Fri Dec 14, 2012, 01:28 AM
The Straight Story (48,118 posts)
Mapping earth's gravity (and the info it gives us about droughts, currents, etc)
Signs of the U.S. Drought Are Underground
A deep and persistent drought struck vast portions of the continental United States in 2012. Though there has been some relief in the late summer, a pair of satellites operated by NASA shows that the drought lingers in the underground water supplies that are often tapped for drinking water and farming.
The maps above combine data from the twin satellites of the Gravity Recovery and Climate Experiment (GRACE) with ground-based measurements to map the relative amount of water stored near the surface and underground as of September 17, 2012. The top map shows moisture content in the top 2 centimeters (0.8 inches) of surface soil; the middle map depicts moisture in the "root zone," or the top meter (39 inches) of soil; and the third map shows groundwater in aquifers.
New Gravity Map Reveals Lumpy Earth
t may look like a mangled lump of Play-Doh, but this colorful object is actually the most accurate digital model yet of Earth's gravity field, scientists say.
Released last week, the gravity map is what's known as a geoid, and it was created by a European satellite called the Gravity field and steady-state Ocean Circulation Explorer, or GOCE.
In general, most people think of Earth as a relatively smooth orb. But matter is not evenly distributed across the planet's surface, and bodies of water are constantly shifting due to winds and currents.
By capturing the planet's gravity rather than its physical appearance, the geoid shows the shape that mean sea level would have if it could somehow be extended over the entire surface of the globe, said John Wahr, a geophysicist at the University of Colorado.
"If you can map out the ocean surface somehow (with satellite radar altimetry, for example), and then take the difference between that surface and the geoid, you can ... determine the direction and strength of ocean currents," said the University of Colorado's Wahr.
For example, studying the geoid can help us understand tectonic processes such as those involved in last month's devastating earthquake in Japan, Watkins said. (See "Japan Earthquake Shortened Days, Increased Earth's Wobble.")
"Large earthquakes move enough mass to change the gravity field," he said, "and those changes can tell us more about the mechanism of the quake and how much slip and uplift occurred, especially in offshore areas where it is difficult to observe Earth's crust directly."
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