This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Gaze at the Moon from Earth's surface and you'd be forgiven for thinking that it's a static, immutable piece of the heavens. Month after month it cycles through its phases. Month after month the same complex patches of lighter and darker coloration are easy to see.
To the human eye the Moon is a piece of a clockwork universe. But of course we know this is an illusion. Every year gravitational tides between the Earth and Moon dissipate a little more of the energy pent up in our paired system, and the Moon slides about 3.8 centimeters further from us. We also know that the Moon experiences regular meteorite impacts, and experiences its own peculiar, and varying, interaction with the solar wind due to localized, weak magnetic fields at its surface.
The Moon is also a massive sponge for cosmic rays - high energy atomic or subatomic particles streaking into the solar system at near light-speed, driven by extraordinarily energetic processes elsewhere in the universe.
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When these particle smash into the lunar surface some of their energy is converted to photons, at very high energies. Some of those photons, in the gamma-ray part of the electromagnetic spectrum, escape to stream back out into the cosmos.
NASA's Fermi Gamma-ray Space Telescope, an orbital high-energy astronomical observatory, operating since 2008, has been steadily accumulating data on that backwash of lunar gamma-rays. The image at the head of this post represents a little over ten-years' worth of data, literally a decade-long exposure. In an energy band some ten million times higher than that of visible light the Moon not only glows in gamma-rays, it actually outshines the Sun as seen from Earth.
The gamma-ray Moon doesn't have phases, since it's being bombarded by cosmic rays from all sides, irrespective of where it is in its orbit around us. But there is a modest 20% or so modulation of its brightness in synch with the Sun's 11-year activity cycle. The reason is thought to be the associated variation in the Sun's magnetic field strength and behavior, which alters the 'shielding' of our solar system from electrically charged cosmic rays. The more particles that penetrate through to the Moon, the brighter its gamma-ray emission.
This dataset is a beautiful reminder that our day-to-day perception of the world is woefully limited. All around us are phenomena playing out, as they have done for billions of years, which connect not just to our nearest astrophysical neighbors but literally to the fundamental rhythms of the universe.
