Titan's thick atmosphere has long kept it a mystery to scientists, who could only guess at the nature of its planetary surface. The sixth moon of Saturn is the second largest moon in the Solar System, 50% larger than Earth's Moon1, but little was known about it until the Cassini mission entered Saturn's orbit in 2004. The Huygens probe (also known as Cassini-Huygens) finally landed on the moon's surface on 14 January, 2005. This node will look at Cassini's mission to explore Titan in some detail, and will then look at our current state of knowledge following its initial findings.


The Cassini Orbiter

Cassini-Huygens is a joint NASA/ESA/ASI (Italian Space Agency) mission. The 5.6 tonne spacecraft used the gravity of Venus, Earth, and Jupiter in a series of fly-bys following its launch in 15th October 1997, at Cape Canaveral. Cassini's objectives2 are to examine Saturn's rings, atmosphere, magnetosphere, and satellites, with particular emphasis on Iapetus and Titan. It therefore carries a barrage of scientific instruments, as well as the Huygens probe. One of the highlights of the mission, of course, was the successful launch and touchdown of the Huygens probe to the surface of Titan.


The Huygens Probe

The probe was named after Christiaan Huygens, Titan's discoverer, and left the orbiter on 25 December 2004, before descending to Titan's surface by parachute, and landing on 14 January 2005. During its descent, Huygens took many spectacular photographs, showing panoramic views of the moon as it descended. It also transmitted data continuously to Cassini by radio, which was in turn transmitted to Earth.

The Huygens photographs showed pale hills, probably of water ice, and dark "rivers" which appeared to flow to a dark plain. The probe landed on the plain which was covered in small rocks and pebbles made of water ice3.


The Surface of Titan

Cassini's first observations of Titan's surface showed a fairly smooth surface, with patches of bright and dark terrain3. Only three impact craters have been found, the largest being the 440-km wide Menrva impact basin. Other circular features appear to be filled in, perhaps by windblown sediment, raining hydrocarbons, or volcanoes. Volcanoes are also responsible for spewing water and ammonia into Titan's atmosphere.3.It has long been speculated that Titan might have seas or lakes of hydrocarbons such as ethane or methane, although some areas of dark terrain have now been found to be sand dunes4,5. We will now look at the evidence for mountains and seas or lakes on Titan.


Seas on Titan?

Cassini's radar has shown several areas of smooth reflections, thought to be lakes and seas of liquid ethane and/or methane. Evaporation from these lakes would help explain the high levels of methane in Titan's atmosphere. One sea covers an area of over 100,000 square kilometres (larger than Lake Superior). More examination is necessary to confirm the liquid nature of these lakes and seas.

Mountains

Cassini's radar has also shown the presence of mountains and mountain ranges, some ranges as long as 150 km, and up to 1500 km high6. Some mountains seem to have been formed by tectonic forces such as compression of Titan's brittle lithosphere. Others may have been ejected by meteorite impacts or volcanoes7, and still others may have been formed by tectonic plates pulling apart, allowing material to well up from below.

Atmosphere

Titan's thick atmosphere is the only developed atmosphere on any known moon in the Solar System, and is denser than that of the Earth. Its surface pressure is more than 1.5 times that of the Earth. The thick cloud layer prevents direct observation of the surface from Earth, and helps lower the moon's temperature by reflecting sunlight3.

Titan's atmosphere is 98.4% nitrogen, not unlike the Earth (78%), the remainder being mainly methane and other gases, including hydrocarbons.

Life

The chemical make-up of Titan's atmosphere, and the possibility of liquid seas, make it a possible habitat for life, either now or in the future. It has been aruged that conditions on Titan are similar to conditions on early Earth, and that chemical evolution on Titan might yet lead to life on the moon. The possibility of liquid water or ammonia would increase the chances for life. It is also possible that microbial life might have travelled to Titan following asteroid and comet impacts on Earth. Titan is therefore seen as one of two possible harbours for life in the Solar System, along with Europa.


References

  1. http://www.lpi.usra.edu/features/seas_on_saturn/
  2. http://en.wikipedia.org/wiki/Cassini-Huygens
  3. http://en.wikipedia.org/wiki/Titan_%28moon%29
  4. http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=10165
  5. http://www.esa.int/SPECIALS/Cassini-Huygens/SEMY3F9RRIF_2.html
  6. http://www.lpi.usra.edu/features/titan_mt/
  7. http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1007.pdf