NASA image shows Earth’s lumpiness — exaggerated 10,000 times

No, the Earth is not flat. But it’s not exactly round either. Nor is it — let us stop you right there, smartypants — an oblate spheroid.

Close enough, but not a geoid

An oblate spheroid is a geometrically smooth model of our planet that, due to its rotation, bulges at the equator and flattens at the poles. While it is close enough to be used by GPS systems, for instance, it’s not quite right: It doesn’t account for the lumpiness of our actual Earth.

The technical term for the model that does is a geoid. This model shows what the ocean surface would look like if the entire planet were covered by water and sculpted only by gravity, leaving the surface slightly lumpy.

What does that look like? Nothing like this. This visualization by NASA exaggerates the Earth’s surface dents and bumps by a factor of 10,000, making our cosmic home look like a slightly deflated beach ball, or a somewhat lumpy potato.

Don’t get us wrong: This is not a shocking new discovery. Scientists have known for a long time about our planet’s inner potato. But this NASA image, released in July by its Scientific Visualization Studio at Goddard Space Flight Center, puts a fresh coat of paint on it.

NASA’s new model draws on the GOCO06s gravity field, a dataset built from more than a billion measurements collected over 15 years by 19 satellites, including NASA’s own GRACE mission and the European Space Agency’s GOCE — a sleek, finned spacecraft dubbed “the Ferrari of space.”

The actual numbers may be less impressive than Planet Spud here suggests, but are still surprising: The geoid has a high point of 85 meters (279 feet) above average sea level, and a low of 106 meters (348 feet) below, south of India.

Wide enough to fit the Gateway Arch

To visualize that: The Brooklyn Bridge’s suspension towers rise about 85 meters out of the water; and if you stand the Hollywood Sign on its side, that’s a height (or a depth) of 106 meters. So the total spread of the geoid’s lumpiness is 191 meters (627 feet), wide enough to fit in the Gateway Arch in St. Louis.

Which is the tallest monument in the U.S., and the tallest arch in the world, so nothing to sneeze at. But all in all, these are fairly modest variations. Don’t we have much taller mountains? This is where we need to do a better job of explaining what a geoid actually is.

The actual, physical surface of the earth varies by about 19.7 kilometers (12.2 miles), from the top of Mount Everest to the bottom of the Mariana Trench. But the geoid does not represent that actual surface: It’s a model of what the Earth’s surface would look like if it was all ocean — minus winds, tides, and landmasses, but plus rotation and gravitation.

And this is where the lumpiness — modest as it is — comes from: Gravity does not exert a uniform pull across every part of the planet. That is because the Earth’s density varies.
Earth is not a homogenous cannonball, but rather a barely-set custard of rock, magma, ocean trenches, mountain ranges, and ancient subducted tectonic plates. Some are denser than others, and as a result, some parts of the ground beneath our feet have a stronger pull on us.

Relatively light subsurface material includes sedimentary rocks like sandstone and shale, granite, and salt domes. The heavier stuff includes rocks like basalt, a rock type called gabbro, and peridotite, a main upper-mantle rock.

Everywhere, even Milwaukee

Park a chunk of extra-dense mantle rock under the seafloor, and it pulls water toward it, piling up in a subtle mound. If elsewhere you have an unusually light crust, the theoretical sea surface will sag into a shallow basin. (Yes, counterintuitively, higher gravity will push the sea up, not down, and vice versa).

Do this across the entire globe, and you get the geoid, with its equipotential surface. Meaning that every point on it has the exact same gravitational potential. Because it finds a theoretical level based on sea level, the geoid’s surface runs underneath all solid continents: under the Andes, under Milwaukee, under your house. The geoid doesn’t care. It’s not a place you can visit and send postcards from — it’s a place you can only calculate.

And as calculated, variations in the geoid’s surface add up to just one Gateway Arch. Why doesn’t a massive range like the Himalayas exert a larger pull on the geoid? Well, those majestic and forbidding peaks do have some effect. But not as much as you’d think from looking at them. Even their combined mass is still relatively small compared to the vast, deep, structural variations in density inside the Earth’s mantle and core.

More importantly, water seeks equilibrium, so if the geoid bulges slightly toward those mountains, gravity will quickly pull it sideways and down. Either way, the geoid’s bumps and dips are tiny next to real terrain — its entire 191-meter range is less than 1% of the nearly 20-kilometer span from Everest’s peak to the ocean’s deepest trench.

For a sense of proportion: If you shrank the Earth down to the size of a billiard ball, the geoid’s 191-meter wobble would be smaller than the irregularities that professional billiard rules actually permit on a real ball.

Which is why, if you play NASA’s to-scale version of the geoid animation, you’ll observe…a seemingly smooth, perfectly boring sphere. Nothing to see here. The kicker is of course the exaggerated version, ripe for exhibition at a planetary freak show. (For the hard of reading, NASA captions the animation of the exaggerated version with the disclaimer: “NOTE: Not to scale!!!”)

As mentioned, astronomers have been mapping the gravitational variations in Earth’s surface for a while now. In 2005, the very first picture of Planet Spud went viral.

More than just a potato fetish

ESA’s GOCE craft was instrumental in producing Earth’s next-generation gravitational portrait picture. Launched in 2009, it was so sleek because it had to fly absurdly low — low enough to capture the clearest picture yet of our lumpy Earth, exaggerated 7,000-fold. NASA’s image, built on an expanded dataset, is the latest and lumpiest entry in the time-honored tradition of revealing our planet’s inner tuber.

But why? Well, the geoid actually is a lot more than just an astrophysicist potato fetish. It does in fact define what surveyors, engineers, mapmakers, and adjacent professionals mean when they say “sea level.” Wherever you are, your elevation is measured against the geoid rather than the actual sea.

In a slightly less practical but even more fascinating way, the geoid is also a readout of the Earth’s insides. That bulge over Iceland lines up nicely with the Mid-Atlantic Ridge, and the hot mantle material that feeds into its many active volcanoes.

That deep dip south of India? Scientists call it the Indian Ocean Geoid Low, and they’re still figuring out why it’s there. The Earth’s largest gravitational dimple may be a kind of tombstone over a graveyard of ancient, dense tectonic slabs sunk deep into the mantle over the past 200 million years — the final remains of the Tethys Ocean, now long vanished.

Which is amazing, when you think of it. Just like life itself on this billiard ball. Precisely because of its imperfections.

Strange Maps #1298

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