White Spots, Lone Mountains, and Other Funny Business on Ceres

Ceres is a wonderland of puzzles, including the white spots (far right) and enigmatic ridges, crater chains, and flows. (Credit: NASA/JPL/UCLA/MPS/DLR/IDA; processing Elisabetta Bonora & Marco Faccin) "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka' but 'That’s funny...'" That quote, delivered by the brilliant science writer Isaac Asimov, keeps popping into my head as I look at the remarkable new images of Ceres. NASA's Dawn spacecraft has been orbiting the dwarf planet since March 6, scrutinizing a landscape that is not quite like anything humans have ever seen before. One detail on Ceres jumped out almost immediately: a bizarre white spot, drastically brighter than its drab surroundings. As Dawn got closer, the probe's camera showed that the white spot is actually a patch of at least eight smaller white areas; there are also smaller white spots and extended light-ish splotches scattered across Ceres's 950-kilometer-wide 590-mile wide globe. What are they? No idea, except not alien landing lights (they don't show up in the dark). Subsequent images revealed a solitary, 3-mile-high mountain; long ridges and apparent streamers of impact debris; giant frozen surface flows; and craters with unusual distorted, vaguely hexagonal shapes. The initial scientific reaction to these Ceres images boils down to three simple words that Asimov would well recognize: "That looks funny." It makes sense that Ceres does not quite fit into any familiar categories. It is a type of object that has never before been explored: considerably larger than any other asteroid but far smaller than any of the classical planets, not rocky like Mars but not fully icy like the moons of Jupiter. Ceres is an in-betweener, a missing link left over from the solar system's early days. Most likely it is a surviving protoplanet, the kind of object that was mostly swept up and absorbed into larger worlds. Somehow Ceres survived intact. Now we get to study it, and take a trip 4.6 billion years into the past, when Earth was growing into its modern self by snacking on objects like Ceres. For help in interpreting this thrilling and baffling world, I called on Andy Rivkin of Johns Hopkins University's Applied Physics Lab. Rivkin studies the surface properties of asteroids, and has a wide-ranging curiosity. He wants me to warn you that a) he is not a member of the Dawn team and b) he's an astronomer, not a planetary geologist. Consider yourself warned. But Rivikin is also an able tour guide to the asteroids, with a keen interest in Ceres, so I asked him to share some thoughts about the most interesting--or should I say, "funny"--Dawn has observed so far.