The dwarf planet Ceres is now unwrapped.
Discovered in 1801, Ceres was a cosmic conundrum for centuries. “A faint smidge of light amidst the stars,” says Marc Rayman of the Jet Propulsion Laboratory in Pasadena, California. “Not a lot was known about it.”
No more. NASA’s Dawn spacecraft—looking like “a big celestial dragonfly,” says Rayman—is approaching a third anniversary orbiting Ceres. Over those 1,000 days, scientists have learned more, immeasurably more, than anything known from the previous 214 years.
The uncloaked Ceres—55,000 NASA photographs later, and now with a fully-mapped surface—presents curious bright spots, a colossal inventory of ice, and an ancient cryovolcano with a muddy magma of water, salt, and rock.
“An exotic alien landscape,” says Rayman, the project’s chief engineer and mission director. “Dawn has provided us with a richly-detailed portrait of a unique world.”
Ceres, almost 600 miles across, and about the size of Texas, is the largest object in the main asteroid belt, between Mars and Jupiter.
Historically, it’s celestially ambiguous; pigeonholing Ceres has never been simple.
The astronomer Giuseppe Piazzi, who first spotted it, called Ceres a comet. Later, astronomers considered it a planet, then an asteroid. They reclassified Ceres again in 2006, this time as a dwarf planet.
Dawn launched the next year—powered by ion propulsion, noted for its cool blue emissions—and with “ten times the efficiency of conventional chemical propulsion,” Rayman says. “It’s like your car getting 300 miles to the gallon.”
The journey to Ceres, seven-and-a-half years, included a 14 month sidetrip to Vesta, the brightest asteroid in the sky and also the biggest (more than 300 miles across).
“We took 31,000 pictures of Vesta,” says Rayman.
As Dawn approached Ceres, scientists saw a multitude of bright spots, four times brighter than the rest of the surface, all within gigantic Occator Crater, 57 miles across.
“Salt flats,” says Rayman, the residue from salty underground water breaking the surface: “The water molecules dissipated, but the salt dissolved in them was left behind.”
Indeed, a vast amount of salt water, nearly all frozen, is under the surface of Ceres, with perhaps “some pockets or pools of liquid water,” Rayman says.
But the biggest surprise was the discovery of organic materials, often called the “building blocks” for life, scattered across the landscape, particularly in and around Ernutet Crater.
Still, Rayman is skeptical that life exists on Ceres—even primitive microbial life.
“It’s possible, but highly unlikely,” he says.
Daytime surface temperature on Ceres is minus 100 degrees Fahrenheit; any atmosphere is transient. “Maybe there’s an occasional, exceedingly thin veil of water vapor,” says Rayman, but even that “is more tenuous than the atmosphere above the International Space Station.”
Next for Dawn—NASA just extended the mission—is a plan to augment its already-extensive photo collection.
“We’re interested in flying down to a lower altitude than we’ve ever been before,” says Rayman. “We’ll get even better pictures.” Ceres, once considered one of the solar system's drab dead rocks, is finally ready for its close-up.
