Carol Raymond/JPL

Thomas Prettyman/Planetary Science Institute/JPL

Thomas Prettyman/Planetary Science Institute/JPL

Thomas Prettyman/Planetary Science Institute/JPL

Thomas Prettyman/Planetary Science Institute/JPL

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Norbert Schorghofer/University of Hawaii/JPL

Norbert Schorghofer/University of Hawaii/JPL

Carol Raymond/JPL

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/S.Brunner

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/S.Brunner

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/S.Brunner

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/S.Brunner

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/S.Brunner

Ceres may be a cold, dark, and dead world today, but scientists poring over a trove of data returned by the Dawn spacecraft have found that has not always been the case. Rather, new discoveries of ice on the surface of Ceres and other emerging clues have led planetary scientists to conclude the dwarf planet once had an inner ocean, and perhaps it even harbored life. They discussed their findings Thursday at the annual American Geophysical Union meeting during a news conference.

Ceres is the largest object in the asteroid belt. Since Dawn reached it in early 2015, the spacecraft has returned 54,000 images, 16 million visible spectra, and 21 million infrared spectra. It mapped out the dwarf planet’s gravity field in great deal. Additionally, Dawn carries a detector to study the collision of neutrons with the surface of Ceres. Based upon the energy of gamma rays produced by such collisions, the spacecraft can detect various elements at the surface and to a depth of about one meter. In the last two years Dawn has found a lot of hydrogen.

An ocean?

Scientists have good reason to believe that, in addition to hydrates such as OH, much of the hydrogen near the surface of Ceres exists in the form of water ice. This is partly because concentrations of hydrogen, by weight, are double at the poles of Ceres compared to the equator. This fits within a model that suggests that, over time, water molecules on the surface of Ceres would bounce around. During this time, they would eventually sublimate in warmer areas or get trapped in permanently shadowed craters at the poles of the world.

When Dawn flew over the poles, it captured images and other data to find bright deposits consisting of water ice in 10 craters. Ice can persist in such craters on an airless world over billions of years because temperatures are very cold, about 110 Kelvin. Dawn also found ice at the edge of a shadowed crater in the bright, small Oxo crater near Ceres’ north pole. Carol Raymond, deputy principal investigator of the Dawn mission, said this unstable ice was likely recently exposed by a landslide in the crater wall.

Some of Ceres’ surface ice probably comes from asteroid and comet impacts, but much of it has probably come from within the dwarf planet. Dawn scientists said all of the new evidence points convincingly toward the existence of an interior ocean in the dwarf planet’s distant past. “We can only investigate for the fingerprints on the surface, but we have ample evidence to say the presence of a subsurface ocean was likely,” Raymond said. “I think the data is pointing toward Ceres being an interesting object equivalent to Europa and Enceladus in terms of its potential habitability.”

Making an ocean in the asteroid belt

NASA sent Dawn to Ceres in order to better understand conditions in the primordial Solar System, as the planets formed along with the asteroid belt. At this early stage, as matter in the churning Solar System began to coalesce due to gravitational forces, scientists say that chunks of ice and rock-forming minerals called silicates would have been accreting to form Ceres.

As Ceres added mass, radiation would have warmed the interior of the world and melted the ice. At some point within the first 1 to 2 billion years of the Solar System’s formation, scientists expect that Ceres therefore had an interior ocean, perhaps a rather large one. Whether this was some sort of very salty, briny layer of water or a more familiar ocean like those on Earth, and whether its pH level and chemistry would have been conducive to life, remains a subject for further study.

Alas, as the dwarf world drifted far from the Sun and, without an atmosphere, cooled, this ocean would have slowly frozen out. Today, some liquid water likely remains inside, although it is very briny, Raymond said, with a much lower freezing point than less salty water. It's likely this briny water is a source of material for Ahuna Mons, the tallest mountain on Ceres at 4km high and a suspected cryovolcano.

Dawn has two working reaction wheels and enough hydrazine fuel to power them, such that the spacecraft should be able to continue operations through 2017. Perhaps more answers about this intriguing world will be found in subsequent data.

Listing image by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA