Post contributed by Solmaz Adeli, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institute of Planetary Research, Berlin, Germany
The Amazonian period on Mars, meaning roughly the last 3 Ga, is globally believed to have been cold and hyperarid [e.g., Marchant and Head, 2007]. Recent geomorphological observations, however, have revealed the presence of well-preserved Amazonian-aged fluvial valleys in both the north and south mid-latitude regions of Mars [Howard and Moore, 2011; Hobley et al., 2014; Salese et al., 2016; Wilson et al., 2016]. These features point to one or several climate change phase(s) during Amazonian which could have sustained liquid water at the martian surface. These climate changes could have been triggered by obliquity oscillations [Laskar et al., 2004] causing the transportation of ice from polar regions and its re-deposition at lower latitudes. Episodic melting events during Amazonian, subsequently, formed valleys and other fluvial features, in the mid-latitude regions.
Image 1 shows the Kārūn Valles channel system which is located on the southeastern slopes of the Ariadnes Colles basin in Terra Cimmeria. Kārūn Valles is the downstream part of a ~340 km-long fluvial system which shows evidence of early to middle Amazonian aqueous activity. This evidence includes erosional features such as deep groove marks, streamlined islands, cataracts, and eroded banks (Image 2), as well as depositional features such as fan-shaped deposits, fan deltas, and alluvial fans (Image 3). The Kārūn Valles alluvial fan is of particular interest because it has a braided morphology (Image 1) which is rarely observed on Mars, whereas on Earth it is a common process in river beds. A braided pattern occurs in alluvial conditions when the bars are deposited and shaped inside the river channel by the flow. The Kārūn Valles alluvial fan has well-preserved elongated bars, which clearly have depositional origin and have been shaped by the flow. The source of water for this fluvial activity, in Terra Cimmeria, has been suggested by Adeli et al.  to be surface runoff due to ice/snow melt.
Adeli et al.  calculated the flow discharge and sediment transport rate of Kārūn Valles, using the method described in Kleinhans et al. , in order to better understand the formation mechanism of the Kārūn Valles alluvial fan and to estimate the time-scale for its formation. The alluvial fan was assumed to have been formed in a single event to simplify the calculation. Two scenarios were considered: a) bank-full flow conditions and b) a 10%-flow-depth. In case (a) the Kārūn Valles alluvial fan would have formed in 19-270 days, and in case (b) in 4,700-65,000 days. These time-frames are both geologically short periods of time. However, the possibility of a sequence of multiple episodic events, each lasting for a shorter time, cannot be ruled out. Therefore, liquid water was present during early to middle Amazonian in Terra Cimmeria, for a short period(s), and its abundance was large enough to form over 340 km of fluvial valley system and related erosional and depositional features.
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