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Ever since we began to explore the Solar System, we have searched for
another Earth. But one by one, the planets have disappointed us: Mars, whose
‘canals’ once hinted at intelligent beings, is a cold, cratered world, and
Venus, whose clouds might have hidden oceans and life, is far too hot to
have either. Although scientists no longer expect to find intelligent life
on other planets around our Sun, they still want to find worlds like Earth.
Their aim is to understand more about the early history of our planet, before
life began to reshape its surface layers.

Their best candidate lies not among the planets but in their satellites:
Saturn’s moon Titan, a world that at first glance seems anything but earthly.
Titan lies over a billion kilometres from the Sun and has a temperature
of -179 °C. But the temperature is the key; it has frozen the satellite
into the past. Titan has a dense nitrogen atmosphere containing many organic
compounds. Scientists believe that present-day Titan resembles ancient Earth
so much that through it they can study the conditions that fostered the
development of life on Earth.

Titan is appropriately named: at 5150 kilo-metres across, not only is
it the biggest moon of Saturn but it is also the second biggest moon of
the 61 known in the Solar System. Titan dwarfs the Moon (which is 3476 kilometres
in diameter), surpasses the planets Mercury (4878 kilometres) and Pluto
(2300 kilometres), and rivals Mars (6787 kilometres). Titan is also fast,
compared to our Moon. At 1 221 850 kilometres from Saturn, a separation
more than three times the distance between the Moon and Earth, Titan completes
an orbit in about half the time that our Moon takes. Saturn has a mass 95
times bigger than that of Earth, so it forces Titan to orbit it much faster
than our Moon orbits.

Because Titan is so big, it was the first of Saturn’s 18 moons to be
found. The Dutch astronomer Christiaan Huygens discovered Titan in 1655,
but astronomers learned little about the world until the 20th century. Titan
may be a big moon, but it is still a distant member of the outer Solar System.

Titan’s most remarkable feature is neither its size nor its distance
but its atmosphere. Titan is the only moon with a dense atmosphere, and
we now know that this atmosphere is strikingly similar to ours. The first
hint of an atmosphere came in 1908, when Spanish astronomer Jose Comas Sola
reported that the edges of Titan looked darker than its centre. This phenomenon
suggests an atmosphere that can absorb enough light to give the body fuzzy
edges. But no one knows if Sola really observed this effect. He also reported
seeing clouds on some of Jupiter’s moons, which we now know are bare worlds
of rock and ice.

The first definite proof that Titan had an atmosphere came in 1944,
when the American astronomer Gerard Kuiper analysed the spectrum of sunlight
reflected from Titan. Kuiper noticed that certain wavelengths present in
sunlight reaching Earth were absent in the light that Titan reflected. The
missing parts of the spectrum matched those absorbed by the gas methane
in experiments, so Kuiper concluded that Titan was surrounded by methane
– making it the first satellite known to have an atmosphere.

Methane is a common gas in the outer Solar System: by the 1930s, astronomers
had discovered it in the atmospheres of Jupiter, Saturn, Uranus and Neptune.
But Titan’s atmosphere was nothing like the thick layers of gas in the giant
planets. If methane were the only gas, Kuiper’s work implied that the atmospheric
pressure on Titan was perhaps 1 per cent of that on Earth, similar to Mars.

The methane atmosphere that Kuiper discovered was nonetheless a novelty,
for it surrounded not a planet but a moon. Planets such as Earth can easily
hold on to the gases in their atmospheres, for they are big enough to have
strong gravity. But how can an atmosphere exist around Titan when a planet
such as Mercury has almost none? Mercury is slightly smaller than Titan,
but it is also denser, so its gravity is greater and it should be able to
hold onto gases better.

Kuiper himself supplied the answer: the ability of a body to retain
gases depends not only on the object’s gravity but also on its temperature.
A cold body like Titan can retain an atmosphere more easily than a hot body
like Mercury: the lower the temperature, the more slowly the molecules in
the atmosphere move, and so the more easily the planet or satellite can
hold onto them. Titan’s low temperature more than compensates for its weaker
gravity.

The picture of Titan that emerged from Kuiper’s work was captured in
vivid form by American artist Chesley Bonestell in 1961. Bonestell’s painting
shows the frigid surface of Titan beneath a huge crescent Saturn suspended
in the dark sky. Toward the horizon, the sky is lighter, suggesting a thin
atmosphere. On Titan’s surface, a brown-red mountain peak, partially covered
with snow, dominates the scene, while methane fog blankets the valley below.

Sunshine and clouds

Methane is the first step towards the formation of complex organic molecules.
When sunlight strikes methane, it forms bigger hydrocarbons, such as ethane
and acetylene (ethyne). Sure enough, astronomers detected some of these
hydrocarbons during the 1970s. And they wondered whether Titan might have
even more complicated organic compounds. Titan is an orange world, quite
a different colour from yellow Saturn. The complex organic compounds that
sunlight creates are also orange, so scientists began to believe – correctly,
as it turned out – that Titan’s colour arises from complex organic compounds.

Astronomers also found evidence of clouds on Titan. Joseph Veverka of
Cornell University and Benjamin Zellner, then at the University of Arizona,
independently observed that sunlight reflected from Titan was polarised.
Sunlight is un-polarised, but reflections and refractions within a cloudy
atmosphere produce light polarised in a distinctive way. The light Veverka
and Zellner observed was polarised, suggesting that Titan has clouds.

As astronomers assimilated these and other observations during the 1970s,
two very different models emerged for Titan’s atmosphere. One model, devised
by Robert Danielson and John Caldwell of Princeton University, called for
Titan to have a methane atmosphere with a pressure 2 per cent that of Earth.
Though thin by our standards, it was nonetheless thicker than the atmosphere
of Mars.

A radically different model was proposed by Donald Hunten of the University
of Arizona. Hunten believed that Titan had a dense atmosphere of nitrogen,
the gas that makes up 78 per cent of Earth’s atmosphere. Methane was only
a minor constituent. No one had ever found nitrogen on Titan, but Hunten
believed that was because this gas, an absorber of ultraviolet rather than
visible or infrared radiation, is so difficult to detect from Earth. Hunten
put the pressure of this nitrogen atmosphere at 20 times the atmospheric
pressure on Earth.

The two different models led to predictions of different temperatures
for Titan’s surface. If Titan had only a thin atmosphere, then it must be
quite cold, because the satellite lies far from the warmth of the Sun. If,
on the other hand, there was a thick atmosphere, it would be warmer, for
the gases would trap what little heat Titan receives from the Sun.

During the 1960s and 1970s, astronomers used infrared and radio frequency
observations to deduce Titan’s temperature, but their results disagreed
with one another. In the late 1970s, instrumental improvement led to the
most trustworthy measurement – which later turned out to be correct. It
showed that Titan is quite cold, about as cold as it should be for a world
so far from the Sun. This result argued against the thick nitrogen atmosphere,
and nearly all astronomers believed that Titan had only a thin methane atmosphere.

When spacecraft reached Saturn, they proved this consensus wrong. The
first spacecraft to reach Saturn was Pioneer 11. Launched in 1973, Pioneer
11 flew past Jupiter in 1974 and Saturn in 1979. Pioneer discovered a new
ring around Saturn and nearly collided with an uncharted moon. But the few
pictures Pioneer took of Titan showed little detail.

The next year, though, the far more sophisticated Voyager 1 spacecraft
reached Saturn. Launched in 1977, Voyager 1 sailed past Saturn on 12 November
1980. Even as it returned stunning images of Saturn and its rings, scientists
waited with great excitement for Voyager’s close flyby of Titan. The spacecraft
passed just 4000 kilometres from Titan, a distance less than the satellite’s
diameter, and nearly everything we now know about Titan came from this encounter.
Voyager 2 also flew past Saturn, in 1981, but it passed far from Titan and
provided little new or surprising information.

Despite Voyager 1’s close approach, Titan was a huge disappointment.
Picture after picture showed the moon wrapped in orange haze and clouds.
No break in the clouds appeared, and no view emerged of the satellite’s
surface. The only thing Voyager’s cameras showed was that clouds in the
northern hemisphere were slightly darker than those in the south.

The exciting results came not from Voyager’s cameras but from instruments
that probed the cloudy atmosphere. To the surprise of nearly everyone involved,
Voyager discovered that Titan’s atmosphere is dense, as Hunten had predicted.
The atmospheric pressure is 1.5 times that on Earth, so the atmosphere is
much thinner than Hunten had predicted. This explains why Titan is so cold;
its atmosphere can trap little heat from the Sun. But Voyager vindicated
Hunten’s other prediction, for it found that nitrogen makes up somewhere
between 82 and 99 per cent of the atmosphere.

The spacecraft also found the methane that had been detected from Earth
nearly 40 years before, but its measurements showed that methane accounts
for no more than a few per cent of the atmosphere. In addition, Voyager
found several previously undetected hydrocarbons as well as hydrogen cyanide,
one of the building blocks of amino acids. The organic compounds that give
the moon its orange colour turned out to be similar to those in the photochemical
smog that plagues Earth’s major cities. But Voyager could find no oxygen.

Nonetheless, the atmosphere Voyager discovered is astonishing. Though
over a billion kilometres from the Sun, Titan’s atmosphere resembles air
on Earth, with its 78 per cent nitrogen, more than any other atmosphere
in the Solar System. Earth’s nearest neighbours are very different. Venus
has a far thicker atmosphere than ours, whereas Mars’s atmosphere is far
thinner, and both are primarily carbon dioxide rather than nitrogen. The
only other known nitrogen atmosphere surrounds Neptune’s moon Triton, but
there the air is much thinner than on either Earth or Titan. Pluto may also
have a nitrogen atmosphere, but it is probably even more tenuous.

How did an atmosphere so like Earth’s develop around such a distant
world? Jupiter and Saturn contain enormous amounts of hydrogen and helium,
which they acquired at birth. All the planets arose from a spinning disc
of gas and dust that surrounded the newborn Sun. This disc was mostly hydrogen
and helium, and the giant planets including Jupiter and Saturn grabbed plenty
of these gases for themselves.

Titan could never do that. If it ever took hydrogen and helium from
the primordial solar disc, they are now long gone, because Titan is too
small to hold onto such light gases. Nitrogen, though, is heavier. A nitrogen
molecule weighs 14 times more than a hydrogen molecule. There was enough
in the solar disc to supply this atmosphere, so could Titan have obtained
its nitrogen at birth, from the primordial cloud of gas?

Astronomers think not. They argue that the element neon was about as
abundant in the solar disc as nitrogen, and neon is heavy enough for Titan
to retain. If Titan took its nitrogen from the primordial disc, it should
also have grabbed plenty of neon at the same time. This neon should still
exist as a gas in Titan’s atmosphere today, for neon does not combine with
other atoms nor does it become liquid or solid at Titan’s temperature. But
Voyager did not find neon in Titan’s atmosphere.

There are two other possible sources of Titan’s nitrogen. One is that
as Titan was forming, nitrogen gas from the solar disc was trapped in the
ice that created Titan. This is plausible, because Titan’s interior is about
half water ice and half rock. Nitrogen can be held within the crystal lattice
of ice in the crystalline form known as a clathrate. Since Titan’s birth,
this nitrogen gas could have leaked out of the ice, creating the dense atmosphere.
There is no neon in Titan’s air, because neon, unlike nitrogen, is not easy
for ice to trap.

Titan may also have received its nitrogen indirectly, from ammonia.
There is ammonia in the atmospheres of both Jupiter and Saturn, and Titan
itself may have acquired ammonia when it formed. Over time, sunlight broke
the ammonia molecules into nitrogen and hydrogen atoms. Because they were
light, the hydrogen atoms escaped, leaving Titan with a nitrogen atmosphere.
This scenario has important implications for Titan’s history, because this
reaction would not happen at the temperatures existing there today. To make
nitrogen molecules, Titan must have been about 50 degrees warmer at some
stage in its life.

Astronomers are currently debating which scenario best explains Titan’s
abundance of nitrogen. Both processes may contribute; some of Titan’s nitrogen
may have been trapped in its ice at birth and then escaped into the atmosphere,
while the rest may have come from ammonia. One way to test for the former
process would be to measure the argon present in Titan’s atmosphere. Although
neon would not have formed a clathrate, argon could, and so should form
a few per cent of the atmosphere if the nitrogen comes from ice, and maybe
a thousand times less if ammonia is the source.

Perhaps the most intriguing facet of Titan’s atmosphere is that it completely
hides the surface of the moon. The only thing we know for certain about
the surface is that it must be dark. Titan is about 10 times farther from
the Sun than Earth, so sunlight striking Titan’s atmosphere has only 1 per
cent of the intensity of sunlight here on Earth. The orange haze and clouds
further attenuate the light, and Titan’s Sun might well look no brighter
than the Moon does on Earth.

The hydrocarbons in Titan’s atmosphere suggest that there will also
be hydrocarbons on the surface. Sunlight striking methane creates the more
complex organic compounds that colour the satellite orange – Titan’s smog.
Many of these organic compounds are so heavy that they sink to the surface.
Some astronomers have then suggested that these compounds fall into a huge
ethane ocean that covers all Titan.

Why ethane? When sunlight strikes methane, the most common product is
ethane. Once created, the ethane sinks to the surface as a liquid, like
rain. Over billions of years, this ethane could accumulate to a depth of
roughly a kilometre and produce a huge ocean. Such an ethane ocean would
solve a puzzle: why does Titan have methane in its atmosphere? After all,
sunlight continually destroys methane, so there should not be any left.
But if the ethane ocean has methane dissolved in it, then Titan has a vast
reservoir of methane that can supply the gas as it is destroyed in the atmosphere.

Another Earth?

Despite its appeal, recent radar results rule against such a Titanic
ocean. In 1990, Duane Muhleman of the California Institute of Technology
in Pasadena reported that the variation in radar signals he and his colleagues
had bounced off Titan indicate that Titan cannot be covered entirely with
liquid. Instead, the scientists found variations in Titan’s topography.
This work does not rule out small rivers and lakes of methane and ethane,
but it does indicate that the whole surface cannot be covered with liquid.

Though the nature of Titan’s surface is a mystery, the composition of
the world itself is not. About half of Titan’s mass is rock, and about half
is water ice. The rock, being denser, forms the satellite’s core, while
the lighter ice surrounds the core and makes a mantle. Astronomers know
Titan’s composition because its density – 1.88 grams per cubic centimetre
– is about halfway between the density of rock and the density of water
ice. Titan’s density is similar to that of Jupiter’s moons Ganymede (1.94
grams per cubic centimetre) and Callisto (1.86 grams per cubic centimetre).

Unlike those much simpler worlds, however, Titan offers a unique glimpse
into the past of our own planet and the conditions that prevailed on Earth
before life arose. Like Titan, ancient Earth had complex organic compounds
on its surface, and next to no oxygen in its atmosphere. Though we humans
need oxygen to live, the first life arose without.

If Titan resembles ancient Earth, why has it not advanced to Earth-like
status? Why, after four and a half billion years, has no life arisen? The
answer is that there is no liquid water on Titan. Liquid water is vital
for life, making up, after all, most of us. Moreover, liquid water is an
excellent medium for chemical reactions. For example, if you threw Titan’s
complex organic compounds into liquid water, you would create amino acids,
the first stage to making protein.

Ironically, of course, Titan does have lots of water, because the satellite’s
interior is about half ice. But at -179 °C this water stays frozen.
Titan’s great distance from the Sun has held the satellite into a ‘pre-life’
stage. Imagine the ultimate scientific experiment: drag Titan closer to
the Sun, where the satellite warms up, the water ice melts, and we have
a chance to watch the chemical reactions that may create life.

To study this ancient Earth, scientists from NASA and ESA are preparing
a new mission to Saturn. Named for the astronomer who studied Saturn’s rings
and discovered four of its moons, the Cassini mission will place a spacecraft
into orbit around the planet. Attached to the spacecraft’s side will be
a probe to study Titan, appropriately named Huygens.

Following its launch in 1996, Cassini will travel a complicated trajectory
to reach Saturn. It will go around the Solar System and pass Earth, using
our gravity to head for Jupiter, whose gravity will then send it out to
Saturn. Cassini’s long trajectory means a long flight time, with arrival
at Saturn slated for the next century, late in the year 2002.

Cassini will take twice as long to reach Saturn as Voyager 1 did. But
the two missions are not comparable: Voyager 1 merely flew past Saturn,
whereas Cassini will perform the more difficult task of orbiting the planet.
For this reason, Cassini must carry more fuel than Voyager did, making the
spacecraft much heavier. When it arrives at Saturn, Cassini will use this
fuel to accelerate into orbit around the planet.

But Saturn is definitely worth the trip. Because it will orbit Saturn
rather than fly past, Cassini can photograph Saturn, its rings and its moons
for many years. One of Cassini’s prime targets will be Titan. After it arrives
at Saturn, Cassini will hurl the Huygens probe toward Titan. The probe will
fall through the atmosphere for several hours, determining composition,
density, and temperature.

The probe also carries a camera. After it breaks through the clouds
and before it lands, this camera should return a few photographs of Titan’s
hidden surface. We may finally see whether or not Titan has seas, lakes
or even oceans. We may also see mountains and valleys on the surface.

The probe may or may not survive its landing, since we do not know whether
it will land in a lake or on solid ground. Even if the probe survives, though,
it will probably not tell us much more, for the probe can communicate with
Earth only by transmitting to the main Cassini spacecraft. Huygens has a
limited range; once Cassini is out of reach, perhaps a few minutes after
the hoped-for landing, the probe cannot send data to Earth.

But the probe is not our only hope. The Cassini spacecraft will send
radar signals through Titan’s haze and bounce them off the surface. Just
as Magellan is now mapping the surface of Venus through opaque clouds, Cassini
will scrutinise the surface of Titan. By giving us our first thorough look
at Titan, Cassini will reveal not only a distant world of the outer Solar
System but also a close cousin to ancient Earth.

Ken Croswell is an astronomer and science writer based in California.