http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/682lobkx.asp
Scars of a lunar comet impact? astronomy.com March 18, 2004 Did a comet create the biomorphic swirl on the Moon known as Reiner Gamma? In western Oceanus Procellarum on the Moon lies a curious feature named Reiner Gamma. Its light-colored, semi-symmetrical swirls look mysterious enough to startle any backyard lunar astronomer who stumbles across it unawares. The feature, measuring about 20 by 40 miles (30 by 60 km), has been ascribed to many causes. The most widely mentioned cause is that buried and magnetized material that has created a magnetic field - in effect, a micro-magnetosphere - that shields the mare surface from weathering (darkening) by the impact of ionized particles from the Sun. Supporters of this theory point to the site's strong magnetic anomaly, which was first mapped during the Apollo 15 and 16 missions in the early 1970s. Patrick Pinet (CNRS/University of Toulouse, France) and a group of coworkers used Clementine imagery to derive the photometric properties of the top-most lunar surface across the southern half of the feature. These data, they report, demonstrate that the feature is "optically immature," meaning that its properties appear to have been exposed to solar darkening for only a geologically brief time, perhaps 10 million years. This apparent newness of the feature led the researchers to suggest it was caused by the impact of a collection of icy fragments - a comet, in other words. According to Pinet, the best candidate is a comet nucleus that had been disrupted by Earth's gravity, much as Jupiter's gravity broke up Comet Shoemaker-Levy 9 a decade ago. In any case, Pinet says, the impacting object was made of a cloud of small particles. This could have produced swirls on the Moon's surface when the impacting gas and dust interacted with the uppermost layer of lunar soil, affecting only its uppermost layer. The magnetization might have resulted from compression of the comet's magnetic field, he suggests. But where is the crater? Pinet says the impacting fragments were small and fluffy, and detecting any craters will call for sharper images than what has been achieved so far. "We are planning to use high-resolution images taken by the SMART-1 spacecraft when it reaches the Moon early in 2005," he says. These images will let the researchers examine the surface of Reiner Gamma closely and perhaps reveal details that will tell an unambiguous story of the feature's origin. There are two Japanese missions headed for the Moon as well, Lunar-A (to be launched this summer) and Selene (slated for 2006). Imagers and other instruments on these spacecraft also may make observations that will help settle the question. "This is a puzzling problem that hasn't yet been solved," Pinet concludes. "And its solution will be applicable to all bodies with airless surfaces that have developed a regolith." Besides the Moon, he points to Mercury and asteroids. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
