Image Credit: NASA
Today, Prof. Don Brownlee of the University of Washington visited our department to give a colloquium. He was talking about the results of NASA's Stardust mission, which sent a spacecraft to a comet, collected pieces of the comet, and brought them back to the Earth in January 2006.
Comets are thought to be pristine relics from the early solar system. 4.6 billion years ago in the outer regions of the disk that was to become our Solar System, ice and dust clumped together to form "dirty snowballs" in a wide range of sizes. Some of these were drawn together by gravity to make icy dwarf planets like Pluto and Sedna, some came too close to giant planets like Uranus and Neptune and were flung out into deep space, and some stayed orbiting the sun near where they formed, remaining in a sort of cryogenic deep freeze to the present day. On occasion, one of these icy bodies is perturbed by gravity and falls toward the inner solar system, where the Sun's rays warm it and melt the ice and gas, resulting in a comet.
Professor Brownlee and his collaborators hoped that the dust and debris in a comet might give us clues about the details with which our Solar System formed. They expected to find that the dust of comets would consist of loose agglomerations of "interstellar grains," tiny dust particles that pervade space. So, they designed the Stardust spacecraft, which was launched into space, visited a comet, collected some dust, and brought it back to Earth.
The reason to bring dust back to Earth is quite simple. In our physics labs, we have big machinery and equipment that can determine the composition of individual parts of a single grain of dust. That equipment is not cheap, it is large in size, it tends to weigh several tons or more, and it requires several people to run. Those factors make it impossible to send such detailed equipment into space. Robots may be excellent at running simple experiments in space, but detailed measurements on dust grains is best done back here on Earth. So, Brownlee and collaborators brought the comet (or at least pieces of it) back to Earth by capturing dust-sized grains in a novel substence called aerogel.
The initial results are very surprising. Instead of delicate aggregates of interstellar dust grains, the comet contains pebbles and specks that were melted at very high temperatures, over 3000 degrees Fahrenheit! That material had to have been processed near the sun, but somehow had to get to the outer regions of the solar system to get encased in ice. More than that, the material in the comet looks to be more processed than the material we find in meteors (most of which come from the asteroid belt).
It's almost like we excavated a woolly mammoth from the Siberian tundra and found a kangaroo in its stomach. Not only would we not expect it and need to explain how kangaroos got from Australia to Siberia, but we'd also have to explain how we've never found any evidence for kangaroos in Southeast Asia and China. Maybe some sort of giant slingshot built by disgruntled wombats?
And, in fact, there is one idea for solar system formation that results in a sort of giant slingshot, called an "x-wind." Basically, the region near the forming sun contained a very powerful wind, and any rocky material that happened to drift too close to the wind would get flung into the far reaches of the Solar System. It sounds simple enough, but in reality it is a very detailed and complex model that has not gained full acceptance in the astronomical community. Discoveries such as those from Stardust may give the x-wind model the observational evidence it needs to become accepted.
There's a lot more work to be done, but one big question is already looming. Stardust visited a specific comet: Comet Wild 2. Is Wild 2 a typical comet (all evidence pointed that way prior to the Stardust mission)? Or did the Stardust planners make an unlucky choice of some cosmic oddball comet? Perhaps we need a Stardust 2 mission to another comet to solve the questions raised by Stardust...