As we
search for extraterrestrial life, Earth in some sense always provides our framework.
The data indicate that life does in fact exist on this planet, and it existed
here sometime before about 3.5 billion years ago (give or take a couple hundred
million years). However, thanks to plate tectonics and other pesky processes,
we're missing some critical information about this early time. This includes information
about how life got started on Earth.
Titan may
come to our rescue. As the largest of Saturn's moons--much larger than our own
moon, larger even than the planet Mercury--Titan has several striking similarities
with Earth. For one, it has a substantial, largely nitrogen atmosphere. For
another, it supports an active volatile cycle. And the effects of this
volatile cycle make Titan appear 'surprisingly Earth-like' (as scientists with
the Cassini-Huygens mission have been saying). For example, images show what
are interpreted to be convective clouds forming and then dissipating, apparently
raining out onto the surface and producing what looked for all the world (pun
intended) like terrestrial rivers and lakes.
But Titan also
has significant dis-similarities from Earth, at least as we know it
today. Instead of a water-based volatile cycle as on Earth, Titan cycles
hydrocarbons. At Titan's temperature, water is about as solid as rock on
Earth, but hydrocarbons--which are gaseous under terrestrial conditions--can
evaporate into the atmosphere, re-condense into rain, and fall veeeery slowly
(Titan's gravity is about 1/7th of Earth's) to the surface, either
to pond or flow over the surface as a liquid. A summer cloudburst on Titan
would be something like standing in a cloud of droplets of liquid natural gas
at -300 degrees Fahrenheit. In addition to N2, Titan's atmosphere
contains significant amounts of methane, and ethane and other hydrocarbon
compounds have been detected.
One thing
Titan's atmosphere does not contain is oxygen (which is probably just as
well--oxygen + hydrocarbons is highly flammable). The story of the rise of oxygen
on Earth is not well understood. Oxygen-rich habitats must have developed
around the time of those cyanobacteria that produce oxygen (>2.7 Ga), but
before that time, that is, during the Archean (3.8-2.5 Ga), life must have developed
under oxygen-poor conditions. So how does that relate to Titan? Well, it
means that Titan is like early Earth in not having oxygen. It's also good for astrobiologists
interested in exploring the chemical, possibly pre-biotic, pathways that may
have lead to the development of biota on early Earth.
This
exploration takes a variety of forms. Some of it is theoretical modeling,
trying to understand the large-scale movement and resultant combinations of
materials that occur on Titan's surface. For example, in some equatorial
regions, Cassini imagery shows what look like giant wind-blown dunes. These
dunes seem to be composed preferentially of fine-grained organic sediments, which
may facilitate interesting (i.e., biologically relevant) chemical
interactions. These organic sediments form by UV photolysis and recombination
of hydrocarbons in Titan's atmosphere, a process that gives Titan its orange
haze (kind of like an L.A. smog on steroids). After these complex organic
molecules are formed, they settle out of the sky onto Titan's surface to form
dunes, fill up depressions, line river channels, and generally add to Titan's air
of being a giant 'Rorschach inkblot test.' (In support of this statement, I
respectfully cite the fact that an area of dark terrain in an early Radar image
was referred to by the Cassini Team itself as 'Si-si the cat.' [See Image.]
Another
approach to discovering pre-biotic chemistry of Titan and early Earth is
experimental. This approach has a long history, dating back to the Miller-Urey
experiment, in which gases believed to represent the atmosphere of early Earth were
cruelly subjected to shock treatment (ie., simulated lightening) as a test to
see how life might have arisen on Earth. Two decades, later in the late
1970's, Carl Sagan and Bishun Khare (who is now, appropriately, a scientist at
the Carl Sagan Center), did a similar style of experiment using gases found in
Titan's atmosphere. They created dark, reddish gunk, which they termed 'tholin'
(from the Greek word for 'muddy'). These types of experiments require
continual 'updating' as we learn more about the true conditions--and the actual
gas species--that existed on early Earth and exist on Titan. A recent study,
the most faithful yet to Titan's conditions, used a deuterium lamp to simulate atmospheric
UV radiation of methane and measured the properties of the resultant aerosols.
This work
is all focused on understanding Titan as an analogue for pre-biotic Earth. A
small and courageous group of scientists have suggested that life may exist on
Titan even today. This runs contrary to the maxim that life as we know it
requires liquid water, of which Titan has none, but is based on the idea that non-aqueous
life might arise through analogue chemistry. For example, ammonia--like water--is
a polar molecule and dissolves other molecules--including hydrocarbons--with
electrical charge.
Continued
research here on Earth may inform us about this possibility for life on Titan
today. Likewise, continued spacecraft investigation of Titan may tell use
about life on Earth in the ancient past. In the exploration for life, Titan
and Earth symbolize spatial and temporal symbiosis.
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