Mercury’s water may have arrived in one impact day

New simulations suggest Mercury’s polar ice was deposited after a single, large but slower-than-expected impact created a brief, water-rich atmosphere. The process could have played out over one Mercurian day—176 Earth days—helping explain why Mercury holds so

For years, scientists have stared at Mercury’s north and south poles and asked the same stubborn question: where did the ice come from?

In the permanently shadowed craters there—places that never see sunlight—ice deposits several metres deep have been known since NASA’s MESSENGER spacecraft orbited Mercury between 2011 and 2015. The puzzle has been how water survived at all. especially given that Mercury’s daytime temperatures can reach upwards of 430°C (806°F).

Now, new simulations point to a striking possibility: Mercury may have gained its unexpected water in the span of a single Mercurian day.

The scenario starts about 100 million years ago. when Mercury’s surface shifted abruptly from relatively dry and ice-free to abruptly ice-bearing. The new work—by Parvathy Prem at the Johns Hopkins Applied Physics Laboratory in Maryland and her colleagues—focuses on how an impact could have delivered water. not by scattering it gradually over time. but by creating the right brief conditions for it to move.

Earlier research suggested the source could have been a comet-like body around 17 kilometres across that struck Mercury at roughly 30 kilometres per second. Prem’s team, however, models a different kind of collision: a larger impactor hitting at a slower speed than previously suggested.

Prem describes it as a “movie” that can be tracked from start to finish. First comes a huge chunk of ice and rock slamming into Mercury. producing the enormous Hokusai crater now visible on the planet’s surface. As the impactor hit the ground, it would have vapourised almost completely. That left Mercury with an extremely tenuous, but water-rich, atmosphere.

Because that atmosphere would have been far too thin to see with the naked eye, the team leans on what happens “in the right wavelengths.” For a short window, the planet might have been glowing.

Then radiation from the Sun starts to take its toll. Most of the atmosphere would have been destroyed quickly. But the simulations find that just over one-fifth of the water vapour from the impactor could have migrated toward Mercury’s poles and found shelter in the permanently shadowed regions.

Prem says this is more than many earlier calculations suggested. She adds that a larger impactor arriving more slowly would be an even better match—trapping more water on the surface.

If the model is right, the timing is just as dramatic as the mechanism. All of this would have happened over one Mercurian day, which lasts 176 Earth days. Emily Costello at the University of Hawaiʻi calls it, in her words, “the most eventful day in the last billion years of Mercury’s history.”

That scale of delivery could help resolve a comparison that has long bothered planetary scientists. Mercury and Earth’s Moon are remarkably similar in nearly every way. yet the Moon does not have Mercury’s polar ice deposits. The new study points to a straightforward difference in outcomes: “Mercury recently experienced a large-scale water delivery. The moon didn’t,” Costello says.

The implications extend beyond Mercury’s poles. Prem says Mercury’s polar ice deposits act like “this interesting geological record” of how and when water came to the inner solar system—and that researchers are now trying to read what it’s telling them. The hope is that understanding Mercury’s record can also inform the broader story of how other worlds. including Earth. acquired water.

That story is likely to get more data soon. The BepiColombo spacecraft launched in 2018 and will enter orbit around Mercury later this year, adding another layer of measurement to what MESSENGER already revealed.

Mercury Mercury ice polar craters permanently shadowed regions MESSENGER BepiColombo Hokusai crater water delivery planetary science impact simulations inner solar system water