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Space Topics: Asteroids and Comets

Asteroid 1 Ceres

Ceres rotation animation
Ceres rotation animation
Credit: NASA, ESA, Joel Parker, Peter Thomas, Lucy Mcfadden / animation by Emily Lakdawalla

Ceres, the first asteroid to be discovered, is by far the largest and most massive asteroid 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.

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.

Highest-resolution global mosaic of Dione
Saturn's moon Dione
Ceres could look something like Dione, with its relaxed craters and linear ridges. Ceres is 85% the size of Dione.
Credit: NASA / JPL / Space Science Institute

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 Facts

Because 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).

Size:
Diameter: approximately 950 kilometers (equatorial diameter 974.6 +/- 3.6, polar diameter 909.4 +/- 3.2 kilometers) (about 1/13 or 7.5% of Earth)
Mass: 9.43 x 10^20 kilograms (0.016% of Earth)
Bulk density: 2.10 grams per cubic centimeter (less dense than rock, more dense than ice)

Orbit:
Average orbital radius: 2.767 astronomical units
Orbital inclination: 9.73 degrees
Orbital period: 4.60 years
Rotational period: 9.076 hours

Earth at a scale of 50 km/pixel

 

 

 

Ceres at a scale of 50 km/pixel

Ceres and Earth compared
Both are shown at a scale of 50 kilometers per pixel.


How Do We Know What Ceres Is Made Of?

First, there is the important clue of Ceres' density.  Several different teams of astronomers have studied the tiny gravitational perturbations of some of the larger asteroids on each other and on the planets Mars and Earth, and from those perturbations were able to derive estimates of the mass of Ceres.  Taken together with the sizes of the asteroid calculated from other astronomical observations, these numbers can be used to compute the bulk density, an important clue to what it's made of.  Here is how the densities of the three largest asteroids compare:

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
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.