W Post NASA’s lunar probes will test theory of why one side of the moon is lopsided
By _Brian Vastag_ (http://www.washingtonpost.com/national/brian-vastag/2011/06/02/AGMEARHH_page.html) , Published: January 1, 2012 < She hangs there nightly, a yellow or white or spookily orange disk, the bringer of tides, the caster of romantic shadows. She waxes and wanes and sometimes she turns ruddy as the shadow of the Earth crosses her face. For all her beauty, though, our moon hides a lumpy, unflattering secret: She’s lopsided. Her backside is much thicker than her front. And no one knows why. It’s unseemly, really. After more than 100 robotic and human missions to the moon, scientists still can’t account for why one half — the half we can’ t see — is taller than the other. SA _probes_ (http://www.nasa.gov/mission_pages/grail/main/index.html) _that arrived_ (http://www.washingtonpost.com/national/health-science/new-years-countdown-twin-nasa-probes-prepare-to-slip-into-orbit-around-the-moon/2011/1 2/28/gIQAFzQtMP_story.html) at our satellite this weekend may finally reveal a shocking truth: that early on, a smaller twin moon smushed into her. As this intruder splatted into its big sister, it shattered “like a mega-avalanche,” said _Erik Asphaug_ (http://es.ucsc.edu/personnel/Asphaug/) , the planetary scientist at the University of California at Santa Cruz who _published_ (http://www.nature.com/nature/journal/v476/n7358/full/nature10289.html) the twin-moon idea in the journal Nature in August. His co-author was Martin Jutzi of the University of Bern in Switzerland. This collision would have spread a wide hump of rock onto the back of the moon. There, the material cooled and hardened into a thick crust: the far-side lunar highlands. “This is one of those ideas that all sorts of people will try to prove wrong,” said _Maria Zuber_ (http://www-geodyn.mit.edu/mtz.html) , the MIT scientist heading up the new NASA moon mission. “But it’s extremely testable.” And so GRAIL will test it. Designed to probe the moon’s interior, the two washing-machine-size spacecraft will reveal the thickness of the moon’s crust, its topmost layer. If the two-moon theory is correct, the backside crust will be much thicker than that of the front side. The hump should taper toward the equator. GRAIL could also spot another hidden feature predicted by the theory. If a second moon did crash into the first, the collision would have occurred when the big moon was young and hot. A thin layer of molten heavy elements including uranium and potassium still burbled just under the crust. The backside impact would have squeezed this liquid, pushing it around to the front side. There it would have cooled and hardened, leaving a telltale layer. The existence of both features — a thick backside crust and a thin, dense layer under the front’s crust — would offer strong support for the twin-moon theory, Asphaug said. When Zuber first heard the notion, she scoffed. “This is going to be nonsense,” she recalled thinking. But computer simulations run by Asphaug and Jutzi were compelling leading Zuber to reverse course. “It’s a plausible scenario,” she said. The idea is also simple, another stroke in its favor. By contrast, other explanations for the moon’s front-back discrepancy tend toward the complicated and unsatisfying. “There are all these theories out there,” Asphaug said, “that have big warts on them.” Such as: Maybe the front side of the moon was terribly unlucky, flattened by seven or eight big space rocks. The problem: Asteroids and comets arrive from all directions; there’s no reason impacts should cluster. “It’s like flipping a coin and getting heads eight times,” Asphaug said. -- Centroids: The Center of the Radical Centrist Community <[email protected]> Google Group: http://groups.google.com/group/RadicalCentrism Radical Centrism website and blog: http://RadicalCentrism.org
