My first Magazine Sky & Telescope - 02.2019

Stratosphere and Troposphere: Abundant Methane Electrons Escape Ions H 2 CH 4 (?) 1,200 1,000 800 1,400 spheric dynamics, which can then affect where the haze particles are located. So by studying the haze, we’ll have a better idea of what’s going on energy-wise in the atmosphere. Sunlight u ATMOSPHERE IN PROFILE Shown is a representa- tive temperature profi le for Titan’s atmosphere (black line), along with some of the major chemical processes. Shorter (more energetic) ultraviolet wavelengths reach shallower depths in the atmosphere. Relativistic par- ticles called galactic cosmic rays might reach all the way down into the troposphere. Thermosphere Ionosphere N 2 and CH 4 ionized Heavy ions HCN C 2 H 2 CH 4 broken down Mesosphere 600 Pre-equinox 400 Detached haze Post-equinox Hydrocarbon Lower down, in Titan’s stratosphere, additional reactions Main haze chemistry is driven by sunlight’s less-energetic 200 Stratosphere ultraviolet photons, which break up some of Hydrocarbon condensation CH 4 clouds Troposphere the larger molecules originally formed higher 0 Lakes in the atmosphere that have sunk to this level. 80 100 140 160 180 200 Galactic 60 Temperature (K) cosmic rays Those molecular fragments then react with the widespread CH 4 , destroying more of it. The major net chemical reaction in Titan’s atmosphere is the conversion of CH 4 into H 2 , which escapes The Huygens probe descended through all these regions of to space because of Titan’s low gravity, and C 2 H 6 (ethane), the atmosphere, ﬁ nally landing near the equator in a rela- tively dry wash, similar in appearance to those in the deserts which condenses into droplets that fall onto the surface, of the U.S. Southwest. As the Huygens probe sat on the sur- where the liquid stays because it cannot evaporate in Titan’s face, it measured methane evaporating from beneath it due surface conditions. to the heat of the probe. This indicates that liquid methane This is a crucial reaction sequence: It means that CH 4 is (and likely other liquids as well) had ponded just irreversibly destroyed in Titan’s atmosphere. below the surface, perhaps the memory of the Computer models indicate that all the existing last rainstorm. methane in Titan’s atmosphere should be used All of the liquid and solid organic compounds up in 10 to 100 million years. Scientists gener- produced in the atmosphere eventually end ally assume this means that CH 4 is somehow up on the surface, where they are either fur- resupplied, perhaps from outgassing of meth- ther modiﬁed by chemical processes or instead ane trapped in cage-like surface compounds #9 Methylacetylene modify the surface themselves, or both. In this called clathrates, and not that it is merely what way, Titan’s surface and atmosphere are uniquely is left of a much larger original abundance of connected. To fully understand what’s going on in the atmo- methane in the atmosphere that is in the act of disappearing. sphere — and what it could tell us about both our planet and The less-energetic photons may also drive photochemis- others beyond the solar system — we’ll need more informa- try within ices that have condensed out of the atmosphere. tion about Titan’s surface, especially the composition. Several One of the most striking examples of condensation in Titan’s teams are working on mission atmosphere occurred as the south pole moved into winter: concepts for returning us to Titan to A polar vortex formed and a giant ice cloud, later identiﬁed explore these questions. as HCN ice, appeared. This is one of many examples of the In the meantime, scientists will seasonal changes that were observed by Cassini, a beneﬁt of #10 dig deeper into the wealth of data having such a long-lived mission, which provided data for Diacetylene returned by Cassini-Huygens, point almost half of a Saturn year. powerful telescopes at Titan to search for new molecules, As we descend into the troposphere, more condensation can occur. Here CH 4 clouds and storms appear seasonally and and run laboratory experiments to better understand how organic materials behave at cryogenic temperatures. With change latitude as they chase the sunlight that drives their hard work we will crack the enigma of Titan. formation. Titan’s dense atmosphere and low gravity result in raindrops that fall slowly and grow to larger sizes than on Earth. Rain appears to be relatively infrequent on Titan — but ¢ When not dreaming of parasailing through Titan’s atmo- when it does rain, it pours, resulting in ﬂash ﬂoods that carve sphere, Johns Hopkins planetary scientist SARAH HÖRST streams and rivers into the organic-coated, water-ice bedrock. investigates organic chemistry throughout the solar system. sk yandtele scope.com • FE B RUA RY 2 019 29

My first Magazine Sky & Telescope - 02.2019 | Page 31