Page last updated at 19:14 GMT, Wednesday, 16 December 2009

'Coldest place' found on the Moon

By Jonathan Amos
Science correspondent, BBC News, San Francisco

Crater (Nasa)
The south-western edge (purple) of the floor of Hermite Crater is brutally cold

The Moon has the coldest place in the Solar System measured by a spacecraft.

Nasa's Lunar Reconnaissance Orbiter has used its Diviner instrument to probe the insides of permanently shadowed craters on Earth's satellite.

It found mid-winter, night-time surface temperatures inside the coldest craters in the northern polar region can dip as low as minus 247C (26 kelvin).

"The Moon has one of the most extreme thermal environments of any body in the Solar System," said Prof David Paige.

"During the middle of the day, temperatures can get up to about 400K (127C) at the equator; and at the poles at night, they can get very cold," the Diviner principal investigator at the University of California, Los Angeles, added.

Prof Paige has been describing his instrument's latest findings here at the American Geophysical Union's (AGU) Fall Meeting, the world's largest annual gathering of Earth scientists.

Diviner was part of the suite of instruments launched on LRO in June this year and has been operating continuously since it was switched on in July.

In October, the spacecraft found itself in the perfect position to witness summer solstice in the Moon's southern hemisphere and winter solstice in the northern hemisphere.

The Moon does have seasons - just about. The tilt of the lunar axis is 1.54 degrees. For most places, this makes no difference, but as Prof Paige explained, at the poles, this gives rise to a small, three-degree change in the elevation of the Sun on the horizon through the course of a year.

Artist's impression of LRO (Nasa)
LRO was launched in June

"This results in a significant variation in the extent of shadows and temperatures," he said.

Diviner observed the lowest summer temperatures in the darkest craters at the southern pole to be about 35K (-238C); but in the north, close to the winter solstice the instrument recorded a temperature of just 26K on the south-western edge of the floor of Hermite Crater.

There were also areas on the southern edges of the floors of Peary and Bosch Craters that got almost as cold.

Calculations suggest one would have to travel to a distance beyond the Kuiper Belt - well beyond the orbit of Neptune - to find objects with surfaces this cold.

"The way you can make something cold is to eliminate all possible other heat sources, and in these craters at the lunar poles they receive no direct sunlight and the coldest places don't even receive any indirect sunlight," Prof Paige said.

"In other words, only what little radiation may be scattered from some distant cliff gets down into these areas; and they just cool off. Finally, they reach an equilibrium temperature down at those low values."

The discovery adds further weight to the idea that some craters on the Moon could harbour water-ices for extended periods, and also more volatile substances that require even colder storage temperatures.

Infographic, BBC
CRaTER - characterises the global lunar radiation environment
Diviner - measures lunar surface temperatures
LAMP - maps the Moon's permanently shadowed regions
LEND - measures the flux of neutrons from the Moon
LOLA - provides a global lunar topographic model
LROC - LRO's camera will help select future landing sites
Mini-RF - uses radar to search for evidence of water-ice

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