Post by Dr Susan Conway, Open University, UK
Gullies on Mars were first discovered in 2000 (Malin and Edgett, 2000) in images taken by the Mars Orbiter Camera on board NASA’s Mars Global Surveyor platform. They are kilometre-scale features and have a striking resemblance to water-carved gullies on Earth (Image 1).
Martian gullies have few superposed impact craters and are thought to have occurred in the last few millions of years (e.g., Johnsson et al., 2014), with some modifications of their morphology having occurred in the last few decades (e.g., Dundas et al., 2014). Gullies are concentrated at the mid-latitudes and are found most commonly on steep (>20°) pole-facing slopes, including crater walls, crater central peaks/pits, mesas, escarpments and dune slip-faces. Gullies at higher latitudes are oriented towards the equator leading researchers to link their formation to variations in solar insolation and climate due to changes in Mars’ axial tilt (which has varied between 15-45° in the last 10 Ma).
Gullies occur over the same latitudinal band as a draping surface unit termed the “latitude dependant mantle”. The mantle appears degraded in the mid-latitudes, where gullies are most common, and remains intact near the poles where the terrain appears smooth or softened. This mantle is thought to be formed by a mixture of ice and dust deposited from the atmosphere and has been dated between 0.1 and 20 Ma. The fact that these two landforms appear together has led many to infer that gullies are formed by melting of the mantle (see this IAG post)(e.g., Bridges and Lackner, 2006).
Where gullies incise into this mantle they have characteristic v-shaped chutes, in which the channels themselves are located (Image 2). These chute-walls are often covered with polygonal fractures (interpreted as thermal contraction cracks in icy-material)(Levy et al., 2009) and can also have fresh collapses (Image 2). The walls of the incisions lie at 10-20° where they have polygons, but up to 30° where there is evidence of collapse. These incisions provide a unique window into the latitude dependant mantle and using high resolution topographic data we have measured their depth and have shown that the mantle on these slopes is between 5 and 30 m in thickness. We have also found that for gullies completely embedded in the mantle (Image 3) the volume of the incision can be up to 17 times that of the depositional fan – giving us an estimate of the ice-content of the mantle of 46% and 95% ice by volume (Conway and Balme, 2014).
Ice exposed at the surface sublimates away quickly at the present day, hence we think this ice is protected by a surface lag, similar to the lag that protects ice within moraines on Earth. Gullies form by melting of ice-cored moraines on Earth (Image 3), whereby a slump exposes the interior ice, which continues to melt and promote further erosion until it becomes covered again by sufficient lag to protect it from insolation-driven melting (Lukas et al., 2005). A similar process could explain the formation of Martian gullies incised into the mantle.
Bridges, N.T., Lackner, C.N., 2006. Northern hemisphere Martian gullies and mantled terrain: Implications for near-surface water migration in Mars’ recent past. J. Geophys. Res. Planets 111, 09014. doi:10.1029/2006JE002702
Conway, S.J., Balme, M.R., 2014. Decametre-thick remnant glacial ice deposits on Mars. Geophys. Res. Lett. Vol. 41, 15, doi:10.1002/2014GL060314.
McEwen, A.S., 2014. Long-Term Monitoring of Martian Gully Formation and Evolution with MRO/HiRISE. Icarus in press. doi:10.1016/j.icarus.2014.05.013
Johnsson, A., Reiss, D., Hauber, E., Hiesinger, H., Zanetti, M., 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. doi:10.1016/j.icarus.2014.03.005
Levy, J.S., Head, J.W., Marchant, D.R., Dickson, J.L., Morgan, G.A., 2009. Geologically recent gully-polygon relationships on Mars: Insights from the Antarctic dry valleys on the roles of permafrost, microclimates, and water sources for surface flow. Icarus 201, 113–126.
Lukas, S., Nicholson, L.I., Ross, F.H., Humlum, O., 2005. Formation, Meltout Processes and Landscape Alteration of High-Arctic Ice-Cored Moraines—Examples From Nordenskiold Land, Central Spitsbergen. Polar Geogr. 29, 157–187. doi:10.1080/789610198
Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335. doi:10.1126/science.288.5475.2330