This week’s guess the planet image comes from Mercury and
was captured by the Messenger spacecraft using two separate instruments. These
are the Mercury Dual Imaging System (MDIS) which mapped the surface, and the Mercury
Laser Altimeter (MLA) which recorded the topography. I’ve talked before about some of the ways of producing topographic data from orbital spacecraft. In this case a somewhat different method has been used. The
instrument used a laser to record the distance between its orbital position and
the ground. Since the movement of the spacecraft is known very precisely it is
possible to time how long it takes a laser to reach the surface and derive distance
from this (since the speed of light is known).
Here this elevation data has been combined with the visual
data from the spacecrafts cameras. The result is a more useful and indeed more
impressive image than either instrument could generate on its own. The elevation
data has been used to create a perspective view across this part of mercury, so
that details of the positive and negative relief can be clearly seen. The false
colour component of the image shows the elevation, with reds indicating high
features and blue showing low elevation
areas. This is a fairly standard colour scheme for showing topography, and thus
allows us to interpret this landscape at a glance. More information about this area of mercury can be found at this link, to the NASA description of this image.
What we see is quite interesting. The area is clearly a
cratered plain, like much of Mercury. However there is a clear divide between
high elevation areas to the left of the image and lower elevation areas to the
right. The divide between these, which cuts across the central crater is the Carnegie
Rupes, a tectonic escarpment. “Rupes” means cliff or escarpment in Latin. This feature
is the result of the gradual shrinking of mercury. It is a small planet, only
slightly larger than Earth’s moon. As a result the core has cooled more rapidly
than that of Earth, and this has resulted in contraction of the planet. As the
planet shrinks a large number of faults and escarpments form as sections of the
crust get crinkled up.
By combining data from a variety of sources we can get a
good look at the structure of the cliff in this area. This allows us to
determine how and, in some cases, when it formed. For example in this location
the cliffs seem to cut through the basin of the Duccio crater. This tells us
that the crater must already have been present when the cliffs formed, and thus
that one feature is younger than the other. Planetary scientist rarely work
from isolated images, but compare and contrast between different datasets, and
use overlays like this to highlight features of the landscape. Any false colour
data can be combined with a visual map in order to make it clearer when
variations match other features. For example instruments which measure chemical
and mineral signatures on the surface can be combined with other data to
indicate where specific patches of materials occur, and whether they are frequently
associated with specific elements of the landscape.
All spacecraft carry a range of different sensors, and try
to collect as much diverse data on each orbit as they can. This is then
transmitted back to Earth and processed and combined by the scientists who
study that planet. The end result are higher level products such as this, which
can be used to study the planet, or demonstrate specific features of its
landscape.
Image Credit:
ASA/Johns Hopkins University
Applied Physics Laboratory/Carnegie Institution of Washington
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