Four-and-a-half billion years ago, a massive world, possibly as big as the Moon or even Mars, orbited our Sun before crashing into another celestial body and shattering into rubble. Now, scientists have reported the first definitive evidence that this lost planetary embryo, or protoplanet, actually existed.
The Angrite Mystery
What gave away the lost world’s secret was a piece of its debris uncovered in the Sahara Desert, known as the Northwest Africa (NWA) 12774 angrite meteorite. Angrites are among the oldest known volcanic rocks in the solar system, forming within just a few million years after the solar system began.
A section of the meteorite under cross-polarized light. Credit: CU Boulder/John Kashuba
They are exceptionally rare. Out of more than 80,000 meteorites discovered on Earth, only 68 are angrites. Unlike Earth and Mars, angrites contain very little silica. For a long time, scientists thought angrites must always come from a small asteroid with a radius of less than 200 kilometers.
Minerals Forged Under Pressure
When Aaron Bell and his colleagues at CU Boulder studied NWA 12774, they found the meteorite contained clinopyroxene that was exceptionally rich in aluminum. This is a telltale sign that the rock formed under enormous pressure deep underground.
A slice with a green olivine crystal. Credit: John Kashuba
X-ray image. Credit: Aaron Bell/CU Boulder
To their surprise, the aluminum-rich clinopyroxene needed at least 17.5 kilobars of pressure to form. For comparison, the crushing pressure at the bottom of the Mariana Trench is only around 1 kilobar. That level of pressure could not have existed inside a small asteroid, suggesting the parent body must have been at least 1,000 km in radius.
The Scale of the Lost World
Other clues pointed to an even more striking possibility. The crystals preserved sharp edges and delicate chemical patterns that would have been erased if they had formed extremely deep underground. This suggested they formed at relatively shallow depths, meaning the world had to be even larger—stretching beyond 1,800 kilometers (Moon-sized) and possibly approaching a Mars-sized world.
A Different Evolutionary Path. It remains unclear how the protoplanet met its end. A catastrophic event likely shattered it, with its fragments later becoming the building blocks of other terrestrial planets. This meteorite points to a distinct and separate evolutionary path in planetary formation in the early history of our solar system.
Source: Paper published in Earth and Planetary Science Letters. Read more at CU Boulder Today.
