Cassini spacecraft still delivers Titan moonscape view from Saturn
By Amanda Jermyn

Ever since the Cassini spacecraft first arrived at Saturn in July, 2004, it has been returning remarkable data and photos of the ringed planet and its moons. A joint venture of NASA and the European Space Agency, Cassini is managed by the Jet Propulsion Laboratory at Caltech. In October 2004, Cassini flew by Saturnís largest moon Titan for the first time, and in January, 2005, its Huygens probe landed on Titan. While descending, it gathered data for a few hours from Titanís atmosphere, and continued to transmit data for about 90 minutes after it landed. Meanwhile, Cassini itself continues on, revealing ever more fascinating details about Saturn and its moons.

Titan, it turns out, is a pretty interesting place. The second largest moon in our Solar System, it is roughly twice the size of Earthís moon, and larger than the smallest planet, Mercury. With its large size, dense atmosphere, sand dunes and liquid rivers and lakes, it seems more planet-like than lunar. The liquids in its rivers and lakes are ethane and methane (which freeze at much lower temperatures than water), and its atmosphere is mostly nitrogen, with ethane and methane clouds. Just as Earth has a water cycle Titan has a methane cycle, with methane rather than water falling as rain.

Ethane and methane are simple hydrocarbon molecules, but hydrocarbons can, in the presence of nitrogen, assemble themselves into complex structures such as amino acids that form the basis of what we call life. While this raises the tantalizing possibility of extraterrestrial life, one should remember that the surface of Titan is extremely cold. With a temperature of about minus 270 degrees Fahrenheit its surface water is all in the form of ice, and it is unknown whether any form of life could exist at such temperatures.

However, in June of this year data from the Cassini Missionís radio science experiment suggested that beneath Titanís organic rich atmosphere and icy crust lies an ocean of liquid water sitting above a layer of high-pressure ice that has below it a water-infused silicate core. The presence of a subsurface liquid ocean was deduced by the detection of large, solid ďtidesĒ about 30 feet high that bulge and compress the surface of Titan as it orbits Saturn. If the planet was comprised entirely of hard rock, the gravitational attraction of Saturn would cause bulges or tides of only 3 feet.

Because Titanís surface is mainly water ice, it is presumed that its subterranean ocean is mostly liquid water. While this is not in itself an indicator of the presence of life, it leaves open the intriguing possibility that life might exist within such an ocean. Researchers think life is more likely to arise when water is in contact with rock, and we canít yet be certain whether the ocean floor is made of rock or ice.

In other news from Titan, Cassini has recently captured images of a huge vortex forming at its south pole. This mass of swirling gas measures about 3,200 miles across, and is spinning at four times the speed of the rest of the moon. It is unclear how long this vortex has been forming because until recently, Cassini was orbiting too far north to have viewed the southern-most part of Titan. A large ďhood,Ē previously observed at Titanís north pole is believed to be the result of cell convection, a process in which dense air sinks towards the surface, pushing air at its edges up to form clouds. Scientists theorize that as the seasons change on Titan, the same mechanism may now be at work at the south pole, creating the recently observed vortex, as the southern winter approaches. Similar convection occurs above Earthís oceans, but this is occurring on Titan at a much higher altitude. As a single year on Titan lasts about 30 Earth years, it takes a long time to observe any changes in seasonal weather.

While the Cassini Mission is due to end with an impact on Saturn in 2017, other space probes to this region of the Solar System are in the works. One such proposal is the Titan Mare Explorer which would land in an ethane-methane sea near Titanís north pole, and float on its surface for three to six months. If it gets the go ahead it could launch as early as 2016 and arrive in 2023. In the meantime, who knows what fantastic images and insights Cassini might yet reveal?