NASA’s Messenger spacecraft closed out a successful four-year tour in Mercury’s orbit Thursday with a cataclysmic crash into the scorching planet after consuming its last gasps of fuel.
The $446 million robotic science mission was the first probe to ever enter orbit around the closest planet to the sun, where it mapped Mercury’s tortured landscape, confirmed the existence of water ice buried inside permanently dark polar craters, and found a world far more dynamic than researchers predicted.
The orbiter launched from Cape Canaveral aboard a Delta 2 rocket in August 2004 and returned to Earth a year later to reshape its trajectory toward Mercury. Two fleeting flybys of Venus and three encounters with Mercury set up for Messenger’s final approach to the innermost planet in March 2011.
Messenger was fitted with a sunshield to weather temperatures up to 600 degrees Fahrenheit during the trek toward the sun.
Designed for a one-year mission, Messenger lasted four years and spent the last 12 months gathering unprecedented data on Mercury from lower altitudes than any mission before it.
Messenger ran out of hydrazine fuel April 6, and it used leftover helium pressurant in a last-ditch effort to counteract the pull of gravity and extend the mission a few extra weeks.
Ground controllers at the Johns Hopkins University Applied Physics Laboratory in Maryland tracked Messenger’s final days, predicting the space probe would strike Mercury around 3:26 p.m. EDT (1926 GMT) Thursday.
Mission managers expected Messenger to impact Mercury at a velocity of 8,750 mph on the planet’s northern hemisphere, out of view of Earth. Messenger’s operations team confirmed the probe’s crash a few minutes later when they did not receive a signal from the spacecraft at the time it would have emerged back in range of ground-based communications antennas.
Scientists said the high-speed impact would carve fresh crater 50 feet wide on Mercury’s surface, and the BepiColombo spacecraft built in Europe and Japan will look for Messenger’s impact scar after its scheduled arrival at Mercury in 2024.
Messenger collected science data up to its last day, with the final downlink of stored observations and images streaming back to Earth on Thursday morning.
“Going out with a bang as it impacts the surface of Mercury, we are celebrating Messenger as more than a successful mission,” said John Grunsfeld, head of NASA’s science directorate. “The Messenger mission will continue to provide scientists with a bonanza of new results as we begin the next phase of this mission — analyzing the exciting data already in the archives, and unravelling the mysteries of Mercury.”
Messenger’s suite of science instruments gathered 10 terabytes of data on Mercury over its four-year mission, completing 4,105 laps of the planet.
“The spacecraft and the instruments have worked virtually flawlessly over those four years,” said Jim Green, director of NASA’s planetary science division. “The data is on Earth, and we have it now. We’re going to continue to make wonderful discoveries with it.”
Before Messenger’s mission, less than half of Mercury was mapped from images during flybys in the 1970s by NASA’s Mariner 10 spacecraft.
“I think it’s good to put this into perspective,” said Nancy Chabot, lead scientist for Messenger’s dual camera system at the Applied Physics Laboratory. “Before this mission, we had seen 45 percent of the planet. I had a globe, and more than half of it was blank. It’s just been huge, changing our understanding of the planet as a whole.”
Images of fault scarps on Mercury taken by Messenger indicate the planet is contracting — a sign that its internal core is still cooling — and measurements show Mercury’s magnetic field is surprisingly offset from the planet’s center, said Sean Solomon, Messenger’s principal investigator from Columbia University’s Lamont-Doherty Earth Observatory.
Messenger also revealed a tenuous atmosphere around Mercury extending into a comet-like tail behind the planet as molecules are blasted away by intense solar radiation. The mission monitored seasonal changes in the atmosphere as Mercury sped around the sun in its 88-day orbit.
Mercury’s reputation as a hellish world is well-deserved — temperatures can get up to 800 degrees Fahrenheit during daytime — but isolated pockets of the planet never see the sun.
Such regions at the bottoms of Mercury’s polar craters are cold enough to support deposits of water ice, and Messenger’s science team verified the ice’s presence at or just below Mercury’s surface after scanning the material with cameras, a laser altimeter and a neutron spectrometer.
Earth-based radars hinted at the ice on Mercury, but evidence from Mercury gave researchers confidence the ice slabs are there.
“Perhaps the most interesting discovery might be the polar deposits,” Solomon said.
The last leg of Messenger’s mission at low altitude gave scientists a chance to see into the shaded bottoms of the craters in more detail, according to Chabot.
“We’re actually seeing into these regions where the sun never shines on Mercury in higher resolution than has ever been possible before,” Chabot said in March.
Snapping long exposures and using sunlight scattered off crater walls, high-contrast images from Messenger’s two-eyed camera exposed sharp features that appear to be soils laid on top of ice.
“Most of these deposits don’t consist of water ice directly at the surface, but rather water ice covered by a dark layer which we think is 20 to 30 centimeters (8 to 12 inches) thick,” Solomon said. “And that dark layer is darker than anything else on Mercury.”
The stuff preserved on Mercury’s hidden crater floors could the same type of mixture of water and organic compounds that ripened into life on Earth.
“The team has put forward the hypothesis that this dark material is, in fact, organic carbonaceous material delivered to Mercury by the same objects that brought the water ice stored in these deep freezes at the north and south poles, giving us a record in these poles of the delivery process from the outer solar system not only of water ice, but of what — on our planet — were once the building blocks of organic chemistry and life,” Solomon said.
Mercury has a larger iron core relative to its overall size than any other planet in the solar system, so scientists assumed its other elements would be blown away by intense heat from the sun.
Messenger found a world rich in lighter elements like sodium, sulfur, potassium and chlorine. Despite Mercury’s iron interior, scientists say Messenger saw relatively little of the element in the planet’s crust.
“So the ideas for how the inner planets got assembled, and how the building blocks of planetary materials were delivered to the inner solar system and survived the process of plaentary accretion, are all being changed by Messenger’s results,” Solomon said.
Larry Nittler, the No. 2 scientist working on the Messenger mission, said the probe’s data archive will be tapped to help refine models on how the solar system’s rocky planets formed.
“One of the big science questions that Messenger set out to answer was why Mercury has this large core and this thin silicate shell, and I would argue we still don’t know the answer to that question a whole lot better than we knew before we went into orbit,” Nittler said. “Part of this is not due to a lack of data, but a lack of quantitative understanding of the different possibilities that could lead to this.
“For example, a very popular model for a long time was the idea that Mercury might have formed much larger and had a giant impact that stripped off much of its pre-existing mantle and crust,” Nittler said. “Then it formed a new crust on its small remaining mantle.
“I think a lot of the questions that were raised — and are still raised — are possibly still answerable with the data we obtained,” Nittler said. “That’s the big one that I really hope we eventually answer.”
Solomon singled out Mercury’s ice deposits as the most interesting discovery by Messenger’s mission, noting the finding’s implications for life.
“I don’t think anybody would count Mercury as habitable in the sense that there’s no part of the surface or the near-subsurface where water can be stable in a liquid form,” Solomon said.
But Mercury has a story to tell in the pursuit of life’s origins, he added, because like Earth, it was bombarded by organic-rich asteroids and comets that left behind water and the chemical constituents of organisms.
“Even a planet that is not in the nominal habitable zone of our own star is a witness to the delivery of the ingredients of habitability from the outer solar system to the inner solar system, including the ice and including the organic building blocks that are capable — on other bodies — of leading to pre-biotic organic chemistry,” Solomon said.
“In terms of understanding what leads to habitability, Mercury has to be included.”
Follow Stephen Clark on Twitter: @StephenClark1.
