Tip o’ the Space Helmet to Geotripper for his Monday post (go and read it at http://geotripper.blogspot.com/2008/03/speaking-of-landslides-caught-on-film.html), which brought to light (for me, at least) a completely phenomenal picture of a landslide CAUGHT ON TAPE from Mars. It’s a little brain-boggling to think that some humans built a machine, lobbed it into orbit around Mars, and now that machine is sending us pictures back of another planet. Just goes to show that when Humans aren’t busy devising new and clever ways to kill one other, we can actually do some pretty nifty things.
Jumpin’ Cats! This picture is from orbit AROUND MARS! (pic from http://hirise.lpl.arizona.edu/PSP_007338_2640).
Anyway, the landslide pictures are pretty spectacular, and look for all the world like a dry, noncohesive granular flow. These images got me thinking about all the talk about water-driven geomorphic (or would that be areomorphic?) processes inferred from Martian pictures. Malin et al. (2000) and Malin et al. (2006) interpreted some gully-features on Mars as having been formed by water flowing on the surface of Mars, perhaps even as recently as 1999 (based on a the bright spots in some of the gulley images). Of course, the problem with water flowing on Mars is that the pressure/temp regime on the surface is such that most water would sublimate straight from ice to vapor.
Kraal et al. (2008) present a combined image-interpretation and sand-box actualistic experiments to suggest that certain features on Mars represent rapid water release (from a subterranean source), thereby getting around the sublimation problem.
Figure 1 from Kraal et al. (2008), pg. 973, showing the interpreted topographic feature. The authors make use of a confusing sort of terminology, seemingly equating deltas and alluvial fans (contra McPherson et al. 1987). Regardless, Kraal et al. (2008) interpret the sediment volume of the feature, and conclude that flows equal to the discharge (instantaneous? Annual? It isn’t ever clearly stated) of the Mississippi River could have produced this step-like feature in the course of a decade. They evoke a sudden release of underground water to account for this high magnitude, but short duration, flood event.
However, a recent paper from Pelletier et al. (2008) calls into question the fluvial origin of the Martian gullies. Pelletier et al. (2008) construct a DEM for an active Martian gully feature (complete with bright spots!), and use it to constrain 1-D and 2-D numerical flow models meant to constrain what flow types could account for the features. They show that liquid flows can produce the needed run-out length for the feature, but can’t produce the distal distributary lobe (of course, one may reasonably ask whether or not the distal distributary lobe is the result of a single event OR if it represents multiple events superimposed on one another; that option is not explored in the paper). However, dry debris flows can be modeled successfully that mimic both the needed run-out length as well as the distal lobate features. Pelletier et al. (2008) also state that wet debris flows could produce the same features as dry debris flows. Despite this, there is no need to evoke water to explain the features.
Looking at these newest pictures showing a large scale avalanche type feature on Mars, it just goes to show that interpreting geomorphic and sedimentary processes is difficult, no matter where you go in the universe!
Kraal, E.R., Van Dijk, M., Postma, G., Kleinhans, M.G., 2008, Martian stepped-delta formation by rapid water release: Nature, v. 451, p. 973-976.
Malin, M.C., and Edgett, K.S., 2000, Evidence for recent groundwater seepage and surface runoff on Mars: Science, v. 288, p. 2330–2335, doi: 10.1126/science.288.5475.2330.
Malin, M.C., Edgett, K.S., Posiolova, L.V., McCauley, S.M., and Noe Dobrea, E.Z., 2006, Present-day impact cratering rate and contemporary gully activity on Mars: Science, v. 314, p. 1573–1577, doi: 10.1126/science.1135156.
McPherson, J.G., Shanmugam, G., Moiola, R.J., 1987, Fan-deltas and braid deltas: Varieties of coarse-grained deltas: GSA Bulletin, v. 99, p. 331-340.
Pelletier, J.D., Kolb, K.J., McEwan, A.S., and Kirk, R.L., 2008, Recent bright gully deposits on Mars: wet or dry flows?: Geology, v. 36, p. 211-214.