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Space Topics: Asteroids and CometsAsteroid 1 Ceres
Ceres, the first asteroid to be discovered, is by far the largest and most massive body in the main asteroid belt. In fact, it comprises about a third of the total mass of the entire main belt. Its size and mass are sufficient to give it a spherical shape, like the planets. However, it is still much smaller than any of the planets, similar in size to Saturn’s medium-sized moons Tethys and Dione. Because of its small size, it does not have sufficient gravitational influence to have cleared its neighborhood of other, smaller bodies in similar orbits, so under the International Astronomical Union's new definition scheme Ceres is considered to be a "dwarf planet," and may very likely be the only dwarf planet in the asteroid belt. (Vesta is the only other candidate; its status will depend upon what Dawn discovers about its shape.) Recent observations by the Hubble Space Telescope suggest that Ceres is a very interesting body, worthy of a dedicated mission to visit it. For example, like the planets and large moons, Ceres has "differentiated," meaning that its interior is separated into crust, mantle, and core layers, with the densest materials being at the center. The outermost layer is likely made of icy material including water and ammonia. The fact that Ceres has not lost these volatile components in destructive impact events means that its composition probably preserves a record of what the solar system was like when it was first condensing from dust into planetesimals and larger protoplanets. Ceres may well be a relict protoplanet. Because of its position in the dynamic main asteroid belt, its surface is
probably heavily cratered. But the Hubble observations also hint at
low topography on Ceres; it lacks large, deep, bowl-shaped craters. If
Ceres has large craters, they must be "relaxed," meaning that their
shape has become flattened over time, with gravity inducing flow in Ceres'
icy crust to even out the crater topography. Such relaxed craters are
very common on the icy moons of Jupiter and Saturn. The Hubble observations
also suggest that there are several large surface features that have markedly
different reflectance from the rest of Ceres, like the light-colored spot visible
in the animation above. Ceres' surface probably also has interesting
tectonic features: compressional (folding) features that could have formed
as the whole body shrank when its water melted and differentiated, and then
extensional (faulting) features that could have formed as the water began to
re-freeze and expand.
The conclusions from Hubble's indistinct observations have whetted our appetite for a mission to go to Ceres and explore it in much more detail. NASA's Dawn mission will launch in 2007 for a 2015 rendezvous. Basic FactsBecause Ceres has not been visited by spacecraft, size estimates depend upon telescopic observations. There has been a lot of variability in published reports as different observers have used different techniques and different instruments to make their estimates. Consequently, numbers published on the Web vary widely. The numbers below come from papers published in 2005 and 2006 (see references). The orbital information, based on more than two centuries of astronomical observations, are much more precise. Size: Orbit:
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| Ceres | 2.10 grams per cubic centimeter |
| Pallas | 2.71 grams per cubic centimeter |
| Vesta | 3.44 grams per cubic centimeter |
| McCord et al., 2006 | |
For comparison, here are the densities of the basic materials that form solid bodies in the solar system:
| Water ice | 0.94 grams per cubic centimeter |
| Silicate (rock) | 3.36 grams per cubic centimeter |
| Metal | 4.80 grams per cubic centimeter |
And here are the densities of three large icy satellites in the outer solar system:
| Ganymede | 1.94 grams per cubic centimeter |
| Titan | 1.88 grams per cubic centimeter |
| Callisto | 1.86 grams per cubic centimeter |
Ceres has a surprisingly low density given its size. Many small asteroids have low bulk densities because they have very low gravity and so cannot compress their interiors enough to squeeze out empty pore space. But Ceres is a big object and should have no real porosity below its surface. In order to have such a low density, it has to be made of a significant quantity of water ice, like the icy satellites of the outer solar system. Depending on what its rockier component is made of, it could be between 17% and 27% water by mass.
Another clue is Ceres' albedo, or reflectivity. Ceres reflects roughly 10% of the sunlight that strikes it, which makes it very dark but not as dark as other low-density asteroids called carbonaceous chondrites. These have albedos of around 3 to 5%. So if Ceres is made at least partly of the same stuff that other asteroids are made of, it must have something brighter mixed into its composition in order to be as bright as it is. Scientists disagree on what this brighter material could be. Exposed water ice would not be stable at the surface of Ceres at its distance from the Sun, but some evidence suggests that there is water or ammonia bound up in Ceres' surface minerals.
Taken together, all of this information suggests that Ceres is likely made of the same source material as a class of meteorites known as carbonaceous chondrites, but that it also has a significant quantity (17 to 27% by mass) of water ice.
How Do We Know Ceres Is Layered?
 Model of the interior of Ceres
The density and shape of Ceres suggest that it is internally layered, with a denser silicate core and an icy mantle. The mantle would have been molten at one point in Ceres' history, and may possibly even be molten today. Credit: after McCord and Sotin, 2005 |
There are two important lines of evidence that suggest that Ceres is divided into layers. The first is a theoretical model developed by scientists Tom McCord and Christophe Sotin. They took what is known about the composition of Ceres and wrote down a mathematical model of an undifferentiated (mixed-up) body with that composition. The mathematical model included the presence of radioactive isotopes of important rock-forming elements that would have existed in the early solar system. The most important one for their model turned out to be aluminum-26, which has a relatively short half-life of 716,000 years. Because of the relatively short half-life, it decays quickly, generating a lot of heat early in the formation of the solar system. Other, longer-lived radioactive isotopes of iron, potassium, thorium, and uranium could also have been important.
McCord and Sotin explored many variables of initial composition and formation time, but regardless of the choice of variables, their model Ceres heated up enough after it formed for its ice to melt. Because of the strong contrast in density between ice and rock, the still-solid rock component would sink and the water would rise, forming a body with at least three layers: a solid rocky core, a liquid water mantle, and a solid icy crust. But the process of melting water absorbs a lot of heat; Ceres' watery composition would have prevented the rocky core from heating up enough to melt. Over time, the crust would thicken as the icy mantle froze, but there could still be a liquid water or ammonia-water layer between the icy crust and solid core on Ceres today.
This model makes an important prediction. Ceres, like all bodies in the solar system, rotates, and the rotation makes its equatorial diameter fatter than its pole-to-pole diameter. However, the "oblateness" (or fatness) of a spinning body depends upon its internal structure. A thoroughly mixed, homogeneous body will be more flattened than a body that has more of its mass concentrated toward the center. McCord and Sotin predicted that if Ceres was actually differentiated, its polar and equatorial axes should differ in diameter by about 64 kilometers (40 miles).
It would take a detailed Hubble Space Telescope observation of Ceres to measure its shape accurately enough to test this prediction. That measurement was performed in 2003 and 2004 by an independent team consisting of Peter Thomas and others, and they found Ceres' shape to be exactly as predicted by McCord and Sotin for the model Ceres with a rocky core and an icy crust. Not only did they find the equatorial and polar axes to be different by about 64 kilometers, but they also found no evidence of extreme topography on Ceres as is found on Vesta. This means that if Ceres has lots of impact craters -- which is very likely, given its location in the solar system -- then those craters must be "relaxed." In other words, the force of gravity has acted to cause the icy surface of Ceres to flow, pushing the crater centers upward to flatten their initially bowl-shaped topography. The medium-sized icy moons of Saturn, like Tethys, Dione, and Rhea, have this kind of relaxed topography.
Exactly where the layers lie inside Ceres depends on how much ice it contains, which depends on how dense its rocky component is. If Ceres is less icy, it has a relatively thin water ice layer of about 70 kilometers (45 miles) in thickness; if Ceres is more icy, its ice layer would be about 120 kilometers (75 miles) thick.
When Dawn enters orbit at Ceres, mission controllers will be able to study the properties of its orbit to improve upon the estimates of Ceres' mass and how that mass is distributed within the protoplanet.
References
McCord, T. B., and C. Sotin, 2005. Ceres: Evolution and current state, Journal of Geophysical Research 110: E05009, doi: 10.1029/2004JE002244
McCord, T. B., L. A. McFadden, C. T. Russell, C, Sotin, and P. C. Thomas, 2006. Ceres, Vesta, and Pallas: Protoplanets, Not Asteroids. EOS, Transactions, American Geophysical Union 87: 10, 7 March 2006.
Thomas, P. C., J. W. Parker, L. A. McFadden, C. T. Russell, S. A. Stern, M. V. Sykes, and E. F. Young, 2005. Differentiation of the asteroid Ceres as revealed by its shape, Nature 437: 224-226, doi: 10.1038.
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