Bits of comet surprise scientists

Friday, January 25, 2008

Tiny samples of a glowing comet, flown back to Earth by a pioneering spacecraft named Stardust, hold remarkably little dust from any ancient far-off stars, but a lot of the stuff that makes up nearby rocky asteroids, Livermore scientists have found to their surprise.

The Stardust mission was the first in space history to snatch material from a distant comet in orbit around the sun, and researchers all over the world had expected the microscopic cometary grains to be older than the sun itself - material from a time billions of years ago when the solar system was merely a cloud of dust and gas swirling around a "proto sun."

Instead, a team of analysts at the Lawrence Livermore National Laboratory has discovered that thousands of the comet's grains are actually bits of the same silicate rocks found in meteorites that fall to Earth from the asteroid belt between Earth and Mars.

The Livermore team's two leaders, materials scientist Hope Ishii and astrophysicist John Bradley, discussed their findings with reporters Thursday. Their full report is being published Friday in the journal Science.

Stardust was launched from Earth nine years ago on its journey of 3 billion miles. It circled the sun three times before it reached its target - a comet named Wild-2 - and grabbed samples of the comet's dusty material in a filmy trap called an aerogel four years ago. The spacecraft finally returned to Earth two years ago and jettisoned its precious cargo by parachute over the Utah desert.

Since then, nearly 200 scientists at more than 75 institutions have been analyzing the particles from the comet, and the Livermore team is one.

Curiously, Ishii said, the comet's dust does not include many of the metals that scientists had expected to find in an object that must have been formed long ago close to the solar system and later was flung far out into a region called the Kuiper Belt.

Most scientists, she said, had expected to find a treasure trove of the solar system's most primitive material, but it's not there in comet Wild-2.

To Bradley, the fact that few of the particles the team analyzed appear to date from the solar system's earliest formation is "somewhat disappointing," but to scientists, he said, "a dose of cold reality is a good thing." It means, he said, that they must now try to understand why this comet - and perhaps many others - hold so little primitive material that formed close to the young sun, and so much that makes up what are known as chondritic matter found in meteorites from the asteroid belt.

Hundreds of other scientists in laboratories around the world are still analyzing other samples of the Stardust spacecraft's priceless cargo, and at NASA's Ames Research Center astrophysicist Scott Sandford has recently reported that some of the comet's particles reveal organic material - the same chemicals from which the ingredients for life are made.

Donald Brownlee of the University of Washington, the Stardust mission's principal investigator, told The Chronicle on Thursday that other analysts had "absolutely" found grains of minerals in the comet that could only have come from the sun's earliest days - perhaps 5 billion years ago.

Although most of the material analyzed by the Livermore team clearly resembles the stuff of asteroids, something less 1 percent of the grains analyzed by others do, in fact, predate the solar system's formation, Brownlee said.

A NASA spacecraft called Deep Impact flew to the comet named Tempel 1 and deliberately crashed into it to kick up a cloud of dust that the spacecraft's instruments analyzed.

Michael O'Hearn of the University of Maryland, that mission's science leader, said in an e-mail Thursday that "these results (from Livermore) make an even stronger case for the need for a new Comet Surface Sample Return mission."

In fact, a spacecraft named Rosetta from the European Space Agency is now on a 10-year journey to a comet even more distant than Stardust's target. Rosetta will fly alongside the comet Churyumov-Gerasimenko for a full year as it orbits the sun. It will then release a lander to the comet's surface and bake samples in an oven for analysis right there.

Rosetta's date with the comet is May 2014.

E-mail David Perlman at dperlman@sfchronicle.com.