Saturn’s moon Titan is the only moon in the solar system that has an atmosphere as thick as Earth’s, consisting of more than 98% nitrogen, roughly 1.4% methane, and smaller amounts of other gases. NASA’s Cassini satellite has been circling Saturn since 2004, witnessing more than one-third of its 29-year orbit around the Sun, allowing it to observe the changing of the seasons. However, a new study finds that the seasons are not the only thing changing Titan’s atmosphere: its chemical makeup fluctuates according to the Sun’s 11-year cycle of magnetic activity.

Westlake et al. analyzed data from 41 flybys of Titan, some at altitudes of less than 1000 kilometers when Cassini dipped into the upper fringes of its atmosphere. The authors found that the amount of methane there varied wildly over time—it dipped from mid-2006 to 2008, then gradually recovered for 2 years, but crashed to roughly half of its 2006 peak by 2011.

These fluctuations correspond neatly to the 11-year solar cycle, in which the Sun’s rotation gradually winds up magnetic field into contorted coils, giving rise to flares and sunspots that emit ultraviolet and X-ray light. Upon reaching Titan, this powerful radiation can tear methane molecules apart.

After reviewing the Cassini data, the authors think that this destruction of methane occurred from 2006 to 2008 during the last phases of the previous solar maximum. Upon reaching solar minimum in 2008, the quiet Sun allowed Titan’s methane to recover its levels. Then, as the Sun once again began gearing up toward its most recent solar maximum in 2013, methane levels declined. This case is bolstered by data from the last mission to make such measurements—the Voyager 1 spacecraft, which swooped by Titan in 1980 during solar maximum conditions and found similarly depleted levels of methane.

By using one- and three-dimensional models, the authors were also able to trace the movements of the different chemicals through Titan’s atmosphere. During solar maximum, the broken-down methane remnants combine to form heavier hydrocarbons that rain down through the atmosphere. During solar minimum, the replenishment of methane in Titan’s upper atmosphere comes from its lower layers. Although it takes only weeks for increased solar radiation to send Titan’s methane levels crashing, it takes years for them to recover. The authors predict methane levels will not reach their previous peak until sometime this year. (Journal of Geophysical Research: Space Physics, doi:10.1002/2014JA020394, 2014)

—Mark Zastrow, Freelance Writer

Citation: Zastrow, M. (2015), When the Sun goes quiet, Titan gets gassy, Eos, 96, doi:10.1o29/2015EO028533. Published on 11 May 2015.