Updated 4:30 p.m. EDT April 30.
Mercury got a new crater Thursday, carved when NASA’s MESSENGER spacecraft lost its battle with gravity and plunged to the planet’s surface at more than 8,750 miles per hour (14,000 kilometers per hour).
The crash happened at 3:26 p.m. EDT, after MESSENGER zoomed behind the solar system’s smallest planet. This time, it didn't reappear on the other side, as it has done 4,104 times before.
After four years of exploring, mapping, and studying the dense little world from orbit, the spacecraft ran out of hydrazine fuel in early April. For the last few weeks, mission engineers boosted MESSENGER’s orbit using helium gas, allowing the craft to fly almost impossibly close to Mercury’s surface and gather a trove of high-resolution data.
It’s a demise that has been foretold for years, but that doesn’t make the mission’s end any less poignant.
“The MESSENGER team has come to regard our resilient spacecraft as a beloved member of our extended family,” said mission principal investigator Sean Solomon of Columbia University’s Lamont-Doherty Earth Observatory. “I'm sure [it] will feel as though we've lost a favorite relative.”
Yet MESSENGER can rest easily and well: The spacecraft’s surveillance of Mercury has been extraordinarily successful.
Surprising, Shrinking Planet
MESSENGER launched in 2004 and pulled into orbit around the the first rock from the sun in 2011. The 1,100-pound spacecraft quickly showed that dense little Mercury isn’t just another boring, roasted sphere. From its cratered surface to its heavy iron heart, Mercury has proven to be a surprisingly active, perplexing world.
Among MESSENGER’s top ten science discoveries—presented by Solomon earlier this month—are volcanic lava flows on Mercury’s surface, the planet’s bizarre halo of energetic electrons, and the fact that the already small world is shrinking.
Topping the list is Mercury's surprising array of volatile elements (such as potassium, sulfur, and sodium), which would normally be expected to evaporate away on a world where temperatures can reach 800 degrees Fahrenheit. The unexpected composition has forced scientists to reconsider how the planet formed, Solomon says.
And curiously, water ice is tucked into permanently shadowed craters dotting the planet’s poles. Perhaps even more strange, covering that water ice is a dark, tar-like substance that Solomon and others suspect is a mix of organic compounds. Together, the ice and organics suggest that Mercury bore witness to the events that brought life-giving materials from the outer solar system to the inner planets.
“I think of Mercury as a place to study all of the processes that lead to astrobiology and habitability,” Solomon said.
Hollows and a Heavy Heart
Among MESSENGER’s other finds are Mercury’s hollows: strange, pitted depressions that frequently show up in or near craters, and appear to be both very young and very bright. “The hollows are one of the most viscerally interesting discoveries from the mission,” says MESSENGER scientist Steve Hauck of Case Western Reserve University. “They were completely unexpected—a new landform—and one that appears to form by loss of rock to space.”
Mercury's enigmas are more than just skin deep. Beneath its cratered, strangely pitted crust lies an enormous iron core—a hunk of metal comprising more than 65 percent of the planet’s mass (Earth’s core, in contrast, is a piddly 32 percent). The metal gives rise to one of the most perplexing magnetic fields in the solar system, Hauck says.
Like Earth’s, Mercury’s magnetic field is generated by a whirling dynamo inside the molten core; but Mercury’s magnetic field is weak and tilted by about 20 degrees. As a result, the planet’s magnetic and geographic poles are nowhere near the same place. It’s a configuration that scientists are still trying to explain, and will continue searching through MESSENGER's data for clues. (Watch Slooh's live coverage of the crash, starting at 3 p.m. ET Thursday. Story continues below video.)
Long Live the Messenger
Even as it crashes, MESSENGER isn't done yet. Indeed, because of the radio transmission time between Earth and Mercury, the spacecraft’s last set of data will still be heading home at the moment of impact, Solomon says. And there are all those observations, made during the spacecraft’s last, low-altitude orbits, to go through.
Its death, too, will create a new beginning, as the 52-foot-wide crater that MESSENGER leaves will be a crucial source of science for ESA’s BepiColombo spacecraft, set to arrive at Mercury in 2024. Because scientists know when and where MESSENGER made the crater, they’ll be able to determine how quickly things like space weathering and micrometeorites sculpt the planet’s surface.
Then, for millions of years, that crater will mark the final resting place of MESSENGER, a stalwart spacecraft that briefly lived around a fantastically odd planet.
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