Titan: Earth's sister world?

If Saturn's largest moon Titan were a planet, it would likely stand out as the most important planet in the Solar System for humans to explore. Titan—fifty percent bigger in diameter than Earth's Moon, and 80% more massive—has a dense atmosphere of nitrogen and methane and a surface covered with organic material. It is the only place in the Solar System other than Earth where liquid is known to exist on the surface in large quantities. Titan therefore is arguably Earth's sister world and the Cassini-Huygens mission considers the study of Titan among its highest priorities.

Although it is far colder and lacks liquid water, the chemical composition of Titan's atmosphere resembles that of early Earth. This, along with the organic chemistry that takes place in Titan's atmosphere, prompts scientists to believe that Titan could provide a laboratory for seeking insight into the origins of life on Earth.

While the diameter of Titan is only 40 percent of Earth's, it's atmosphere extends 10 times further into space than Earth's. Earth's atmosphere reaches a mere 60 kilometres from its surface, while Titan's atmosphere reaches nearly 600 kilometres into space.

The atmospheres of Titan and Earth are both primarily made of nitrogen. Titan's atmosphere is 95 percent nitrogen, while Earth's is 78 percent nitrogen. The next most abundant gas in Earth's atmosphere is oxygen (21 percent). Scientists believe that Titan's atmosphere might be similar to conditions on primordial Earth—that is, what Earth was like before life emerged. Titan might hold clues to the early atmosphere on Earth and studying it could uncover some of the mysteries of how Earth's atmosphere has evolved throughout the ages.

Data from the Huygens probe, which touched down on Titan's surface in January 2005, and the Cassini orbiter has shown that many of the processes that occur on Earth also apparently take place on Titan—wind, rain, volcanism, tectonic activity, as well as river channels, and drainage patterns all seem to contribute in shaping Titan's surface.

However, at an inhospitable -179 degrees Celsius, the chemistry that drives these processes is fundamentally different from Earth's. For example it is methane that performs many of the same functions on Titan that water does on Earth.

The Huygens probe landed near a bright region now called Adiri. During it's descent through Titan's thick atmosphere, it photographed light hills with dark riverbeds that empty into a dark plain. It was believed that this dark plain could be a lake or at least a muddy material, but it is now known that Huygens landed in the dark region, and it is solid. The image at left was taken by Huygens when it reached the surface. Scientists believe it only rains occasionally on Titan, but the rains are extremely fierce when they come.

Only a small number of impact craters have been discovered. This suggests that Titan's surface is constantly being resurfaced by a fluid mixture of water and possibly ammonia, believed to be expelled from volcanoes and hot springs. Some surface features appear to be volcanic structures. Volcanism is now believed to be a significant source of methane in Titan's atmosphere. However, there are no oceans of hydrocarbons as previously hypothesised. Dunes cover large areas of the surface.

The existence of oceans or lakes of liquid methane on Titan was predicted more than 20 years ago. Radar and imaging data from Cassini's Titan flybys have provided convincing evidence for large bodies of liquid. With Titan's colder temperatures and hydrocarbon-rich atmosphere, these lakes and seas most likely contain a combination of liquid methane and ethane (both hydrocarbons), not water.

The Cassini-Huygens mission, using wavelengths ranging from ultraviolet to radio, is methodically and consistently revealing Titan's secrets and answering long-held questions regarding the moon's interior, surface, atmosphere, and the complex interaction with Saturn's magnetic field. While many pieces of the puzzle are yet to be found, with each Titan flyby comes a new data set that furthers our understanding of this world as scientists attempt to pin down scenarios for the formation and evolution of Titan and its atmosphere.

Titan facts:

Adapted from information issued by NASA.

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