This week’s “guess the planet” image comes from comet 67P/Churyumov-Gerasimenko.
Credit for this image goes to ESA’s Rosetta Team.
Rather than talking about the geology in this image I drew
your attention to the dark smudge like feature in the bottom part of the image.
This feature is entirely man made as it is actually the shadow of the Rosetta
spacecraft, cast on the surface of the comet during a close flyby in 2015. ESA
report that the shadow is 20 by 50 metres, indicating the scale of the image.
At the time this image was taken the Rosetta orbiter came
within six kilometres of the comet’s surface allowing it to take images of an unprecedentedly
high resolution. As ESA outline on their website the fact that the spacecraft is
casting a shadow during its close approach is also very significant. This means
that Rosetta was passing between the comet and the sun, and thus the sun was
directly overhead.
This meant that the surface features didn’t cast shadows
across the image and allowed the Rosetta team to determine the reflectance of
the materials which make up the comets surface. As planetary scientists we are
often interested in the “albedo” of a surface. This is the proportion of light
which it reflects, and can tell us a lot about the sort of materials the
surface is made of. However the lighting conditions at the time an image is
taken will have just as much of an effect on the brightness of an imaged
surface as its albedo. Because there are few shadows in this image (apart from
the obvious one) the Rosetta team know that the reflectance of the surface materials
reflects their true albedo.
The angle of the sun is very important when examining remote
sensing images. A couple of weeks ago I talked about how you can use the
direction of shadows to distinguish between positive and negative relief
features. The length of shadows can also be used to add a third dimension to
what would otherwise be a flat image. So long as you know the angle of the sun
you can use some basic trigonometry to determine the height of the feature
casting the shadow.
The sun is thus a very useful tool in interpreting the
surface of another planet.
Image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The Original image, and information about it's properties can be found here:
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