The solar system's innermost planet, a world known for its torrid temperatures, may conceal water ice inside permanently dark polar craters, and NASA's MESSENGER spacecraft aims to resolve the question as it begins an extended mission at Mercury.

Artist's concept of the MESSENGER spacecraft at Mercury. Credit: NASA
 
A year after it arrived in orbit around Mercury, MESSENGER started the second phase of its science mission March 18, and controllers plan to lower the craft's orbit in April through a series of thruster firings.

The aggressive orbit-lowering maneuvers were not planned during MESSENGER's prime mission because they could empty the spacecraft's oxidizer propellant tank, according to Ralph McNutt, MESSENGER project scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

"It will actually enable us to do more with [measuring] topography, and we'll be able to do more geochemical analysis when we bring it in close," McNutt said.

Goals for the next year include tracing the history of Mercury's geology, estimating when volcanism ceased, reconstructing the planet's recent terrain changes, and probing Mercury's tenuous atmosphere and magnetosphere.

As the solar cycle ramps up, scientists will track the sun's changing effects on Mercury.

"We're in a front-row seat at the 50-yard line with respect to what's going with the sun," McNutt said.

Another central focus of MESSENGER's extended mission will be confirming whether water ice is hidden deep inside Mercury's polar craters, permanently shadowed regions where frigid conditions persist as other regions of the planet are scorched by temperatures up to 800 degrees Fahrenheit.

Repeated observations of Mercury by the Arecibo telescope have revealed bright signatures near the rocky planet's poles, leading scientists to speculate the luminous radar returns may indicate the presence of water ice trapped on cold crater floors.

Craters near the moon's poles also harbor ice. Some polar crater floors never see sunlight.

"Just from the Earth-based radar observations of Mercury, it's always been very, very striking that it looks like water on Mercury, and even stronger so than we see on the moon," said Nancy Chabot, lead scientist for MESSENGER's dual-imaging system, a set of color wide- and narrow-angle cameras.

MESSENGER completed a global map of Mercury in its first year in orbit, filling in gaps unseen during earlier flybys.

Sean Solomon, MESSENGER's principal investigator from the Carnegie Institute of Washington, said the orbiter snapped more than 87,000 images in its first year at Mercury, collecting data equaling 730 flybys.

The probe's camera took an image of Mercury's south pole every other Earth day, tracking the planet's rotation over the course of one Mercury day, equivalent to 176 Earth days - or two Mercury years.

The imagery allowed scientists to piece together what parts of Mercury never receive sunlight - crater floors which are always black in visible images.

An illumination map of Mercury's south pole. Black regions see the least sunlight. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institute of Washington
 
"The shadows change from image to image, and you average them all together and what you're left with is this, which is an illumination map," Chabot said. "It's just an average of how often a surface is illuminated. The important point here is the things that are shown in black never see sunlight. This is fascinating because it's mapping out these areas in permanent shadow by Mercury's south pole."

Researchers compared the Arecibo radar data with MESSENGER's images. They match, Chabot said.

"It's lining up beautifully with these areas of permanent shadow," Chabot said in a March 21 press conference. "This is very, very consistent with the water ice hypothesis. These are cold regions in permanent shadow where water ice can be cold-trapped."

MESSENGER imagery of Mercury's north pole showed similar results. Earth-based observations and the orbiter's camera seem to show the bright radar signatures are all located inside permanently dark craters.

According to Chabot, scientists will continue crunching data from MESSENGER's neutron spectrometer to determine the mineral composition of the dark crater floors. The orbiter's laser altimeter is measuring the topography, or roughness, of the craters and helping researchers develop thermal maps.

Comprehensive observations with MESSENGER's suite of sensors could conclusively determine if the radar-bright material inside Mercury's lightless polar craters is water ice or another type of volatile compound.

"The story is still just starting for MESSENGER, and it will be great to incorporate all of these instruments together," Chabot said. "What is the source of that volatile and what is the identity of the volatile? I don't have the answer yet today, but people are working on that."

"These are really interesting questions about the availability of water in our solar system," Chabot said.