Mercury makes no sense. It is a bizarre piece of rock with a composition that is different from the neighboring rock planets.
“It’s way too close,” 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 a thick mantle like Earth, and no one knows exactly why. One possibility is that the planet used to be much larger — perhaps twice its current size or more. Billions of years ago, this fledgling proto-Mercury, or super Mercury, could have been hit by a large object, stripping its outer layers and leaving the remnant we see.
While it’s a nice idea, there’s never been 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 colleagues said pieces of this proto-Mercury may be hiding in museums and other meteorite collections. By studying them, the mysteries of the planet can be unlocked.
“We currently have no samples of mercury,” said Dr. cartier. Obtaining such specimens “would be a minor revolution” in understanding the natural history of the solar system’s smallest planet.
According to the Meteoritical Society, nearly 70,000 meteorites around the world have been collected from remote places like the Sahara and Antarctica, and have made their way into 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.
Notably absent from these documented space rocks are confirmed meteorites from our solar system’s inner planets, Venus and Mercury. It is typically assumed that it is difficult, though not impossible, for debris closer to the sun and its gravity to get further into the solar system.
Among a small number of meteorite collections is a rare type of space rock called aubrites. Named after the village of Aubres in France, where the first meteorite of this type was found in 1836, aubrites are pale in color and contain small amounts of metal. They are low in oxygen and appear to have formed in an ocean of magma. About 80 aubrite meteorites have been found on Earth.
For these reasons, they seem to match scientific models of conditions on the planet Mercury in earlier days of the solar system. “We have often said that aubrites are very good analogues for Mercury,” said Dr. cartier.
But scientists have stopped saying that they are actually bits of Mercury. Klaus Keil, a scientist at the University of Hawaii at Manoa who died in February, argued in 2010 that aubrites came from other types of asteroids rather than anything ejected from Mercury, with some scientists preferring a group of asteroids in the belt called E-type asteroids. Among his evidence were signs that aubrites had been blown up by the solar wind — something Mercury’s magnetic field should have protected against.
dr. However, Cartier has a different idea. What if aubrites originally came from Mercury?
Echoing the hypothesis that a large object collided with a younger Mercury, Dr. Cartier that a large amount of material would have been thrown into space, about a third of the mass of the planet. A small amount of that 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 stayed for billions of years, occasionally bumping into each other and being blown up constantly by the solar wind, explaining the solar wind fingerprint seen in aubrites. But eventually, she suggested, some pieces were pushed to Earth and fell on our planet like aubrite meteorites.
Low levels of nickel and cobalt in aubrites are consistent with what we would expect from the proto-Mercury, says Dr. Cartier, while data from NASA’s Messenger spacecraft that orbited Mercury from 2011 to 2015, supports similarities between Mercury’s composition and aubrites.
“I think aubrites are the shallowest parts of the mantle of a large proto-Mercury,” said Dr. cartier. “This could solve the origin of Mercury.”
If true, it would mean we’ve had bits of Mercury – albeit a much older version of the planet – hiding in drawers and display cases for over 150 years.
“It would be fantastic,” said Sara Russell, a meteorite expert at the Natural History Museum in London who was not involved with Dr. cartier. The museum has 10 aubrites in its collection.
Other experts have reservations about the hypothesis.
Jean-Alix Barrat, a geochemist at the University of Western Brittany in France and one of the few aubrite experts in the world, thinks there isn’t enough aubrite material in meteorite collections to find out if their contents match models of the super Mercury .
“The authors are a bit optimistic,” he said. “The data they use is not sufficient to validate their conclusions.”
In response, Dr. Cartier that she had removed possible contaminating rocks from her aubrite samples to get representative levels of nickel and cobalt, which she “convinced” were correct.
Jonti Horner, an expert in asteroid dynamics at the University of Southern Queensland in Australia, was also unsure whether material from Mercury could enter a stable orbit in the asteroid belt and hit Earth billions of years later. “It just doesn’t make sense to me from a dynamic standpoint,” he said.
Christopher Spalding, an expert in planet formation at Princeton University and co-author of the study by Dr. Cartier, says his modeling shows that the solar wind can push material away from Mercury enough to link it to E-type asteroids.
“The young sun was very magnetic and spinning quickly,” he said, turning the solar wind into a “vortex” that could send chunks of Mercury into the asteroid belt. Another possibility, yet to be modeled, is that the gravitational forces of Venus and Earth further dispersed the material before some made their way back to our planet.
dr. Cartier’s proposal could soon be put to the test. A joint European-Japanese space mission called BepiColombo is currently en route to orbit Mercury in December 2025. Dr. Cartier presented her idea to a group of BepiColombo scientists in early May.
“I was impressed,” said Dr Rothery, a member of the BepiColombo science team. He said their mission could look for evidence of nickel in Mercury’s surface that would more convincingly link the planet to collected aubrites.
It won’t be “easy,” he notes, as Mercury’s surface today will only resemble what’s left behind from proto-Mercury. But he said the results “would contribute to the modeling.”
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 evidence of an active and violent early solar system.
“It will show that giant impacts are very common,” he said, and that they “play an important role in shaping the architectures of planetary systems.”
dr. Cartier tests her ideas further by melting some aubrite samples under high pressure. If these experiments and BepiColombo’s data bolster her hypothesis, aubrites could suddenly be promoted from a quirk in our meteorite collections to some of the most remarkable meteorites ever collected — pieces of the Solar System’s inner world.
