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.
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.

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.

Four figures illustrating change in Titan's haze layer.
Figure 2. The change in Titan’s haze layer is illustrated in this figure, derived from data obtained by Cassini. The picture in panel a was taken in 2006, while the picture in panel b was taken several months after Titan’s equinox in August of 2009. Pictures c and d magnify the outer regions and show the difference in altitude of the haze layer (near 500 kilometers [310 miles] in c and near 380 km [235 miles] in d).
Titan's seasonal changes are tied to Saturn's orbital year, lasting 29.5 Earth years. Saturn's orbital motion caused the sun to cross Saturn's ring plane, and both Saturn's and Titan's equators, from south to north, on Aug. 11, 2009 (UTC). As on Earth, the northbound crossing of the equator marks the beginning of spring in the northern hemisphere and the beginning of autumn in the southern hemisphere.

Both Voyager flybys showed that Titan's northern and southern atmospheres could look brighter or darker than the other (Figure 1). Scientists think the differences arise primarily because of differences in the abundance of the haze in the two hemispheres.

Cassini's images show that Titan's top haze layer makes a dramatic change in altitude, from 500 kilometers (310 miles) above the surface to 380 kilometers (240 miles) above the surface (Figure 2), in just a few months. It changes most rapidly right around equinox, a very prompt response to the approach of spring/autumn in the opposing hemispheres. In contrast, the seasonal change in the apparent brightness between Titan's northern and southern hemispheres lags the equinox by about one Saturn season, more than 7 years, completing the change around the solstice. By the solstice the winter hemisphere gets darker due to the increased haze there.

Haze is caused by particles that are relatively small, but much larger than molecules, that are suspended in an atmosphere. Hazes differ from clouds by being more generally present across a geographic area or atmospheric volume with little variation in the density of particles suspended in the atmosphere. On Earth, haze can give the atmosphere an opalescent appearance that subdues colors.

Clouds usually have boundaries. On Earth, unless specified otherwise (for example, dust cloud, smoke cloud, or volcanic ash cloud), clouds are composed of water droplets or ice crystals. Fog is a variation of a cloud: it is a cloud whose base is on Earth's surface.
On Earth, we see hazes resulting from suspended, tiny water droplets, wildfire smoke, volcanic ash and a variety of pollutants. Indeed, long before Los Angeles and its smog, the Native Americans living in the region called the area the "valley of smoke" because of the natural haze they frequently saw in the basin.

Rather than being composed of water droplets or pollutants, Titan's haze is composed of tholins: tar-like particles involving carbon, hydrogen and nitrogen. Both solar ultraviolet radiation and Saturn's magnetosphere, the magnetic bubble around the planet, play a role in forming these materials high in the atmosphere by providing the energy to convert the simple molecules (molecular nitrogen and methane) making up Titan's atmospheric gases into more complex molecules.

In the new study, Cassini scientists used 81 images of Titan, taken between Oct. 23, 2004, and Aug. 7, 2010. The image collection shows Titan's complete disk at all phases from thin crescent to almost "full moon" and all were made using ultraviolet, blue, or green filters.

Figure 3. In this Cassini image, Titan's atmosphere’s seasonal hemispheric dichotomy returned to the configuration of a darker northern hemisphere and lighter southern hemisphere seen by NASA’s Voyager spacecraft in the early 1980s. The contrast in brightness is subtle, but visible in this natural color view taken shortly after Saturn's August 2009 equinox.
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