Very small and closer to the Sun than it should be, Mercury has long puzzled astronomers precisely because it defies much of what is known about the formation of worlds. A space mission arriving there in 2026 could finally provide answers, according to the BBC.
At first glance, Mercury seems like the least spectacular planet: a barren globe with a barely detectable atmosphere and no trace of water or conditions for life. But, looked at more closely, it becomes much more interesting, because it is a cosmic object full of contradictions.
The smallest planet in the Solar System is 20 times lighter than Earth and barely exceeds Australia in diameter. However, it is surprisingly dense, second only to Earth. Almost all of its mass is concentrated in a huge metallic core.
Its position doesn’t match current models either. The extremely close orbit to the Sun cannot easily be explained by classical theories of planet formation.
“There is something essential we are missing”admit the researchers, who consider the origin of Mercury one of the biggest enigmas of the Solar System.
A mission with the potential to rewrite the textbooks
Hopes now turn to BepiColombo, a European and Japanese mission launched in 2018. After a prolonged journey due to technical problems, the probe will enter orbit in November 2026. It would be the first visitor to the planet in more than a decade, and one of its main goals is to clarify how Mercury came to be.
Understanding this planet is important not only for the history of the Solar System, but also for the study of exoplanets.
With its extreme features, Mercury is more similar than any other world to the planets discovered around other stars.
The first clues: something is not right
The signs that Mercury is “otherwise” they began to appear in the 1970s, when the Mariner 10 probe made three flybys. Gravitational measurements revealed an unusual internal structure: a core that occupies about 85% of the planet’s radius, covered only by a thin layer of rock. No other terrestrial planet looks like this, the BBC reports
The Messenger mission, which orbited the planet between 2011 and 2015, revealed an even more complicated situation.
Although the planet Mercury is subject to extreme temperatures, from over 430°C during the day to -180°C at night, the probe detected volatile elements such as potassium and thorium, which should have long ago been vaporized by solar radiation. Ice has even been identified in the permanently shadowed craters at the poles.
These findings fueled the idea that Mercury did not form where it is now.
Computer simulations of planet formation start from a disk of gas and dust around a young star. Planets grow by accreting material from around them.
But no matter how they adjust the parameters, the researchers fail to obtain an object with the characteristics of Mercury. Models produce “Zero Mercury”as some specialists joke.
The hypothesis of a huge impact
The most popular theory holds that Mercury was originally much larger, perhaps almost as large as Mars, and formed farther from the Sun.
In the first few million years, the planet would have been struck by a massive body, which would have torn off much of its mantle and crust, leaving behind only the dense core.
The hypothesis explains the current structure, but raises other problems: such a violent impact should also have removed the volatiles detected by Messenger. Furthermore, the velocities required for such a collision are improbable in the context of the dynamics of the early Solar System.
Mercury as a cosmic “projectile”.
Some researchers propose another hypothesis, that Mercury was not the victim, but the aggressor: a body that hit another planet, perhaps even Venus, and was deflected into its current orbit.
Such an impact “in passing” could explain the loss of the mantle.
Some questions remain though: where did the ejected material go? Why didn’t it return to the planet or form a satellite system?
A possible answer would be “collision grinding”: the fragments would have been gradually pulverized into fine particles, then scattered by the solar wind. But the process should have been extremely efficient, perhaps more efficient than is realistic.
Born of fire?
Another theory dispenses with the idea of impact altogether. According to it, Mercury formed right near the Sun, in a region so hot that only iron-rich particles survived, according to the BBC. The planet would have resulted from the aggregation of these heavy materials.
But this scenario also has a major flaw: If iron was so abundant, why didn’t Mercury continue to grow?
