Some cosmic rays originate within solar system, researchers

by Harvey Leifert hleifert@agu.org of the American Geophysical Union

WASHINGTON - Researchers have found that a portion of anomalous cosmic rays -- charged particles accelerated to enormous energies by the solar wind -- results from interactions with dust grains from a belt of comet-sized objects near Pluto's orbit. These objects make up what is known as the Kuiper Belt, a remnant of the formation of the solar system. "This novel finding shows how dust in the cosmos may play an important role for producing the most energetic particles known," says Dr. Nathan Schwadron, a senior research scientist in the Space Science and Engineering Division of Southwest Research Institute (SwRI) in San Antonio, Texas. The study by Schwadron and colleagues at SwRI and the University of Michigan was published October 30 in Geophysical Research Letters, a journal of the American Geophysical Union.

"Dust grains are produced in vast amounts through collisions of Kuiper Belt objects," says Schwadron. "These particles give us a glimpse of the early stages of our solar system when the dust content was much larger, and could parallel other more dusty stellar systems that exist now."

Recent observations of anomalous cosmic rays are puzzling because of the unexpected presence of iron, silicon and carbon, notes Schwadron. "This finding varies from the traditional explanation of anomalous cosmic rays which were thought to be devoid of easily charged elements."

The interstellar medium has lots of carbon, silicon and iron atoms, but electrical charging (ionization) of these elements prevents them from penetrating deeply within the solar system. "Our team looked for a source already inside the solar system to account for the unusual anomalous cosmic rays -- and we found one in the tiny comet-like grains from the nearby Kuiper Belt," says Schwadron.

As the grains produced by collisions in the Kuiper Belt drift in toward the sun, they are bombarded by solar wind particles, which causes sputtering and frees the carbon, silicon and iron atoms from within. At that point, those particles interact with solar radiation, transforming them into ions (charged particles). The solar wind then sweeps them out and accelerates them to anomalous cosmic ray energies at the edge of the solar system, where they are bounced to and fro by magnetic fields in the solar wind and in the medium beyond the solar system, according to Schwadron.

Tom Bogdan, program director in the NSF Division of Atmospheric Sciences, which partly funded the research, says, "This is a big step toward solving the long-standing mystery of the origin of the anomalous component of cosmic rays. The research underscores the power of remote sensing: Sampling of Kuiper Belt material with unmanned space probes is a huge and difficult enterprise. The detection locally of the anomalous cosmic ray component provides information on the conditions that prevail in this remote region of our solar system."

"Anomalous cosmic rays" are so named because they form in the relative vicinity of the Earth, near the sun, and have lower energy than galactic and intergalactic cosmic rays, which form in the far reaches of the galaxy and beyond. Cosmic rays, the most energetic particles in the cosmos, move throughout the universe at light speed and constantly bombard the Earth.

"The discovery that anomalous cosmic rays can be generated from material in the Kuiper Belt provides a tool for understanding its mass distribution and composition and for probing the plasma-dust interactions in space," says Schwadron.

Cosmic rays also are believed to play a role in evolution. "Cosmic rays are a double-edged sword. They cause genetic mutation and are harmful to living organisms, but on the upside stimulate biological evolution," Schwadron says. "Cosmic rays are our only available sample of matter from the far reaches of the distant galaxy, and from other galaxies. They can tell us a lot about what's in the universe, and we can now use them to study what's in the Kuiper Belt. Their relationship to the creation or maintenance of life is also worth a closer look."

This program was supported with funding from NSF, NASA, and SwRI.