The Largest Delta on Mars?

Post by Jacob Adler, School of Earth and Space Exploration, Arizona State University.

Ancient river deltas are found in many locations on Mars [see Di Achile & Hynek, 2010 and references therein], and are formed as sediment drops out of suspension in water as it approaches a wider shoreline of a lake, sea, or (debatably) an ocean. Some proposed deltas on Mars are found in closed basins (e.g. an impact crater) away from the Martian dichotomy boundary, implying an ancient climate during which the crater ponded with water [e.g. Eberswalde or Jezero]. Occasionally, inlet and outlet river valleys are seen at different elevations along the crater rim, lending further evidence to the hypothesis that the crater filled with liquid water at least up to the outlet elevation. Deltas found in open basins, on the other hand, imply a larger body of standing water, and Mars scientists look for other clues to support the deltaic rather than alluvial fan formation mechanism. In our recent papers, we tested whether the Hypanis fan-shaped deposit (Image 1) could be a delta, and discussed whether this supports the hypothesis that there was once a large sea or ocean in the Northern plains of Mars [Adler et al., 2018; Fawdon et al., 2018].

Image1

Image 1: a) The Hypanis deposit stands out as light-toned in the center of this Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) mosaic of our study region. Also marked are Lederberg crater, and the Sabrina deposit in the closed basin of Magong crater. (b) Hypanis and Sabrina have a low nighttime temperature (dark) as recorded by the THEMIS instrument on Mars Odyssey, suggesting it is mostly composed of small grain size material. Image from the Nighttime IR 100m Global Mosaic v14.0 [Hill et al. (2014); Edwards (2011)] and from the Northern Hypanis Valles Night IR Mosaic [Fergason (2009)]. NASA/JPL/ASU. (c) Our proposed fluvial sequence discussed in the paper. Main lobe (A) could once have had continuous layered beds spanning to the distal island deposits (E). The cross-cutting relationships we observed are consistent with hypothesized shoreline regression to the north. Flow migrated to the northern lobe (B), then to braided inverted channels (C and D) as water retreated. NASA/MSSS/USGS. d) Our digital elevation mosaic shows the topography of Hypanis and surrounding features. Elevations are colored from white (-2500 m) to light green (-2800 m).

The Hypanis deposit is roughly 1100 km2 in size, which would make it the largest potential delta on Mars, and comparable in the size of the Colorado river delta on Earth. Recent orbital high-resolution images, multispectral data, and topographic models allowed us to investigate this deposit in great detail for the first time, which had been proposed as a candidate landing site for NASA’s Mars 2020 rover, the ESA’s ExoMars rover, and NASA’s future human missions. Using low sun angle images with large shadows, we could see paleoflow lineations that indicated avulsion node migration forming different lobes. We measured low downstream gradients of beds within the deposit (0-2°), and identified distal deposits over 150 km away from the main lobe, indicating the feature was once much larger and had been eroded. The gently sloping beds were found to be continuous for many kilometers (Image 2).

Image2

Image 2: The interior layers of Hypanis are roughly flat and continuous for many kilometers. The beds form cliffs and terraces of ~0.5-2.0 meters in thickness as visible from orbit. The beds near the bottom of the image appear blockier than the upper strata near the top of the image. In this region, strata appear to be covered by transverse aeolian ridges (TARs) or sand dunes, perhaps originating from the erosion of the deposit. HiRISE ESP_036161_1920, NASA/JPL/UA.

The above observations, and our measurement of a very low thermal inertia and lack of large boulders, indicate that Hypanis was formed in a prolonged relatively calm aqueous depositional environment (i.e. a delta). We also consider the context of Hypanis near other proposed deltas and paleoshorelines (see Image 3) to be supportive of the deltaic hypothesis. If Hypanis was indeed an open basin delta as the regional topography suggests, then a large body of water would have existed in the Chryse basin at least 3.6 billion years ago.

Image3

Image 3: The global distribution of potential deltas on Mars is shown over a Mars Orbiter Laser Altimeter (MOLA) colorized elevation map. Blue is topographic low, red is high. Yellow boxes are locations of deltas mapped by Di Achille & Hynek (2010) and references therein. Hypanis is indicated by the white arrow. NASA/JPL/ GSFC.

Further Reading

Adler, J.B., et al. (2018), Hypotheses for the Origin of the Hypanis Fan-Shaped Deposit at the Edge of the Chryse Escarpment, Mars: Is it a Delta?, Icarus, In Press, doi: 10.1016/j.icarus.2018.05.021.

Di Achille, G. & Hynek, B.M. (2010), Ancient ocean on Mars supported by global distribution of deltas and valleys, Nature Geoscience, 3, 459-463.

Fawdon, P., et al. (2018), The Hypanis Valles Delta: The last highstand of a sea on early Mars? Earth and Planetary Science Letters, In Press.

Goudge, T.A., et al. (2015), Stratigraphy and paleohydrology of delta channel deposits, Jezero crater, Mars, Icarus, 301, 58-75.

Hauber, E., et al. (2009), Sedimentary deposits in Xanthe Terra: Implications for the ancient climate on Mars, Planetary and Space Science, 57, 944-957.

Hauber, E., et al. (2013), Asynchronous formation of Hesperian and Amazonian-aged deltas on Mars and implications for climate, Journal of Geophysical Research, 118, 1529-1544.

Irwin, R.P. III, et al. (2015), Paleohydrology of Eberswalde crater, Mars, Geomorphology, 240, 83-101.

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