Mercury doesn’t make sense.
It is a strange lump of rock with a composition that is different from its neighboring rocky planets.
“It’s too much densesaid David Rothery, a planetary scientist at the Open University of England.
Most of the planet, the closest to the sun, is occupied by its core.
It lacks a thick mantle like Earth has, and no one is sure why.
One possibility is that the planet was much larger, perhaps twice its current volume or more.
Billions of years ago, this fledgling proto-Mercury, or super-Mercury, could have been hit by a large object, ripping off its outer layers and leaving behind the remnant we see.
While it’s a good idea, there has never been direct evidence of it.
But some researchers think they’ve found something.
In a paper 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 they can be hidden in museums and other meteorite collections. Studying them could unlock the mysteries of the planet.
“We don’t have any samples of Mercury” at this time, 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 gathered around the world from places as remote as the Sahara and Antarctica, and have found 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 are from Mars.
Notably absent from these documented space rocks are confirmed meteorites from our solar system’s innermost planets, Venus and Mercury.
It is generally hypothesized that it is difficult, though not impossible, for debris closer to the sun and its gravity go further into the solar system.
Among a small number of meteorite collections is a rare type of space rock called aubrites.
Named for the town of Aubres in France, where the first such meteorite 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 appear to match 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 Cartier.
But scientists stopped short of saying that they are 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 originate from other types of asteroids than something that was ejected from Mercury, with some scientists favoring a group of asteroids in the belt called E-type asteroids.
Among their evidence were signs that the aubrites had been attacked by the sun Wind, something that Mercury’s magnetic field should have protected against.
Cartier, however, has another idea.
What if aubrites originally came from Mercury?
Following the hypothesis that a sizable object collided with a younger Mercury, Cartier said that a large amount of material would have been thrown into space, about a third of the planet’s mass.
A small amount of that debris would have been blown 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, occasionally colliding and continually being pummeled by the solar wind, which explains the solar wind imprint seen in the aubrites.
But eventually, he suggested, some pieces were pushed toward Earth and fell to our planet as aubrite meteorites.
The low levels of nickel and cobalt found in aubrites match what we would expect from proto-Mercury, says Cartier, while data from NASA’s Messenger spacecraft that orbited Mercury between 2011 and 2015 support similarities between the composition of Mercury and aubrites.
“I think the aubrites are the uppermost parts of the mantle of a large protomercury,” Cartier said.
“This could solve the origin of Mercury.”
If true, it would mean we’ve had pieces of Mercury, albeit a much older version of the planet, stashed away in drawers and display cases for more than 150 years.
“It would be fantastic,” said Sara Russell, a meteorite expert at the Natural History Museum in London, who was not involved in Cartier’s work.
The museum has 10 aubritas 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, doesn’t think there is enough aubrite material in meteorite collections to determine whether its content matches models of super Mercury. .
“The authors are a bit optimistic,” he said.
“The data they use are not enough to validate their conclusions.
In response, Cartier said she removed potential contaminant rocks from her aubrite samples to obtain representative levels of nickel and cobalt, which she was “confident” were correct.
Jonti Horner, an asteroid dynamics expert at the University of Southern Queensland in Australia, wasn’t sure 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 dynamics standpoint,” he said.
Christopher Spalding, an expert on planet formation at Princeton University and a co-author of the Cartier study, says his model shows that the solar wind can blow material away from Mercury enough to link it to E-type asteroids.
“The young sun was highly magnetic and spinning fast,” he said, turning the solar wind into a “swirl” that could send pieces of Mercury into the asteroid belt.
Another possibility, yet to be modelled, is that the gravitational weights of Venus and Earth dispersed the material farther before some returned to our planet.
Cartier’s proposal could be put to the test soon. A joint European-Japanese space mission called BepiColombo is on track to orbit Mercury in December 2025. Cartier pitched his idea to a group of BepiColombo scientists in early May.
“I was impressed,” said Rothery, a member of the BepiColombo science team.
He said his mission could search for evidence of nickel on Mercury’s surface that would link the planet more conclusively to the collected aubrites.
It won’t be “simple,” he notes, since Mercury’s surface today will only resemble what remains of proto-Mercury.
But he said the results would “help feed the model.”
Willy Benz, an astrophysicist at the University of Bern in Switzerland who first proposed the idea of a proto-Mercury, says that if the aubrites come from Mercury, they will add to the evidence for an active and violent early solar system.
“It will show that giant impacts are quite common,” he said, and that they “play a significant role in shaping the architectures of planetary systems.”
Cartier is further testing his ideas by melting some samples of aubrite under high pressure.
If these experiments and the BepiColombo data support their hypothesis, aubrites may go from being a rarity in our meteorite collections to becoming some of the most remarkable meteorites ever collected: pieces of the innermost world of the solar system.
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