There could be much more ice on Mercury's surface than previously thought, suggests new research adding to previous findings that some of its craters permanently shadowed from the Sun's blistering rays could have frozen water.
The study, published in Geophysical Research Letters, adds three new members to the list of craters near Mercury's north pole that appear to harbour large surface ice deposits.
But in addition to those large deposits, the research also shows evidence that smaller-scale deposits are scattered around Mercury's north pole, both inside craters and in shadowed terrain between craters.
Those deposits may be small, but they could add up to a lot more previously unaccounted for ice.
"The assumption has been that surface ice on Mercury exists predominantly in large craters, but we show evidence for these smaller-scale deposits as well," said the study's lead author Ariel Deutsch, a Ph.D. candidate at Brown University in Providence, Rhode Island, US.
"Adding these small-scale deposits to the large deposits within craters adds significantly to the surface ice inventory on Mercury," Deutsch said.
The idea that Mercury might have frozen water emerged in the 1990s, when Earth-based radar telescopes detected highly reflective regions inside several craters near Mercury's poles.
The planet's axis does not 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 probe entered Mercury's orbit in 2011. The spacecraft detected neutron signals from the planet's north pole that were consistent with water ice.
For this new study, Deutsch worked with Gregory Neumann from Nasa's Goddard Space Flight Center to take a deep dive into the data returned from MESSENGER.
They looked specifically at readings from the spacecraft's laser altimeter. The device is mostly used to map elevation, but it can also be used to track surface reflectance.
The researchers identified three new craters near Mercury's north pole that appear to harbour large surface ice deposits.