New images of unsurpassed clarity have been obtained with the ESO Very Large Telescope (VLT) of formations on the surface of Titan, the largest moon in the Saturnian system. They were made by an international research team during recent commissioning observations with the "Simultaneous Differential Imager (SDI)", a novel optical device, just installed at the NACO Adaptive Optics instrument.

The clearest view of Titan's surface, available so far. It was obtained through a "transparent", narrow spectral window with the 8.2-m VLT YEPUN telescope and the NACO adaptive optics instrument operated in the Simultaneous Differential Imager (SDI) mode . It covers about three-quarters of the full surface and has an image resolution (sharpness) of 0.06 arcsec, corresponding to 360 km on the surface. Credit: ESO

With the high-contrast SDI camera, it is possible to obtain extremely sharp images in three colours simultaneously. Although mainly conceived for exoplanet imaging, this device is also very useful for observations of objects with thick atmospheres in the solar system like Titan. Peering at the same time through a narrow, unobscured near-infrared spectral window in the dense methane atmosphere and an adjacent non-transparent waveband, images were obtained that are virtually uncontaminated by atmospheric components. They map the reflectivity of a large number of surface features in unprecedented detail.

The images show a number of surface regions with very different reflectivity. Of particular interest are several large "dark" areas of uniformly low reflectivity. One possible interpretation is that they represent huge surface reservoirs of liquid hydrocarbons.

Whatever the case, these new observations will be most useful for the planning of the delivery of the Huygens probe - now approaching the Saturn system on the NASA/ESA Cassini spacecraft and scheduled for descent to Titan's surface in early 2005.

Views of Titan - the quest for the surface

Titan, the largest Saturnian moon and the second largest moon of the solar system (only Jupiter's Ganymede is slightly larger), is the only satellite known with a substantial atmosphere. It is composed mainly of nitrogen (like that of the Earth) and also contains significant amounts of methane. Opaque orange hazes and clouds of complex organic molecules effectively shield the solid surface from view, cf. e.g. the Voyager images.

Simultaneous views of Titan's surface and atmosphere. Credit: ESO

Recent spectroscopic and radar observations suggest that there are huge surface reservoirs of liquid hydrocarbons and a methane-based meteorological cycle similar to Earth's hydrological cycle. This makes Titan the only known object with rainfall and potential surface oceans other than the Earth and thus a tantalizing research object for the study of pre-biotic chemistry and the origin of life on Earth.

The Huygens probe launched from the NASA/ESA Cassini-Huygens mission will enter Titan's atmosphere in early 2005 to make measurements of the physical and chemical conditions, hopefully surviving the descent to document the surface as well.

Coordinated ground-based observations will provide essential support for the scientific return of the Cassini-Huygens encounter. However, only 8-10 m class telescopes with adaptive optics imaging systems or space-borne instruments can achieve sufficient image sharpness to attain a useful level of detail.

The new map of a large part of Titan's surface, shown above, represents an important contribution in this direction.

A question of atmospheric windows

The first intriguing views of Titan's surface were obtained by the Hubble Space Telescope (HST) in the 1990's. From the ground, images were obtained in 2001-2 with the Keck II and Gemini North telescopes and more recently with the ESO Very Large Telescope (VLT). All of these observations were made through a single narrow-band filter at a time.

The wavelengths used for such observations are critical for the amount of surface detail captured on the images. Optimally, one would look for a spectral band in which the atmosphere is completely transparent; a number of such "windows" are known to exist. But although the above observations were made in wavebands roughly matching atmospheric windows and do show surface features, they also include the light from different atmospheric layers. In a sense, they therefore correspond to viewing Titan's surface through a somewhat opaque screen or, more poetically, the sight by an ancient sailor, catching for the first time a glimpse of an unknown continent through the coastal haze.

One narrow "window" is available in the near-infrared spectral region near wavelength 1.575 nm. In February 2004, an international research team working at the ESO VLT at the Paranal Observatory (Chile) obtained images of Titan's surface through this spectral window with unprecedented spatial resolution and with the lowest contamination of atmospheric condensates to date.

They accomplished this during six nights (February 2, 3, 5, 6, 7 and 8, 2004) at the time of the commissioning phase of a novel high-contrast imaging mode for the NACO adaptive optics instrument on the 8.2-m VLT YEPUN telescope, using the Simultaneous Differential Imager (SDI) [2]. This novel optical device provides four simultaneous high-resolution images at three wavelengths around a near-infrared atmospheric methane absorption feature.

The main application of the SDI is high-contrast imaging for the search for substellar companions with methane in their atmosphere, e.g. brown dwarfs and giant exoplanets, near other stars. However, as the present photos demonstrate, it is also superbly suited for Titan imaging.

Mapping Titan's surface in unprecedented detail

Titan is tidally-locked to Saturn, and hence always presents the same face towards the planet. To image all sides of Titan (from the Earth) therefore requires observations during almost one entire orbital period, 16 days. Still, the present week-long observing campaign enabled the team to map approximately three-quarters of the surface of Titan.

Six nightly views of Titan's surface. Credit: ESO

A new map of the surface of Titan (in cylindrical projection and covering most, but not all of the area imaged during these observations) is shown in top image. For this, the simultaneous "atmospheric" images (at waveband 1.625 nm) were "subtracted" from the "surface" images (1.575 and 1.600 nm) in order to remove any residual atmospheric features present in the latter. The ability to subtract simultaneous images is unique to the SDI camera.

This truly unique map shows the fraction of sunlight reflected from the surface - bright areas reflect more light than the darker ones. The amount of reflection (in astronomical terms: the "albedo") depends on the composition and structure of the surface layer and it is not possible with this single-wavelength ("monochromatic") map alone to elucidate the true nature of those features.

Nevertheless, recent radar observations with the Arecibo antenna have provided evidence for liquid surfaces on Titan, and the low-reflection areas could indicate the locations of those suspected reservoirs of liquid hydrocarbons. They also provide a possible source for the replenishment of methane that is continuously lost in the atmosphere because of decomposition by the sunlight.

Presumably, the bright, highly reflective regions are ice-covered highlands.