Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p44c..05h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P44C-05
Other
5415 Erosion And Weathering, 5419 Hydrology And Fluvial Processes, 5464 Remote Sensing
Scientific paper
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 from MOLA, along with images, has shed new light on valley formation. These datasets show many more valleys are present than previously observed. In an updated global map, >4 times as many valleys have been identified, totaling a summed length 2.5 times greater than earlier estimates. 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. Additionally, some valley networks appear to be in different stages of preservation, indicating multiple periods of formation spread over up to 400 million years. As already understood, most of the valleys occur on Noachian terrains. 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 (1995) mapped roughly 92% of all valleys on Noachian terrain. The newly-mapped Noachian-age valleys are constrained in elevation and have a rough Gaussian distribution centered around 1500 m. Phillips et al., (2001) showed that most of these valleys follow the long-wavelength topography set-up by the development of the Tharsis bulge, helping constrain its age to the Noachian epoch. The younger valleys, while comprising a small part of the total, developed more evenly across all elevations. These 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 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.
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