Exploring the atmosphere

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Titan’s Methane Cycle

Titan, the largest of Saturn’s moons, is the only satellite in the solar system that possesses a substantial atmosphere. The chemistry in Titan’s atmosphere may be similar to what occur in the primeval Earth several billion years ago. 


Titan’s temperature profile. The solid line represents measurements of HASI instrument on the Huygens probe. The symbols are from Voyager radio occultation data. [Lunine and Atreya, 2008]

Photochemistry of CH4

CH4 will undergo photolysis process at the altitude of organic haze layer, leaving ethane (C2H6) behind. In Titan, where the CH4 mixing ratio is large, the limiting factor for ultraviolet photolysis is the availability of photons, and shielding by the photochemically produced haze is limited. Additional loss of methane occurs owing to the impact of charged particles in the ionosphere. Hydrogen is removed from CH4, and escapes to space. The remaining fragments form higher-order hydrocarbons (ethane, acetylene, propane, polyynes, benzene, and so on) and nitriles. The process is irreversible because of the escape of the H2 and the formation of more complex molecules polymerize which subsequently condense into aerosols and fall to the surface.


One of Titan’s greatest mysteries is the origin of CH4 in the atmosphere. With a lifetime of 20 million years, CH4 must be regularly resupplied to the atmosphere to be as abundant as it is today. Another mystery is the sink of C2H6. Continuously CH4 photolysis over the age of solar system will have created a global liquid layer of ethane ocean hundreds of meters deep, which is not seen in any image of Titan. So, to fully understand Titan’s methane cycle, we need to find out

1. The source of CH4 that resupply the atmosphere against photolysis;

2. The sinks of  C2H6 in the absence of a deep surface layer.


Temperature profile

Titan is far away from the sun and therefore its temperature is much lower than that on the Earth. The temperature profiles detected by Voyager and Cassini Huygens show consistently that Titan’s surface temperature is around 93 K, with a decrease trend up to an altitude of 50 km and a increase trend from above. Hence, the ‘troposphere’ of Titan’s atmosphere is defined as the layer below this critical height.. The temperature gradient in Titan’s troposphere is significantly shallower than in the equivalent part of the Earth’s atmosphere. The lapse rate is 1.3 K/km compared with 9.8 K/km on Earth, and vertically large extended convection is not very possible.

Titan at a glance

On the right is Titan in the view of Voyager during its 1980 flyby. Titan is  covered by a opaque cloud layer, which prevented Voyager from seeing its surface. The cloud covers the southern hemisphere looks lighter in color than over the northern hemisphere, a dark hood over the north pole.  The southern hemisphere is slightly brighter than the northern, possibly the result of seasonal effects. When the Voyagers flew past, the season on Titan was the equivalent of mid-April and early May on Earth, or early spring in the northern hemisphere and early fall in the south. To sea through Titan’s thick atmosphere, we need to look at the near-infrared wavelength at which atmospheric absorption is the least. 

Voyager's close approach and diametric radio occultation show Titan's surface diameter is only 5,150 kilometers (3,200 miles) - - slightly smaller than Ganymede, Jupiter's largest satellite. The atmospheric pressure near the surface is about 1.6 bars, 60% greater than Earth's.


A detached haze layer hundreds of kilometers above the surface,  is seen by Voyager looking across the edge of  Titan.


 Left:  Titan and thick haze. Photo Nov. 12, 1980. Range 435,000 km. Click the picture to enlarge

Atmospheric composition

With a surface temperature of 93.7K, water in the atmosphere is almost completely frozen out. The major atmospheric compounds of Titan are 98.4% Nitrogen(N2), 1.6%Methane(CH4) and tracer amount of other gases such as hydrocarbons. CH4 comprising 5% by number of gas molecules above the surface, and 1.5% at temperature minimum some 50 km above the surface.