You Gotta Be Kidding!
So, I was standing in the on-board theater after one of my talks on the Celebrity Eclipse a couple of weeks ago and a guy came up to me and expressed his disbelief that comets could have made Earth’s oceans. He thought it improbable and wanted me to prove it to him right there. I didn’t have my handy planetary science textbook with me (there ARE weight limits on how much baggage you can bring on planes these days), so I told him that the evidence pointed very strongly toward the idea that comets played a role in forming our oceans, and that the role is still being explored. I also told him it wasn’t a matter of “belief”, it’s a matter of following the evidence where it takes you, regardless of your personal feelings or limited understanding of what you’re trying to figure out.
Well, lo and behold, the next day as I was checking my email, I found a press release from the folks who run the Herschel Space Observatory, and it was all about evidence in data taken as they studied Comet Hartley 2. This comet originated in a distant region of the solar system that stretches out to the limits of the solar system from the orbit of Neptune. This area is packed with countless icy bodies such as comet nuclei (conglomerations of ice infiltrated with dust and rock), rock-and-ice bodies larger than comets, and objects like Pluto and its sister dwarf planets. It’s far from the Sun, and so the temperatures out there are pretty cold, which have helped preserve the ancient icy relics of solar system formation. That’s important to help understand some aspects of planetary formation—and, as it turns out—the birth of our oceans.
So, the chain of evidence from comet to Earth ocean starts with the formation of our solar system. Roughly 4.5 billion years ago, the Sun and planets began coalescing out of a cloud of gas (including water ices, water vapor, and so on), and dust. As things heated up near the Sun, materials that could only exist in a cold environment were either sublimated (sort of like dry ice), melted, or otherwise destroyed by the heat. That left a small amount of water in the inner solar system, but a HUGE reservoir of water ice-rich material existed in the chilly safety of the outer solar system. Much of it still exists in its primeval state (that is, it hasn’t been heated, and it’s largely as it was when the solar system began to form). So, as planetary scientists have long known, if you want to know what conditions were in the early solar system, that information you most want to study is locked away in the icy bodies in the outer solar system. Since we haven’t yet studied that region of space personally, the next best thing we can do is study things that come to the inner solar system from it—like comets.
Where does Earth come in? At its formation, our home world was likely a hot, dry desert. It, like all the other worlds of the inner solar system, went through a period of bombardment after it formed. The stuff doing the bombarding came from all over the solar system, including the outer reaches where all these primeval chunks of ice exist. And, that outer “stuff” contained water. Lots of it. Bombard our planet with enough of it, and you have the makings of oceans.
Now, that sounds like a good idea, but where are the data to support it? This is where the Herschel data play a role. The spacecraft used special instruments to peer into the comet’s coma—that thin, gaseous atmosphere that makes a comet look all cloudy. The coma forms as frozen materials inside the comet vaporize as they get warmer (as it nears the Sun). This glowing envelope surrounds the comet’s “icy dirtball”-like nucleus (essentially, its core). As the comet gets closer to the Sun, the action of the solar wind pushes the material away, forming that windsock-like tail we see.
Herschel detected vaporized water in the tail of Hartley 2. That’s not unusual. Comets almost always have some element of water ice in their nuclei. What was interesting was the chemical analysis of that water vapor. It contained something called “heavy water”, which is a fancy way of saying that instead of a normal H2O atom that has two normal hydrogen atoms, heavy water has one normal hydrogen atom and one atom of something called deuterium. The ratio of heavy water to regular water (the D/H ratio) in Hartley 2 is the same as Earth’s ocean water D/H ratio. Not all comets have the same D/H ratios, so this narrows the ocean-forming water to a specific family of comets, and tells us that comets like Hartley 2 participated in the formation of Earth’s oceans
And, equally importantly, the deuterium to hydrogen ratio imprinted into water ice of comets such as Hartley 2, which formed out in the distant reaches of the solar system, may well have been quite different from the D/H ratio of water that existed in the warmer inner Solar System.
So, the connection between Earth’s oceans and the Hartley 2-type comets is in the deuterium ratios they share in their water. And, the amount of deuterium in that ratio tells us that Hartley 2’s birthplace was out beyond the orbit of Neptune. It was most likely in the Oort Cloud—a reservoir of ice that is the ancient history treasure chest of the solar system.There’s a lot of that material out there. It would take a very small percentage of that material to contribute to the oceans we have here on Earth.
Now, the whole story isn’t completely finished. Planetary scientists are still sussing out the details about exactly WHERE these comets come from in the outer reaches, and the mechanism that caused so much bombardment to bring water to Earth. But, these observations suggest that Earth’s oceans came from a specific family of comets that exist “out there”, that all have the same D/H ratio that matches Earth’s ocean waters. The next steps are already underway to have Herschel study more comets, and provide data about their D/H ratios.
Stay tuned! It’s looking more and more likely that comets played an important role in populating our planet with ocean water. How big that role was and how long they played it—those are details that planetary scientists can continue to flesh out the story of Earth’s oceans and their history.