March 28, 2018 : What Do We Know About Titan Today ?

Titan, the largest moon of the Gas Giant Saturn, was discovered in 1655 by the Dutch astronomer Christiaan Huygens. There are numerous moons in the Solar System and Titan appears to be the second largest moon in the Solar System. The largest moon of the Gas Giant Jupiter is a little bit bigger than Titan. Titan draws the whole attention of the general public because the giant natural satellite of the Ringed Planet is covered by a significant atmosphere. The astronomer Gerard Kuiper had identified the presence of methane in Titan's atmosphere as soon as 1944. We know today that the atmosphere of the Orange Moon is dominated by molecular nitrogen like the atmosphere of the Earth. However, oxygen is absent or quasi-absent in the atmosphere of the Opaque Moon. Methane, whose chemical formula is CH4, is the second most abundant gas in Titan's atmosphere. A complex haze of organics prevents us from discerning surface features in the visible spectrum from any spacecraft.

The atmosphere of Titan is clearly one of the most striking features of the Orange Moon. Titan is relatively small compared to Earth but the atmospheric pressure on Titan's surface is higher than that of the Earth at sea level. At first sight, Titan seems to have the right combination of gravity and environmental temperature to allow the presence of a significant atmosphere over the presumed icy surface. The Huygens lander, proposed by the European Space Agency, had recorded a surface temperature of minus 179 degrees Celsius or minus 290 degrees Fahrenheit and an atmospheric pressure of 1,467 hPa at the level of the ground on January 14, 2005. Researchers believe that the Titanian atmosphere looks like the atmosphere of the Early Earth. The atmosphere of the Opaque Moon is particularly deep, thick and dense. The gas blanket extends about 370 miles or 600 kilometers high which is significantly higher than the atmosphere of the Blue Planet.

Prior to the Voyager mission involving the Voyager 1 spacecraft and the Voyager 2 spacecraft, planetologists believed that Titan was the largest moon in the Solar System due to the fact that the Titanian atmosphere is remarkably deep and opaque. Where was the surface since we could not see it in the visible spectrum from outer space ? The Voyager mission allowed us to conclude that Titan is in fact smaller than Ganymede, the giant moon of Jupiter, which is devoid of any significant atmosphere and which has its own magnetic field as opposed to Titan which doesn't possess any significant magnetic field. Planetologists or meteorologists are particularly interested in the organic chemistry of Titan's atmosphere which undergoes the action of ultraviolet light from the Sun in particular. Elements like carbon, hydrogen and oxygen can combine or recombine to form various types of molecules, hydrocarbons or organics. Can this soup of elements, molecules or ions evolving in Titan's atmosphere engender a prebiotic chemistry ? Can this soup lead to life ?

Researchers are surprised to notice a significant fraction of methane present in the orange atmosphere of Saturn's largest moon because methane tends to be destroyed by UV light from the Sun over time. Is there a stable methane cycle on Titan ? Where do methane molecules come from on Titan ? Are there internal sources like subsurface reservoirs of methane or subsurface pockets of methane occasionally or regularly fueling the atmosphere ? Are there active cryovolcanoes spewing methane or hydrocarbons ? The gases from Titan's atmosphere can escape into space under the action of solar winds in particular. The Cassini orbiter had identified polar winds that push charged particles or molecules like methane and nitrogen along Saturn's magnetic field beyond Titan's atmosphere. That kind of process can be observed on Earth, our home which has its own magnetic field. The magnetosphere appears to be closely related to the internal structure of the planetary body and to the speed of rotation. It takes about 24 hours for the Earth to rotate whereas it takes about 16 Earth-days for Titan to rotate and to turn around Saturn.

The Titanian atmosphere appears relatively dynamic with seasonal phenomena clearly observed during the Cassini-Huygens mission. There is a methane cycle implying evaporation processes, condensation processes, cloud formation and precipitation processes like in the atmosphere of our planet. However, the hydrological cycle of Titan doesn't involve water. The hydrological cycle of the giant moon involves methane which can appear in its liquid form on Titan. Hydrocarbons like ethane and propane can also appear in their liquid form on the surface of Titan. Water on Titan can only appear in its solid form or in the form of ice and the soil of the Orange Moon may be rich in water. One can imagine that in the harsh environmental conditions of Titan, water must be as hard as rock on Earth. To a certain extent, Titan looks like the Earth with familiar landscapes. One can encounter rain on Titan and there are winds which can be relatively strong at low latitudes. Strong erosional processes or dissolution processes can be observed on Titan's surface from outer space.

Researchers had suspected the presence of lakes, seas or oceans of methane in the dark areas found at low or mid-latitudes. Thanks to images acquired from the Huygens probe on January 14, 2005 and thanks to radar data obtained from the Cassini orbiter during the Cassini mission, we know, now, that the low-albedo areas of the equatorial or tropical region are dominated by Seif dunes or linear and parallel dunes extending over long distances. One can compare the dunes of Titan to the dunes found on Earth. The dunes of Titan look like the Seif dunes found in the Namibian desert in Africa. However, the composition of the dunes may be particularly exotic since they may contain hydrocarbons or organics like benzene. Some molecules from Titan's haze may fall to the surface to form a red or dark sludge called tholin. The radar data captured from the Radar Mapper of the Cassini orbiter during the Cassini mission clearly show the action of prevailing winds which influence the orientation of the linear dunes that can be diverted by topographic obstacles like hills, mountains or ice chunks.

The aerial views acquired from the Huygens probe during the atmospheric descent had revealed bright hills or mountains made of dark drainage channels and a dark plain which looked like a sea. However, the images of the soil, taken after the landing phase, unveiled a field of pebbles and an orange sky. The eroded stones imply that the Huygens probe may have landed onto an ancient river. There may be rainfall events from time to time. Are there seasonal rainfall events, there, during the long Titanian year which almost represents 30 Terrestrial years ? Later, we realized that lakes, seas or rivers were mostly found at high latitudes on Titan. Ontario Lacus, which can be found in the south polar region, was the first body of liquids identified on Saturn's largest moon. Thanks to radar data, we have been in a position to determine that the most humid area on Titan is located in the north polar region of Titan where major bodies of liquids like Kraken Mare, Ligeia Mare and Punga Mare are found. Jason Hofgartner who is a planetary scientist at Cornell University argued in 2014 :  What I think is really special about Titan is that it has liquid methane and ethane lakes and seas, making it the only other world in the solar system that has stable liquids on its surfaces.  He added :  It not only has lakes and seas, but also rivers and even rain. It has what we call a hydrological cycle, and we can study it as an analog to Earth's hydrological cycle and it's the only other place we know of where we can do that.

The high latitudes of Titan's northern hemisphere unveil a remarkable network of lakes, seas and rivers which can be interconnected. Researchers are wondering whether there are internal sources to the liquids present in the pools. We had identified a giant ethane cloud engulfing the north polar region. Dynamic clouds have been seen, recently, in the area of the north polar lakes and seas as well. So, we know that there must be a methane cycle or an ethane cycle there. In 2014, researchers had identified a new bright feature which seemed to be an island in Ligeia Mare. The shape of that bright feature had rapidly evolved and dissipated. What was it ? Was it really a transient island ? Or was it something else like a field of nitrogen bubbles for instance ? Some planetologists have advanced that the Magic Island phenomenon may correspond to a kind of iceberg, to the outcome of a cryovolcanic activity or to an area of strong waves. Scott Edgington who is Cassini deputy project scientist and who works for NASA's Jet Propulsion Laboratory in Pasadena, California, pointed out :  Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water. 

The luminosity on the surface of Titan is relatively weak due to the low level of energy received from the Sun and due to the opaque atmosphere which tends to make the environment darker. However, there are dynamic processes on the surface of Titan. Researchers suspect the presence of an internal ocean of liquid water or of an internal ocean of liquid water and ammonia beneath the icy crust of the giant moon. Seasonal factors apparently play a key role in the environment of Titan. The atmosphere of Saturn's largest moon can help planetologists better understand or analyze the atmosphere of exoplanets because the atmosphere of Titan serves as a model of cloudy worlds. Tyler Robinson of NASA's Ames Research Center advanced :  It turns out that there's a lot you can learn from looking at a sunset.  Since the Saturn Orbit Insertion of June 30, 2004, we have been in a position to identify remarkable features on Titan such as relatively high mountains more than 10,000 feet high. The mission of the Cassini probe ended on September 15, 2017 with a crash against Saturn thanks to a targeted flyby of Titan.

During the mission, the Cassini probe performed 127 encounters with Titan. Earl Maize who is Cassini project manager at JPL pointed out :  Cassini has been in a long-term relationship with Titan, with a new rendezvous nearly every month for more than a decade.  Titan may become habitable, with seas or lakes of liquid water, when the Sun becomes a red giant in a distant future. Will the moon engender life one day ? Murthy Gudipati, a researcher at JPL, advanced :  Scientists previously thought that as we got closer to the surface of Titan, the moon's atmospheric chemistry was basically inert and dull.  He added :  Our experiment shows that's not true. The same kind of light that drives biological chemistry on Earth's surface could also drive chemistry on Titan, even though Titan receives far less light from the sun and is much colder. Titan is not a sleeping giant in the lower atmosphere, but at least half awake in its chemical activity.  Some planetologists have advanced that Titan may harbor a methane-based life or a water-based life in the presumed subsurface ocean of water. We'll have to go back to Titan !

The composite image above reveals a giant ethane cloud system engulfing the north polar region of Titan during the flyby of December 29, 2006. At the time of the observation, the northern hemisphere of Saturn's largest moon was experiencing the Winter season. The image was produced on the basis of data obtained with the Visual and Infrared Mapping Spectrometer of the Cassini spacecraft. A south polar vortex composed of hydrogen cyanide or HCN was also observed in 2012 during the Autumn season of the southern hemisphere. Image Credit: NASA/JPL/University of Arizona/LPGNantes.

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