Transparency of 2 mu m window of Titan's atmosphere

Thèmes de recherche:
Année:
2018
Auteurs:
Journal:
PLANETARY AND SPACE SCIENCE
Volume:
151
Pages:
109-124
Mois:
Février
ISSN:
0032-0633
Résumé:
Titan's atmosphere is optically thick and hides the surface and the
lower layers from the view at almost all wavelengths. However, because
gaseous absorptions are spectrally selective, some narrow spectral
intervals are relatively transparent and allow to probe the surface. To
use these intervals (called windows) a good knowledge of atmospheric
absorption is necessary. Once gas spectroscopic linelists are well
established, the absorption inside windows depends on the way the far
wings of the methane absorption lines are cut-off. We know that the
intensity in all the windows can be explained with the same cut-off
parameters, except for the window at 2 mu m. This discrepancy is
generally treated with a workaround which consists in using a different
cut-off description for this specific window. This window is relatively
transparent and surface may have specific spectral signatures that could
be detected. Thus, a good knowledge of atmosphere opacities is essential
and our scope is to better understand what causes the difference between
the 2 mu m window and the other windows. In this work, we used scattered
light at the limb and transmissions in occultation observed with VIMS
(Visual Infrared Mapping Spectrometer) onboard Cassini, around the 2 mu
m window. Data shows an absorption feature that participates to the
shape of this window. Our atmospheric model fits well the VIMS data at 2
mu m with the same cut-off than for the other windows, provided an
additional absorption is introduced in the middle of the window around
similar or equal to 2.065 mu m. It explains well the discrepency between
the cut-off used at 2 mu m, and we show that a gas with a fairly
constant mixing ratio, possibly ethane, may be the cause of this
absorption. Finally, we studied the impact of this absorption on the
retrieval of the surface reflectivity and found that it is significant.