Titan, Saturn's largest moon, attracted astronomers interest because
of its thick atmosphere dominated by molecular nitrogen. A complex
photochemistry is at play in the stratosphere of Titan producing
various organic compounds which make it fully opaque in the
visible. Even the close fly-by by Voyager1 could not penetrate this
thick veil. Titan's surface was expected to be covered, at least
partially, by liquid hydrocarbons in form of lakes or even a global
ocean. A very different picture emerged from recent observations in
the near-infrared range.
We show here the first images of Titan's surface between 1.3 and
1.6 microns obtained with the adaptive optics system PUEO
at CFHT. Narrow band images were obtained first at wavelengths
sensitive mostly to the emission from Titan's atmosphere (1.18 microns
J2, 1.64 microns H2), then in bands where both atmospheric and surface
emissions contribute (1.29 microns J1, 1.6 microns H1). The images
obtained with PUEO are of excellent quality with contrasts of 35-45 %
allowing a detailed analysis of Titan's disk. The J2 and H2 images
allow to subtract properly the atmospheric contribution at 1.3 and 1.6 microns from the J1 and H1
maps. The resulting difference images (presented here) recover pure
emission from the surface of the satellite at these wavelengths.
Both images display remarkable similar morphology. They show in
particular an extended bright region located close to the Equator,
previously identified at longer wavelengths. The high resolution of
the PUEO images reveal at least three enhancements within this region,
peaks towering over a mountainous plateau ? A preliminary analysis
shows that only methane ice seems to reproduce the reflective
properties of the bright region observed between 1 and 2 microns. The
presence of such a compound in equatorial regions could be related to
the presence of a high mountain range.
The images were obtained with PUEO at the CFHT in 1997 and 1998, the
latter during excellent seeing conditions (with peaks at 0.3 arcsec in the
visible). The narrow-band filters used are centered at: 1.29 (J1), 1.6 (H1),
1.18 (J2) and 1.64 (H2) microns with widths of 0.1-0.15 microns. Integration times were 100 and 200 sec for a set of 10 images in J1 and J2 respectively and about twice as much for H1 and H2. Thanks to the high contrast of the images,
sophisticated deconvolution processes could be applied, allowing to recover
about 10 individual resolution elements on Titan's disk.
Technical description: