Saturn's moon Titan belongs to a very select club within the solar system. It
is one of only four "terrestrial" planets or moons—those with solid
bodies, as opposed to those made largely of gas, like Jupiter and
Saturn—that has a substantial atmosphere. The other three that wear
blankets of gas are Venus, Mars, and our own Earth.
Why just these four? Why not also, say, Mercury, Jupiter's biggest moons, our
moon? How did those lucky four come by their atmospheres?
It turns out that getting an atmosphere—and holding on to it—really
comes down to how big and how close to the sun you are (or, for Titan, how
close you are to a really big planet). For astrophysicists, it's infinitely
more complex than that, but if you just want the quick and dirty answer, that's
it, and here's why:
Original gas
The story of planetary atmospheres begins back at the beginning of
our solar system, when the planets were forming. During that period, the
so-called inner planets—Mercury, Venus, Earth, and Mars—all
developed the same kind of air, a so-called primary atmosphere. It
consisted mostly of hydrogen and helium, the two elements that today make up 98
percent of the sun and gas giants like Jupiter.
Like planet-sized magnets, the proto-planets had sufficient gravity to draw
these two gaseous elements in from the solar nebula, the vast cloud of gas and
dust that surrounded the sun early in the solar system's history. In that
primordial time, the sun was not very bright and thus not very hot, and this
allowed the four inner planets to hold onto those atmospheres.
Three factors play into a gas's ability to
escape the pull of a planet's gravity: temperature, molecular mass, and escape
velocity (the speed a molecule needs to achieve to escape into space). Hotter, lighter, and faster particles more easily slip out of a
planet's gravitational grip into space than cooler, heavier, and slower
particles.
Hydrogen and helium are two of the lightest molecular-weight molecules out
there. And as the sun grew brighter and hotter, the molecules of hydrogen and
helium that the four inner planets had been able to retain became hotter and
faster, finally reaching escape velocity. When that happened, perhaps within a
few hundred million years after the formation of the inner planets, these gases
escaped into space, leaving Earth and its three companions little more than
balls of rock in space.
The four giant outer planets, meanwhile—Jupiter, Saturn, Uranus, and
Neptune—were able to keep their hydrogen and helium because of their
size: their gravitational pull is mighty enough to contain those two light
gases, and the sun is too far away for its heat to make any difference.
So those four gas giants still host their primary atmospheres.
Putting on air
Fortunately for us, there are secondary atmospheres, otherwise we
wouldn't be here. These are atmospheres that arise long after a planet's
primary atmosphere has vanished into the ether. Yet not all rocky bodies have
the means to sustain them. (Mercury, for one, is too close to the sun to hold
onto any type of gas.) How did the four solid bodies that have them win the
atmospheric lottery?
Leaving Titan aside for the moment, Earth, Mars, and Venus all began developing
their secondary atmospheres in the same way. Over time their envelopes of air
would become as unlike as heaven and hell—in the case of Earth and Venus,
for example—but initially they likely appeared largely the same. The
reason is that, despite their differences today, these three planets lie in
roughly the same neighborhood of the solar system and are thought to consist of
roughly the same mix of elementary stuff.
Earth became heavenly, Mars froze solid, and all hell broke loose on
Venus. What happened?
While Earth, Mars, and Venus eventually got to the point where they could no
longer embrace hydrogen and helium, they did have sufficient gravity and cool
enough surface temperatures to retain heavier molecular-weight gases like
carbon dioxide and water vapor. And they had plenty of these two substances
stored away in one form or another within their stony bodies. The
CO2 and H2O came from two sources: the original building
blocks out of which the planets formed as well as comets that regularly slammed
into the planets early in their history.
Fortunately, again, for us, these crucial substances of CO2 and
H2O—and also nitrogen, which comprises 78 percent of our
atmosphere—were not irretrievably locked in the rocks. These substances
had a catalyst that helped free them: heat. Within each planet, a molten core
created during the planet's initial formation released heat, and so did the slow
decay of radioactive elements deep beneath the surface. This heat kept each
planet toasty enough to produce
volcanic eruptions, which spewed these gases out of the interior.
Despite increased warmth from the sun, these heavier molecules could not escape
the gravity of Earth, Mars, and Venus, respectively, and so they began building
up just above each planet's surface. The result was a secondary
atmosphere—or what most of us know simply as the air.
But, in time, Earth became heavenly, Mars froze solid, and all hell broke loose
on Venus. What happened?
From heaven to hell
This is where the how-close-you-are-to-the-sun part comes in. On Earth, all
that water vapor belched out of volcanoes condensed in the young atmosphere
into liquid water, then fell to the surface as rain. Over eons, this formed the
oceans. Most of the CO2, meanwhile, became incorporated into the
seas and into sedimentary rocks. Most, but not all, and this is crucial: enough
CO2 remained as gas in the atmosphere to create the greenhouse
effect that keeps our planet a life-sustaining average global temperature of
about 59°F. Everything eased into a wonderful balance—all brought
about by our ideal distance from the sun.
As for Mars, its secondary atmosphere had two strikes against it from the
start: the planet's size (too small) and its distance from the sun (too far).
In its first 500 million years or so, the Red Planet had a warm atmosphere and
liquid-water oceans, just like Earth. But Mars is so small that its internal
heat engine burned out early on, and it is so far away from the sun that all
the water vapor that its once-active volcanoes had erupted eventually froze out of
the atmosphere, becoming trapped beneath the surface as ice. All this left the
Red Planet as cold and barren and apparently lifeless as the moon. Mars still
has an atmosphere, but its pressure is 100 times less than Earth's and it's
almost entirely composed of CO2—about the last thing we'd want
to breathe.
Venus has roughly the same concentration of CO2 as Mars, yet its
atmosphere went in precisely the opposite direction. Size wasn't an issue:
Venus has about the same mass as Earth so is plenty hot within. But distance
from the sun has made all the difference. Venus is near enough to our star that
all the water vapor released from its volcanoes burned off long ago, and
without liquid water, the planet could not form oceans that could absorb the
CO2.
The result has been a runaway greenhouse effect. While a greenhouse effect
raises the temperature of Mars by about 5°F and Earth by about 35°F,
on Venus it has jacked up the temperature by around 500°F. The resulting
atmosphere is truly nasty from our perspective: hotter than a self-cleaning
oven, with a density about 10 percent that of water and a pressure about what
you'd feel a half mile down in the ocean.
A moon with atmosphere
And what about Titan? Why did it get an atmosphere when, for example, none of
Jupiter's big moons, which are a lot closer to the sun, did? Well, in this
case, distance from the sun doesn't really come into it; the moons of the outer
planets are so far away that it's a moot point. But distance does factor
in—distance to a giant planet. And, again, size matters. In fact, a moon
needs the right balance of nearness to a giant neighbor and adequate
gravity—that is, size—to gain and hold an atmosphere, and of all
the moons in the solar system, only on Titan did Nature strike that balance.
Whether we humans could ever severely or permanently alter our own
atmosphere is unknown, but do we really want to take that chance?
Titan is close enough to Saturn that it gets squeezed by tidal forces powerful
enough to heat up its interior. So the volcanic activity that long ago died
out, for instance, on our similarly sized moon, has continued there. That
activity releases CO2 and water vapor, but since Titan's mean
surface temperature is -289°F, both of those quickly fall out as ice on
the surface. That leaves nitrogen, which remains a gas at that temperature, and
methane, which builds up in an interaction between sunlight and CO2
ice. The result is an atmosphere that is roughly 90 percent nitrogen and 7
percent methane. (Interestingly, as radically different as Titan's atmosphere
is to our own, it is still worlds closer in composition and pressure to Earth's
nitrogen-rich air than are the CO2-dominant atmospheres of either
Mars or Venus.)
Saturn makes Titan's gases come out; Titan's size ensures some of them stick
around in an atmosphere. Jupiter's moon Io, being so close to its humungous
neighbor, has plenty of volcanic activity, but the moon's mass is too small to
wield the kind of gravity needed to maintain a hold on the gases that gush out
of its insides.
Up in the air
Some atmospheric scientists say that the different tacks the four terrestrials
with atmospheres took should offer a cautionary tale to us as we
unintentionally monkey with ours. By burning fossil fuels, we are releasing far
more CO2 into the atmosphere than Nature has done anytime in the
recent geologic past—an atmosphere that has been likened in thinness to a
dollar bill wrapped around a standard globe. This may upset the exquisite
equilibrium between carbon in the air and carbon in the rocks and seas that our
planet has maintained to one degree or another for billions of years, with
unknown but potentially dire consequences.
Clearly atmospheres can change drastically: look at Mars. Whether we humans
could ever severely or permanently alter our own atmosphere is unknown, but
some experts are now asking: do we really want to take that chance?
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How did Saturn's moon
Titan secure an atmosphere when no other moons in the solar system did? The
answer lies largely in its size and location. Here, Titan as imaged in May 2005
by the Cassini spacecraft from about 900,000 miles away.
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Notwithstanding its rocky core, one
might say that Saturn, seen here in an image taken by the Voyager 2 spacecraft,
is nothing but atmosphere, like its fellow "gas giants" Jupiter, Uranus,
and Neptune.
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While the air on both Mars and Venus
is over 95 percent carbon dioxide, atmospheric CO2 on our planet
amounts to just 0.03 percent—just enough to give us a pleasant global
average temperature of about 59°F.
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These clouds, photographed on Mars
by the Viking 1 lander, are not condensed water vapor as they would be on Earth
but condensed carbon dioxide. Any water long since froze out of the atmosphere
and is now locked as ice beneath the planet's surface.
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Venus is a furnace of a planet,
with a noxious atmosphere bearing a pressure 90 times that on Earth.
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