Meteorites are fascinating for a variety of reasons, but one that has been particularly exciting is their ability to give us new clues about what the primordial Solar System was like. Within them, scientists can discover unexpected windows into the past.
In a new study published in Science Advances, researchers report the discovery of a peculiar structure within meteorite Acfer 094, which landed in the Algerian desert and was discovered in 1990. Detailed analysis of the internal structure of the object suggests that there are tiny pores through the rock, each roughly 11 micrometers across. For scale, human hair can easily be 10 times thicker.
The international team of researchers has a very simple theory for the formation of this material, which they dubbed ultraporous lithology (UPL). What we are seeing here is the space left by primordial ice. The UPL is the fossilized imprint of this primordial component.
Ice is very important in the formation of objects in space. From small asteroids to large planets, everything starts as small grains of dust. At a certain distance from a star, water molecules will begin to freeze into ice crystals. This is the snow-line. Beyond that threshold, ice will form on dust grains, a characteristic that allows them to grow quickly as they stick to each other more easily.
The main clue that ice is responsible for the UPL is the structure of the material itself. Without a solid material that eventually disappeared, the porous structure would have collapsed on itself and we wouldn’t be able to see it.
The structure also gave the team clues as to what the meteorite's parent object might have been like. They believe that the planetesimal had an ice-rich core and ice-poor mantle meaning that it would have formed in the outer Solar System, with the UPL assembling near the boundary between the two. As it moved closer to the Sun, its internal ice would begin to sublimate and the water would alter the rock, leaving distinct marks.
Eventually, this planetesimal was broken and at least a fragment, Acfer 094, ended up on Earth. But this object is not the only one that bears the mark of its icy past.
“At present, a large number of aqueously altered meteorites are known," the researchers write in the paper. "However, it has remained unknown how the ice responsible for the aqueous alteration was distributed in meteorite parent bodies. Our work suggests that UPLs, hosting the primordial ice, were distributed homogeneously in the Acfer 094 meteorite.”
Meteorites continue to be full of surprises. Only recently researchers discovered the presence of biologically important sugars in space rocks.