My first Magazine Sky & Telescope - 02.2019 | Page 26

Why do we care how complex the atmospheric chemistry
We need to know what kinds of
is? One of the big questions we are interested in answering
molecules are made only by life
is how far organic chemistry can proceed in the absence of
life. We know from analyzing meteorites that compounds like so that we can use them to detect
amino acids (the building blocks of proteins) can be produced
life both in the solar system and
by abiotic processes. With Titan, we can begin
to understand how far abiotic organic chemis-
elsewhere.
try can proceed in an atmosphere.
This is important for two reasons. First,
#2
Carbon
we need to know what kinds of molecules are
was complex, we knew we needed to study Titan’s atmosphere
monoxide
made only by life so that we can use them to
further to investigate these questions.
detect life both within the solar system and elsewhere in
To explore what the Cassini orbiter and Huygens probe have
the universe. Many of the molecules used by life on Earth,
taught us about chemistry on Titan, let’s take a trip into the
like amino acids and nucleobases (the building blocks of
atmosphere, starting at the top and working our way down.
DNA and RNA), are not unique to biotic processes: They’re
also produced without life’s involvement and are found in
Ionosphere: Surprise Super-Molecules
meteorites and comets. But their relative amounts in these
The Ion and Neutral Mass Spectrometer (INMS) was the pri-
environments are different than what we find
mary instrument designed to measure the composi-
when they’re created by life on Earth. So we need
tion of Titan’s ionosphere as Cassini repeatedly flew
to study abiotically produced organic molecules
through it during the spacecraft’s mission. INMS
in order to be able to tell the difference between
measured the composition of the atmosphere by
abiotic sources and biotic ones when looking at
measuring the masses of the molecules it ingested.
other planets.
The team had designed the instrument to cover a
#3
Second, we want to understand the role that
mass range of 1 to 99 atomic mass units, which
Ethane
planetary atmospheres play in the origin and evo-
roughly covers atomic hydrogen (1) to molecules
lution of life. One potential role is to provide material (like
with approximately six to seven “heavy atoms” like carbon,
organic molecules) to help life get started and/or to evolve.
nitrogen, or oxygen. This mass range easily included the
Since we realized, even before the arrival of Cassini-Huygens,
heaviest molecule known to exist in Titan’s atmosphere when
that the organic chemistry occurring in Titan’s atmosphere
it was designed (C 2 N 2 , with an atomic mass of 52) and the
q TITAN’S SURFACE These infrared images combine 13 years of Cassini’s visible and near-infrared imagery into smooth mosaics of the surface
that hides beneath Titan’s thick atmosphere. Reddish brown marks the moon’s equatorial dune fields, while the bluish and purplish regions might be
enriched in water ice. The central longitudes of each view are (from left to right) approximately 30°, 60°, 120°, 210°, 300°, and 330° west.
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FE B RUA RY 2 019 • SK Y & TELESCOPE
Alien “Earth” Next Door