Move over, Mars: Evidence is mounting that water exists even on the hot, barren planet closest to the sun, Mercury.
Based on new data from a NASA probe, astronomers say they're close to proving that mysterious "bright" spots at Mercury's poles are patches of water ice.
Ice on Mercury might seem strange. After all, the planet's surface can get hot enough to melt lead on the side facing the sun.
But Mercury doesn't tilt very far on its axis, raising the prospect that the bottoms of some polar craters might be in permanent shadow.
Since Mercury lacks an atmosphere to trap heat, being in constant shade could turn these craters into "cold traps"—zones in which debris or vapor drifting in from interplanetary space could be permanently captured as ever deepening rimes of frost.
Similar cold traps exist near the moon's south pole, where scientists recently found substantial quantities of water ice.
Discovered in 1991, some parts of Mercury's polars are referred to as bright not because they are white, but because they strongly reflect radar waves—a classic sign of water or ice.
In fact, the patches may contain quite a bit of ice, according to Anthony Colaprete, a planetary scientist from the NASA Ames Research Center in Moffett Field, California, who was not part of the Mercury study teams.
"To get this kind of radar reflectance, most folks suggest one needs clean ice (maybe more than 90 percent pure) ... [that's] several meters or more thick," Colaprete said in an email.
Peering Into the Dark
One of the goals of NASA's MESSENGER spacecraft, which has been orbiting Mercury since March, is to peer into the planet's cold traps.
Already, two new studies based on MESSENGER data strongly link Mercury's cold traps with the radar-bright patches, said Sean Solomon, a planetary scientist with the Carnegie Institution of Washington and a member of the MESSENGER team.
One study used laser altimetry to create a topographical map of the planet's north polar region.
The MESSENGER team converted this data into a three-dimensional model of Mercury's cratered north and rotated the model beneath simulated sunshine.
The experiment showed that there did indeed appear to be zones of permanent shadow at the north pole, and "[these] areas coincide with the radar-bright deposits," Solomon said last week at a meeting of the American Geophysical Union in San Francisco.
The second study used six months' worth of MESSENGER's orbital photos to examine the south polar region, tabulating areas that were dark each time they were photographed.
Again, Solomon said, the south-pole study showed that "all of the areas that have radar-bright deposits are indeed in permanent shadow."
Slow Neutrons Will Lead to Water?
But the jury is still out on whether the bright spots are in fact water ice, because water isn't the only substance that might collect in cold traps.
"A number of years ago, Ann Sprague [a planetary scientist at the University of Arizona] suggested that another possible compound that might make similar bright spots is sulfur dioxide," NASA Ames's Colaprete told National Geographic News.
The next step is to look for hydrogen on Mercury by examining neutrons—electrically neutral particles—emitted from the planet's surface, the Carnegie Institution's Solomon said.
Neutrons are created when cosmic rays hit Mercury, breaking atoms into their component particles.
Scientists can determine the abundance of hydrogen on Mercury by mapping the relative flux of neutrons at different energy levels. Hydrogen in Mercury's soil would slow down the particles, making them less energetic.
And the most likely source of hydrogen in the solar system is H2O, aka water.
"Exciting Times" on Mercury
Currently scientists are working to calibrate MESSENGER's neutron spectrometer to obtain a good map of neutron flux.
"We promise results within three or four months," Solomon said, although even finding hydrogen wouldn't be definitive proof of water.
Scientists ultimately confirmed water on the moon by crashing a spent rocket cylinder into one of the permanently shadowed craters and watching the resulting plume—an experiment not currently possible on Mercury.
For now, "who knows what will be found on Mercury?" said Peter Schultz, a planetary scientist at Brown University who was involved in the moon-crash experiment.
"With the discovery of water and other volatiles on the moon, the search by MESSENGER for hidden polar ices [on Mercury] surely reflects exciting times."