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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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