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Protoplanet frozen in time

Science / AAAS
Click for video: The left view is a computer model of Pallas' surface, based on
the Hubble imagery at right. The circle indicates a large crater that is likely deeper
than shown in the model. Click on the image to watch a 3-D animation.

Images from the Hubble Space Telescope suggest that the asteroid Pallas should be grouped along with two other big space rocks as protoplanets - "planetary embryos" that were big enough to stay pretty much as they were during the formation of the solar system, but too small to progress to the next stage of development.

"These are the first really high-resolution images of Pallas that come from Hubble," said Britney Schmidt, a planetary scientist at the University of California at Los Angeles and lead author of the study in Friday's issue of the journal Science. "This was a suite of observations that haven't been made before."

The imagery was collected in 2007 by a camera that's no longer on the Hubble, the Wide Field and Planetary Camera 2, or WFPC2. That instrument was replaced with a next-generation wide-field camera during May's final Hubble servicing mission.

Getting the pictures down from WFPC2 was just the beginning of a two-year-long process to put together a picture of Pallas, similar to the Hubble imagery already collected for its asteroidal siblings Ceres and Vesta.

Schmidt and her colleagues painstakingly looked at the asteroid's profile from different perspectives, then fit all those views together using a computer modeling program called Maya. They also analyzed the slight color differences in the pictures and matched them up with Pallas' 3-D shape.

The result? Pallas turns out to be almost but not quite round, falling just short of a state known as hydrostatic equilibrium. Its mean radius is 170 miles (272 kilometers), which makes it the second-largest asteroid after Ceres, which has a 297-mile (475-kilometer) radius. But at 449 quintillion pounds (2.04 × 1020 kilograms), it's the third most massive asteroid, ranking behind Ceres as well as Vesta.

B.E. Schmidt and S.C. Radcliffe
An artist's conception shows an impact event on the asteroid Pallas.

The asteroid has several depressions of various sizes, including what appears to be a monster crater about 150 miles (240 kilometers) wide. Such craters were likely caused by early impacts that knocked loose a family of asteroidal fragments linked to Pallas, the researchers said.

The analysis also turned up bright and dark spots in ultraviolet light, suggesting differences in composition. "What's great to see is that heterogeneity actually exists, because it gives us some idea that there's some processing going on," Schmidt said.

Pallas' composition suggests that it had liquid water and an active geology at an early point in its multibillion-year history. "There aren't going to be volcanoes on Pallas, and there aren't going to be continents, but it's heading in a direction where it's going to be a planet," Schmidt said.

Much of that activity was frozen in place, making Pallas something of a planet interrupted - what the researchers call "an evolved body with planetlike properties," or a protoplanet. Scientists believe even the biggest planets in the solar system passed through the protoplanetary stage, gravitationally glomming onto bigger and bigger chunks of material until they got where they are today.

Worlds such as Ceres, Pallas and Vesta were stuck in a state of arrested development because nearby Jupiter pushed the asteroids around and grabbed a lot of the good stuff for itself. At least that's how the favored scenario plays out.

Ceres had grown large enough to keep a roundish shape, even after numerous impacts, and thus is now considered a dwarf planet alongside Pluto, Eris and potentially scores of other worlds beyond Neptune. Pallas and Vesta, however, aren't quite in the same league. "They're not quite perfectly round, and potentially because of impact," Schmidt said.

The bottom line is that Pallas is, well, right on the line when it comes to the important features dividing the solar system's big planets and dwarfs (and, for that matter, roundish natural satellites such as our moon) from irregular objects such as small asteroids and comets. The researchers say it's closer to a planet than to a typical asteroid, but Schmidt said the most interesting thing about Pallas isn't its precise classification.

"What's more interesting than just the classification is to think of the process," Schmidt said. "What's unique about this object is that it probably stayed almost completely intact from the early days of the solar system. It hasn't been broken up, and there are only a few of those kinds of objects left."

Protoplanets such as Pallas - and Ceres and Vesta - can thus serve as a fossil record for an important time in our solar system's development. "They were not only the building blocks of planets, but they're also what planets looked like for a short period of time," Schmidt said. "They just never really got to form into something bigger."

For comparison's sake, Eris' mean radius is an estimated 800 miles (1,300 kilometers); Pluto's is 721 miles (1,153 kilometers); and Earth's is 3,959 miles (6,371 kilometers). All those worlds are thus more than twice as wide as Ceres, Pallas and Vesta.

For more about the planet quest, check out my book, "The Case for Pluto." You can also join the Cosmic Log team by signing up as my Facebook friend or following b0yle on Twitter.