The Moon could become a detector for the most energetic particles in the Universe, according to two physicists based in the US and Spain.
They suggest the pings as 鈥渃osmic ray鈥� particles drill their way into the lunar soil could be picked up by radio telescopes.
The highest-energy cosmic rays, primarily protons and atomic nuclei, are millions of times more energetic than those from Earth-bound particle accelerators with energies of more than 1020 electronvolts.
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Such particles are also exceptionally rare: each square kilometre of the Earth鈥檚 surface will be struck by one only once a century on average.
Ice cube
The best way to improve the chances of detecting one of these particles is to have a very large collecting area. The ICECUBE detector in Antarctica, for instance, makes use of vast expanses of ice. 鈥淏ut the biggest volume of material in our neighbourhood is the Moon,鈥� says Jaime Alvarez-Mu帽iz of the University of Delaware in Newark.
Alvarez-Mu帽iz and his colleague Enrique Zas of the University of Santiago de Compostela in Spain have calculated in detail the electromagnetic signal that should be generated when an ultra-high-energy particle hits the nearside of the Moon.
The cosmic rays burrow a few centimetres into the lunar surface, generating a shower of particles. These particles produce 鈥淐erenkov radiation鈥�, an electromagnetic emission given off by particles travelling through a medium faster than the speed of light in that medium.
The radiation is emitted across the whole electromagnetic spectrum, but radio waves escape the soil most easily. 鈥淲e predict a nanosecond burst of radio waves, which should be easiest to detect at a wavelength of a few tens of centimetres,鈥� says Alvarez-Mu帽iz. 鈥淭he best place to look is near the circumference of the Moon.鈥�
Netting neutrinos
The two physicists also simulated the impact of ultra-high-energy neutrinos on the Moon. 鈥淭hese also generate showers of particles which produce radio waves,鈥� he says.
According to Alvarez-Mu帽iz and Zas鈥檚 calculations, radio telescopes looking at the Moon should be able to detect as many as 250 ultra-high-energy cosmic rays a year. 鈥淐urrently, only 17 particles with energies greater than 1020 electronvolts have ever been detected, so the technique could be very important,鈥� says Alvarez-Mu帽iz.
A radio search for ultra-high-energy particles is planned by a team led by Peter Gorham of the NASA鈥檚 Jet Propulsion Laboratory in Pasadena, using two dishes at Goldstone in California. 鈥淲e are quite hopeful we will see something within the next two years,鈥� says Gorham.
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