Titan, Saturn's largest and only substantial moon, shown below, is the only moon in our solar system that has a significant atmosphere, and it is the only object other than Earth that has stable liquid on its surface.
This is a natural colour composite (what the moon looks like to our eyes) of Titan, taken by Cassini in 2005:
Titan, slightly larger than Earth's moon, is blanketed in a thick orange haze. It has a surface gravity about one seventh that of Earth and an atmospheric pressure about 60% higher than Earth's. It's atmosphere is almost entirely composed of nitrogen, about 95%, making it much more similar compositionally to Earth than either Venus or Mars, which have atmospheres dominated by carbon dioxide and are either crushingly thick (Venus) or insubstantial (Mars). Ttitan's atmosphere also contains significant methane, and this is what makes it really interesting, because this molecule undergoes a variety of photochemical reactions in its upper atmosphere. It breaks down in sunlight and reacts to form organic compounds such as benzene, acetylene and hydrogen cyanide, smog chemicals in other words. And it is this smog that makes Titan's atmosphere appear opaque orange. There is much evidence, described in detail in my article about the evolution of Earth's atmosphere, that Earth's atmosphere once had a similar complement of organic molecules and that is how life first formed. Although Titan is a moon rather than a planet, its unique atmosphere makes it more of a "sister planet" to Earth than either Venus or Mars.
Titan is truly a world of mystery. It has a substantial atmosphere while other moons that are of similar size and internal composition, such as Ganymede, Callisto and Europa, all moons of Jupiter, have none at all. The first question to answer then is, "Where did Titan's atmosphere come from?"
The Origin of Titan's Atmosphere
Titan is composed of about half water ice and half rocky silicate-based material and it is probably differentiated into layers, as shown here in this diagram:
Titan's core may still be hot enough to support a layer of liquid water and ammonia beneath its water ice crust (1h ice is a physical state of ice just like the water ice here on Earth). This subsurface ocean, analogous to Earth's magma, is located above a slightly thicker layer of high-pressure ices. There are special phases of water ice that are actually denser than liquid water and these can form under high pressure, even in an environment that is warmed by the core. This high-ressure layer is likely composed of a combination of Ice VI and ammonia dihydrate ice. While Titan's core may have some heat, Titan is very cold. Its surface temperature is about -179°C. The presence of ammonia can keep liquid water from freezing down to a temperature of about -97°C, so this is why a liquid subsurface ocean may be possible on Titan. At the distance of around Saturn, its parent planet, from the Sun, compounds that would have been strictly gases on the rocky planets can accumulate in all three phases - solid, liquid and gas - in the material that formed planets and moons. Titan was made from about half silicates (rocky material) and half volatiles such as ices and ammonia hydrates, a much higher ratio of total volatiles than Earth.
Although it is exciting to have another nitrogen-rich atmosphere in our solar system, Titans' atmospheric nitrogen likely has a different origin than Earth's. Ammonia, which readily dissociates, might be its original source, rather than gradual outgassing which is the proposed origin of Earth's nitrogen gas. If nitrogen had originated from the outgassing of rock, one would also expect other gases from outgassing, particularly argon gas. Unlike Earth, which has about 1% argon in its atmosphere, none has been detected on Titan, with the exception of an argon isotope that is the product of radioactive decomposition of krypton.
There are a few different possibilities why only Titan, of all the Jovian and Saturnian moons, has an appreciable atmosphere. Temperatures in the disk of gases that was forming Saturn and, we assume, Titan and the other moons, would have been cold enough to allow the accretion of ammonia in solid form, whereas temperatures around Jupiter as it was accreting could have been much higher because of the higher gravitational field around this even more massive planet. Callisto and Ganymede therefore may not have accreted as much solid ammonia and their resulting atmospheres may have been too thin to withstand solar wind erosion over time. Another explanation may be that early comet impacts on Callisto and Ganymede would have been of higher energy because of Jupiter's higher gravity. These more forceful impacts could have eroded their atmospheres, while impacts on Titan would have been less energetic and actually deposited material in its atmosphere instead. However, this latter theory is weakened by the isotopic ratio of hydrogen in Titan's atmosphere. Its ratio of deuterium (2H) to hydrogen (1H) is 1.5 times lower than that of comets, so comets are unlikely to be major contributors to its atmosphere.
The high isotopic ratio of lighter nitrogen (14N) to heavier nitrogen (15N) suggests that, even though Titan's atmosphere is fairly dense, much of it has been lost over time. Lighter isotopes are a bit more easily stripped away by solar radiation through Jeans escape, as discussed in the previous article. The young Sun would have had about 70% of today's solar output but it would have had a much higher output of X-ray and ultraviolet photons. Much of Titan's nitrogen loss could have occurred very soon after it formed, within about 50 million years of accretion.
Composition of Titan's Atmosphere
Titan's upper atmosphere contains many layers of haze as shown in this ultraviolet image:
Titan has atmospheric layers analogous to those of Earth, described in a previous article. The purplish upper layers are composed almost entirely of nitrogen and hydrogen (0.1% - a trace gas in Titan's atmosphere). Methane condenses out of Titan's atmosphere above 32 km. This is Titan's tropopause, analogous to Earth's tropopause, above which water vapour condenses out. Below this, the abundance of methane increases to a maximum of about 5% between 8 km altitude and the surface. Titan's troposphere, extending from the tropopause down to the surface, is where a methane convection cycle occurs, analogous to the water cycle bound within Earth's tropopause. This graphic of Titan's atmosphere and its sometimes-called "methanologic cycle gives you an idea of how it works:
Above the layer of methane weather, a relatively small amount of stratospheric methane exists and it is in this layer that sunlight breaks methane molecules apart. The products react with each other to create various organic molecules. The opaque orange haze may come from tarlike organic precipitates from these reactions, called tholins. Tholins are abundant on the surfaces of icy bodies in the solar system, giving them a reddish brown appearance. The outer edge of the orange haze marks the edge of Titan's stratosphere.
If you look at the curving yellow line in the graphic above, you can see that Titan's surface is actually much colder than its upper atmosphere. The reason for this is that the orange haze creates an anti-greenhouse effect by reflecting what little sunlight Titan receives (about 1% of what Earth receives) back into space. Methane on the other hand is a very potent greenhouse gas, so it has a warming effect on Titan's surface. Without atmospheric methane, Titan's surface would be far colder than -179°C. Liquid methane rains down from methane clouds creating seasonal lakes and even seas. Its water-hydrocarbon ice surface, perhaps coated in tholins, is as hard as rock. Its dim surface, seen by the Huygens probe after it landed in 2005, looks much like this artist's rendition:
There is evidence that Titan's ice rock is weathered by methane just like stony Earth rock is weathered by water.
The Dynamics of Titan's Atmosphere
There are a variety of ways in which Titan should be continuously losing its atmosphere. A robust and continuous replenishment of methane must counteract this.
Titan has no magnetic field of its own but it is sometimes enveloped in Saturn's powerful magnetosphere as it orbits the planet. This should be protective but there is a problem. The difference between Saturn's rotational period (10.7 hours) and Titan's orbital period (16 days) means that Saturn's magnetized plasma strikes Titan at a speed of 100 km/s. That kind of buffeting likely speeds up its atmospheric loss rather than protecting it from solar wind.
Solar radiation should have converted all of Titan's methane into more complex hydrocarbons within about 50 millions years of its formation, based on the expected rates of various reactions in this cold environment. This means that methane must be continuously replenished and most researchers believe that the source is cryovolcanic activity. Cryovolcanoes, like volcanoes on Earth, release thermal energy from the mantle to the surface. Most of the thermal energy within Titan's mantle may come from tidal flexing, as the moon is continuously stretched and deformed by Saturn's gravity. Cryovolcanic activity may also simply be pressure release from higher-pressure liquid ammonium sulphate escaping up through the lower pressure ice layer above it . Mysteriously, Titan doesn't appear to have very many of these volcanoes. In fact, none have yet been definitively identified. One researcher, Jeffrey Moore, proposes that methane in Titan's atmosphere might not come from cyrovolcanic activity at all, but from slow diffusion out of a cold stiff interior. In this case Titan could still have a liquid subsurface ocean, and internal radioactivity could be sufficient to heat it enough to stay liquid at -97°C. Unlike Venus and Mars, Titan also shows evidence of active tectonic movement in a north-south direction. It doesn't appear to undergo subduction, like plates on Earth do, but areas of plate compression and stretching are evident. The resulting ridges are also likely the result of tidal flexing.
Titan: Organic Chemistry → Life?
Some researchers think that Titan's atmospheric methane could at least in part come from living organisms. Astrobiologist Chris McKay suggests that methanogenic life could exist in Titan's methane lakes. These cold-world organisms would take in hydrogen gas instead of oxygen, react it with acetylene instead of glucose and release methane as a waste gas rather than carbon dioxide. There is much debate about whether this kind of biochemistry is feasible. There is a possibility too that life might exist within Titan's ammonia-water subsurface ocean, with energy supplied by tidal friction and radioactive decay heat. There is also some speculation that while Titan is now a very cold environment, it could be much warmer billions of years from now as the Sun evolves into a red giant. The Sun's evolution would be gradual enough to supply a relatively stable warm environment with surface temperatures at around -70°C, perhaps lasting long enough for microorganisms to evolve. By then, much of Titan's anti-greenhouse haze will be depleted as the Sun's ultraviolet output decreases and Titan's methane greenhouse effect takes over as a primary warming mechanism. It may not be methane lakes that populate this warmer world. It will likely sport a liquid ammonia water surface instead.
Regardless of whether life exists on Titan, its mix of complex organic molecules is very exciting to researchers interested in how life made its appearance on early Earth. Scientist David Grinspoon believes that Titan and Earth are examples of bodies with surfaces and atmospheres that are, or have the potential to be, intimately interconnected with life. These kinds of atmospheres have an active cycling of a compound between gas, liquid and solid states that tends to maintain equilibrium and they provide opportunities for chemical interactions with other compounds. There is an active ongoing chemistry in other words, in which life has an opportunity to evolve right along with its abiotic environment. Although Titan does not have any appreciable carbon dioxide in its atmosphere and this gas was likely both abundant and played an important role in the prebiotic chemistry and the formation of life on early Earth, Titan does have a carbon source for complex organic molecules to form and that is methane. Using this molecule, Titan maintains a chemical balance analogous to Earth's.
From Titan to Extrasolar Planets
Life on planets in other solar systems may be expected to be associated with environments that have similar active cycling. This is the basis of the Gaia Hypothesis, and we will be taking a closer look at how scientists can use hypotheses like this one to focus their search for other planets in the universe that could support life.
Titan provides us with yet another laboratory in which to study how initial formation conditions determine what kind of atmosphere develops, what kinds of factors are important in determining how atmospheres evolve, and how composition and environment determine atmospheric activity. Venus and Mars seem to be worlds that have failed to achieve an equilibrium state as their atmospheres now move unchecked toward extremes. Titan, on the other hand, offers us an exciting example of how a world under physical circumstances greatly different from those on Earth can develop atmospheric cycling mechanisms and achieve a state of equilibrium. It is perhaps these kinds of mechanisms that provide conditions stable enough for life to evolve.
If you would like to know more in general about Venus, Mars, Earth or Titan, please take a look at these previous Scientific Explorer articles:
Earth
Mars
Venus
Titan (as well as Saturn and some of its other moons)
Next, we will find out what scientists are looking for as they search for distant planets that could harbour life, in Earth's Atmosphere Part 7.
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