Valleys, Deltas, and Lacustrine Sediment in the South-western Melas Basin, Valles Marineris, Mars

Post contributed by Joel Davis, Department of Earth Sciences, University College London, UK.

During the last few decades, dry river valley networks and delta fan structures have been found to be increasingly common on ancient terrains on the martian surface (e.g. Goldspiel and Squyres, 1991; Hynek et al., 2010). They are considered to be one of the main lines of evidence that Mars once had Earth-like precipitation and surface runoff (e.g. Hynek and Phillips, 2003). One such location is the south-western Melas basin, part of a collapsed graben structure on the southern wall of Melas Chasma, Valles Marineris – Mars’ equatorial canyon system (Images 1 & 2). The basin likely formed in the early Hesperian period (~ 3.7 – 3.5 Ga), soon after Melas Chasma opened.

Image 1

Image 1: Context Camera image-mosaic of western portion of palaeolake sequence in the south-western Melas basin. In the left of the image, valley networks can be seen converging on a delta-like structure at the centre of the image. Layered lacustrine deposits are well exposed in the right of the image; about 40-50 packages are visible at this resolution. [Image numbers: G22_026866_1710_XN_09S077W & P07_003685_1711_XI_08S076W]

The basin contains a complex stratigraphy (Image 1), including multiple networks of valleys at both its western and eastern edges, which converge on a topographic depression at the centre of the basin. Here, an array of sedimentary fans that resemble delta structures grade into a thick sequence of layered sediment, suggesting that the depression is a palaeolake (Quantin et al., 2005; Williams and Weitz, 2014). With no obvious egress point for any potential water, the palaeolake forms a closed-basin system, meaning that water may have pooled for long periods of time without overspilling.

Image_2

Image 2: MOLA topography map of Melas Chasma, Valles Marineris. The south-western Melas basin is highlighted. It is situated about two kilometres off the main valley floor

The high drainage density and dendritic nature of the valley networks are supportive of them having been formed by Earth-like precipitation and surface runoff, as oppose to groundwater related formation mechanisms (Quantin et al., 2005; Williams and Weitz, 2014). The delta structures show multiple overlapping lobes and originate from both sides of the palaeolake. There is also evidence, both in the valley networks and in the palaeolake sequence, that the basin may have dried out at least once before refilling at a later stage (Williams and Weitz, 2014). Volume estimates suggest some of the fan structures may have taken up to 10,000 years to form (Metz et al., 2009), but given a potential hiatus, lacustrine processes could have been active in the basin for much longer periods of time. More recently, the basin has been extensively modified by mass wasting and aeolian activity.

It is for these reasons that the Melas basin is currently one of the candidate landing sites for NASA’s upcoming Mars 2020 rover mission (Williams et al., 2014). The lacustrine sediment is considered a former habitable environment, which could have the potential to preserve biosignatures, the exploration of which is one of the mission’s main science goals.

Further Reading

Goldspiel, J. M., and Squyres, S. W. (1991). Ancient aqueous sedimentation on Mars, Icarus, 410, 392–410.

Hynek, B. M., & Phillips, R. J. (2003). New data reveal mature, integrated drainage systems on Mars indicative of past precipitation. Geology, 31(9), 757–760.

Hynek, B. M., Beach, M., and Hoke, M. R. T. (2010). Updated global map of Martian valley networks and implications for climate and hydrologic processes. J. Geophys. Res. 115, 1–14.

Quantin, C., Allemand, P., Mangold, N., Dromart, G., and Delacourt, C. (2005). Fluvial and lacustrine activity on layered deposits in Melas Chasma, Valles Marineris, Mars. J. Geophys. Res. 110, E12S19.

Metz, J. M., Grotzinger, J. P., Mohrig, D., Milliken, R., Prather, B., Pirmez, C., and Weitz, C. M. (2009). Sublacustrine depositional fans in southwest Melas Chasma. J. Geophys. Res. 114, E10002.

Williams, R. M. E., & Weitz, C. M. (2014). Reconstructing the aqueous history within the southwestern Melas basin, Mars: Clues from stratigraphic and morphometric analyses of fans. Icarus, 242, 19–37.

Williams, R. M. E., Weitz, C. M., Quantin, C., Dromart, G., Grindrod, P. M., and Davis, J. (2014). In Situ investigation of the Southwestern Melas Basin. Mars 2020 Landing Site Workshop, Crystal City, VA.

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