Everything Science Forum

Why some planets (like earth) have atmosphere and others (like mercury) don't?
Why moon doesn't have atmosphere?
Other natural sattelites (in other planets), do they have atmosphere?
What is the vital presupposition in order a planet to have atmosphere?

Planets like the earth have the mass, and consequently the gravitational field, to retain the gaseous elements like nitrogen and oxygen, which are the principal constituents of earth's atmosphere.  Mars has much less mass and has not atmosphere.  Venus with a mass of 0.815 earth mass, also has sufficient gravity to retain its atmosphere.  The planets, Mercury and Pluto,  have insufficient mass to retain atmosphere, as is the case with the moon.

Titan has an atmosphere, 0.0226 earth mass or 1.35e23 kg (Earth mass = 5.972e24 kg), but it has the gas composition and the cold temperatures to retain an atmosphere.

The heavy planets are so-called gas giants - Jupiter, Saturn, Uranus and Neptune - all have large quantities of hydrogen, helium and traces of carbon, nitrogen and oxygen.

The hydrogen gas that makes up the atmosphere slowly transitions to liquid as pressures increase. Below the liquid hydrogen rest the heavier liquid helium. Deep in the depths of the body of Saturn the hydrogen, now under tremendous pressure, is transformed to liquid metallic hydrogen. It is believed that at the center of Saturn exists a rocky core about ten times the mass of the Earth.  Saturn is approximately 75% hydrogen and 25% helium with traces of other substances like methane and water ice.

from http://physics.pdx.edu/telescope/site4/composition.html

As for Jupiter -

Atmosphere: Jupiter is a gaseous planet; it does not have a solid surface like the Earth does (but probably has a solid, rocky core 10 to 15 times the mass of the Earth). When we look at Jupiter, we are seeing icy clouds of gases moving at high speeds in the atmosphere. Jupiter's atmosphere is composed of about 90% hydrogen and 10 % helium. There are only minute traces (0.07%) of methane (CH3), water, ammonia, and rock dust.

Mantle: Pressurized hydrogen in the mantle may generate electric currents which generate Jupiter's powerful magnetic field. The outer mantle is liquid hydrogen; the inner mantle is liquid metallic hydrogen. The layers of extraordinarily-compressed hydrogen are in a state so extreme that it has never been produced on Earth. The pressure is so great that the hydrogen molecules inside Jupiter conduct heat and electricity very well, in a metal-like fashion (they do not do this under Earth-like condition). Inside Jupiter, electrons from hydrogen molecules move freely from molecule to molecule (like the electrons of a metal); this is what allows the electrical and heat conductivity.
Core: At the center of the planet is a molten rock core which is many times bigger and more massive than the entire Earth. It is 20,000 °C, about three times hotter than the Earth's core.

Internal Heat: Jupiter is a heat source; it radiates 1.6 times a much energy as it receives from the Sun. This energy is produced by Jupiter's shrinking due to gravity, and this produces heat. Also, it is still cooling down, losing its initial energy (the energy it received as the Solar System formed).

from http://www.enchantedlearning.com/subjects/astronomy/planets/jupiter/jupiterinside.shtml

(The information on Saturn and Jupiter is subject to verification from additional sources).

In order for a planet to have an atmosphere, it must have the gaseous elements such as hydrogen (which could be in ammonia, water, methane), helium, nitrogen, oxygen, at the least.  Then it must have mass (and gravity) to retain the gaseous elements under the temperatures.

The Joker in the deck is Titan. No object smaller than Mars has an appreciable atmosphere, except Titan. It has an atmosphere one and a half times as dense as the Earth's.

Fascinating place!

yale

part of it is because it is very cold at Titan. according to Jeans, the gasses have less energy and therefore will not escape as fast.

Ganymede is also VERY cold and a bit larger, yet has no serious atmosphere.
The question is whether the somewhat colder environment, coupled with only a fourth as much sunlight, and a much less energetic environment around Saturn vs. Jupiter, is sufficient to explain the enormous difference.

http://www.spacedaily.com/news/cassini-04zzzt.html

The key question about Titan's atmosphere is how is its methane being generated. It is the same question currently intriguing scientists about Mars, where atmospheric methane also was discovered recently.

Methane poses a potentially exciting problem. The gas cannot exist indefinitely in a planet's atmosphere because sunlight breaks it down. That means any atmospheric methane must have a source of replenishment.

The most common source of methane is living organisms, which produce it as a byproduct of their metabolism. Cattle, for example, digest the plant material they eat and excrete - through their burps and other means - methane. In fact, cattle excretions account for nearly one-fifth of the methane in Earth's atmosphere - the second-biggest greenhouse gas.

Does the methane on Titan mean living organisms are at work somewhere on the surface? Not necessarily. There is that aforementioned possibility of a methane ocean, which could be supplying the gas to the atmosphere through evaporation - and creating the truly alien specter of methane raining back down to the surface.

 :beer

Methane poses a potentially exciting problem. The gas cannot exist indefinitely in a planet's atmosphere because sunlight breaks it down. That means any atmospheric methane must have a source of replenishment.

The most common source of methane is living organisms, which produce it as a byproduct of their metabolism. Cattle, for example, digest the plant material they eat and excrete - through their burps and other means - methane. In fact, cattle excretions account for nearly one-fifth of the methane in Earth's atmosphere - the second-biggest greenhouse gas.

On the other hand, Titan is far from the sun, so sunlight is very low out there.

What applies to Earth, with an atmosphere of nitrogen (78%) and oxygen (~22%) does not apply to Titan.  Jupiter and Saturn, and of course Uranus and Neptune, are rich in light elements - e.g. H, C and N, so the presence of methane (in general alkanes) and ammonia is expected.  Also, it should be expected that some of the moons would be rich in these elements too.