The following is a short paper entitled
The Origin of Titan's Atmosphere I wrote for a planetary physics
class I took several years ago (1996, I think). I found it in my notebook
recently when looking for
something else and reread it. The professor liked it,
so I present it to you for posterity.
1: Introduction
Titan is a unique object in our solar system. It is not only the
second-largest satellite among the planets, but it also has the
second-densest atmosphere of all the terrestrial planets and
satellites, barring only Venus. Titan's atmosphere
is primarily molecular nitrogen and methane, with
trace amounts of other compounds including hydrocarbons
and "smog," like the hydrocarbons that give the Jovian planets their
color. In addition, Titan's atmosphere is very cold (about
100 kelvins), which not only keeps the rate of thermal escape
of the atmosphere very low, but also keeps many other atmospheric
constituents (or possible constituents) frozen on the surface.
The presence of a significant atmosphere on Titan was suspected as
early as the turn of this century, and confirmed by
Kuiper by
1944. While the atmosphere has been well-studied throughout
the latter half of this century (especially by the Voyager 1
rendezvous), there is still debate about the origin of the dense,
nitrogen-rich atmosphere. Currently there are several theories
on the origin of Titan's atmosphere, none of which have been
conclusively proven or dismissed. In this paper, I will summarize
and discuss each of these theories. In addition, I will discuss
the upcoming Cassini/Huygens mission to Saturn and Titan, which
may offer more clues to the origin of Titan's atmosphere.
Section 2 of this paper will give a brief overview of the status of
observations and experimentation on the atmosphere of Titan. Section
3 will discuss current theories on the formation of Titan's atmosphere
and the pros and cons of each. Section 4 will discuss the
Cassini/Huygens probe, and the contributions which it may make
to the debate over Titan's atmosphere.
2: Observations of Titan's atmosphere
The pre-Voyager and Voyager observations are thoroughly discussed
in Hunten (1984) and Owen (1982), and will be summarized here.
Suggestions of an atmosphere were first made by Solà in
1908 and were found to be theoretically possible by Jeans in 1925, but the existence of an atmosphere was not proven until Kuiper
observed methane red and near-IR absorption lines in
1944. This discovery was significant not only because it requires a dense
atmosphere with a significant fraction of methane, but because it also requires that the atmosphere be chemically evolved, since methane requires
hydrogen in the presence of carbon, and molecular and atomic hydrogen would
have escaped from Titan's weak gravitational field since the formation
of the solar system. Laboratory experiments assuming a pure methane
atmosphere again predicted a dense atmosphere, but one much less
dense than is now known.
Until the 1970's, few significant advances were made in the
study of Titan, other than
the possible detection of mid-infrared bands of more hydrocarbon
compounds and the development of several atmospheric models. In the
1970's however, major refinements to the popular theories about the
atmosphere were made. The detection of vibration-rotation
spectral bands of hydrocarbons such as acetylene, ethane, and
deuterated methane were confirmed by better
high-resolution IR spectra, and wavelength-dependent continuum opacities
suggested the presence of a rich hydrocarbon "smog" in the atmosphere.
However, ammonia -- common in the giant planets --
was not detected. In addition, radio
observations of Titan set its temperature at 87 K, cold enough for
liquid methane and ethane on the surface. Before the Voyager encounter,
we therefore knew that Titan had a cold, dense atmosphere, rich in
methane and other hydrocarbon compounds. At this point,
the presence of molecular nitrogen had
not been confirmed, although it was strongly suspected to be the major
atmospheric constituent.
Finally, the Voyager 1 and 2 encounters set firm limits on the
temperature, structure and composition of Titan's atmosphere. UV
observations of the upper atmosphere and radio occultation measurements
of the mean molecular weight determined that molecular nitrogen
is the primary constituent of the atmosphere with a surface pressure of
about 1.5 bars. Infrared measurements set the surface temperature
at 95 K. Radio occultation measurements also established
the structure of the atmosphere, with a well
defined stratospheric
temperature minimum of about 75 K. In addition, Voyager also
established that the partial pressure of methane was low in comparison
to that of nitrogen, so the presence of significant condensed
methane on the surface of Titan is unlikely. Finally, precise
measurements of the mean molecular weight indicated that
it is slightly higher than 28, indicating
the presence of heavier gases, possibly argon.
While we now have a fairly complete picture of Titan's atmosphere,
we do not know how and why it formed. As mentioned in the introduction,
Titan has an atmosphere denser than all of the terrestrial planets
and satellites other than Venus, so it is likely that its atmosphere
formed under special conditions, probably at or near the beginning
of the solar system. Currently, there are several theories on the
origin of Titan's atmosphere, including accretion of gas from
the solar nebula, and outgassing of atmospheric constituents from
the interior. Currently, observations made to date cannot confirm
or eliminate any of these theories. The following sections will discuss
conditions during the formation of Titan and theories on the
development of a Titanian atmosphere, along with constraints on
the likelihood of these theories being correct.
3: The origin of Titan's atmosphere
3.1: Nebular composition and the origin of Titan
Before discussing theories on the origin of Titan's atmosphere, the
conditions near Saturn and Titan at the time the planets and satellites
formed should be discussed, as they are crucial to the formation
of Titan itself as well as its atmosphere. As was discussed in lecture,
the materials composing the nebular disk at the time the
Sun and planets formed had a composition gradient of solids and of
molecular species dependent upon the temperature in the disk. This also
implies a radial dependence upon distance from the Sun. At the
distance
of Saturn, the composition of Saturn's primordial accretion disk was a
combination of rocky materials and volatile species. The
temperature of the nebula due to the Sun would have been roughly
Tambient = (500 K/AU)/RSaturn's orbit) ~ 50 K,
which implies that in the absence of an accreting Saturn, the local
nebula would have been cold enough to form ices, hydrates, and clathrates
of volatile species in addition to dust grains and rocky materials.
IR radiation from the accreting proto-Saturn and
proto-Titan could substantially increase this temperature, but there
would still be volatile species out of which to create the atmosphere.
Prinn and Fegley (1981) derived the chemical equilibrium conditions for
both hot and cold accreting nebulae. They found that under the
conditions expected around the accreting Jovian planets and their
satellites, the levels of volatile gases and hydrocarbons is actually
enhanced in some cases. The result is that the nebula out of which
Titan (and possibly its atmosphere) formed would be rich in methane
and ammonia, and depleted in simpler species like CO, hydrogen, and
nitrogen. In this case, the rocky material and water ice out of
which Titan likely formed would be very rich in both methane and
ammonia. This is crucial to all of the theories of the formation
of Titan's atmosphere, with the only difference being in where
the majority of the atmosphere came from - either from direct
accretion, by outgassing and cryogenic volcanism, or by
impactors
releasing hydrocarbons from the surface clathrate/hydrate ices.
3.2: Formation of Titan's atmosphere
Currently, there are three possible explanations (with some variations)
to account for Titan's dense atmosphere: that it formed via accretion
of nebular gas onto the developing proto-Titan, that it formed
via cryogenic volcanism and outgassing of warm materials from Titan's
core, or that it formed via melting and outgassing caused by
impactors or accretion energy. In all of
the theories for the formation of the atmosphere, it formed
during or soon after Titan itself grew out of the nebula forming
Saturn, and in most cases, the theories most consistent with
observations involve some combination of accretion and outgassing.
The key points in determining whether the theories are valid are
that they not only match what we know from previous observations
and the Voyager flybys, but that they are physically possible. Each
theory and variations will be discussed along these lines in the
following sections.
3.2.1: Accretion of the Atmosphere
This theory states that Titan's atmosphere was primarily accreted
from the proto-Saturn and proto-Titan nebulae. In the case
of accretion, the nebula composed of either molecular nitrogen and
methane or ammonia and methane is accreted directly onto the surface
forming a dense atmosphere. In the case of the latter, the ammonia
could be photodissociated via solar radiation over a long period
of time (~ 0.1 - 1 Gigayear), generating the current nitrogen-rich
atmosphere, with some methane. (The resulting molecular hydrogen
from photodissociation of ammonia would escape via Jeans escape.)
This has two problems. First it would require that Titan
accreted significantly more ammonia than methane, since the measured
nitrogen column is much higher than the methane column. This is not
likely since there would not be significantly more ammonia than
methane in the nebula.
Second, and most importantly, the observed abundances of elements other
than nitrogen in the atmosphere do not match the cosmic abundances
expected from simple accretion
from the nebula. One example mentioned by Owen (1982) is the
abundance of neon. The mean molecular weight is very close to
28, the molecular weight of molecular nitrogen. However, neon
has nearly the same cosmic abundance as nitrogen, but
it cannot be a major constituent of Titan's atmosphere since the mean molecular
weight would be much lower. It might be possible to combine
several gases such as neon and argon to create a local mean molecular
weight of 28, but this would be impossible to maintain over the entire
height of the atmosphere. Therefore, an atmosphere created by simple
accretion of the nebula is not likely.
3.2.2: Cryogenic volcanism and outgassing
The outgassing theory states that Titan's atmosphere arose primarily
via outgassing of volatiles accreted onto the surface as clathrates
or hydrates. In this scenario, volatiles are accreted onto the surface
primarily as clathrates, or are incorporated into surface clathrates
during the accretion phase and into clathrates making up the
interior. The atmosphere could then be fully explained by outgassing
of methane and ammonia. This scenario actually requires
the presence of a small accreted atmosphere during the outgassing
phase because a tenuous atmosphere created only by outgassing and
melting of surface ice would likely take longer to grow (due to
heat loss) and may not form at all.
Outgassing might have occurred in
several ways. Hunten (1984) describes a scenario where accreted
ammonia and methane clathrates are melted by the relatively high
surface temperature due to accretion. The resulting ocean of
water, ammonia and methane releases a significant amount of
methane and ammonia into the atmosphere, and the ammonia is
converted to nitrogen and hydrogen via photodissociation. As
accretion stops, the surface temperature drops, and as the
ocean freezes, it reincorporates the atmospheric methane into
clathrate much faster than it reincorporates nitrogen. The
result is a nitrogen-rich, cold atmosphere with some methane
and other trace gases.
Another possibility is discussed by Lunine and Stevenson (1987)
whereby a dense atmosphere does not form immediately, but instead
comes from core material in a cryogenic volcanic process. Their
scenario depends upon the fact that Titan's interior would
(most probably) not have been stably stratified immediately after
accretion, such that there may have been significant layers of
dense rock on top of icy material in the core. Since
this would be unstable, the core of Titan would "overturn" generating
significant heating, and allowing methane and ammonia trapped in
clathrates in the interior to percolate through the outer layers
and be outgassed. Again, generation of nitrogen
could occur via photodissociation, or as the authors suggested,
via shock heating by impactors such as comets.
3.2.3: Formation of atmosphere by impactors
This theory has been suggested by several authors (Lunine and
Stevenson (1987), Jones and Lewis (1987)); it suggests that
the atmosphere could have been generated via shock-heating of the
ice or liquid surface of Titan caused by infalling planetesimals
or comets. As mentioned in the preceeding section, impactors
would not only liberate volatile gases locked in surface clathrates
but they would also be able to convert ammonia to nitrogen, and
thus explain the current atmosphere. (McKay et al. 1988) This process
probably
would have been common during the accretion phase while there was
still a significant amount of material in the Saturnian and Titanian
accretion disks, so it is possible that the atmosphere may have
been generated in this way. However, Hunten claims the process is
less likely to generate a dense atmosphere than it is to eject
mass from Titan. Massive impactors moving at high velocity
relative to Titan (e.g. comets) would likely have enough energy to
eject a significant portion of Titan's mass and atmosphere.
4: Future Prospects - the Cassini/Huygens probe
Of the three theories for the formation of Titan's atmosphere,
it seems that the outgassing/volcanism theory is most likely
to be correct. However, there is still significant debate
whether the formation of the present atmosphere was driven by
vaporization of methane and ammonia from a primordial atmosphere,
by outgassing from Titan's interior, or some combination of the
two. One way that this might be resolved is with the Huygens probe on the Cassini mission to Saturn, due to drop into Titan's
atmosphere in 2004. The Huygens probe has several instruments
designed to study the atmosphere and surface in great detail;
one of these, the Gas Chromatograph and Mass Spectrometer (GCMS)
has the potential to help determine which model for the formation
of the atmosphere is correct.
One of the most-discussed methods for distinguishing between
formation models is the abundance of argon and its isotopes in
the Titanian atmosphere. We know that the mean molecular weight
in the atmosphere is slightly higher than that of pure nitrogen,
so there must be material heavier than nitrogen in the atmosphere;
currently, argon is a strong candidate for this component. However
its origin is ambiguous since it may have either been accreted
with the clathrates out of which the atmosphere was formed, or it
might have been outgassed from Titan's core as a radioactive decay
product of potassium. Therefore the abundance of argon, and more
specifically of each of its isotopes, is linked to the formation
process that generated Titan's atmosphere. The GCMS experiment on
Huygens may be able to distinguish between the two possibilities by
measuring the ratios of the more abundant isotope argon-36, and the
potassium-40 decay product argon-40. If argon-36 is
the primary argon isotope observed on Titan, then it is more likely that
the argon was accreted (since argon-36 is the most cosmically abundant
isotope of argon). If argon-40
is the most abundant, then it is probable that it arose from outgassing
of potassium by volcanic silicates from the core regions, where some of
the potassium would have been radioactive potassium-40. (Engel and
Lunine 1994)
There are several other instruments on board Huygens which may also
help. Engel and Lunine also mention mention the possibility of obtaining
images of the surface with the Descent Imager and Spectral
Radiometer
(DISR). It is possible that volcanic outflows may be visible on the
surface, which would be a strong indicator that outflows may have
played a role in the formation of the atmosphere. The DISR would be
capable of imaging these regions, assuming they would be located in the
region where the Huygens probe will land. In addition, the Surface
Science Package (SSP) may be able to make compositional measurements
of the surface ices. This could determine whether the
surface ice, presumably in nearly the same state as it was when the
primordial oceans froze, is significantly enhanced in methane relative
to ammonia. This could determine whether the global ocean
played a significant role in the formation of the atmosphere.
5: Conclusions
Titan and its atmosphere have been well studied, especially since the
early 1970's and the Voyager missions. Since Voyager, there has been
significant progress in the understanding of Titan's atmosphere, and
there have been several possible explanations for its
formation. Currently, the most favored hypothesis on the origin
of Titan's atmosphere is the outgassing and volcanism theory, where
the atmosphere formed from clathrates either on the surface or in the
core of Titan, during or immediately after the accretion phase
when Titan was still warm. However, it will not be possible to distinguish
between these two theories until we can make measure the composition
of the atmosphere and surface. The Huygens probe may be able to make
this measurement, and answer the question of how Titan's
atmosphere originated.
References
S. Engel and J.I. Lunine, 1994, "Silicate interactions with
ammonia-water fluids on early Titan,"
Journal of Geophysical Research,
vol. 99, no. E2, 3745
D.M. Hunten et al., 1984, "Titan," in
Saturn, University
of Arizona Press, 671
T.D. Jones and J.S. Lewis, 1987, "Estimated impact shock
production of N
2 and organic compounds on early Titan,"
Icarus, 72, 381
G.P. Kuiper, 1944, "Titan: a Satellite with an Atmosphere,"
Astrophysical Journal, 100, 378
J.I. Lunine and D.J. Stevenson, 1987, "Clathrate and ammonia hydrates at high pressure - Application to the origin of methane on Titan,"
Icarus, 70, 61L
C.P. McKay et al., 1988, "High-temperature shock formation of N
2 and organics on primordial Titan,"
Nature, 332, 520
T. Owen, 1982 "The composition and origin of Titan's atmosphere,"
Planetary and Space Science, 30, 833
R.G. Prinn and B. Fegley Jr., 1981, "Kinetic inhibition of CO and
N
2 reduction in circumplanetary nebulae - Implications for satellite
composition,"
Astrophysical Journal, 249, 308
(There are two figures which I obviously can't include, and both are just
pie slices showing the radial structure of Titan at the time of
accretion - undifferentiated rock and ice in the core, a rocky mantle,
and a clathrate crust possibly with an ocean beneath the icy surface layer.
The (possible) oceans and volcanism have long since frozen.)