Poor Pluto. First it gets kicked out of the planet club, now it can no longer claim to be the coldest place in the solar system. Dark craters near the moon's south pole have snatched that title – which is good news for the prospects of finding water ice on Earth's companion.
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The craters' towering rims block the sun from reaching their centres, like the long shadows cast by tall buildings at dusk. In this permanent darkness, they stay at a constant -240°Celsius – more than 30°C above absolute zero and 10°C cooler than Pluto, which was measured at -230°C in 2006.
"The lunar south pole is among the coldest parts of the solar system and may be in fact colder than what we expect from places like Pluto," NASA scientist Richard Vondrak said at a press conference on Thursday.
The cold temperature bodes well for the prospect of finding water ice deposits in the moon's shadowy pockets. Previous calculations had shown that water and other volatile gases would dissipate into space at temperatures above about -220°C.
The measurements come from NASA's Lunar Reconnaissance Orbiter (LRO), which launched in June.
The satellite's temperature sensor, DIVINER, measures the amount of emitted and reflected radiation given off by the surface. LRO has a number of other instruments designed to map properties such as topography and neutron levels – another possible indicator of water.
In July, the satellite sent back pictures of the Apollo landing sites to commemorate the 40th anniversary of humans on the moon. On Thursday, LRO's primary mission began to collect data that could be used to plan a possible return to the moon.
The temperature finding raises hopes that NASA's other current lunar satellite mission, LCROSS, the Lunar Crater Observation and Sensing Satellite, will find evidence of water when it crashes into a crater near the moon's south pole on 9 October.