Titan’s detached haze as a window on Titan’s mesosphere
Haze vertical structures in planetary atmospheres are particularly interesting because they reveal interactions between atmospheric dynamics and haze microphysical processes, including photochemistry. Here we report on what can be learned about Titan's seasonal behavior from its 'detached' haze.
The full contribution is found here: http://rdcu.be/KrqG
Among the various features found in planetary atmospheres, the "detached haze layers" are quite remarkable. They are found in the earth’s atmosphere, on Titan, Venus, Mars, Jupiter, Saturn and Pluto. In most cases, they are transient cloud layers or dust plumes produced by a particular circulation or wave activity. Except for Titan, these layers have a typical lifetime of a few hours or days, quite short compared to the seasonal cycle. For Pluto, the wave activity seems to modulate the haze layer and create small detached structures similar to the secondary structures that streak the detached layer of Titan.
The detached layer of Titan (Fig. 1) is special and differs from those found on other bodies because it is long-lived and nearly global. During its multi-year stable phase, it is a continuous "shell" of haze at all longitudes, covering the main haze layer from the summer pole to the winter polar region, where it merges with the winter polar haze structure around 55-degrees latitude (Figure 1). It was observed by the Voyager cameras in 1980 and 1981 and again by Cassini cameras beginning in 2004. Between 1981 an 2004 the layer rose from an altitude of 350 km to 515 km, which was initially mysterious. Climate models show that this layer is structured by the wind and is good tracer of atmospheric circulation. It is also the place where products of chemistry (organic and prebiotic molecules, aerosol precursors) formed at very high altitude and older aerosols of the main layer converge and mix together. This idea, however, has been challenged and competes with other scenarios based on the role of haze particle microphysical processes alone.
With Cassini images, we monitored the altitude of the detached layer over the period 2005 -2017, almost 1/2 of Titan’s year (29.5 Earth years). This period includes the equinoctial transition in 2009, when atmospheric circulation in Titan’s mesosphere reversed, profoundly affecting the detached haze. While stable since the beginning of the Cassini arrival until 2008, around equinox the detached haze layer underwent a rapid drop, due to weakening of the vertical winds, which brought it to 350 km in 2011, exactly one Titan year after the Voyager visits. The haze altitudes measured one Titan year apart matched to within uncertainty of the measurements (about 10 Km). In 2012 the detached layer disappeared completely. Titan climate models predicted its reappearance near the next solstice (April 2017), around 500 km altitude. The return of the detached layer, under the predicted conditions, constituted an important test to validate the dynamic origin of this layer. This is not the end of the story, however. Some details about the timing of the re-appearance, the evolution of the haze and the fine-structure of the haze at smaller spatial scales are not explained by models. These remain for further study.
The paper in Nature Astronomy is here: https://go.nature.com/2q0eSCa