NASA's Messenger spacecraft has found strong evidence for vast ice deposits in ultra-cold, permanently shadowed craters near the poles of hellish Mercury, the solar system's innermost planet, scientists said Thursday.

View of Mercury's north polar region with the radar-bright regions shown in yellow. Scientists have found compelling evidence the radar-bright regions are water ice. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
 
The results of observations carried out over the past year and a half indicate between 100 billion and one trillion metric tons of ice are present on Mercury, delivered by impacting comets and asteroids falling into the inner solar system from its outermost regions.

"We can ask the question, how much ice is there? And for this, we can combine both north and south poles because the situation in the south is very similar to the north," said David Lawrence, a Messenger scientist at the Johns Hopkins University Applied Physics Laboratory.

"Well it turns out, if you add it all up, you have on the order of a hundred billion to one trillion metric tons of ice. The uncertainty on that number is just how deep it goes. We think it's at least 50 centimeters deep, it could be as deep as 20 meters."

Translating those numbers into slightly more understandable terms, Lawrence said enough ice is present on Mercury to bury Washington, DC under a column of frozen water two to two-and-a-half-miles high.

Messenger is the only spacecraft ever to orbit Mercury. The $427 million mission was launched in 2004, dropping into the inner solar system for three Mercury flybys over six-and-a-half years, using the planet's gravity to help it slow down enough to brake into orbit in March 2011.

Since then, the spacecraft has been collecting data around the clock to help scientists understand how Mercury, believed to be 60 percent iron, ended up with an oversize core, a thin shell of a crust and the highest density in the solar system.

Other objectives include learning what materials are present in the crust, what powers the planet's magnetic field and how that field interacts with the solar wind and Mercury's tenuous, ultra-thin atmosphere.

A long-standing question centered on whether permanently shadowed craters near Mercury's poles, where temperatures are as low as minus 350 degree Fahrenheit, might harbor ice deposits.

A high-latitude impact crater illuminated by the angled rays of the Sun creates a region of very warm temperatures on the illuminated rim, lower temperatures on the illuminated floor of the crater, and extremely cold temperatures in regions of permanent shadow, where water ice is stable. Credit: NASA/UCLA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
 
Observations by the Arecibo radio telescope in Puerto Rico in 1991 revealed radar-bright areas near Mercury's poles that matched up with craters photographed by NASA's passing Mariner 10 spacecraft in the 1970s.

"One of the major objectives of the Messenger mission when we were selected and launched was to test the idea, more than 20 years old, that the polar deposits on Mercury, discovered by Earth-based radar, consist of dominantly of water ice," said Principal Investigator Sean Solomon at Columbia University's Lamont-Doherty Earth Observatory.

To find out, Messenger carried out three sets of observations.

A gamma-ray and neutron spectrometer measured the abundance of neutrons blasted away from the surface by high-energy cosmic rays. Over the poles, the neutron "flux" dropped off in a manner consistent with interactions with hydrogen atoms in water ice.

Messenger also measured infrared reflectivity over the poles, which again was consistent with the presence of ice, as were the surface and near-surface temperatures, measured with unprecedented accuracy.

"We subjected that hypothesis to three very stringent tests," Solomon said. "Does it have the neutron spectrometry signal that you would expect for water ice? Yes it does. Does it have the near infrared reflectance that you would expect for water ice? Yes, it does. And finally, does it match the very detailed thermal models that we can now construct? And the answer is yes, it does."

Solomon said no other known material "matches the radar, the neutron reflectance and the thermal characteristics that we have documented with the Messenger spacecraft."

Artist's concept of the Messenger spacecraft at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
 
Scientists believe water ice and other materials are constantly delivered to the inner solar system by comets and asteroids that occasionally collide, depositing raw materials that, in Mercury's case, can get trapped in the ultra-cold polar craters.

"Messenger has revealed a very important chapter in the story of how water ice and other volatile materials have been delivered to the inner planets, including Mercury, we think by the impact of comets over time and volatile-rich asteroids," Solomon said.

"It's extraordinary that this chapter is so well preserved on the planet closest to the sun."

Orbiting the sun at a distance of just 36 million miles, Mercury's temperature extremes are unmatched in the solar system, ranging from a broiling 840 degrees to 350 degrees below zero in craters that are never exposed to sunlight.

The Messenger findings were published in Thursday's edition of Science Express.