Today on Earth, people across the globe will be reflecting on water for World Water Day.
—NASA Goddard Space Flight Center Image by Reto Stöckli
We are a water world—just about any gradeschooler can tell you that water covers roughly 70 percent of Earth’s surface.
The trick is that just 2.5 percent of that water is fresh, and about two-thirds of our fresh water is locked away in glaciers.
Organizers of the UN-sponsored campaign point out that there’s a limit to the clean, drinkable water available to the human race, and we’re making things worse via pollution, climate change, and wasteful consumption.
One interesting suggestion is getting to know where your water comes from, on the theory that once you’ve seen the source, you’ll care a bit more about keeping it safe.
So on a cosmic scale, where did Earth’s water come from?
Really, there’s a heck of a lot of H2O in our solar system, which is perhaps unsurprising, considering that hydrogen is the most abundant element in the known universe.
In addition to the recent discovery of water on our own moon, several moons of other planets are suspected of having vast subsurface oceans, and lots more astronomical bodies definitely have large amounts of water ice. For example:
Go Googling for moons with oceans, and the first one you’re probably gonna stumble upon is Jupiter’s moon Europa. It’s only slightly smaller than our moon, and it has much the same components as other rocky bodies in our solar system, i.e., water and silicates. In fact, the top layer seems to be all water down to about 62 miles, or a hundred meters.
At great distance from the nice warm sun and no atmosphere to trap surface heat, the very top is clearly solid ice. But thanks to internal heat from tidal forces, Europa appears to have a liquid layer. It’s unclear exactly how deep the liquid goes and how warm it gets—but at least one scientist thinks the alien ocean is saturated with enough oxygen for fish-size things to be swimming around down there.
Jupiter’s biggest moon also shows signs of having an ocean under an icy crust, albeit a pretty salty one.
Magnetic data collected by the Jupiter probe Galileo hinted that a layer of liquid water or slushy ice lurks deep below the solid ice surface, while oddly young-looking surface features suggested that relatively warm, salty material had not long ago oozed up through fractures in the crust. In Ganymede’s case, however, heat to maintain this liquid would come from natural radioactivity in the rocky parts of the moon’s interior.
Saturn’s largest moon is pretty unique, being the only known moon with an atmosphere and the only body other than Earth with stable bodies of liquid on its surface. Of course, Titan’s lakes are filled with methane …
Still, very recent research suggests that deep down inside, Titan is pretty watery, maybe all the way down to its core.
You’d think that a moon with huge icy geysers would be a pretty good candidate for having an underground ocean.
But scientists have been surprisingly divided about whether the insides of Enceladus are liquid or solid ice, with a process called sublimation (solid turning directly to gas) generating the water vapor-filled plumes.
The latest data from the Cassini orbiter suggest there’s some liquid sloshing around under the surface, based on the discovery of water ions in the plumes that appear on Earth only where water is in motion.
Often called dirty snowballs, comets are balls of ice and dust left over from the formation of the solar system. Unlike asteroids, which formed between the orbits of Mars and Jupiter, comets likely took shape at the far reaches of the solar system.
The recent Stardust mission to Comet Wild 2 suggests that comets formed when rocky material long ago burned by the sun somehow got flung beyond Neptune’s orbit, where it got cemented together by primordial ices.
The comets we see in the sky as “stars with hair” were shook lose from their distant orbits by the gravity of larger bodies, such as Neptune, and sent hurting toward the inner solar system. These visitors display their glorious tails because proximity to the sun is heating volatile ices, and solar wind is pushing the resulting gases.
As for Earth, it’s entirely possible, with all this water floating about the solar system, that we’ve always had oceans. Or have we? Our closest neighbors—Mercury, Venus, and Mars—are conspicuously dry by comparison. Water that exists in quantities that count is mostly found out among the gas giants.
In fact, a recent study made the case that Earth would have been so hot in its infancy that any water we had to start with was vaporized.
Which brings us back to comets.
About 3.8 billion years ago, the inner solar system went through a period called the Late Heavy Bombardment, as evidenced by ancient impact craters on the moon. By that time, Earth would have cooled down a bit, so if the swarm of projectiles was full of comets, those icy bodies may have delivered Earth’s oceans.
“We may sip a piece of the impactors every time we drink a glass of water,” the study authors wrote in a 2009 paper in the journal Icarus.
Food, or drink, for thought. So the next time a comet swings past Earth, go take a gander, and maybe seeing the source with your own eyes will provide inspiration to protect our planet’s precious water supply.
—Moon interiors and Comet Wild 2 all courtesy NASA/JPL