Earth Is a Potato
Earth, in most renderings, is a smooth sphere with a glossy complexion—a blue marble, as pictures snapped from space have shown us. Earth scientists know that’s not exactly true. Earth, in fact, is an ellipsoid, a little bit squashed at the poles and fat around the equator, not to mention speckled with mountain ranges. And then you have the geoid people—the ones who think of Earth less as an imperfect sphere and more as a lumpy potato.
C. K. Shum, a professor at Ohio State University, is one such person. He works in geodesy, which involves the study of Earth’s gravity field, and he sees the planet through a different lens than the rest of us. Imagine that the shape of Earth is subject only to the influence of its own gravity—no tides, no winds, no currents—and that water is distributed accordingly over the whole planet. Protrusions correspond to the denser parts of the planet, which exert more gravity; the indentations indicate the less dense bits, which wield less gravity. Scientists call this the geoid. The differences in Earth’s gravity are actually quite small, so most visual representations of the geoid are exaggerated to highlight the bumpiness, Shum told me. In this view, Earth looks ready to be drizzled with some olive oil and sprinkled with herbs, and then thrown into the oven.
Potato Earth has popped up on the internet over the years, each time confounding some people who mistakenly believe that this is what Earth looks like stripped of water. Really, our planet’s surface isn’t anywhere near so tuberish. But the geoid is a reminder that Earth is wonkier than we might have realized—delightfully so. Behold our imperfect potato planet! It’s weird, but it’s ours.
The geoid is a complicated concept, especially if you’re not one of the several experts I begged to explain it. After all, it’s not a shape we can see naturally. “We cannot measure it,” E. Sinem Ince, a scientist at the GFZ German Research Center for Geosciences, told me. “We can only collect gravitational data and model it approximately.” The idea originated in the early 19th century with the German mathematician Carl Friedrich Gauss, but it wasn’t until satellite technology came along, in the 20th century, that geodesists could outline the potato with real precision. A widely known visualization of the geoid, constructed from tens of thousands of space-based measurements of Earth’s gravitational field, comes from the institute where Ince works, in Potsdam, Germany. The geodesy community calls it the Potsdam potato.
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The Potsdam potato may not be what Earth really looks like, but it’s also not entirely imaginary. The geoid has applications on the ground as we know it; according to the geodesy experts, an imaginary ocean at rest is a fantastic reference for infrastructure planning. For example, taking the geoid into account ensures that every floor of your brand-new skyscraper is level, Shum said—if it were built today, with the power of the potato, the Tower of Pisa would probably be closer to upright.
Potato Earth is not a static model, Marcelo Santos, a geodesy professor at the University of New Brunswick, in Canada, told me. That’s because Earth itself is always churning through volcanism and plate tectonics, shifting mass around. “Any mass has its own gravity, so if you move mass, you move gravity,” Santos said. Some shifts unfold over millennia; parts of Earth are still rising after the previous ice age ended, like a sofa puffing back up after its occupant stands, he said. Other changes occur over shorter periods, such as during floods and droughts.
Monitoring those changes has real scientific value. Studying the geoid helps scientists measure the loss of ice in polar regions as a result of climate change, the geodesy experts told me, and better understand what happens when our planet becomes a baked potato. It also helps them learn more about the Earth’s mysterious depths; by studying spots on the planet that exhibit higher- or lower-than-average gravity, researchers can investigate the mass-shifting processes unfolding there. The 3-D geoid rendering most recently made the rounds attached to a story about a “gravity hole” in the Indian Ocean, and the undersea magma plumes that might be causing it.
Earth is not alone in its irregularity: The solar system is full of potato worlds, gravitationally speaking. Mercury, Mars, Venus, and even our own moon “show rugged potato features too,” Shin-Chan Han, a geodesy professor at the University of Newcastle, in Australia, told me. Compared with Earth, Mars is more uneven, thanks to its giant volcano Olympus. Celestial tubers likely exist everywhere in the universe; the odds are that potatoes are more common than gravitationally uniform balls. (Whether any alien geodesists are likening them to alien root vegetables is another question.)
[Read: Our solar system in true color is really something else]
I asked Santos what life would be like if Earth were a homogenous ball of rock, and the geoid a perfect sphere as a result. Nothing catastrophic would happen, he said. In fact, things might be a little easier. Scientists wouldn’t have to refine their measurements of the shape-shifting geoid over and over. “It’d be boring, and I’d be unemployed,” Santos said. A lumpier world is far more interesting. It’s already changed how I see the world. For starters, I’ll never look at a potato in the same way again.