Titan's Winter Polar Vortex Structure Revealed By Chemical Tracers

Supplemental References

Flasar, F. M., et al. (2005), Titan's atmospheric temperatures, winds, and composition, Science, 308, 975-978.

Teanby, N. A. et al (2008)'Titan's winter polar vortex structure revealed by chemical tracers'Journal of Geophysical Research - Planets, Vol 113, Article number E12003

Titan's Chemical Tracers in the North Polar Region

Schematic of possible dynamical processes occurring in Titan's north polar region.

Research by: N. A. Teanby, R. de Kok, P. G. J. Irwin, and S. Osprey, Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK; S. Vinatier Observatoire de Paris, LESIA, Meudon, France; P. J. Gierasch Department of Astronomy, Cornell University, Ithaca, New York, USA; P. L. Read Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK; F. M. Flasar, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA B. J. Conrath, Department of Astronomy, Cornell University, Ithaca, New York, USA; R. K. Achterberg Department of Astronomy, University of Maryland, College Park, Maryland, USA; B. B?zard, Observatoire de Paris, LESIA, Meudon, France; C. A. Nixon Department of Astronomy, University of Maryland, College Park, Maryland, USA; S. B. Calcutt, Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK

Figure 3. Cross sections of temperature and composition through Titan's atmosphere constructed using two years of Cassini-CIRS limb observations. Black contours indicate zonal wind speeds and lambda gives this lifetime of each species. The polar vortex located around 50N creates a strong mixing barrier.'

Cassini's Composite Infrared Spectrometer (CIRS) is a Fourier transform spectrometer that records spectra in the far- and mid-IR (10-1500 cm-1, 1 mm to 7 um). This spectral range covers emission features of most of Titan's photochemical inventory and spectra can be used to derive atmospheric temperature and composition. The full spectral range of CIRS is covered by three separate focal planes, which use the same telescope and scan mechanism: FP1 10-600 cm-1 (far-IR); FP3 600-1100 cm-1 (mid-IR); and FP4 1100-1500 cm-1 (mid-IR)

Scientists analyzed two years of observations from CIRS to determine cross sections of five independent chemical tracers (HCN, HC3N, C2H2, C3H4, and C4H2), which are then used to probe dynamical processes occurring within the vortex. Results revealed compelling evidence that the vortex acts as a strong mixing barrier in the stratosphere and mesosphere, effectively separating a tracer-enriched air mass in the north from air at lower latitudes. In the mesosphere, above the level of the vortex jet, a tracer-depleted zone extends away from the north pole toward the equator and enrichment is confined to high northern latitudes. The results suggest that a residual polar circulation may be present.

The scientists also observed an unexpected enrichment of C4H2 in the northern stratosphere, which suggests photochemical polymerization of C2H2. The observations provide stringent new constraints for dynamical and photochemical models and identify key polar processes for the first time. Some of the processes we see have analogues in Earth's polar vortex, while others are unique to Titan.

Implications

The winter polar vortex on Saturn's largest moon Titan has profound effects on atmospheric circulation and chemistry and for the current northern midwinter season is the major dynamical feature of Titan's stratosphere and mesosphere. The main implication is a better understanding of Titan's polar processes. Observed composition gradients provide strong evidence for a middle atmosphere mixing barrier around 60 degrees N. This is further corroborated by strong potential vorticity gradients in the same region, suggesting a dynamical origin analogous with Earth's polar vortices.

Significance to Solar System Exploration

Titan is unique in the solar system because it has a thick atmosphere dominated by nitrogen and methane with an active photochemical cycle. Understanding processes occurring within Titan's atmosphere is crucial to understanding the origins of Saturn's largest moon, as well as in comparing Titan to a pre-biotic Earth.

For more information about NASA science or how to get your research highlighted, please contact Samantha Harvey, Senior Science Writer, NASA's Jet Propulsion Laboratory.

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