Evaluating the role of volatiles in bedrock chute formation on the Moon and Mars

Abstract

Steep channel-like landforms—referred to here as bedrock chutes—line the rocky walls of some craters on the Moon and Mars. The role of volatiles, such as H₂O or CO₂, or dry rockfall in the formation of bedrock chutes is unknown on either planetary body. To test whether bedrock chute formation on Mars involved volatile activity, we used digital elevation models of Mars generated from HiRISE and CTX stereo-imagery to survey 4–9 km diameter craters globally and measure chute morphology as a function of latitude and orientation—properties that might co-vary with volatile activity. We also analyzed bedrock chutes on the Moon, which is presumably devoid of significant erosion due to volatile activity, using LROC NAC data, to serve as a volatile-free endmember for comparison with Mars. Martian bedrock chutes occur at all latitudes and have median values of chute spacing (wavelength) of ~300 m, relief of ~15 m, and slope of ~33°. Chutes on the Moon are less common and are generally steeper (~41°) with less relief (~5 m) as compared to Mars. While dry rockfall might have formed bedrock chutes on both the Moon and Mars, martian chutes are systematically deeper on pole-facing slopes between 10°S- 30°S indicating a likely role for volatile activity in chute formation. Bedrock chutes are also deeper where they co-occur with well-incised martian gully channels. The latitude-dependence for deeper bedrock chutes on pole-facing slopes extends to lower latitudes than gullies—within the contemporary tropics—indicating the potential for volatile-related activity closer to the equator than documented for gullies or other ice-related features on Mars. Chutes carved into bedrock likely form slowly compared to gully channels, which are incised into more erodible ice-cemented sediment, and therefore might provide a longer record of environmental conditions over larger swaths of Mars.

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