Particles from outer space have taught us about the inner workings of the subatomic world and promise to solve other puzzles on a cosmic scale. Closer to home, however, we have learned to harness cosmic rays in ingenious ways. They are helping us to unravel secrets of our past. Yet, ultimately, they may limit our future ability to explore the solar system.
Time capsules
Over the aeons, cosmic rays have left an imprint in the Antarctic snow from which we can determine the history of the sun and possibly our climate.
Advertisement
Primary cosmic rays from deep space have to penetrate the sun鈥檚 solar wind before being caught by the Earth鈥檚 magnetic field and hitting the upper atmosphere. So the solar wind acts like a protective shield, called the heliosphere, which fills the solar system and deflects cosmic rays.
However the sun鈥檚 activity goes through cycles and this influences the intensity of cosmic rays arriving at Earth. When the sun is quiet, the heliosphere is weaker, which enables more cosmic rays to penetrate the solar system and collide with atoms in the Earth鈥檚 atmosphere. The collision between incoming cosmic-ray protons and atmospheric oxygen nuclei leads to nuclear transmutations and, in particular, to two isotopes of beryllium: beryllium-7 and beryllium-10. The momentum of the primary ray is transferred to the beryllium isotopes which fall to Earth. Any of these isotopes landing in Antarctica are deposited in the snow, 鈥渇ootprints鈥� which accumulate in layers over the centuries.
Beryllium-10 has a half-life of about 1.4 million years and decays to boron-10, while beryllium-7鈥檚 half-life is a mere 53 days before decaying to lithium-7. The ratio between the two isotopes in the ice today reveals how long has elapsed since they were formed in the atmosphere. By drilling ice cores and gathering samples at various depths in the Antarctic, it may be possible to measure the concentration of beryllium isotopes, and deduce how solar activity has varied over thousands of years.
Many scientists suspect that whereas solar activity has had only a minor role in climate change over the last century, it may have played a much more significant role over many centuries. The beryllium footprints left by cosmic rays may reveal if there is indeed a relationship between solar activity and climate change on Earth.
Cloud in a bottle
The idea that there is a correlation between solar activity and the Earth鈥檚 climate has been around for nearly half a century, but it has never been totally convincing. In recent years, Henrik Svensmark at the Danish National Space Institute in Copenhagen and colleagues have proposed that solar variations modulate the cosmic ray intensity at the Earth, which in turn may affect cloud formation and, through this mechanism, climate. An experiment at the CERN laboratory near Geneva, Switzerland, is now attempting to test this hypothesis.
鈥淐osmic rays may affect cloud formation and, through this mechanism, Earth鈥檚 climatex鈥�
Cosmic-ray protons passing through the atmosphere can ionise volatile compounds, leading to airborne droplets, or aerosols. These droplets may then become the seeds for clouds. While these basic facts are generally agreed, it is not known whether cosmic rays play a significant role in cloud formation on a large scale.
CERN physicist Jasper Kirkby is leading an attempt to study the effects that cosmic rays may have on atmospheric chemistry. The experiment is called Cosmics Leaving OUtdoor Droplets (CLOUD, see picture). It consists of a custom-built chamber filled with ultra-pure air and laced with the molecules that are believed to seed clouds, such as water vapour, sulphur dioxide, ozone and ammonia. Protons are then fired into the chamber in an attempt to simulate collisions between cosmic rays and the real atmosphere.
After this irradiation, the CLOUD team samples the 鈥渁tmosphere鈥� in their apparatus to see what effect the protons have had. The experiment is ongoing, but early results suggest that collisions between high-energy protons and the CLOUD atmosphere lead to the copious production of nanometre-scale droplets. These are far too small to serve as seeds for clouds, however.
While this is an important first step, it says little so far about a possible cosmic-ray effect on clouds and climate.
CLOUD will continue to take data, probably for at least five years, and the apparatus will be refined as researchers learn more about the impact of protons in its atmosphere. They are also planning experiments with larger aerosol particles in CLOUD鈥檚 atmosphere, which they hope will eventually produce artificial clouds large enough to study. This will hopefully settle the question one way or the other, says Kirkby.
Radiocarbon dating
The technique of radiocarbon dating is used widely in archaeology and is possible thanks to cosmic ray collisions in the atmosphere. The collisions produce a number of unstable isotopes, such as carbon-14, which has a half-life of 5730 years. Were it not for cosmic rays replenishing carbon-14 in the atmosphere, this isotope would long ago have disappeared from Earth. When plants die or are consumed by humans or animals, their accumulation of carbon-14 stops. Over time, the concentration of this isotope falls. Comparing the relative amounts of carbon-14 and stable isotopes in an archaeological artefact allows its age to be determined.
Electronic damage
When a cosmic ray passes through an electronic circuit, its energy may cause a transient error to occur. This can be a problem for the electronics found in satellites, spacecraft and even aircraft. A cosmic ray may have caused the flight-control system of a Qantas flight to malfunction in 2008. The aircraft plunged hundreds of metres, causing injuries to many of those on board, but was landed safely.
Software systems in aircraft have since been redesigned to average out sudden power spikes of the kind that a cosmic ray might induce. A cosmic ray is believed to have caused a malfunction two years ago aboard the Voyager 2 spacecraft, which was launched in 1977.
Biological damage
The accumulation of radiation from exposure to cosmic rays can ionise molecules, leading to adverse health effects. Cosmic rays are a significant fraction of the radiation exposure of humans, about 15 per cent of the total background at sea level. This rises rapidly with altitude, however, and in high places such as Denver and Mexico City can be up to five times as high.
Airline crews, who fly long-distance high-altitude routes regularly, may receive double the exposure to ionising radiation. Above the atmosphere, the ubiquitous presence of cosmic rays is a major impediment to human space exploration. In a round trip flight to Mars, crew members would be exposed to accumulated radiation at the limits of what radiological studies regard as safe.