Sunday, August 17, 2014
Clouds Moving Across a Lake on Saturn's Moon Titan
Recently released images from the Cassini probe in the Saturn system show fresh clouds moving slowly above a giant lake on Saturn's moon Titan. The lake itself is not water, but probably liquid methane and ethane (methane is what we call natural gas on Earth and ethane is an ingredient in fuel).
Now you might say, big deal, so there are clouds on top of a lake. I can see that anytime on Earth. But the big difference is that the temperature on this distant moon is so cold, that water there is always frozen harder than rock. So lakes, rivers, rain drops, and clouds can only form from substances that can stay liquid and gas at very cold temperatures. Methane plays the same role on Titan that water plays on Earth -- it can be solid ice, flowing liquid, and gas in the atmosphere. The clouds we see are condensations of droplets of methane. (As always, click on the picture to see it bigger.)
Even just having the images is a real tribute to our technology. When we first got close-up images of Titan from space missions that flew by, its surface was not visible. The entire giant moon is covered with a thick haze. Light cannot make it through such haze (as people in Los Angeles on bad air days know all too well :-)), so we equipped the Cassini spacecraft with special instruments that can "see" infrared (heat rays) and radar -- waves that do get through the thick atmosphere of Titan.
The lake above which we see the new clouds is called Ligeia Mare (the Ligeian Sea), the second largest known body of liquid outside the Earth. It's so big that if you walked around it, you'd walk 1240 miles. Winds on Titan are pushing the clouds over the lake at speeds of about 7 to 10 miles per hour.
A large storm in 2010 cleared the atmosphere of Titan of most clouds, and astronomers who follow the weather on this distant moon have been waiting for the summer cloud patterns in Titan's northern hemisphere to return.
NASA's imaging wizards even put together a little movie of the cloud motions, which you can see at: http://photojournal.jpl.nasa.gov/archive/PIA18420.gif
The movie is not going to win any Oscars, but as you watch the few frames with different amounts of detail showing, remind yourself that you are watching images taken 900 million miles from Earth by a fragile spacecraft that has been orbiting through the Saturn system for almost 10 years.
By the way, if you want to see the shape of Lake Ligeia, see the image below, taken with radar, that shows it especially well: