Brrr. Purples near the moon's South Pole denote permanently frigid spots in both daytime (left) and nighttime (right).

NASA/UCLA

And the Solar System's Coldest Spot Is ...

What's the coldest spot in the solar system? For now, that distinction belongs to permanently shadowed craters near the moon's south pole, according to the first results from the Lunar Reconnaissance Orbiter (LRO) spacecraft announced today at a NASA press conference.

Shivering in at a mere 33 degrees above absolute zero, the regions are likely places to find deposits of water ice, a resource that would be in demand if astronauts ever live on the moon. Another instrument has returned hints of water ice in some of these cold spots, scientists announced at the press conference, but it also showed signs of water ice in impossibly hot places, too. More data are needed, as every scientist in the press conference managed to note.

David Paige, the principal investigator for the Diviner thermal emission instrument onboard NASA's LRO, had predicted such cold temperatures with simple models. But now, said the University of California, Los Angeles, planetary scientist, there's solid proof. Although 33 kelvin is colder than almost any place in the feebly heated outer solar system, Paige says, he does allow that there may be similarly shadowed spots out there that could be colder.

Thirty-three kelvin is plenty cold enough to trap stray water molecules over the eons from the impact of comets, as earlier remote sensing of permanently shadowed lunar craters had suggested (Science, 15 May, p. 878). And LRO's LEND neutron spectrometer has detected hydrogen--presumably part of water-ice molecules--within the top meter of lunar soil in some polar permanently shadowed craters, said LRO project scientist Richard Vondrak of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The catch is that LEND also seems to be picking up signals from hydrogen well outside of permanently shadowed craters, where daytime temperatures soar toward a distinctly nonicy 380 kelvin. So perhaps the hydrogen has nothing to do with water. "I have no comment," says Paige. "I take a wait-and-see attitude."