Cosmic Rays
Cosmic-ray composition
Cosmic rays
are a puzzling form of radiation that is constantly raining down on us
from space. They are made up of electrically charged, subatomic
particles that crash into our atmosphere, where they are broken up and
fall to Earth in even smaller fragments.
Because cosmic rays are electrically charged, they are deflected by
various magnetic fields throughout the galaxy. Due
to this deflection, the rays don't point directly back to their
sources. Scientists must use indirect
methods to determine the source of the cosmic rays and the way they
have traveled (or
"propagated") through the galaxy. The chemical composition of the
cosmic rays
provide a surprisingly rich source of information. The chemical
composition of the solar system has been determined from a combination
of spectroscopy
on the Sun, studies of the solar wind and by chemical
analysis of meteorites. These meteorites likely have a purer sample of
the early solar system's components than terrestrial rocks, which have had
many years of constant pressure and change here on Earth to distort
their original forms.
The composition of cosmic rays is important because these rays are
a direct sample of
matter
from outside the solar system and contain elements that are much too rare to be seen in
spectroscopic lines from other stars. They also provide important
information on the chemical evolution of the universe.
If we look at the elemental composition measured for cosmic rays and
compare it to our best understanding of the composition of the solar system, we
quickly see some large differences.
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Solar and Galactic Cosmic Ray (GCR)
Composition
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Cosmic-ray secondaries
In the figure above, we take the abundance of silicon as a "standard
candle" or reference point, and compare the abundances (relative to silicon)
of the elements in the solar system and in galactic cosmic rays.
Silicon is used as the reference because it is a common intermediate-weight
element that is easy to measure. We see that there is less hydrogen and helium in the
cosmic rays than in the solar system, we think because hydrogen and
helium are harder to accelerate to high energies than heavier elements. We
also see that some light elements (lithium, beryllium, and boron) that are
rare in the solar system (and in the rest of the universe) are quite common in
cosmic rays. We also see more cosmic ray elements between silicon and iron
than in the solar system.
The accepted reason for all the observed cosmic ray lithium,
beryllium, and boron is that these are pieces of heavier cosmic ray
elements, especially carbon and
oxygen, that have had high speed collisions with the very tenuous gas
in interstellar space. Likewise, the elements between silicon and iron
have been supplemented by fragments of heavy cosmic rays such as iron and nickel.
These fragments are known as secondary cosmic rays, or simply secondaries.
From the number of secondaries observed at Earth, and with knowledge
of the probability of these collisions (which can be measured in particle
accelerators here on Earth), it is possible to calculate the amount of matter that
the cosmic rays have traveled through. More matter would break up more
primary cosmic rays. If the cosmic rays have stayed in the galaxy, the amount
of matter that they have passed through divided by the average density of
interstellar space (about one atom per cubic centimeter) gives
the age of cosmic rays. With this method, we determine an average cosmic ray
age of about two million years. But this turns out to be inaccurate.
Radioactive clocks
Another way to obtain the age of cosmic rays is to use radioactive
isotopes as clocks in a way very similar to the way carbon-14 is used by
archaeologists. There are several isotopes, beryllium-10, aluminum-26, chlorine-36,
etc., which are almost entirely secondaries. After they are created, they begin to
decay, and the fraction that reaches us at Earth gives the age of the cosmic
rays. With this method, the average age of cosmic rays comes out to approximately
ten million years. The reason the two million year age from the previous paragraph is
wrong is that cosmic rays don't just stay in the regions where the
density is one atom per cubic centimeter
(such as the galactic disk). Cosmic rays spend a large portion of
their time in the low-density galactic
halo, bouncing back and forth through the galactic disk many times.
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EGRET Gamma Ray All Sky Survey
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As the cosmic rays interact with interstellar gas, they can produce
gamma rays, which can be seen in the EGRET gamma ray image of the
Milky Way galaxy shown above.
Last Modified: March 2011
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