When it comes to gamma radiation, the moon is actually brighter than the sun

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The sinister, infernal glow from the moon may seem unreal in this picture because it is invisible to us. But the instruments that detect the gamma rays tell us that it's real. More than a red and grainy image, it is a living reminder that not everything is needed for the human eye.

It is also a reminder that all humans who visit the Moon must be protected from this high-energy radiation.

Cosmic rays with gamma rays

NASA's Fermi Gamma-Ray Space Telescope captured these images from the gamma rays of the moon. In this part of the electromagnetic spectrum, the moon is brighter than the sun. This is because the Sun produces most of its energy in other parts of the spectrum, although it emits gamma rays, especially during solar thinnings.

Most of the gamma rays in our solar system come from distant sources such as quasars and active galactic nuclei (AGNs). The Moon is an indirect source of gamma radiation and produces gamma rays through its interaction with cosmic rays.

Cosmic rays are a type of high-energy radiation that, for the most part, is produced outside our solar system. They are produced by things like supernovae and active galactic nuclei. When cosmic rays hit matter, like the surface of the moon, they create gamma rays.

Artist concept of an active galactic core hosting an energy blazar. Active galactic nuclei are a source of cosmic rays. When these rays hit the moon, gamma rays are created. Credit: Conceptual Image Lab from NASA / Goddard Space Flight Center.
Artist concept of active galactic cores hosting an energy blazar. Active galactic nuclei are a source of cosmic rays. When these rays hit the moon, gamma rays are created. Credit: Conceptual Image Lab from NASA / Goddard Space Flight Center.

Mario Nicola Mazziotta and Francesco Loparco, two scientists at Italy's National Institute of Nuclear Physics, are studying the gamma radiation of the moon as a means of understanding cosmic rays. Cosmic rays are particles that move rapidly and draw their acceleration from their sources, such as the aforementioned supernovae and AGN.

"Cosmic rays are mainly protons accelerated by some of the most energetic phenomena in the universe, such as the shock waves of stars and explosive jets produced when matter falls into black holes," explained Mazziotta in a press release from NASA.

The particles that make up cosmic rays are electrically charged. When they strike a magnetic field, like the Earth's magnetosphere, they are mostly deflected. But the moon lacks a magnetic field. As a result, even the weakest cosmic rays strike directly on the Moon's surface, producing gamma rays. The Moon actually absorbs most of the gamma rays it creates, but some escape into space.

And the Fermi telescope can see them, turning the moon into a kind of inadvertent particle detector.

The Fermi Gamma Ray Telescope (FGRST) has been operating for 11 years now. Mazziotta and Loparco studied the images of the Moon from the duration of the telescope's mission and, over time, the view has improved.

"… The moon would never go through its monthly cycle of phases and would still look full."

Francesco Loparco, National Institute of Nuclear Physics of Italy.

The strength of the gamma rays of the moon is not always the same. It varies with time. Mazziotta and Loparco collected data on the moon's gamma rays exceeding 31 million electron volts, or 10 million times more than visible light, and organized them over time. This resulted in the next image, which shows that the view improves with time.

<img src = "https://www.universetoday.com/wp-content/uploads/2019/08/MoonvsTimesingleimageen.jpg" alt = " These images show the ever-improving vision of the gamma ray glow of the moon from NASA's Fermi Gamma space telescope. Each 5 by 5 degree image is centered on the moon and shows gamma rays of energies greater than 31 million electron volts, tens of millions of times higher than those of visible light. At these energies, the moon is brighter than the sun. The brighter colors indicate a larger number of gamma rays. This sequence of images shows how a longer exposure, ranging from two to 128 months (10.7 years), improved eyesight.
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These images show the ever-improving vision of the gamma ray glow of the moon from NASA's Fermi Gamma space telescope. Each 5 by 5 degree image is centered on the moon and shows gamma rays of energies greater than 31 million electron volts, tens of millions of times higher than those of visible light. At these energies, the moon is brighter than the sun. The brighter colors indicate a larger number of gamma rays. This sequence of images shows how a longer exposure, ranging from two to 128 months (10.7 years), improved eyesight.
Credit: NASA / DOE / Fermi LAT Collaboration

"Seen at these energies, the moon would never go through its monthly cycle of phases and would still look full," Loparco said.

The fact that the moon emits these gamma rays is a warning. NASA's Artemis program will see more astronauts on the moon for potentially longer periods than other missions on the moon. They will have to be protected from both the cosmic rays that hit the moon and the gamma rays of the moon that result.

A complex interaction

The interaction between cosmic rays, gamma rays, the moon and the sun can be complex. Gamma rays can have different levels of energy. For example, these FGRST images only capture gamma rays exceeding 31 million electron volts (MeV) by a certain amount. But the gamma rays can be much more energetic than that, and can represent billions, even trillions of MeV.

The Fermi Gamma-Ray space observatory is not the only observatory to see the gamma radiation of the moon. The Compton gamma ray observatory captured this image of the gamma rays of the moon. Image Credit: By DJ Thompson, DL Bertsch (NASA / GSFC), DJ Morris (UNH), R. Mukherjee (NASA / GSFC / USRA) - ur = http: //heasarc.gsfc.nasa.gov/ docs / cgro / epo / news / gammoon.html., Public Domain, https://commons.wikimedia.org/w/index.php?curid=19838942
The Fermi Gamma Radiation Observatory is not the only observatory to observe the gamma radiation of the Moon. The Compton gamma ray observatory captured this image of the gamma rays of the moon. Image Credit: By DJ Thompson, DL Bertsch (NASA / GSFC), DJ Morris (UNH), R. Mukherjee (NASA / GSFC / USRA) – ur = http: //heasarc.gsfc.nasa.gov/ docs / cgro / epo / news / gammoon.html., Public Domain, https://commons.wikimedia.org/w/index.php?curid=19838942

Since the electric charge of cosmic rays means that they can be deflected by magnetic fields and the Sun has a strong magnetic field, only the most powerful ones can hit the Sun. In turn, these powerful cosmic rays hit the dense part of the Sun's atmosphere and create very powerful gamma rays. The sun is therefore brighter than the moon with gamma rays greater than 1 billion electron volts.

The 11-year cycle of the Sun also affects the cosmic rays that hit the moon and the gamma rays that result. During this cycle, the sun undergoes variations of its magnetic field. As a result, sometimes more cosmic rays hit the moon than other times. This variability of the cosmic rays striking the lunar surface creates a variability of the lunar gamma rays. According to Fermi data, it can vary by 20%.

The Sun's magnetic field varies in intensity and complexity over its eleven-year cycle. This comparison shows the relative complexity of the solar magnetic field between January 2011 (left) and July 2014. In January 2011, three years after the solar minimum, the field is still relatively simple, with open field lines concentrated near the poles. At solar maximum, in July 2014, the structure is much more complex, with closed and open field lines that stand out everywhere.

The gamma rays from the Moon and the cosmic rays that cause them pose a threat to astronauts because they are both ionizing radiation of great penetrating power. It takes a lot of protection to prevent them from hitting the astronauts. Materials with high atomic numbers are effective shields. Lead (atomic number 82) is a good shield because it is also very dense.

For low energy gamma rays, the risk to astronauts is due to exposure over time. Think of a radiology technician versus a patient on x-ray. The exposure of patients to x-rays in their lifetime is not very high, so they accept the risk. For the technician though, things are different. They are exposed every working day, so they leave the room and are protected from X-rays by materials like lead.

It's the same for astronauts. The more time they spend on the moon in a gamma / cosmic ray environment, the more they need to limit their exposure. Not only by protecting, but by timing.

Try to understand the radiation environment of the moon

These data from the Fermi gamma ray space telescope help scientists understand the risk of gamma / cosmic rays on the moon. If there are times when the moon emits 20% fewer gamma rays because of the Sun's 11-year cycle, it may be wise to take advantage of this period.

If we saw the sky in the form of gamma rays, in the same way as the Fermi space telescope, it would be unrecognizable. This image was built from six years of Fermi observations. It shows the whole sky at energies between 50 billion (GeV) and 2 trillion electron volts (TeV). The luminous band in the middle is the central plane of the Milky Way. Some of the brightest sources are pulsar wind nebulae and supernova remnants in our galaxy, as well as distant galaxies called blazars powered by supermassive black holes. The labels indicate the highest sources of energy, all localized in our galaxy and emitting gamma rays greater than 1 TeV. Image Credit: NASA / DOE Collaboration / Fermi LAT
If we saw the sky in the form of gamma rays, in the same way as the Fermi space telescope, it would be unrecognizable. This image was built from six years of Fermi observations. It shows the whole sky at energies between 50 billion (GeV) and 2 trillion electron volts (TeV). The luminous band in the middle is the central plane of the Milky Way. Some of the brightest sources are pulsar wind nebulae and supernova remnants in our galaxy, as well as distant galaxies called blazars powered by supermassive black holes. The labels indicate the highest sources of energy, all localized in our galaxy and emitting gamma rays greater than 1 TeV. Image credit: NASA / DOE / Fermi LAT collaboration

Radiation exposure is one of the major obstacles to space travel and long-term space missions. The magnetosphere and the atmosphere of the Earth are both radiation shields. Even in low Earth orbit, astronauts may be exposed to greater radiation.

If we want to have a human presence on the moon, it is imperative that we understand the radiation environment there. NASA has already been looking at the lunar radiative environment as early as 2005 in anticipation of a human outpost on the moon. When they launched the Lunar Reconnaissance Orbiter (LRO) in 2009, it contained an instrument called the Cosmic Ray Telescope for Radiation Effects (CRaTER).

CRaTER's work consists in characterizing the Moon's radiation environment and its biological impact on astronauts. It uses plastics to mimic human tissue and places them behind different protective materials. Harlan Spence, Senior Researcher at CRaTER, said, "We will not only measure radiation, we will use plastics mimicking human tissue to see how these extremely energetic particles penetrate and interact with the human body."

Fermi's images of the gamma rays of the Moon are another piece of the radiation puzzle. And it's a puzzle that must be solved before realistically hopes for a long-term lunar base or crewed Mars missions.

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