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10th of September 2020
Moonlight and Earthlight
by Emil Lenc
Just like our car radio, we can tune our radio telescopes to observe at specific frequencies - it is the equivalent of seeing different colours with an optical telescope. Observing different radio "colours" allows us to understand the nature of the radio emissions and the likely physical processes causing them. In the case of the Moon, when we observe it with the Australian Square Kilometre Array Pathfinder (ASKAP), what we see is a mostly featureless disk (the image on the left). Interestingly, this is the case regardless of the actual phase of the Moon. This is because we are "seeing" thermal emission from many metres below the surface of the Moon - unlike the surface temperature, it remains relatively constant and so appears relatively uniform across the lunar surface. When we tune ASKAP to observe at certain other frequencies, the nature of the Moon changes abruptly - a very bright spot appears at its centre (the image on the right). This is the radio equivalent of earthlight. The earthlight (also known as the Moon's ashen glow) we are most familiar with is when sunlight reflected from the Earth's surface and clouds dimly illuminate the otherwise dark part of a crescent Moon. However, the earthlight we see in radio is different - what we are seeing is evidence of humanity! That bright spot is a reflection of all of our human-made radio transmissions from the side of the Earth facing the Moon i.e. all of our TV transmissions, radar, mobile communications, etc. This emission is of particular interest for those searching for extra-terrestrial intelligence as it allows us to measure how much human-made radio emission is "leaking" into space and the nature of the emission. Understanding how our civilisation looks like from space may help us detect similar emission from neighbouring planetary systems where intelligent life may exist. (Image credit: E. Lenc, V.A. Moss, & K.W. Bannister (CSIRO) and D.C. Price (Curtin/Berkeley))

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