Post contributed by Dr. Joe McNeil, Natural History Museum, London
Mars is a planet of geographic contrast. The southern highlands, which comprise the lower two-thirds of the planet form a rugged, heavily cratered landscape that are high in elevation and are some of Mars’ oldest terrains. The upper third of the planet – the northern lowlands – form lightly cratered plains, Mars is a planet of geographic contrast. The southern highlands, which comprise the lower two-thirds of the planet, form a rugged, heavily cratered landscape that are high in elevation and are some of Mars’ oldest terrains. The upper third of the planet – the northern lowlands – form lightly cratered plains, are lower in elevation, and are much younger in age. Where the two hemispheres meet, they form a planetary-scale feature called the dichotomy boundary. One of the most important – and enigmatic – regions of Mars lies in the highlands just south of the boundary at a place called Mawrth Vallis (Image 1). The 650 km-long Mawrth Vallis channel carves its way through Noachian-aged (4.1—3.7 billion years ago) clay-bearing, layered deposits that underpin much of our understanding of the role of liquid water on early Mars (e.g. Bishop et al., 2008; Loizeau et al., 2012). Ongoing work has revealed that these clay-bearing, layered deposits were once much more extensive than they are today. The evidence for this is contained in a population of thousands of mesas, buttes, and hills (‘mounds’) that exist in the lowlands just north of the dichotomy. Image 1A shows the mound population north of the dichotomy within the study area, as well as an example of three individual mounds in Image 1B.
Image 1: Similar strata, different settings. (A) shows a topographic map of the Mawrth Vallis region, with lowlands (teal), highlands (brown), and dichotomy boundary (dashed red line). The mounds are shown as black shapes in the northern lowlands. Inset MOLA (Mars Orbiter Laser Altimeter) globe shows location and global context of the study area. Panels B1 and C1 are CTX (Context Camera) and HiRISE (High Resolution Imaging Science Experiment) images showing a typical mound and a crater in the highlands, respectively, which both exhibit similar dark and light-toned layering at the metre scale (panels B2 and C2; HiRISE IRB).
Three key lines of evidence link the origin of the mounds to the highlands despite the present-day discontinuity: Firstly, the mounds possess morphologically identical outcrops to the nearby exposures in the highland region (Image 1). Bedrock in both regions is layered at the metre-scale with dark and light strata that can be traced laterally for several kilometres. Secondly, hyperspectral CRISM data reveal the highland plateau and mounds both contain the same variety of Fe/Mg-rich phyllosilicates – indicating that similar aqueous environments have interacted with mafic material to alter the original chemistry in both locations (Image 2). Finally, the mounds and plateau occur proximally to one another at similar elevations, and are approximately the same height, containing similar thicknesses of these morphologically and mineralogically similar clay-bearing, layered deposits.
Image 2: Mound mineralogy mimics Mawrth! CaSSIS (Colour and Stereo Science Imaging System) NPB (NIR-PAN-BLU) image of a 450 m-tall mound draped over CTX Digital Terrain Model: 2x elevation exaggeration. CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) I/F spectra extracted from the mound and from the plateau show absorptions at 1.4, 1.9, 2.3 and 2.4 µm, which are diagnostic of Fe/Mg-rich phyllosilicates.
This evidence suggests a physical connection between the mounds and highlands in the past, indicating that the highland plateaus around Mawrth Vallis once extended further north. The clay-bearing mounds that lie the furthest from the highlands (e.g. Image 3) are hundreds of kilometres away from the modern dichotomy boundary, so the plateau has in some places retreated by at least this much. The erosion that drove this retreat is not well understood and is still an area of active research, but possible candidates include fluvial incision along pre-existing fractures, intense, short-lived flooding events, or glacial action. Because these mounds are rich in phyllosilicates, alteration – the process by which liquid water interacts with rocks and changes their chemistry to form clay minerals – must also have been much more regionally extensive than previously thought.
Image 3: Animated gif of a cluster of mounds in the lowlands. These mounds are the same as in the previous figures, and lie approximately 200 km from the dichotomy boundary. They exhibit morphological, stratigraphic, compositional, and geometric similarities with the highland plateau, suggesting it once extended at least this far. CTX image draped over CTX DTM; 2x exaggeration. Field of view approximately 12 km across centre of image.
Ongoing efforts aim to unravel the implications of these mounds for early martian climate, landscape evolution, and hydrology. Additionally, because the mounds occur in Oxia Planum, the future ExoMars Rover landing site (McNeil et al., 2022; Quantin-Nataf et al., 2021), their geology may be one day explored with ground-based observations. One undeniable conclusion has emerged so far: these mounds serve as a previously unrecognised archive of Noachian geology, offering valuable insights into the nature and evolution of early planetary environments.
Further reading:
Bishop, J.L. et al. (2008), Phyllosilicate Diversity and Past Aqueous Activity Revealed at Mawrth Vallis, Mars, Science, 321, 5890, pp.830-833
Loizeau, D. et al. (2012), Chronology of deposition and alteration in the Mawrth Vallis region, Mars, Planetary and Space Science, 72, 1. pp.31-43
McNeil, J.D. et al. (2021), Morphology, Morphometry and Distribution of Isolated Landforms in Southern Chryse Planitia, Mars, JGR: Planets, 126, 5. e2020JE006775.
McNeil, J.D. et al. (2022), Mounds in Oxia Planum: The Burial and Exhumation of the ExoMars Rover Landing Site, JGR: Planets, 127, 11. e2022JE007246.
Quantin-Nataf, C. et al. (2021), Oxia Planum: The Landing Site for the ExoMars “Rosalind Franklin” Rover Mission: Geological Context and Prelanding Interpretation, Astrobiology, 21, 3. pp.345-366.
Planetary Geomorphology Image of the Month 
Ian Reid
/ February 1, 2024Joe McNeil provides an interesting and convincing explanation for what he terms ‘mounds’ that lie in the northern lowlands of Mars several hundred kilometres from the ‘Dichotomy boundary’. He suggests that they are remnants of a one-time more extensive southern highland plateau.
Anyone familiar with cratonic parts of Earth’s continents will recognise similarities with inselbergs or kopjes that rise from the surrounding peneplane. The most celebrated, of course, is Uluru, though this has no immediately local companions, unlike the Martian examples.
Looking at the Image 1A, the spatial density of mounds, decreasing northwestwards, does indeed suggest a retreating plateau scarp (the Dichotomy Boundary), whether this is a one-time coastal or an inland feature. But, of additional interest is why the distant ‘mounds’ (Images 1B1 and 3) have persisted in the topography. It might not be possible to discount local tectonics and speculate that they are horsts, though their planform does not lend itself readily to this. Ordinarily, one might then look to local structural flexure or mineralogy as reasons for countering weathering and erosion.