NASA PROBE FINALLY ARRIVES AT AN ICY ALIEN WORLD

This artist's concept shows NASA's Dawn spacecraft arriving at the dwarf planet Ceres, the most massive body in the asteroid belt. Dawn is the first mission to visit a dwarf planet. NASA/JPL-Caltech

LARRY LEBOFSKY HAS always been fascinated with Ceres, the biggest object in the asteroid belt between Mars and Jupiter. Even the license plate on his 2005 Toyota Matrix reads “1CERES.” In the late 1970s, the planetary scientist made some of the first observations that suggested Ceres—which is a dwarf planet, like Pluto—has bits of water ice on its surface. Now, as NASA’s Dawn spacecraft arrives at the planet today, he may finally get proof. “That would be great,” Lebofsky says. “For the last 30 years, there’s never been enough evidence to convince anybody that this is true.”

Dawn’s exploration of Ceres officially began at 5:36 am PST/8:36 am EST this morning, when mission controllers confirmed that the spacecraft entered an orbit around Ceres.1 Over the next several months, Dawn will continue to spiral down within 233 miles of the surface. From its orbital perch, it will study an icy world that could prove to be the best place in the solar system to find extraterrestrial life.

When Lebofsky first looked at Ceres through a 28-inch telescope atop Mt. Lemmon in Arizona, all he saw was a point of light. But as Dawn got closer over the last few weeks, it has revealed Ceres to be a round, cratered world—with two unexpected bright spots that are confounding scientists. “We’ve never seen anything like this on another body,” says Christopher Russell, the Dawn mission’s principal investigator.

One idea that scientists have tossed around is that the luminous spots are cryogenic volcanoes, spewing out water vapor that reflects sunlight. That could mean that liquid water is leaking to the surface—an exciting possibility. Along with elements like carbon and hydrogen present on Ceres (the ingredients necessary to make organic molecules), and enough heat from sunlight and the warm radioactive elements in its core, liquid water on Ceres just might be enough to sustain life. “Basically you equate life as we know it with water,” says Lebofsky, who isn’t part of the Dawn team.

Ceres rotates in this sped-up movie built from images taken by the Dawn probe during its planetary approach.  NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Now, scientists already know that Ceres contains a huge amount of potentially life-sustaining water: more than 43 million cubic miles of it. But most of it is probably frozen. No good for extraterrestrial life.

So scientists are hoping that Dawn will provide evidence of a subsurface liquid ocean on Ceres. So far, the evidence is unclear. Last year the Herschel space telescope detected wisps of water vapor around Ceres, which suggested possible water plumes, but Dawn hasn’t seen any. And another explanation for those mysterious bright spots—some say the simplest and best one—is that they’re reflective patches of ice, exposed when space rocks flew in and chipped a couple of holes in the planet’s surface. That would explain why the spots brighten as the sun rises overhead and dims as the sun sets, Russell says. If those icy spots contain minerals like carbonates and salts, they could indicate the presence of liquid water—but they won’t tell you if that water was melted ice from a space-rock impact or if the water comes from a reservoir below the surface, Russell says.

Depending on what Dawn finds, Ceres could be the most hospitable place in the solar system other than Earth. For example, one clear sign of liquid water would be an ice volcano, a mound of ice created when liquid water leaks onto the surface and freezes. “Ceres is in a fairly benign environment,” Russell says. Unlike Europa, Enceladus, or the other icy moons of Jupiter and Saturn, Ceres isn’t engulfed by a planet’s strong magnetic field, which generates a belt of harmful radiation. Ceres is also much closer to the sun and therefore warmer—and it’s easier to get to from Earth. “Ceres is a very obvious place to send a lander,” Russell says. A lander could find the definitive evidence for a subsurface ocean.

“If you look farther out, it wouldn’t surprise me if Ceres would be the next place after Mars where astronauts might go,” says Andy Rivkin, a planetary scientist at Johns Hopkins University Applied Physics Laboratory. “This is just the first mission to go Ceres. I don’t think it’ll be the last.”

Dawn is now still about 37,000 miles from the surface, hidden behind Ceres opposite the sun. It won’t be sending back any pictures or data until late April, when it reemerges from Ceres’s dark side. When it does, it will start to map the surface, analyze the chemical composition, and make the most accurate measurements yet of the planet’s mass. And, it may prove Lebofsky’s ideas from decades ago correct. “New discoveries may confirm what you may have thought in the past—and that’s great,” Lebofsky says. “But what’s more exciting is discovering things you may not expect.” As with visiting any alien world, that’s a given.

1Updated at 10:48 am EST on 3/6/15: After receiving a signal from the Dawn spacecraft at 8:36 am EST, mission controllers confirmed that the Dawn spacecraft entered orbit around Ceres at 7:39 am EST.