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.
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