Post contributed by Dr Andreas Johnsson, Department of Earth Sciences, University of Gothenburg, Sweden.
The question whether Martian gullies formed by fluvial processes or by dry mass wasting have been a source of heated debate ever since their discovery (Malin and Edgett, 2000). Intense research within the last decade however points to a fluvial origin for a majority of gully landforms on Mars.
This is supported by Earth-analog studies (e.g., Reiss et al., 2011), modelling and morphology (e.g., Mangold et al., 2010) and their regional context (e.g., Balme et al. 2006; Kneissl et al. 2010). Recently Dickson et al. (2009) suggested that gullies are due to degradation of a dust-ice mantle which drapes the underlying bedrock. The latitude-dependent dust-ice mantle extends from ±30° and poleward. The dust-ice mantle is proposed to represent the equatorward migration of volatiles from the polar caps during high orbital obliquity excursions (Laskar et al, 2004).
Martian gullies consist of an alcove (source area), channel (transport) and fan (depositional area). Gullies themselves do not denote the forming mechanism but are instead a descriptive term for a degrading-aggrading landform. The seemingly fine-grained nature of the mantle material makes interpretation of the fan structure problematic since small-scale morphology is beneath the resolution of the HiRISE instrument (25 cm/pixel).
In terrestrial water-limited environments such as on Svalbard a majority of gullies are formed by debris flows. Debris flows on Earth are formed by a flowing mixture of clastic debris of a range of grain sizes and water. Once a debris flow stops it display distinct morphologies such as lobate debris deposits, paired lateral deposits (leveés) and medial deposits (debris plugs) (Johnson and Rodine, 1984). These diagnostic attributes have been elusive to observe on Mars. Except for a few debris flow-candidates (Lanza et al., 2010; Levy et al, 2010; Reiss et al., 2011) no unambiguous observations of debris flow deposits have been made so far.
A newly discovered well-preserved 4.7-km diameter crater in the Aonia Terra region on Mars contain remarkably pristine deposits which display all the morphological attributes of being formed by debris flows (Image. 1 A to E)(Johnsson et al., 2014).
Located on the rampart ejecta of a nearby crater, the Aonia crater has sharp crater rims with distinct ejecta rays that suggest the crater is very young. The crater-size-frequency distribution of the ejecta yielded an absolute model age between 0.1–1 Ma and a best-fit absolute model age of ∼0.19 ± 0.04 Ma (Johnsson et al., 2014). These results suggest that the debris flows formed in the waning stages of the last proposed martian ice-age which ended about 0.4 Myr ago (Head et al., 2003). Degradation of a dust-ice mantle (melting of the H2O ice content) which has been invoked to explain gullies elsewhere on Mars is absent within the young Aonia crater.
We thereby propose that atmospherically derived snow/ice deposition and melting is the source of water in the Aonia crater. These conditions may have been fulfilled during high orbital obliquity excursions within the last million years. The distinct north-south asymmetry in degradation within the crater further demonstrates that insolation-controlled slope processes have been surprisingly efficient on Mars during the last Myr.
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. doi: 10.1016/j.icarus.2014.03.005.
Balme, M.R., et al. (2006), Orientation and distribution of recent gullies in the southern hemisphere of Mars: Observations from High Resolution Stereo Camera/Mars Express (HRSC/MEX) and Mars Orbiter Camera/Mars Global Surveyor (MOC/MGS) data. J. Geophys. Res., 111, E05001. doi:10.1029/2005JE002607.
Dickson, J.L., Head, J.W., 2009. The formation and evolution of youthful gullies on Mars: Gullies as the late-stage phase of Mars’ most recent ice age. Icarus 204 (1), 63-86. doi:10.1016/j.icarus.2009.06.018.
Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent ice ages on Mars. Nature 426, 797–802.
Kneissl, T., Reiss, D., van Gasselt, S., Neukum, G., 2010. Distribution and orientation of northern-hemisphere gullies on Mars from the evaluation of HRSC and MOC-NA data: Earth Planet. Sc. Lett. 294, 357–367. doi:10.1016/j.epsl.2009.05.018.
Lanza, N.L., Meyer, G.A., Okubo, C.H., Newsom, H.E., Wiens, R.C., 2010, Evidence for debris flow gully formation initiated by shallow subsurface water on Mars. Icarus 205, 103–112. doi:10.1016/j.icarus.2009.04.014.
Levy, J.S., Head, J.W., Dickson, J.L., Fassett, C.I., Morgan, G.A. Schon S.C., 2010. Identification of gully debris flow deposits in Protonilus Mensae, Mars: Characterization of a water-bearing, energetic gully-forming process. Earth Planet. Sci. Lett. 294, 368–377. doi:10.1016/j.epsl.2009.08.002.
Malin, M.C., Edgett K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335.
Mangold, N. et al., 2010. Sinuous gullies on Mars: Frequency, distribution, and implications for flow properties, J. Geophys. Res., 115, E11001. doi:10.1029/2009JE003540.
Reiss, D., et al. 2011. Terrestrial gullies and debris-flow tracks on Svalbard as planetary analogs for Mars. In Garry, W.B., and Bleacher, J.E. (eds), Analogs for Planetary Exploration. Geological Society of America Special Paper, Vol. 483, 165–175.