NASA's Messenger spacecraft has swung around its namesake planet for three years, beaming observations of Mercury back to Earth, but next March it will smash into the cratered surface it has been studying from afar.
The satellite's oblong orbit around the solar system's innermost planet brings it gradually closer and closer as it looks into Mercury's mysterious volcanoes, craters and magnetic field. With dwindling fuel to counteract the dense planet's pull, the scientists managing the mission at Johns Hopkins Applied Physics Laboratory in Laurel can only delay its fall for so long.
It's now orbiting as close as several dozen miles above the planet's gray, dimpled crust — and soon, closer. Data collected in the final months of the decade-long mission to explore the inner solar system could help prove the presence of ice in polar craters and provide more detailed accounts of what volatile elements are contained in lava flows or the mysterious depressions on the planet's surface.
Those "bonus" observations depend on Messenger's instruments holding up to the 800-degree heat radiating from Mercury's surface during the slow-rotating planet's prolonged days. The planet, named for the Roman god of messages, makes one revolution around the sun every 88 Earth days, and yet it rotates so slowly that it takes 176 Earth days for one solar day to pass on Mercury.
As long as the instruments hold up, they could add detail and nuance to data that is expected be used by scientists for decades to come, providing insights into how planetary systems form around stars across the universe.
"Every time we've gone somewhere in the solar system and looked with higher resolution, we've made new discoveries," said Larry Nittler, a scientist at the Carnegie Institution for Science who is deputy principal investigator of the $500 million project's science team.
Since its launch in 2004, Messenger has made plenty of discoveries.
Much of what was known about the planet closest to the sun came from NASA's Mariner 10 mission, which flew past Mercury and Venus in 1974 and 1975. Mariner found early signs of Mercury's iron core, small magnetic field and minimal atmosphere.
Messenger began sending photographs and other observations of the planet in January 2008, eventually cataloging the entire surface for the first time. The closer it got to the planet, the more theories it confirmed and disproved.
When Messenger began an elongated, elliptical orbit around Mercury in 2011, the data was even more dramatic.
Scientists predicted they would find few "volatile" elements — abundant here on Earth but with boiling points too low to be expected to withstand Mercury's heat — in the planet's crust. But they found astonishingly high levels of sodium and potassium, as well as surprisingly low levels of iron. They also found signs of significant volcanic activity, perhaps not as distant in the past as once expected.
They mapped its magnetic field and found it to be asymmetrical, unlike Earth's, with its magnetic equator located about 20 percent of the way toward the pole.
And they believe they have confirmed that some craters at the planet's poles contain ice — the holes are deep enough to remain in permanent shadow.
"In many cases, a lot of our original ideas about Mercury were just plain wrong," Nittler told the Baltimore Sun in June 2011, after just three months of data collection in orbit.
Now scientists hope to prove more theories wrong, or right, as the case may be.
A closer view could prove the presence of ice in the polar craters and provide greater detail of what elements are in different geographic features. Images from the first year of orbit showed, at best, 10 meters per pixel, but are now approaching 2 meters per pixel. Observations of magnetic fields and elemental composition are similarly getting increasingly detailed.
One feature scientists are particularly interested in getting a closer look at is what are known as hollows, irregularly shaped depressions with bright, flat floors and halo-like markings around them. They believe a combination of the planet's heat, its constant bombardment by tiny meteors, and the powerful effects of charged solar particles could be causing materials in surface rocks, likely sulfur or potassium, to sublimate — transforming from solid to gas without first becoming liquid — leaving the curious craters behind.
"We've always had to squint at the images to see the details," said Nancy Chabot, instrument scientist for Messenger's cameras, the Mercury Dual Imaging System. "We're getting new insights into the depths of these features and what the edges look like in more detail."