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Morning Drizzle at Xanadu

Xanadu seems to be a misty, drizzly place. So say new images showing a persistent light rain of methane over the western foothills of this, the major continent of Titan. Titan’s day is sixteen Earth days long, so if we say the drizzle or mist dissipates after about 10:30 AM local time, we’re saying that it lasts until three Earth days after sunrise. As much as the Sun ever rises on this frigid, cloud-bound world.

The work, conducted using data from the Keck Observatory (Hawaii) and Very Large Telescope (Chile), was presented today at the Division for Planetary Sciences meeting in Orlando (FL). The findings mark the first direct observation of methane rain, although precipitation has been presumed to occur for some time now. Features near Titan’s poles have been interpreted as lakes of liquid hydrocarbons, presumably replenished by just such precipitation.

Drizzle found on Xanadu

Image: VLT and Keck near-infrared images of Titan’s surface and lower
troposphere can be subtracted to reveal widespread cirrus-like clouds of frozen methane (lower images) and a large patch of liquid methane (dark area within box) interpreted as clouds and morning drizzle above the huge continent of Xanadu (outline). At left is a chart of Titan’s aerosol haze versus altitude, indicating higher density haze over portions of the south pole and the heights of frozen and liquid methane clouds. Credit: Mate Adamkovics/UC Berkeley, W. M. Keck Observatories, ESO.

The researchers, Máté Ádámkovics and Imke de Pater (UC Berkeley), see the widespread drizzle as a possibly dominant mechanism for returning methane to the surface, thus closing the methane cycle in a way similar to how the water cycle operates on Earth. The first images of actual clouds on the moon were the work of de Pater’s group, finding frozen methane clouds at an elevation of some 30 kilometers over Titan’s south pole. That work was done in 2001, since which time much data on the clouds has accumulated. Liquid methane clouds seem to occur below 20 kilometers.

In any case, getting a read on precipitation is hard work. Says Ádámkovics:

“The stratiform clouds we see are like cirrus clouds on Earth. One difference is that the methane droplets are predicted to be at least millimeter-sized on Titan, as opposed to micron-sized in terrestrial clouds – a thousand times smaller. Since the clouds have about the same moisture content as Earth’s clouds, this means the droplets on Titan are much more spread out and have a lower density in the atmosphere, which makes the clouds ‘subvisible’ and thus hard to detect.”

You need to do this kind of work at infrared wavelengths because Titan’s haze then becomes relatively transparent rather than presenting the impenetrable layers of smog seen in optical wavelengths. Using different infrared wavelengths, the researchers could probe the atmosphere at a range of altitudes, creating a methane absorption profile that allowed analysis of the clouds and their attendant drizzle. Tricky work, this, its methodology fully explained in today’s issue of Science Express, where the paper is Ádámkovics et al., “Widespread Morning Drizzle on Titan” (abstract).

Related: New Cassini views of Titan’s lakes and seas, with lakes also discovered around Titan’s south pole, have just become available.

Comments on this entry are closed.

  • Adam October 12, 2007, 2:10

    Hi Paul

    So like Earth, so not like Earth. Such a strange little world. I wonder how visible such spread-out clouds would be from the surface. Millimetre sized droplets is huge for cloud – does it precipitate at centimetre size? And the critical velocity would be quite low – roughly a metre a second. Rain falls at a walking pace on Titan rather than a quick jog like Earth. Wonder if there’d be hail?

    Those orange skies could get irritating after a while. I’d be craving blue.

  • ljk December 22, 2007, 16:21

    Mountains Discovered On Titan, Saturn’s Largest Moon

    ScienceDaily (Dec. 21, 2007) — By analyzing images from NASA’s Cassini Radar instrument, a Brigham Young University professor helped discover and analyze mountains on Saturn’s largest moon, additional evidence that it has some of the most earthlike processes of any celestial body in the solar system.

    Full article here:


  • ljk January 2, 2008, 11:26

    Titan Unveiled: Saturn’s Mysterious Moon Explored

    Ralph Lorenz and Jacqueline Mitton


  • ljk February 20, 2008, 11:34

    Titan’s rotational state : The effects of a forced “free” resonant wobble

    Authors: B. Noyelles

    (Submitted on 14 Sep 2007 (v1), last revised 19 Feb 2008 (this version, v3))

    Abstract: In Noyelles et al. (2008, Astron. Astrophys., 478, 959-970), a resonance involving the wobble of Titan is hinted. This paper studies this sc enario and its consequences.

    The first step is to build an accurate analytical model that would help to find the likely resonances in the rotation of every synchronous b ody. In this model, I take the orbital eccentricity of the body into account, as well as its variable inclination to Saturn’s equator. Then an an alytical study using the second fundamental model of the resonance is performed to study the resonance of interest. Finally, I study the dissipat ive consequences of this resonance.

    I find that this resonance may have increased the wobble of Titan by several degrees. For instance, if Titan’s polar moment C is equal to 0.35MR_T^2 (M and R_T being respectively Titan’s mass and radius), the wobble might be forced to 28 degrees. Thanks to an original formula, I find that the dissipation associated with the forced wobble might not be negligible compared to the contribution of the eccentricity. I also s uspect that, due to the forced wobble, Titan’s period of rotation may be somewhat underestimated by observers.

    Finally, I use the analytical model presented in this paper to compute the periods of the free librations of the four Galilean satellites as well as the Saturnian satellite Rhea. For Io and Europa, the results are consistent with the previous studies. For the other satellites, the per iods of the free librations are respectively 186.37 d, 23.38 y and 30.08 y for Ganymede, 2.44 y, 209.32 y and 356.54 y for Callisto, and 51.84 d, 2.60 y and 3.59 y for Rhea.

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0709.2293v3 [astro-ph]

    Submission history

    From: Beno\^it Noyelles [view email]

    [v1] Fri, 14 Sep 2007 13:08:29 GMT (140kb)

    [v2] Tue, 18 Sep 2007 11:48:46 GMT (113kb)

    [v3] Tue, 19 Feb 2008 19:55:12 GMT (132kb)