Just over nine hundred million miles away, a pale sun rises over a great hydrocarbon lake. Clouds brush against the low mountain sides, perhaps even raining and feeding systems of rivers and channels. Winds blow across vast areas of dunes, constantly changing the look of the landscape. The surface of Titan has the potential to be, without a doubt, far more like Earth than any other body in the solar system. Titan is not just any ordinary moon; it could be described as being ‘planet-sized’, as it is larger than the planet Mercury. What makes it even more unique is the fact that it is the only moon to have a substantial atmosphere, mostly composed of nitrogen much like our own. This atmosphere is even thicker than Earth’s, with a surface pressure measured to be about 1.5bar.[1]
Titan was discovered in 1655 by the Dutch astronomer Christiaan Huygens, who was inspired by the discoveries of the Galilean moons by Galileo with his telescope forty years earlier.[2] In 1908, a Spanish astronomer named José Comas Solà observed limb darkening of the moon and believed it to provide evidence for an atmosphere.[3] In 1944, Gerard Kuiper measured methane absorption bands at 6190Å and 7250Å with the spectrograph at the McDonald Observatory, which further provided evidence of an atmosphere, composed partly of methane.[4] We did not have direct observational evidence of Titan’s atmosphere until the Pioneer 11 and Voyager missions flew by in the late 70s and early 80s as they explored the planet Saturn. The photographs they sent back to Earth showed that the atmosphere was optically thick and the surface of the moon could barely be seen.
After the great success of the Voyager missions in increasing our understanding of Saturn and its moons, work began on creating a pair of more sophisticated probes with instruments that would allow us to see through the obscuring haze of Titan’s atmosphere. A joint program between NASA and ESA, the Cassini-Huygens probe was completed and launched in 1997. Cassini was built to orbit around Saturn and was equipped with radar among other instruments. As Titan’s atmosphere is not opaque to radio waves, radar can be used to examine the topography of the surface. Cassini was also equipped with the Visible and Infrared Mapping Spectrometer (VIMS). Again, due to the low opacity of the atmosphere in certain small windows of the infrared region, we can use this instrument to get an idea of what compounds are present on the surface. The Huygens probe was built to be transported by Cassini and then dropped from orbit onto the planet’s surface with the hope that by dropping through the atmosphere, images and readings of the surface could be obtained.
Since taking measurements of Titan’s surface temperature and pressure during the flyby of Voyager 1, we have known that it is thermodynamically possible for the methane detected by Kuiper to be present as a liquid on the surface of the moon as well as other hydrocarbons such as ethane. It is only over the past five years, with the arrival of Cassini, that we have been able to determine to what extent liquid methane covers the surface of Titan. The first observations were made from Cassini in 2005 using its on-board radar around the moon’s equatorial regions. This initial data showed no direct observational evidence of any liquid hydrocarbon on the moon’s surface. Also, with the decent of the Huygens probe, no liquid bodies of water were imaged around the landing site, around 10° south of the equator. However, surface features which resembled drainage canals or rivers were found on both the radar imagery and the optical imagery from Huygens. In a later Cassini flyby in mid-2006, radar imaging of an area of latitude 70°-83° North found ‘more than 75 radar dark patches’[5] amid much brighter areas. These dark patches correspond to areas where very little of the emitted radio waves are reflected back to the probe. This is entirely consistent with what we would expect to see from a liquid such as methane. Furthermore, small dark lines were observed feeding into these lakes of liquid methane, which could be proposed to be the drainage channels of precipitated liquid into the liquid lakes. Clouds and thick haze have also been observed from Earth-bound observations, by the Cassini probe, and during the descent of Huygens. Despite there being no direct evidence for rain on the surface of Titan, the presence of these clouds certainly hint toward an element of precipitation involved in the methane cycle on the moon.
Of course, Titan is not the only body in the solar system to show evidence of drainage canals and other such fluvial processes. There are very obvious analogues with our hydrological processes here on Earth. However, we also see what appear to be various channels, dried river beds, and lake beds on Mars. The various probes and orbiters that have visited Mars over the past ten years have observed many terrain features which suggest that it had a much more fluvial past.[6]
Another very familiar feature that Cassini found while making radar observations of Titan was sand dunes. Around the equatorial regions of Titan, large amounts of radar dark material are observed. Theories before the arrival of Cassini suggested that it could be interpreted as a vast sea of liquid hydrocarbons, which naturally fuelled the excitement for the probe’s eventual arrival. The radar images from Cassini allowed us to see these dark areas in far more detail. Unlike the radar dark areas around the poles, which were smooth in their appearance, these areas had a much finer structure. Long straight stripes can clearly be observed in the radar imagery, which has been identified not as a sea of hydrocarbons, but rather a sea of sand. These longitudinal sand dunes are very similar to those seen in the Namib and Sahara Deserts here on Earth. In fact, despite having different air densities and gravity to Earth, the geometry of the sand dunes is the same and the dunes even exhibit ‘complex interaction with pre-existing topographic features’ as they mould around underlying hills.[7] From our knowledge of longitudinal dunes on Earth, we know that they typically arise due to fluctuating surface winds.[8] After analysis of the geometry and direction of the sand dunes in equatorial regions of Titan it can be seen that the prevailing wind is around 0.5ms-1 and in an Eastward direction, which likely varies due to tidal winds caused by Saturn as well as various passing pressure cells.[9] Data taken with VIMS on board Cassini shows that the sand itself has very strong similarities to ‘solid organic materials’, or at least that the sand is ‘coated with a layer of organics several microns thick.’[10]
One thing that scientists will be looking to understand more over the coming years is the question of where these sand particles come from. Meteor impact rates are not significant enough to produce the amount of sediment to fill the dune seas, especially with Titan having such a thick atmosphere. Some sand may come from fluvial processes but again, that would not be enough. It is possible that they may arise as products of organic photochemistry, where the energy of photons of light from the Sun goes into breaking down organic molecules in the moon’s hydrocarbon rich atmosphere.[11] Or perhaps there are some other processes which we are yet to discover. What is for certain is that by further study of the Aeolian and fluvial influences on Titan’s dunes, we can further understand the physical processes that go on in longitudinal sand dunes here on Earth.
In Titan’s atmosphere, methane is constantly being converted by UV photons from the Sun into other organic compounds, yet we still see large amounts of methane in the atmosphere. One of the great mysteries of Titan is the question of where all this methane comes from. There must be some sort of source providing methane to continually enrich the atmosphere. Many theories have been proposed but perhaps the most popular is that of Cryovolcanism. Cryovolcanism is a kind of volcanism which occurs on very cold worlds, such as those in the outer solar system, where volatiles such as water, ammonia and methane erupt instead of molten lava. We have seen evidence of such Cryovolcanism on both Triton[12] and Enceladus[13] and we are just beginning to see evidence for it on Titan too. In a press release made on 17th December 2010, the Cassini team announced that they have found what they believe to be a cryovolcano on the surface of Titan named Sotra Facula. With ‘finger-like’ flows and deep craters within its large mountainous sides, it certainly looks similar to volcanoes we would find here on Earth.[14]
Research on Titan is very much an active field with new and exciting discoveries and realisations coming on a regular basis. The success of the Cassini-Huygens mission has been fantastic for the field, allowing scientists to probe through Titan’s unforgiving atmosphere and uncover its many hidden secrets. The program has been so scientifically valuable that, despite recent cuts to the NASA budget, Cassini has received funding to continue flying for another seven years. During this time it will complete another 21 flybys past Titan, no doubt revealling even more about this fascinating moon.
As we learn more about Titan, we become more in awe of its similarity to our own home. With lakes, deserts, volcanoes and mountains; perhaps one day that far away moon will become a home away from home. Whatever the future holds for the exploration of Titan, I am certain that many more exciting discoveries lie ahead.
References
[1] Titan: Exploring an Earthlike World – Coustenis, Taylor – 2008 – Pg 130
[2] Christiaan Huygens: Discoverer of Titan – ESA News Release – 2003 …http://www.esa.int/esaSC/SEMJRT57ESD_index_0.html
[3] Titan from Cassini-Huygens – Brown, Lebreton, Waite – Published by Springer – 2009
[4] Titan: A Satellite with an Atmosphere – Kuiper – 1944 – The Astrophysical Journal, Vol 10, Pg 378
[5] The Lakes of Titan – Stofan E.R. – Nature Vol. 445 pg. 61-64 – 4th January 2007
[6] An Intense Terminal Epoch of Widespread Fluvial Activity on Early Mars – Howard A. et al. – Journal of Geophysical Research 110 – 2005
[7] RADAR IMAGING OF GIANT LONGITUDINAL DUNES : NAMIB DESERT (EARTH) AND THE BELET SAND SEA (TITAN) – 37th Lunar & Planetary Science Conference – 2006
[8] Dynamic processes acting on a longitudinal (seif) sand dune – Haim Tsoar – Sedimentology Vol. 30 Issue 4 pg. 567-578 – August 1983
[9] The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes – R.D. Lorenz et al. – Science Vol. 312, No. 5774 pg. 727-727 – 5th May 2006
[10] Cassini/VIMS Near Infrared Imaging and Spectroscopy Of Titan’s Sand Dunes – J.W. Barnes et al. – Bulletin of the American Astronomical Society, Vol. 39 pg. 501
[11] The Sand Seas of Titan: Discovery and Implications for Methane Climatology and Wind Patterns – R.D. Lorenz et al. – Planetary Dunes Workshop: A Record of Climate Change – 2007
[12] Cryovolcanism on Triton – Kargel J.S. et al. – Abstracts of the Lunar and Planetary Science Conference, Vol. 21, pg. 599 – 1990
[13] Enceladus: An Active Cryovolcanic Satellite – Spencer J.R. et al – Saturn from Cassini-Huygens – Published 2009
[14] Cassini Spots Potential Volcano on Saturn Moon – Brown D.C. et al – http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20101214/ – 17th December 2010
