Overlapping Lobate Deposits in Martian Gullies

Post contributed by Rishitosh K. Sinha, Planetary Sciences Division, Physical Research Laboratory, India.

Gullies are found on steep slopes on the surface of Mars and appear as a linear-to-sinuous channel linking an alcove at the top to a fan at the bottom. The most interesting interpretation of the past two decades has been that the Martian gullies were carved by the flow of liquid water as discovered from the high-resolution images returned by the Mars Orbiter Camera (MOC) onboard “Mars Global Surveyor (MGS)” in 2000 (Malin and Edgett, 2000). Subsequent observations using MOC and the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) images revealed that Martian gullies are active today and that sublimation of seasonal carbon dioxide frost – not liquid water – could have played an important role in their formation. In our recent work using HiRISE images we reported global distribution of overlapping lobate deposits in gullies (Image 1) showing that a debris-flow like process may be responsible for gully formation (Sinha et al., 2020).

Image 1: Top: 3D view of gullies on the pole-facing wall of ~8 km diameter Los crater (35.08˚ S, 76.22˚ W) on Mars. HiRISE image ESP_020774_1445 (0.25 m/pixel) is draped over 1 m/pix HiRISE elevation model. The depth of crater floor from the crater rim is ~1 km in elevation and the image spans ~4 km from left to right. The box shows the location of bottom panel. Bottom: Image shows the gully fan surface within Los crater with overlapping lobate deposits, including convex-up and tongue shaped terminal lobes with lateral levees. HiRISE image credits: NASA/JPL/University of Arizona.

We define an overlapping lobate deposit (Image 2a) as a lobe that partially superposes another formerly deposited lobe in the downslope direction (Image 2b). We found that these deposits in martian gullies have morphological characteristics typically observed in terrestrial debris flows, as described below. The overlapping lobes have a convex-up tongue-shape, and commonly display a straight to slightly sinuous flow pattern in plan view (Image 2b). The overlapping lobes are younger as they are at the top of the stratigraphy of the gully fans and there is an abundance of clasts in the lobes (the number of clasts diminishes with increasing age – de Haas et al., 2015). There can be series of terminal lobes stacked together, often regressively by backstepping, or fanning out laterally, thereby forming broad deposit complexes (Image 2b). Morphological evidence of channel plugging, backstepping, and avulsion are often observed, which is typical for the spatio-temporal evolution of fans formed by lobate debris flow deposits on Earth (de Haas et al., 2016). Lobes are often connected to channels if not buried by subsequent flows, and the channels are mostly flanked by lateral levees (Image 2c). Where levees occur, the channels and lobate deposits are elongated.

Image 2: (a): Pole-facing wall of a ~6.5 km unnamed crater (43.24˚ N, 134.20˚ W) with overlapping lobate deposits in gullies. HiRISE image ESP_046028_2235. (b) Detailed view of the overlapping lobate deposits in gullies. Boxes represent the overlapping lobate deposits. Lobes are characterized by a convex-up tongue shape (black arrows). Morphologic evidence for channel backfilling by backstepping of subsequent lobe deposits is represented by white arrows. (c) Examples of lateral levees (arrows) in elongated channelized flows. HiRISE image credits: NASA/JPL/University of Arizona.

We analysed 1726 impact craters (988 craters in the northern hemisphere and 738 craters in the southern hemisphere) between 30˚ and 75˚ latitude range (Image 3). We found 278 craters hosting gullies in the northern hemisphere and 487 craters in the southern hemisphere. Among these craters hosting gullies, we identified and examined overlapping lobate deposits in gullies in 20 craters (6 in the northern hemisphere and 14 in the southern hemisphere). Previously, only 6 craters hosting overlapping lobate deposits in gullies had been reported in the mid-latitudes of Mars. As a result, there are now 26 craters in which overlapping lobate deposits are known to occur in gullies.

Image 3: Locations of overlapping lobate deposits in Martian gullies between 30˚-75˚ N and S. The colors of the plotted symbols specify: blue circle – gullied craters with overlapping lobate deposits, red triangle – previously reported gullied craters with overlapping lobate deposits, white circle – craters with gullies in HiRISE, and black circle – craters without gullies in HiRISE. Background colours show Mars Orbiter Laser Altimeter gridded topography (red is high elevation and green is low elevation) in cylindrical projection. HiRISE image credits: NASA/JPL/University of Arizona.  MOLA credits: NASA/JPL/GSFC.

The craters hosting lobate deposits in Martian gullies span a broad range of ages from ~2 to 300 Ma in the northern hemisphere and ~10 Ma – 3 Ga in the southern hemisphere. Furthermore, the gullies with overlapping lobate deposits are not distinct from the overall gully population in their distributions, geomorphic settings, or the slope angles on which they occur (Sinha et al., 2020). We infer that the debris-flow like process forming lobate deposits has been active more widely, but its morphological expression is only preserved where lobate deposits have formed very recently. In the older gully fans, the morphological signatures of the lobate deposits have been erased by post-depositional processes (wind erosion in particular). We have not observed any such lobate deposits forming today, so it still remains under investigation as to whether the sublimation of seasonal carbon dioxide frost can create such deposits and therefore whether it can explain the formation of martian gullies.

Further Reading

Malin, M. C., & Edgett, K. S. (2000). Evidence for recent groundwater seepage and surface runoff on Mars, Science, 288(5475), 2330-2335.

Sinha, R. K., et al. (2020). Global documentation of overlapping lobate deposits in Martian gullies, Icarus, 352, 113979.

Conway, S. (2010). Debris flows on Earth and Mars (Doctoral dissertation, The Open University).

de Haas, T., et al. (2015). Sedimentological analyses of Martian gullies: the subsurface as the key to the surface, Icarus, 258, 92-108.

de Haas, T., et al. (2016). Autogenic avulsion, channelization and backfilling dynamics of debris‐flow fans, Sedimentology, 63(6), 1596-1619.

Johnsson, A., et al. (2014). Evidence for very recent melt-water and debris flow activity in gullies in a young mid-latitude crater on Mars, Icarus, 235, 37-54.

Lanza, N. L., et al. (2010). Evidence for debris flow gully formation initiated by shallow subsurface water on Mars, Icarus, 205(1), 103-112.

Levy, J. S., et al. (2010). Identification of gully debris flow deposits in Protonilus Mensae, Mars: Characterization of a water-bearing, energetic gully-forming process, Earth and Planetary Science Letters, 294(3-4), 368-377.

Reiss, D., et al. (2011). Terrestrial gullies and debris-flow tracks on Svalbard as planetary analogs for Mars, Geological Society of America Special Papers, 483, 165-175.

Dundas, C. M., et al. (2015). Long-term monitoring of martian gully formation and evolution with MRO/HiRISE, Icarus, 251, 244-263.

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1 Comment

  1. Ben Everitt

     /  November 2, 2020

    Very nice analysis. Great images! The issue of channel formation on mars seems to be following the same path as our earlier understanding of the “geomorphology” of the moon. For a brief time in the mid 1960’s, enthusiasm for “water on the moon” was spawned by discovery, in the lunar orbiter mission, of the long meandering channel in Schroter’s Valley, and the avalanche chutes in the large craters like Copernicus, and considerable energy was expended toward explaining how liquid water could have existed on the now airless moon. It is an interesting sidenote in the history of planetary science. In succeeding years, I have lost track of those pre-digital references to “water on the moon”. If anyone has them, I would love to see them posted.

    Reply

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