Dust grains in the outer solar system are the source of some of the cosmic rays that bombard Earth, planetary scientists report in the Oct. 30 Geophysical Research Letters. The grains are located in the Kuiper belt, which lies within the solar system beyond the orbit of Neptune and consists of comets and other icy objects from the solar system's formation.

It's the composition of certain cosmic rays that suggests they originate in the solar system, says Nathan A. Schwadron of the Southwest Research Institute in San Antonio. These so-called anomalous cosmic rays are energetic charged particles that strike Earth with only about one-hundredth the energy of cosmic rays from the far reaches of our galaxy or beyond. They consist of carbon, silicon, and iron ions. These ions are common in space between stars, but they would have difficulty penetrating the solar system because the solar wind would repel many of them.

When objects in the Kuiper belt collide, they generate debris ranging from dust grains a few micrometers in diameter to city-size objects, Schwadron notes. As the grains drift toward the inner solar system, they're buffeted by the solar wind. It shakes carbon, silicon, and iron atoms loose from the grains. The atoms become ionized by the sun's ultraviolet radiation and are then accelerated to enormous energies by the solar wind. Some of the ions ultimately bombard Earth, Schwadron and his colleagues calculate.

The discovery that anomalous cosmic rays can be generated from material in the Kuiper belt provides a new tool for gauging the composition and mass of this relic from the solar system's formation, Schwadron says.--R.C.