Scientists led by the Southwest Research Institute have identified a probable new asteroid parent of meteorites by studying a small fragment that reached Earth 12 years ago.
The composition of a piece of the Almahata Sitta (AhS) meteorite indicates that its parent body was an asteroid roughly the size of Ceres, the largest object in the main asteroid belt, and it formed in the presence of water at intermediate temperatures and pressures.
“Carbonaceous chondrite (CC) meteorites record geological activity during the early stages of the Solar System and provide information on the histories of their parental bodies,” explains Dr. Vicky Hamilton, SwRI scientist and first author of an article published in ‘Nature Astronomy ‘that describes this research.
“Some of these meteorites are dominated by minerals that provide evidence of exposure to water at low temperatures and pressures – he adds -. The composition of other meteorites points to warming in the absence of water. Evidence of metamorphism in the presence of water under intermediate conditions it has been practically absent, until now. ”
Asteroids, and the meteors and meteorites that sometimes come from them, are remnants of the formation of our Solar System 4.6 billion years ago. Most reside in the main asteroid belt between the orbits of Mars and Jupiter, but collisions and other events have ruptured them and ejected debris into the Solar System.
In 2008, a 9-ton, 4-meter-diameter asteroid entered Earth’s atmosphere and exploded in about 600 meteorites over the Sudan. This was the first time that scientists predicted an asteroid impact prior to entry and allowed for the recovery of more than 10 kilograms of samples.
“We were assigned a 50-milligram sample of AhS to study,” recalls Hamilton. “We assembled and polished the tiny fragment and used an infrared microscope to examine its composition. Spectral analysis identified a variety of hydrated minerals, particularly amphibole, that target at intermediate temperatures and pressures and a prolonged period of aqueous disturbance in a parent asteroid at least 600 and up to 1,800 kilometers in diameter. ”
Amphiboles are rare in the CC meteorites, as they were only previously identified as a trace component in the Allende meteorite. “AhS is a serendipitous source of information about the earliest materials in the Solar System that are not represented by CC meteorites in our collections,” notes Hamilton.
Orbital spectroscopy of the asteroids Ryugu and Bennu visited by Japan’s ‘Hayabusa2’ spacecraft and NASA’s ‘OSIRIS-REx’ spacecraft this year is consistent with the water-altered CC meteorites and suggests that both asteroids differ from most. of known meteorites in terms of their hydration status and evidence of large-scale, low-temperature hydrothermal processes. These missions have collected samples from the surfaces of asteroids for their return to Earth.
“If the compositions of the ‘Hayabusa2’ and ‘OSIRIS-REx’ samples differ from what we have in our meteorite collections it could mean that their physical properties make do not survive the processes of expulsion, transit and entry through the atmosphere terrestrial, at least in its original geological context – explains Hamilton, who is also part of the ‘OSIRIS-REx’ scientific team -. However, we believe there are more carbonaceous chondrite materials in the Solar System than our collections of meteorites represent. ”