Ceres’ interior revealed by gravity data
NASA’s Dawn spacecraft does not have the necessary equipment capable of studying Ceres’ interior, but remote measurements of subtle changes in the impact of the dwarf planet’s gravity on the spacecraft have revealed its internal structure.
Mission scientists measure the effect of Ceres’ gravity on Dawn by sending radio signals to the spacecraft, which then returns those signals to Earth.
The signals are received via a global network of large antennas known as the Deep Space Network (DSN), used by NASA to communicate with spacecraft exploring the Solar System.
They allow scientists to obtain extremely precise measurements of Dawn‘s speed, up to a minute 0.004 inches (0.1 millimeters) per second.
With this gravity data, scientists have created a detailed map of Ceres’ gravity field that encompasses the smallest variations.
This map provides crucial information about the dwarf planet’s internal structure, data that scientists can use to begin putting together a geological and thermal history of the small world.
Dawn scientists learned that Ceres is less dense than other rocky worlds in the Solar System and that it is geologically differentiated into layers comprising a core, mantle, and crust.
They also confirmed Ceres is in hydrostatic equilibrium, meaning its gravity squeezes it into a round or nearly round shape. This was determined via comparisons between its form and its gravity field.
Less dense than the Earth, the Moon, or protoplanet Vesta, which Dawn initially orbited, Ceres has long been suspected of harboring water ice and other low-density materials.
At some point, likely early in Ceres’ history, the water ice and other light materials separated from the rock, with the heavier rock sinking to the core and the lighter materials rising to the crust’s outer layers.
“We have found that the divisions between different layers are less pronounced inside Ceres than the Moon and other planets in our solar system,” noted Ryan Park, supervisor of the Solar System dynamics group at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “Earth, with its metallic core, semi-fluid mantle, and outer crust, has a more clearly defined structure than Ceres. The new data suggest that Ceres has a weak interior, and that water and other light materials partially separated from rock during a heating phase early in its history.”
Understanding the structure of Ceres’ density could be the key to understanding and reconstructing its history.
The new findings confirm that history involves interactions between water and rock in Ceres’ interior, said Dawn deputy principal investigator Carol Raymond, also of JPL.
The new data also showed that mass in Ceres’ interior is displaced by regions of high elevation on the dwarf planet’s surface, much like water is displaced by a floating boat.
Mountains and geological features at high elevations put pressure on Ceres’ weak mantle almost as if the former are floating on top of the latter, a phenomenon seen on other solar system worlds but never before confirmed on Ceres.
Water was fluid in Ceres’ interior during the period early in the dwarf planet’s history, the new data confirms.
While the interior of early Ceres was hot, its temperatures never reached the point at which silicates melt, which would have formed a metallic core.
The study’s findings have been published in the journal Nature, with Park as lead author and Raymond as a co-author.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.