MESSENGER's about to crash into Mercury, and the last views from its death spiral are amazing

The MESSENGER spacecraft is getting its last look at Mercury before fuel runs out and gravity takes over. This recently released 41-image mosaic shows Duccio Crater bisected by a tectonic feature called Carnegie Rupes, which formed when Mercury shrunk as its interior cooled. // NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Close-up views of Mercury’s mysterious “hollows” reveal a lack of craters, implying the features are far younger than the rest of the planet’s surface. Scientists suspect something in the rock is sublimating. // NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Long flute-like gullies line the steep walls of a volcanic vent on Mercury in this high-resolution image from NASA’s MESSENGER spacecraft. // NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

"The spacecraft is finally running out of fuel, and at this point it's just sort of skimming the planet's surface," says William McClintock, principal investigator of the Mercury Atmospheric and Surface Composition Spectrometer.

Among the close-ups are scenes of ice deposits in crater shadows, linear fault scarps formed as the planet shrinks, and an abundance of strange depressions that pockmark the surface across anywhere from a few dozen feet to several miles. (For a full gallery, visit the latest series on the MESSENGER website.)

“These features, given the name ‘hollows,’ were a major surprise, because while we had been thinking of Mercury as a relic — a planet that wasn’t really changing anymore — hollows appear to be younger than the planet’s freshest impact craters. This finding suggests that Mercury is a planet whose surface is still evolving,” says MESSENGER scientist David Blewett. The team suspects these hollows form as something in the rock sublimates, which typically happens when a substance changes from an ice to a gas without melting into a liquid.

When MESSENGER arrived at our solar system’s inner frontier in 2011, much of Mercury was still a mystery because spacecraft had imaged less than half its surface. Astronomers thought the planet would resemble Earth's cold, dead Moon. Scientists hoped to find out how the planet formed and explain why it has a magnetic field when Mars and Venus do not.

Thanks to more than four years and 4,000 orbits, scientists have mapped Mercury’s entire surface and gathered more than a quarter-million images. X-ray and gamma-ray spectrometers on board allowed for the first global geochemical maps of surface composition, deciphering the planet’s history of impacts and volcanism. And MESSENGER’s instruments watched Mercury’s diminutive magnetic field grow and shrink seasonally in response to the active Sun. Another surprising find is that despite 800° F (425° C) temperatures, the planet may hold as much as 1 million metric tons of ice in its crater shadows. Recent research also shows that the world was painted black by carbon from comets.

"A lot of people didn't give this spacecraft much of a chance of even getting to Mercury, let alone going into orbit and then gathering data for four years instead of the original scheduled one-year mission." says McClintock. "In the end, most of what we considered to be gospel about Mercury turned out to be a little different than we thought."

One big question still remains: How did Mercury get its large iron core?

Mission managers initially expected their spacecraft to make its final plunge at the onset of spring. But in March, mission managers at Johns Hopkins University Applied Physics Laboratory managed to extend that demise by a month, pushing MESSENGER’s D-Day out to the end of April. Astronomers hope that analyzing those last few weeks of high-resolution images will uncover new secrets as to how the innermost planet formed and evolved.

"Before long it's going to be in pieces scattered across the surface of Mercury. But I don't think anyone who has worked on the project will ever forget it," says McClintock. "It has been an extremely exciting mission, and a once-in-a-lifetime experience."

Last week, mission managers knocked off their fifth orbital correction maneuver to boost MESSENGER's orbit using propellant not designed to keep the spacecraft afloat. But the last of these maneuvers happens on Friday. 

"Following this last maneuver, we will finally declare MESSENGER out of propellant, as this maneuver will deplete nearly all of our remaining helium gas," says MESSENGER Mission Systems Engineer Daniel O'Shaughnessy said. "At that point, the spacecraft will no longer be capable of fighting the downward push of the Sun's gravity. After studying the planet intently for more than four years, MESSENGER's final act will be to leave an indelible mark on Mercury, as the spacecraft heads down to an inevitable surface impact."

Eric Betz is associate editor of Astronomy magazine. Follow him on Twitter: @EricBetz.