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On Titan, the Sky is Falling!

Haze is observed in atmospheres all over the Solar System, from pristine wild lands and smoggy cities on Earth to the poles of Mars, the heights of Saturn, and throughout Titan's atmosphere. In fact, it is haze and not clouds that obstructs our view of Titan's surface. Observations through Titan's haze can be made through a few selected "windows" in infrared wavelengths or in microwaves transmitted and received by Cassini's radar.

This image of Titan obtained by Voyager 2
Figure 1. This image of Titan obtained by Voyager 2 in 1981 shows the southern hemisphere appearing lighter in contrast and a dark 'collar' is evident around the north pole.

As common as haze is in the solar system, the behavior of Titan's topmost haze layer has added a new wrinkle for meteorologists to explain. A recent study led by Bob West, a member of the Cassini imaging team, who is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif., finds that haze on Titan is behaving in a way never observed anywhere else in the solar system. With the recent change of seasons on Titan, its highest haze layer has dipped to a lower altitude.
Titan's haze affects its appearance and plays a role in the temperatures and movements of various layers in Titan's atmosphere, and in the exchange of energy between layers in its atmosphere. The haze also is involved in chemical reactions in the atmosphere and serves as a source of organic material on Titan's surface. Its behavior provides clues to how the atmosphere circulates and how it changes with the seasons.

Earth and Titan share some analogous weather – as recent findings on Titan's seasonal rainstorms and cirrus clouds show -- but their atmospheres have some significant differences that enable scientists to learn what makes each body so singular. Titan's atmosphere is much more massive and dense at Titan's surface and the upper layers rotate much faster than the satellite itself. It is enriched in organic molecules in a much colder setting and it is embedded in the magnetic bubble around Saturn (most of the time) without the protection of its own magnetic field. These differences make Titan a laboratory for helping to understand atmospheric and meteorological properties of Earth and other planets.

The researchers measured the position of the haze layer and fitted a circle generated by computer around Titan to the measurements, usually with an accuracy of one pixel. The measurements are not simple to make. Note in Figure 2 that the upper haze layer is not continuous (panels a and b) and it doesn't have sharp boundaries or an obvious line of maximum density (panels c and d). Measuring the haze layer takes time and care.

Measurements at the start of the study interval found that the haze layer neatly fit a circle centered on Titan. By Nov. 15, 2008 -- 269 days before Saturn's and Titan's seasonal equinox -- the haze layer became noticeably non-circular, with the haze over the equator higher than the haze over the pole. The maximum difference between the altitudes of the equatorial and polar haze occurred near equinox, August 11, 2009 (UTC), when the equatorial haze was 30 kilometers (20 miles) higher than the polar haze.

Not all features of the models proposed to explain the behavior of the haze and the atmosphere match well with what has been observed so far. By the end of Cassini's mission, shortly after Saturn's and Titan's next solstice, haze monitoring (Figure 3) should reveal what is needed for a satisfactory model of Titan's atmospheric behavior.

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publication:

"The Evolution of Titan's Detached Haze Layer near Equinox in 2009," Robert A. West (JPL), Jonathan Balloch (JPL), Philip Dumont (JPL), Panayotis Lavvas (Lunar and Planetary Laboratory, University of Arizona, Tucson), Ralph Lorenz (Applied Physics Laboratory, Johns Hopkins University, Laurel, MD), Pascal Rannou (GSMA, UMR CNRS 6089, Université de Reims Champagne-Ardenne, and LATMOS, UMR CNRS 8190, Université de Versailles St-Quentin, Verrières le buisson, France) , Trina Ray (JPL), and Elizabeth P. Turtle (Applied Physics Laboratory, Johns Hopkins University, Laurel, MD), Geophysical Research Letters, Vol. 38, L06204, doi:10.1029/2011GL046843, 2011.

-- Stephen Edberg, Cassini science communication coordinator