It shouldn’t be very easy to find ice on Mercury. It’s true that temperatures on the planet’s night side drop to -280°F (-173°C), but daytime temperatures soar to a hellish 800°F (427°C) or so, and days on Mercury last for nearly three months. That’s plenty of time for the ice to melt and vaporize. Yet for decades, scientists have suspected that Mercury might be icy all the same — provided you looked in the right place. Back in the 90’s, astronomers used the giant Arecibo radio telescope in Puerto Rico as a huge radar gun and pointed its barrel toward Mercury, specifically toward its poles. Craters there might be just deep enough — and their rims just high enough — that their basins would remain in permanent shadow. Ice once deposited there by meteors or comets would remain forever. The bright radar reflections that bounced back to Aricebo provided some tentative confirmation of that theory.
Now there’s no more “tentative” about it. A set of three papers just published in Science, based on observations by the Messenger spacecraft in orbit around Mercury, makes the nearly ironclad case that there is indeed water at Mercury’s north pole, at least — and also some mysterious dark material that could be made of the same tarry organic compounds that contaminate comets. “The idea has been out there for long time,” says David Lawrence of Johns Hopkins University, author of one of the papers, “and one of the big science goals for Messenger was to try to confirm it.”
Ice in permanently shadowed craters is not unique to Mercury. In 2009, NASA‘s LCROSS mission established that the phenomenon occurs on the moon, a fact it proved by crashing a projectile into a crater and analyzing the debris that was blasted out. Messenger did its work more subtly, with three pieces of circumstantial evidence combining to make the case — hence the three separate Science papers. The first bit of proof came from the spacecraft’s neutron spectrometer, Lawrence’s specialty. When cosmic rays hit Mercury’s surface, he explains, “they bust apart atomic nuclei, sending neutrons bouncing around like billiard balls.”
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When the neutrons hit objects of similar mass — the nucleus of a hydrogen atom, for example — they kind of stop in their tracks, like two billiard balls running into each other on a table. When they hit the nuclei of heavier elements, it’s more like hitting a bowling ball: the neutrons go flying off at high speed, and some of them should end up in Messenger’s spectrometer. But the instrument recorded relatively few neutrons from the poles, and, says Lawrence, “the only thing that can cause that is a lot of hydrogen.” And by far the most likely form of hydrogen is H20.
The second piece of evidence comes from calculating how cold the craters at Mercury’s poles should be and that required determining their depth and and the height of their rims. UCLA’s David Paige, lead author of Mercury paper number two, used precision topographic data from Messenger to do that cosmic surveyor’s work, then ran the data through computer models that spit out a temperature estimate. “What we find,” Paige says, “is that in these cold impact craters, it gets way cold enough such that if there were water molecules they’d stick there, form ice deposits.”
The same laser altimeters that have been mapping Mercury’s topography provided the third line of evidence. The time it takes an infrared laser beam to get to the surface and back tells scientists how high or low a particular point is. But the brightness of the reflection tells them how bright or dark the surface is (darker areas absorb more and reflect less, brighter areas vice versa). Because they suspected ice was present, says Gregory Neumann of the Goddard Spaceflight Center, lead author of the third paper, “we expected reflectance in the permanently shadowed craters to be high.”
(More: The Hot Rock: Mysterious Mercury)
That was true, partly, but curiously enough, there were significant patches that were darker than average. “One of the paper’s reviewers quipped ‘scientists discover that it’s dark where there’s no light,’” says Neumann. But when he discussed the oddity with Paige, he recalls, “Dave said ‘excellent, I’m sure I know what this is.’” Since comets are the likeliest explanation for ice deposits, and since they’re well known to be more or less filthy with organic compounds, the dark and light patches tie the whole theory together with a tidy bow.
By itself, this discovery would be important enough, but, says Paige, “it’s also part of the bigger scientific quest to figure out where the original water and organic compounds came from that made life on Earth possible in the first place.” Comet impacts are a very likely explanation, and while the evidence of those impacts is long gone, the primordial debris left on Mercury, largely unchanged for billions of years, could provide valuable clues. “We need to look in places like this,” he says, “to get a better idea of how life got started here — and maybe even how it might have gotten started in other planetary systems.”