CFHT, Current Image of the Week


September 18th, 2000

First Images of TITAN's surface at 1.3 microns

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First Images of TITAN's surface at 1.3 microns

Credit: Image courtesy of A. Coustenis
Observatoire de Paris-Meudon, Meudon, France

(for additional details see:

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.

Technical description:

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.

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editors: François Ménard, Jean-Charles Cuillandre & Catherine Dougados
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.