Weather watching… on Titan.

Nasa’s Messenger spacecraft may have finally arrived in orbit around Mercury, but the Cassini probe has been exploring Saturn’s system of moons since 2004 – some archived posts here.  [Image Credit: NASA/JPL/SSI]

The latest images show seasonal weather patterns, and apparently associated surface changes.  From the Nasa/JPL press release

Extensive rain from large cloud systems, spotted by Cassini’s cameras in late 2010, has apparently darkened the surface of the moon. The best explanation is these areas remained wet after methane rainstorms. The observations released today in the journal Science, combined with earlier results in Geophysical Research Letters last month, show the weather systems of Titan’s thick atmosphere and the changes wrought on its surface are affected by the changing seasons.

“It’s amazing to be watching such familiar activity as rainstorms and seasonal changes in weather patterns on a distant, icy satellite,” said Elizabeth Turtle, a Cassini imaging team associate at the Johns Hopkins University Applied Physics Lab in Laurel, Md., and lead author of today’s publication. “These observations are helping us to understand how Titan works as a system, as well as similar processes on our own planet.”

The Saturn system experienced equinox, when the sun lies directly over a planet’s equator and seasons change, in August 2009. (A full Saturn “year” is almost 30 Earth years.) Years of Cassini observations suggest Titan’s global atmospheric circulation pattern responds to the changes in solar illumination, influenced by the atmosphere and the surface, as detailed in the Geophysical Research Letters paper. Cassini found the surface temperature responds more rapidly to sunlight changes than does the thick atmosphere. The changing circulation pattern produced clouds in Titan’s equatorial region.

Here’s a short video of clouds in the mid-southern latitudes of Titan from seven images captured between December 13 and 14 2009.

And another short video, this time of clouds above the large methane lakes and seas near Titan’s north pole – the four images used were captured between September 22 and 23, 2009.

And here’s a series of images of Titan’s equatorial region

And the same images annotated.

From the associated Nasa/JPL press release

The brightest objects seen in these images are methane clouds in the troposphere, the lowest part of the atmosphere, which are most visible on the left of panel B, the lower half of panel C, and the right of panel D. Surface features appear in shades of gray. These images show changes (outlined area) along the southern boundary of a dune field near the equator named Belet. Dark Belet occupies most of the top of these images. (Belet looks dark because it is made from different materials than neighboring areas.)

Titan’s equatorial latitudes are mostly arid. However, scientists interpret the changes seen in these images to be evidence of methane rain wetting the surface. Scientists have monitored the brightness of Titan’s surface, including this area, for years and have ruled out other possible causes of the changes. In these images, some of the dark areas grow larger and then recede within weeks. The maximum extent of the changes is shown with a blue outline.

Years ago, images from the European Space Agency’s Huygens probe and the Cassini radar instrument revealed dry channels near the equator (see Mosaic of River Channel and Ridge Area on Titan and Xanadu: Rivers Flowed onto a Sunless Sea). The new observations suggest the climate here is similar to that in the southwestern United States, where infrequent rain carves washes and riverbeds.

Titan’s weather has been changing with the seasons, and storms now are more common at low latitudes, such as those observed here. An arrow-shaped storm cloud several hundred kilometers (miles) across was observed on Sept. 27, 2010. See Titan’s Arrow-Shaped Storm for an image of that storm. See The Rite of Spring to learn how the sun’s illumination of the Saturnian system changed during the transition to spring in the northern hemispheres and to fall in the southern hemispheres of the planet and its moons. See Titan’s Moving Mid-Latitude Clouds to learn more about Titan’s changing weather.

The first image in this montage, panel A on the left, was taken early in the Cassini mission on Oct. 22, 2007, and shows how this region had appeared before the storms. The second image, panel B, was taken on Sept. 27, 2010. The huge arrow-shaped cloud is just out-of-frame to the left in panel B. The arrow-shaped cloud was quickly followed by extensive changes on the surface that can be seen in panel C, an image captured on Oct. 14, 2010. These changes cover an area of 500,000 square kilometers (193,000 square miles), roughly the combined area of Arizona and Utah in the United States.

The wet terrain can still be seen about a month after the storm in panel D, which was taken on Oct. 29, 2010. But by Jan. 15, 2011, which was the date of panel E, the area mostly appears dry and bright, with a much smaller area still dark, i.e. wet.