Recent Knowledge: Surface of Titan
Titan's surface seen by Cassini VIMS (image credit: NASA/JPL/University of Arizona)
Knowledge prior to Cassini/Huygens
For many years Titan has been the most mysterious moon in the Solar System because the surface was obscured by the stratospheric haze and strongly absorbing methane. In planetary atlases there was no page for Titan, whereas detailed maps were available for numerous small moons. In the 1980s several speculations arose concerning the properties of Titan’s surface. The most prominent idea was that the whole surface could be covered by a global ocean of liquid hydrocarbons as deep as a kilometre. This hypothesis was based on implications of models of atmospheric chemistry according to which, among others, ethane and acetylene are produced from nitrogen and methane and deposited as liquids on the surface throughout billions of years. In the 1990s the surface could be recognized for the first time by the Hubble Space Telescope in the near-infrared spectrum. The images revealed a rather heterogeneous distribution of surface brightness. Therefore, a global surface ocean became questionable. Later observations by different ground-based telescopes confirmed this trend. Consequently, it was generally assumed that the brightness contrast may simply reflect a continent-ocean distribution like on Earth.
By analogy with similar moons of the outer Solar System, an icy crust was considered as the most likely component of the surface. However, the entirety of spectral and other physical characteristics of the surface derived from various astronomical observations was not compatible with any known surface of a planet or moon in the Solar System. Hence Titan remained mysterious until the arrival of the Cassini/Huygens mission.
Discovery by Cassini/Huygens
The mystery of Titan was eventually unveiled by the Cassini/Huygens mission since 2004. Huygens made impressive images during the descent phase: an Earth-like landscape with hills, rivers and river mouths. The rivers were most likely caused by flowing liquid methane, perhaps after rainfall, although the rivers seem dried out. Later on similar dendritic systems were discovered by Cassini at many places across Titan and some of them may actually contain liquids. Many dry rivers are found in areas where no clouds have so far been observed. It remains to be determined under which circumstances heavy rainfall could develop in largely dry areas.
Vicinity of the landing site of Huygens (image credit: ESA/University of Arizona, Tomasko et al., 2005)
Landing site of Huygens (image credit: ESA/University of Arizona, Tomasko et al., 2005)
The diverse landscape of Titan definitely rules out a global hydrocarbon ocean on the surface. An Earth-like continent-ocean mixture was also not found. Instead numerous hydrocarbon seas/lakes were detected in the polar regions. Most lakes are located near the north pole. The three largest seas are referred to as Maria (plural of Mare), all other lakes are called Lacus. Liquid ethane was unambiguously detected in the spectra. Titan’s seas/lakes thus constitute the only known large standing bodies of liquids outside the Earth. Since the beginning of the Cassini mission some evidence for an increase of the lake level has been found, but also evidence for a retreat of some lakes, possibly due to precipitation and evaporation. The polar liquid lakes may play the role of polar caps, while polar caps of frozen material have not been found.
Radar image of polar lakes (image credit: NASA/JPL, Stofan et al., 2007)
The discovery of extensive dune fields in the tropics and subtropics was one of the surprises of the Cassini mission. Titan’s parallel dunes extend in west-east direction, have heights of ~100 m and are aligned with a dune spacing of a few kilometres. They cover about one fifth of the globe, i.e. a larger portion than the dunes on Earth. The material of the particles of the dune is unknown. The alignment and appearance of the dunes provide valuable information on the winds that shaped these dunes.
Radar image of equatorial dune fields (image credit: NASA/JPL, Lorenz et al., 2006)
Titan does not have topography as pronounced as the Earth. Large-scale topography amounts to merely a few hundred metres. However, mountainous areas contain mountains as high as 2000 m. The polar flattening is small because of the slow rotation of Titan. On the other hand, Titan has two equatorial bulges of similar size caused by the tidal force of Saturn.
Even recent spectral measurements of the surface by Cassini and Huygens can still not reveal the exact composition of Titan’s surface. The spectra are most consistent with ice, but several unknown impurities are clearly present. Huygens found after the landing evidence of liquid methane evaporated from the surface. Although the surface itself is a hard crust, viscous material that could also contain methane is perhaps present in the soil.