Post by Dr. Stéphane Le Mouélic, Laboratoire de Planétologie et Géodynamique, CNRS UMR6112- University of Nantes, Nantes, France.
Titan is one of the most fascinating bodies of our Solar System. Bigger than Mercury, this satellite of Saturn is veiled by a thick atmosphere of nitrogen containing a few percent of methane. Aerosols formed in the atmosphere by a complex chemistry triggered by the solar UV irradiation produce a global haze totally masking the surface to the naked eye. During 13 years, from July 2004 to September 2017, the Cassini spacecraft orbited Saturn. It took advantage of gravity assist maneuvers to perform 127 equatorial and polar flybys of Titan. Data from the Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini revealed the distribution of the main compositional units of the surface of Titan (Image 1). The inset in Image 1 shows the 84 km-diameter Selk crater, one of the primary targets chosen for the next New Frontier “Dragonfly” mission, a mobile rotorcraft-lander planned to be launched in 2026.
Image 1: False color composite of Titan with the red controlled by the 1.59/1.27 µm, green by the 2.03/1.27 µm and blue by the 1.27/1.08 µm band ratios. The equatorial dune fields appear in a consistent brown color. Selk crater is shown in the inset. Credits NASA/JPL/Univ. Arizona/CNRS/LPG.
Three instruments onboard Cassini were used to map the surface through the scattering haze and absorbing methane. A Radar provided SAR images covering two thirds of the surface, with a resolution up to ~300m/pixel, (Elachi et al., 2005, Lopes et al., 2019). The ISS multispectral camera allowed global observations thanks to its 0.93 µm infrared filter (Porco et al., 2005, Karkoschka et al. 2017), and the VIMS imaging spectrometer acquired observations in 352 wavelengths between 0.3 and 5.1 µm using a two-axis scanning mirror to build images up to 64×64 pixels (Brown et al., 2004). The interest of VIMS resides in the capacity to observe at infrared wavelengths where the atmospheric methane is not too absorbing and where the scattering by aerosols is reduced (Sotin et al., 2005). Once corrected from acquisition geometry and atmospheric effects (Le Mouélic et al., 2019), global mosaics of all the VIMS images acquired during the Cassini mission show the diversity of the main terrains on Titan. Cassini revealed the presence of geomorphological earth-like features such as rivers (carved by liquid methane), lakes and seas (filled by hydrocarbons). The brownish equatorial regions that we see in the VIMS image in Image 1 correspond to huge dune fields (Rodriguez et al., 2014, Brossier et al., 2018). Only few tens of impact craters have been identified at the surface (Wood et al., 2010, Neish et al., 2012, Hedgepeth et al., 2018), which implies that the surface is geologically young.
Image 2: The Huygens landing site seen by VIMS onboard Cassini, and by DISR onboard Huygens (top right inset). Adapted from Le Mouélic et al. (2019).
In January 2005, the Huygens probe landed on Titan, after taking images and measurements during its descent under a parachute. These stunning images revealed the presence of dried ramified riverbeds (Tomasko et al., 2005). Image 2 shows the VIMS color mosaic of the Huygens landing site (marked by a red cross) as seen from orbit. A black and white panorama acquired by the DISR instrument on Huygens from an altitude of ∼34 km is shown for comparison (top right). If everything goes well, the forthcoming Dragonfly spacecraft will fly again over Titan landscapes in 2034, investigating dune fields and the floor of an impact crater. Craters are particularly interesting, as both liquid water and complex organic materials once existed there together for possibly thousands of years. Who knows what kind of new wonders will come out of the Dragonfly images? Let’s be patient…
Further Reading
Barnes, J.W., B. J Buratti, E. P Turtle, J. Bow, P. A Dalba, J. Perry, R. H Brown, S. Rodriguez, S. Le Mouelic, K. H Baines, C. Sotin, R. D Lorenz, M. J Malaska, T. B McCord, R. N Clark, R. Jaumann, P. O Hayne, P. D Nicholson, J. M Soderblom and L. A Soderblom, Precipitation-Induced Surface Brightenings Seen on Titan by Cassini VIMS and ISS, Planetary Science, 2:1, doi:10.1186/2191-2521-2-1, 2013
Brown, R.H., et al. The Cassini visual and infrared mapping spectrometer (VIMS) investigation. Space Sci. Rev. 115, 111–168, 2004
Brossier, J., S. Rodriguez, T. Cornet, A. Lucas, J. Radebaugh, L. Maltagliati, S. Le Mouélic, A. Solomonidou, A. Coustenis, M. Hirtzig, R. Jaumann, K. Stephan, and C. Sotin, Geological Evolution of Titan’s Equatorial Regions: Possible Nature and Origin of the Dune Material, JGR, https://doi.org/10.1029/2017JE005399, Vol 123, 5, pp 1089-1112, 2018.
Elachi, C., et al.. Cassini radar views the surface of Titan. Science 308 (5724), 970–974. https://doi.org/10.1126/science.1109919, 2005
Hedgepeth, J.E., Neish, C.D., Turtle, E.P., Stiles, B.W.. Impact Craters on Titan: Finalizing Titan’s Crater Population, 49th Lunar and Planetary Science Conference, Lunar and Planetary Institute, Woodlands, Texas, 2018.
Karkoschka, E., A. McEwen, J. Perry, Producing the best global mosaic of Titan’s surface albedo using Cassini images, 48th LPSC, abstract #2518, 2017
Le Mouelic, S., Cornet, T., Rodriguez, S., Sotin, C., Seignovert, B., Barnes, J. W., Brown, R.H., Baines, K. H., Buratti, B. J., Clark, R. N., Nicholson, P. D., Lasue, J. Pasek, V., Soderblom, J. M.., The Cassini VIMS archive of Titan: from browse products to global infrared color maps, , Icarus, 319, 121-132, doi: 10.1016/j.icarus.2018.09.017, 2019
Lopes et al., Titan as revealed by the Cassini Radar, Space Science Reviews, Vol 215, Issue 4, article id. 33, 50 pp., 10.1007/s11214-019-0598-6, 2019
Neish, C.D., Lorenz, R.D.. Titan’s global crater population: A new assessment. Planetary and Space Science 60, 26–33. https://doi.org/10.1016/j.pss.2011.02.016, 2012
Porco, C.C., et al., Imaging of Titan with the Cassini spacecraft. Nature 434,159–168, 10.1038/nature03436, 2005.
Rodriguez, S. et al., Global mapping and characterization of Titan’s dune fields with Cassini: Correlation between RADAR and VIMS observations, Icarus, Volume 230, p. 168-179, 10.1016/j.icarus.2013.11.017, 2014
Seignovert, B., Le Mouélic, S., Brown, R. H., Karkoschka, E., Pasek, V., Sotin, C., & Turtle, E. P. Titan’s global map combining VIMS and ISS mosaics, 50th Lunar and Planetary Science Conference, held 18-22 March, 2019 at The Woodlands, Texas. LPI Contribution No. 2132, id.1423, 2019
Sotin et al., Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan, Nature, doi:10.1038/nature03596, p786-789, 2005
Tomasko, M.G., et al., Rain, winds and haze during the Huygens probe’s descent to Titan’s surface. Nature 438, 765–778. doi.org/10.1038/nature04126, 2005.
Wood, C.A., Lorenz, R., Kirk, R., Lopes, R., Mitchell, K., Stofan, E., The Cassini RADAR Team, Impact craters on Titan. Icarus 206, 334–344, 2010.
Planetary Geomorphology Image of the Month 