Exploratour - Evolution of the Solar System
If a planet cannot bring new molecules into the atmosphere with volcanoes, then eventually the whole atmosphere can drift away. But for the atmosphere to drift away, the planet has to be small. The size of the planet matters. This table shows the time it takes for a few common atmospheric gases to escape the gravity of a planet. Like astronauts jumping long distances in the low gravity of the Moon, atmospheric gases can rebound large distances in a low gravity environment, if they have enough energy. Eventually, given enough time, atmospheric molecules will make their way out of an atmosphere and escape into space.
The table shows that as the planets get larger (more massive), it is harder for the gases to escape. Gravity acts as a "lid" to keep the particles close to the planet. The table shows that the average time for any gas to escape from Jupiter's gravity is billions of years. Thus it is likely that all the gases in Jupiter's atmosphere are probably the original gases swept up by the proto-Jupiter. Nothing can escape. On the other hand, the reason that the atmosphere of the planet Mercury, with no active volcanoes, has drifted away is because the planet is so small. Mercury, being about the size of the Moon, has about the same gravity as the Moon.
Mercury |
Venus |
Jupiter |
||
Hydrogen (H) | less than 1 hour | days | over the age of the Universe | |
Helium (He) | roughly 1 day | 100 times the age of the solar system | over the age of the Universe | |
Oxygen (O) | 800 million years | over the age of the Universe | over the age of the Universe | |
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