The picture above is not actually a map of the topology (height variation) of the Indian Ocean sea floor, as much as it looks like it.
It’s actually a map of the change in the Earth’s gravity field across the Indian Ocean sea floor, and it’s so cool I can hardly stand it.
Why is it cool? For a lot of reasons. One is how it was made: using data from a bunch of different satellites, including Jason-1 and CryoSat-2. These satellites used various methods to determine their exact altitude above the sea surface at any given time. Since the orbits of the satellites are well known, variations in the altitude above the sea surface correspond to changes in the height of the surface. So, for example, if there’s a wave a meter high moving across the ocean, a satellite would measure its own altitude as being a meter lower, since the distance from the satellite to the top of the wave is one meter less than the average distance to the sea surface.
So how does this map the sea floor? Get this: If there’s a mountain under the ocean, then it has more gravity than the water around it (rock is denser than water, so it has more mass per cubic centimeter, which means it has more gravity than the same volume of water). This slight increase in gravity draws in water on the surface around it, piling it up on the surface — water is incompressible, so it doesn’t just flatten out. So when the satellite flies over a seamount, it sees a little bump in the sea surface.
If there’s a chasm or trench in the sea floor, then it has slightly lower gravity than the rock around it, so there’s a corresponding dip in the sea surface above it.
Mind you, these bumps and dips in the water surface are tiny, on the order of a millimeter. This is usually completely swamped by currents, waves, chop, and the like. The only way to get good measurements is to take a lot of them, and then the bumps and dips that change over time will cancel out. It’s like flipping a coin; do it a few times and they might all come out heads, but do it a million times and you can be pretty sure you’ll get extremely close to half heads and half tails.
This was the first thing to amaze me: Scientists can measure the sea surface height to incredible accuracy. The map is based on new techniques that improve the measurements by a factor of two or so.
I got a fun surprise as I read more about this, too: A new unit to think about, called a “Gal”, which is short for “Galileo”. It’s a measure of acceleration, and is equal to one centimeter per second per second. What does that mean?
Gravity is a force that accelerates a mass. On the Earth’s surface, the strength of gravity is enough to accelerate a mass by about 10 meters per second every second. If you drop a rock, after one second it will fall at 10 meters per second (22 mph). After another second it will have accelerated to 20 meters/second (44 mph), and then after another to 30 meters/second (66 mph).
A Gal is an acceleration of 1 cm/s every second, so the strength of Earth’s gravity at the surface is roughly 1000 Gals (it varies a bit from place to place due to density differences in the ground, changes in latitude, and so on).
OK, so what? Well, the maps of the sea floor need some sort of unit attached to them. They’re not really showing the heights of the mountains and the depths of the trenches, because there’s no way to directly measure that this way. They’re displaying the change in gravity. So the maps display this in Gals — actually in milliGals, 1/1000th of a Gal. The darkest red parts of the map are an increase in strength of 90 mGals, and the darkest blue -90 mGals.
Again, this is phenomenal. A milliGal is one-millionth the strength of Earth’s surface gravity! But this is the sort of thing engineers and scientists can measure using the satellite data.
The result is a very detailed and accurate map of the sea floor, which has tremendous value. It can be used by ships and submarines to navigate, of course. But it also maps out where tectonic plates meet, and that has quite a bit of interest to geologists. In fact, a new feature was discovered using these maps: a microplate.
The crust of the Earth floats on the mantle below it, and is broken up into a bunch of chunks called plates. These are pretty big, thousands of kilometers across. As they float, they move around very slowly, a couple of centimeters per year. They collide, grinding into each other, slipping past one another, or one sliding under another. This puts a lot of stress on the surrounding material, and can shear off parts of a plate to create a smaller microplate. Late last year, an oceanic microplate was discovered in the maps, and was dubbed the Mammerickx microplate, after Jacqueline Mammerickx, who was a pioneer in sea floor mapping.
I find this all terribly exciting. Why? Because we live on the surface of Earth, and we’re still discovering new things about it! In many ways, we’ve mapped the surface of the Moon, Mars, and even some comets and asteroids better than we have our own planet. With so much of Earth’s surface under several kilometers of water, it’s hard to know what’s what.
As we’ve seen so many times, the best way to understand our planet is to get off it. By going up above the surface and looking back down, we learn far more than if we never venture off it.
T. S. Eliot was a poet, but he had the heart of a scientist:
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.
He may not have been talking specifically about venturing into space to map our world, but he may as well have been.
from Bad Astronomy http://ift.tt/1ZmEcdV
via IFTTT
沒有留言:
張貼留言