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Comparison of Martian Gullies and Lunar Crater-Wall Landslides

I present lunar landslides that bear a striking resemblance to some martian gullies, despite the lack of water on the Moon both today and in the past. This observation indicates that gully features can be formed via a dry landslide mechanism. Thus on Mars, morphology alone is insufficient to determine the mechanism of formation of these features. Introduction: In 2000, Malin and Edgett (1) reported the observation of martian gullies on steep terrain. Gullies have three distinctive characteristics: an alcove, a channel, and a deposition apron. The alcove is the highest on the slope and the source for the channel. If more than one gully is on the slope, their alcoves frequently emanate from the same layer in the cliff. The channel heads appear to be clear of debris, suggesting efficient material transport. The channels are incised into the cliff; they are topographic, not merely albedo features. In some cases they form meanders and branching patterns. The deposition aprons at the bottom of the cliff resemble terrestrial alluvial fans. (Gullies are morphologically distinct from slope streaks, which are observed both on Mars (2) and the Moon (3). Slope streaks are formed by dry landslides and are detected as albedo features, not topographic channels. They move straight down the slope ignoring impediments and gradually fan out downslope.) Malin and Edgett (1) proposed that the martian gullies formed by groundwater seepage in the recent past. They based their hypothesis primarily on gully morphology, comparing martian gullies with terrestrial gullies whose formation involves the action of liquid water. Although liquid water is not stable at current martian temperatures and pressures (4), Mellon and Phillips (5) found that normal geothermal heating could raise the temperature of subsurface water to liquid temperatures if the overlying regolith was sufficiently insulating. However, liquid water may not be necessary to form the gullies. Treiman (6) proposed gully formation by avalanches of fine granular material without the presence of liquid water. Furthermore, it has been shown experimentally (7) that the flow of dry granules can exhibit features similar to those seen on Mars. In contrast to Mars, no lunar features formed by the action of liquid water. The Moon has no evidence for liquid water floods or valley networks. Furthermore, the samples returned from the Moon are drier than earth rocks. Hence, if the dry landslide hypothesis for the formation of martian gullies is correct, we might expect to see similar features on the Moon, where there is no liquid water. We do. In this abstract I report the observation of lunar crater rim landslides which are morphologically similar to the martian gullies, suggesting that the martian gullies need not have been formed by liquid water. Comparison of Martian and Lunar Features: One lunar example is crater Dawes (Fig. 1), located at 27.4◦E, 12.2◦N. Dawes was photographed by Lunar Orbiter V, image 070-H2. The crater is near the edge of Mare Tranquilitatis, near Mare Serenitatis. It is flat floored, with a diameter of 17 km and a 3.6 km