Mercury may have gained all of its unexpected water in a single day

Approximately 100 million years ago, Mercury’s surface experienced a sudden transformation. Previously dry and devoid of ice—unsurprising given the planet’s extreme daytime temperatures reaching over 430°C (806°F)—the landscape changed dramatically within just one Mercurian day.

Mercury’s poles feature craters that remain in perpetual shadow, known as permanently shadowed regions. Thanks to data from NASA’s Messenger spacecraft, which orbited Mercury from 2011 to 2015, we know these craters are filled with ice deposits several meters thick. The origins of this ice, however, remain a mystery.

Earlier studies proposed that a comet-like body, approximately 17 kilometers wide, collided with Mercury at a speed of about 30 kilometers per second, depositing the ice. Recent simulations led by Parvathy Prem at the Johns Hopkins Applied Physics Laboratory in Maryland suggest a different scenario: a larger object impacting the planet at a slower velocity.

“We’ve long been aware that Mercury’s poles contain ice. The hypothesis that an impactor might have laid down these ice deposits isn’t new, but this is the first time we’ve modeled and visualized the entire process from start to finish,” says Prem. “This is the first detailed examination of how this scenario might have unfolded.”

This scenario begins with a massive ice and rock body crashing into Mercury, forming the large Hokusai crater seen today. Upon impact, the object would have almost entirely vaporized, leaving behind a thin, water-rich atmosphere on Mercury.

“If we viewed Mercury with the naked eye, the atmosphere would likely be too thin to see. However, observing it at specific wavelengths might reveal a brief glow,” explains Prem.

Although most of this atmosphere would have been quickly destroyed by the sun’s intense radiation, researchers discovered that over one-fifth of the water vapor from the impactor could have migrated to the poles, settling in permanently shadowed areas. This amount surpasses previous estimates and aligns more closely with Messenger’s observations, notes Prem. A larger, slower impactor would likely trap more water on Mercury’s surface.

If the findings are accurate, this entire event would have transpired within one Mercurian day, equivalent to 176 Earth days. “This would undoubtedly be the most significant day in the past billion years of Mercury’s history,” comments Emily Costello from the University of Hawaiʻi.

This discovery could clarify why Mercury’s polar craters are rich in ice, while Earth’s moon, which shares many similarities with Mercury, is not. In simple terms: “Mercury had a recent large-scale water delivery, unlike the moon,” states Costello.

Understanding Mercury’s ice deposits could also shed light on how and when water arrived in the inner solar system, including Earth. “Mercury’s polar ice serves as a geological record of how and when water entered the inner solar system. We are deciphering this record to uncover its insights,” says Prem. The BepiColombo spacecraft, launched in 2018, is expected to aid this mission when it enters Mercury’s orbit later this year.