Spectral and emissivity analysis of the raised ramparts around Titan's northern lakes

Anezina Solomonidou (1, 2, 3) , Alice Le Gall (4) , M. J. Malaska (5) , S. P. D. Birch (6) , R. M. C. Lopes (5) , A. Coustenis (3) , S. Rodriguez (7) , S. D. Wall (5) , R. J. Michaelides (8) , M. R. Nasr (9) , C. Elachi (2) , A. G. Hayes (6) , J. M. Soderblom (9) , A. M. Schoenfeld (10) , C. Matsoukas (11) , Pierre Drossart (3) , M. A. Janssen (5) , K. J. Lawrence (5) , Olivier Witasse (12) , J. Yates (1, 3, 13) , J. Radebaugh (14)

1 ESAC - European Space Astronomy Centre
2 CALTECH - California Institute of Technology
3 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics
4 PLANETO - LATMOS
5 JPL - Jet Propulsion Laboratory
6 Cornell University [New York]
7 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
8 Physics Department [Stanford]
9 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]
10 EPSS - Department of Earth, Planetary and Space Sciences [Los Angeles]
11 KTH - KTH Royal Institute of Technology [Stockholm]
12 ESTEC - European Space Research and Technology Centre
13 CZU - Czech University of Life Sciences Prague
14 Department of Geological Sciences [BYU]

Some of Titan's small northern hemisphere lakes show raised rampart features (which are distinct from raised rims), and appear as SAR-bright mound-like annuli extending away from the lake for up to tens of kilometers from the shoreline. We investigate the infrared and microwave characteristics of these features using Cassini Visual and Infrared Mapping Spectrometer (VIMS) and RADAR data. A spectral comparative analysis is performed among the lakes, their ramparts, and the surrounding regions. We overcome the profound difference in spatial resolution between VIMS and SAR data by using a method that provides overlays between the spectral images and SAR, thus enabling the correct selection of VIMS pixels. The surface properties of the selected areas are obtained using a radiative transfer analysis on the selected VIMS pixels, in addition to emissivity obtained from the RADAR in radiometry mode. Analysis of these combined and co-registered data provides constraints for the formation mechanism(s) of raised ramparts. The results show that the emissivity of the raised ramparts is close to that of Titan's labyrinthic terrains and to that of empty lake floors in the northern polar regions. This is confirmed by the VIMS analysis that also shows that the infrared spectral response of the raised ramparts is very similar to that of some empty lake floors. This suggests that both areas are made from or are covered by a similar material. In addition, two out of the eight lakes with raised ramparts show spectral differences at three specific wavelengths, 1.6, 2.0, and 5.0 μm, between the ramparts and the surrounding terrain. We hypothesize that this could be due to some component, or mixture of components in the ramparts that is less absorbent at these specific wavelengths, or it could be an effect of different grain sizes. These observations provide first insights into the possible mechanisms leading to the formation of the raised ramparts that are discussed here.

Mots clés: Radiative transfer; Titan surface; Titan hydrology; Radar observations; Spectroscopy; Radiative transfer

Domaines: Astrophysique [astro-ph]; Planétologie et astrophysique de la terre [astro-ph.EP]

Anezina Solomonidou, Alice Le Gall, M. J. Malaska, S. P. D. Birch, R. M. C. Lopes, et al.. Spectral and emissivity analysis of the raised ramparts around Titan's northern lakes. Icarus, 2020, 344 (July), pp.113338. ⟨10.1016/j.icarus.2019.05.040⟩. ⟨insu-02157141⟩