Titan, the Enigmatic Moon

The European Southern Observatory’s Very Large Telescope has obtained new images of the surface of Titan, Saturn’s largest moon. A number of surface regions reflect light differently, and the new images map the varying reflectivity of these features. Of particular interest are several large "dark" areas of uniformly low reflectivity that may represent huge liquid hydrocarbonate oceans.

Simultaneous views of Titan’s atmosphere (top) and surface (bottom). Credit: ESO

To identify these dark features, the team gave them provisional names. Official names will be assigned later by the Working Group on Planetary System Nomenclature of the International Astronomical Union (IAU WGPSN). The temporary names include: the "lying H", the "dog chasing a ball", and the "dragon’s head".

Titan is the second largest moon of the solar system; only Jupiter’s Ganymede is larger. Like Earth, Titan’s substantial atmosphere is composed mainly of nitrogen. An opaque orange haze and clouds of complex organic molecules shield the solid surface from view. But spectroscopic and radar observations suggest there are huge surface reservoirs of liquid hydrocarbonates on Titan, as well as a methane-based meteorological cycle similar to Earth’s hydrological cycle. This makes Titan the only known solar system object with rainfall and potential surface oceans other than the Earth, and therefore a tantalizing possibility for pre-biotic chemistry and the origin of life.

The first views of Titan’s surface were obtained by the Hubble Space Telescope in the 1990s. 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. All of these observations were made through a single narrow-band filter.

The wavelengths used for such observations determine 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.

The previous observations were made in wavebands roughly matching atmospheric windows. While they allowed scientists to view surface features, they also included the light from different atmospheric layers. In a sense, it was as though they were viewing Titan’s surface through an opaque screen. Titan scientists therefore were like ancient sailors, catching a glimpse of an unknown continent through the coastal haze.

european southern observatory
The image features have been named informally from left to right, "lying H", dog chasing a ball, and dragonhead. Credit: ESO

The new images were made with the "Simultaneous Differential Imager (SDI)", a new optical device just installed at the NACO Adaptive Optics instrument. The high-contrast SDI camera can obtain sharp images in three colors simultaneously.

In addition to the dark areas that may be liquid hydrocarbonate oceans, the SDI observations mapped a single bright region in Titan’s southern hemisphere, centered at approximately 15° longitude. This is different from the "bright feature" seen in the Hubble images at longitude 80° – 130°, an area that was not covered during the recent observations. Presumably, bright, highly reflective regions are ice-covered highlands.

Although mainly conceived for exoplanet imaging, the SDI device also is useful for observing objects with thick atmospheres in the solar system, like Titan. By peering at the same time through a narrow, unobscured near-infrared spectral window in the dense methane atmosphere and an adjacent non-transparent waveband, the images are virtually uncontaminated by atmospheric components.

Because Titan is tidally-locked to Saturn, it always presents the same face towards the planet. To image all sides of Titan from the Earth, observations need to be made for the moon’s entire orbital period of 16 days. The recent observing campaign lasted for six nights (February 2, 3, 5, 6, 7 and 8, 2004), enabling the team to map approximately three-quarters of the surface of Titan.

Huygens landing probe to Saturn’s moon, Titan.
Image Credit: ESA


What’s Next

These new observations could help plan the delivery of the Huygens probe. Now approaching the Saturn system on the NASA/ESA Cassini spacecraft, the probe is scheduled to descend into Titan’s atmosphere in early 2005. The Huygens probe will make measurements of the physical and chemical conditions of the atmosphere, and scientists are hoping the probe will survive the entire descent and document Titan’s surface as well.

The team that made the recent observations include Markus Hartung (ESO-Chile), Laird M. Close (Steward Observatory, University of Arizona, Tucson), Rainer Lenzen, Tom M. Herbst and Wolfgang Brandner (Max-Planck Institut for Astronomie, Heidelberg, Germany), Eric Nielsen and Beth Biller (Steward Observatory, University of Arizona, Tucson), and Olivier Marco and Chris Lidman (ESO-Chile). Their work is described in detail in a research paper "First surface map of Titan at 1.575 microns" by M. Hartung et al., submitted to the European research journal Astronomy & Astrophysics.