Mercury is meaningless. It is a bizarre piece of rock with a composition that is different from its neighboring rocky planets.
“It’s way too dense,” said David Rothery, a planetary scientist at the Open University in England.
Most of the planet closest to the sun is occupied by its core. It lacks an Earth-thick mantle, and no one really knows why. One possibility is that the planet was much larger – possibly twice its current mass or more. Billions of years ago, this nascent proto-Mercury, or super Mercury, could have been hit by a large object, stripping away its outer layers and leaving behind the rest we see.
Although this is a good idea, there has never been any direct evidence for it. But some researchers think they’ve found something. In work presented at the Lunar and Planetary Science Conference in Houston in March, Camille Cartier, a planetary scientist at the University of Lorraine in France, and her colleagues said pieces of this proto-Mercury could be hiding in museums and other meteorite collections. Studying them could unravel the mysteries of the planet.
“We don’t have any mercury samples” at this time, Dr. Cartier said. Obtaining such specimens “would be a small revolution” in understanding the natural history of the smallest planet in the solar system.
According to the Meteoritical Society, nearly 70,000 meteorites have been collected around the world from places as distant as the Sahara and Antarctica, ending up in museums and other collections. Most come from asteroids ejected from the belt between Mars and Jupiter, while more than 500 come from the Moon. More than 300 come from Mars.
Confirmed meteorites from the deepest planets in our solar system, Venus and Mercury, are conspicuously absent from these documented space rocks. It is generally assumed that it is difficult, but not impossible, for detritus closer to the sun and its gravity to make their way further into the solar system.
Among a small number of meteorite collections are a rare type of space rock called aubrites. Named after the village of Aubres in France, where the first such meteorite was discovered in 1836, aubrites are pale in color and contain small amounts of metal. They are low in oxygen and seem to have formed in an ocean of magma. About 80 aubrite meteorites have been discovered on Earth.
For these reasons, they appear to fit scientific models of conditions on the planet Mercury in the early days of the solar system. “We have often said that aubrites are very good analogues of Mercury,” said Dr. Cartier.
But scientists stopped short of saying that they were actually pieces of Mercury. Klaus Keil, a scientist at the University of Hawai’i at Manoa who died in February, argued in 2010 that aubrites were more likely to come from other types of asteroids than from something that was ejected from Mercury, some scientists favoring a group of asteroids. belt asteroids called E-type asteroids. Among its evidence were signs that the aubrites had been blown by the solar wind – which Mercury’s magnetic field should have protected against.
Dr. Cartier, however, has another idea. What if aubrites originally came from Mercury?
Assuming that a large object collided with a younger Mercury, Dr Cartier said a large amount of material would have been thrown into space, about a third of the mass of the planet. A small amount of this debris would have been pushed by the solar wind into what is now the asteroid belt, forming the E-type asteroids.
There, the asteroids would have remained for billions of years, sometimes crashing into each other and being continually blown by the solar wind, explaining the solar wind imprint seen in aubrites. But eventually, she suggested, some pieces were pushed towards Earth and fell on our planet as aubritic meteorites.
The low levels of nickel and cobalt found in aubrites are consistent with what we’d expect from proto-Mercury, Dr Cartier says, while data from NASA’s Messenger spacecraft that orbited Mercury from 2011 to 2015 confirms the similarities between the composition of Mercury and aubrites.
“I think aubrites are the shallowest parts of a large proto-Mercury mantle,” Dr. Cartier said. “That might solve Mercury’s origin.”
If true, that would mean we’ve had pieces of Mercury – albeit a much older version of the planet – hidden away in drawers and display cases for over 150 years.
“That would be fantastic,” said Sara Russell, a meteorite specialist at the Natural History Museum in London, who was not involved in Dr Cartier’s work. The museum has 10 aubrites in its collection.
Other experts have reservations about the assumption.
Jean-Alix Barrat, a geochemist at the University of Western Brittany in France and one of the few aubrite experts in the world, does not believe there is enough aubritic material in meteorite collections to determine whether their content matches to the super Mercury models. .
“The authors are a bit optimistic,” he said. “The data they use is not sufficient to validate their conclusions.”
In response, Dr Cartier said she had removed any contaminating rocks from her aubrite samples to obtain representative levels of nickel and cobalt, which she was “confident” about.
Jonti Horner, an expert in asteroid dynamics from the University of Southern Queensland in Australia, was also unsure that material from Mercury could enter a stable orbit in the asteroid belt and hit Earth billions. years later. “It just doesn’t make sense to me from a dynamic perspective,” he said.
Christopher Spalding, an expert in planet formation at Princeton University and co-author of Dr Cartier’s study, says his modeling shows that the solar wind can push material away from Mercury enough to bind it to E-type asteroids. .
“The young sun was highly magnetic and spinning rapidly,” he said, turning the solar wind into a “whirlwind” that could send chunks of Mercury toward the asteroid belt. Another possibility, which has yet to be modeled, is that the gravitational weights of Venus and Earth dispersed the material further before some returned to our planet.
Dr. Cartier’s proposal could be put to the test soon. A joint Euro-Japanese space mission called BepiColombo is currently on its way to orbit Mercury in December 2025. Dr. Cartier presented his idea to a group of BepiColombo scientists in early May.
“I was impressed,” said BepiColombo science team member Dr Rothery. He said their mission could look for evidence of nickel on Mercury’s surface that would more conclusively link the planet to collected aubrites.
It won’t be “simple,” he notes, given that Mercury’s surface today will only look like what’s left of proto-Mercury. But he said the results would “help feed into the modelling”.
Willy Benz, an astrophysicist from the University of Bern in Switzerland who first proposed the idea of a proto-Mercury, says that if aubrites come from Mercury they will add to the evidence for an early solar system. active and violent.
“This will show that giant impacts are quite common,” he said, and that they “play an important role in shaping the architectures of planetary systems.”
Dr. Cartier is still testing his ideas by melting samples of aubrite under high pressure. If these experiments and BepiColombo’s data support his hypothesis, aubrites could suddenly be promoted from an oddity in our meteorite collections to some of the most remarkable meteorites ever collected – chunks of the world’s deepest solar system.