Careful tracking of NASA's Dawn spacecraft reveals that Ceres has a rocky core and a thick ice-and-salt crust. Maybe a briny ocean still lies buried inside.

All of the interplanetary space missions over the past half century can be grouped into two general types: those that provided an initial reconnaissance of an object (the Voyager flybys of the outer planets, for example) and in-depth study (the Galileo and Cassini orbiters that followed).

Dawn spacecraft around Vesta
A portrayal of Dawn in orbit around the giant asteroid 4 Vesta. The spacecraft's huge solar-cell "sails", spanning 20 m (65 feet), provide several kilowatts of electricity to power the spacecraft's xenon-fueled ion-fueled thrusters.

In this sense, NASA's Dawn spacecraft provided planetary scientists with a "twofer" — it traveled to the big, unexplored asteroids 4 Vesta and 1 Ceres and, in each case, spent many months scrutinizing that body intensively. The key was Dawn's ion-drive propulsion system, which provided enough thrust to make the spacecraft a long-term tenant and allow its instruments to examine every lump and crater in detail.

Mission scientists get a major bonus from orbital exploration, because they can probe a body's interior simply by tracking how its gravity causes the spacecraft's motion to very slightly speed up or slow down. In Dawn's case, the team used the spacecraft's radio transmissions to measure changes in its orbital velocity to within 0.1 mm (0.004 inch) per second during a two-month-long High Altitude Mapping Orbit in late 2015. The results, published August 3rd in Nature by Ryan Park (Jet Propulsion Laboratory) and 13 colleagues, reveal much about the asteroid belt's largest body. Here's a recap:

• Ceres is big enough for gravity to draw it into a spherical shape. But it spins fast: once every 9.07 hours, which has distorted its shape into an ellipsoidal that's squashed pole to pole by about 7½%. Its equatorial diameter is at most 966 km (600 miles), though a bit less elsewhere, but through the poles it's 892 km. Given its rotation period, this is pretty much exactly the expected shape — what geophysicists call hydrostatic equilibrium.

Interior of Ceres
Careful tracking of NASA's Dawn spacecraft allowed its scientists to map the gravity field of Ceres and, with that, to probe the dwarf planet's interior. The core might be dry rock or a mix of rock and water-bearing clay minerals. The ice crust probably is strongly enriched with salt compounds.

• Ceres has a center of mass that's offset from its geometric center by just 1 km, again matching expectations. Mission scientists think this small offset probably arises from a thick, rigid outer shell that varies slightly in thickness.

• The overall density is 2.16 g/cm3, slightly higher than estimates from ground-based observations. Even so, it's far lower than that for a rocky body like the Moon (3.34 g/cm3). So either the interior of Ceres is full of holes (it's not) or there must be a lot of ice in there (most likely frozen water).

• Finally, careful mapping of the gravity field reveals that the rock and ice aren't mixed together but instead have differentiated (separated) into a core and crust — though Ceres is not as completely stratified as is, say, Ganymede. Prior to Dawn's arrival, geophysicists were split on whether that would be the case. Park's team estimates that the crust has a thickness of near 190 km (120 miles), but it can't be pure water ice. Most likely it's strongly enriched with various salts.

Occator crater on Ceres
A patchwork of bright splashes inside Occator crater on Ceres continues to puzzle planetary scientists. NASA's Dawn spacecraft recorded this composite view in August 2015.

So, early in its history, Ceres had to be warm enough for the water to flow, and at some point this dwarf planet must have been covered with a deep, salty ocean. And maybe some of that ocean remains deep inside. After all, Ceres has more than 100 curious white spots — most associated with craters and most dramatically on the floor of Occator — and for now scientists are leaning toward salt deposits of some kind as the explanation for these.

Conceivably the impacts created enough heat to melt the ice locally and leave behind a salty residue. But other observations — like Dawn's imaging of hazes suspended above the surface and the detection of water vapor by ESA's Herschel space observatory — are building a strong case that there's a briny deep somewhere below the surface.


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