How to Get an Atmosphere
By Peter Tyson
Posted 04.04.06
NOVA
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 Mercury, or
Jupiter's biggest moons, or our own 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
How did Saturn's moon Titan secure an atmosphere when no
were forming. During that period, the so-called inner
other moons in the solar system did? The answer lies largely in its
planets—Mercury, Venus, Earth, and Mars—all
size and location. Here, Titan as imaged in May 2005 by the
developed the same kind of air, a so-called primary
Cassini spacecraft from about 900,000 miles away. Enlarge Photo
atmosphere. It consisted mostly of hydrogen and
credit: Courtesy NASA/JPL/Space Science Institute
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
Notwithstanding its rocky core, one might say that
the means to sustain them. Mercury, for one, is too close
Saturn, seen here in an image taken by the Voyager 2
to the sun to hold onto any type of gas. So how did the
spacecraft, is nothing but atmosphere, like its fellow
four solid bodies that have them win the atmospheric
"gas giants" Jupiter, Uranus, and Neptune. Enlarge
lottery?
Photo credit: Courtesy NASA/JPL/Space Science Institute
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.
These clouds, photographed on Mars by the Viking 1
Fortunately, again, for us, these crucial substances of CO 2
lander, are not condensed water vapor as they would
and H2O—and also nitrogen, which comprises 78 percent
be on Earth but condensed carbon dioxide. Any water
of our atmosphere—were not irretrievably locked in the
long since froze out of the atmosphere and is now
rocks. These substances had a catalyst that helped free
locked as ice beneath the Red Planet's surface.
them: heat. Within each planet, a molten core created
Enlarge Photo credit: Courtesy NASA
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, which maintains our planet at 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.
Venus is a furnace of a planet, with a noxious
atmosphere bearing a pressure 90 times that on
Earth. Enlarge Photo credit: Courtesy NASA/JPL
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.
Clearly atmospheres can change drastically—look at Mars.
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 CO 2 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 CO2dominant 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 CO 2 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-sized
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.
While the air on both Mars and Venus is over
Whether we humans could ever severely or permanently
95 percent carbon dioxide, atmospheric CO2 on
alter our own atmosphere is unknown, but some experts are
our planet amounts to just 0.03 percent—just
now asking, Do we really want to take that chance?
enough to give us a pleasant global average
temperature of about 59°F. Enlarge Photo credit:
Courtesy NASA