Meteorite fragments land across every corner of Earth. But among the roughly 80,000 meteorites discovered so far, only a handful are known as angrites—and at least one of them might be the remnant of a long-lost protoplanet in our solar system, a new study suggests.
Researchers studying NWA 12774, an angrite retrieved in the Sahara Desert, noticed that the pressures required to form the fragment’s chemical structure could not have existed inside smaller asteroids. What’s more, the sharp, delicate crystals inside the angrite indicate that it was born at relatively shallow depths. In a recent paper published in Earth and Planetary Science Letters, the team presented an unexpected possibility—did the angrite once belong to a long-lost planetary embryo in our solar system?
“Meteorites are essentially a library of information about the formation and evolution of the early solar system,” Aaron S. Bell, the study’s first author and an earth scientist at the University of Colorado Boulder, told Gizmodo. “Angrites, in particular, preserve a record of processes that occurred at the very beginning of planetary formation.”
Ancient oddities
Angrites are among the most ancient, basaltic meteorites, with isotopic studies dating them at around 4.56 billion years old, Bell explained. However, their chemical composition sets them apart from other basalts found on Earth, the Moon, or even Mars. For instance, angrites have low levels of silica and a distinctive mineral chemistry, he added.
But NWA 12774 was arguably weirder. This particular angrite contained clinopyroxene, a mineral found beneath Earth’s surface, with exceptionally high levels of aluminum—a “chemical signature typically associated with high-pressure crystallization,” Bell noted. “That observation suggested formation under conditions not expected for a small asteroid and instead pointed toward a much larger parent body.”
When the rock cracks
To really drive this point home, Bell and colleagues designed an experiment to confirm how much pressure it would take to crystalize aluminum-rich clinopyroxine. Specifically, the team developed and tested a computational thermodynamic tool and plugged in the compositions measured in NWA 12774. Doing so allowed the researchers to “take the composition of individual mineral grains and use thermodynamics to infer the conditions under which they formed and from that constrain the size of the body they came from,” Bell said.
The tests revealed that the crystals would have needed at least 17.5 kilobars of pressure. To put this into perspective, the pressure at the bottom of the Mariana Trench adds up to around one kilobar, according to a university statement on the findings. Additional calculations indicated that the angrite would have had to come from a parent body with a radius of at least 621 miles (1,000 kilometers). For comparison, the Moon’s mean radius is about 1,080 miles (1,738 km).
A lost planet?
That might actually be a conservative estimate. X-ray images of the fragment showed well-preserved, sharp crystal edges that wouldn’t have survived being deep underground, the statement explained. In that case, the parent body could have been as big as Mars, which has a radius of 2,050 miles (3,300 km).
“These meteorites preserved evidence of a completely different pathway through which early planets developed,” Bell added in the statement. “There are many meteorites sitting in drawers that haven’t been thoroughly studied, so there were likely more of these protoplanets we don’t know about.”
Of course, it’s not like we can turn back time to see if NWA 12774 really fell out of a long-lost part of the solar system. Still, angrites—and for that matter, any ancient meteorite—are a unique artifact from the early solar system that reveals valuable insight into the geochemistry of the early solar system. And that’s a “remarkable opportunity,” Bell told Gizmodo.
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