PROVIDENCE, R.I. [Brown University] — With its scientific mission complete and the last of its fuel spent, NASA’s MESSENGER spacecraft crash-landed today on the surface of Mercury, a planet we now know much more about thanks to MESSENGER’s four years of orbital investigation.
“It was a phenomenally successful mission by any measure,” said Jim Head, the Louis and Elizabeth Scherck Distinguished Professor of the Geological Sciences in Brown’s Department of Earth, Environmental and Planetary Sciences and a member of the MESSENGER science team. “What we learned from MESSENGER has completely changed our view of this enigmatic planet.”
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) was launched in 2004 and entered Mercury’s orbit in 2011 after flying by Mercury three times. The original mission called for one year in orbit, but MESSENGER far outlived that plan. It continued transmitting scientific data until just a few days ago.
Head and his students played key roles in examining the data returned from the spacecraft, and their work turned up more than a few surprises about our innermost planet. Among those surprises was Mercury’s active volcanic past. In 2011, Head and his colleagues confirmed that the planet’s smooth northern plains were formed by an epic outpouring of volcanic lava — enough to bury Texas four miles deep.
And while the lava that resurfaced the smooth plains likely came from a relatively fluid flow, MESSENGER also revealed that Mercury was once home to violent volcanic explosions. Head and his colleagues saw the first evidence of explosive volcanism in 2008, during one of MESSENGER’s pre-orbit fly-bys. In 2014, the team used the more detailed orbital data to show that volcanic explosions rattled Mercury for much of its early history.
“The presence of explosive volcanism tells us something really interesting about Mercury’s interior,” Head said.
Volcanic explosions are associated with the presence of volatiles — compounds with low boiling points that expand in volume causing violent bursts. It had long been assumed that as Mercury formed so close to the sun, most of its volatiles would have burned off. But a history of explosive volcanoes, combined with MESSENGER’s detection of volatile sulfur, potassium, and sodium on Mercury’s surface, helped paint a completely new picture of Mercury’s formation.
But those weren’t Mercury’s only surprises. During its time in orbit, MESSENGER also revealed water ice hiding in permanently shaded crater floors at Mercury’s poles. It was a remarkable find, considering temperatures on Mercury’s surface can reach 800 degrees Fahrenheit. “Not only that,” said Head, “the ice appears to be covered with some type of organic material, perhaps related to the building blocks of life.”
MESSENGER also showed that the solar system’s smallest planet is getting smaller — shriveling due to cooling and changes in its unusual interior. Mercury has a huge iron core, much greater than half its diameter. Mapping and dating Mercury’s mountain-like “shrinkage scarps” provides clues to the amount and timing of these changes in its interior.
Mission managers did all they could in MESSENGER’s final days to keep the mission alive. Last month, engineers used the helium that normally pressurizes the craft’s empty propellant tanks to give MESSENGER one last orbital boost. The move added a few precious days to a mission that had already far outlived expectations.
“It was truly a heroic effort on the part of the engineers to extend the mission,” Head said. “We were able to get data during the orbital decay that will prove absolutely invaluable.”
Head says the body of data produced by MESSENGER will continue to inform scientist’s evolving understand of Mercury for years so come.
“Mercury is one of a set of Earth-like planets in the inner solar system, which also includes Venus, Earth, and Mars,” Head said. “MESSENGER has provided unprecedented insights into the early history of these Earth-like planets, a part of the history of Earth no longer preserved on our own home planet. Slowly our formative years are coming into focus thanks to this historic mission.”