# Icy lanes on Titan

S. Douté

► To cite this version:

S. Douté. Icy lanes on Titan. Nature Astronomy, 2019, 3 (7), pp.590-591. 10.1038/s41550-019-0842-8. [https://hal.science/hal-02394963v1](https://hal.science/hal-02394963v1)

HAL Id: hal-02394963

https://hal.science/hal-02394963v1

Submitted on 2 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
OUTER SOLAR SYSTEM

Icy lanes on Titan

Multivariate analysis of infrared hyperspectral images of the Saturnian satellite Titan reveals widespread ice-rich terrains in the tropics. They are related to a variety of contemporary or past geological processes.

Sylvain Douté

Titan, the largest satellite of Saturn, is a fascinating evolved object with a surface shaped by active geologic and climatic processes¹. It also possesses a thick and dynamic atmosphere dominated by N₂ and CH₄. With a mean atmospheric pressure of 1.5 bar and temperatures in the range 90–95 K at the surface, rainfall and runoff of liquid hydrocarbons occur. The atmosphere is also the location of photochemical reactions leading to the synthesis of complex organic matter that precipitates on the surface. As a consequence, the atmosphere shows considerable haziness, making it difficult to observe the surface and to determine its composition. Writing in *Nature Astronomy*, Caitlin Griffith and colleagues analyse a series of infrared hyperspectral images and obtain some new insights about the nature of Titan's surface².

Although different instruments of the Cassini orbiter acquired large datasets of observations during the 126 Titan flybys, access to surface properties is still a daunting task because of the high opacity of Titan's atmosphere at many wavelengths. Radar waves penetrate down to the surface and return to an orbiter, allowing the use of mapping techniques such as synthetic aperture radar imagery or passive radiometry, which reveal structures and basic material composition³, but lack the ability to detect individual molecular species. In the optical domain, a significant fraction of solar photons reflected by the surface can escape the atmosphere through very narrow spectral windows in the infrared. Despite these difficulties, Le Mouelic and co-authors recently managed to create global infrared colour maps with a minimum atmospheric contribution from individual observations by the Visible and Infrared Mapping Spectrometer (VIMS)⁴. Nevertheless, the spectral features, which are indicative of the chemical nature of the components at the surface, remain weak and subtly convolved with the dominant spectral features of the atmosphere. Therefore, large uncertainties on the surface composition remain. This is a challenge that any research aimed at determining the composition of

Titan's surface has to overcome, including the present study by Griffith et al.

Different techniques are used to extract and exploit this weak signal, from simple spectral ratios to complex radiative transfer modelling. The latter method is based on a physical model that calculates the radiance measured by the instrument using a complete description of the atmosphere's scattering and absorbing properties. Such properties are variable on Titan and, due to the complexity of the physics at the

low Titan temperatures, always present significant uncertainties. As a consequence, inversion of such a nonlinear model to extract information from a weak (and thus noisy) signal from orbital measurements is an extremely difficult and time-consuming task. Ultimately the number of spectral images that can be treated is restricted, limiting our vision of Titan's surface to spatially limited regions of interest. Besides, radiative transfer modelling and retrievals is a pixelwise mapping.

<figure><img src="image_2.png" /></figure>

**Fig. 1** | Infrared colour map of Titan's hemisphere that contains most of the ice-rich corridor detected by Griffith et al. The map was generated using NASA's Titan Trek website (https://trek.nasa.gov/titan/) fed with data by Le Mouelic et al.⁴
A different approach to the problem is to consider the spatial coherency that exists in the image, where the signature of a compound can be subtly expressed in hundreds of pixels with varying magnitude. This redundancy is the key to extract ‘pure’ signatures of surface components and, possibly, to quantify their abundances. The major contribution of the research presented by Griffith et al. is to use a well-known multivariate transformation technique, principal component analysis, not only to exploit the redundancy within a given image but also to be able to treat 37 spectral cubes acquired above Titan by VIMS onboard Cassini. Such an analysis allowed the authors to present the first large-scale compositional characterization of Titan’s equatorial belt from tropics to tropics, with a focus on water ice versus non-icy materials. Predominant H₂O spectral features are detected over a 6,300-km-long tropical corridor, indicating the persistent occurrence of ice-rich terrains. This corridor follows the boundary between dark dune fields made of organic material sedimented from the atmosphere and elevated bright plateaus, some of the oldest terrains on Titan’s surface (for example, south of Chusuk Planitia in Fig. 1). According to Griffith et al., the bright-dark margins are probably escarpments, where the high bright terrain looms over the dunes. These different pieces of evidence point towards a tectonic origin of the escarpment, a fault that reveals Titan’s icy bedrock. The strongest H₂O ice features

also occur in the Sotra complex, a region displaying a 500-m-high mountain (Doom Mons), possibly of volcanic origin, from which emerge fluvial-like features associated with deposits. Griffith et al. explain these results with an erosion process that reveals solidified cryo-lavas that are otherwise buried beneath organic sediments. Erosion is also at work over an extensive region of enhanced water ice, distinct from the corridor that surrounds Titan’s largest crater Menrva, as well as around smaller craters. In such cases, icy materials have been excavated by impact and only later exposed by erosion. The analysis and interpretation of VIMS imagery by Griffith et al. thus appeals to varied geological processes, that is, tectonics, volcanism, impacts and erosion due to Titan’s hydrological cycle. This work thus contributes to a big picture that emerges about the geological evolution and activity of the satellite.

This picture is still preliminary, as the uncertainties on the surface composition and properties are still large and inconsistencies exist between different kinds of data, leaving several open questions. For example Griffith et al. mention that the long icy corridor, although a feature of planetary proportion, is hardly correlated to any obvious topographical or gravitational signature. Also geophysical observations by Cassini show no evidence that Titan is geologically active at present, although atmospheric CH₄ must be regularly replenished because its lifetime in the atmosphere is of the order of 20 Myr.

More sensitive multivariate techniques⁵ and efficient massive inversion schemes of physical models⁶ could improve the exploitation of the extensive Cassini dataset. In the future, giant ground-based telescopes such as the European Extremely Large Telescope will be able to provide both the required spatial resolution (approximately 30 km per pixel) and a spectral resolution fine enough to pick up photons unaffected by absorption occurring in both Earth’s and Titan’s atmosphere. Of course, the best way to map in detail the composition of Titan’s surface would be a new in situ mission, with delivery of aerial platforms or aircrafts below 40 km altitude that would be able to observe a variety of terrains with minimal atmospheric contamination⁷.

<figure><img src="image_3.png" /></figure>

### Sylvain Douté

Institut de Planétologie et d'Astrophysique de Grenoble, Univ. Grenoble Alpes, CNRS, Grenoble, France.

e-mail: sylvain.doute@univ-grenoble-alpes.fr

Published online: 5 July 2019

https://doi.org/10.1038/s41550-019-0842-8

### References

1. Hörst, S. M. *J. Geophys. Res. Planets* **122**, 432–482 (2017).

2. Griffith, C. A. et al. *Nat. Astron.* https://doi.org/10.1038/s41550-019-0756-5 (2019).

3. Le Gall, A. et al. *J. Geophys. Res. Planets* **121**, 233–251 (2016).

4. Le Mouelic, S. et al. *Icarus* **319**, 121–132 (2019).

5. Moussaoui, S. et al. *Neurocomputing* **71**, 2194–2208 (2008).

6. Ceamanos, X. et al. *J. Geophys. Res. Planets* **118**, 514–533 (2013).

7. Pauken, M. et al. Science at a variety of scientific regions at Titan using aerial platforms. In *Planetary Science Vision 2050 Workshop 2017 Contribution No. 1989*, id. 8177 (LPI, 2017).