The Answer May Involve Several Sources

In my last posting, I talked about the news that Comet 67P/Churyumov-Gerasimenko carries water that is isotopically different from Earth water, and the implication that this has for cometary water delivery to early Earth’s oceans. In other words, based on the Rosetta mission’s study of that comet, it’s not likely that the type of comets that 67P came from could have delivered the water that Earth needed. This conclusion (among others) has led planetary scientists and geologists to look at other sources for water. Water-rich asteroids are one possible source, and they’re being looked at in greater detail. Generally speaking, it seems an interesting line of research to follow up on, since Earth accreted from planetesimals in the early solar nebula, and bits and pieces of those early planet “seeds” still exist in the Asteroid Belt and inner solar system.

However, there’s yet another proposition for Earth’s water is catching some attention. This week researchers at Ohio State University announced results from a study they’ve done that suggests early Earth made some of its own water through known geological processes. So, essentially, our planet formed from rocks that had the “stuff” of water (hydrogen and oxygen) bound up in them. When subjected to the heating induced by plate tectonics, the rocks can be made to release water, and that may be what has fed (and continues to nourish) our oceans.

This doesn’t discount the idea that comets also delivered water to the planet, but the evidence this week of wildly differing D/H ratios in some comets (particularly Comet 67P/Churyumov-Gerasimenko) certainly pokes a hole in the idea that ALL the water on our planet came from the infall of comets over time. Yes, there are comets that have D/H ratios closer to Earth’s D/H in its water, but not all comets do. And, if the D/H ratios don’t match, then you have to look at other sources of water. That’s where the Ohio State study comes in. You can read more about it here.

D/H Ratios: a Very Brief Primer and Some References

I want to go back briefly to this whole thing about D/H ratios. The term is a shorthand reference to a whole body of study that looks at hydrogen (H) and deuterium (D), both different isotopes of the same element, and shows how their relative abundances (that is, how much of each one there is) in the universe can tell us something about the evolution of an object. This is a subject that could (and has) filled whole chapters of books and whole papers in journals, so I won’t go into huge details, but will try to give you some factoids about it to help you better understand this part of the whole “where did Earth’s water come from” story.

Most of the naturally occurring deuterium in the universe was made in the Big Bang. That inventory is essentially all the universe has. It can be destroyed in high-energy (and hot) environments, such as in or near stars. So, one hallmark of a warm environment is less (or no) deuterium.

In the case of the Earth water discussion, let’s focus on the D/H ratio for water. It’s a measure of how much hydrogen there is in water compared to how much deuterium there is. You can have D₂O and H₂O in the same environment. If there’s a LOT of D₂O, we call it “enriched” in deuterium compared to hydrogen.

On Earth, the water is low in deuterium compared to comets, which (by comparison) are enriched. The lower amount on Earth can be due to several reasons, among them the warmer environment that Earth enjoys (close to the Sun, plate tectonics supply interior heat, etc.). Earth’s D/H ratio is 1.5 × 10^-4, a ratio that reflects that heating, plus the escape of deuterium to space from the upper atmosphere, and other factors.

Many comets (such as 67P. which has a D/H ratio of 5.3 × 10^-4) have high levels of deuterium. This implies they’ve not been “processed” (heated) by close passages to the Sun. For comets that live in the distant reaches of the solar system, their D levels are very close, if not identical, to the deuterium abundances that they had when the solar system first formed. The comets formed in cooler parts of the solar nebula and thus haven’t been stripped of their deuterium. Comets that live (or pass close) by the Sun can show lowered amounts of deuterium. Studying D/H ratios of comets show us that not all the comets are identical in their isotopic abundances. For example, Comet Halley’s D/H is around 3.08 × 10^-4, still higher than Earth water.

So, the big differences in D/H ratios between Earth water and comet water tell us that the comets were not the sole source of water on our planet. They may have contributed SOME water. But, asteroids are also water-rich remnants of the same material that created the rocky planets,  and  D/H ratios for some asteroids are much closer to Earth’s measurements. So it’s a good idea to look to them for a contribution.

The new result from Ohio may well bolster the idea that the early seeds of Earth delivered water to the still-forming planet back “in the day”, and that our planet is slowly (on billion-year cycles) squeezing that water from the rocks (very doable at the mantle and core boundaries) and delivering it to the surface oceans. That heat-based cycling would also separate (and/or destroy) deuterium in the water, delivering a D/H ratio that we see today. Of course, this doesn’t neatly tie up the story — there is a LOT of research to be done studying more comets, asteroids, and the Earth’s mantle boundary to nail down the whole story of water on Earth.

If you want to read more about water, deuterium, the D/H ratio, check out these sources:

Solar System Deuterium/Hydrogen Ratio (a downloadable PDF survey paper) and  Water on Mars and Venus (another PDF)

Also, the Wikipedia article on deuterium gives you a good background look at this isotope of hydrogen.