Supersonic winds in Titan’s upper atmosphere

Titan, the biggest moon of Saturn, rotates slowly on itself with a 16 Earth’s day period, while its atmosphere, which extends up to 1500 km, rotates in the same direction but much more quickly. For instance, at an altitude of 300 km the zonal winds that blow eastward parallel to the equator can reach a speed of 200 m/s. Their rotation period around Titan’s axis is then only about 24h. This situation – called atmospheric super-rotation – is observed on only to bodies in the solar system: Venus and Titan. Modelers think that it is caused by the combined effect of a solar-driven meridional circulation, forming high-latitude jets, and propagating waves, transporting angular momentum equatorward. The Cassini spacecraft, which observed Titan from 2004 to 2017 did not carry any instrument dedicated to the wind measurements. Indirect measurements of wind speed could still be obtained from the temperature fields (determined from the analysis of the thermal emission of the atmosphere) and their seasonal changes were determined up to 400 km. But above this altitude, the wind regime was up to now very poorly known.

How is the wind speed measured in the upper atmosphere?

Thanks to the unprecedented spectral and spatial resolution of the ALMA interferometer, researchers now have access to a direct and precise measurement of the wind speed. This makes use of the Doppler shift of the lines emitted at submm wavelengths by the numerous molecules present in Titan’s atmosphere.

Figure 1: Doppler shift observed on the West limb (red) and East limb (black) of Titan in the equatorial region for two lines of CH3CN (one centered at 349.44699 GHz, corresponding to zero on the frequency scale) and one line of HNC (isomer of HCN, centered at 362.63030 GHz). The Doppler shift between the two limbs is due to fast winds rotating in the same direction as the surface of Titan.

The researchers used data with sufficient spatial resolution to map the disk of Titan (1 arcsec including its atmosphere) and even isolate the limb emission. They analyzed the emission lines and their Doppler shifts for six molecules: HCN, DCN, CH3CN, CH3CCH, HC3N et HNC. The derived wind maps obtained from each molecule show strong prograde(1) winds varying from 250 to 350 m/s and with different structures from one molecule to another.

Figure 2: Wind speed maps measured from CH3CN (left) and HNC (right). Distance is expressed in Titan’s radius. Blue color shows approaching winds and yellow-orange shows recessing winds. Winds derived from CH3CN display a hemispheric asymmetry with faster winds in the southern hemisphere (in autumn) than in the northern hemisphere. Winds measured from HNC are more localized around the equator with speeds as high as 350 m/s, which is 1.4 times the speed of sound!

Where do these strong winds occur in the atmosphere?

Modelling of the line shape and its intensity provides molecular abundance vertical profiles, which in turns permits to assign altitudes from which the emission – and therefore the wind - comes from. As each of the six studied molecules probes a different altitude region, it is possible to derive for the first time the vertical profile of the wind speed from 300 km to 1000 km above the surface. This profile shows winds that regularly increase with altitude.

Figure 3: Equatorial wind speeds measured from the line Doppler shifts of each of the six studied molecules: HNC (violet), HCN (red and yellow), HC3N (cyan), CH3CN (black), DCN (green) et CH3CCH (blue). Altitude ranges probed by each of these molecules are shown as vertical bars.

What is the cause of the winds?

The existence of 350 m/s zonal equatorial winds in Titan’s thermosphere (above 600 km) was totally unexpected based on density measurements from Cassini, and its physical origin remains to elucidate. Pre-Cassini models predicted a thermospheric wind regime blowing from the day side toward the night side, like on Venus. But the Cassini mission measured temperatures in the Titan thermosphere: no correlation between the temperature, latitude, longitude nor solar insolation were found, which suggests that the temperature in this region is not controlled by the UV solar flux absorption. The measured fast wind speeds therefore can’t be explained by the solar heating of the upper atmosphere.

Can the energy source of this wind be linked to the impact of ions and electrons coming from Saturn and transported in its magnetosphere? The magnetospheric plasma that rotates with Saturn impacts Titan at a speed of 120 km/s. This drives convection in Titan’s ionosphere, perhaps ultimately generating winds in the neutral upper atmosphere from the collision of ions with neutral molecules. However, these magnetospheric winds are unlikely to penetrate the atmosphere below 1000 km.
Perhaps more likely, a source « from below » is to be envisaged. Indeed, waves produced in the stratosphere in response to the diurnal variation of the solar insolation can propagate toward the upper atmosphere. These gravity waves, which were observed by Cassini-Huygens and from the ground, could transfer momentum from the deepest atmospheric layers towards the upper atmosphere and accelerate equatorial winds. This qualitative scenario still needs to be modeled.

Additional observations to characterize possible wind variability, and theoretical investigations to assess the quantitative plausibility of these mechanisms are urgently needed.

Note
(1) Prograde winds rotate in the same direction as the rotation of Titan’s surface, they are blowing from West to East for somebody who would be on the surface of Titan.

Reference

An intense thermospheric jet on Titan, Lellouch et al 2019, Nature Astronomy

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