Does Titan's Atmospheric Makeup Fluctuate With the 11-Yr. Solar Cycle?

Titan observed at infrared and UV wavelengths. Note the purplish haze at the periphery identified as the methane atmosphere.


Astronomical periodicities are of great interest in astronomy, and perhaps the most noteworthy entail the resonances found in respect of planetary orbits. For example, in the case of Jupiter, gaps in the asteroid belt correspond to discrete resonances with Jupiter's orbit, i.e. defining cases for which asteroids orbit twice for each Jupiter orbit, three times for each Jupiter orbit and five times for every three Jupiter orbits. These gaps, called "Kirkwood Gaps" are of great interest in seeing the effects of periodicities in the asteroid belt.

Now, there appears to be evidence that Saturn's moon Titan, exhibits periodical fluctuations of its methane atmosphere in terms of the solar 11-year cycle.  Titan is the only moon in the solar system that has an atmosphere as thick as Earth's consisting of 98 percent nitrogen and roughly 1.4 percent methane.

But now we know it isn't "fixed". Observations made over nearly one third of Saturn's 29-year orbital period by NASA's Cassini space craft disclose Titan's atmosphere has a chemical makeup that fluctuates according to the Sun's 11-year cycle of magnetic activity.

Westlake et al (Journal of Geophysical Research, Space Physics, 2014) have analyzed Cassini data from 41 flybys of Titan, some at altitudes of less than 1,000 km when the spacecraft dipped into the upper fringes of the atmosphere.  The team found that the amount of methane varied widely over time, dipping from mid-2006 to 2008 then gradually recovering for two years before crashing to one half its 2006 peak by 2011.

These fluctuations were found to correspond neatly to the 11- year solar cycle in which the Sun's rotation gradually winds up the magnetic field into tightly wrapped coils giving rise to sunspots as well as flares. The latter are capable of emitting ultraviolet and x-radiation that can tear methane molecules apart.

After reviewing the Cassini data, the authors hypothesize such destruction of methane occurred from 2006-08 during the final phase of the previous solar maximum. On reaching the solar minimum in 2008, the "quiet" Sun then allowed Titan's methane to recover its previous levels. The methane levels began declining again after solar activity began increasing toward the next maximum in 2013.

The Westlake at al case is also bolstered by the Voyager 1 mission, the last to make similar measurements, when it swooped by Titan in 1980 during the famous Cycle 20 maximum. It found similarly depleted levels of methane.

Interestingly, through the use of 3-dimensional models, Westlake at al were also able to trace the movements of the different chemicals through Titan's atmosphere. They found that during solar maximum the broken down methane remnants combined to form heavier hydrocarbons that rain down into the atmosphere. 

Not surprisingly, though it takes mere weeks for Titan's methane levels to plummet it takes years to build them back up. The authors predict they won't return to their previous peak until sometime later this year.