Online seit: 12. April 2006
X-ray CT of select locations of Sutter's Mill SM73 carbonaceous chondrite with high resolutions between 1.6 to 4 micron/voxel, showing details of local mineralogy.
This is where history begins to blur. The weathering of the meteorite fragments suggested the space rock was only 50,000 to 90,000 years old when it hit Earth — basically a blink of an eye in the solar system's nearly-5-billion-year history. This is fortunate, as Jenniskens' team discovered carbonaceous chondrites are fragile and begin to break up around that time frame.[...] "This thing is diverse, even on a small scale,"
Its trajectory shows it was traveling on a very unusual, stretched-out orbit. Only one other space rock, which fell in 2009 in Denmark, has been seen following a similar path. Both may trace back to a family of asteroids called Eulalia, which orbit the sun in the asteroid belt between Mars and Jupiter, circling three times for every one time Jupiter orbits. On a close pass, Jupiter’s gravity may have kicked Sutter’s Mill on a path to hit Earth, Jenniskens says.[...] The rock’s beat-up nature suggests that many of the Eulalia asteroids may have suffered more than previously thought. More answers may be coming soon; some properties of the Sutter’s Mill meteorite, like how it reflects light, are similar to the carbonaceous asteroid that Japan’s Hayabusa-2 spacecraft plans to visit in 2018.
But in three rocks collected before a heavy rainstorm, which bathed the other pieces in earthly contaminants, organics are less abundant by a factor of 1000 than in previously studied CM chondrites.These three rocks could not have lost organics due to space "weathering": analysis of the meteorites' exposure to cosmic rays suggests the original meteor was flying through space for only about 50,000 years before hitting Earth.Based on its trajectory and its relatively short flight time, Jenniskens thinks the meteor can be traced back to a family of asteroids dominated by 495 Eulalia, a group known as a possible source of CM chondrites. It is probably a piece that broke off during an impact, revealing the relatively pristine material inside.So what happened to its organics? Jenniskens' team found that the meteorites are breccia – smaller rocks cemented together – which suggests that the asteroid from which they came took a series of beatings. Those impacts, or possibly other processes inside the asteroid, could have heated it enough to destroy most organic material. The result might have implications for the organics delivery theory, says Bill Bottke of the Southwest Research Institute in Boulder, Colorado."It shows that not all asteroids can deliver sufficient quantities. One of the disappointments is that, from a prebiotic organic chemistry perspective, it was very limited," says Bottke. "But this is an unusual case. Most [CM chondrites] are loaded with organic compounds."
Analyses performed using different techniques at other institutions were in agreement: the mineralogy and other geochemical features of these fragments are unexpectedly diverse and complex. This suggests that the surface of the asteroid that spawned the CM chondrites, their "parent body," is more complex than previously thought.
"Amino acids were few in this meteorite because this particular meteorite appears to have been slightly heated in space before it arrived at Earth," said Danny Glavin of NASA’s Goddard Space Flight Center, Greenbelt, Md.It appears that different parts of the meteorite had a different thermal alteration history. Heating also removed some of the water that used to move salts around in the asteroid."Samples collected before it rained on the meteorite fall area still contained such salts," said George Cooper of NASA Ames, "but Sutter's Mill was less altered by water in the asteroid itself than other CM type meteorites." "Only 150 parts per billion of Sutter's Mill was actual gold," said co-author and cosmochemist Qing-zhu Yin of U.C. Davis, Davis, Calif., "but all of it was scientific gold. With 78 other elements measured, Sutter's Mill provides one of the most complete records of elemental compositions documented for such primitive meteorites."
Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.
This is a color rendering of the interior structure of the SM73 meteorite sample. The X-ray CT was performed at Center for Molecular and Genomic Imaging at UC Davis by Doug Rowland for Prof. Qing-zhu Yin with Xradia MicroXCT-200. Image resolution: 22.57 micron/voxel, 1024x1024x1024. QZY thanks Robinson family for making this specimen available for scientific research.
Dr. Peter Brown, from the Centre for Planetary Science and Exploration and the Department of Physics and Astronomy at Western University London [Ontario], was part of the team that analyzed and tracked the object.
