The Titanian Paradox ( Pre-Cassini / Huygens Approach )
How to explain that Titan harbors an atmosphere? It seems paradoxical when we make a comparison between Ganymede, Mars and Titan.Indeed, Titan is a sphere smaller than Ganymede and Mars and yet, it is covered with a thick, opaque and deep orange atmosphere, by far denser than the Mars's atmosphere.Furthermore, Ganymede doesn't retain any atmosphere.On the basis of these observations, one can postulate that the distance of the body to the Sun and its surface temperature may play a key role in the development of an atmosphere.
The astrophysicist Sir James Jeans developed a theory of gases as soon as 1904 that explained very well this paradox: his dynamical (or kinetic) theory of gases showed the mechanisms with which the mass of the celestial body and its surface temperature acted on the gases.The first things one have to know are:"The hotter the gas, the faster the molecules go and the gas molecules are pulled towards the planet by gravity".If any of the molecules near the top of the atmosphere gather enough speed to exceed the escape velocity, they can disappear into space. To summarize the theory, a body has the best chance of hanging on to its atmosphere if it is massive, if the gas molecules are themselves relatively heavy and if it is cold. In a colder environment, the molecules generally move slowly and don't reach escape velocity.As James Jean's calculations showed, hydrogen and helium are too light to be retained by Titan.These kind of gases are very abundant in the upper atmosphere of more massive planets like the gas giants, Jupiter, Saturn, Uranus and Neptune.
Ganymede which is a little wider than Titan ( 5260 km in diameter compared to 5150 km for Titan) would certainly be covered with a similar atmosphere if its environmental temperature was 30 degrees lower: according to the Stefan's law, the theoritical temperature at the surface of a dark body as far from the Sun as Ganymede is around -150°c ( 122.5K or -238°F).At the same distance as Titan from the Sun, this theoritical temperature reaches -182°c that is 90.5Kor -296°F.
Titan benefits from the conjunction of several other favorable factors: notably, its rotation speed is lower than that of Ganymede and the meteors that crash into the surface have a lower speed than the meteors that crash into Ganymede. This last idea comes from Kevin Zahnle who explained that impacts at high speeds could blow off part of an atmosphere while impacts at lower speeds were mor likely to add material to an atmosphere than blow it away into space.One knows that comets gather speed as they approach the inner solar system. In other words, any comet will hit Titan more slowly than Ganymede or Mars which are closer to the Sun.
The previous theoritical explainations must not hide that we know very little about Titan's atmosphere. The composition of the upper atmosphere is roughly known: Voyager 1, in 1980, detected a large amount of Nitrogen in the atmosphere, probably in a proportion between 85% and 95%. The second major gas seems to be methane with a proportion up to 5% of Titan's atmosphere. Argon may also be present in a relatively high proportion, up to 6%.The atmospheric pressure at the surface level is around 1.5 bars that is 1.5 times the atmospheric pressure on Earth at the sea level. Are there clouds of hydrocarbon molecules in the atmosphere?: this intriguing question remains wide open since the opaque, dull, red haze has not yet revealed his secrets.We have no significant indices of the possibility of clouds developing in the titanian sky: for instance, Voyager 1didn't detect any radio emissions from lightning on Titan. Scientists try to detect brightness variations in Titan's disk to deduce a circulation of clouds. In the 1990's, Dan Wenkert and Glen Garneau of JPL claimed that they had detected faint clouds on Titan, moving at speeds of 25-40 m/s, in a prograde direction: it didn't persuade the scientific community.In 1995, Caitlin Griffith and Toby Owen identified a bright area covering 10% of the Titan's disk: they assimilated it with clouds located at an altitude around 10 to 15 km above the surface. The idea that Titan's atmosphere might be supersaturated with methane appeared: the cloud droplets would grow quickly and fall as rain. But all these interpretations remain uncertain: a brighter area is likely to mean an icy surface, a cloud layer or high mountains. Darker spots are likely to mean seas of methane, ethane, ammonia or acetylene,or dry areas.
Even if we have no irrefutable evidence for a meteorology with clouds on Titan, numerous models on the topic have been developed.Since there is a large amount of methane in the atmosphere, it is admitted that methane might behave in a similar way as water on Earth: an evaporation process from the hydrocarbon lakes would occur and clouds of methane or ethane would form generating rain from time to time.The ultraviolet light is expected to split the methane gas ( CH4), in the atmosphere, into various fragments, called radicals ( CH2, CH, H, CH3 ). These radicals recombine into various organic molecules, the most abundant of which are acetylene (C2H2) and ethane (C2H6).They can react with other hydrocarbon radicals and nitrogen radicals from the break up of nitrogen molecules to form more complex materials, including tholins and hydrogen cyanide (HCN).But where does the red haze come from? Carl Sagan, probably inspired by Stanley Miller, had carried out a key experience on this subject: he had submitted electric sparks to methane gas and he obtained as a result a reddish sludge that might indicate what is really going on in the Titanian atmosphere.
Jonathan Lunine envisaged a model in which huge oceans of methane are slowly evaporating into the atmosphere where methane is converted irreversibly into ethane which drizzle back into the oceans.It is here a pure photochemistry process, methane spliting into molecules that recombine to form methane, under the action of ultraviolet radiations from the Sun. Over time,the oceans become progressively richer in ethane, less volatile and deeper.This model was presented in "Science Magazine" ("Ethane ocean on Titan").
What about the seasons on Titan and their influence on the meteorology? The seasons are more marked if the inclination of the rotation axis of the planet to its orbit is higher and if the orbit of the planet around the Sun is more elliptical ( significant distance changes from the Sun ).The tilt of the rotation axis of Earth is 23.5°: that's the reason why there are seasons on Earth.The distance variation from the Sun plays a limited role as the distance changes are small.For Titan, that is the opposite: the distance variations are greater and the rotation axis of Titan is tilted by 26° to the Sun's plane.It is sufficient to change the strength of sunlight by 20%. So, Titan's seasons may be more exaggerated than Earth's.The temperature change resulting from season changes might have a significant impact on the atmosphere: if the temperature ranges from -170°c and -200°c, a Sun fainter means that the temperature coming decreases, gases of nitrogen and methane will fall to freeze, the atmosphere will be thinner, the pressure will drop, the greenhouse effect will diminish triggering a drop in temperatures.On the other hand,a Sun warmer means higher temperatures coming, that nitrogen and methane boil, go up, the pressure increases, the greenhouse effect increases triggering a rise in temperatures.
What about the atmospheric circulation on Titan? When Voyager 1 passed near Titan in 1980, it revealed that the northern hemisphere was to a certain extent darker than the southern hemisphere but the first images from the Hubble Space Telescope in 1990 by Caldwell showed the opposite that is the south was darker at blue wavelengths.The idea that more haze was produced in the hemisphere where it is summer because there is more ultraviolet light from the sun was questioned. Most researchers now favour the idea of haze being transported by winds from north to south and vice versa.What about the winds on Titan?According to the General Circulation Model, it seems that the atmosphere and the winds are oriented into a prograde direction. The winds are influenced by numerous factors and especially sunshine,the diameter and the rotation of the planet. On Earth, the solar energy is higher around the equator: the ait heats up, rises and go to the polar regions.On Titan, the energy incoming from the Sun is weak and the atmosphere is opaque and thick (as on Venus). Most models show that opaque atmospheres around slowly rotating bodies seem to lead to zonal winds following lines of latitude: so Titan's atmospheric circulation is probably dominantly zonal. If the wind movement is weak, the difference of temperature between high latitude and low latitude is likely to be higher.