Scientists have finally shown what has been postulated for decades: the planet Mercury holds billions of tonnes of water ice at its north pole.
A report in Science shows evidence from the Messenger spacecraft that craters in constant shadow host water.
The findings may help explain how these ingredients first arrived on Earth.
Messenger was the first spacecraft to orbit Mercury, and since its arrival in March 2011 has been feeding back the best images of the planet that scientists have ever seen.
The principal evidence for water ice comes from the craft's "neutron spectrometer", which can detect the subatomic particle neutrons as they stream from Mercury.
"Neutrons are generated when cosmic rays hit a planet," Sean Solomon, Messenger principal investigator, explained to the Science podcast.
"Hydrogen is the best absorber of neutrons, so a neutron spectrometer looks for the signature of hydrogen near the surface by looking for decrease in the flux of neutrons coming from the planet."
This dip in the neutron count showed vast amounts of hydrogen in specific places at the planet's pole, consistent with deposits of water.
But further measurements using a laser and looking for reflections showed that much of the ice is covered with a layer of dark material tens of centimetres thick.
"The guess is that both the water and the dark material, which we think is organic-rich material, were delivered by the same objects impacting Mercury: some mixture of comets and the kinds of asteroids that are rich in organic and volatile material like water ice," Prof Solomon said.
"These are very common objects in the Solar System - we know many of them have orbits that bring them very close to the Sun."
Prof Solomon said that what Messenger finds not only unlocks secrets about the innermost planet in our Solar System, but could also shed light on those of other planets.
"The surprise that we received on making the first chemical measurements of Mercury was that none of the theories for how Mercury was assembled are correct," he said.
"So we're having to rewrite the books on how Mercury was assembled, and by implication how all the inner planets were assembled.
"The ice at the poles is only a recent chapter in that history but it's one that might be very informative."