A new giant cosmic ray detector could be the most massive planet in our Solar System. Three scientists at the University of St. Andrews in the UK suggest that Jupiter, with its thick atmosphere and colossal size, could be the ideal place for hunting the highest energy cosmic rays, yet.
Cosmic rays are high-energy particles that are thought to originate from objects outside of our Solar System. Because they consist mainly of high-energy protons and atomic nuclei, scientists think that some of the highest-energy sources in the universe could produce such rays. Supernovae and active galactic nuclei are two possible origins.
 |
| X-ray, Optical and Infrared composite of Kepler’s Supernova Remnant. Credit: NASA/ESA/JHU/R. Sankrit and W. Blair |
So far, the highest-energy cosmic rays that scientists have observed reach about 1020 electron volts. That’s 40 million times the energy that accelerated particles in the Large Hadron Collider attain.
Particles with this kind of energy will destroy living tissue and wreak havoc on microelectronics. Thankfully, these high-energy cosmic rays are incredibly rare and the highest energy rays, which can do the most damage, are the rarest.
The more rare the particle, the larger surface area scientists need in order to better their chances of observing a single signature, which brings us to Paul Rimmer, a research fellow at the University of St. Andrews who led the study.
 |
| Illustration of how the Pierre Auger Observatory works. Cosmic rays enter the atmosphere creating a shower of particles that then rain down onto the observatory’s detectors depicted here as the grey cylindrical objects. Credit: Pierre Auger Observatory. |
Rimmer is reaching into a rare realm, searching for cosmic rays with energies that surpass 1020 electron volts. Such a task, however, is extremely limited by current cosmic ray detectors on Earth.
“If you go to ten times more energy [from 1020 to 1021 eV], you would expect there to be fairly close to a thousand times less particles,” Rimmer said. “If you have less particles, you need larger area in order to detect them.”
With a collecting area of about 1,200 square miles, the Pierre Auger Observatory in Argentina is designed for detecting high-energy cosmic rays with energies between 1018 and 1020 eV. To detect cosmic rays of even greater energies, scientists would need a detector about ten times larger, Rimmer and his colleagues explain in their paper recently published in The Astrophysical Journal Letters.
 |
| Jupiter with Great Red Spot taken by NASA’s Voyager 1 in 1979. Credit: NASA |
In terms of large collective area, Jupiter fits the bill. To observe high-energy cosmic rays on Jupiter, Rimmer and his two colleagues suggest using the Large Area Telescope (LAT), which is the primary instrument aboard the Fermi Gamma Ray Space Telescope.
When cosmic rays bombard Jupiter, they will interact with particles throughout the planet’s atmosphere. A shower of particles results from this interaction, but Rimmer is particularly interested in the gamma rays that would be part of that shower. And LAT has the right equipment to observe the desired energy range of the gamma rays in which Rimmer is interested.
“We could aim Fermi at Jupiter and what Fermi would look at is gamma rays, the very highest energy particles of light produced from extensive air showers,” Rimmer said. “The cosmic rays that produce these sorts of air showers would be so rare that you would be talking about a span of days between events.”
 |
| Illustration of air showers from high-energy cosmic rays. Credit: Pieter Kuiper |
No one has observed cosmic rays with energies as high as those Rimmer and his colleagues discuss. There is some reason to believe that such high-energy cosmic rays do not exist or are impossible to detect because they would interact with other matter in space long before reaching our Solar System.
“If they do exist this would have two implications,” Rimmer said. “Either they would have to be produced close to us so they wouldn’t have to travel so far that they would be destroyed or that relativity breaks down and there’s some sort of new physics going on.”