The idea that Mercury might have frozen water emerged in the 1990s, when radar telescopes detected highly reflective regions inside several craters near the planet’s poles.

Mercury’s axis doesn’t have much tilt, so its poles get little direct sunlight, and the floors of some craters get no direct sunlight at all. Without an atmosphere to hold in any heat from surrounding surfaces, temperatures in those eternal shadows have been calculated to be low enough for water ice to be stable. That raised the possibility these ‘radar-bright’ regions could be ice.

That idea got a boost after NASA’s MESSENGER spacecraft entered Mercury’s orbit in 2011. The orbiter detected neutron signals from the planet’s north pole that were consistent with water ice.

In the new study, lead author Ariel Deutsch, a Ph.D. candidate at Brown University, and colleagues took a deep dive into the MESSENGER data.

They looked specifically at readings from MESSENGER’s Mercury Laser Altimeter (MLA) and detected high reflectance deposits consistent with surface ice in three large craters.

“The total area of the three sheets to be about 3,400 km² — slightly larger than the state of Rhode Island,” Deutsch said.

The researchers also looked at reflectance data for the terrain surrounding those three large craters.

That terrain isn’t as bright as the ice sheets inside the craters, but it’s significantly brighter than the average Mercury surface.

“We suggest that this enhanced reflectance signature is driven by small-scale patches of ice that are spread throughout this terrain. Most of these patches are too small to resolve individually with the altimeter instrument, but collectively they contribute to the overall enhanced reflectance,” Deutsch said.

To seek further evidence that such smaller-scale deposits exist, the team looked though the MLA data in search of patches that were smaller than the big crater-based deposits, but still large enough to resolve with the altimeter. They found four, each with diameters of less than about 5 km.

“These four were just the ones we could resolve with the MESSENGER instruments. We think there are probably many, many more of these, ranging in sizes from a kilometer down to a few centimeters,” Deutsch said.

Knowing that these small-scale deposits exist, and that they’re likely the source of the slightly brighter surface outside craters, could dramatically increase the ice inventory on Mercury.

“How this polar ice may have found its way to Mercury in the first place remains an open question,” Deutsch said.

“The leading hypothesis is that it was delivered by water-rich comet or asteroid impacts. Another idea is that hydrogen may have been implanted in the surface by solar wind, later combining with an oxygen source to form water.”

The study is published in the journal Geophysical Research Letters.

Source: Sci News