A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.



Fog! Titan! Titan Fog! (and a peer review experiment)


Look! Titan has fog at the south pole! All of those bright sparkly reddish white patches are fog banks hanging out at the surface in Titan's late southern summer.
I first realized this a year ago, but it took me until now to finally have the time to be able to put all of the pieces together into a scientific paper that is convincing enough that I can now go up to any person in the street and say: Titan has fog at the south pole!
I will admit that the average person in the street is likely to say hmph. Or yawn. Or ask where Titan is. So let me tell you why finding fog at the south pole of Titan has been the scientific highlight of my summer.
Titan is the only place in the solar system other than the earth that appears to have large quantities of liquid sitting on the surface. At both the north and south poles we see large lakes of something dark. Oddly, though, we don’t actually know what that dark stuff is. At least some of it must certainly be ethane (that’s C2H6, for all of you who have forgotten your high school chemistry). Ethane slowly drips out of the sky on Titan, sort of like soot after a fire, only liquid soot in this case. Over geological time, big ponds of ethane could accumulate into the things that look like lakes on Titan. Odd as they sound, big lakes of liquid ethane are, at least to me, the least interesting possibility. They are the least interesting because ethane is a one way street. Once the liquid ethane is on the ground, it can’t evaporate and is there pretty much forever, unless it somehow sinks into the interior.
Why does all of that ethane drip out of the sky? Because sunlight breaks down methane (CH4) to form ethane much the same way it breaks down car exhaust fumes to form smog in big cities. There’s plenty of methane in the atmosphere, so the supply of ethane is near endless. The dripping will not end soon.
But the methane is where all of the potential action is. Methane is to Titan what water is to the earth. It’s a common component in the atmosphere and, at the temperature of Titan, it can exist in solid, liquid, or gas form. Like water on the earth, it forms clouds in the sky. Like water on the earth, it probably even forms rain. But what we don’t know is whether or not that rain makes it to the surface and pools into ponds or streams or lakes which then evaporate back into the atmosphere to start the cycle over again. In short, we don’t know if Titan has an active methane atmosphere-surface hydrological cycle analogous to the water atmosphere-surface hydrological cycle on the earth.
Until now.
Because there is fog.
Fog – or clouds – or dew – or condensation in general – can form whenever air reaches about 100% humidity. There are two ways to get there. The first is obvious: add water (on Earth) or methane (on Titan) to the surrounding air. The second is much more common: make the air colder so it can hold less water and all of that excess needs to condense. This process is what makes your ice cold glass of water get condensation on the outside; the air gets too cold to hold the water that is in it, and it condenses on the side of your glass.
Terrestrial fog commonly forms from this process. That fog that you often see at sunrise hugging the ground is caused by ground-level air cooling overnight and suddenly finding itself unable to hang on to all its water. As the sun rises and the air heats, the fog goes away. You can also get fog around here when warm wet air passes over cold ground; the air cools, the water condenses. And, of course, there is mountain fog that is causes by air being pushed up a mountain side, where it cools and – you get the pictures – can no longer hold on to all of its water so it condenses.
Interestingly, none of this works on Titan.
It’s really really hard to make Titan air colder fast. If you were to turn the sun totally off, Titan’s atmosphere would still take something like 100 years to cool down. And even the coldest parts of the surface are much too warm to ever cause fog to condense.
What about mountain fog? A Titanian mountain would have to be about ~15,000 feet high before the air would be cold enough to condense. But Titan’s crust, made mostly of ice, can’t support mountains more than about 3000 feet high.
We’re left with that first process: add humidty.
On Titan, as on earth, the only way to add humidity is to evaporate liquid. On Titan this means liquid methane.
Liquid methane! There it is!
Evaporating methane means it must have rained. Rain means streams and pools and erosion and geology. Fog means that Titan has a currently active methane hydrological cycle doing who knows what on Titan.
But there’s one more twist. Even evaporating liquid methane on Titan is not sufficient to make fog, because if you ever made ground-level air 100% humid the first thing it would do after turning into fog would be to rise up like a massive cumulous cloud. There’s only one way to make the fog stick around on the ground for any amount of time, and that is to both add humidty and cool the air just a little. And the way to cool the air just a little is to have it in contact with something cold: like a pool of evaporating liquid methane!
Only final fun part of the story. The fog doesn’t appear to prefer hanging around the one big south polar lake or even around the other dark areas that people think might be lakes. It looks like it might be more or less everywhere at the south pole. My guess is that the southern summer polar rainy season that we have witnessed over the past few years has deposited small pools of liquid methane all over the pole. It’s slowly evaporating back into the atmosphere where it will eventually drift to the northern pole where, I think, we can expect another stormy summer season. Stay tuned. Northern summer solstice is in 2016.
Our paper describing these results (written by me, Alex Smith and Clare Chen, two Caltech undergraduate students, and Mate Ádámkovics, a colleague at UC Berkeley) was recently submitted to the Astrophysical Journal Letters. The paper will shortly go out for peer review, which is an integral part of the scientific process where the paper is vetted by experts. Peer review, as implemented in the current world of over-stressed astronomers, has some serious flaws, though. One problem is that the peer review is performed by one person! Sometimes that one person is thoughtful and insightful and provides excellent insight and commentary. Sometimes that one person misses or misunderstands crucial points. It is rare, though, that any one person can be a broad enough expert in all of the topics in a scientific paper to provide adequate review of the whole thing. Plus people are busy.
What is the solution? I don’t know. There has been much talk recently about all of this, and even some interesting experiments done by scientific journals. I thought I would try an experiment of my own here. It goes like this: feel free to provide a review of my paper! I know this is not for everyone. Send it directly to me or comment here. I will take serious comments as seriously as those of the official reviewer and will incorporate changes into the final version of the paper before it is published.
What kinds of things would I look at closely if I were a reviewer of this paper? Probably things like: is the claim of discovery of something fog-like convincingly made? Is the fog-like feature really at the surface rather than simply a cloud? Is our argument of how fog must form convincing? Is it correct? These were, at least, the things I thought hardest about as I was writing the paper. Perhaps you will find more!

14 comments:

  1. Fantastic news! Congratulations on this discovery.

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  2. Not sure I can contribute anything useful with respect to your paper. However, a comment on this blog article:

    ...make the air colder so it can hold less water and all of that excess needs to condense.

    and

    ...the air gets too cold to hold the water that is in it...

    Though they are, in a way, literally correct phrases like this make me cringe as they reinforce the myth that the air is somehow like a sponge which can hold so much water vapour at a given temperature.

    This gives people badly misleading ideas in two areas related condensation that I'm peripherally interested in: building science and meteorology related to gliding and aviation in general. I've also heard some confusion expressed about this with respect to dew on amateur telescopes, though in those cases it didn't make any difference to the substance of the discussion.

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  3. Fascinating paper. While I'm not able to contribute anything from a scientific standpoint, there's a possible typo in section 2 Observations, Paragraph 1, Line 8,:
    "We also added an addition synthetic filter......." Should read " an additional" ?

    Otherwise, a good experience for a layperson to get a look at an academic paper, and a great idea to try out this kind of mass participation.

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  4. If you want online reviewing, why didn't you submit to an EGU climate journal, where open reviewing comes standard?

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  5. Second to last word in the abstract needs an -ly on the end.

    “The best examples are shown in Figure 1.” God bless you for not calling your best example ‘typical’.

    Your argument would be stronger if you could show that your fog appeared in areas where rain had previously occurred. Otherwise, any source of cold methane could give you fog (e.g. an endothermic methane-producing chemical (or photochemical) reaction). Will 2NH3 (s) + 3C2H6 (l) -> 6CH4 (g) + N2 (g) proceed to the right under Titanian surface conditions, assuming an ammonia component in the surface ice?

    Your references are very self-referential. Is no-one else in the world studying methane weather on Titan?

    If ethane is a stable dead-end liquid, then why is there so little of it on the surface?

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  6. Chuck --
    I didn't know about the EGU journals. Very cool sounding. Taking a quick look at the actual journals, though, it doesn't look like weather on other planets would fit anywhere, would it?

    I understand your comment about self-referencing. It's an interesting balance. In general many of the things one does are related to things in the past, so you build this common body of work that you then often cite. But it can also be a sign of laziness: you know your own work best and it comes to mind instantly when you're looking to throw a reference in. I will go make sure I didn't do that!

    Mike

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  7. Hi, Mike :)

    I had to sleep on this for a while.

    On page two, it says, "...methane should be seasonally transported from summer
    pole to summer pole (Mitchell 2008)". OK, that would mean that the methane doesn't go anywhere. Didn't you mean, "...from summer pole to winter pole..."?

    The chart on page 5 is pretty interesting, especially the region between 2.5 and three microns. I guess analyzing detailed differences would be beyond the scope of the paper. You guys think you could model up a graph of what pure methane ought to look like? Maybe that's beyond the scope of this paper, grist for some future paper.

    I need to get busy with differential equations homework, so this is really a quick read through rather than a review.

    I'd like to comment on the low depth of Titan's mountains. I'm wondering: how deep does the aquifer go on Titan? That's obviously beyond the scope of the paper but it seems both Titan and Earth have "the right" amount of liquid.

    Thanks, Chuck! :D

    -Michael C. Emmert

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  8. Very good article. Everything is remarkably well explained. Thanks :D

    About the paper, I'm afraid I can only detect typos, like Alcareru did. Here's a list:

    Several "Titans" instead of "Titan's": near the end of the Introduction, 4th and 6th line of Observations, 2nd line of Analysis and 3rd line of Discussion.
    Last paragraph on Observations: "ofpotential" and "-surface".
    Analysis: "calcuations" (3rd line).
    Discussion: "earth" instead of "Earth" (1st line).


    Also, I see a lot of JPEG compression on figures 1 and 2, and it seemed to be there before the addition of arrows and circles. This makes features in the images difficult to see…

    But that's it. Good and interesting work!

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  9. Argh. First "official" comment back from the jounral. They won't print a 12 panel figure like Figure 1. Guess I won't show the one low-res one.

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  10. OK. Resubmitted with a smaller fig 1 and the typos pointed out here fixed (thanks all!).

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  11. Do you need to explicitly rule out geothermal methane (or is that just a special case of methane pools, albeit with a rather different, and no longer so 'hydrological', cycle)? Being hotter, it might make mist more easily.

    I found it hard to understand why you needed three filters to do the initial identification. What does 5μm add that the surface filter doesn't, and vice versa? It looks as if the answer is that nearly everything (but not quite everything) is dark at the surface at 5μm whereas there's a lot of greyness in the surface filter, but I wasn't quite sure. That might just be me, and the intended audience not have any difficulty.

    I also didn't understand (not having followed up the references) what you mean by 'identical' synthetic filters in the first para of Section 2. Does it refer to the way the wavelengths are weighted?

    I take it there aren't any other possible volatiles? I looked around for information on C2H2, C2H4, HCN, etc and they all seemed as involatile as ethane. Couldn't find data on CH2NH but it seems implausible on the face of it.

    Hope that helps - not being a scientist I find it hard to know when something is a convention that I'm not familiar with.

    In principle, any paper on Arxiv is open for review - does that happen in practice?

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  12. Quick question and observation. From what I last read the surface temperature of Titan is roughly -290F, yet methane freezes at roughly -296F. Isn't that a little close, wouldn't this be more snow like, or did I miss something. If you cool things down a little are you not getting closer to that freezing number?

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  13. Are we sure ethane is a one way street?

    Someone already suggested a chemical rxn that would break it down, sounds implausible/ineffective at those temperatures but I don't know that it is. But in any case those aren't pools of pure ethane, there has to be methane dissolved in it how much difference could that make?

    -M

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  14. Congatulations! Interesting and exciting experiment.

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