Data collected by NASA's Messenger probe indicates "the presence of one billion to one trillion metric tonnes of ice" at Mercury's poles, said Sean Solomon, principal investigator with the Messenger program.
"These are very exciting results... but there is more to come," he added.
The ice is in the permanently shadowed region of Mercury’s north pole and is thought to be between 50 centimetres and 20 metres deep.
Scientists suspect there is also ice near the south pole, though the Messenger probe was unable to take a sufficiently close look. Messenger was launched in 2004 and arrived in orbit around Mercury in March 2011. It is the first spacecraft to orbit the planet — the Mariner 10 probe flew past in the 1970s — and has been studying the surface in unprecedented detail.
The presence of water raises the question of whether Mercury could house, or could ever have housed, life.
“No one is saying there’s life on Mercury,” said Solomon, though he admitted the planet is now much more “astro-biologically interesting.”
The findings, he said, are more likely to shed light on the early processes of how life formed on Earth and might form elsewhere.
“In the book of life there are early chapters and Mercury may well inform us about those early chapters,” he said.