Titan in 2005 photographed by the Cassini spacecraft.
Credit: Wikimedia
SYDNEY: Saturn's moon Titan is more Earth-like than thought, with two distinct layers in the lower atmosphere that affect the moon's wind patterns, dune spacing and cloud formation.
In a new study published in Nature Geoscience today, researchers used a three-dimensional climate model of Titan's atmosphere together with an analysis of dune locations and measurements from the Voyager 1 spacecraft, the Cassini orbiter and the Huygens probe to discover how the moon's lowermost atmosphere interacts with its surface.
Titan is located nine times farther away from the Sun than the Earth is, so its solar insolation - the solar radiation energy received on the surface that determines the variation in temperature from night to day - is quite weak. Despite this, in terms of its daily cycle, this strange moon is closer to an Earth-like world than previously anticipated.
"We were surprised to find that the daily cycle on Titan is really efficient and observable. The solar insolation on the surface of Titan is 1,000 times weaker than on Earth, so it was not believed [to] have any effect on Titan's dense atmosphere," said lead author Benjamin Charnay from the French National Centre for Scientific Research (CNRS) in Paris.
Titan's mysterious boundary layer
With a diameter approximately 50% larger than the Earth's Moon, Titan is the second largest moon in the Solar System. One Titan year is equivalent to 29.5 Earth years; it is the only moon known to have a dense atmosphere; and like the Earth's atmosphere, is dominated by nitrogen.
In 1981, the Voyager 1 spacecraft made the first detailed observations of Titan's atmosphere. In 2004, the Cassini orbiter arrived at Saturn, followed by the Huygens probe, which landed on Titan a year later. But even after information was gleaned from all three, it has proven difficult for scientists to consolidate the data into a single, coherent picture of Titan's climate.
Charnay and colleagues were interested in the boundary layer of Titan's atmosphere, which is the layer that is influenced by the surface below. In this layer, winds are slowed down due to surface friction, and as the Sun heats the ground, turbulent air motions are created.
The boundary layer on Earth rises during the day because of the heating of the surface by the Sun, generally up to an height between 500 m and 3 km. "This layer is very important for climate and weather. Most of the clouds (cumulus, stratus and fog) are ... produced by the boundary layer," said Charnay.
Before this study, the boundary layer of Titan was a mystery because the moon's thick, opaque atmosphere makes it difficult to see what's happening in the lower parts. By using a computer simulation to judge the interactions between the atmosphere and Titan's surface, the researchers were able to fill in the blanks to reveal an atmosphere that is strongly structured, with two distinct layers that affect wind patterns, dune spacing and cloud formation.
Unique atmosphere for Titan
"The most interesting point is that their model shows the presence of two different boundaries, the lower one caused by the daily heating and cooling of the surface - and varying in height during the day - and the higher one caused by the seasonal change in global air circulation," commented Paulo Penteado from the Institute of Astronomy, Geophysics and Atmospheric Science at the University of São Paulo in Brazil.
According to Charnay, this link between the lower atmosphere's layers and the moon's daily and seasonal cycle has never been seen on another moon or planet besides the Earth. "This unprecedented organisation of the boundary layer has several consequences. It controls the atmospheric circulation and wind patterns in the lower atmosphere; it controls the size and spacing of dunes on Titan; it could imply the formation of boundary layer clouds (of methane on Titan). Such clouds seem to have been observed but not explained," he said.
Penteado added that using models for this kind of analysis can yield incorrect conclusions because of inadequate constraints or approximations, so these results will need to be tested against other independent models. He also said that because Charnay's model also lacks the hydrological cycle (of methane clouds, rain, lakes and evaporation), further work is needed to explore the effects of methane.
Could Titan support life?
The temperature on Titan is around -180°C, which makes it impossible to have liquid water on the surface to support life. "But Titan remains one of the most interesting objects in the Solar System concerning the question of the habitability," he said.
Numerous chemical reactions are occurring in Titan's atmosphere, which leads to the formation of many organic molecules and perhaps amino acids - the building bricks of life. "Similar reactions could have happened in the atmosphere of the early Earth leading to life. Titan is often seen as a frozen early Earth."
