A European probe approaching a comet in deep space has found the body's surface to be relatively warm, suggesting it has a mostly "dark, dusty crust", mission controllers say.
Thermal readings were taken by the unmanned spacecraft Rosetta as it neared Comet 67P/Churyumov-Gerasimenko on the final leg of a six-billion-kilometre, 10-year pursuit.
Using an infrared spectrometer, Rosetta scanned the comet between July 13 and 21, when the distance between them closed from 14,000km to just over 5000km, the European Space Agency (ESA) said on Friday.
At the time, the comet was about 555 million kilometres from the Sun.
Its average temperature was minus 70 degrees Celsius, the measurements found.
This may sound bitterly cold, but is in fact 20-30 degrees Celsius warmer than would have been expected if the comet had an ice-only surface, said an ESA statement.
"The temperature measurements provide direct confirmation that much of the surface must be dusty, because darker material heats up and emits heat more readily than ice when it is exposed to sunlight," it said.
Comets are believed by astrophysicists to be relics left from the building of the Solar System around 4.6 billion years ago.
Famously dubbed "dirty snowballs" by the late US astronomer Fred Whipple, they are theorised to be clusters of primeval ice and dust.
The 1.3-billion-euro ($A1.9-billion) Rosetta mission aims to pierce the secrets of these wanderers, to explain their composition and the chemistry of their ancient molecules.
Launched in 2004, Rosetta should come within 100km of the comet next Wednesday.
It has spent much of the last 10 years shuttling between Earth and Mars, using the planets' gravitational force as a slingshot to build up speed for the rendezvous.
The highlight of the mission will come in November, when Rosetta drops a refrigerator-sized lab, Philae, on to the comet's surface.
Philae is designed to last for six months, but the mother ship will stay close by, orbiting around the comet as it zips around the Sun and then heads out towards the orbit of Jupiter in 2015.
The spacecraft is named after the famous stone, now in the British Museum, that explained Egyptian hieroglyphics, while Philae is named after an obelisk that in turn helped decipher the Rosetta stone.
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