Titan’s hazes unveiled

Now that Saturn is visible in the sky at dusk, it is fairly easy to spot its biggest satellite Titan with a small telescope. Don’t expect nothing more than a bright point in the sky but it is provocative to imagine the Huygens probe descending into its thick atmosphere in the beginning of 2005. It returned the first images from an exotic and cold world, covered by a dense, orange haze and showing river-like channels on its surface. Liquid water is absolutely unexpected at the extremely low Titan’s temperatures (just 90 K) and so other compounds must play the role of Earth’s water. In fact, it is very likely that Titan has what we can call a methane cycle with evaporation and precipitation even in the form of heavy rain.

All those secrets, however, remained elusive for many years and visible images as those returned by the Voyager spacecrafts were only able to capture an orange disk covered by hazes, as shown in Figure 1. It was proposed in the 80s that complex organic molecules play a central role in the composition of Titan’s haze, as the works by Carl Sagan and many others claimed. This idea made Titan a very interesting world for astrobiology, since we had what has sometimes been called a pre-biotic Earth with a single, though strong, handicap: its low temperature. Anyway, Titan’s organic chemistry gained and deserved much attention that was strongly supported by the Cassini/Huygens mission and its data. But where those organics come far? How did they form?

Even though we had been there and in spite of Cassini spacecraft efforts, there had been no experimental confirmation of the origin of Titan’s hazes so far. Numerical models strongly supported the idea of these being constituted by polyciclic aromatic hydrocarbons (PAHs) but the empiric demonstration was lacking yet. A major contribution to this problems has been provided by a recent publication by Lopez-Puertas and collaborators in The Astrophysical Journal [1]. This paper not only addresses the vertical distribution of PAHs and supports the idea of the main haze layer of Titan being formed in the upper atmosphere, but it also gives a sounded explanation for another mystery in Titan’s spectrum.

During two flybys of Titan by Cassini in July and August 2007, the Visual and Infrared Mapping Spectrometer (VIMS) onboard the spacecraft recorded a number of limb daytime observations which were used to infer the abundance of methane in the satellite’s upper atmosphere. In a recent paper by Dinelli et al. [2] a very strong emission at 3.28 mm seen in those data was reported. The intensity was high, and no species expected in Titan’s upper atmosphere were able to produce such a strong emission. The emission was estimated to be in the upper atmosphere, from 600 to 1250km, with a maximum at 950km. Dinelli and collaborators proposed the aromatic compounds as the main suspect although it was unclear how would these rare molecules be able to produce as much radiation as the abundant methane.

The missing key to this problem was found in the work by López-Puertas. The main question was: is it possible to produce such a strong emission with a realistic amount of PAHs? The PAHs can be excited by the much more energetic UV radiation coming from the Sun and they are very efficient in redistributing it among vibrational models which later emit in the near infrared at the observed wavelength. The first part of the paper is thus devoted to calculate the efficiency of such redistribution and therefore to find how to relate the observed radiance at longer wavelengths with the abundance of the PAHs.

Once it was possible to think of a mechanism to account for the observations, they used the NASA Ames PAH IR Spectroscopic Database to fit the observed radiance as shown in Figure 2. This method provides the clue to determining for the first time the abundance of PAHs as a function of height and it also gives a good insight into some of their general properties, such as the average number of rings. The results show that the PAHs can be found in the upper atmosphere in large concentrations, mostly at 950km, where the strong emission was identified. The PAHs found there have between 10 and 100 carbons and a mean mass of more than 400 u.

The main conclusion that can be drawn from this work is that finding such large concentrations of PAHs at high altitudes strongly supports the thick lower hazes being formed from above, as many suspected and none had been able to prove yet. The general scheme is summarized in Figure 3: UV photons coming from the Sun and energetic particles generated in the Saturnian system ionize methane and nitrogen found very high in Titan’s atmosphere. The heavy ions are transformed through complex ionic and photo- chemistry processes into the PAHs shown in the paper discussed here. Those PAHs then literally fall into Titan’s lower atmosphere, or more technically they are said to precipitate or be transported by other mechanisms into Titan’s lower hazes.

This conclusion also opens new perspectives on the astrobiological problem of Titan. The methane snow possibly falling into the satellite’s surface might be strongly contaminated by fairly complex organic compounds of which we now have a much better information. May something similar happened in the young Earth? Conditions are possibly way too different to establish any parallelism but it is certainly exciting to think of such a complex process starting at more than one thousand kilometers high and ending in Titan’s ground.

References:

[1] López-Puertas M., Dinelli B.M., Adriani A., Funke B., García-Comas M., Moriconi M.L., D’Aversa E., Boersma C. & Allamandola L.J. (2013). LARGE ABUNDANCES OF POLYCYCLIC AROMATIC HYDROCARBONS IN TITAN’S UPPER ATMOSPHERE, The Astrophysical Journal, 770 (2) 132. DOI: 10.1088/0004-637X/770/2/132

[2] Dinelli B.M., López-Puertas M., Adriani A., Moriconi M.L., Funke B., García-Comas M. & D’Aversa E. (2013). An unidentified emission in Titan’s upper atmosphere, Geophysical Research Letters, 40 (8) 1489-1493. DOI: 10.1002/grl.50332