By Bob Silberg, Science Writer
Where did the world come from? According to Iroquois legend, a pregnant woman fell from the clouds and needed a place to land, so a muskrat brought mud from the bottom of the sea and laid it onto the back of a turtle, where it grew into the Earth. The Vikings, meanwhile, had it that the Earth was built from the body parts of a giant frost ogre.
Modern science offers a much stranger story:
An immense gas cloud, peppered with atoms made by stars, squeezed itself together so tightly that its center ignited into a nuclear furnace and became our sun. The remaining bits and pieces of the cloud, known as the solar nebula, clumped together and formed the planets, moons, asteroids, and comets.
Scientists have plenty of evidence that this is correct, but an important chunk of the story is missing. According to Don Burnett, principal investigator for the Genesis mission, scientists think they understand how collisions of kilometer-size asteroids can ultimately build a planet. "But," he said, "how you go from a dust grain to a kilometer is pretty much an open issue at the present time."
Gravity alone doesn't seem to do the trick. "People have tried to do the theories," Burnett explained. "There are gravitational instabilities that apparently happen in a thin disk of gas (that, under the right circumstances, could lead to dust and gas accreting into larger bodies), but it is widely accepted that the amount of turbulence under the conditions of the solar nebula would prevent these gravitational instabilities from working. We don't know -- it's a wide-open question -- how you go from a micron to a kilometer."
Another puzzle, which has been perplexing cosmochemists for decades, may help to point the way to an explanation of how the planets began to form. In most ways, the ingredients that make up the solar system appear to have been very well-mixed, so that the proportions of isotopes of most elements look the same everywhere. But scientists have found surprising differences in the isotopes of certain elements. And this anomaly is particularly striking with regard to isotopes of the most abundant element of the planets in the inner solar system: oxygen.
Scientists have proposed various chemical processes to help explain these differences, and identifying the right ones could lead to a good theory of how those first dust grains began to stick together and form the seeds of today's planets. But one vital piece of information has been missing: a thorough analysis of the composition of the solar nebula. Learn how the solar system started out, and you can attempt to explain how it evolved to what we see today. How can anyone analyze a cloud of gas and dust that existed some 4.6 billion years ago? The key, scientists believe, is the Sun.
"The composition of the Sun is the starting composition for everything in the solar system," Burnett said. That's because nearly all of the original solar nebula exists today as the Sun, which has more than 99% of the solar system's mass. And while the Sun's core undergoes great changes as it turns hydrogen into helium and, in the process, liberates the energy that makes our lives possible, the atoms that make up the outer portion of the Sun are thought to be virtually unchanged since the birth of the solar system. Analyze the Sun's surface, and you produce a picture of the original cloud of gas and dust from which the solar system formed.
But a mission to fly to the Sun, scoop up a sample of its surface, and return it to Earth is well beyond today's capabilities. How then to put the Sun under the microscope?
The Answer is Blowing in the Wind
Cosmochemists caught a lucky break when nature came up with the solar wind, an exceedingly thin but furious outflow of the solar surface. The nonstop solar wind is what pushes comet tails away from the Sun, and its interaction with Earth's magnetic field produces the beautiful auroras sometimes seen at northern and southern latitudes.
The solar wind is representative of the Sun's surface, and the Sun's surface is representative of the ancient solar nebula. So the Genesis spacecraft collected samples of the solar wind and brought them to Earth, where scientists and engineers had been developing new technologies to examine them.
The nature of the examination process is such that any atoms that undergo analysis become unavailable for further analysis, so the samples are husbanded very carefully. The plan is to take full advantage of today's cutting-edge technologies, but also to preserve samples for future technologies.
As Burnett put it, though the instruments created for the Genesis mission are among the most advanced technologies of their kind on the planet, they're "barely good enough" to measure such small numbers of atoms. "Ten years from now, 20 years from now," he said, "it may be a lot easier to analyze these things."
The atoms will keep. They've waited more than four billion years to help us understand our origin. They can easily wait a few years more.
Last Updated: 4 December 2012