The Chandrayaan-1 folks had a session yesterday, and people streamed into the room to see what Paul Spudis had to say about ice on the moon. Spudis is an LPI scientist leading the mini-SAR radar instrument on Chandrayaan, which is a prelude to a bigger radar instrument on the Lunar Reconnaissance Orbiter, which will launch in May. Both instruments will search for radar reflections, consistent with ice, in the permanently shadowed craters of the moon’s poles; Spudis has been heavily involved in this search for years.

A quick review of history (Spudis has a nice review, colored by his perspective, here): In the 1990s, the Clementine mission got everyone excited by a strange double-bouncing radar reflection from inside some polar craters. Spudis calls this double reflection “CPR” for circular polarization ratio. Blocks of ice — or rough regolith — can cause this change in the polarity of the radar signal.

A few years later, Lunar Prospector brought both good and bad news: it detected an excess of hydrogen atoms — consistent with water in the regolith (shown in the image here). But only at the level of a few percent, which meant that it was uncertain whether it could represent microscopic bits of water in the pores of the soil and rock, or actual chunks of water ice.

And then in a series of papers over the last decade, people like Don Campbell, using ground based radars like Arecibo, weighed in and cast doubt on the Clementine interpretation.

But where Arecibo can only see along the rims of the polar craters, Chandrayaan can look straight down in. Spudis was very coy about what mini-SAR had seen in its first few months of its operations. But he tantalized the crowd with maps of a few small, young-ish polar craters that had high CPR signals inside the rim, but not outside. Normally, you expect the CPR signal to be high both inside and outside the rough fringe of the impact crater. He didn’t say if the anomalous result could signal ice. “I don’t want to speculate on what we’re seeing until we’ve got these numbers pinned down.” Campbell, listening intently in the audience, was also intrigued. “I think it’s very interesting, very nice looking data,” he says. “But we need to wait and see.”

Image: NASA