A team led by Southwest Research
Institute (SwRI) has 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 SwRI
Space Science and Engineering Division. “Dust grains are produced in vast
amounts through collisions of Kuiper Belt objects. 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.
Said Tom Bogdan, program director in the NSF Division of Atmospheric
Sciences, which funded the research, “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,” he 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 the National Science
Foundation, NASA and SwRI. The paper “The Outer Source of Pickup Ions and
Anomalous Cosmic Rays” by Dr. Nathan A. Schwadron (SwRI), Dr. Michael R.
Combi (University of Michigan), Dr. Walter F. Huebner (SwRI), and Dr. David
J. McComas (SwRI) appears in the October 30[MM1] issue of Geophysical
Research Letters.
