Wednesday, January 2, 2013

River Found on Saturn's Large Moon Titan

Scientists working with the Cassini space probe orbiting Saturn have recently identified a river more than 200 miles long on the moon Titan. You can see the river valley running into a large sea (to the right) on the radar image. Like the seas on this frigid moon, the river is not filled with liquid water (much too cold for that), but probably liquid methane (swamp gas) and ethane.

Titan has a thick, smoggy atmosphere, so the spacecraft can't just look for features like this using ordinary light. Instead, one of its instruments does the same thing that airport controllers do to keep track of aircraft. It bounces radar beams off the moon's surface and then makes an image using the radar information. On such radar pictures, dark means smooth, light means rough. You can see the smooth dark surface of the river on the accompanying picture, which makes scientists think that the river basin really is full of a flowing liquid with a smooth surface.

This is the first flowing river found on another world besides Earth. Mars has dried-up river beds, but its atmosphere is much too thin to allow liquid rivers or seas today. Titan, on the other hand, has an atmosphere thicker than Earth's, so the pressure is enough to allow liquids to flow.

Earlier radar (and infrared) images of Saturn have identified a number of lakes on Titan, including one as large as Lake Ontario.  There are also images that over time, show liquid areas getting smaller and larger, as if they evaporated and then were filled again -- perhaps by rainfall.  (Once again, this would be a rain of cold ethane and methane, not water.)  Just like the Earth is at "the triple point" of water (boasting temperatures and pressures where water can be solid, liquid and gas), so Titan appears to be at the triple point of methane and ethane and so we can have these substances as vapor, bodies of liquid, and icebergs.

I'm just about to teach my introductory astronomy class on the planets at Foothill College starting Monday and Tuesday, and between the new pictures from Mars and Saturn (to say nothing of Mercury), we'll have lots of astronomy news to keep us occupied. If you can't attend, don't worry. I'll also continue to share the news on this blog as we go along.