Earth’s Atmosphere: Newsletter Challenge (December 2011)

Gravity.

The surface gravity of the larger planets is able to hold the lighter gases.

This is probably a partial restatement of your answer, but another factor is that the earth is a metallic rocky planet, with a higher percentage of heavier elements. As a result we have a higher percentage of heavy gasses in our atmosphere than say Jupiter or Saturn, and the lighter gasses naturally float to the top of the atmosphere where they are somewhat more exposed to the solar wind, which is of course much stronger nearer to the sun than in the outer reaches of the solar system where the gas giants live. I'd guess that the hydrogen and helium concentrations on Venus are lower still, even though it's atmosphere weighs several times as much as ours. Poor Mercury has been pretty much stripped naked.

One of the 3 primary fission products is alpha particles, i.e., helium nuclei. Once they grab a couple electrons to become electrically neutral they are then helium atoms.

GA from me even though Pluto is not a planet anymore. Planetary title decay....

There's not much hydrogen in the earth's atmosphere because of the presence of oxygen.

A lot of the Earth's hydrogen is in the water.

Not sure about the helium.

Hydrogen and Helium are light gases. The more massive the molecules of a gas are, the lower the average velocity of molecules of that gas at a given temperature and the lower is the chance that any of them reach the so called escape velocity. Escape velocity out of earths atmosphere is lower as from f.e. Jupiters atmosphere. Earths atmosphere is warm enough and the escape velocity of Hydrogene and Helium is high enough to leave our planet and to give me (and my Kois in the frozen pond) a chance to breave...

pinball-wizzard

The earths atmosphere is too dense with heavier elements, eg nitrogen, oxygen etc. for the two lightest elements to remain in suspension.

Also, it's warmer on earth. Even at the coldest points on/in our planet and in our atmosphere, the light gasses are fully gasses. On the colder outer planets, there is probably equilibrium between liquid and vapor of those gasses, especially deep in the atmosphere where the pressure would be high. I can envision something like the water cycle on earth, with the light gasses or hydrocarbons on other planets.

the earth's gravitation is not hard enough to hold the light elements hydrogen and helium in its gravitational area is one reason (v>11210m/s). Is there ever been enough pure hydrogen or helium in the earth's matter to leave appreciable amounts of it in the earths atmosphere or did the jupiter (or maybe the sun) in their creation state fetch all the hydrogen and existing helium into their gravitational field?

It's all been used up in balloons and dirigibles...and for people to talk like chipmunk

Simpley the Earth does not have enough gravitational pull to hold all of the Hydrogen and Helium. They are lighter then air and escape the atmosphere.

The combination of the high volatility of the element and the scarcity (only naturally produced by the decay of thorium and uranium) may contribute to the low amount of helium in Earth's atmosphere.