How Mercury Might Have Gained Its Surprising Water Supply in Just One Day

Approximately 100 million years ago, dramatic changes transformed Mercury’s surface. Previously dry and ice-free, the planet’s conditions shifted significantly within a single day, coinciding with daytime temperatures soaring above 430°C (806°F).

At Mercury’s poles lie craters known as permanent shadow regions, where sunlight never reaches. NASA’s Messenger spacecraft, which orbited Mercury from 2011 to 2015, confirmed these craters host ice deposits several meters deep. However, their origin remains a mystery.

Prior studies have proposed that a comet-like object, roughly 17 kilometers in diameter, may have impacted Mercury at a speed of approximately 30 kilometers per second. Recent simulations by Parvathy Prem and her team at the Johns Hopkins Applied Physics Laboratory suggest that a larger, slower impactor might be responsible.

“We’ve known for a while that Mercury has ice at its poles, and the idea that these deposits stemmed from impactors isn’t new. However, this is the first time we’ve modeled the entire process and visualized it comprehensively,” Prem stated. “This is my first in-depth look into it.” What happens in this simulation?

The simulation begins with a massive ice and rock chunk colliding with Mercury, creating the expansive Hokusai crater visible on its surface today. Upon impact, the object would have evaporated nearly completely, resulting in a thin but water-rich atmosphere around Mercury.

“To the naked eye, Mercury might seem faint, but at the right wavelength, the planet may emit a brief but noticeable shine,” Prem explained.

While much of the atmosphere would have dispersed due to intense solar radiation, the researchers discovered that slightly more than one-fifth of the water vapor generated by the impactor traveled to the poles, potentially taking refuge in the permanently shadowed regions. This finding aligns more closely with Messenger’s measurements and suggests a larger, slower impactor trapped more water on the surface than previously thought.

If Prem and her team are correct, all these events would have unfolded in just one Mercuryian day, the equivalent of 176 Earth days. “This day may well be the most significant in Mercury’s billion-year history,” remarked Emily Costello from the University of Hawaii.

This research could answer the long-standing question of why Mercury’s polar craters contain ice whereas Earth’s moon has none, despite their striking similarities. “Mercury experienced a massive influx of water recently; the Moon did not,” Costello remarked.

Understanding Mercury’s polar ice deposits may also shed light on the broader narrative of when and how water became present throughout our solar system, including Earth. “Mercury’s polar ice deposits serve as a fascinating geological record of when and how water emerged in the solar system’s interior, and we are now deciphering this record,” Prem noted. The ongoing mission will be supported by the BepiColombo spacecraft, launched in 2018 and set to enter Mercury’s orbit later this year.