GLY 2100-Homework #2: Did it ever rain on Mars? Overview: This homework activity follows a series of lectures on the conditions necessary for life to evolve (one of which is water in liquid phase), where such suitable conditions are found in our solar system today, and where these conditions (and life) may have occurred in the deep past. Homework questions, links, references, diagrams, and maps are included at the end of this document. Did it ever rain on Mars?? Some of the Viking images sent back from Mars in the 1970s show tantalizing evidence of dendritic valley networks in some of the oldest terrains on the planet. One of the big questions ever since has been whether it might have rained early in Mars history. One of the ways of deciding whether the Mars valley networks might have been produced by rainfall is to find out how similar they are to valley networks on Earth, which we know are produced by rainfall. The standard method for analyzing drainage basins is comparison of how many streams there are per square kilometer (drainage density) and how much branching there is. On Earth, areas with lots of rainfall have a high drainage density with intricately branching tributaries. By the mid-1990s, after examining many Viking images, most workers had concluded that the Martian valley networks weren’t branching enough or integrated enough to suggest past rainfall (e.g., Carr, 1995; Carr and Chuang, 1997). What would happen if we were to revisit these areas but look at them on new, higher resolution images?? In this exercise, we’re going to evaluate the characteristics of the valley networks of an area on both the older Viking images and on new higher resolution images and data from the Mars Orbiter Camera and Mars Orbiter Laser Altimeter to see whether better resolution images might change our interpretations or not. On pages 4 and 5, you’ll find two images. Both are of the same area in the Arabia Terra region of the ancient, heavily-cratered Southern Highlands. Start by using the lat/lon values to locate the region in question on a map of Mars. The image on page 5 is a 1970s Viking Mars Digital Image Model (MDIM). Traced in red are the valley networks mapped by Carr (1995). The image on page 6 is a combination of a Mars Global Surveyor MOC image mosaic with a color-coded MOLA elevation map. Traced in yellow are the valley networks mapped by Hynek and Phillips (2003). You can clearly see that, at higher resolutions, many more drainage segments can be detected, and the drainage system appear to be more integrated and more mature than one would think from the valley networks that can be mapped on the Viking MDIMs. The question is whether or not the valley networks are integrated and mature enough to allow one to argue that rainfall was likely involved in their formation. An alternative hypothesis is that the water is coming not from above but from below in a process called groundwater sapping. Groundwater sapping (see image of sapping on Earth – page 6) is what most likely has shaped many tributary canyons of the Grand Canyon.

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