THE WOODLANDS, Texas—Ceres is a cold, dead world today. But it wasn't always so, and as researchers delve deeper into images and data collected by the Dawn spacecraft, they continue to find intriguing hints about a world that likely had a large, subsurface ocean during the early days of the solar system. They also have found a number of features on the surface of Ceres they cannot yet explain, deepening the mystery of the dwarf planet’s evolution.

The largest object in the asteroid belt, measuring some 950km across, Ceres is roughly the size of Texas. So perhaps it's fitting that scientists gave their most detailed briefing yet about the planet in a darkened conference room in The Woodlands, just north of Houston, at the Lunar and Planetary Science Conference on Tuesday. Here the mysterious bright spots of Occator crater and other parts of the small world shone brightly.

These parts also intrigued. As the Dawn spacecraft approached Ceres in 2015, it revealed a large bright spot in Occator crater, but as the spacecraft has flown down to within 400km of the world’s surface and produced a global map at a resolution of 35 meters per pixel, scientists have found many bright spots in the crater likely due to salt deposits. Still, the brightest one at the center remains most interesting.

This bright area measures about 10km across, explained Carol Raymond, the NASA Jet Propulsion Laboratory scientist who is the Dawn mission’s deputy principal investigator. It is brighter than the other spots in the Occator crater by a factor of two. In the center of the spot, a multi-hued dome rises above a pit area, with concentric fractures radiating away from the dome to suggest recent activity. “It appears material has come up from below and been placed in the central pit,” she said. “By what process did the material come to the surface?”

Scientists would like to know. There are other unexplained features as well. Some craters have a blueish tinge, and others have pits that do not appear to have been caused by impacts from the asteroid belt but rather seem to be depressions where water ice has evaporated. One crater, Haulani, has a small mountain range in its center. Flow features, likely from mass wasting, appear at a number of locations on Ceres’ surface. Then there is Ahuna Mons, rising 5km above the surface with nearly flat walls, defying expectations. “Clearly we have a lot of work to do to put together a self consistent story,” Raymond said.

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

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

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

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

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

NASA/JPL-Caltech

NASA/JPL-Caltech

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

NASA/JPL-Caltech

NASA/JPL-Caltech

NASA/JPL-Caltech

NASA/JPL-Caltech

NASA/JPL-Caltech

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

That story?

Before Dawn ever arrived at Ceres, scientists believed the dwarf planet was probably an ocean world during the early solar system, when it was hotter. But unlike existing ocean worlds in the solar system today, such as Europa with its interior tidal heating, Ceres did not have a consistent heat source. So the world likely froze, from the outside in, over time.

In that sense, Dawn is on a detective mission to find out what happened, what conditions might have been like back then, and whether Ceres might ever have been conducive to life beneath its surface. The salt deposits on the surface offer one important clue. They may very well be the remnants of a frozen ocean with the salt left over from brines as the ocean froze out.

But some features on Ceres today, including the very bright spot in Occator crater, indicate that the world remains geologically active. The crater is about 80 million years old, Raymond said, yet the bright material at its heart remains extremely so. It is not easy to keep things that bright on an airless planetary surface for a long period of time.

How to explain the persistence of the bright materials? There must be some process related to the energy and heat from impacts that mobilizes salt hydrates, interior water ice and rock into some type of cryomagma. But as with the stunning features New Horizons has found on Pluto, scientists are only beginning to understand the exotic behavior of ices at very, very cold temperatures.

Neutron detector

Scientists are hoping to get some answers about the possibility of subsurface ice from a gamma ray and neutron detector carried by the Dawn spacecraft. This instrument has only been able to collect data during the last three months as Dawn has reached its lowest altitude orbit, said Thomas Prettyman, a scientist with the Planetary Science Institute who is a Dawn mission co-investigator.

The process works like this: Galactic cosmic rays strike the surface randomly over time, with a few particles pinging each cubic centimeter of the surface each second. These high energy particles will sometimes strike the nucleus of an atom, breaking it apart. Sometimes neutrons from these strikes will then bump into the nucleus of a hydrogen atom. These collisions create characteristic gamma rays, Prettyman said.

Dawn carries a gamma ray detector to study the result of these cosmic rays whacking the surface of Ceres. Using gamma ray spectroscopy, the instrument can begin to determine the composition of the topmost meter of the regolith on the surface of Ceres. “Boy, is it a wonderful data set,” Prettyman said.

Unfortunately, the data analysis is only beginning, and it’s difficult to tease out the chemical composition from neutrons and gamma rays. But so far, the analysis has found considerably fewer neutrons near the poles of Ceres than at the equator, which suggests a much higher concentration of hydrogen closer to the poles. This may well indicate the presence of subsurface ice. Additional data should allow the researchers to fully test the hypothesis that Ceres once held a water ocean.

The spacecraft’s overall health remains good, Raymond said, and it’s operating perfectly with two reaction wheels and its hydrazine jets. Even if the reaction wheels fail, the vehicle can collect all the data scientists want and complete its primary mission by August. If the wheels hold up, the instrument could continue taking data for another year.

And then the scientists will have all of the information they could have hoped for from an orbiting spacecraft. As for the next step, Raymond offered this statement: “It would be nice to land there, wouldn’t it?”