Mapping Mars by Oliver Morton
Author:Oliver Morton
Language: eng
Format: epub
Publisher: Picador
Published: 2009-03-14T16:00:00+00:00
Shorelines
The more clearly the immensely speculative nature of geological science is recognized, the easier it becomes to remodel our concepts of any inferred terrestrial conditions and processes in order to make outrages upon them not outrageous.
—William Morris Davis,
“The Value of Outrageous Geological Hypotheses”
In 1984, shortly after landing a job at JPL, a geologist named Tim Parker found that he had a couple of weeks without too much to do. He decided to spend them in the image facility, looking at Viking photographs of Chryse Planitia. As part of a generation of geologists that had entered planetary science after Mariner and Viking, Parker was quite happy with the idea that vast floods had streamed into Chryse through the outflow channels to the south. What intrigued him was what had happened to the floodwaters after they left the channels. If they pooled as big, shallow, ice-covered lakes before draining into the permeable rocks below, those lakes would have had shores. If so, traces of those shores might still be visible. And if such shorelines were still visible, then Parker’s eyes might be peculiarly attuned to their discovery.
Today, North America is considerably better endowed with lake shores than it is with lakes. In the comparatively recent past—during the decline of Earth’s most recent Ice Age, between sixteen thousand to ten thousand years ago—the continent was home to many massive lakes that today have more or less completely vanished. Lake Agassiz, stretched out along the edge of the retreating Laurentide ice sheet that sat over eastern Canada, was at its greatest extent four times the area of Lake Superior today. Lake McConnell, to the north of Lake Agassiz, was more than six hundred miles long. On the south edge of the smaller Cordilleran ice sheet to the west was Lake Missoula, roughly the size of today’s Lake Ontario. And far from the ice sheets themselves, but fed by the rains that were part of the glacial climate system, were the great lakes of the West: Lake Manlius, part of the bed of which is now Death Valley; Lake Lahontan, in western Nevada; and, largest of all, Lake Bonneville, the shriveled remains of which persist today as Utah’s Great Salt Lake. Lake Bonneville has a particular place in American geological history because in the late nineteenth century it became the subject of the great monograph in which G. K. Gilbert deduced the lake’s past immensity by measuring its former shorelines—shorelines in the plural, because the lake disappeared in stages, leaving behind the distinctive rugby-shirt striping of a bathtub in a house full of students—and showed how the continental crust had risen up as the vast mass of water was removed.
In the early 1980s the shores of Lake Bonneville again became a site of geological interest. The Reagan administration planned to deploy a new generation of land-based nuclear missiles, the MX or “Peacemaker.” Unlike earlier American ICBMs, the Minutemen and Titans (whose descendants sent the Vikings to Mars), the MX was being brought forth into a
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