What if someone said that the moon is brighter than the sun? It might bamboozle you but if you see these celestial bodies through NASA’s Fermi Gamma-ray Space Telescope, our moon would appear brighter than the sun. Scientists in the USA space agency have observed our neighbors through the FGST for over a decade and have found the sun isn’t the brightest object in our neighborhood. In fact, a celestial body that doesn’t has its own light is the brightest object in our neighborhood.
Gamma-ray observations are not sensitive enough to clearly see the shape of the Moon’s disk or any surface features. In order to see it, scientists took help of Fermi’s Large Area Telescope (LAT) and found a prominent glowing center on the Moon’s position in the sky.
To develop a better understanding of cosmic rays present in the universe, Mario Nicola Mazziotta and Francesco Loparco, both at Italy’s National Institute of Nuclear Physics in Bari, have been analyzing the Moon’s gamma-ray glow. “Cosmic rays are mostly protons accelerated by some of the most energetic phenomena in the universe, like the blast waves of exploding stars and jets produced when matter falls into black holes,” explained Mazziotta.
These particles of cosmic rays are strongly affected by magnetic fields as they are heavily electrically charged. Since our moon lacks magnetic fields, even low-energy can reach the surface of our natural satellite which eventually turns the moon into a particle detector. Scientists explained that cosmic rays collide with Moon’s surface as it has no magnetic field. Collision eventually results in the production of gamma rays as Moon has a powdery surface called regolith. The soft surface of Moon absorbs majority of gamma rays produced but some of them escape into the atmosphere.
After analyzing the data obtained by the Fermi-LAT, Mazziotta and Loparco compiled the data and created several images showing how the
view has improved during the mission. They rounded up data for gamma rays with energies above 31 million electron volts — more than 10 million times greater than the energy of visible light — and organized them over time, showing how longer exposures improve the view.
“Seen at these energies, the Moon would never go through its monthly cycle of phases and would always look full,” said Loparco.
As NASA sets its sights on sending humans to the Moon by 2024 through the Artemis program, with the eventual goal of sending astronauts to Mars, understanding various aspects of the lunar environment take on new importance. These gamma-ray observations are a reminder that astronauts on the Moon will require protection from the same cosmic rays that produce this high-energy gamma radiation.
While the Moon’s gamma-ray glow is surprising and impressive, the Sun does shine brighter in gamma rays with energies higher than 1 billion electron volts. Cosmic rays with lower energies do not reach the Sun because its powerful magnetic field screens them out. But much more energetic cosmic rays can penetrate this magnetic shield and strike the Sun’s denser atmosphere, producing gamma rays that can reach Fermi.
Although the gamma-ray Moon doesn’t show a monthly cycle of phases, its brightness does change over time. Fermi LAT data show that the Moon’s brightness varies by about 20% over the Sun’s 11-year activity cycle. Variations in the intensity of the Sun’s magnetic field during the cycle change the rate of cosmic rays reaching the Moon, altering the production of gamma rays.