The sun sets on the Pacific Ocean in La Jolla, California.Mike Blake / Reuters

Mars wasn’t always so dry and dusty. The Red Planet’s ancient ocean was sprayed right off the planet in violent bursts of solar wind, billions of years ago. The rest of its atmosphere was blasted away, too. Martian waterfalls, rivers, lakes, and the piles of clouds reflected in them—all gone, forever.  

When scientists announced earlier this month they’d figured out the mechanisms behind Mars’s atmospheric loss, several people had the same question: Will Earth end up like Mars someday?

It makes sense to ask. After all, when a huge ocean covered about half of Mars’s Northern Hemisphere, the planet may have looked a lot like Earth does now.

An artist’s rendering of Mars four billion years ago (M. Kornmesser / ESO)

Today, Earth’s atmosphere is fairly well protected by a global magnetic shield. But it won’t be that way forever. Earth will someday lose its oceans, too. What will the planet be like when that happens? And which ocean will be the last to go?

Planetary scientists have several theories about how Earth might dry up, but little else to go on. “To my knowledge there’s no evidence that it’s ever happened in the last 500 million years, for which we have good records,” said Paul Renne, the director of the Berkeley Geochronology Center. “If it did happen, it’s hard to imagine that water would survive anywhere on Earth—no lakes, rivers, glaciers. This would presumably mean 100 percent extinction of biota.”

To imagine an Earth without oceans is to visualize an unrecognizable version of our planet—not a blue marble, but perhaps a brown one. New topography would reveal itself: towering mountain ranges and vast craters that were once submerged by oceans.

Would the last droplets of water evaporate from the deepest Pacific trenches? “Since the ocean is global, there’s really only one ocean. So they’ll all vaporize over the same time frame,” said David Osleger, a planetary scientist at the University of California Davis.

Unless they all freeze at the same time, that is. “As long as there’s water on Earth as a liquid, there will be oceans,” Osleger told me in an email. “The only ways that the water could be lost is via complete freezing—which may have happened long ago, a phase of Earth history called Snowball Earth—or via complete evaporation.”

Snowball Earth might explain why there are glacial deposits in tropical regions. A slightly less extreme variation of this theory, known as Slushball Earth, explains how life on Earth could have survived such a scenario. But neither reveals how the water that covers most of our planet might one day disappear for good.  

Those who believe Earth’s oceans are on an evaporation course say they have about 4 billion years left. By then, our aging sun will have swelled into a red giant, 100 times its size.

By this point, Earth’s temperatures will be in the thousands of degrees. “The proximity of the Sun’s heat will vaporize our oceans and the entire biosphere,” Osleger said. But, humans will almost certainly be long gone by then.

In other words, when Earth’s oceans boil up, the death of the planet won’t be far behind, geologically speaking. Many scientists believe that the sun, in its own death throes, will torch our planet and then engulf it, before collapsing into a white dwarf. Just another dense point of light in a messy universe.

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