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comet, Herschel Space Telescope

Comets Made Oceans???!?

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You Gotta Be Kidding!

Earth's oceans likely contain elements of ancient comets. Image courtesy of NOAA/Vents, Korea Polar Research Institute (KOPRI)

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.

This illustration shows the orbit of comet Hartley 2 in relation to those of the five innermost planets of the Solar System. The comet made its latest close pass of Earth on 20 October, coming to 19.45 million km. On this occasion, Herschel observed the comet. The inset on the right side shows the image obtained with Herschel’s PACS instrument. The two lines are the water data from HIFI instrument. Credits: ESA/AOES Medialab; Herschel/HssO Consortium

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.

astronomy, astronomy news, comet, comet elenin, conspiracies, planetary science

Comet Elenin: Scientific Facts vs. Bravo Sierra

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Get Your Straight Skinny Right Here

I used to study comets for a living.  They’re iceballs, mixed with a little dirt.  They’re pretty small as solar system bodies go — often not more than a couple of miles (or kilometers across). They orbit the Sun just like planets do, and once you know a comet’s orbit (or any solar system object’s orbit), you can predict it pretty well.  They don’t suddenly change their orbits without reason (see Kepler’s laws of Planetary Motion, which apply to comets and asteroids in general (see discussion under “First Law”) as well, to understand why).

To really “get” what a comet is and does, the next time it snows in your neighborhood, take a handful of snow and mix it with some dirt.  If it doesn’t snow, then go get a snowcone or get some chipped ice and mix it with dirt to make an iceball.

Heft it in your hand. Look at it.  It’s not very dangerous on its own, is it? Common sense tells you that it doesn’t have much mass, it doesn’t have a strong gravitational pull.  If you could build a snowball maybe a mile or two across and put it into orbit around the Sun, you’d have a comet.  Most comets are made of water ice, with traces of other ices mixed in (carbon dioxide ice, methane ice, stuff like that that we know the physical principles of).  They orbit the Sun, often in very long orbits that take them out beyond the orbits of Mars, or Jupiter or even Neptune.  There are many, many comets and each one has the same basic makeup and long orbits.  I findthem fascinating because of what they are and where they came from, and what they tell us about the solar system’s history.

The true value of comets is really what they tell us about the conditions in which they formed.  that’s what always kept me interested in the comets we studied. Each one carries a treasure trove of chemical information about the elements in and conditions prevailing in the early solar system.  In the original solar nebula, the cloud of gas and dust from which the Sun and planets formed, gases such as hydrogen, oxygen, nitrogen and so forth were pretty abundant. So were grains of dust and water and other molecules. Because space temperatures are cold, many of the molecules existed as frozen ices.

As the conditions at the center of the nebula warmed up (where the Sun was forming), the hot bright radiation of the protosun destroyed any icy material that existed nearby.  Only the icy materials and gases in the far reaches of the solar system (mostly out beyond Jupiter,where the temperatures were cold enough to support icy objects) survived.

Comets come from a reservoir of icy chunks that has existed beyond Neptune’s orbit since the very earliest epochs of solar system history.  All these objects — collectively grouped as Oort Cloud objects — orbit the Sun, but at very huge distances.  And, as I mentioned above, they carry the chemical evidence of what it was like in the early solar nebula. That makes each comet a treasury of information.

So, how do comets get to the inner solar system?  Their orbits are changed by entirely normal and scientifically understandable circumstances. Since they’re small, it doesn’t take much to nudge a cometary nucleus from its orbit into a slightly different orbit — one that takes it closer to the Sun. The most logical and commonsense suspects for such gravitational nudges would be nearby planets (dwarf or otherwise), or possibly a passing star (and yes, stars can do that) at the very edges of the solar system.  Spacecraft (alien or otherwise) would not be big enough to nudge a cometary nucleus, but a close pass with a body the size of Pluto, for example, would.

Anyway, once nudged, the cometary nucleus is on a new orbit — and often times that orbit is one that takes it in toward the Sun and through the orbital paths of other planets and asteroids.  As a comet gets closer to the Sun, it feels more of the Sun’s gravitational pull, and—at that point, you can see how Kepler’s laws really do work—a comet’s orbit is shaped by the gravitational tug of the Sun and any planetary bodies it flies close to.  If it happened to get close to Earth, it might be affected by that, for example.

This is all very natural and, if you understand what orbits are and how they evolve over time due to natural forces, then you “get” what comets do. They’re frozen chunks of ice and dust, following paths set in motion a long time ago

So, there’s this comet called Elenin doing its closest pass to the Sun during its elliptical orbit.  It’s doing what all things in orbit around the Sun do—which is completely normal and nothing to be worried about.   Its path will take it close enough so that we could spot  it, but not close enough that it’s going to do anything to us.  Even if it passed really close to Earth, its mass is so small and its body so inconsequential that nothing would happen. Really.

Comet Elenin as seen by STEREO spacecraft, August 6, 2011. From Earth, presently the comet is a faint smudge of light in deep sky exposures. By late August comet Elenin could be visible to the naked eye as a dim "fuzzy star" with a tail.

So, here’s the skinny on Elenin’s appearance in our skies. On October 16 of this year, it will be approximately 22 million miles (35 million kilometers) from Earth. That is 90 times the distance between Earth and the Moon (which lies around 238,000 miles (~333,000 kilometers) away).  It’s probably not going to be very bright in the sky, and you may need binoculars to see it. So, it’s not really the brightest comet to come into the inner solar system. Certainly many amateur and not a few professionals will take a look at it, and measure its tail and gas out put to help understand its chemical makeup. But, that’s about it.  Another entirely normal cometary appearance in the solar system.

There are a LOT of people out there, posting on the Web about how Elenin is going to blot out the Sun, or align with some other celestial body and cause trouble for Earth in some other way. Some of the stuff I’ve read even invokes unknown aliens, UFO fleets (that nobody except the Bravo Sierra vanguard can see), suddenly appearing and disappearing mysterious spacecraft, and other ad hoc fantasies. It’s like reading about the Bermuda Triangle or voodoo economics—lots of Bravo Sierra, few (if any) provable facts.

It really is all nonsense. There’s no other polite way to put it.  These fantasies are written by people who haven’t taken the time to learn the basic laws of physics and Kepler’s motions. It’s kind of like reading financial news from people who don’t understand how money works or soccer stories written by people who don’t know the rules of the game.

How an object as small as Elenin could blot out the Sun from a distance of 22 million miles makes me laugh. This is a really small comet. If you were looking directly at the Sun (never a good idea though—since it would burn your retinas in a few seconds, so don’t even think about it) and the comet passed between us and the Sun, I doubt you’d even see the difference.  That is, if you could see at all after staring at the Sun that long. Do you really want to trust your eyesight to idiots on the Web who post such nonsense?  So, why trust their “scientific knowledge”?  That’s right. You wouldn’t.

You probably should read all the nonsense though—it’s always good fine-tune your Bravo Sierra Detector(TM), especially as we head into an election year in the United States. And, in these tough economic times, a little laughter at silliness can be a good thing, as long as you know it’s silliness.  I know that logic and the laws of science are sometimes less enticing and entertaining than out-and-out nonsense.

Before you do wade through the Web-enabled fantasies about this comet, arm yourself with some scientific facts.  Check out the Comet Elenin FAQ, written by people who know the science of comets. The more you know, the less likely it is you’ll be taken in by purveyors of Bravo Sierra.