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A team of researchers led by a NASA Lunar Science Institute (NLSI)member based at Southwest Research Institute has discoveredevidence that the giant impact crater Rheasilvia on Asteroid (4)Vesta was created in a collision that occurred only about 1 billionyears ago, much more recently than previously thought. This resultis based on the analysis of high-resolution images obtained withthe Dawn spacecraft, which entered orbit around Vesta in July 2011. In addition to creating the crater, the impact is believed to havelaunched a large number of fragments into space, some of whichlater escaped the main belt and possibly hit the Earth. Vesta, the second-most massive body in the main asteroid belt, isbelieved to have formed within the first few million years afterthe earliest solar system solids (~4.6 billion years ago).According to models, its early evolution occurred in an environmentwhere collisions with other asteroids were much more frequent thanthey are today. It was thought that one such early collision on Vesta created aswarm of fragments, which we now call an asteroid family. AlthoughVesta and its family are located between Mars and Jupiter, smallerpieces of these asteroids can be found in meteorite collections onEarth, including most eucrite, howardite and diogenite meteorites. Several large craters on Vesta were first inferred by Hubble SpaceTelescope imaging. However, a photographic survey by the Dawnspacecraft revealed a collision-dominated history, as well as astrong north-south dichotomy in the asteroid's cratering record. Vesta's heavily cratered northern terrains retain much of theirearliest history, but the southern hemisphere was reset by twomajor collisions in more recent times. The Dawn survey revealed high-resolution details of these craters,allowing scientists to estimate their ages on the basis of thenumber of younger craters that have been superposed on the crater'sfloor since their formation. Rheasilvia, the youngest of these impact structures, is about 505kilometers (314 miles) across. The number of smaller craters foundwithin Rheasilvia can be used like a clock to estimate itsformation age. The best estimates suggest it is only about 1 billion years old.For reference, this is nearly 3 billion years after the barrage ofcomets and asteroids that produced the so-called Late HeavyBombardment of the Moon (and Solar System). Before this time, the asteroid belt is believed to have beensubstantially larger than it is today. The volume of material excavated by the impact that formedRheasilvia is larger than the estimated volume of known asteroidalmembers of Vesta's dynamical family, suggesting that most of theobserved family was formed in this single event. "An age of about 1 billion years for Rheasilvia is unexpectedlyyoung. This result has important implications for our understandingof the evolution of the Vesta, its asteroid family and the innermain asteroid belt in general. We have just started exploringVesta's secrets, and I'm sure other intriguing results will comealong shortly," said NLSI team member Dr. Simone Marchi, leadauthor of "The Violent Collisional History of Asteroid (4) Vesta,"published in the May 11 issue of the journal Science. Dr. William Bottke, who is the assistant director of the SpaceStudies Department in the Space Science and Engineering Division atSouthwest Research Institute (SwRI), is also the director of theCenter for Lunar Origin and Evolution (CLOE), one of NASA's newLunar Science Institutes located at SwRI in Boulder. I am an expert from led-flashlightkeychains.com, while we provides the quality product, such as China Novelty Led Lights , Led Head Lamps, Led Flashlight Keychains,and more.
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