Last year, the Cassini spacecraft found solid (haha) evidence for the existence of lakes of liquid methane and ethane on the giant moon Titan. Of course, Titan is barely a moon at all — bigger than Mercury, it would be a planet in its own right if it weren’t orbiting Saturn. It has an atmosphere with almost twice the surface pressure as Earth’s, which is mostly nitrogen and a trace of hydrocarbons.
But that trace is important: because Titan is so cold, methane and ethane can rain from the Titanian sky, forming river systems and lakes. But there’s a problem: the north pole of the moon has far more lakes than the south pole. Seven times as many!
Why?
First, methane on Titan goes through cycles something like water does on Earth. During Titan’s summer, the northern lakes lose methane to evaporation, and the gas gets transported to the colder south pole, where it recondenses. The opposite happens in the southern summer. But that should balance out, so that each pole over the course of time has the same amount of liquid. They don’t. So, clearly, something else is going on. And it isn’t topography; the north and south polar regions of the moon have roughly the same overall shape, so you don’t expect liquids to flow into or out of one of those regions more than the other.
However, some scientists think they may now know what it is. Their idea, not yet proven but still very interesting, is that the reason is due to the seasons on Titan, coupled with Saturn’s elliptical orbit.
Saturn orbits the Sun once every 29 or so years. Its orbit is decidedly elliptical; it varies from about 1.35 to 1.5 billion kilometers from the Sun, a variation of 150 million or so kilometers! When Titan’s north pole is in summer, Saturn is farther from the Sun, and the southern summer is when Saturn is closest. That means that summers are cooler in the north, lowering the methane transport to the southern pole… and summers are warmer for the south pole, increasing methane transport.
In other words, Titan’s south pole is selling low and buying high. That’s not what you want to do if you want to keep yourself rich in hydrocarbon lakes.
Interestingly, again like the Earth, the geometry of Saturn’s orbit changes slightly over time. Over the course of tens of thousands of years, it changes such that the situation is reversed: Titan’s north pole will experience summer when Saturn is closest to the Sun, and the south pole when Saturn is farthest. If this idea of asymmetrical seasonal flow is correct, this will reverse that flow, putting more lakes at the Titanian south pole. On the Earth, these orbital variations are called Milankovitch cycles, and are most likely tied to very long term (like thousands of years long) global climate change. It’s possible, too, that the Milankovitch cycle may be tied to regions on the Earth periodically becoming deserts and then turning more humid once again.
Titan is sometimes seen as an analog of the young Earth, with methane taking the place of water in its geology and chemistry. It’s interesting to see that perhaps we have even more in common than we thought!
November 30th, 2009 at 2:19 pm
Of course, the big question is, when are we going to invade Titan for all that natural gas?
November 30th, 2009 at 2:27 pm
Hey, my house is in that picture! LOL
November 30th, 2009 at 2:36 pm
Imagine if we could steal it from Saturn’s grasp and move it closer to Sun…
November 30th, 2009 at 2:58 pm
Interesting reading the linked article: I’d always thought we were just seeing wintry lakes in the north and summer drought in the south and that the situation would reverse in the next few years. Thanks.
November 30th, 2009 at 3:34 pm
What Mchl, dreams of lighting intergalactic farts?
November 30th, 2009 at 4:04 pm
Absolutely fascinating. A great example of how asking questions leads to more questions as well as answers.
November 30th, 2009 at 4:14 pm
3. Mchl:
Closer? I think we should just smash it into MArs. Yeah, then we’d have one good sized planet(still a LITTLE less mass than Earth) and with all those hydrocarbons even Exxon would be happy.
GAry 7
PS. How does that song go? Hail, Titania,,,or was that Britannia? Then it would need a new name. How about Tars, or MarTan?
November 30th, 2009 at 6:06 pm
@Larian
I can already imagine sometimes in the future a space war for the resources of Titan.
November 30th, 2009 at 6:10 pm
I like to daydream that life on Titan moves at a much slower rate than we do and a being is STILL running from the Huygens probe to alert the local town council.
November 30th, 2009 at 6:11 pm
Has anyone seen this story yet?
http://www.timesonline.co.uk/tol/news/science/space/article6934078.ece
Awesome awesome awesome!!!!
November 30th, 2009 at 6:40 pm
It’s neat that one region has exaggerated seasonal variation with short summers and long winters while the opposite region has mitigated variation with short winters and long summers. Mars is in the same predicament.
November 30th, 2009 at 10:28 pm
One thing I find interesting is that the liquid is at the poles. Maybe some extraterrestrial life exists where liquids condense at the poles just as ices exist on terrestrial planets like Mars and Earth.
December 1st, 2009 at 12:20 am
“would be a planet in its own right if it weren’t orbiting Saturn”
true of Ganymede (Jupiter) as well?
Speaking of which, Phil, what is your preferred way to classify Ceres and Eris?
December 1st, 2009 at 2:33 am
So there is a fixed ratio between the time Saturn moves around the sun and Titan moves around Saturn? Otherwise it wouldn’t really matter where Saturn is for Titan’s winter, right?
December 1st, 2009 at 5:22 am
#1 Larian:
But we’ll have to ask it, before we invade it…
December 1st, 2009 at 10:42 am
I’ve seen Titan protryed in one late 80s science fiction film called creature ( a combination Alien, Invation of the body snachers, and the Thing50s version with a little 2001 thrown in ) and of course it not like the real moon Titan as we have discoverd . p.s. I think if Phill has not seen this he should it’s a good and scarey but has some science problems 1 I will mention is a secen were the captian is out on the surface without his helmet he can’t breath the Titan atomosphere and is saved by anthor crew member but has no ill efects from trying to breath mathane gas for several minutes
December 1st, 2009 at 10:52 am
I’ve always been impressed by the one narrow picture of the surface of Titan taken by the Huygens lander: http://ael.gsfc.nasa.gov/images/saturn/titanSurface2.jpg
It almost looks like there’s a stream flowing way off in the distance.
December 1st, 2009 at 1:53 pm
Did I read that correctly? Saturn’s orbit varies by an entire AU?? Didn’t think it was that eccentric.
December 1st, 2009 at 4:55 pm
Mike Burkhart,
Phil does know about the movie, Creature. He helped identify the movie for me a number of years back. I saw most of the movie on a 13 inch black and white television one summer evening while in my early teens. That was kinda fun but I would not recommend that movie at all.
December 1st, 2009 at 8:24 pm
“Saturn orbits the Sun once every 29 or so years.”
that means next year i will be one saturnian year old. wooohoo. and i am going to be sure to have a saturn birthday cake for the big 29
December 1st, 2009 at 10:57 pm
I saw one of the paper’s coauthors [Lunine] give a great talk on this at UofAriz. just a couple of weeks ago. It was part of the math dept.’s lecture series; yeah, math(s), not planetary sciences.
December 2nd, 2009 at 7:34 am
This is like reading the descriptions of planets in Mass Effect, except x1,000 more awesome because its real.
I love this stuff, thank you Mr. Plait!
December 2nd, 2009 at 7:02 pm
[...] Discover: The reason for the Titanian season [...]