Larger than the planets Mercury or Pluto, Titan is the third largest moon in our solar system with a radius of 2,575 kilometers, which lies at a mean distance of 1,221,850 km from its parent the ringed planet Saturn. This is a rather peculiar moon that has a dense fogy atmosphere hiding its surface. Titan takes 15 days 22 hours 41 minutes to orbit Saturn, and we know that its average density is 1.88 grams per cubic centimeter.
The Voyager space probes visited Saturn in the mid 1980s and made many observations of Titan. Many of the images returned only show Titan's yellow-orange cloud tops, and little in the way of atmospheric features. However, Voyager-I did observe that there were distinct differences in brightness between Titan's northern and southern hemispheres, and polar hoods, that were thought to be seasonal variations.
Voyager's solar and Earth occultation data, acquired as the spacecraft passed through Titan's shadow, revealed that the dominant atmospheric constituent is nitrogen. Methane, the atmospheric gas detected from Earth, represents several per cent of the composition of the atmosphere. There are also hydrocarbons of Acetylene 2.2 parts per million, Ethylene 0.1 parts per million, Ethane 1.3 parts per million, and Propane 0.7 parts per million. Nitrites of Hydrogen cyanide 160 parts per billion, and Cyanoacetylene 1.5 parts per billion were also confirmed.
Titan's surface pressure, one and a half bars, is 50 per cent greater than Earth's in spite of Titan's smaller size. The surface temperature was found to be -180 degrees centigrade, indicating that there is little greenhouse warming. The temperature profile in Titan's atmosphere has a shape similar to that of Earth: warmer at the surface, cooling with increasing altitude up to the tropopause at 42 kilometres (-203 degrees centigrade), then increasing again in the stratosphere.
The Cassini Huygens space probe arrived at Saturn on 1 July 2004 and successfully entered orbit around the planet and promptly send back 'post cards' showing the rings, and Saturn's atmosphere in amazing detail. However, we had to wait until 26 October for Cassini's first real close approach of Titan when the probe passed within 1,200 Km of the moon returning the first close up photographs, while about 1% of the moon's surface were also mapped by radar. The infrared images reveal breaks in the cloud cover and some surface features, while radar determined lava like flows of a cryogenic cold substance. Scientists were excited by the findings since they determined Titan was still geologically active with Volcanic caldera and cryogenic cold volcanoes showing some similarity to the surface of the planet Venus. A person standing on Titan's surface in the daytime would experience a level of daylight of a town or city during twilight with an overcast sky.
Cassini Encounters With Saturns Moons
Dates for your diary --- Best flyby dates over the Next 12 months.
Orbit -B Titan .......... 13 Dec 2004 ...... (2,358 km)
Orbit -C Iapetus ..... 01 Jan 2005 ...... (65,000 Km)
Orbit 04 Enceladus .. 09 Mar 2005 ....... (500 Km)
Orbit 12 Mimas ....... 02 Aug 2005 ....... (45,100 Km)
Orbit 15 Tethys ....... 24 Sept 2005 ..... (33,000 Km)
Orbit 15 Hyperion .... 26 Sept 2005 ..... (990 Km)
Orbit 16 Dione ........ 11 Oct 2005 ....... (500 Km)
Orbit 18 Rhea ......... 26 Nov 2005 ..... (500 Km)
Cassini will next fly by Titan on 13 December at a distance of 2,358 kilometres that will be the last close encounter of 2004, when the Huygens landing site will be confirmed prior to the probes release from its mother-ship on Christmas Eve. The tiny Huygen's craft is expected to enter Titan's atmosphere and begin its decent to the surface on 14 January 2005, coinciding with Cassini's close fly by of the moon at a distance of only 60,000 km.
Much of Huygen's science will take place during its atmospheric decent, which will be relayed to the Cassini craft, then transmitted back to earth's waiting scientists and the media, and If the Huygen's probe lands successfully on Titan it will be a major bonus for the mission.
The Huygen's Lander will be attempting to determine the origin of Titan's molecular nitrogen atmosphere. Planetary scientists want to answer the question: "Is Titan's atmosphere primordial (accumulated as Titan formed) or was it originally accreted as ammonia, which subsequently broke down to form nitrogen and hydrogen?"
If nitrogen from the solar nebula (out of which our solar system formed) was the source of nitrogen on Titan, then the ratio of argon to nitrogen in the solar nebula should be preserved on Saturn's largest moon. Such a finding would mean that we have truly found a sample of the "original" planetary atmospheres of our solar system
The Huygen's probe will also try to detect Lightning on Titan. The extensive atmosphere of Titan may host Earth-like electrical storms and lightning. Although no evidence of lightning on Titan has been observed so far, the Cassini Huygens mission provides the opportunity to determine whether such lightning exists. In addition to the visual search for lightning, the study of plasma waves in the vicinity of Titan may offer another method. Lightning discharges a broad band of electromagnetic emission, part of which can propagate along magnetic field lines as whistler-mode emission.
So exciting times lie ahead
Acknowledgements: I would like to thank NASA/JPL for providing updated physic-chemical data for this news item and for the use of the latest images.
Science Correspondant YAHOO Instant Messege ID: richard_pearson2