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Not just a storm in a teacup

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The cloud that emerged above the south pole of Saturn's moon Titan in 2012 has been found to consist of hydrogen cyanide particles. This unexpected result prompts fresh thinking about the atmosphere of this satellite. See Letter p.65

In May 2012, a large spinning cloud appeared over the southern pole of Saturn's moon Titan, where it persists still1 (Fig. 1). The composition of the cloud has eluded identification until now. On page 65 of this issue, de Kok et al.2 provide strong evidence for an unexpected answer: the cloud is made of hydrogen cyanide (HCN) ice particles. This result is difficult to refute because two spectral features indicate the presence of HCN, rather than of a HCN polymer3, and the cloud's mass is consistent with that predicted for a cloud composed of HCN. The only problem is that the cloud, at an altitude of 300 kilometres, is not where it is supposed to be.

Figure 1: Titan's south polar cloud.
figure1

Space Science Inst./JPL-Caltech/NASA

The cloud rises above the winter darkness of Titan's southern pole and catches the glint of sunlight. De Kok et al.2 find that this spinning cloud, which completes an orbit in 9 hours — much faster than Titan's 15-day orbit — is composed of hydrogen cyanide. This orbital period signals the formation of a polar vortex as the southern pole enters into winter darkness14. The image, taken by the Cassini spacecraft's Imaging Subsystem, is centred near the south pole and extends over a distance of about 1,000 kilometres.

HCN is expected to condense in Titan's atmosphere at an altitude of 80 km; indeed, a nearly imperceptible tropical haze layer at this height matches the anticipated effects of HCN condensation4. By contrast, as discussed by de Kok and colleagues, the south polar HCN cloud emerged in the satellite's southern polar winter and resides in a region where, only three months earlier, the Cassini spacecraft's infrared spectrometer measured a temperature of 170 kelvin, which is 45 K too warm for HCN to condense5.

The next temperature measurement by Cassini will occur in 2015. Perhaps these data will reveal that Titan's atmosphere is appropriately cool at an altitude of 300 km, arming theorists with enough information to understand the complex conditions of the polar winter. Barring this inconvenience with the temperature, HCN clouds at high polar altitudes can form by processes that are typical of Earth's atmosphere, a point that becomes apparent when considering the broader context of Titan's atmospheric chemistry and dynamics.

Titan's atmospheric composition resembles certain models of early Earth, before oxygen was a significant component of our atmosphere and when methane (CH4) may have carried much of the atmospheric carbon6. Titan's two most abundant constituents, nitrogen (N2) and methane, control the atmospheric make-up. These molecules are broken apart in the upper atmosphere (at roughly 1,000 km altitude) by solar ultraviolet radiation in a process called photolysis, thereby yielding reactive radicals, which initiate the production of complex organic molecules. The main nitrogen-containing molecule produced, HCN, regulates the production of nitrogen species.

The photochemically produced molecules mix down to the lower atmosphere while chemically mingling to form new molecules. Eventually, they settle on the moon's surface in organic lakes and puddles, with a 'soup base' of methane and ethane (C2H6) that resembles natural gas7,8. As these nitrogen-bearing molecules diffuse downward, they are steered towards Titan's winter pole by atmospheric circulation9. Here they enter cooler regions and condense into clouds at different atmospheric layers, depending on their different thermodynamic properties.

Two distinct kinds of cloud cap the winter pole, both identified from measurements made by Cassini's Visual and Infrared Mapping Spectrometer: one at 55 km altitude, which is consistent with a C2H6 composition10, and one at 300 km, found by de Kok and colleagues to be made of HCN. These stacked clouds, composed of the most abundant nitrogen and carbon photochemical species, track seasonally with the winter pole. An HCN cloud was previously identified8 above Titan's north pole at the end of the northern winter, and the C2H6 cloud has vanished from this region in preparation for its southerly migration for the winter11.

The winter pole on Titan is a peculiar place. Here the atmosphere radiatively cools during winter's darkness, triggering a suite of dynamical atmospheric responses. As discussed by de Kok and colleagues, the polar temperature is regulated by the intertwined effects of atmospheric chemistry, radiation and dynamics, which control the atmosphere's absorption and emission of radiation, and the compression, expansion and mixing of Titan's gases. A potential explanation for the polar HCN clouds is that they form from a process known as open-cell convection, in which cool, dense air sinks and warms slightly while the surrounding air rises and cools, thereby forming clouds (Fig. 1). The cool polar atmosphere also contrasts with the warmer lower latitudes. The resulting decrease in temperature with increasing latitude affects the atmosphere's pressure structure, which, when combined with Titan's spin, implies circumpolar winds that become more strongly westerly with altitude12. Titan's atmospheric polar vortex, witnessed by the rapid spin of the south polar HCN cloud, isolates the polar air from the rest of the atmosphere, allowing the pole to cool further. The presence of clouds spinning in a vortex can thus naturally emerge at a winter pole.

However, the detailed operations of Titan's winter pole, such as the seasonal evolution of the chemistry and temperature at 300 km altitude, are complicated and far from understood12. Better grasped is Earth's polar atmosphere, where the winter polar vortex is a repository of unique composition and clouds. The polar chemistry evolves from winter to spring with the production and loss of many molecular species that ultimately control the polar ozone abundances.

Laboratory simulations suggest that the photochemistry in Titan's atmosphere produces amino acids and nucleotide bases13. How far the chemistry evolves in Titan's upper atmosphere is unclear, and would probably require detailed in situ sampling of the upper atmosphere. However, further understanding of Titan's organic chemistry will entail studies of the abundance, phase and particulate composition of the main nitrogen photochemical product, which affects the overall nitrogen chemistry. The presence of an HCN maelstrom opens investigations into a new avenue of planetary-satellite organic chemistry — that of a cold and dark polar vortex stocked with nitrogen and methane photolysis products, which are typical of Titan and, perhaps, of early Earth.

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Correspondence to Caitlin A. Griffith.

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Griffith, C. Not just a storm in a teacup. Nature 514, 40–41 (2014) doi:10.1038/514040a

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