation space.
4.4. Deposition and Overburden Removal in Oxia Planum: Rates and Timing
Our calculated minimum deposition/erosion rates are similar to previous estimates of middle-late Noachian
global erosion rates (0.8–1mMy
−1, see summary in Golombek etal., 2006), and are considerably lower than
previous crater obliteration estimates for Oxia Planum itself (8mMy
−1, Quantin-Nataf etal.,2021). Our esti-
mates differ from those in Quantin-Nataf etal. (2021) primarily because our estimates are for the minimum
mean erosion rates, whereas theirs are mean crater obliteration rates (which is the rate at which craters are
removed through both deposition and erosion). Our calculated erosion rates are a similar order of magnitude but
lower than the median erosion rate of outcrops in arid to polar environments on Earth (∼6mMy
−1; Portenga &
Bierman,2011), which is perhaps not surprising given the different atmospheric conditions. However, Amazo-
nian estimates for erosion rates in Oxia Planum are much lower: 0.01–0.03mMy
−1 (Kite & Mayer,2017) and
0.08mMy
−1 (Quantin-Nataf etal.,2021). The difference between these Amazonian rates and our minimum rates
supports our interpretation that most of the mound material erosion took place in the Noachian/Hesperian rather
than during the Amazonian.
Assuming an average Noachian erosion rate of approximately 0.9mMy
−1 (Golombek etal.,2006), this would
mean that the estimated ∼130m of mound material would have been removed in ∼150Myr before embayment
by the dark plains material. Deposition would have had to occur over the prior ∼50Myr, with an average rate of
approximately 2.6mMy
−1, similar to Holocene sedimentation rates in some of the deepest parts of Earth's oceans
(Piñero etal.,2013). The true erosion and deposition rates are likely to be more complex, with alternating periods
of deposition, erosion and inactivity, as indicated by the layering in some mounds (Figure3e). Factors that could
affect the depositional and erosion regime include sediment availability and source, environmental conditions
that would influence the amount highland runoff, and impact gardening, amongst others. Furthermore, we do not
know how early Martian environmental conditions including putative ocean chemistry might have affected these
deposition rates. Mound-derived Transverse Aeolian Ridges (Balme etal.,2008) around the bases of mounds
and across the study area (Favaro etal.,2021) demonstrate that sediment transport, and thus probably also wind
erosion, is occurring here at the present time.
4.5. Implications for the Landing Site and Rosalind Franklin
Our calculations show that the clay-bearing plains unit in the landing site of the Rosalind Franklin rover was
buried by ∼130m of overburden during the Noachian—an order of magnitude shallower burial than calculated
by Quantin-Nataf etal.(2021). Hydraulic fracturing of rocks on Mars has been calculated to occur at depths of
1km or greater (Caswell & Milliken,2017), so the vertical stress imparted to the CBU from ∼130m of overlying
material alone is unlikely to have been great enough to produce the fractures seen in the CBU. The maximum
thickness of the mound-forming layer in Chryse Planitia was around 550m (McNeil etal.,2021a), or around half
of what would be needed to fracture the underlying bedrock. It seems unlikely therefore from our reconstruction
that Oxia Planum, which is closer to the edge of the Chryse basin, would be able to accommodate twice the thick-
ness of material than deeper areas of the basin—especially when the evidence presented here suggests that the
layer was considerably thinner in Oxia Planum. There are two possibilities: (a) hydraulic fracturing of the CBU
was the result of deformation of an underlying ductile layer (Quantin-Nataf etal.,2021); or (b) fractures in
the CBU are not hydraulic but are the result of horizontal tensile stresses generated by contractional processes
such as desiccation, syneresis, or thermal contraction (Parkes Bowen etal.,2022). Identification and analysis
of sub-HiRISE-scale mound-proximal fractures in the CBU using the PanCam instrument aboard the Rosalind
Franklin rover (Coates etal.,2017) could allow for a better understanding of their origins.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
14 of 17
The similarities in height difference between the top of the present-day mounds and the paleosurface defined by
the contact between the mounds and CBU suggest that when the mound layer was emplaced in the Noachian, the
geography of Oxia Planum was broadly comparable with its modern low-relief topography. Despite this, there are
small differences, as the geological boundary often occurs near the base of the topographic mound slope (e.g.,
Figure5b) suggesting that these mounds have protected a more complete section of the CBU.
The mounds appear to be the second-oldest unit in the landing site after the CBU. Therefore, the regions of CBU
that occur directly below still-intact mounds have remained buried and unexposed since burial in the early middle
Noachian and have had greater protection from solar radiation, impact events, and atmospheric alteration than
exposed CBU. Any potential biosignatures would also have been afforded this protection. The mounds also could
record water-rock interaction as their upstanding lineations are interpreted as indurated fractures, suggesting
interaction with fluids (McNeil etal., 2021a). We propose that the base of mounds—particularly tall, layered
mounds with indurated fractures that show evidence of active erosion—and subordinately, areas around DRU
mesas, may be among the best places in the landing ellipses to detect biosignatures in the CBU.
It is feasible that the Rosalind Franklin rover will land close enough to a mound to be able to image it in situ with
the PanCam instrument, including the High Resolution Camera and Wide Angle Cameras (Coates etal.,2017).
Ground-based imaging of strata exposed on individual mound edifices could allow us to determine the deposi-
tional environments in which the mound-forming unit was formed, for example, by identifying sedimentological
features such as clast size, geometry and orientation, horizontal- and cross-stratification, scours, channels or grad-
ing. Whilst the sedimentology and stratigraphy of individual mounds will be able to inform us about the origin
of the deposits, images of multiple mounds will allow for a better understanding of the wider mound-forming
deposit and could elucidate whether it was deposited through primary deposition, ashfall processes, the rework-
ing of pre-existing material, or any combination of these. If the mounds were part of a regional layer, we would
expect to be able to see consistent sedimentological features across different examples, and we would also expect
a compositional difference between the upper member and the lower member/middle member. If the mounds
are distal outliers of Oxia Planum (Coogoon Valles) sediment fan material (e.g., Quantin-Nataf etal.,2021) or
progradation of any highland detrital sediments, we might expect to see large-scale sedimentary features exposed
in mound flanks. The Perseverance and Curiosity rovers both imaged the flanks of Kodiak Butte and the Murray
Buttes in their respective landing sites, revealing large-scale sedimentary features which have contributed to
our understanding of past depositional environments in these locations (e.g., Banham etal.,2021; Mangold
etal.,2021). Similar analyses using PanCam data from the Rosalind Franklin rover may allow for an improved
understanding of the Noachian environments of Oxia Planum.
5. Conclusions
The morphology of mounds in Oxia Planum is more likely to be the result of erosional processes than
constructional processes. The Oxia mounds are morphologically and morphometrically continuous with the
population of larger mounds further out in the Chryse basin, and comprised part of a Noachian-aged, layered
deposit that extended around the circum-Chryse region.
The mounds contain as many as three distinct members, possibly separated by unconformities: the lower
mound member, a bright, layered, blue-toned material, which could be uneroded sections of the uppermost
clay-bearing plains; the middle mound member, a thin, low-relief blue-toned material; and the upper mound
member, a yellow-toned material of variable roughness which forms most of the mound topography.
In Oxia Planum, the mound-forming layer had a minimum mean thickness of ∼40m but is likely to have had
a mean thickness of ∼130m. This is considerably thinner than elsewhere in Chryse Planitia, suggesting that
the mound-forming layer thinned toward Arabia Terra to the south, following the elevation of both the paleo-
surface and the present-day elevation. Alternatively, the Oxia Planum mounds may represent the uppermost
part of the circum-Chryse mound-forming deposit.
Our minimum erosion rate estimates are an order of magnitude higher than Amazonian estimates, a similar
order of magnitude but slightly lower than present-day erosion rates in arid and polar deserts on Earth, and
comparable to previous estimates of erosion rates on Noachian Mars.
Mounds in Oxia Planum show positive-relief linear to curvilinear features that are similar to examples in the
circum-Chryse region. These are interpreted to be indurated fractures, suggesting interaction with fluids and
precipitation of resistant minerals.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
15 of 17
It is highly unlikely that the mound-forming layer alone provided enough vertical stress to hydraulically
fracture the CBU, suggesting either that the overburden in the region was considerably thicker than our calcu-
lations suggest, or that hydraulic fracturing was achieved through non-overburden-induced stress, or that the
fractures were formed “near-surface” through horizontal tensile stresses.
Those areas of the clay-bearing plains which are directly covered by mounds have been continually protected
from the Martian environment since the Noachian, and are likely to be the most pristine clay-rich materials in
the landing site. The areas directly adjacent to the mounds (and mesas of dark resistant material), are likely to
have been more recently exposed than other areas away from the mounds through the erosion and backstep-
ping of these younger units. These are therefore the most promising locations for Rosalind Franklin to search
for subsurface biosignatures.
In situ imaging of the flanks of mounds by Rosalind Franklin's PanCam instrument is likely to help reveal the
origin of the mound material, and aid in our understanding of the early Martian depositional environments
present in Oxia Planum.
Data Availability Statement
Supporting InformationS1 including tabulated mound morphometric and location data as well as geospatial
data vectors (shapefiles, .shp) for mounds are available in McNeil etal. (2021a) and McNeil etal. (2021b).
See Supporting InformationS1 for supporting figures and tables. CTX and HiRISE image data are publicly
available at the NASA Planetary Data System repository in the Mars Reconnaissance Orbiter section (https://
pds-imaging.jpl.nasa.gov/volumes/mro.html). CaSSIS data are publicly available at the ESA Planetary Science
Archive (http://archives.esac.esa.int/psa/ftp/ExoMars2016/em16_tgo_cas/data_raw/) and (http://archives.esac.
esa.int/psa/ftp/ExoMars2016/em16_tgo_cas/data_calibrated/), with relevant Oxia Planum-specific CaSSIS data
available at Fawdon etal.(2021b). CaSSIS observations are also available at https://cassis.halimede.unibe.ch/
observations. A guide to downloading and viewing CaSSIS data is available at https://issues.cosmos.esa.int/
socciwiki/display/PSAPUB1/CaSSIS+Quick+Start+Guide.
References
Abramov, O., & Kring, D. A. (2005). Impact-induced hydrothermal activity on early Mars. Journal of Geophysical Research, 110(12), 1–19.
https://doi.org/10.1029/2005JE002453
Balme, M., Berman, D. C., Bourke, M. C., & Zimbelman, J. R. (2008). Transverse aeolian ridges (TARs) on Mars. Geomorphology, 101(4),
703–720. https://doi.org/10.1016/j.geomorph.2008.03.011
Banham, S. G., Gupta, S., Rubin, D. M., Edgett, K. S., Barnes, R., Van Beek, J., etal. (2021). A rock record of complex aeolian bedforms in a
Hesperian desert landscape: The Stimson formation as exposed in the Murray buttes, Gale crater, Mars. Journal of Geophysical Research:
Planets, 126(4), e2020JE006554. https://doi.org/10.1029/2020JE006554
Bibring, J.-P., Soufflot, A., Berthé, M., Langevin, Y., Gondet, B., Drossart, P., etal. (2004). OMEGA: Observatoire pour la Minéralogie, l'Eau,
les Glaces et l'Activité. In Presented at the Mars express: The scientific payload, (Vol. 1240,pp.37–49).
Brossier, J., Altieri, F., De Sanctis, M. C., Frigeri, A., Ferrari, M., De Angelis, S., etal. (2022). Constraining the spectral behavior of the
clay-bearing outcrops in Oxia Planum, the landing site for ExoMars “Rosalind Franklin” rover. Icarus, 386, 115114. https://doi.org/10.1016/j.
icarus.2022.115114
Brož, P., Čadek, O., Hauber, E., & Rossi, A. P. (2014). Shape of scoria cones on Mars: Insights from numerical modeling of ballistic pathways.
Earth and Planetary Science Letters, 406, 14–23. https://doi.org/10.1016/j.epsl.2014.09.002
Brož, P., & Hauber, E. (2012). A unique volcanic field in Tharsis, Mars: Pyroclastic cones as evidence for explosive eruptions. Icarus, 218(1),
88–99. https://doi.org/10.1016/j.icarus.2011.11.030
Brož, P., & Hauber, E. (2013). Hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on Mars: Phreatomag-
matic eruptions on Mars. Journal of Geophysical Research: Planets, 118(8), 1656–1675. https://doi.org/10.1002/jgre.20120
Brož, P., Hauber, E., Conway, S. J., Luzzi, E., Mazzini, A., Noblet, A., etal. (2022). New evidence for sedimentary volcanism on Chryse Planitia,
Mars. Icarus, 382, 115038. https://doi.org/10.1016/j.icarus.2022.115038
Brož, P., Hauber, E., van de Burgt, I., Špillar, V., & Michael, G. (2019). Subsurface sediment mobilization in the southern Chryse Planitia on
Mars. Journal of Geophysical Research: Planets, 124(3), 703–720. https://doi.org/10.1029/2018JE005868
Carter, J., Loizeau, D., Quantin, C., Balme, M., Poulet, F., Gupta, S., etal. (2015). Mineralogic context of the circum-chryse planitia candidate
landing sites for the ExoMars rover mission. In Presented at the 46th Lunar and Planetary Science Conference (Vol. 1988).
Carter, J., Quantin, C., Thollot, P., Loizeau, D., Ody, A., Lozach, L., et al. (2016). Oxia Planum, A clay-laden landing site proposed for the
ExoMars rover mission: Aqueous mineralogy and alteration scenarios. In Presented at the 47th lunar and planetary science conference (p.2).
Caswell, T. E., & Milliken, R. E. (2017). Evidence for hydraulic fracturing at Gale crater, Mars: Implications for burial depth of the Yellowknife
Bay formation. Earth and Planetary Science Letters, 468, 72–84. https://doi.org/10.1016/j.epsl.2017.03.033
Coates, A. J., Jaumann, R., Griffiths, A. D., Leff, C. E., Schmitz, N., Josset, J.-L., etal. (2017). The PanCam instrument for the ExoMars rover.
Astrobiology, 17(6–7), 511–541. https://doi.org/10.1089/ast.2016.1548
Davis, J. M., Balme, M. R., Fawdon, P., Grindrod, P.M., Favaro, E. A., Banham, S. G., & Thomas, N. (2022). Ancient alluvial Plains at Oxia
Planum, Mars (preprint). Planetology. https://doi.org/10.1002/essoar.10511552.1
De Toffoli, B., Mangold, N., Massironi, M., Pozzobon, R., Mouélic, S. L., L’Haridon, J., & Cremonese, G. (2019). Fluid migration through
fracture networks, Gale crater (Mars). In Geophysical Research Abstracts, (Vol. 21,pp.1–1.1).
Acknowledgments
JDM acknowledges STFC for support
under Doctoral Training Grant SA215664
and the Open University SRA. MRB and
PF acknowledge UK Space Agency Fund-
ing: ST/V001965/1, ST/R001413/1, and
ST/W002736/1. An anonymous reviewer,
Solmaz Adeli, Daniela Tirsch, Thomas
Früh, Kristen Bennett, and the editors
Andrew Dombard and Bradley Thomson,
are thanked for their insightful comments
which greatly helped to improve this
manuscript. The authors wish to thank
the CaSSIS spacecraft and instrument
engineering teams. CaSSIS is a project
of the University of Bern and funded
through the Swiss Space Office via ESA's
PRODEX programme. The instrument
hardware development was also supported
by the Italian Space Agency (ASI)
(ASI-INAF Agreement I/2020-17-HH.0),
INAF/Astronomical Observatory of
Padova, and the Space Research Center
(CBK) in Warsaw. Support from SGF
(Budapest), the University of Arizona
(Lunar and Planetary Lab.) and NASA are
also gratefully acknowledged. Operations
support from the UK Space Agency
under Grant ST/R003025/1 is also
acknowledged.
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
16 of 17
Favaro, E. A., Balme, M. R., Davis, J. M., Grindrod, P.M., Fawdon, P., Barrett, A. M., & Lewis, S. R. (2021). The aeolian environment of the land-
ing site for the ExoMars Rosalind Franklin rover in Oxia Planum, Mars. Journal of Geophysical Research: Planets, 126(4), e2020JE006723.
https://doi.org/10.1029/2020JE006723
Fawdon, P., Balme, M. R., Davis, J. M., Bridges, J. C., Gupta, S., & Quantin-Nataf, C. (2022). Rivers and lakes in Western Arabia Terra: The
fluvial catchment of the ExoMars 2022 rover landing site. Journal of Geophysical Research: Planets,127(2), e2021JE007045. https://doi.
org/10.1029/2021JE007045
Fawdon, P., Grindrod, P., Orgel, C., Sefton-Nash, E., Adeli, S., Balme, M., etal. (2021a). The geography of Oxia Planum. Journal of Maps, 17(2),
621–637. https://doi.org/10.1080/17445647.2021.1982035
Fawdon, P., Grindrod, P., Orgel, C., Sefton-Nash, E., Adeli, S., Balme, M., etal. (2021b). The geography of Oxia Planum 02 CASSIS data [Data-
set]. The Open University. https://doi.org/10.21954/OU.RD.16451217.V1
Fawdon, P., Skok, J. R., Balme, M. R., Vye-Brown, C. L., Rothery, D. A., & Jordan, C. J. (2015). The geological history of Nili Patera, Mars.
Journal of Geophysical Research: Planets, 120(5), 951–977. https://doi.org/10.1002/2015JE004795
Gary-Bicas, C. E., & Rogers, A. D. (2021). Geologic and thermal characterization of Oxia Planum using Mars Odyssey THEMIS data. Journal
of Geophysical Research: Planets, 126(2), e2020JE006678. https://doi.org/10.1029/2020JE006678
Golombek, M. P., Grant, J. A., Crumpler, L. S., Greeley, R., Arvidson, R. E., Bell, J. F., etal. (2006). Erosion rates at the Mars exploration rover
landing sites and long-term climate change on Mars: Climate change from the Mars rovers. Journal of Geophysical Research, 111(E12),
E12S10. https://doi.org/10.1029/2006JE002754
Ivanov, M. A., Slyuta, E. N., Grishakina, E. A., & Dmitrovskii, A. A. (2020). Geomorphological analysis of ExoMars candidate landing site Oxia
Planum. Solar System Research, 54(1), 1–14. https://doi.org/10.1134/S0038094620010050
Jaumann, R., Neukum, G., Behnke, T., Duxbury, T. C., Eichentopf, K., Flohrer, J., etal. (2007). The high-resolution stereo camera (HRSC)
experiment on Mars Express: Instrument aspects and experiment conduct from interplanetary cruise through the nominal mission. Planetary
and Space Science, 55(7–8), 928–952. https://doi.org/10.1016/j.pss.2006.12.003
Kite, E. S., & Mayer, D. P. (2017). Mars sedimentary rock erosion rates constrained using crater counts, with applications to organic-matter
preservation and to the global dust cycle. Icarus, 286, 212–222. https://doi.org/10.1016/j.icarus.2016.10.010
Loizeau, D., Werner, S. C., Mangold, N., Bibring, J. P., & Vago, J. L. (2012). Chronology of deposition and alteration in the Mawrth Vallis region,
Mars. Planetary and Space Science, 72(1), 31–43. https://doi.org/10.1016/j.pss.2012.06.023
Malin, M. C., Bell, J. F., Cantor, B. A., Caplinger, M. A., Calvin, W. M., Clancy, R. T., etal. (2007). Context camera investigation on board the
Mars reconnaissance orbiter. Journal of Geophysical Research, 112(5), 1–25. https://doi.org/10.1029/2006JE002808
Mandon, L., Parkes Bowen, A., Quantin-Nataf, C., Bridges, J. C., Carter, J., Pan, L., etal. (2021). Morphological and spectral diversity of the
clay-bearing unit at the ExoMars landing site Oxia Planum. Astrobiology, 21(4), 464–480. https://doi.org/10.1089/ast.2020.2292
Mangold, N., Gupta, S., Gasnault, O., Dromart, G., Tarnas, J. D., Sholes, S. F., etal. (2021). Perseverance rover reveals an ancient delta-lake
system and flood deposits at Jezero crater, Mars. Science, 374(6568), 711–717. https://doi.org/10.1126/science.abl4051
McEwen, A. S., Eliason, E. M., Bergstrom, J. W., Bridges, N. T., Hansen, C. J., Delamere, W. A., etal. (2007). Mars reconnaissance orbiter's
high resolution imaging science experiment (HiRISE). Journal of Geophysical Research, 112(5), 1–40. https://doi.org/10.1029/2005JE002605
McNeil, J. D., Fawdon, P., Balme, M. R., & Coe, A. L. (2021a). Morphology, morphometry and distribution of isolated landforms in southern
Chryse Planitia, Mars. Journal of Geophysical Research: Planets, 126(5),e2020JE006775.https://doi.org/10.1029/2020JE006775
McNeil, J. D., Fawdon, P., Balme, M. R., & Coe, A. L. (2021b). Oxia Planum ExoMars 2022 rover landing site mounds: Morphometric data
[Dataset]. The Open University. https://doi.org/10.21954/ou.rd.16832266.v2
Molina, A., López, I., Prieto-Ballesteros, O., Fernández-Remolar, D., de Pablo, M. Á., & Gómez, F. (2017). Coogoon Valles, western Arabia
Terra: Hydrological evolution of a complex Martian channel system. Icarus, 293, 27–44. https://doi.org/10.1016/j.icarus.2017.04.002
Murchie, S., Arvidson, R., Bedini, P., Beisser, K., Bibring, J. P., Bishop, J., etal. (2007). Compact connaissance imaging spectrometer for Mars
(CRISM) on Mars reconnaissance orbiter (MRO). Journal of Geophysical Research, 112(5), 1–57. https://doi.org/10.1029/2006JE002682
Oehler, D. Z., & Allen, C. C. (2010). Evidence for pervasive mud volcanism in Acidalia Planitia, Mars. Icarus, 208(2), 636–657. https://doi.
org/10.1016/j.icarus.2010.03.031
Okubo, C. H., & McEwen, A. S. (2007). Fracture-controlled paleo-fluid flow in Candor Chasma, Mars. Science, 315(5814), 983–985. https://
doi.org/10.1126/science.1136855
Pan, L., Quantin-Nataf, C., Breton, S., & Michaut, C. (2019). The impact origin and evolution of Chryse Planitia on Mars revealed by buried
craters. Nature Communications, 10(1), 1–8. https://doi.org/10.1038/s41467-019-12162-0
Parkes Bowen, A. P., Bridges, J., Tornabene, L., Mandon, L., Quantin-Nataf, C., Patel, M. R., etal. (2022). A CaSSIS and HiRISE map of the
clay-bearing unit at the ExoMars 2022 landing site in Oxia Planum. Planetary and Space Science, 214, 105429. https://doi.org/10.1016/j.
pss.2022.105429
Piñero, E., Marquardt, M., Hensen, C., Haeckel, M., & Wallmann, K. (2013). Estimation of the global inventory of methane hydrates in marine
sediments using transfer functions. Biogeosciences, 10(2), 959–975. https://doi.org/10.5194/bg-10-959-2013
Portenga, E. W., & Bierman, P.R. (2011). Understanding Earth's eroding surface with
10Be. Geological Society of America Today, 21(8), 4–10.
https://doi.org/10.1130/G111A.1
Quantin-Nataf, C., Carter, J., Mandon, L., Thollot, P., Balme, M., Volat, M., etal. (2021). Oxia Planum: The landing site for the ExoMars
“Rosalind Franklin” rover mission: Geological context and prelanding interpretation. Astrobiology, 21(3), 345–366. https://doi.org/10.1089/
ast.2019.2191
Roberts, A. L., Fawdon, P., & Mirino, M. (2021). Impact crater degradation, Oxia Planum, Mars. Journal of Maps, 17(2), 569–578. https://doi.
org/10.1080/17445647.2021.1976685
Ruff, S. W., & Christensen, P. R. (2002). Bright and dark regions on Mars: Particle size and mineralogical characteristics based on Thermal
Emission Spectrometer data. Journal of Geophysical Research, 107(E12), 2-1–2-22. https://doi.org/10.1029/2001JE001580
Sibson, R. (1981). A brief description of natural neighbour interpolation. Interpreting Multivariate Data.
Silvestro, S., Pacifici, A., Salese, F., Vaz, D. A., Neesemann, A., Tirsch, D., etal. (2021). Periodic bedrock ridges at the ExoMars 2022 landing
site: Evidence for a changing wind regime. Geophysical Research Letters, 48(4), e2020GL091651. https://doi.org/10.1029/2020GL091651
Skok, J. R., Mustard, J. F., Ehlmann, B. L., Milliken, R. E., & Murchie, S. L. (2010). Silica deposits in the Nili Patera caldera on the Syrtis Major
volcanic complex on Mars. Nature Geoscience, 3(12), 838–841. https://doi.org/10.1038/ngeo990
Smith, D. E., Zuber, M. T., Frey, H. V., Garvin, J. B., Head, J. W., Muhleman, D. O., et al. (2001). Mars Orbiter Laser Altimeter: Exper-
iment summary after the first year of global mapping of Mars. Journal of Geophysical Research, 106(E10), 23689–23722. https://doi.
org/10.1029/2000JE001364
Tanaka, K. L., Skinner, J. A., & Hare, T. M. (2005). Geologic map of the northern plains of Mars. U.S. Geological Survey Geologic Investigations,
2888, 80225. Retrieved from https://pubs.usgs.gov/sim/2005/2888/
Journal of Geophysical Research: Planets
MCNEIL ETAL.
10.1029/2022JE007246
17 of 17
Thomas, N., Cremonese, G., Ziethe, R., Gerber, M., Brändli, M., Bruno, G., etal. (2017). The colour and stereo surface imaging system (CaSSIS)
for the ExoMars trace gas orbiter. Space Science Reviews, 212(3–4), 1897–1944. https://doi.org/10.1007/s11214-017-0421-1
Uthus, T. N. (2020). Crater statistics and geological history of Oxia Planum, landing site for ExoMars2022. University of Oslo.
Vago, J. L., Westall, F., Coates, A. J., Jaumann, R., Korablev, O., Ciarletti, V., etal. (2017). Habitability on early Mars and the search for biosig-
natures with the ExoMars rover. Astrobiology, 17(6–7), 471–510. https://doi.org/10.1089/ast.2016.1533
ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
In preparation for the operations of the ExoMars Rosalind Franklin rover, characterising its landing site in Oxia Planum is essential. Of particular interest is the extensive Clay-bearing Unit present at the site, a key target in the search for biosignatures. In this paper we provide a map based on variations in colour and spectral information within this unit, covering the 1σ landing envelope of the rover along with a 1 km buffer to account for minor shifts of the landing envelope ahead of launch (referred to going forward as the 1σ+ landing envelope). We used imagery from the Colour and Stereo Surface Imaging System (CaSSIS) and High Resolution Imaging Science Experiment (HiRISE) instruments, along with CaSSIS Band Ratio Composites with enhanced colour sensitivity to the presence of ferric (Fe³⁺) and ferrous (Fe²⁺) iron bearing materials. Our map is of a far higher resolution (map-scale 1:2000) than those previously available and, in contrast to previously available maps of this unit, differentiates between an Orange Subunit and a Blue Subunit which make up the clay-bearing unit. This mapping covered the ∼91% of the 1σ+ landing envelope where there was CaSSIS coverage and split the clay-bearing unit into three categories: one for each of the clay subunits, and another for exposures of the clay-bearing unit where either both subunits were too intermixed to reliably separate, or where it was difficult to determine which of the two were present. The results from our mapping shows that at least ∼35% of the 1σ+ envelope is covered by exposures of the Clay-bearing Unit: ∼18% by the Orange Subunit, ∼9% the Blue Subunit, and ∼12% were classified as Indeterminate. The spread of these two subunits varied substantially over the 1σ+ landing envelope, with the south-east half of the landing envelope dominated by the Orange Subunit (∼70% exposures in this area belonging to the Orange Subunit, ∼10% to the blue and ∼20% to the Indeterminate), while the north-west has more sporadic exposures of the Clay-bearing Unit (∼22% Orange, ∼37% Blue and ∼41% Indeterminate). The colour distinction between the two subunits is thought to be due to constituent mineralogical differences rather than differences in dust coverage of the two subunits. The scale of the fracturing present in the two subunits has also been assessed in this study, via qualitative observations of the fracture length and quantitative mapping out of fracture networks. While there were differences in the scale of fracturing between the two subunits, these were not as great as had previously been identified.
Article
Full-text available
Oxia Planum, the landing site for the ExoMars rover mission, is a shallow basin on the southern margin of Chryse Planitia that hosts remnants of fan‐shaped sedimentary deposits associated with the ancient channel system Coogoon Vallis. This indicates runoff from a catchment in Arabia Terra has transported sediment into the landing site. To explore this fluvial system we created a model catchment for Oxia Planum and, using 6 m/pixel ConTeXt camera orbital remote sensing image data, we digitized the fluvial and lacustrine landforms in Western Arabia Terra in and around this catchment. We find: (a) The catchment has a minimum area of ∼2.1 × 10⁵ km² and has been episodically deformed by tectonic activity; (b) There were at least two phases of fluvial activity. The first created a mature landscape associated with Coogoon Vallis, which may have deposited alluvial or deltaic deposits in the Oxia Basin. After a substantial hiatus, a second phase of activity incised U‐section channels into the pre‐existing landscape and channel systems; and (c) Evidence for numerous possible paleolake deposits within the catchment. These are not well connected to the fluvial system and were probably sustained by groundwater activity contemporaneous with both phases of fluvial activity. This groundwater might have modified the mineralogy of Oxia Planum. Oxia Planum probably experienced an alluvial or distal deltaic/lacustrine depositional environment during the mid Noachian, which was later overprinted by a younger phase of fluvial activity.
Article
Full-text available
We present the geography of Oxia Planum, the landing site for the ExoMars 2022 mission. This map provides the planetary science community with a framework to understand this, until recently, unexplored area. The map comprises (1) a mosaic of the panchromatic Context Camera (CTX) Digital Elevation Models (DEM) and Ortho Rectified Images (ORI) controlled to the High Resolution Stereo Camera (HRSC) multiorbit Digital Elevation Models (DEM) and (2) a mosaic of Colour and Stereo Surface Imaging System (CaSSIS) synthetic colour data products, registered to the CTX ORI mosaic. We define a grid of exploration quadrangles (quads) and an informal group of geographic regions to describe Oxia Planum. These regions bridge the scale gap between features observed on large areas (∼100s km²) and the local geography (10s km²) relevant to the Rosalind Franklin rover’s operations in Oxia Planum.
Article
Full-text available
The main goal of the European Space Agency (ESA) and Roscosmos ExoMars rover mission is to collect samples from the near-subsurface of Mars. The rover will look for any physical or chemical evidence of ancient life in the near subsurface. This map shows the distribution of impact craters at this proposed landing site in Oxia Planum on Mars. The map records 1199 impact craters > 500 m in diameter in a 5.0° × 2.5° region around Oxia Planum. The impact craters are symbolised based on the way different aspects of their morphology have degraded since their formation. The distribution of degradation and burial morphologies of impact craters can be used to determine where burial and erosion processes have occurred. Because the formation of impact craters is well constrained, occurs instantly and with a predictable flux, future studies could use this knowledge and our dataset to constrain when these events occurred.
Article
Full-text available
The margin of Chryse Planitia, Mars, contains >10⁵ kilometer‐scale mesas, buttes, and plateaus (“mounds”), many of which are found in and around Oxia Planum, the ExoMars 2022 Rover landing site. Despite this, their origins and evolution are unknown. We have analyzed the morphologies and morphometries of 14,386 individual mounds to: (1) classify them based on their geomorphology; (2) constrain when they formed based on their stratigraphic and spatial relationships; and (3) develop hypotheses for their geological history. The mounds are classified as compound mounds, mesas, clustered mounds, and hills. Mound heights show that their elevations above the plains tend to a maximum height of 500 m. We interpret this as the thickness of a previously continuous layer that extended several hundred kilometers from the southern highlands into Chryse Planitia. Stratigraphy constrains the deposition of this layer to the Early‐Middle Noachian, correlatable to the phyllosilicate‐bearing strata of Mawrth Vallis, with similar layering also observable in some mounds, suggesting a genetic relationship. The mounds sometimes occur in circular arrangements, interpreted as an association with buried impact structures. We propose that the mounds formed through differential erosion after the premound layer was indurated by mineralization from groundwater in areas superposing underlying crustal weaknesses, for example, at buried crater margins. The subsequent differential erosion of this layer preferentially removed areas unaffected by this induration in the Late Noachian‐Early Hesperian leaving the mound population seen at present. These features present accessible three‐dimensional exposures of ancient layered rocks, and so are exciting targets for future study.
Article
Full-text available
Lithified aeolian strata encode information about ancient planetary surface processes and the climate during deposition. Decoding these strata provides insight regarding past sediment transport processes, bedform kinematics, depositional landscape, and the prevailing climate. Deciphering these signatures requires detailed analysis of sedimentary architecture to reconstruct dune morphology, motion and the conditions that enabled their formation. Here we show that a distinct sandstone unit exposed in the foothills of Mount Sharp, Gale crater, Mars, records the preserved expression of compound aeolian bedforms that accumulated in a large dune field. Analysis of Mastcam images of the Stimson formation shows that it consists of cross‐stratified sandstone beds separated by a hierarchy of erosive bounding surfaces formed during dune migration. The presence of two orders of surfaces with distinct geometrical relations reveals that the Stimson‐era landscape consisted of large dunes (draas) with smaller, superimposed dunes migrating across their lee slopes. Analysis of cross‐lamination and subset bounding surface geometries indicate a complex wind regime that transported sediment toward the north, constructing oblique dunes. This dune field was a direct product of the regional climate and the surface processes active in Gale crater during the fraction of the Hesperian Period recorded by the Stimson formation. The environment was arid, supporting a large aeolian dune field; this setting contrasts with earlier humid depositional episodes, recorded by the lacustrine sediments of the Murray formation (also Hesperian). Such fine‐scale reconstruction of landscapes on the ancient surface of Mars is important to understanding the planet’s past climate and habitability.
Article
Oxia Planum (335.5°E, 18.2°N) is selected as the landing site for ExoMars rover mission (ESA/Roscosmos), where the “Rosalind Franklin” rover is scheduled to land in the decade. The region reveals several extensive clay-bearing outcrops recently exhumed, where biosignatures are possibly preserved. The objectives of the mission are to search for organics and investigate traces of past or extant life on Mars. Preliminary surveys of these outcrops show infrared absorptions typical of Fe,Mg-rich clays in the 1.0–2.6 μm range (1.4, 1.9, 2.3 and 2.4 μm) and an additional absorption at 2.5 μm implying a possible mixture with other mineral phase(s). Here we provide a detailed description of absorptions of the clay-rich materials detected in Oxia Planum, and map their strength and distribution throughout the region using hyperspectral data gathered by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard NASA's Mars Reconnaissance Orbiter (MRO) mission. Our analysis suggests that the Fe,Mg-rich clays identified in Oxia Planum mainly correspond to either Fe-bearing saponites (e.g., Griffithite) or vermiculite ores (i.e., vermiculite associated with a hydrobiotite component). Conversely, large clay-bearing outcrops found in the catchment area (337°E, 16.7°N) are rather consistent with nontronites in association with Al-rich clays and kaolins, in agreement with previous identification in the Mawrth Vallis – west Arabia Terra province. Presence of Fe,Ca-rich carbonates is recognized with the absorption near 2.53 μm and the observation of a broad peak in the 3–4 μm range, supporting their co-occurrence with the clays in Oxia Planum and its catchment area. Although we favor a pedogenesis alteration for the clays found in the catchment area, the origin of those studied in Oxia's basin remains enigmatic, where alternative scenarios could be either lacustrine and deltaic sedimentation, groundwater circulation, or even hydrothermal fluid circulation. Future in-situ measurements by “Rosalind Franklin” rover will indubitably provide new insights on the mineralogical diversity seen in the region and their origins.
Article
Kilometre-sized flows (KSFs) have been observed in many regions on Mars and have been typically interpreted as lava flows. However, sedimentary volcanism has been proposed as an alternative origin for some KSFs. Remarkable examples of such hypothesized sedimentary KSFs are located at the southern margin of Chryse Planitia. There, the flows are associated with conical and dome-shaped edifices; however their formation mechanism remains enigmatic due to the absence of ground truth. Previous studies revealed that these KSFs consist of three morphological elements: a central depression, leveed central channels, and a distal portion of the fading channel(s). Here, we present new morphological results obtained on these KSFs using seven newly available Digital Elevation Models computed from HiRISE stereo pairs. Our investigation confirms that these features are aggradational and formed by the transport of a liquid. This material emerged from identified depressions and the presence of subtle mounds inside them is interpreted to mark the position of feeder vents. We also observe that the margins surrounding the central large channels are not continuous. They are cut by meter-sized troughs linking the central channels to units which have distinctive albedo and roughness compared to their surroundings. These bright units do not have a clear topographical expression, suggesting that the effused material originally flowing away from the central channel was easily removed after its emplacement. Such surface features are unlikely to be related to igneous deposits, since once lava is released from a main channel, it would rapidly solidify due to the heat loss and hence result in topographically distinct features. In contrast, such morphological expressions are more likely related to sedimentary volcanism and the emplacement of low viscosity water-rich mud. Sublimation, evaporation, infiltration or a combination of these processes should lead to water loss from the flows without leaving a detectable topographic expression but changing the roughness and hence albedo of the surface. The southern part of Chryse Planitia is a region on Mars where subsurface sediment mobilization could have operated in the past and hence represents a promising site for future exploration where deeper-sourced sedimentary deposits are exposed at the surface.
Article
Perseverance images of a delta on Mars The Perseverance rover landed in Jezero crater, Mars, in February 2021. Earlier orbital images showed that the crater contains an ancient river delta that was deposited by water flowing into a lake billions of years ago. Mangold et al . analyzed rover images taken shortly after landing that show distant cliff faces at the edge of the delta. The exposed stratigraphy and sizes of boulders allowed them to determine the past lake level and water discharge rates. An initially steady flow transitioned into intermittent floods as the planet dried out. This history of the delta’s geology provides context for the rest of the mission and improves our understanding of Mars’ ancient climate. —KTS