Water, Earth, and Comets: Science Tells Us How Things Are

The latest news from the Rosetta mission is that comet 67P’s water is different from Earth’s water. This is challenging a long-held idea that comets supplied most of the water in our oceans, lakes, and rivers. The latest proof comes from measurements of an isotope of hydrogen called “deuterium”, made by the mission’s ROSINA instrument at Comet 67P/Churyumov-Gerasimenko.  It’s telling us that, in the words of the old jazz favorite, the idea of Earth water from comets “…ain’t necessarily so.”

 

Let’s look at the evidence for this claim. The graphic here from the Rosetta mission illustrates what I’ll call “elemental compositions” of water in various solar system bodies. The element in question is hydrogen (which makes water (H₂O)) , and the differences about where they appear on the graphic are due to the presence of an isotope of hydrogen called deuterium.

Earth’s water is in blue, and it lies on a blue line stretching across the graph. That blue line is a measurement of Earth water’s D/H ratio (which indicates how much deuterium is in the water) and its value is around 1.5 × 10^-4

The comet water (in two shades of pink) is plotted on a blue line, stretching out from the y-axis, at a particular value equal to 5.3 × 10^-4.  Without getting into a lengthy explanation of the details, this really means that the comet’s water has MORE deuterium than the Earth water does. In a nutshell, it implies very strongly that Earth water did NOT come from the comets.

Notice something else?  Look at the asteroids.

Yup, the values for Earth water and are very, very close. In fact, for water measurements in some asteroid groups, the D/H ratio is identical to Earth water’s.

Hmmmm….

That points to asteroids being a much more likely source of Earth water than comets. This becomes particularly interesting when you look at what D/H ratios tell us about something that happened a long time ago—the formation of objects in the solar system.

Let’s step back a little and talk this through.

For a long time, planetary scientists wondered about where our water did come from. Comets seem like an obvious source, since they’re largely water, right? And, it was assumed that they just crashed onto Earth (and other worlds) early in the solar system’s history. That’s a logical assumption to make, but in science, you can’t just assume stuff; you have to prove it with observations and experiments. And, people have done that, studying studying Earth water, comet particles and water vapor measurements made by the Rosetta spacecraft at Comet 67P, as well as asteroid chunks.

Why asteroids? As it turns out, asteroids have water in them, too. And, they are a major component of the rocky material that created our planet (as well as Mercury, Venus, and Mars).  It makes sense that they delivered their water, too. Add that in to whatever gases (“volatiles” in science-speak) that outgassed from Earth to create the primordial atmosphere and oceans, and you have – perhaps – another piece of the water puzzle on Earth.

Based on that evidence, it looks more and more like the solution is that Earth’s water didn’t come primarily from comets, and even more interesting, that most (if not all) comets have water that is significantly different from Earth’s. This is also requires us to look at where comets formed in the original solar nebula and where they “live” now, something that planetary scientists are still figuring out.

I spoke with Alan Stern about this latest finding. He’s on the Rosetta mission, and is an expert on outer solar system bodies and comets (and PI on the New Horizons mission to Pluto and beyond) in particular. He pointed out that it could well turn out that the comet population in our solar system is heterogenous in D/H. If so, most (if not all) comet populations may turn out to have the same D/H ratios, and their water will be more like Comet 67P’s water than like Earth water.

So, where does that leave us?  It’s looking more and more like asteroids played a more important part in delivering water to the ancient Earth. Comets haven’t been ruled out, yet. And, that’s what has planetary scientists buzzing. If the D/H ratios hold up across large populations of comets, that has pretty important implications for our understanding of conditions in the ancient solar system. It’s not a done story yet, there is still a lot of work and many observations to be done to nail down the origins of Earth’s water. But, the evidence against comets as water sources is really starting to look compelling.

That’s the main story. I’ve got some background information below if you’d like to delve more deeply into this fascinating story. Also check out the press release from the Rosetta mission.

Background: What’s D/H and How Does it Explain Ancient History?

Scientists study and compare the different isotopes of hydrogen that makes up the water at Earth, comets, and asteroids. (In particular, they’re interested in hydrogen and deuterium, which has a different neutron number than hydrogen.) Astronomers are finding tiny but incredibly significant differences in those samples. Those variations point away from comets as being the sole-source suppliers and possibly toward asteroids as playing a larger role in creating Earth’s water supply.  Not that comets didn’t supply some water, but it’s quite likely that asteroids contributed most of it.

The measurements found tiny differences in what are called the D/H ratios of the water. D/H stands for “deuterium to hydrogen”, with deuterium being a “heavier” type of hydrogen.  It’s a key marker for understanding the origin of Earth’s water (among other things).  To give you an idea of the differences between Comet 67P water and Earth water, Comet 67P has a D/H ratio of right around 5.3 x 10^-4.  Earth water has a D/H ratio of right around 1.5 x 10^-4.  That means that the comet’s water is more enriched by deuterium than Earth water.

Earth water D/H ratios are much more similar to the D/H ratio of asteroids located between Mars and Jupiter, which could mean that the water in our oceans and lakes and rivers and atmosphere may have come mainly from asteroids and possibly certain comets that resided in much closer to the Sun. Meteorites (pieces of asteroids that fall to Earth) that originally came from asteroids in the Asteroid Belt also match the composition of Earth’s water. Thus, despite the fact that asteroids have a much lower overall water content, impacts by a large number of them could still have resulted in Earth’s oceans.

What about the comets?

The great mass of comets in our solar system date back to the earliest epochs of solar system history. They now reside in reservoirs very far from the Sun. Those collections are the Oort Cloud, which stretches out to over 10⁵ AU (astronomical units, 1 AU is the distance between Earth and the Sun), and is the source of long-period comets such as Halley. The Kuiper Belt, located at a distance of over 50 AU, is known to be the origin of the comets of the family that 67P/Churyumov-Gerasimenko belongs to.

According to the Rosetta mission results (made using the ROSINA instrument), the comets in this family may not all come from a single source region, the Kuiper Belt. Some may have originated in the Oort Cloud. That could explain their higher amounts of deuterium, and thus show that they couldn’t be part of Earth’s water supply. As we study more comets, planetary scientists  will get more D/H ratios of outer solar system bodies (and of course, next year’s New Horizons mission flyby of Pluto will be looking at this problem, too).  This is how we’ll find out if the idea that all (or most) cometary bodies have a different D/H ratio is true.

In general, different solar system bodies have different D/H ratios. The amount they contain depends on where they are found in the solar system, and if they have been subjected to heating (by the infant Sun, for example, or during orbital passes by the Sun) or through other processes that disrupt or destroy deuterium. Much of the material in the most distant regions of the solar system has hardly changed since the very earliest epochs of solar system formation, so objects from the Kuiper Belt and Oort cloud will still have their original D/H ratios. It also means that such objects that wander into the inner solar system couldn’t have been big contributors to water on Earth (and other rocky bodies). Kuiper Belt and Oort cloud objects are “pristine”, which means they open a window into what the conditions and chemistry of materials were like in the solar system some 4.5 billion years ago.

On the other hand, heat is a destroyer of deuterium, which could explain why closer-in comets and asteroid have different ratios in their water than materials from more distant-residing comets. This could help scientists understand the conditions in the solar nebula in the regions where Earth and the other rocky planets formed and eventually gained their atmospheres (and in the case of Earth, its water inventory).