My first Magazine Sky & Telescope - 02.2019 | Page 25

and an important place to study the chemistry that might have occurred in our planet’s atmosphere before life as we know it produced the large amount of molecular oxy- gen (O 2 ) we now breathe. It might even give us the tools to recognize habitable planets around other stars. A Hydrocarbon Laboratory After molecular nitrogen, methane is the most abundant gas in Titan’s atmosphere (2% to nitrogen’s 98%). Ultra- violet photons from the Sun break N 2 and CH 4 molecules into pieces that react with each other to produce heavier byproducts, which eventually form Titan’s thick organic haze. This photochemistry is similar to the way that sun- light spurs the formation of smog here on Earth. Some of the molecules formed by Titan’s photochem- istry are very familiar to us — molecules like propane (C 3 H 8 ) or hydrogen cyanide (HCN) — and they fall into broad categories of chemical com- pounds called hydrocarbons (molecules with hydrogen atoms attached to carbon atoms) and nitriles (molecules #1 Hydrogen with a carbon atom triple-bonded to a nitrogen atom). More broadly, we refer p TOP 10 Here to the compounds in Titan’s atmosphere and on the follow- ing pages are the (and, eventually, on its surface) as 10 most abundant organic, which simply means the mol- photochemically ecules contain hydrogen atoms bonded made molecules to carbon atoms. in the stratosphere Prior to the arrival of the Cassini- above Titan’s equatorial regions. Huygens spacecraft, the heaviest mol- ecule that scientists had ever detected in White is hydrogen, black is carbon, Titan’s atmosphere was benzene (C 6 H 6 ), red is oxygen, and discovered using data from ESA’s Infra- blue is nitrogen. red Space Observatory in the late 1990s. Benzene is a pretty complicated mol- ecule to result from atmospheric chemistry, although we do also see it at Jupiter’s poles, where it is made via ener- getic-particle chemistry in the aurorae. Thus we knew that the organic chemistry in Titan’s atmosphere was very complicated even before Cassini-Huygens. Indeed this is one of the reasons why we have been interested in Titan since the Voyager encounters fi rst revealed that complex organic reactions are happening in its atmosphere. ORGANICS An organic compound is any molecule that contains carbon bonded to hydrogen. The molecule can contain other elements, too. Hydrocarbons are a type of organic compound and consist of only hydrogen and carbon. Most carbon-bearing molecules are organic. sk yandtele scope.com • FE B RUA RY 2 019 23