Is Titan's atmosphere biogenic in origin?

Titan is unique among all the planetary moons in having a dense
atmosphere. This atmosphere has a number of unusual characteristics
that it shares with Earth's atmosphere, and which I think are
consistent with it being biogenic.

Titan is the largest moon of the gas giant, Saturn, and is similar in
size to Earth’s moon. When the solar system formed (4.6 billion years
ago), the planets condensed out of a disk of material swirling around
the sun. Near the sun, solar heat vaporised the more volatile
components of this accretion disk, leaving behind the nonvolatile
(rocky and metallic) components. Hence, the "terrestrial" planets
nearest the sun - Mercury, Venus, Earth, Mars - are composed mainly of
rock and metals such as iron (the metals are concentrated mainly in
the core of these planets). Their inventory of volatile substances
(such as water, ammonia, carbon dioxide) is modest or nonexistent.

At the distance the gas giants including Saturn (and presumably Titan)
formed from the sun, volatile substances were present in abundance.
The largest component of the accretion disk at that distance was
hydrogen, which would have had a tendency to hydrogenate any substance
capable of undergoing hydrogenation. Free oxygen would tend to be
converted into water, nitrogen into ammonia and carbon into
hydrocarbons. Temperatures at these distances from the sun are much
lower than they are where the terrestrial planets formed, and here
water and ammonia exist as nonvolatile ices rather than liquids or
gases.

Large gas giants such as Saturn have a gravitational field so powerful
that they can hoover up and retain even highly volatile, lightweight
gases such as hydrogen. With their weak gravitational field, small
bodies such as Titan (which is about the same size as Earth's moon)
would have trouble attracting and retaining volatile gases. The moons
of the gas giants are thus formed mainly of rock, water/ammonia ices,
carbonaceous residues and perhaps solid carbon dioxide. Titan is the
only one with a substantial atmosphere.

Titan's atmosphere consists mainly of nitrogen, with a hydrocarbon
component (primarily methane). Like Earth's atmosphere, it has a
stratosphere, a zone where the atmospheric temperature rises rather
than falls with altitude. Earth and Titan are the only bodies in the
solar system with an atmosphere having this structure. On Earth, the
presence of the stratosphere is the reason we have oceans of liquid
water today - if the stratosphere wasn't there, water vapour would be
able to reach the upper atmosphere, where it would be rapidly
photodissociated and escape into space. On Titan, the stratosphere
probably performs a similar role. In fact, methane has a sufficiently
low molecular weight that it could probably escape from Titan's feeble
gravity even without being photodissociated.

Our atmosphere is easily recognised as biogenic because it is in a
state of chemical disequilibrium. Its two main constituents are
nitrogen and oxygen. The most stable state for these two substances is
not as separate gases, but as the nitrate ion. In the cores of
lightning bolts, temperatures are high enough for oxygen and nitrogen
to combine to produce nitric oxide. Nitric oxide then rapidly
undergoes further oxidation to form nitrate. This reaction would
slowly but surely remove Earth’s atmosphere, if it weren’t for the
continual activity of denitrifying bacteria (which convert nitrate
back into gaseous nitrogen). Denitrifying bacteria are responsible for
the nitrogen in our atmosphere. The other major constituent, oxygen,
is produced by photosynthesis. Apart from a minor inert gas component,
our atmosphere today is entirely biogenic in origin.

The atmosphere of Titan would also appear to be in disequilibrium. Its
two main constituents are nitrogen and methane. Although not as
powerful an oxidising agent as oxygen, nitrogen does nonetheless
combine exothermically with hydrogen to form ammonia (in fact I think
the reaction 3CH4 + 2N2 - 4NH3 +3C may be exothermic - can anyone
confirm this?). Titan is known to have a "hydrological" cycle based
around methane and ethane, so presumably it has storms and lightning
bolts as well. Some experiments on the effects of electric discharge
through gas mixtures were carried out in the 1950s, AFAIK a lightning
bolt passing through a gaseous mixture of nitrogen and methane
produces a tarry, nonvolatile residue containing a complex mix of
hydrocarbons and amines (and probably some ammonia and hydrogen as
well). At Titan’s near-liquid air temperatures, this mixture will be
completely solid apart from any hydrogen produced (which will escape
to space because Titan’s gravity is too weak to hold it). Titan’s
atmosphere would long since have been converted into tar and ammonia
ice without some continual process of renewal. In fact, I doubt
whether Titan’s gravitational field would be strong enough to capture
significant amounts of gaseous nitrogen from the accretion disk
present when the planets formed, instead I think it is far more likely
to have been produced by the denitrification of ammonia ice. Likewise,
methane has such a low molecular weight that I can’t see very much of
it being captured from an accretion disk, even if Titan could hold
onto it afterward (and I’m not sure that it can without a
stratosphere). To me it looks far more likely that the nitrogen and
methane are both byproducts of an ongoing process, which have
accumulated over time to produce the atmosphere and oceans we see on
Titan today. I think that process is very probably life itself - what
else could keep regenerating nitrogen and methane unfailingly for
billions of years? The data on atmospheric composition from the
Huygens probe will be very interesting, since with luck it could find
further evidence of chemical disequilibrium, strengthening the case
for life. It will be even more interesting if it can analyse Titan’s
seas - does anyone know if it can do this?

If life is present on Titan, it must be completely different from life
as we know it. Titan is so cold that water ice is a quartzlike mineral
there, it certainly couldn’t play an active role in any biological
processes. Liquid methane and ethane would have to take the place of
water, which I think is quite feasible since hydrocarbons are
excellent solvents for a wide range of organic chemicals. The low
temperatures would allow a lot of unstable compounds that can’t exist
at room temperature to be used, and so could allow a far richer
chemistry than is available to organisms living at room temperature.
Without catalysis, chemical reactions are extremely slow at these
temperatures, which would reduce unwanted side reactions and allow
Titanian organisms far better control of their internal chemistry than
organisms living at room temperature (it might mean that they have a
slow metabolism compared to those on Earth, though).

I’d imagine that Titanian life is more likely to be consist of
microscopic single-celled organisms than green bug-eyed monsters,
after all the only living organisms on Earth for most of its history
were bacteria. It wasn’t until there was high levels of free oxygen in
the atmosphere that large, highly metabolically active multicellular
life forms appeared here. Titan doesn’t have oxygen in its atmosphere,
instead its atmosphere closely resembles the anoxic atmosphere of
Earth a couple of billion years ago (with all of the CO2 and H2O
frozen out of course).

I'm looking forward to the results from the Cassini/Huygens mission,
they should be very interesting. Does anyone know if any missions are
planned to Neptune, which I understand also has lots of nitrogen and
methane in its atmosphere (and looked very Earthlike in the Voyager
photos)?

Hugh

In article ,
Hugh wrote:

If life is present on Titan, it must be completely different from life
as we know it. Titan is so cold that water ice is a quartzlike mineral
there, it certainly couldn’t play an active role in any biological
processes.

I nitpick only to be polite: water can't exist as a liquid on
the surface of Titan, but what about underground, in a subtitanian
liquid water zone analogous to Ganymede's hypothetical ocean? Am I
remembering correctly that a water-ammonia mix can have a melting
point as cold as 175ish K?
--
"The keywords for tonight are Caution and Flammable."
Elvis, _Bubba Ho Tep_

(Hugh) wrote in message ...
gases such as hydrogen. With their weak gravitational field, small
bodies such as Titan (which is about the same size as Earth's moon)

Titan is not about the same size the same size as earths moon, nor
even close.

Titan is, in fact, larger than Mercery, and the second largest moon in
the solar system. Not trying to be rude, just pointing it out

Very nice post otherwise though

(Hugh) wrote in message ...

... With their weak gravitational field, small
bodies such as Titan (which is about the same size as Earth's moon)
would have trouble attracting and retaining volatile gases. The moons
of the gas giants are thus formed mainly of rock, water/ammonia ices,
carbonaceous residues and perhaps solid carbon dioxide. Titan is the
only one with a substantial atmosphere.


Nice article, mightyhugh, except where you compare the size of Titan
to Earth's moon. Titan is much, much larger than Earth's moon, and is
in fact larger than the planet Mercury. You put out a lot of good
info otherwise though

On 14 Jul 2004 16:51:33 -0400, (James Nicoll)
wrote:

In article ,
Hugh wrote:

If life is present on Titan, it must be completely different from life
as we know it. Titan is so cold that water ice is a quartzlike mineral
there, it certainly couldn’t play an active role in any biological
processes.

I nitpick only to be polite: water can't exist as a liquid on
the surface of Titan, but what about underground, in a subtitanian
liquid water zone analogous to Ganymede's hypothetical ocean? Am I
remembering correctly that a water-ammonia mix can have a melting
point as cold as 175ish K?

You're absolutely right, I'd forgotten that ammonia can greatly reduce
the freezing point of water. There could well be a substantial
water/ammonia ocean beneath Titan's surface, so there could be
subterranean water-based life on Titan. I still like the idea of
hydrocarbon-based life in Titan's methane oceans though :-)


Hugh

In article ,
Hugh wrote:
On 14 Jul 2004 16:51:33 -0400, (James Nicoll)
wrote:

In article ,
Hugh wrote:

If life is present on Titan, it must be completely different from life
as we know it. Titan is so cold that water ice is a quartzlike mineral
there, it certainly couldn’t play an active role in any biological
processes.

I nitpick only to be polite: water can't exist as a liquid on
the surface of Titan, but what about underground, in a subtitanian
liquid water zone analogous to Ganymede's hypothetical ocean? Am I
remembering correctly that a water-ammonia mix can have a melting
point as cold as 175ish K?

You're absolutely right, I'd forgotten that ammonia can greatly reduce
the freezing point of water. There could well be a substantial
water/ammonia ocean beneath Titan's surface, so there could be
subterranean water-based life on Titan. I still like the idea of
hydrocarbon-based life in Titan's methane oceans though :-)

A little googling shows that not only do some models of Titan
have eutectic water-ammonia mixes, with water ice above and a mixture
of water-ice and clathrates below but far more depressingly, I knew
that at one point and it wasn't all that long ago.

I wasn't aware that Titan ever had open seas (first 10^8 years),
though. Makes one think...

In any case, a world-ocean of ammonia and water could be one
of several biomes, and variation with selective pressure might keep a
continual process of organisms trying to colonize the colder surface
regions. Admittedly there are limits to what v&ns can accomplish.
--
Take the piston rings out of my stomach, And the cylinders out of my brain
Extract from my liver the crankshaft, And assemble the engine again!

[from 'The Dying Aviator']

Mordecai wrote:
(Hugh) wrote in message ...

gases such as hydrogen. With their weak gravitational field, small
bodies such as Titan (which is about the same size as Earth's moon)

Titan is not about the same size the same size as earths moon, nor
even close.

Titan is, in fact, larger than Mercery, and the second largest moon in
the solar system. Not trying to be rude, just pointing it out

Titan is nearly as large as Mars, with about 75% of
its diameter, a bit less than half its volume, and
about a quarter of its mass (Titan is made of lighter
stuff).