Titan Weather: Cloudy Every 15 Years
Talk by Chris McKay, Part II
About two years ago, before the Huygens probe arrived at Titan, Henry Roe, a graduate student I was working with at Berkeley, discovered clouds on Titan. He was the first person to get images of what he thought were clouds on the south pole. We sent his results off for publication, which is what a good graduate student is supposed to do, and the editor sent it out for review. But the reviewer said, "Nope, those aren't clouds. That's a mistake in the data processing. It's just an artifact." So I had the student assemble the raw data as well as the processed data, and the clouds were still there in the raw data.
In fact, in the equatorial mid-latitude regions of Titan, the temperature hasn't changed in 20 years. It's exactly the same to half a degree. So it's very easy to predict. If you were a weatherman working on Titan, in the mid-latitude, your report would be, "Temperature today and tomorrow will be exactly like it was yesterday. And we can predict that for the next 20 years it will be exactly the same." The only place on Titan where you can have a light-dark contrast, which is what drives this kind of storm activity, is in the polar regions, not at the equator. In that sense, it's different from the Earth in its meteorology.
These clouds are now gone. We don't see them any more. They were only there during the height of Titan's southern summer, when the southern pole was getting 24 hours of sunlight. That's when the clouds came. Now that fall has come to Titan, the clouds have gone away. They were apparently only there for a couple of years. And our prediction is that in another 15 years, clouds will form at the north pole, as that becomes sunlit summer. So, I'm going too try to stick around to see that.
The other possibility is that there is a cryovolcano at 40 degrees south, a source of methane, a volcano ejecting methane. Remember the reason we once thought Titan had an ocean was that something had to be resupplying the atmosphere with methane. Well, there's no ocean. So you might be wondering, How is Titan resupplying the atmosphere then? Good question. It's not coming from an ocean; what could it be coming from? Henry suggested that these clouds could be caused by a cryovolcano. And if you calculate how much methane this volcano would have to put out to make these clouds, it turns out to be just the right amount to keep the atmosphere in methane.
So there are dueling views. Some people think these clouds are caused by atmospheric circulation, some people think it's a cryovolcano. We don't know. We'll just have to wait and see.