Baker-Geomorphology-water - Geomorphological Evidence for...

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INTRODUCTION Despite 30 years of accumulating, increasingly abundant and unequivocal geomorphological evidence, the case for past water-related activity on Mars remained immensely controversial until very recently (Baker 2004). Ingenious models were proposed (e.g. Hoffman 2000) to ascribe non- aqueous origins to individual Martian landforms that other- wise had striking similarities to water-generated features on Earth. Nevertheless, this view is profoundly changed because of recent developments, notably the nuclear physical measurement of abundant, extant, near-surface ice (Boynton et al. 2002) and direct chemical analyses of aqueous minerals associated with sedimentary rocks (Squyres et al. 2004). Recent dampening of hydrophobic theorizing makes it appropriate to reassess the geomorphological evidence for water on Mars, and this brief overview will emphasize developments. Geomorphology concerns the nature and origins of land- forms and landscapes. Its application to extraterrestrial planetary surfaces relies upon known associations of form and process on Earth. However, the rationale for ascribing genesis to a newly discovered planetary landform is not simply a matter of comparative image analysis. Landforms and landscapes exist in complex, interrelated assemblages, in which the different elements relate to one another in time and space because of their generation by a unique sequence of formative processes. Scientifically productive alternative explanations must not merely satisfy individual, simple-minded, “look-alike” criteria. Instead, they must account holistically for the entire, genetically related assem- blage, in the same way that the solution of a crime depends, not on a single clue, but on the whole interrelated web of evidence that is built up through connections of time and space by a master detective. CHANNELS, VALLEYS, ALLUVIAL FANS, AND SEDIMENTS As recognized early in the era of spacecraft exploration, channels and valleys extensively dissect the surface of Mars. Channels are elon- gated troughs that display clear evidence for large-scale fluid flow across their floors and on parts of their walls or banks (F IG. 1A) . Immense channels, with widths of tens of kilometers and lengths of up to a few thousand kilometers, display a suite of morphological attributes that are most consistent with genesis by cataclysmic flows of water and sediment (Baker 2001). On Earth such flows produced the distinctive landforms of the Channeled Scabland (F IG. 2A) . An important recent discovery is that Martian flood channel activity, involving outbursts of water and associated lava flows, occurred in the Cerberus Plains region on the order of 10 million years ago (Berman and Hartmann 2002; Burr et al. 2002). The huge discharges associated with these floods and the temporally related volcanism should have introduced considerable water into active hydrological circulation on Mars.
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This note was uploaded on 04/15/2008 for the course HIST 101 taught by Professor Wormer during the Spring '08 term at University of Arizona- Tucson.

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Baker-Geomorphology-water - Geomorphological Evidence for...

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