Titan's Weather

[sheeny]

I got this from this morning's Nature Contents.

Enjoy,

Al.

Editor's Summary

27 July 2006
It's raining methane

Saturn's moon Titan and the Earth are the only Solar System worlds where rain reaches the surface. Titan's rain may not be much like ours, as it's methane rain, and the atmospheric cycles of water and methane are very different. Two papers this week offer clues as to what a rainy day on Titan might be like. Hueso and Sánchez-Lavega use a numerical model to show that severe methane convective storms accompanied by intense precipitation may occur in certain conditions. These storms would be comparable to flash flood events on Earth. Tokano et al. present methane distribution and temperature data from instruments on board the Huygens probe. Huygens recently took images of landscapes suggestive of rivers or lake-beds, but the camera did not show any liquid. The new data point to the presence of weak drizzle-like rain. In contrast to clouds observed by telescopes or the Cassini spacecraft, the barely visible clouds encountered by Huygens are widespread, suggesting that rainfall occurs globally and may affect Titan's surface structures.
News and Views: Planetary science: Titan's exotic weather

Titan is viewed as a sibling of Earth, as both bodies have rainy weather systems and landscapes formed by rivers. But as we study these similarities, Titan emerges as an intriguingly foreign world.
Caitlin A. Griffith

Letter

Nature 442, 428-431(27 July 2006) | doi:10.1038/nature04933; Received 6 February 2006; Accepted 23 May 2006
Methane storms on Saturn's moon Titan

R. Hueso1 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04933.html#a1) and A. Sánchez-Lavega1 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04933.html#a1)
Top of page (http://www.nature.com/nature/journal/v442/n7101/abs/nature04933.html#top) The presence of dry fluvial river channels and the intense cloud activity in the south pole of Titan over the past few years1,2,3 suggest the presence of methane rain. The nitrogen atmosphere of Titan therefore appears to support a methane meteorological cycle that sculptures the surface and controls its properties1,4. Titan and Earth are the only worlds in the Solar System where rain reaches the surface, although the atmospheric cycles of water and methane are expected to be very different5. Here we report three-dimensional dynamical calculations showing that severe methane convective storms accompanied by intense precipitation may occur in Titan under the right environmental conditions. The strongest storms grow when the methane relative humidity in the middle troposphere is above 80 per cent, producing updrafts with maximum velocities of 20 m s-1, able to reach altitudes of 30 km before dissipating in 5–8 h. Raindrops of 1–5 mm in radius produce precipitation rainfalls on the surface as high as 110 kg m-2 and are comparable to flash flood events on Earth6.


Letter

Nature 442, 432-435(27 July 2006) | doi:10.1038/nature04948; Received 29 March 2006; Accepted 6 June 2006
Methane drizzle on Titan

Tetsuya Tokano1 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a1), Christopher P. McKay2 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a2), Fritz M. Neubauer1 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a1), Sushil K. Atreya3 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a3), Francesca Ferri4 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a4), Marcello Fulchignoni5 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a5),6 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a6) and Hasso B. Niemann7 (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#a7)
Top of page (http://www.nature.com/nature/journal/v442/n7101/abs/nature04948.html#top) Saturn's moon Titan shows landscapes with fluvial features1 suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole2,3,4 or cloud streaks at southern mid-latitudes5,6 and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models7,8,9,10,11. Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.

[Greg Bryant]

These papers tie in very well with Cassini's discovery of what appear to be multiple lakes near Titan's north pole.

http://www.astmag.com.au/news20060728_Titan.htm

Saturn and its moons are truly a wonderful system.

[sheeny]

Got this from this morning's news@nature.

Enjoy,

Al.

Titan: swimming in the rain

Signs of lakes, flash floods, storm clouds and drizzle seen on Saturn's moon.
Richard Van Noorden
http://www.nature.com/images/spacer.gifhttp://www.nature.com/news/images/spacer_pink.gifhttp://www.nature.com/images/spacer.gifhttp://www.nature.com/news/2006/060724/images/060724-7.jpg
http://www.nature.com/news/images/spacer.gif
A close sweep of the northern reaches of Titan shows 'great lakes' on Saturn's moon.
© NASA/JPLWhat's the weather like on Titan, Saturn's largest moon? New research this week suggests it is pretty wet. Papers published in Nature show evidence of a light drizzle, and forecast the potential for occasional flash floods from storm clouds. Meanwhile, recent radar images from the Cassini spacecraft have revealed a land of lakes in Titan's northern hemisphere.

When Cassini reached Saturn's moon Titan in 2004, researchers discovered a number of interesting geological features, including apparent river canyons, and immediately began to debate how they were formed. Clues have so far come in dribbles, with some studies showing "probable" rain, and others seemingly ruling out large bodies of water, at least in the bits of the moon seen thus far (see Titan disappoints ocean hunters (http://www.nature.com/news/2005/050606/full/050606-10.html)).

But Cassini's seventeenth close flyby of the moon, which happened on 22 July and brought the craft into more northern regions, provides the most solid evidence yet that there are big lakes — the size of North America's Great Lakes — on Titan.

It's raining methane

Titan is often compared to Earth: larger than the planet Mercury, it has a dense atmosphere, and shows features such as clouds, hills, dunes, river channels and now lakes. But the atmosphere is ten times thicker than Earth's, and much colder: at –179 °C on the surface, it is methane, not water, that dominates Titan's weather.

http://www.nature.com/news/images/columns_qt_left.gif Lakes? We won't be 100% sure until we can dip our toes in one. http://www.nature.com/news/images/columns_qt_right.gifhttp://www.nature.com/images/spacer.gif
Jonathan Lenine
University of Arizonahttp://www.nature.com/images/spacer.gifhttp://www.nature.com/images/spacer.gifWhen the Huygens probe dropped through Titan's haze on 14 January 2005, there was probably a light methane drizzle. So say Tetsuya Tokano and colleagues who publish their results in Nature today1 (http://www.nature.com/news/2006/060724/full/060724-7.html#B1). "We analysed data on the temperature, pressure and methane concentration profiles as the probe descended," explains Tokano, from Cologne University's Institute for Geophysics and Meteorology, Germany.

They found two distinct layers of cloud: one holding methane ice, and then underneath a liquid methane/nitrogen mix that drizzled to the surface. "But drizzle cannot produce the geological-like features seen on Titan," says Tokano. "We would need much heavier rainstorms at other times."

Such storms are predicted by Ricardo Hueso and Agustín Sánchez-Lavega from the University of the Basque Country in Bilbao, Spain2 (http://www.nature.com/news/2006/060724/full/060724-7.html#B2). "We simulated methane clouds under a range of conditions to try and reproduce cloud observations at Titan's south pole," says Hueso. Their models predict bouts of heavy rainfall.

"The model is quite complete, though I'm not sure it is optimized for polar conditions," comments Pascal Rannou, of the University of Versailles in St Quentin, France, who has worked on models of methane circulation around Titan.

Land of lakes

These scientists were looking at Titan's south pole, where it is now summer and where most clouds have been observed. In the wintry north pole, where liquid methane would evaporate less quickly from the colder surface, Cassini's newest radar pictures seem to show liquid methane lakes. The images were taken at 950 kilometres from the surface, the closest Cassini can get to Titan's atmosphere.

Images of dark lake-shaped patches and channels are not in themselves conclusive. But a variety of other evidence convinced the scientists. "For two days we asked the team to talk themselves out of seeing these pictures as lakes, but no one was able to," says Steve Wall from NASA's Jet Propulsion Laboratory in Pasadena, California.

The darkness of the radar image indicates that the surface of the patches is very smooth, a good indication of liquid. And whereas the darkness could be explained by chemicals derived from the atmosphere snowing down on the ground, this wouldn't create a patch with very sharp edges; a body of liquid would.

The patches also look to be surrounded by little drainage shelves, notes Jonathan Lunine, a planetary scientist from the University of Arizona, Tuscon. This indicates that a liquid has been eroding the sides as the lake level moved up and down over time. Finally, the microwave background suggests that the lakes are warmer than their surroundings, so are probably liquid methane rather than ice, adds Lunine.

"We won't be 100% sure until we can dip our toes in one, but all of the tests we could perform now come up positive for lakes," Lunine concludes.

What will happen to Titan's weather in the future? Because it takes Titan 29.5 Earth years to orbit the Sun, Cassini will have to hang around to spot any seasonal variations. "We need to 'follow the methane' and find out where it comes from and how it moves around Titan," says Rannou. The spacecraft's Saturn tour was planned up to 2008, but may be extended to 2010. The next Cassini flyby is planned for 7 September 2006.