LOS ANGELES - Despite its proximity to the sun, portions of Mercury appear to be covered in ice, scientists said Thursday after analyzing 20,000 new images of the solar system's smallest planet.
The pictures beamed to Earth by the Messenger spacecraft strongly suggest that frozen water - and perhaps other frozen substances - coat portions of impact craters near the planet's north and south poles. Permanently enshrouded in shadow, these surfaces are typically a chilly 300 degrees below zero Fahrenheit.
"One of the great ironies is that Mercury may have more ice at its poles than even our own moon," Jim Green, director of NASA's planetary science division, said during a news conference.
Once considered "the burned-out cinder of the solar system," as Green put it, the pictures taken by Messenger reveal that the planet closest to the sun is a world unlike any other.
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The spacecraft, which entered Mercury's orbit in March, is providing panoramic views of expansive, smooth volcanic plains that cover an area half the size of the continental U.S. Scientists can see, in great detail, the faults formed when pieces of Mercury's crust were pressed together and numerous impact craters that have been covered by lava flows and are now ghost craters.
They are also getting a close-up view of the dark area around the central peak of Degas Crater in the northern hemisphere and the scarlike remnants of the planet's pyroclastic flows, once fast-moving rivers of sizzling gas and boiling rock.
The pictures were taken with Messenger's Mercury Dual Imaging System, which has narrow- and wide-angle cameras that use a technology similar to that found in digital cameras. The craft also has instruments to collect data on the planet's chemical composition, topography and magnetic field.
Mercury is the densest and smallest of the solar system's eight planets. It has the oldest surface and most extreme daily temperature fluctuations. Because of the intense heat associated with its proximity to the sun, Mercury is also the least explored.
The Messenger mission, conceived 40 years ago and launched in 2004, is supposed to unravel some of the central mysteries surrounding the innermost planet: How did it evolve? Why is it so dense? Why does it have a magnetic field, when larger planets like Mars do not?
Data collected by Messenger show that Mercury's magnetic field is not simply a miniature version of the one on Earth, as scientists had thought. Instead of having a magnetic field equator that is the same as its geographical equator, as Earth does, the one on Mercury is shifted toward its north pole. That may open up highways for charged particles to travel from space to the tiny planet. As these particles crash into Mercury, material from its upper crust is kicked off, contributing to the formation of its atmosphere and to changes in the planet's chemistry.
A closer look at the planet's surface also suggests that the original building blocks that formed Mercury were different from those that gave rise to our own planet. For instance, it has 10 times as much sulfur as is found in the crust of the Earth or the moon - a byproduct, perhaps, of volcanic gasses and a potential clue about the history of volcanism on Mercury.
Messenger orbits Mercury twice in one Earth day and has completed about one-quarter of its mission. It will run out of fuel in about nine months and eventually crash into the planet.
