An analysis of the Moon’s gamma-ray brightness performed by Mario Nicola Mazziotta and Francesco Loparco of the National Institute of Nuclear Physics, Bari, Italy, using NASA’s Fermi Gamma-ray Space Telescope’s Large Area Telescope (LAT) instrument revealed that at certain electromagnetic wavelengths the Moon is brighter than the Sun. It’s the result of the interaction of the Moon with cosmic rays so the observations of those gamma rays offer new information on very energetic cosmic events but also on the Moon’s environment. Studying that environment has become more important following the new projects of manned Moon missions.
Studies of the origin of cosmic rays, consisting for about 90% of protons and the rest of electrons and nuclei of various atoms at very high energies, have been going on for decades and various studies collected evidence that they come from supernovae, supermassive black holes and other very energetic phenomena that can be in other galaxies as well. The particles’ strong charge makes them strongly influenced by magnetic fields but the Moon doesn’t have a magnetosphere therefore it’s a target for cosmic rays.
When cosmic rays hit the Moon’s surface layer, the so-called regolith, the impact’s energy is released in the form of electromagnetic radiation at very high energies, gamma rays. Most of it gets absorbed by the Moon but part of it returns to space, sometimes reaching the Earth’s orbit.
The Fermi Gamma-ray Space Telescope’s LAT instrument allowed to collect from its orbit at an altitude between about 526 and 544 kilometers the gamma rays with energies higher than 31 million electron volts, over 10 million times higher than that of visible light.
The organization of the observations of the Moon at gamma rays showed how longer exposures improved the result. The image (NASA/DOE/Fermi LAT Collaboration) shows the Moon observed by the Fermi Gamma-ray Space Telescope with exposures ranging from 2 to 128 months.
The Moon’s brightness at gamma rays varies over time, not with the monthly lunar phases in visible light but following the 11-year solar activity cycle with variations of about 20%. The intensity changes of the Sun’s magnetic field during that cycle determine changes in the amount of cosmic rays that reach the Moon.
The observed gamma rays make the Moon even brighter than the Sun but only at certain frequencies. There are gamma rays at energies higher than a billion electron volts generated by cosmic rays at very high energies that can penetrate the Sun’s magnetic field and strike our star’s atmosphere.
The study of the Moon’s environment has become more important following NASA’s new mission projects to send astronauts starting in 2024. If humans start staying long on the Moon and in the following years they’ll travel long into deep space they’ll require protections from cosmic rays as well.
