Mystery of Mars’ Missing Ocean: Revealed

Paper Title: Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs

Authors: G.L. Villanueva, M.J. Mumma, R.E. Novak, H.U., Käufl, P. Hartogh, T.Encrenaz, A. Tokunaga, A. Khayat, M.D. Smith

First Author’s Institution: NASA Goddard Space Flight Center

Paper status: accepted by Science

I must first say that although it took a lot of self-control, I refrained from posting any sort of fallacy in today’s (April Fool’s) post. So the reader can rest assured that the results presented today are 100% accurate.

There have been more missions to Mars than to any other planet in the Solar System yet there is still so much to be learned about our closest neighbor. The presence of valley networks suggest that there was once liquid water running rampant on its surface but we know that present day Mars is much too cold to sustain liquid water. So what was going on 4.5 billion years ago? Unfortunately, I have no answers for you in this regard, as it remains a 30-year-old debate.

On one side, there are the scientists who will fight to the death defending the theory that Mars was actually cold but temporarily heated by impacts from asteroids during the Late Heavy Bombardment (a period of time when asteroid activity was quite large). In the opposing corner are the scientists who hypothesize that early-Mars had massive quantities of greenhouse gases, which made it possible to have prolonged existence of liquid water. I personally, agree with the latter argument. The idea that Mars had a stable warm climate makes the search for life (or fossils) very compelling. And, given the results from today’s paper, it looks like I’ll be able to continue with my high hopes. Six years worth of observations prove that Mars had a steady ocean, which covered 19% of the planet’s surface (the Atlantic covers 17%).

Looking for something when it clearly doesn’t exist = Insanity?

How do you look for the presence of water if the water isn’t there anymore? Well, if we can’t search for the water directly, we can do the second best thing and try to figure out how it left and where it went. In the case of Mars, there is really only one place the water could’ve gone: up and out! As the water escapes to space, it leaves behind a chemical signature in the atmosphere that can be measured remotely, here on Earth. In this case, astronomers used instruments at the ESO’s Very Large Telescope in Chile and instruments at the W.M. Keck Observatory to do global spectroscopy of the entire planet. Pretty cool but what chemical signature are they physically measuring?

The oceans on Earth are made up of two different kinds of water: H2O and HDO. HDO is a slightly heavier form of water because, instead of having two hydrogen atoms, it has one hydrogen atom and one deuterium atom (a hydrogen with a neutron). When the ocean starts evaporating, gravity tells us that more H2O will escape as compared to HDO based on a weight argument alone. As Mars begins to evolve, evaporation progresses, more HDO will be left behind in the atmosphere as opposed to H2O. Lucky for us, H2O and HDO have completely different spectral signatures and so we can measure the ratio of hydrogen to deuterium (D/H ratio). By looking at the global spectroscopy of the planet, we can predict how much water was lost and calculate how much surface water there was billions of years ago (check out this bite for more on this process).

Patience is Key

The Mars Science Laboratory (Curiosity) has also done D/H measurements. The key difference between those measurements is that they are representative of one specific location. Although that is valuable, Villanueva et. al. blew those measurements out of the water. Over six years, they mapped the global D/H ratio. This allowed them to look at differences between the Polar Regions and the equatorial regions, as well as to get a sense for any existing seasonal changes.  Intuitively, one would expect differences in D/H ratio throughout Mars, especially considering Mars has prominent north and south polar caps. Just as suspected there were large seasonal and latitudinal variations. Some regions had D/H ratios about 1-3 times the value of Earth’s ocean while others, including the water ice in the polar reservoirs were enriched by at least a factor of 8 times what we see here on Earth.  In order to get this level of enrichment, Mars must have lost at least 6.7 times what is currently being stored in the polar caps.

The discovery of the missing ocean

All the maps described above tell us that early Mars had an ocean of at least 20 million cubic kilometers. This is about 6.7 times the amount of water frozen in the Martian northern cap but it is also only about 1% of the amount of liquid water on Earth. Not a lot, you might think. Well, the last point I will emphasize is that the quantity of water estimated by the authors is only a lower estimate. It is entirely possible that Mars once had much more water. Why? Because our estimate hinges on the assumption that all the water left on Mars is tied up in the polar caps. Other Mars scientists believe that there might be a significant source of water tied up below the surface. Whether or not that is true remains a mystery.

Either way, I am glad the scientists in favor of warm early mars can take this victory home. I only hope that one day, soon, we will uncover what the atmosphere of early Mars was doing in order to make this ocean possible.

About Natasha Batalha

Born in Brazil, raised in CA... I somehow ended up in the tundra of Ithaca, NY for undergrad at Cornell studying physics. I spent my undergraduate days dancing and studying exoplanets and am now a 3rd year graduate student at Penn State studying astrophysics and astrobiology. I'm currently working on atmospheric photochemistry with Jim Kasting, a JWST exoplanet simulator with Jason Kalirai and Avi Mandell, and am conducting some laboratory experiments on biosignature gases with Chris House. Outside of academia I'm involved in an awesome organization called "Learn to Be", which provides online tutoring for underserved communities. We are always looking for more tutors, so please contact me if you are interested!

19 Comments

  1. This is really exciting! Are there any ways that we know of to get an upper bound on the volume of the oceans that Mars once had?

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  2. Thanks for this cool post! Are there any ways we might test some scientists’ theory that there might be a significant source of water tied up below the surface of Mars?

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  3. Great post! Are there any estimates for potential upper bounds on the amount of water Mars once had?

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  4. In the theory that Mars was cold but heated by impacts, where was all of the ice needed to melt to create the liquid water whose effects we can see today? Does this theory also include means for the water to escape the surface to see the decline in ice that we see today?

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  5. There was a recent astrobite about the migration of Jupiter around the time of planet formation. Is it possible that Mars experienced a climate change due to post-formation era planetary migrations?

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  6. How does one correct for variations in H2O and HDO abundance in that region of the solar system, or is such an abundance fairly regular across the solar system? Additionally, how much credibility do you think there is for the arguments for water trapped beneath the martian surface?

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  7. This is fascinating! What are the next steps in this research? How will we find out if there is water below the surface?

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  8. How does the atmospheric D/H ratio remain stable enough to tell us about long-lost oceans for so long? I would have expected any impact Martian oceans might have had on the Martian atmosphere to have faded by now.

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  9. Do we currently have any theories about where to look for fossils/other signs of life on Mars? Could this new information help narrow down the scope for this sort of search?

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  10. In terms of a possible source of water below the surface of Mars, how do we know Mars’ composition? Do we have information about seismology that could tell us the chemical components of the planet and could this inform whether or not water could be beneath the surface?

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  11. Very nice post! 🙂 Since the atmosphere’s composition seems to have remained quite stable, is it possible to draw a good picture of where this ocean was in Mars?

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  12. In regards to the possibility of water below Mars’ surface, could this water always have been here, when there was a surface liquid ocean, or did it migrate there somehow because of climate change?

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  13. I’m surprised we were able to trace water from here on Earth. More so surprised how even though mars is hit almost defenseless against solar winds and Mars’ thin atmosphere, we managed to still give us some information

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  14. Based on your discussion, there seems to be ample proof that there was indeed water on Mars at some point in its past. How do those scientists who maintain that the temperature necessary to sustain liquid water was brought about by temporary asteroid bombardment prove their theory as opposed to the one you hold?

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  15. most probably Mars loses its atmosphere because of solar wind. mars has very weak magnetic field to be protected from it. after my research the solar corona disapears every some tens of thousands of years for another tens of thousands duration. here on earth we fall into an ice age. we should expect Mars to form an atmosphere and oceans during these intervals.

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  16. I’m not an astronomer, but I have an interest in astrobiology and since there were so many unanswered questions I’ll attempt those. Caveat emptor.

    @Jonathan: “Do we currently have any theories about where to look for fossils/other signs of life on Mars?”

    1. Planetary biosignatures can be widespread. As you may know, the Curiosity team has announced finding a methane background (global) compliant with impact events, as well as spikes (very local, it is thought) indicating a currently active Mars. (Even if the activity may be confined to methane release from billion of years old ice traps.)

    2. Fossils on the other hand are mostly found in specific locales on Earth. Fossilization is rare, and the environment can influence. (E.g. resulting in fossil beds from a constrained place and time.)

    – Organic matter can get trapped in fluvial mudstone and sandstone on Earth, and that is why Curiosity specifically drilled for them in the Gale crater fluvial environment.

    The Curiosity team has announced finding organics, chlorinated aromatics indicating simple organics at +3.5 billion years ago. The recent LPSC-15 has a conference abstract on some possible more complex organics find, fatty acids even.* [ http://www.bbc.co.uk/news/science-environment-31988540 ]

    It is hard to say if any of those are signs of life or merely signs of ancient habitability, but good news.

    – MISS (microbially induced sedimentary structure) signs is more controversial, which like early stromatolites looks good as fossils for many but not all. As far as I know such a survey for them has been suggested before and is suggested presently by Curiosity scientists.

    There is one paper in Astrobiology from Nora Noffke, a MISS expert on such signatures, from the Gale lake beach environment. She can’t finish her detection list (need microanalysis) but there are three correlated lines of evidence that is hard to explain else. (Such alternative explanations do not appear on Earth, she claims … but mind the lacking microanalysis as well as lack of much comparative geological material.) The mudstone organics was found about half a meter away and some centimeters or so “down” sediment-wise from there. [ http://online.liebertpub.com/doi/pdf/10.1089/ast.2014.1218 ]

    The paper isn’t from the Curiosity team however. Their current project scientist, Aschwin Vasavada, find it it interesting but the journalist describes him saying that “the rover’s scientists … did not reach the same conclusion.” [ http://www.theage.com.au/technology/sci-tech/astronomy/photos-show-possible-signs-of-ancient-microbial-life-on-mars-20150112-12mt27.html ] Their own MISS expert, Dawn Summers, mentions it in passing as interesting (IIRC, and I haven’t time to check) in an LPSC talk. [ https://virtualoptions.agu.org/media/B41P-01.+Sagan+Lecture%2C+Presented+By+Dawn+Sumner/0_ohzla2kg ; you may have to create a guest account to watch it.]

    I don’t think anyone has published a detailed critique of Noffke’s paper, so it remains as an interesting possibility.

    The overall plan is to let the 2020 sampling rover put away samples that a putative later sample return mission can retrieve back to labs. A survey for microfossils need many samples and massive instruments.

    * In principle fatty acids can be made by abiotic Fisher-Tropsch reactions and then water chemistry I think, I’m vague on the actual chemistry.

    But I haven’t been able to pin down a reference telling me abiotic fatty acids have been observed in nature before. As part of the recycling biomass (the kerogen cycle) it may happen I guess. But this is supposedly produced from scratch.

    @Erin: “How will we find out if there is water below the surface?”

    As I understand the D/H technique can tell of a (extinct or extant) water cycle partitioning between the polar region water ice and the atmospheric results. And I remember seeing papers claiming that there are indications of more (ground) water or ices.

    And the recurrent slope lineae, features that come and go with the martian seasons, are most likely constrained to be (salinated) water. If so, there is so much that it tells of active aquifers at places. [ http://www.space.com/29007-mars-dark-streaks-mystery-water.html ]

    The upcoming InSight and ExoMars missions will drill 2-5 meters deep and look for thermal profiles respectively chemical samples. At least the latter should move the question along.

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  17. I’m not sure if my previous comment will come out of moderation, but if it does Sumner’s talk was from the AGU Fall meeting of course. Also, I english bad and had lots of other details wrong today. I blame lack of coffee…

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  18. I think you did a good job, Torbjörn – especially if you were coffee deprived!
    I’m doing the current offering on Coursera (a free course) of Caltech’s excellent Science of the Solar System and we have just spent three weeks looking at the question of water on Mars – so see that the questions here don’t really admit of one page answers.

    But if anyone wants to explore more, it is possible to join the course even tho it’s already begun and view the video lectures by Dr Mike Brown. It’s really worth the effort – I promise you!

    Just Google Coursera, Solar System and that will get the URL.

    Reply
  19. What sort of follow-up studies are being done one this? Is there any way for us to search for signs of past life in these oceans?

    Reply

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