Posted by Dr P. Senthil Kumar, National Geophysical Research Institute, Council of Scientific & Industrial Research, Hyderabad 500007, India.
Gullies are well-known geomorphic features on Earth where they are mainly formed by erosion due to flow of liquid water. They are also detected on Mars and the Moon and their origin on those bodies are under discussion (Malin and Edgett, 2000; Senthil Kumar et al., 2010). The gullies consist of alcoves (erosional features), channels (features indicating transportation) and fans or debris aprons (depositional structures). These features are clearly observed on the interior walls of impact craters on Mars and widely on the mountain slopes of Earth. Hence, geomorphologists use these features to examine the characteristics of liquid water flow either in the present or past geological records.
The gullies can also form by dry-granular flows, as seen in some locations on Mars (Treiman, 2003; Shinbrot et al., 2004). Indeed, the observation of gullies on the interior walls of lunar impact craters spurred new views about the origin of gullies (e.g., Bart, 2007). The recently concluded and on-going lunar missions (Chandrayaan-1 and Lunar Reconnaissance Orbiter) revolutionized our understanding of these landforms because of very high-resolution imaging techniques. Based on these new data, Senthil Kumar et al. (2013) have reported new observations of gullies and landslides on the Moon and proposed a mechanism for their formation. In their study, high-resolution images from Chandrayaan-1 Terrain Mapping Camera reveal landslides and gully formation on the interior wall of a 7 km-diameter simple crater emplaced in Schrödinger basin on the farside of the Moon (Image 1). These features occur on the steep upper crater wall, where the slope is ~35°.
Further examination of the Lunar Reconnaissance Orbiter Narrow Angle Camera (LRO NAC) images gullies show that the gullies are composed of typical alcove-channel-fan morphology, similar to those seen on the surface of Mars (Image 2a). Spectral characteristics inferred from data obtained by Hyperspectral Imager and Moon Mineralogy Mapper on board Chandrayaan-1 show that the gullies and landslides are devoid of prominent spectral absorption features related to water or hydroxyl molecules, suggesting they formed by dry-granular flow processes. In addition mass movements on the crater wall led to the formation of arcuate ridges and ponding of fine-grained sediments on the crater floor. Runout flows from small impact craters on the slopes indicate that impact-induced seismic shaking was responsible for the downslope mass movements, similar to those observed on the surface of asteroids (Robinson et al., 2001; Richardson et al., 2004). Crater size-frequency distributions indicate a minimum age of ~18–2 Ma for the gullies and ~2 Ma for the landslides, while age of the host crater ejecta was inferred to be ~175 Ma. The gullies and landslides are also found to occur on the interior wall of impact craters in the low-latitudes on the Moon and probably formed by similar processes (Images 2b and 2c).
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Senthil Kumar, P., V. Keerthi, A. Senthil Kumar, J. Mustard, B. Gopala Krishna, Amitabh, L. R. Ostrach, D. A. Kring, A. S. Kiran Kumar, and J. N. Goswami (2013), Gullies and landslides on the Moon: Evidence for dry-granular flows, J. Geophys. Res. Planets, 118, 206–223, doi:10.1002/jgre.20043.
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