Exobiology in the Solar System & The Search for Life on Mars - ESA

SP-1231
Fig. I.5.2.2/2. Modelled Titan surface spectrum
(Coustenis et al., 1999, <strong>in</strong> preparation).
70
surface morphology, ra<strong>the</strong>r than atmospheric cloud structures.
<strong>The</strong> Titan surface spectrum, as modelled from <strong>the</strong> observations (Fig. I.5.2.2/2), is
remarkably consistent with a mostly solid surface dom<strong>in</strong>ated by water ice and it
<strong>the</strong>re<strong>for</strong>e resembles that of o<strong>the</strong>r satellites, such as Hyperion or Callisto (Coustenis et
al., 1995).
Spatially-resolved images of Titan, us<strong>in</strong>g <strong>the</strong> European Sou<strong>the</strong>rn Observatory
(ESO) 3.6 m. telescope and ADONIS optics, were obta<strong>in</strong>ed <strong>in</strong> October 1993,
September 1994, October 1995 and November 1996 (Coustenis et al., 1997). <strong>The</strong>
images were recorded through a set of narrowband filters (K1, H1, J1), def<strong>in</strong>ed <strong>in</strong>
order to maximise <strong>the</strong> contributions of <strong>the</strong> light reflected by Titan’s surface <strong>in</strong> <strong>the</strong> total
signal recorded at <strong>the</strong> centre of <strong>the</strong> 2.0, 1.6 and 1.28 µm transparency w<strong>in</strong>dows of
Titans atmosphere. At those three wavelengths, Titan’s atmosphere is transparent
because of very low molecular methane absorption. In order to allow <strong>the</strong> subtraction
of <strong>the</strong> stratospheric contribution and hence improve <strong>the</strong> visibility of ground features,
images were recorded <strong>in</strong> adjacent narrowband filters (K2, H2, J2) <strong>in</strong> <strong>the</strong> w<strong>in</strong>gs of <strong>the</strong>
strong methane bands.
<strong>The</strong> 2.2 µm ADONIS images show <strong>the</strong> expected north-south asymmetry <strong>in</strong> <strong>the</strong>
stratosphere, <strong>the</strong> South Pole brighter than <strong>the</strong> North Pole, reversed with respect to
Voyager and due to seasonal effects. After deconvolution, correction of centre-limb
effects and subtraction of <strong>the</strong> atmospheric component, <strong>the</strong> 2.0 µm surface images
show a bright well-def<strong>in</strong>ed equatorial spot at around 120º LCM (lead<strong>in</strong>g hemisphere)
extend<strong>in</strong>g over 60º <strong>in</strong> longitude and 30 º <strong>in</strong> latitude, and some smaller and less
contrasted features near <strong>the</strong> poles, all rotat<strong>in</strong>g over six consecutive nights at <strong>the</strong>
expected rotation rate of Titan’s solid body. <strong>The</strong> area attributed to ground features is
<strong>in</strong> agreement with HST images (Smith et al., 1996) taken near 1 µm, both as to <strong>the</strong>
location and <strong>the</strong> shape. <strong>The</strong> spatial resolution is at <strong>the</strong> limit of diffraction (0.13 arcsec
at 2.0 µm), very similar to <strong>the</strong> HST images. Moreover, <strong>the</strong> ADONIS images of Titan’s
trail<strong>in</strong>g hemisphere show high-latitude bright zones with a north-south asymmetry;
<strong>the</strong> nor<strong>the</strong>rn latitudes appear brighter by about a factor two with respect to <strong>the</strong> south
(Fig. I.5.2.2/3). <strong>The</strong>se bright zones might exist over Titan’s entire disc.
<strong>The</strong> entire surface of Titan appears quite dark <strong>in</strong> <strong>the</strong> water ice bands, <strong>in</strong> agreement
with spectroscopic measurements by Coustenis et al., 1997. <strong>The</strong> model of a global
ocean cover<strong>in</strong>g <strong>the</strong> surface must be ruled out. It is <strong>in</strong>ferred that large cloud structures