|MadSci Network: Astronomy|
The existence of an atmosphere on a planet depends on a lot of factors other than just a planet’s size. For example, Venus and the Earth are about the same size (Venus is slightly smaller), yet Venus has an atmosphere with about 100 times the pressure of Earth’s atmosphere. Titan has an atmosphere, yet worlds of roughly the same mass such as Ganymede (satellite of Jupiter) and our own Moon have little to no atmospheres.
Atmospheres are formed either as gas which is gravitationally attracted onto a planet during the formation of the solar system (such as the atmospheres of the gas giant planets, like Jupiter), or atmospheres may be formed as a result of “outgassing.” Outgassing is the escape of gases from the interior of a planet after the planet has formed. The atmosphere on Earth is primarily composed of gas has been belched out of volcanoes (escape of gas is what makes volcanic eruptions so dramatic!), and these gases have chemically evolved into the air which you are breathing right now (if you’ve ever been near a volcano, you know that the gases coming straight out of the Earth smell pretty bad - they can even kill you). In order for a planet to outgas, there have to be gases in its interior to begin with. These are usually trapped within the planet as it grows by gas, dust, asteroids, comets, small planets, etc., falling onto the planet. If the planet heats up too much during this time, many of the gases can escape right away, instead of slowly being released over the planet’s lifetime.
Once the gases have gotten to the surface of the planet, enough of them
have to stay there in order for an atmosphere to form. It’s been four and
a half billion years since the beginning of the solar system, so gases
released early in the planet’s history have to stick around for a long time
in order to still be part of the atmosphere today! There are many ways
that gases can leave the atmosphere:
1) They can just leave out the top. The only thing that’s holding them is gravity. At the top of the atmosphere, gases can be broken up, ionized, and heated to very high temperatures by solar radiation and the solar wind. Light atoms, such as hydrogen, can escape very easily.
2) They can freeze. For example, during a Martian winter, some of the carbon dioxide in the atmosphere is deposited as frost at the pole. Gases also freeze at the poles of icy satellites. You can watch this happen yourself on a winter day, as water freezes out of the atmosphere and falls as snow.
3) They can chemically react with other gases, changing into different compounds and “raining out,” or they can react with the surface of the planet and become “locked” into another compound. For example, much of the carbon dioxide in the Earth’s atmosphere has reacted with the rocks on the Earth’s surface and with living organisms to become limestone (calcium carbonate) or even oil (which we are now pumping back out of the ground to fuel our cars, turning it back into carbon dioxide). Over 95% of the Earth’s outgassed carbon dioxide is estimated to be locked into rocks through this process (perhaps this is part of why Venus has such a thick carbon dioxide atmosphere and we don’t).
So getting back to the original question, you can see that Titan’s atmosphere must be maintained by a delicate balance of temperature, composition, and the initial conditions under which the planet formed. There are many factors which must be just right for an atmosphere to exist for the long term. It is thought that on Ganymede, many of the volatile gases (the same ones which compose Titan’s atmosphere) were “baked out” during the formation of the planet. Titan was just far enough away from the Sun and the conditions around Saturn were cold enough for these gases to form the atmosphere. We will know much more when the Cassini spacecraft, due to be launched in about a month, arrives at Saturn and sends a probe down into Titan’s atmosphere.
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