‘Ice fossils’ from the desert
Algerian meteorite offers new insights into early asteroid formation.
By Richard A. Lovett
Scientists studying an 82-gram meteorite discovered in the mountains of southern Algeria in 1990 have found fossils of primordial snowflakes.
Not that they’re actual snowflakes. Rather, these fossils are tiny pores left behind billions of years ago, when snowflake-like ice grains that once filled them melted away.
The pores measure only 10 microns in size and were found by examining the meteorite known as Acfer 094 with high-resolution CT imaging.
Megumi Matsumoto, from Tohoku University, Japan, says Acfer 094 was chosen because it is 4.6 billion years old and extremely primitive – meaning that, other than the melting of the ice, it has been little altered since its parent asteroid was formed.
All of this is important, says Humberto Campins, an asteroid researcher at the University of Central Florida, US, because the entire meteorite “is basically a fossil of the [parent asteroid’s] accretion process from pieces that contained both ice and dust”.
Understanding that process, he says, is an important step in understanding the formation of planets, themselves composed from agglomerations of similar pieces.
Scientists have long believed that beyond a certain distance from the Sun, known as the snow line, these primordial bits were a mix of ice and dust. (Closer in, there was no ice, because there was too much solar heat for ice crystals to form.)
“[But] this is the first time this has been identified inside a meteorite,” Campins says.
But Matsumoto says her team’s finding shows even more; nanoscale microscopic observations of the meteorite found evidence of minerals formed by the interaction of water with rock.
That in itself isn’t surprising, given that her team had found the meteorite to be riddled with fossil pores from now-melted ice.
But it’s not that simple, she says, because the pores couldn’t have contained enough water to account for the minerals her team found.
“Additional ice is required,” she says. “This means that the distribution of ice in the parent body was heterogeneous and that ice was much more abundant elsewhere in [it].”
The best way to explain this, she says, is if the parent body migrated toward the Sun during its formation, crossing the snow line in the process.
This would cause its core – composed of materials accreted early in its formation, far out from the Sun – to have more ice, and therefore more water, than its outer layers.
“The Acfer 094 meteorite was probably derived from the outer part of the parent body,” she says.
All of this, Matsumoto and Campins say, is also important for understanding how the Earth got its water.
That water, they say, had to have been brought here from somewhere farther out in the Solar System.
“It is this type of meteorite [and similar, larger asteroids] that is most likely to have contributed,” Campins says.
The new study is published in the journal Science Advances.