This week’s guess the planet image comes from Mars, and on
this occasion I’ve chosen a feature related to my own research. This is a
HiRISE image of scalloped depressions near Peneus Patera in the southern hemisphere
of Mars. Credit goes to the NASA HiRISE team. This image is just under a kilometre
wide.
The depressions in this image are interesting for a number
of reasons. They are likely to form as a result of the sublimation of ground
ice. At this latitude on Mars there is frequently a thick layer of ice and
dust. When temperatures rise, during the martian summer, this ground ice can
sublimate, changing from a solid straight to a gas without the need to become
liquid first.
This results in the loss of a substantial volume of material
in the ground, leaving the large depressions seen in this image. You can see
that there are several edges in the larger image above, and that the ground is
littered with smaller pits, which have yet to form full depressions. The size
of these features suggests that a lot of water ice must once have been in the
ground here. So how would you go about working out the volume of material (both
dust and ice) which has been lost?
One technique which planetary scientists use to determine
depth is to look at the length of shadows. We know the time of day at which any
image is taken, and thus the height of the sun in the martian sky at that time.
This means that if we have some clearly defined shadows in an image we can use
some simple trigonometry to work out the height of the feature casting them.
Unfortunately in this case there aren’t any particularly
good shadows to use. The steep slopes are all on the northern edge of the
feature, which is quite irregular due to its scalloped shape. Furthermore these
features have an asymmetric profile. The pole facing slope (in this case the
one at the top of the image) is typically very steep, while the equator facing
slope is much shallower. This all means that there aren’t any particularly good
shadows to measure.
Fortunately we have another tool at our disposal, a second
image of the same area. With this it is possible to actually create a 3d model
of the site. When we see things, our eyes each receive a slightly different
view of whatever we are looking at. The slight differences between what each
eye is seeing are interpreted by the brain as a 3d image. We can do the same
thing with a computer, by comparing the differences between two images to
figure out the real shape of the objects being imaged.
This is similar to how a 3d film is made. Filmmakers use two
cameras simultaneously, a process which goes right back to the 19th
century when “stereoscopic photographs” were all the rage. In our case we need
to wait for the spacecraft to pass over the site again, and take a second image
from a slightly different angle. This was done for this site, creating what is
called a stereo pair between this image and PSP_005698_1225.
By putting the two images
together, and doing a lot of painstaking processing, we can not only produce a
3d image, but what is called a “digital terrain model”. This is a 3d map of the
area, which will show us the height of the walls of the depressions as
determined by the differences between the two images. We can use models like
this to make very precise measurements of height and depth.
Hopefully some of you
reading this have a pair of the 3d glasses with the red and blue lenses. If so
take a look at this image, which is called an “anaglyph”. Here the two offset
images have been combined, with each having a different colour. The coloured
lenses in the glasses allow each eye to look at a different image, and trick
the brain into seeing it in 3d. Obviously this isn’t as good for making precise
measurements, but can give you a better sense of the landscape than you would
get from a flat image.
Image Credit: NASA/JPL/University of Arizona HiRISE image
PSP_004274_1225 http://www.uahirise.org/PSP_004274_1225
Anaglyph Credit: NASA/JPL/ University of Arizona
http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/ANAGLYPH/PSP/ORB_004200_004299/PSP_004274_1225_PSP_005698_1225/PSP_004274_1225_PSP_005698_1225_RED.browse.png
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