Source of Earth’s water: A little idea about D/H ratios

You couldn’t have missed the headlines: Due to Rosetta’s measurements of the ratio of hydrogen and deuterium in 67P/Churyumov-Gerasimenko’s water vapor, the hypothesis that Earth’s water had been brought here by comet impacts sometime around the Late Heavy Bombardment suffered a heavy hit, so to say. In short: Most comets have a much higher ratio of D/H (deuterium/hydrogen) than Earth. The only known exception is comet 103P/Hartley 2. Of course, we don’t know how representative the current sample may be of comets hitting Earth nearly four billion years ago, but it’s now supposed that Earth’s water had been more likely brought here by asteroid impacts (as most investigated asteroids have a D/H ratio similar to Earth), or possibly that most of our water has been here since Earth’s formation (only not always on the surface). According to seismic measurements, about twice as much water as is in our oceans, lakes, glaciers and atmosphere may be contained in high-pressure mantle rock. So far, it’s not very clear if this water gets to the surface due to tectonics or is stored there for billions of years. However, slow degassing of the mantle rock may have formed Earth’s oceans. We may not need to explain the abundance of water on Earth by impacts of other bodies after all. It’s still in the early stages of research, we shall see later how each of the hypotheses fares in the light of new results.

But summing this up is not why I’m writing this post – you can read so much more in any popular science article on the web. I have an idea, which may or may not be completely off – I’m not a geologist or a chemist, so I’d be glad if a more qualified person told me whether it’s really stupid, clever or somewhere in between. Suppose for a moment that comets had indeed brought most of our water on Earth. How do we get from an average comet’s D/H ratio to Earth’s?

That’s where mantle rock comes back into the game. Could it have been more depleted in water in earlier stages of Earth’s existence, only gradually enriched in it until reaching some equilibrium?

So. In this scenario, we have water with more deuterium in it than now and mantle rock gradually absorbing water from the surface reservoirs.

What intrigues me is this: Is there any reason why heavy water would be absorbed in the mantle disproportionately to water containing hydrogen atoms?

We don’t need to devise any kind of preferential intake into chemical reactions due to the little higher molecular height of water containing deuterium instead of hydrogen (I don’t even know of any inorganic chemistry process that would distinguish between izotopes well enough – some enzymes are capable of that, but let’s leave biochemistry aside here). When I discussed this little idea of mine yesterday, Professor Markoš offered an easy solution when he mentioned studies showing that normal water evaporates more easily than heavy water (suggested also by the very high D/H ratio on Venus). If we evaporate a significant portion of the oceans of “comet water” and the water still in its liquid form is being incorporated into the bedrock, we may get a disproportional amount of heavy water stored in the mantle compared to what’s left on the surface. The Earth cools, the water whose molecules haven’t dissolved and atoms reached the escape velocity, gradually rains back. The D/H ratio of surface water is now lower than at the beginning.

That’s it in a nutshell. Now, this scenario is far too simplified. We’d need to put some constraints on the intake of water into the mantle, rate of evaporating under the presumed temperature and atmosphere composition in the given time range, create a model, etc., and finally try to falsify the hypothesis by measuring D/H ratio in our mantle water. Which is, you know, kind of difficult to do…

What I’m interested in is: Is it at least remotely possible, or am I far off on this track? Geologists, chemists, astronomers – fire at will!


For those interested in D/H ratios in different parts of our solar system, this graph from Altwegg et al. 2014 shows it quite nicely:



Book launch at Fenixcon

My newest novel, “Bez naděje”, is having a book launch ceremony at Fenixcon this Saturday afternoon. More info and a snippet from the novel (in Czech) is to be found here.

In another news, I’m invited as a speaker at University Pardubice’s seminar (on December 17) about the relationship of philosophy and science. I’ll be speaking about this topic from a biologist’s perspective, and if I have enough time, I’ll dive into the topics of formulation of hypotheses, development of science and (dis)advantages of reductionism a bit.

Science-fictional & real science

If we have seen further, it’s largely by standing on layers of previous errors.

I had a lecture last week about cognitive biases, their possible adaptiveness and also impacts on science. It also led me to think about the old “hyper-competent scientist” trope so typical for SF. Science-fictional scientists can often recite complex information verbatim and know the answer to every question, even if it’s unrelated to their subfield of research – but then again, there are few molecular biologists focused on studying only one class of receptors in SF. Science-fictional scientists are usually either “generalists”, or very well-informed about basically every single subject of their field. A biologist can easily identify any plant or animal, run various analyses, create a model of a protein’s active site as well as an ecosystem simulation. And if they by chance don’t know something, they are able to quickly look the relevant information up or find out.


Continue reading

A new article: Realms of Dark, Deep and Cold

These places never see sunlight, are buried deep under thick ice crusts and warmed mostly by radioactive decay and tidal forces: subsurface oceans of celestial objects far from their stars – if they have any. Decades ago, they were the domain of science fiction, until such places were hypothesized in our solar system thanks in part to Voyager flybys of Europa in 1979. Shortly after, the idea was popularized when it appeared in Arthur C. Clarke’s Space Odyssey saga. Since then, we learned much more about characteristics of possible subsurface oceans, discovered that they probably exist on more worlds than we dared to expect just a few years ago, and that they’re more fascinating than even SF authors hoped.

My article on the topic of subsurface oceans was published today in Clarkesworld Magazine. I wrote about moons and dwarf planets in our system as well as extrasolar planets; however, the topic is so vast that I couldn’t have possibly covered everything of interest – especially when virtually any piece of information is interesting and thought-provoking. If you’ve read the article and want to go deeper and learn more, you can read some of the following material I’ve used. Many of the scientific papers can be downloaded without any special access (use Google Scholar). The rest should be accessible from most university libraries.

If you don’t want to dig into the scientific articles at first, I can recommend the popular science book Alien Seas: Oceans in Space. It doesn’t deal just with subsurface oceans of icy objects; it concerns nearly any conceivable kind of oceans in a broad sense of the word, in our system as well as in the rest of the galaxy. It’s an excellent introductory read, well-written and an interesting food for thought.

There is plenty of resources about Europa but it’s never a bad way to start with a relatively recent good review. That’s the case of Kargel et al. (2000); very comprehensive information about Europa’s history, geology, characteristics of both the crust and the ocean and its prospects for life can be found there. Specifically conditions for methanogenesis as an energy source for possible life on Europa are discussed in McCollom (1999). More about all three Galilean moons possibly containing bodies of liquid water and consequences of different parameters is to be found in Zimmer et al. (2000) and Spohn and Schubert (2003).

A lot has been published about Saturn’s moons Enceladus and Titan; this is just a tip of the iceberg: Titan’s probable internal structure is described in Tobie et al. (2005). Regarding the tiny Enceladus, Roberts and Nimmo (2007) investigated the long-term stability of its ocean; analysis of ice grains from its geysers in Saturn’s E-ring is present in Postberg et al. (2009); shear heating as a heat source for the ocean is discussed in Nimmo et al. (2007); possible conditions for life in Parkinson et al. (2007); this along with possible biomarkers in McKay et al. (2008).

A paper by Hussmann et al. (2006) dealt with modeling the interior of icy satellites of the giant planets and trans-Neptunian objects. This work represents a turning point of a kind – a subsurface ocean even in very far Kuiper belt bodies like Eris and Sedna (sometimes also considered an inner Oort cloud object) was first officially proposed here. Thermal evolution and possible cryovolcanism of KB objects is also investigated in Desch et al. (2009).

Concerning Pluto, Robuchon and Nimmo (2011) modeled Pluto with several different initial condition sets and proposed what observable features might tell us about the possible presence of the ocean during the New Horizons flyby. Spectroscopy of Pluto, its moon Charon and Neptune’s Triton is described in Protopapa et al. (2007), including the detection of crystalline water ice on Charon’s surface.

A very good overview of possibilities of life in the Solar System, including subsurface oceans, and opportunities of energy cycles and geoindicators of life detection can be found in Schulze-Makuch et al. (2002).

Speaking of even further places, Ehrenreich and Cassan (2006) investigated the possibilities of existence of bodies of liquid water (both surface and subsurface) on extrasolar planets throughout the galaxy. Information specifically about the GJ 667C system can be found in Anglada-Escudé et al. (2012). Exomoons are discussed very well in Scharf (2006).

I hope you enjoyed the Clarkesworld article and this list of resources will be of interest to you. If I’ve managed to ignite even one spark of fascination and curiosity, I’m happy.

3rd April 2014 update: Results from Cassini’s measurements of the gravitational pull of Enceladus suggest a large pocket of liquid water near the south pole, as published in the newest issue of Science (Iess et al. 2014); it adds to the indirect (albeit extremely important) evidence of the moon’s intense cryovolcanism. So – good news! Also, discoveries of three dwarf planets were announced in the last couple of days. 2013 FY27 is might be even larger than Sedna (between 760 and 1500 km compared to about 1000 km) so we can expect quite significant radiogenic heating – according to Hussman et al. (2006) model maybe sufficient for a liquid ocean. Let’s hope for even more amazing discoveries like these.


Anglada-Escudé, G., Arriagada, P., Vogt, S. S., Rivera, E. J., Butler, R. P., Crane, J. D., … & Jenkins, J. S. (2012). A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone. The Astrophysical Journal Letters751(1), L16.

Desch, S. J., Cook, J. C., Doggett, T. C., & Porter, S. B. (2009). Thermal evolution of Kuiper belt objects, with implications for cryovolcanism. Icarus,202(2), 694-714.

Ehrenreich, D., & Cassan, A. (2007). Are extrasolar oceans common throughout the Galaxy?. Astronomische Nachrichten328(8), 789-792.

Hussmann, H., Sohl, F., & Spohn, T. (2006). Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects. Icarus185(1), 258-273.

Kargel, J. S., Kaye, J. Z., Head III, J. W., Marion, G. M., Sassen, R., Crowley, J. K., … & Hogenboom, D. L. (2000). Europa’s crust and ocean: Origin, composition, and the prospects for life. Icarus148(1), 226-265.

McCollom, T. M. (1999). Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa. Journal of Geophysical Research: Planets (1991–2012)104(E12), 30729-30742.

McKay, C. P., Porco, C. C., Altheide, T., Davis, W. L., & Kral, T. A. (2008). The possible origin and persistence of life on Enceladus and detection of biomarkers in the plume. Astrobiology8(5), 909-919.

Nimmo, F., Spencer, J. R., Pappalardo, R. T., & Mullen, M. E. (2007). Shear heating as the origin of the plumes and heat flux on Enceladus. Nature,447(7142), 289-291.

Parkinson, C. D., Liang, M. C., Yung, Y. L., & Kirschivnk, J. L. (2008). Habitability of Enceladus: Planetary conditions for life. Origins of Life and Evolution of Biospheres38(4), 355-369.

Postberg, F., Kempf, S., Schmidt, J., Brilliantov, N., Beinsen, A., Abel, B., … & Srama, R. (2009). Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature459(7250), 1098-1101.

Protopapa, S., Herbst, T., & Böhnhardt, H. (2007). Surface ice spectroscopy of Pluto, Charon and Triton. Messenger129, 58-60.

Roberts, J. H., & Nimmo, F. (2008). Tidal heating and the long-term stability of a subsurface ocean on Enceladus. Icarus194(2), 675-689.

Robuchon, G., & Nimmo, F. (2011). Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean. Icarus216(2), 426-439.

Scharf, C. A. (2006). The potential for tidally heated icy and temperate moons around exoplanets. The Astrophysical Journal648(2), 1196.

Schulze-Makuch, D., Irwin, L. N., & Guan, H. (2002). Search parameters for the remote detection of extraterrestrial life. Planetary and Space Science50(7), 675-683.

Spohn, T., & Schubert, G. (2003). Oceans in the icy Galilean satellites of Jupiter?. Icarus161(2), 456-467.

Tobie, G., Grasset, O., Lunine, J. I., Mocquet, A., & Sotin, C. (2005). Titan’s internal structure inferred from a coupled thermal-orbital model. Icarus175(2), 496-502.

Zimmer, C., Khurana, K. K., & Kivelson, M. G. (2000). Subsurface oceans on Europa and Callisto: Constraints from Galileo magnetometer observations.Icarus147(2), 329-347.

On the awesomeness of 2012 VP113

Earlier today, NASA announced the discovery of a probable new dwarf planet 2012 VP113. Its perihelion is even futher than Sedna’s, at around 80 AU. Though not nearly as eccentric as Sedna (e = 0.698 compared to 0.859) and therefore with a much closer aphelion (around 450 AU), it’s still one of the few most eccentric relatively large bodies of the Solar System. Why is it so amazing? It might continue to tell us a story Sedna started: a story about the origins of our system and its early days. It shows that Sedna is not a unique anomaly and might point us towards further evidence for some of the theories what might have happened to perturb their orbits so much. Was it a close passing of another star from the Sun’s birth open cluster? Or possibly a massive rogue planet? And should we expect bodies even larger than Earth in the inner Oort cloud? Could it all together tell us more about the characteristics of the early accretion disc around the Sun and help us with the theory of planet formation, extended a bit lately by observation of other systems in various phases?

Quite a lot of objects similar to 2012 VP113 might be lurking beyond the Kuiper belt, waiting for their discovery; with periods lasting thousands of years, catching them all seems rather a long-time job. And a wonderful one too; a quest after the wild ancient history of our own home.

Amazing discoveries are being made every day and one cannot possibly learn them all, let alone try to comment on them. I usually don’t tend to comment since I usually would only be repeating what was said somewhere else at a greater length (or I’d offer my own perspective, wrap it up in further knowledge and publish it somewhere I’d get paid – such is the reality of a poor time-constrained writer) but I made a tiny exception today for several reasons. Sedna has likely lost its primacy as a dwarf planet with the furthest perihelion, and I’m quite fond of this object. It was the first one of its class, the one that kindled the imagination of scientists and suggested that a large-scale event like another star’s passing less than 1000 AU from the Sun might have occurred long time ago. I love the outer Solar System very much (not that I didn’t love the rest of it too, after all, I live in the inner part quite happily) so this finding excited me at the greatest level. I was extremely excited about gravitational waves lately, but this one just fell into an area much closer to me. Quite literally, too.

After all, both of these discoveries are fascinating narratives: one about the history of our universe, the other about our system. They’re mysteries; results of the work of brilliant dedicated detectives, developing still better methods and finding more and more clues. What more can a storyteller want?