Post by Samantha Peel Department of Earth and Planetary Sciences, University of Tennessee, USA.
Central pit craters are a crater type that contain an approximately circular depressions in their floor or central peak (Image 1). These craters have been found on Mars, Ganymede, and Callisto (e.g., Barlow, 2010; Alzate and Barlow, 2011; Bray et al., 2012). On Mars, a subset of central pit craters has been found to contain valleys that terminate in central pits (Peel and Fassett, 2013). These “pit valleys” are believed to have formed as ancient rivers transported water and sediment to the central pits.
Image 2 shows the range in valley morphologies found in craters. The valleys have a variety of forms, from sinuous to linear, and exhibit a range in both width and depth of incision. Additionally, some of the valleys have extensive tributaries while others have a limited number of tributaries. This range in morphologies may be due to different water sources, the presence or absence of faults, water availability and/or the degree of feature preservation.
Many of the observed valleys terminate at the apices of fan-shaped features whose toes are located on the wall or floor of the central pit (Image 2). These features have been identified as sedimentary in nature and may be preserved deltas. If this is the case, then the central pits hosting these fans must have once held standing water.
Some central pit craters with interior valley networks have sedimentary features located closer to their source areas. The crater in image 1 contains numerous examples of this. Some of these pit valleys are partially covered by sedimentary features identified as alluvial fans, while other valleys appear to cut through alluvial fans within the same crater. These relationships suggest that the availability of water was variable. Derived discharge estimates (based on a channel-to-valley ratio of 0.14 (Penido et al., 2013) for valleys within five sample craters were found to range between 10 and 104 m3/s. The source(s) of the water is currently under investigation, but results to date suggest that precipitation was likely a significant contributor.
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Peel, S. E., Fassett, C. I. (2013), Valleys in pit craters on Mars: Characteristics, distribution, and formation mechanisms, Icarus, 225, 272-282, doi:10.1016/j.icarus.2013.03.031.
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