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Mercury may have been robbed by Venus in the early days of the solar system. A series of close passes between the two planets when they were young could have stripped away Mercury鈥檚 outer layers, leaving behind a world that is mostly dense core.

Mercury鈥檚 iron core makes up 70 per cent of its mass, a much higher proportion than any other rocky planet in the solar system. To end up like this, something must have happened to Mercury to strip away its mantle, the thick layer between the crust and core.

The simple answer is that a young Mercury underwent a collision that melted its outer layers 鈥� we think a similar collision on Earth formed the moon. But the Messenger spacecraft, which orbited Mercury between 2011 and 2015, spotted signs of elements such as potassium that would have vaporised in the heat of a collision. If a smash-up made Mercury so strange, it shouldn鈥檛 have those elements.

Now Hongping Deng at the University of Cambridge has used simulations to show that it may have been a series of near misses instead. 鈥淚f you pass by without direct contact, there is much less heat generated,鈥� he says. 鈥淚t just peels off the mantle.鈥�

You can see this in the simulation video below, where Mercury is the smaller of the two bodies. Purple corresponds to the outer mantle, while green is the dense core.

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Mercury probably started out about 30 per cent iron by mass, which is the iron fraction of the oldest rocks in the solar system. Deng found that if the young Mercury was spinning in the right way, its mantle would have been relatively easy to detach, meaning it would take four close encounters with the young Venus to raise the iron fraction to its present-day value.

Previous simulations of the early solar system have predicted multiple near passes between the two planets as they grew, so this is a plausible scenario, Deng says.

If this drive-by theft did occur, it would be hard to catch Venus with the stolen goods 鈥� Mercury is roughly 10 per cent the mass of Venus, and its mantle just a fraction of the whole mass, meaning material originating from Mercury would be hard to find on present day Venus.

Instead, we need to study Mercury more closely to聽tell for certain how it formed. 鈥淚 think obtaining in situ geochemical analyses or analysing a sample in Earth-based laboratories [would] be extremely useful for determining its formation mechanism,鈥� says Kathleen Vander Kaaden at the Lunar and Planetary Institute in Texas.

We won鈥檛 have to wait too much longer for a closer look. The BepiColombo spacecraft is on its way to Mercury and will arrive in 2025. As it orbits Mercury, it is expected to reveal hints from the surface of how the planet was born, says Vander Kaaden.