Titan Weather: Cloudy Every 15 Years
There is only one moon in our solar system that has clouds, Saturn`s giant moon Titan. First discovered by a scientist using an earthbound telescope, the clouds were later confirmed by NASA`s Cassini spacecraft.
Chris McKay, a planetary scientist at NASA Ames Research Center, recently gave a public lecture, sponsored by the Planetary Society, about what scientists have learned about Titan from the Cassini-Huygens mission. In this, the second part in a four-part series, McKay talks about Titan`s mysterious clouds, some of which are thought to appear only briefly every 15 years.
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.
So we were predicting clouds on the southern rim of Titan. When Henry first saw them, they had just appeared there. They hadn`t been there before. So the Cassini orbiter flew under Titan and took pictures of the south pole looking up - and there were the clouds. It`s really spectacular. It was a real vindication for Henry Roe. And it shows the advantage of actually being there to see them, of being able to get underneath Titan and look up at them, rather than looking at them edge-on.
In the images that were taken over a five-hour period in July, you can see that the clouds have changed. If you fly in and out of Dallas in the summer, you`ll often see thunderstorm lines up along the plains. They change, too, over a period of about an hour or so. So the clouds on Titan changed on about the same time scale as a thunderstorm system on Earth. You look at them and they could be clouds on Earth. They`re white, fluffy, billowing. But they`re not water. There`s no water anywhere near here. These are methane, liquid methane clouds. And the temperature there is about minus 200 Centigrade. And there are clouds and they`re moving.
On Earth, clouds tend to form mostly in the tropics, where the sun is the brightest and the thermal contrast between the dark cycle and the light cycle, night and day, is the strongest. That causes upwelling, which drives clouds to form near the equator.
On Titan, we don`t see clouds at the equator. We see clouds at the pole. Titan is different from Earth in that the place on Titan which has the strongest contrast between dark and light is the poles. At Titan`s equator, the sun goes up and sets, but the atmosphere is so thick that at nighttime the temperature is the same as the daytime. The thermal response of the atmosphere is much, much longer than the Titan day, which is 16 Earth days. But it`s shorter than the Titan year, which is 30 Earth years.
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.
NASA
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