*Click here for Slideshow: Meteor streaks over Siberia*
* Green rock from S. Morocco may be 1st meteorite from Mercury*
NBC News, by Alan Boyle, Science Editor, and Jason Major, Universe Today (Feb. 5 & 16, 2013) — Scientists have raised their estimates of the size and power of what turns out to be the most widely witnessed asteroid strike in modern history. The size estimate puts the object that caused Friday’s meteor blast over Russia in a troublesome category of asteroids: big enough to cause damage, but small enough to evade detection.
The new estimates, based on additional readings from a sensor network built to detect nuclear blasts, suggest the meteor released the energy equivalent of nearly 500 kilotons of TNT. That’s about 30 times the power of the Hiroshima atomic bomb.
Experts have been assessing the level of the meteor explosion using a network of infrasound sensors that were set up under the terms of the Comprehensive Test Ban Treaty to check for changes in atmospheric pressure caused by nuclear blasts.
“These new estimates were generated using new data that had been collected by five additional infrasound stations located around the world — the first recording of the event being in Alaska, over 6,500 kilometers away from Chelyabinsk,” NASA’s Jet Propulsion Laboratory said in a statement.
NASA now says the Chelyabinsk object must have been about 55 feet wide (17 meters wide) with a mass of 10,000 tons before it entered Earth’s atmosphere.
“We would expect an event of this magnitude to occur once every 100 years on average,” Paul Chodas of NASA’s Near-Earth Object Program Office said in the statement. “When you have a fireball of this size, we would expect a large number of meteorites to reach the surface, and in this case there were probably some large ones.”
Searchers have been focusing on a frozen lake about 50 miles (80 kilometers) west of Chelyabinsk, where they suspect meteorite fragments made a 20-foot-wide (6-meter-wide) hole in the ice. Searchers have found fragments up to a half-inch wide (1 centimeter wide) that might have come from the meteor, but nothing bigger yet, according to reports from Russia.
The space agency said Friday’s Russian meteor was the largest reported since 1908, when an asteroid roughly the size of 2012 DA14 exploded over a remote wooded area in Siberia’s Tunguska region. That blast flattened millions of trees over a 820-square-mile area, but was not widely seen. Friday’s event, in contrast, took place over a city of 1.1 million inhabitants, and hundreds of millions more watched the videos that were distributed over the Internet.
Greenish rock recovered from southern Morocco may be first meteorite from Mercury
Chemical makeup is mostly in line with observations from Messenger probe, scientists say
Pieces of the Moon and Mars have been found on Earth before, as well as chunks of Vesta and other asteroids — but what about the innermost planet, Mercury? That’s where some researchers think this greenish meteorite may have originated, based on its curious composition and the most recent data from NASA’s Messenger spacecraft.
NWA 7325 is the name for a meteorite fall that was spotted in southern Morocco in 2012, comprising 35 fragments totaling about 345 grams. The dark green stones were purchased by meteorite dealer Stefan Ralew, who operates the retail site SR Meteorites. Ralew immediately made note of the rocks’ deep colors and lustrous, glassy exteriors.
Ralew sent samples of NWA 7325 to researcher Anthony Irving of the University of Washington, a specialist in meteorites of planetary origin. Irving found that the fragments contained surprisingly little iron but considerable amounts of magnesium, aluminum and calcium silicates — in line with what’s been observed by Messenger in the surface crust of Mercury.
Even though the ratio of calcium silicates is higher than what’s found on Mercury today, Irving speculates that the fragments of NWA 7325 could have come from a deeper part of Mercury’s crust, excavated by a powerful impact event and launched into space, eventually finding their way to Earth.
In addition, exposure to solar radiation for an unknown period of time and shock from its formation could have altered the meteorite’s composition somewhat, making it not exactly match up with measurements from Messenger. If this is indeed a piece of our solar system’s innermost planet, it will be the first Mercury meteorite ever confirmed.
But the only way to know for sure, according to a research paper written by Irving and his colleagues, is to conduct further studies on the fragments and, ultimately, samples that are returned from Mercury.
Irving’s team’s findings on NWA 7325 will be presented at the 44th Lunar and Planetary Science Conference, to be held in Houston from March 18 to 22. Read more in this Sky & Telescope article by Kelly Beatty.