McCord didn’t get around to looking at Vesta until the early 1970s, and even then he didn’t give it much thought until his team got around to processing the data. “I was in the lab one day and one of the guys pulled the Vesta spectrum out of the computer, which we had observed a week or a month before,” he says. “And my God, it had one of the most beautiful absorption features you ever saw on a planetary object.” The data indicated that Vesta was basaltic, which suggested that Vesta’s rocks had been heated to melting at some point and then cooled. The discovery also established that the HED meteorites and Vesta shared the same composition.
McCord and his researchers also looked at Ceres but didn’t get far. Ceres was darker and murkier, and it didn’t have the clearly identifiable spectrum of Vesta. McCord’s grad students set to work on the data and came up with some preliminary findings: Ceres was a carbonaceous chondrite (a type of asteroid composed of water locked in minerals and carbon-based materials), and it had not been thermally altered. In other words, it had never melted and cooled, as Vesta had. This posed more questions than it answered. How did a large asteroid evolve and retain significant amounts of water? Nobody had any theories to explain it, and the researchers dropped the subject.
When the Dawn mission was approved, much of the focus was on Vesta. “You’re human, so you’re generally interested in things you know about,” Russell admits. “If you don’t have any information, you don’t have that thing to grab your interest.” That attitude began to shift in 2002, when McCord took a sabbatical to Nantes, on France’s west coast. “I got to thinking about Ceres, and I learned that the people who had been doing the most careful orbit and mass determinations were at the University of Bordeaux, a two-hour drive to the south.” McCord went down and learned that researchers there had been able to make accurate estimates of Ceres’ density. Pure water has a density of 1 (measured in grams per cubic centimeter). A conventional dry asteroid, made of silicates with some iron mixed in, would have a density of 3 or 3.5; Vesta’s is thought to be in this range. Ceres has a density only slightly higher than 2. That means there is a lot of water in the mix.
McCord found the work of Christophe Sotin and his graduate students at the University of Nantes even more intriguing. Sotin had developed a computer model of how Saturn’s biggest moon, Titan, could have formed without its liquids boiling off. Although Titan is chemically very different from Ceres, it too contains a lot of water. Perhaps, thought McCord, some version of Sotin’s model could explain how Ceres could have formed with its water intact. “We began to see that it was easy for Ceres in the early, early history to have created a liquid ocean,” McCord says.
Here’s how the theory goes: In the early solar system, dust particles glommed together to form bigger dust particles, which formed pebbles, then rocks, and so forth, until they combined into an object up to several hundred miles in diameter. The original dust particles were made largely of silicates mixed with other materials, including water and aluminum 26, a radioactive isotope with a half-life of about 700,000 years. That’s just long enough to make a big difference in how an asteroid evolves. Vesta and most other asteroids, the theory goes, accreted quickly and accumulated a lot of aluminum 26 that had not yet decayed. The aluminum 26 produced so much heat inside the asteroid that any water evaporated into space. Ceres, by contrast, accreted more slowly, so by the time it formed, the aluminum 26 had already mostly burned itself out. As a result, Ceres retained most of its water—and a memory of the solar system’s original composition.
These findings ignited McCord’s interest in Ceres, to the point where “I kept demanding we go to Ceres first,” he says. Russell sympathizes. “If we had to pick which was the most interesting, Ceres or Vesta, it’s not clear which one would win,” he says.
The argument is moot: Vesta is closer than Ceres, and therefore it must be Dawn’s first stop. But Ceres may make the bigger headlines. Vesta seems like Mercury or the moon, writ on a smaller scale. Ceres is unique. Imaging of the surface may reveal whether there is indeed an ocean beneath an icy crust. Observing the surface should allow scientists to glean some idea of how the interior behaves—if there’s volcanic activity that could provide the heat to sustain life, for instance. Dawn’s spectrometer will be able to detect the presence of organic molecules.
Unfortunately, Dawn isn’t equipped to search for past or (dare we dream?) present life on Ceres. That would require penetrating the surface and taking and analyzing samples. “To detect life, you need a pretty sophisticated lab on the surface or in the interior or wherever the environment is,” McCord says. “That’s technically a major challenge and virtually impossible—nobody’s willing to spend the amount of money to do that.”
For now, at least. After Dawn’s visit, attitudes might change.