Passage to a Ringed World - NASA's History Office

CONSTITUENTS OF THE
TITAN ATMOSPHERE
Chemical Common Atmospheric
Constituent Name Concentration
N 2 Nitrogen 90–97 percent
Hydrocarbons
CH 4 Methane 2–10 percent
C 2 H 2 Acetylene 2.2 parts per million
C 2 H 4 Ethylene 0.1 parts per million
C 2 H 6 Ethane 13 parts per million
C 3 H 8 Propane 0.7 parts per million
Nitriles
HCN Hydrogen cyanide 160 parts per billion
HC 3 N Cyanoacetylene 1.5 parts per billion
To summarize, prior to Cassini, the
primary constituents of Titan’s atmosphere
have been detected, the process
by which photolysis of methane
has produced a smoggy haze is fairly
well understood and the pressure–
temperature profile as a function of
altitude in the atmosphere has been
determined. We do not know the
source of the atmosphere, if there is
active “weather” (clouds, rain, lightning)
nor how the atmosphere circulates.
These are the intriguing science
questions the Cassini Orbiter and the
Huygens Probe will soon investigate.
Atmospheric Origin and Chemistry.
What is the source of molecular nitrogen,
the primary constituent of Titan’s
current atmosphere? Is it primordial
(accumulated as Titan formed) or
was it originally accreted as ammo-
nia, which subsequently broke down
to form nitrogen and hydrogen? Or
did the nitrogen come from comets?
These important questions can be
investigated by looking for argon in
Titan’s atmosphere.
Both argon and nitrogen condense
at similar temperatures. If nitrogen
from the solar nebula — out of which
our solar system formed — was the
source of nitrogen on Titan, the ratio
of argon to nitrogen in the solar nebula
should be preserved on Titan.
Such a finding would mean that we
have truly found a sample of the
“original” planetary atmospheres.
Argon is difficult to detect, however,
because it is a noble gas — it was
not detectable by Voyager instrumentation.
The upper limit that has been
set observationally is one percent relative
to nitrogen; the solar nebula ratio
is close to six percent.
Methane is the source of the many
other hydrocarbons detected in Titan’s
atmosphere. It breaks down in
sunlight into fragments such as CH2 and H . The CH fragments recom-
2 2
bine to produce hydrocarbons.
Ethane is the most abundant by-product
of the photochemical destruction
of methane. The leftover hydrogen
escapes from Titan’s atmosphere. This
is an irreversible process, and the
current quantity of methane in Titan’s
atmosphere — if not replaced — will
be exhausted in 10 million years.
The hydrocarbons spend time as the
aerosol haze in Titan’s atmosphere
obscuring the surface. Polymerization
can occur at this stage, especially for
hydrogen cyanide and acetylene,
forming additional aerosols that eventually
drift to the surface. Theoretically,
the aerosols should accumulate on
the surface, and, over the life of the
solar system, produce a global ocean
of ethane, acetylene, propane and so
on, with an average depth of up to
one kilometer. A large amount of liquid
methane mixed with ethane could
theoretically provide an ongoing
source of methane in the atmosphere,
analogous to the way the oceans on
Earth supply water to the atmosphere.
Radar and near-infrared data obtained
from Earth-based observations
show, however, that there is no global
liquid ocean, although there
could be lakes and seas, or possibly
subsurface reservoirs. The ultimate
fate of Titan’s hydrocarbons, which
are expected to exist as liquids or
solids on its surface, is a mystery.
THE MYSTERIOUS TITAN 33