The reason for the Titanian season

Bad Astronomy
By Phil Plait
Dec 1, 2009 1:00 AMNov 20, 2019 5:23 AM


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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!

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