Hynek, Brian M听1听;听Beach, Michael听2听;听Hoke, Monica RT听3
1听University of Colorado
2听University of Colorado
3听University of Colorado
Valleys on Mars are thought to have been formed by precipitation and surface runoff as well as groundwater processes, and the relative contributions of these mechanisms have long been debated. Topography and recent high resolution images have shed new light on valley network formation. These datasets show many more valleys are present than previously observed. We have remapped valleys across Mars using global datasets and compared our results to previous ones that used lower-quality images from the Viking Missions [1] (Fig. 1). In the updated global map >4 times as many valleys have been identified, totaling a summed length 2 times greater than earlier estimates (e.g. Fig. 2). Corresponding drainage densities of networks are almost always much higher. Most of the valleys have characteristics consistent with formation by precipitation, including dendritic form; meandering channels that occasionally exhibit braiding, and terracing; tributaries reaching up to drainage divides; and high stream order (Fig. 2). Additionally, some valley networks appear to be in different stages of preservation, indicating multiple periods of formation spread over up to 200 million years.
As already understood, most of the valleys occur on Noachian terrains (Fig. 1). In terms of age, ~84% of valleys in the new global map of networks lie entirely on Noachian terrains (>3.7 Ga ago), 10% cross into or are entirely contained in Hesperian-aged surfaces (3.7-3.0 Ga) and 6% occur on Amazonian units (<3.0 Ga). This is a shift to younger ages from previous work; for example, Carr [1] mapped roughly 92% of all valleys on Noachian terrain. The younger valleys formed after climatic conditions favored precipitation and stable surface water on a global scale and they probably originated from processes such as volcanism or asteroid impacts that could force local climate change. All valleys with an age <2.9 Ga occur on the Tharsis Rise volcanic complex and most of these likely had a hydrothermal origin. In summary, new global analysis of valley networks has pointed to punctuated times of precipitation in the Noachian, with a shift to local formation in more recent epochs.
[1] Carr, M. H. (1995) JGR, 100, 7479鈥�7507.