Saturn's moon Titan belongs to a really select club within the solar system. It is among only four "terrestrial" planets or moons—those through solid bodies, as opposed to those made mainly of gas, choose Jupiter and also Saturn—that has a an extensive atmosphere. The various other three the wear blankets that gas space Venus, Mars, and also our own Earth.

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Why just these four? Why not additionally Mercury, or Jupiter's greatest moons, or our own moon? just how did those lucky 4 come by your atmospheres?


It turns out that obtaining an atmosphere, and also holding on to it, yes, really comes under to how large and just how close to the sun you are—or, because that Titan, exactly how close you room to a really big planet. For astrophysicists, it's infinitely more complex than that. Yet if you just want the quick and also dirty answer, that's it, and here's why:


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how did Saturn's moon Titan certain an atmosphere when no various other moons in the solar mechanism did? The prize lies mainly in the size and also location. Here, Titan as imaged in may 2005 by the Cassini spacecraft from about 900,000 mile away.

Original gas

The story of planetary environments begins back at the beginning of our solar system, as soon as the planets were forming. During that period, the so-called inner planets—Mercury, Venus, Earth, and also Mars—all occurred the very same kind that air, a so-called main atmosphere. It consisted mostly of hydrogen and also helium, the two aspects that today make up 98 percent that the sun and gas giants like Jupiter.

Like planet-sized magnets, the proto-planets had enough gravity to attract these two gaseous facets in from the solar nebula, the large cloud that gas and also dust that surrounding the sun early in the solar system's history. In that primordial time, the sunlight was not really bright and also thus not very hot, and also this permitted the four inner planets to host onto those atmospheres.

Three factors play into a gas's capacity to escape the traction of a planet's gravity: temperature, molecule mass, and also escape velocity, the speed a molecule demands to attain to escape into space. Hotter, lighter, and also faster particles an ext easily slip the end of a planet's gravitational grip into room than cooler, heavier, and also slower particles.

Hydrogen and also helium room two of the lightest molecular-weight molecules the end there. And as the sun grew brighter and also hotter, the molecules of hydrogen and helium the the 4 inner planets had actually been able come retain came to be hotter and faster, lastly reaching escape velocity. As soon as that happened, probably within a couple of hundred million years after the development of the inner planets, these gases escaped into space, leaving Earth and also its 3 companions little an ext than balls of rock in space.


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The 4 giant external planets, meanwhile—Jupiter, Saturn, Uranus, and also Neptune—were maybe to keep their hydrogen and also helium because of their size. Your gravitational traction is mighty sufficient to contain those two light gases, and also the sunlight is too much away because that its heat to make any kind of difference. So those 4 gas giants still hold their primary atmospheres.

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notwithstanding its rocky core, one might say that Saturn, seen here in picture taken by the Voyager 2 spacecraft, is nothing however atmosphere, prefer its other "gas giants" Jupiter, Uranus, and also Neptune.
Courtesy NASA/JPL/Space science Institute

Putting top top air

Fortunately because that us, over there are second atmospheres, otherwise us wouldn't be here. These are environments that arise long after a planet's primary environment has vanished right into the ether. Yet not all rocky bodies have actually the way to sustain them. Mercury, because that one, is also close come the sunlight to host onto any type of gas. So how did the four solid body that have actually them success the atmospheric lottery?

Leaving Titan aside for the moment, Earth, Mars, and also Venus all began emerging their an additional atmospheres in the same way. Gradually their envelopes of air would end up being as unlike as heaven and also hell—in the instance of Earth and Venus, because that example—but at first they likely appeared largely the same. The reason is that, despite their distinctions today, these 3 planets lie in about the same ar of the solar system and also are thought to consists of approximately the same mix of elementary school stuff.

Earth ended up being heavenly, Mars froze solid, and all hell broke loosened on Venus. What happened?

While Earth, Mars, and Venus eventually got come the suggest where they might no longer embrace hydrogen and helium, they walk have sufficient gravity and cool enough surface temperature to retain more heavier molecular-weight gases favor carbon dioxide and also water vapor. And also they had actually plenty that these 2 substances stored away in one type or one more within your stony bodies. The CO2 and H2O come from two sources: the original building blocks the end of which the planets formed as well as comets that on regular basis slammed right into the planets early in their history.

Fortunately, again, because that us, these crucial substances that CO2 and H2O—and likewise nitrogen, which comprises 78 percent of our atmosphere—were not irretrievably locked in the rocks. This substances had a catalyst that helped free them: heat. Within every planet, a molten core produced during the planet's initial development released heat, and also so did the slow decay of radioactive aspects deep in ~ the surface. This warmth kept each earth toasty sufficient to produce volcanic eruptions, i m sorry spewed these gases out of the interior.

Despite raised warmth native the sun, these more heavier molecules might not to escape the gravity of Earth, Mars, and also Venus, respectively, and so they began building up just above each planet's surface. The an outcome was a second atmosphere—or what many of us understand simply together the air.

But, in time, Earth ended up being heavenly, Mars froze solid, and also all hell broke loosened on Venus. What happened?

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these clouds, photographed top top Mars by the Viking 1 lander, room not condensed water vapor as they would certainly be on Earth yet condensed carbon dioxide. Any type of water long due to the fact that froze the end of the atmosphere and is currently locked as ice beneath the Red Planet's surface.

From sky to hell

This is where the how-close-you-are-to-the-sun component comes in. ~ above Earth, all that water vapor belched out of volcanoes condensed in the young setting into fluid water, then dropped to the surface ar as rain. End eons, this created the oceans. Most of the CO2, meanwhile, became incorporated right into the seas and also into sedimentary rocks. Most, yet not all, and also this is crucial. Sufficient CO2 continued to be as gas in the setting to create the greenhouse effect, i m sorry maintains our planet at a life-sustaining average worldwide temperature of about 59°F. Everything eased into a wonderful balance, all brought around by our appropriate distance from the sun.

As for Mars, its second atmosphere had actually two strikes against it native the start: the planet's size (too small) and its street from the sun (too far). In its an initial 500 million years or so, the Red Planet had actually a heat atmosphere and also liquid-water oceans, similar to Earth. But Mars is so small that the internal heat engine melted out at an early stage on, and it is so far away native the sunlight that all the water vapor the its once-active volcanoes had erupted eventually froze the end of the atmosphere, becoming trapped beneath the surface as ice. All this left the Red planet as cold and also barren and also apparently lifeless as the moon. Mars still has actually an atmosphere, but its press is 100 times much less than Earth's and also it's virtually entirely composed of CO2—about the last point we'd want to breathe.

Venus has approximately the exact same concentration of CO2 as Mars, yet its environment went in precisely the the contrary direction. Size wasn't an issue: Venus has around the very same mass as planet so is plenty warm within. Yet distance from the sun has actually made all the difference. Venus is near sufficient to our star that all the water vapor exit from that volcanoes shed off lengthy ago, and without liquid water, the planet could not kind oceans that could absorb the CO2.

The an outcome has to be a runaway greenhouse effect. If a greenhouse effect raises the temperature that Mars by about 5°F and Earth by around 35°F, ~ above Venus it has actually jacked up the temperature by roughly 500°F. The resulting environment is truly nasty from our perspective: hotter 보다 a self-cleaning oven, with a density about 10 percent the of water and also a pressure about what you'd feel a half-mile under in the ocean.

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Venus is a heating system of a planet, with a noxious atmosphere bearing a pressure 90 times the on Earth.

A moon with atmosphere

And what about Titan? Why go it gain an environment when, because that example, nobody of Jupiter's huge moons, which space a lot of closer to the sun, did? Well, in this case, street from the sunlight doesn't yes, really come right into it; the moons of the external planets are so much away that it's a moot point. However distance does aspect in—distance come a large planet. And, again, dimension matters. In fact, a moon requirements the ideal balance that nearness come a huge neighbor and adequate gravity—that is, size—to gain and hold an atmosphere, and of all the moons in the solar system, just on Titan walk Nature strike that balance.

Clearly environments can change drastically—look in ~ Mars.

Titan is close enough to Saturn that it gets squeezed through tidal forces powerful enough to heat up the interior. Therefore the volcanic activity that long back died out, because that instance, ~ above our likewise sized moon has ongoing there. That activity releases CO2 and water vapor, but since Titan's mean surface temperature is -289°F, both that those conveniently fall out as ice top top the surface. That leaves nitrogen, which stays a gas at that temperature, and also methane, which builds up in one interaction between sunlight and CO2 ice. The result is an atmosphere that is around 90 percent nitrogen and also 7 percent methane. (Interestingly, together radically various as Titan's setting is to our own, that is still people closer in composition and also pressure come Earth's nitrogen-rich air than room the CO2-dominant settings of one of two people Mars or Venus.)

Saturn makes Titan's gases come out; Titan's dimension ensures few of them stick approximately in an atmosphere. Jupiter's moon Io, being so close to its humungous neighbor, has actually plenty of volcano activity, however the moon's massive is too small to wield the kind of gravity needed to keep a host on the gases the gush the end of that insides.

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when the air on both Mars and Venus is end 95 percent carbon dioxide, atmospheric CO2 on our planet amounts to just 0.03 percent—just sufficient to provide us a pleasant an international average temperature of around 59°F.

Up in the air

Some atmospheric researchers say the the various tacks the 4 terrestrials with environments took need to offer a cautionary story to united state as we unintentionally monkey through ours. By burning fossil fuels, we are releasing far much more CO2 right into the setting than Nature has done anytime in the current geologic past—an atmosphere that has been likened in thinness to a dollar bill wrapped approximately a standard-sized globe. This might upset the exquisite equilibrium between carbon in the air and carbon in the rocks and also seas the our planet has maintained come one degree or an additional for billions the years, v unknown yet potentially dire consequences.

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Clearly environments can change drastically—look in ~ Mars. Whether we humans might ever severely or permanently change our own environment is unknown, but some professionals are currently asking, carry out we really want to take the chance?


Editor's Notes

This function originally showed up on the site for the NOVA regime Voyage come the an enig Moon.