It feels strange to take a funicular to a planetary science meeting. But it’s a steep walk up from the hotel rooms near the sea, and there are no steps, so here at the El Conquistador Hotel in Fajardo, Puerto Rico, I take a slowly climbing tram to the annual meeting of the American Astronomical Society’s Division for Planetary Sciences.

It is damp — humid — outside and in one of the large, darkened meeting halls, Carle Pieters of Brown University is talking about dampness on Earth’s moon.

Her team’s main findings that water permeates the lunar surface come from three spectrometers on three different spacecraft — Cassini, Deep Impact and Chandrayaan-I — were published online September 24 in Science and were reported in Science News.

“So how much water is on the surface?” someone in the audience asks Pieters. For now, she says, the answer depends on who you ask. Quantifying the amount is highly model dependent but, she says, “regardless, it’s not a lot of water; it’s not a wet surface but it has water and OH [the hydroxyl radical] on it,” she says. “It’s drier than the driest desert.”

Roger Clark of the U.S. Geological Survey in Denver says there’s some water even on the moon’s equator and he estimates that the concentration of water on the lunar surface could be anywhere between 10 and 1000 parts per million. But he suggests that some of the variations in the water concentration seen by the Deep Impact spacecraft may be due to the craft’s viewing angle, rather than to interactions between the lunar surface and hydrogen carried by the solar wind, as Deep Impact researchers have suggested.

Deep Impact’s Jessica Sunshine of the University of Maryland in College Park disagrees. And if there’s water on the surface of the moon, due to interaction with the solar wind, there ought to be water on Mercury, the solar system’s hottest planet, and even on asteroids, she asserts.

Amid the controversy, one question that does have an answer, says Sunshine, is: How many spacecraft do you need to prove there’s water on the moon?

Three.

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