2: Why study the oceans from space?
Most of what we have learned about the oceans over the years has come
from people going out to sea in boats and tossing things over the side
and collecting whatever happens to get caught in their nets. Now
to be honest, there are a few serious drawbacks to this. First, the
ocean is a very big place and ships don't move all that quickly.
Second, conditions in the ocean can change pretty quickly and that fact combined
with their enormous size means that the chances of really characterizing
the conditions in any given place are pretty difficult. And third,
being out at sea for any length of time can be pretty uncomfortable even under
the best of circumstances and most oceanographic research vessels can certainly
NOT be classified as the best of circumstances. Although the
conditions aboard ships and the ways that measurements are made
have improved over the past 100 or more years since the
Challenger expedition opened the era of descriptive oceanography, ship-based
oceanographers are still generally limited to sampling things in a pretty
small area with often a great deal of difficulty.
Satellites on the other hand are wonderful for looking at very large areas of the world in a very short time. This is called synoptic; being able to look at a large region at the same time. To give you one example that clearly demonstrates the differences between ship sampling and satellite observations, let's take a look at a pretty unique part of the world - Tasmania. Located off the southeast coast of the continent of Australia, the ocean around the island of Tasmania is in a constant state of turmoil. Ocean currents collide with one another around the island producing regions of intense mixing which ultimately result in very complex patterns of phytoplankton distributions which can be seen in this image of the plankton chlorophyll concentrations taken by the Coastal Zone Color Scanner (CZCS) way back in 1981. The success of the CZCS mission is what led NASA to develop the SeaWiFS Program...but more on that later.
This satellite image is composed of many individual measurements that the satellite acquired as it scanned the Earth from space. These individual samples (called pixels) are spaced approximately 1 kilometer apart. This particular image represents a portion of the Earth that is about 1000 kilometers wide by 1000 kilometers high which means that there are 1 MILLION individual satellite measurements in this single image. Now imagine that you are on that research ship steaming back and forth across the waters around Tasmania at a constant 10 kilometers per hour (a reasonable speed for a typical research ship as it takes samples) steaming east along a 1000 kilometer cruise track. When you reach the end of one row, you steam north for 1 kilometer and turn and the head west for another 1000 kilometers. You repeat this process of heading back and forth until you have matched the coverage that you see in the satellite image. However, while it took the satellite just a little more than 1 MINUTE to make all those 1 million measurements, it would take you MORE THAN 10 YEARS to make the same number of measurements from the ship.
Not only would you NOT want to spend the next 10 years of your life steaming
back and forth on the waters around Tasmania, but you can easily see that
it would be impossible to capture the kind of variability that you can see
in that satellite image over such a large area from the traditional shipboard
survey. However, the real strength in satellite measurements come when
they are combined with the very detailed measurements that can only
be made by sampling the ocean directly.
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