A rock pulled from the Sahara is offering a rare glimpse of a lost early world.
A new study says the meteorite Northwest Africa 12774 contains the first definitive evidence of a long-lost body that may have rivaled the moon in size and existed just a few million years after the solar system formed 4.5 billion years ago.
The roughly 454-gram meteorite was found in the Sahara Desert in 2019. Scientists classify it as an angrite, a rare type of meteorite that ranks among the oldest volcanic rocks in the solar system.
Researchers say NWA 12774 preserves an unusual chemical signature that suggests some of the solar system’s earliest worlds developed differently from other rocky planets.
“The materials that formed the angrite parent body are fundamentally different from the ingredients of Earth and Mars,” study lead author Aaron Bell, a geoscientist at the University of Colorado Boulder, said in a statement. “These meteorites preserved evidence of a completely different pathway through which early planets developed.”
Scientists know angrites formed alongside the young sun more than 4.5 billion years ago by measuring tiny radioactive elements within them that act like natural clocks, according to NASA. They are also extremely rare. Only 68 of more than 80,000 meteorites recovered on Earth are known angrites.
Their chemistry has long puzzled scientists. Unlike Earth, Mars and most other rocky worlds, angrites contain very little silica, a major component of planetary crusts throughout the solar system. Because of that composition, scientists had assumed they came from a relatively small asteroid.
But when Bell and his colleagues analyzed NWA 12774, they found crystals of a mineral called clinopyroxene that were “exceptionally rich” in aluminum, which the study says is a sign the rock formed under immense pressure.
The team reconstructed the conditions in which the meteorite formed and found the mineral required pressures of at least 17.5 kilobars. The study says that is more than 17 times the pressure at the bottom of the Mariana Trench, the deepest point on Earth.
Those conditions could not have existed inside a small asteroid, the study says, so the parent body must have been much larger.
The crystals also preserved sharp edges and chemical patterns that scientists say would have been erased if they had spent long periods deep inside a hot planetary interior. The study says that points to the minerals forming at relatively shallow depths, which means the parent body would have needed to be substantially larger to generate the same pressures near its surface.
Under that scenario, the lost world may have exceeded 1,800 kilometres in radius, making it comparable in size to Earth’s moon and potentially approaching Mars, according to the study.
“It points to a distinct and separate evolutionary path in planetary formation in the early history of our solar system.”
“It’s incredible to think there was once a world this large,” Bell said in the statement. “We only know it existed because a few fragments of it happened to land on Earth.”
What happened to that world is still unclear. The researchers say one possibility is that violent collisions in the young solar system destroyed it, with fragments such as NWA 12774 later incorporated into other rocky planets, including Earth.
“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,” Bell said.
The research was published in Earth and Planetary Science Letters.
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