Radiation on The Moon’s surface

Space radiation on The Moon’s surface has the same sources as on the Earth: The Sun and Galactic Cosmic Rays, but our natural satellite does not have own, strong magnetic field and no atmosphere – the two main factors which protect us against ionization radiation from The Space.

Why radiation on The Moon is important?

Our natural satellite is the best candidate to extend some human activities from The Earth to the other celestial body. Researches in the space environment laboratories, mining materials rare on The Earth, tourism are examples of possible utilization of The Moon. Ionization radiation affects a human’s body as well as electronic devices, both of them may be damaged by the rays, so we need to understand how to protect humans and their devices against the radiation. To effectively protect we need to understand the threat.

Structure of space radiation

The Moon is under bombardment by the same space particles as The Earth is. Particles from The Sun ( solar wind and solar eruptive events) and Galactic Cosmic Rays ( particles emitted outside the solar system ). Because The Moon does not have a strong magnetic field it does not contain belts of trapped charged particles (an equivalent of Van Allen belts around The Earth), rest of the radiation soup is similar to this in other regions of The Solar System.

Structure of primary space radiation about The Moon
Primary space radiation structure around The Moon

As we can see in the picture above, the solar wind has a very dense flux of particle with relatively small energy and GCR has low flux but the energy of particles may have giant energy. In the middle between solar wind and GCR, we have radiation produced by solar eruptive events.

Secondary radiation

The Earth’s magnetic field and atmosphere stop space particles, the radiation hit the atmosphere’s atoms and produce showers of secondary radiation, which reach the ground. In the case of The Moon, the particles coming from space are not stopped and directly hits its surface.

Space radiation particles interacts with The Moon’s ground

Regolith is a mixture of dust, broken rocks, and other related materials covered solid rocks on Mars, Earth and The Moon. Space radiation particles hit it on The Moon surface and are stopped. Depends on the energy of the particle different depth of penetrations and different effects are happening.

  • Solar Wind
    Particles have relatively low energy and are almost immediately stopped when hit the regolith – they penetrate it for less than 1um. If a dose of solar wind particles is intense enough, then can change crystalline regions on the surfaces of grains into amorphous layers. The particles which are retained in the material are the major source of elements rare on The Moon like hydrogen, carbon and nitrogen.
  • Solar Eruptive Events
    The most particles penetrate few centimetres of the Moon’s surface and are stopped by ionization energy loses. In the first centimetre of the regolith solar particles induces a few reactions and secondary particles are emitted, but they do not penetrate deeply. Produced shower of secondary particles is not meaning in overall radiation on The Moon, except during the peak of intense, hard eruptive events on the Sun, when the effects may be similar for this for GCR described below.
  • GCR
    We can distinguish two kinds of particles in the spectrum of Galactic Cosmic Rays: heavy ions and light particles -protons and alpha particles. Heavy ions cannot penetrate materials deeply unlike protons and alpha particles. Heavy ions are stopped in the first 10 centimetres of the regolith and because they carry big energy they produce heave damage in the material structure. Protons and alpha particles penetrate the regolith deeper and produce showers of secondary particles which that extends meters into the lunar surface. Dominant secondary particles are neutrons, and some of them are escaped into space – so-called lunar neutron albedo is created. Beside of neutrons GCR can produce gamma rays.

GCR and intense solar eruptive events, as well as secondary radiation produced by them, are a hazard for humans and electronic equipment on The Moon.

Hazard for humans and inventory

Ionizing radiation on the Moon has been of interest to national space agencies from the start of plans to send people there. Almost every mission to the Moon and its orbit was equipped with a scientific instrument for measuring rays. When the astronauts landed on the silver globe on NASA’s Apollo program, these were risky but relatively short missions. Years later, it turned out that cosmic rays affected the health of the lunar astronauts, nearly half of them died from heart disease, what is exceptional in comparison to the rest of the astronauts.

Few facts about radiation on The Moon

Nowadays, when mankind wants to back to The Moon and explore it, the problem of space radiation backs and several scientific missions research lunar environment. The most important missions which deliver information about the lunar environment are Lunochod missions, Apollo missions, Lunar Prospector, Lunar Reconnaissance Orbiter (LRO) and Chinese Chang missions. I will summarize a few important things here:

  • The primary space radiation structure is similar to this in other regions of the solar system
  • Intense of GCR and its secondary radiation depends on The Sun activity, less solar activity means greater cosmic rays
  • Daily radiation dose on the lunar surface is 2.6 times greater than on the International Space Station
  • The Moon does not have its global, big magnetic field, however, it contains local areas with their own local magnetic fields which can protect the surface against particles from the space
  • Neutron fluxes emitted by the lunar surface depends on how much iron (Fe) is in the ground, for that reason the flux of neutrons is greater in mountains than in mares. Beneath I put maps of neutral fluxes for the two sites of the lunar surface from publication [‘Global maps of lunar neutron fluxes from the LEND instrument’ M. L. Litvak and others ] based on Lunar Reconnaissance Orbiter
  • The Moon emits gamma rays, in this spectrum, it is lighter than The Sun.
The Moon's gamma ray emmission
The Moon gamma rays visible by Fermi Gamma-ray Space Telescope

References

  • (2008) Managing Space Radiation Risk in the New Era of Space Exploration.
  • C. Zeitlin N. A. Schwadron H. E. Spence A. P. Jordan M. D. Looper J. Wilson J. E. Mazur L. W. Townsend (2019) Update on Galactic Cosmic Ray Integral Flux Measurements in Lunar Orbit With CRaTER.
  • GRANT H. HEIKEN, DAVID T. VANIMAN, BEVAN M. FRENCH (1991) LUNAR SOURCEBOOK A User’s Guide to the Moon.
  • J.H. Adamsa, M. Bhattacharyab, Z.W. Linb, G. Pendletonc, J.W. Watts (2007) The ionizing radiation environment on the moon.
  • M. L. Litvak, I. G. Mitrofanov, A. Sanin, A. Malakhov, W. V. Boynton (2011) Global maps of lunar neutron fluxes from the LEND instrument.
  • Shenyi Zhang, Robert F. Wimmer-Schweingruber, Jia Yu (2020) ‘First measurements of the radiation dose on the lunar surface’ [online]. Available at: <https://advances.sciencemag.org/content/6/39/eaaz1334>. Accessed: 2020-09-27

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