Incoming small celestial bodies: more may not be needed to explain the presence of water ice on Mercury.

In the interior of our solar system we find the planet Mercury. You might not expect ice cream there; after all, the planet is three times closer to the sun than the earth and therefore has to endure much more radiation and heat. And yet there is ice on Mercury. In deep craters near the poles, the bottoms of which never see sunlight. There, in those craters, is ice many meters thick.

Origin of the ice

But where does that ice actually come from? Researchers at the Netherlands Institute for Space Research (SRON) have now examined this question. And their findings can be read in the magazine icarus†

Two options

When it comes to the ice on Mercury, there are roughly two possibilities. Either the water molecules originated from space and were deposited on Mercury by impacted asteroids and comets or whirling down space dust. Or the water molecules are ‘endogenous’, ie originate from the interior of Mercury or originated on Mercury itself. “Endogenous sources are volcanic activity or the outgassing of the crust and mantle,” said researcher Kateryna Frantseva. Scientias.nl† That such events can bring water to the surface is due to the fact that some of the material from which Mercury was formed harbored water. Later, for example, some of that material may have come to the surface through eruptions. “In addition, the interaction between the planet’s surface and the solar wind can also generate water.”

In their study, however, Frantseva and colleagues do not focus on these endogenous water sources, but rather on the possibility that asteroids, comets and space dust brought water to Mercury. They explored that option using simulations. Using special software, they imitated the solar system – containing the sun, eight planets and hundreds of thousands of asteroids, comets and dust particles. “In our simulations, we tried to simulate the dynamics in the solar system, i.e. how planets and small celestial bodies move around the sun. And every time there was a collision between a small celestial body and a planet or the sun, we noted that. The simulation is statistically representative of reality: the number of small celestial bodies colliding with planets corresponds to what we see in reality, but we cannot use the simulation to predict which specific celestial body collides with a planet at any given moment. coming. In fact, the software allows us to simulate how many small celestial bodies, on average, collide with a given planet within a given time frame.”

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And the simulations show that Mercury is going to have to endure quite a bit. In fact, over a period of 1 billion years, enough small water-bearing celestial bodies hit the planet to explain the amount of ice we now find on the planet. “We can’t rule out endogenous water sources, such as volcanic activity and gases escaping from the crust and mantle, but this result shows we don’t need them,” Frantseva said. “We can suffice with impacts from small celestial bodies to explain the water we see on Mercury.”

Ten thousand kilograms of water

Space dust plays a particularly important role in this; the simulations suggest that interplanetary dust particles deposit more than ten thousand kilograms of water on Mercury per year. Asteroids and comets together transport about 1000 kilograms of water to the planet every year. “The water deposited by small celestial bodies on Mercury will only partially remain on the surface,” Frantseva said. “Some of the water is lost during the impact, some is lost through interaction with UV radiation from the sun and the rest reaches the permanently shadowed crater near the poles.”

BepiColombo

As mentioned, it is not yet entirely certain whether incident small celestial bodies when it comes to the thick ice layer in Mercury’s polar craters, actually deserve all the credit. “A simulation is not the same as an observation,” emphasizes Frantseva. “But we do know for sure that small celestial bodies collide with planets.” And with that, it seems very plausible that those comets, asteroids and that space dust deserve at least some credit. The exact size of their role will have to become clear in the near future. “To validate our model, we need to compare the water in the polar craters with the water we see on small celestial bodies,” says Frantseva. BepiColombo, a probe currently en route to Mercury and scheduled to arrive there in 2025, may have a role in this.

There is no question for Frantseva that the questions about the origin of Mercury’s ice are worth the time and effort of researchers. “It’s important to understand how water — one of the key ingredients for life as we know it — is distributed throughout our solar system.” What makes Mercury an extra interesting research object is that ideas about water transport on or to this planet can be tested during future missions to the planet. “In addition, Mercury is also a wonderful example of a planet on which water can survive, despite its small distance from the parent star.” The findings of current and future research on Mercury’s ice are of course not only important for our understanding of our own solar system; it could also have implications for planets orbiting other stars and their possible habitability.