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.”
Rosetta’s measurement of the deuterium-to-hydrogen ratio (D/H) measured in the water vapour around Comet 67P/Churyumov-Gerasimenko. The measurements were made using ROSINA’s DFMS double focusing mass spectrometer between 8 August and 5 September 2014.Credits: Spacecraft: ESA/ATG medialab; Comet: ESA/Rosetta/NavCam; Data: Altwegg et al. 2014 and references therein. Click here to get an enlarged view.
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.
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.