Science & Technology

Cosmic-rays make the Moon glow brighter than the Sun

Researchers can use the Moon as a natural, space-based particle detector in order to better understand cosmic rays bombarding us from deep space. (NASA)
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Researchers can use the Moon as a natural, space-based particle detector in order to better understand cosmic rays bombarding us from deep space.

It’s hard to imagine the moon’s glow dominating that of the Sun, but it turns out that if our eyes were attuned to see gamma rays–the night sky would glow brighter than the daytime.

These images show the steadily improving view of the Moon’s gamma-ray glow from NASA’s Fermi Gamma-ray Space Telescope. Each 5-by-5-degree image is centred on the Moon and shows gamma rays with energies above 31 million electron volts, or tens of millions of times that of visible light. At these energies, the Moon is actually brighter than the Sun. Brighter colours indicate greater numbers of gamma rays. This image sequence shows how longer exposure, ranging from two to 128 months (10.7 years), improved the view. (NASA/DOE/Fermi LAT Collaboration)

That counter-intuitive view of the sky is exactly the way NASA’s Fermi Gamma-ray Space Telescope sees our region of space. Currently, gamma-ray observations are not sensitive to see the shape of the moon clearly or any of its surface details, meaning that Fermi’s Large Area Telescope (LAT) detects a prominent glow centred on the Moon’s position in the sky.

Mario Nicola Mazziotta and Francesco Loparco from Italy’s National Institute of Nuclear Physics have been analyzing the Moon’s appearance in the gamma-ray segment of the electromagnetic spectrum. The aim of these observations is to better understand cosmic rays–radiation from deep space in the form of particles moving at near-light-speed.

Mazziotta explains: “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.”

As the particles that make up cosmic rays are electrically charged, this means they are strongly affected by magnetic fields. Earth’s magnetic field protects the surface from a large proportion of the cosmic rays that bombard it, but the Moon has no such protection.

This means that even low-energy cosmic rays can strike the Moon’s surface, meaning the Earth’s natural satellite can serve as a rudimentary space-based particle detector.

When the cosmic rays strike the Moon’s surface, they interact with fine powder which coats it–regolith–to produce gamma-ray emissions. Even though the Moon absorbs most of the gamma-rays some escape.

If we were able to see the Moon in the gamma-ray section of the electromagnetic spectrum its phases would cease to be, as it would always appear ‘full’ to our eyes.

Loparco explains: “Seen at these energies– 31 million electron volts, more than 10 million times greater than the energy of visible light — the Moon would never go through its monthly cycle of phases and would always look full”

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.

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.

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.

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