When Stardust landed in the Utah desert in January 2006, it was already a scientific triumph—during its seven years in space, the craft used a gravity assist from Earth to intercept comet Wild-2, collected gas and dust surrounding the comet's icy nucleus and returned the samples intact after nearly 3 billion miles of travel (earning it a Popular Mechanics Breakthrough Award). But it was after scientists had the chance to examine the findings that this mission became even more groundbreaking.
In Stardust's tennis racketshaped aerogel collectors, Michael Zolensky of NASA's Johnson Space Center and his colleagues found lots of surprises. While most astronomers had presumed comets formed at the far edge of the solar system, Stardust found compounds that form at hot temperatures near to the sun as well as organic compounds that form at cold temperatures away from the sun, meaning they might form in more peculiar ways than initially thought. "It's a grab bag of the entire solar system," Zolensky tells PM. Researchers are still grappling with the implications, trying to learn how materials like calcium aluminum
formed near the sun could find their way to the outer solar system so quickly.
In true scientist fashion, Zolensky relishes the unforeseen. "If it was exactly what we expected, that'd be boring," he says. There were other questions raised by the Stardust samples as well. Spectroscopic measurements from Earth had always suggested the presence of organic material in Wild-2, and the first studies from Stardust turned up evidence of these building blocks of life. But they couldn't eliminate doubt surrounding the detection of glycine, the simplest amino acid, according to Danny Glavin of the Goddard Space Center, who was called in to work on the Stardust Preliminary Examination Team.
The problem was that Stardust's samples, though historic, were incredibly small. That made them difficult to measure and sensitive to contamination. A Spanish research team raised the possibility that osbornite, one of the exotic chemicals scientists were surprised to find in Stardust, could have gotten there by contamination from rocket fuel. Stardust scientists rebutted this criticism by pointing out that they found osbornite encased in mineral grains, so it couldn't have come from the outside. The amino acid team, however, had a tougher go of it—with only billionths of a gram of glycine, they had to prove that the amino acid truly came from the comet.
It was Jamie Elsila of Goddard who finally figured it out, and earlier this week announced her research. The key to confirming its origin was to measure isotopes, she tells PM. Chemicals from deep space tend to contain heavier isotopes because the cold temperatures favor them, so glycine from space should show higher ratios of carbon-13 to carbon-12 than glycine here on Earth. After two years of study that pushed the limits of her equipment's sensitivity, Elsila found exactly that in the glycine Stardust brought home, confirming its extraterrestrial origin.
The confirmation of glycine in comets opens all kinds of new questions, she says. Comets reach different, farther-off regions of space than other objects, like asteroids, that are known to harbor organic materials. So, she tells PM, that increases the number of planets a body carrying organic material could strike, a boon to those hoping life on Earth isn't the only life there is. And while it would be mere speculation to say that life on our planet began with organic materials brought by comets, finding glycine in a comet's tail certainly does nothing to diminish that idea.
Stardust might not be done providing answers, either. Elsila says that her isotropic measurements wouldn't have been possible a decade ago, and so technological advances could make it possible to learn more from the miniscule Stardust samples still available. NASA assigned the Stardust spacecraft to another mission, called Stardust NExT, to sample a comet called Tempel-1. Comets aren't identical dirty snowballs, Zolensky says—readings from the Spitzer Space Telescope show Tempel-1 is a very different object than Wild-2, carrying compounds like clay minerals and carbonates that would have required liquid water to form. In 2005, NASA's Deep Impact craft intentionally crashed into the surface of Tempel-1 to analyze the aftermath, and now Stardust will provide the opportunity to measure how the comet has changed over the years.
Zolensky is still trying to make sense of everything Stardust has told us. Much of the current research, he says, focuses on dates—figuring out how and where comets might have formed to carry this hodgepodge of materials from across the solar system. The answers, he says, would tell scientists not just about why two comets are so different from each other, but also more about the early history of the solar system. For Elsila and Glavin, Stardust's measurements of a comet's tail were eye-opening, and now the biggest remaining secrets lie on the inside. A comet nucleus, both agreed, should have a richer mix of chemical compounds, including amino acids. But the next step in unraveling comets won't be easy—it would take a bold new mission to land on a comet and start drilling.