Ceres in Colors

To the naked eye, dwarf planet Ceres looks dark gray all over. However, data from NASA's Dawn spacecraft reveal significant differences in surface composition.

The surface composition of dwarf planet Ceres is far more diverse than can be discerned by the naked eye. The body with a diameter of approximately 950 kilometers must therefore have seen an eventful past. Scientists from the Max Planck Institute for Solar System Research (MPS) in Germany today present these results at the annual meeting of the European Geosciences Union (EGU) in Vienna. They are based on data from the approach phase of NASA's Dawn spacecraft to Ceres. On 6 March, 2015 Dawn reached its destination and became the first spaceship in history to orbit a dwarf planet.

After Dawn’s successful capture into Ceres’ gravity field on 6 March, 2015, the space probe has been approaching its destination so to speak through the back door: since Dawn has been proceeding towards Ceres from the side facing away from the Sun, the dwarf planet was hidden in darkness for several weeks. During this time, the Framing Cameras onboard the spacecraft could therefore not record any new images.

"In the meantime, we have not been idle”, says Andreas Nathues from the MPS, Framing Camera Lead Investigator. "Data from the approach phase to Ceres already contain valuable information that we have evaluated further." The MPS-researchers have especially focused on images obtained with the help of the Framing Camera’s seven color filters. They allow to single out certain wavelength ranges of the light Ceres reflects into space and investigate them individually. In this way, differences in surface composition become visible, that cannot be seen by the naked eye. The scientists represent these differences in false-color maps.

Ceres' surface consists mainly of carbon-rich materials. However, the exact composition varies regionally. This indicates that the body's surface has changed repeatedly over the past 4.6 billion years. "This dwarf planet was not just an inert rock throughout its history. It was active, with processes that resulted in different materials in different regions. We are beginning to capture that diversity in our color images," said Chris Russell, principal investigator for the Dawn mission, based at the University of California, Los Angeles.
Exactly what substances can be found on the surface is still unclear. To find out, the researchers are counting on more highly resolved data that Dawn sends to Earth from the end of April. Then, only 13500 kilometers will separate the spacecraft from the dwarf planet.

"In this early phase of the mission our new images mainly reveal one thing," says Martin Hoffmann from the MPS, who presents the new results in Vienna today. "There is a huge difference to the giant asteroid Vesta." Vesta, Ceres’ smaller sister, has an orbit that lies approximatey 60 million kilometers closer to the Sun and was Dawn's first scientific destination. The spacecraft accompanied the asteroid with a diameter of about 530 kilometers from July 2011 to September 2012. On color maps that were produced according to the same principles as the color image of Ceres shown today, Vesta presents itself as a colorful world with myriads of different surface materials.

"Unlike Ceres, Vesta resembles the four inner planets Mercury, Venus, Earth, and Mars," says Hoffmann. On Vesta's surface primarily various basalts are found, which leave clearly distinguishable fingerprints in visible light. “The carbonaceous compounds which are more common in the outer part of the Solar System show more subtle differences”, he adds. In addition, Vesta's surface displays minerals such as serpentine and olivine, probably the results of impacts.

"Ceres offers surprisingly little evidence of such extraneous materials”, says Hoffmann. There is not yet an explanation for this. After all, Ceres like Vesta was exposed to a constant bombardment of larger and smaller chunks during the past 4.6 billion years. Proofs for this are the numerous craters covering the dwarf planet's surface.

"All in all, Vesta and Ceres are an almost unbelievable piece of luck for us," Nathues concludes. The two inhabitants of the asteroid belt are so close together that they can be reached by a single spaceship within one mission - and yet fundamentally different. Both bodies represent the transition that takes place between the orbits of Mars and Jupiter - from the rocky planets of the inner solar system to the water-rich outer ones. "Dawn gives us the unique opportunity to compare these bodies and thus learn more about the origin and evolution of the solar system," says Nathues.

Dawn's mission to Vesta and Ceres is managed by the Jet Propulsion Laboratory for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK, Inc., of Dulles, Virginia, designed and built the spacecraft. JPL is managed for NASA by the California Institute of Technology in Pasadena. The framing cameras were provided by the Max Planck Institute for Solar System Research, Gottingen, Germany, with significant contributions by the German Aerospace Center (DLR) Institute of Planetary Research, Berlin, and in coordination with the Institute of Computer and Communication Network Engineering, Braunschweig. The framing camera project is funded by the Max Planck Society, DLR, and NASA.