Astronomers have detected a coating of ice and organic chemicals on one of the largest asteroids in the solar system.
The space rock, called 24 Themis, is roughly the size of Sicily and orbits the sun in the main belt of asteroids between Mars and Jupiter, more than 300 million kilometres from Earth.
The discovery supports the idea that asteroids may have brought plentiful supplies of water and organic material to Earth in the distant past and so set the stage for the emergence of life.
Two independent groups confirmed the composition of the asteroid's surface after observing the 200km-wide rock using Nasa's Infrared Telescope Facility (IRTF) which sits on the summit of Mauna Kea in Hawaii.
Analysis of infrared light glinting off the surface of the asteroid revealed that some wavelengths were being absorbed by water molecules. Further investigation suggested complex organic molecules were also present. The findings are reported in two papers in the journal Nature.
"The organics we detected appear to be complex, long-chained molecules," said Josh Emery, a planetary scientist at the University of Tennessee and lead author on one of the studies. "Raining down on a barren Earth in meteorites, these could have given a big kickstart to the development of life."
The discovery of frozen water on the asteroid has surprised some scientists because the sun warms the surface enough for ice to melt. One possible explanation is that ice in the core of the asteroid is heated into water vapour, which seeps through pores in the rock and freezes temporarily when it reaches the surface.
In the second study, a team led by Humberto Campins at the University of Central Florida timed its observations to take account of the asteroid's rotation every eight hours and produce a crude map of the surface. It shows that the entire surface of the asteroid is coated with a layer of frost no more than one ten-thousandth of a millimetre thick.
In an accompanying article, Henry Hsieh, a planetary scientist at Queens University in Belfast, likened the ice to a "living fossil": a remnant of the solar system that many considered long gone.
"This is a thin layer of ice. It's not like going outside on a snowy day," he told the Guardian. "But we didn't really think water would survive in the asteroid belt, and certainly not on the surface of an asteroid."
The discovery is intriguing because it may finally explain how two thirds of the Earth came to be submerged in water, turning a parched rock into a haven for life.
The Earth formed close to the sun as a dry boulder 4.5bn years ago, but asteroids from cooler regions of space would have slammed into the surface for millennia, releasing any water they contained on impact. At the time, asteroids were more numerous and may have carried far more water than has been found on 24 Themis.
Some scientists believe asteroids may have delivered water to every planet in the solar system, but Earth's rocky surface, size and orbit ensured water condensed and remained on the ground, ultimately forming vast seas and oceans.
"Each asteroid might not have carried a lot of water, but if you strike a planet with a few thousand or million of them, it would gradually build up," Hsieh said.
The finding of frozen water as far out as the main asteroid belt suggests water might also be spread throughout alien solar systems. "The building blocks of life – water and organics – may be more common near each star's habitable zone," said Emery. "The coming years will be truly exciting as astronomers search to discover whether these building blocks of life have worked their magic there as well."