http://www.space.com/scienceastronomy/solarsystem/mars_knocks_021107.html
Why a Mars Rock Hits Earth Every Month By Robert Roy Britt space.com 07 November 2002 Every month, on average, a rock from Mars lands on Earth. Most are never found, but those that have been picked up suggest that the theory for how they get here - having been booted from the Red Planet by very large asteroid impacts - is not fully accurate. Now a new computer simulation appears to solve the puzzle by showing that relatively small collisions can do the trick. Scientists know that space rocks ranging from the size of a car to that of a city have hit Mars many times throughout history. In some of these collisions, chunks of Mars are flung into space and never return. Some go on journeys that can last millions of years before being captured by our own planet's gravity. Meteorite hunters have found about 26 rocks on Earth that have been identified as having come from Mars (some of these broke apart upon entering the atmosphere, so the 26 rocks were found as about 40 separate pieces). Scientists had thought it took a serious wallop to instigate these interplanetary exchanges. Yet the new research finds that craters as small as 1.9 miles (3 kilometers) wide on Mars could have been the starting points for rocky odysseys. This minimum crater diameter is at least four times smaller than previous estimates, the scientists write in an account published today in the online version of the journal Science. The study was done by James Head and Jay Melosh of the University of Arizona, with Boris Ivanov of the Russian Academy of Sciences. The scientists said terrain covered by weaker material, which might be created in previous impacts, requires larger events to scoot stuff all the way to Earth. That means, they say, that Martian meteorites found on Earth should tend be from a young Mars, a projection that fits with the dating done on actual rocks that have been collected. In an interview with SPACE.com, Head, who also works for Raytheon Missile Systems, explained what the new simulation reveals. An asteroid one-and-a-half times the size of a football field slams into Mars at 22,370 mph (10 kilometers per second). The energy of the impact is equal to about 60 megatons of TNT, comparable to the largest nuclear devices ever tested. A strong shock wave begins to form. The leading edge of the shock wave reflects off the surface from below and interferes destructively with the rest of the incoming shock wave, canceling out the high pressure near the surface. At the surface, the pressure is zero, according to the simulation. Just below the surface, however, the pressure is great. "The pressure difference accelerates the material to high speed," Head said. "About 10 million fragments averaging 5 centimeters across [2 inches] are accelerated to speeds in excess of 5 kilometers per second [11,180 miles per hour]." That is the escape velocity of Mars, the speed needed to leave the planet without going into orbit around it. "According to the celestial mechanics people, about 7.5 percent of this material is destined to land on the Earth," Head says. "More than half of that lands in the first 10 million years after the impact." Impacts of this size and larger occur every 200,000 years or so on Mars. About once every 2 million years, an impact of this size occurs on terrain suited to the scenario Head and his colleagues lay out. This means fragments from several impacts are in transit all the time. "This works out to about one Martian meteorite landing on Earth each month," Head said. These are not the only space rocks that hit Earth, Head points out. While only a few dozen Mars meteorites have been discovered, the total number of space rocks collected on our planet is about 20,000. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
