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SYFY WIRE Bad Astronomy

Incredible Gravity Map of the Earth’s Seafloors

By Phil Plait

The picture above is not actually a map of the topography* (height variation) of the Indian Ocean seafloor, 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 seafloor, 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 seafloor? 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 seafloor, 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 are dips in the water are subtle: They may have a height of several meters, which sounds like a lot, but they’re spread out over a long distance, sometimes hundreds of kilometers. The slope of the water is incredibly small and difficult to measure, and this is complicated by currents, waves, chop, and the like. The satellites have to find that gently sloping trend up (in the case of seamounts) or down (in the case of trenches) despite all that noise. The only way to get good measurements is to take a lot of them, and then the noise that changes 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 over the old 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 1,000 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 seafloor 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 seafloor, 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 seafloor 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.

*Correction, Jan. 14, 2016: Well, for Pete's sake. I originally wrote "topology" here, not "topography." What's maddening is I know the difference, but for some reason my brain convinced me I had written the correct word. My apologies to all topologists out there; sometimes my brain twists itself into knots like that.

** Correction, Jan. 14, 2016: Arg! OK, I made a mistake. When I originally wrote this, I said that the measurements had to be accurate on the order of a millimeter to measure the water piling up or flowing away from gravitational anomalies. However, the reality is subtly different: The satellites measure the slopes of the mounds or troughs in the water over great distances to map the topography beneath them. I have to admit: This is even more amazing to me. You're not just looking for a bump in your data that pokes up or down; you're looking for a looooong, slooooow trend, which is in many ways more difficult. So my admiration for this work stand; it just points in a slightly different direction. For more info, read this, and this, and my thanks to Peter Guth for his help.

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