Abstract
There is widespread evidence across Mars of past flows in major channel systems as well as more than one palaeo ocean level. However, evidence for the timing of channel flows and ocean levels is based on geographically diverse sources with a limited number of dates, making reconstructions of palaeo flows and ocean levels patchy. Here, based on high-resolution topography, image analysis and crater statistics, we have dated 35 different surfaces in Kasei Valles, that are predominantly found within erosional units enabling us to reconstruct a fascinating timeline of episodic flooding events (ranging from 3.7 to 3.6 Ga to ca. 2.0 Ga) interacting with changing ocean/base levels. The temporal correlation of the different surfaces indicates five periods of channel flows driving the evolution of Kasei Valles, in conjunction with the development of (at least) two ocean levels. Furthermore, our results imply that such ocean rose in elevation (ca. 1000 m) between ca. 3.6 Ga and 3.2 Ga and soon afterwards disappeared, thereby indicating a complex ancient Martian hydrosphere capable of supporting a vast ocean, with an active hydrological cycle stretching into the Amazonian.
Highlights
There is widespread evidence across Mars of past flows in major channel systems as well as more than one palaeo ocean level
If we can reconstruct the history of how Martian fluvial geomorphology has evolved, this can be a powerful tool to understand past environmental conditions on Mars
We use these high resolution topographic data with crater size-frequency distribution statistics to date when channel surfaces and features were created and last re-worked. This has allowed us to reconstruct a three dimensional history of the development of the Kasei Valles from the early Hesperian to early Amazonian—ages based on Hartmann chronology[20]. This reveals a fascinating timeline of the long-term evolution of Kasei Valles, showing five episodes of channel incision/activity, in conjunction with a changing global ocean/base level during the Hesperian period
Summary
The Kasei Valles are the largest outflow channels on Mars, with a channeled area extending 2000 km in length and a channel width of up to 150 k m5 We extended our mapping between cross sections where there were common flat surfaces and we identified any additional/ abandoned channels for further analysis To date these surfaces and channels, we imported Mars Reconnaissance Orbiter high-resolution Context Camera (CTX) imagery into Arc-GIS and analyzed the crater population within each surface. We classified each absolute model age as either date of first exposure/formation or date of last resurfacing We defined the former as being the period of time elapsed since craters started collecting on the surface, whereas the latter as being the time elapsed since a minor modification of the crater population occurred For counting areas < 5,00 k m2, we visually inspected the surface using CTX and HiRISE imagery to determine if the observations correspond to the most surficial unit (last resurfacing period) or were an underlying unit partly buried by younger one/s
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