Last week’s guess the planet image comes from the Earth.
However, it isn’t a view which many of us will be familiar with. This is a
section of a map of the ocean floors, showing the central Indian Ocean. The red
areas show regions of high ground, while the blue patches indicate lower
regions. The full version can be found at this
link. This map is one of the most detailed views of the entire ocean floor to
have been produced, and was a vital contribution to the science of bathymetry,
the study of underwater topography. The map was produced using gravity
measurements. These show how the water in the oceans is displaced by variations
in the height of the sea floor. This is called a geodesic method, as it relies
on the shape of the earth, and the variations in gravity, rather than direct
imaging of the sea floor.
This study highlights an important trade off that must be
considered when looking at data of this sort. Ideally we want the best quality
of data available, the most detailed images and the highest resolutions.
However coverage is also vital. If we only have high resolution data of a few
key places that can often be less useful than less detailed images which cover
a larger region. The latter can give us a better understanding of the context
and environment as a whole. This trade off is very important when considering
spacecraft images. Many probes only have a short time to record data, so it is important
to carefully consider how much high resolution, low coverage data should be acquired,
and when it is more important to get larger scale, lower resolution images.
Mapping the ocean floors is no different. There are many
methods for recording bathymetric data, but many rely on ship mounted
instruments to image the depths. Techniques like sonar can be used for this but
they require a ship to actually sail over the area being studied taking
measurements as it does. However, relatively few survey ships are available to
chart the sea floor, and so most maps of that sort have been made for major
shipping routes, rather than trying to systematically build up a global
picture. It is easy to forget how large the world is, or how much of it is
covered by water. Surveying the entire ocean floor by this means would produce
a very detailed dataset, but would take far too much time and effort to be
achievable.
The gravity based method cannot match the resolution of sonar
measurements, but is the best way to get a global picture of the ocean’s depth.
The global picture is vitally important when examining the ocean floor, because
the topography of much of the sea bed is defined by the global process of plate
tectonics. The pattern of highs and lows shown in this map indicates the boundaries
of the Earth’s tectonic plates, allowing the way they meet and interact to be
studied in unprecedented detail.
The spreading centres where plates pull apart are marked by
ocean ridges, while subduction zones are often accompanied by deep trenches
like the Marianas Trench, where the lowest point on Earth can be found. I
actually chose the Indian Ocean because the mid Atlantic ridge might have been
a little too conspicuous; whereas the region shown above has a range of
topography, reflecting its complex tectonic history.
Image Credit: NASA Earth Observatory
Apologies for the delay in bringing you this blog. Things have been very hectic, and its taken longer than usual to get this one ready. I will probably wait until December before posting the next guess the planet, so that I have a looming deadline out of the way.
