It may be the closest planet to the Sun, but even Mercury has water ice. A lot in fact—as much as 100 billion to 1 trillion tons, according to a trio of studies published today by scientists working with NASA's MESSENGER orbiter. The findings, out today in the journal Science, are the first to give strong evidence of water in Mercury's polar regions, following decades of speculation.
Using the neutron spectrometer aboard MESSENGER—which in March became the first spacecraft enter Mercury's orbit—scientists detected unprecedented high concentrations of hydrogen within the craters in Mercury's polar regions. Combined with the temperature models created from topographic data taken with the orbiter's laser altimeter, the researchers had their evidence: Pure water deposits on Mercury.
"If it was the neutrons alone, it would be harder to say, 'Bang, it's water ice,'" says David Lawrence, MESSENGER participating scientist. "But when you combine that with the radar data, the topography, thermal maps, they're all pointing to the same conclusion—it's got to be ice."
It may seem improbable that a planet where temperatures can register as high 800 F at the equator would have ice, but the surface of Mercury is a place of extremes. Since Mercury's rotational axis isn't titled like the Earth's, craters within the polar regions of the planet are permanently shaded from the Sun's rays, causing temperatures within their depths to sink as low as -370 F. That's certainly cold enough to retain any ice from a comet impact millions of years ago, Lawrence says.
While the recent discovery doesn't improve our understanding of how the planet initially formed—a topic that continues to confound scientists to this day—it does pose a new set of questions for researchers.
"In the planetary science community we've been living with this idea of water ice for 20 years," Lawrence says. "But the new questions are, how do we measure this is more detail? How deep does it go?" Further study into the ice will hopefully reveal the source of the ice—perhaps comet impacts—and its extent. With enough fuel to last it until 2015, MESSENGER will give scientists plenty of time to find out.