COE - The Atmosphere Lesson
The Atmosphere
Gases are the least dense of all of the phases of matter, so the first layer that one would encounter when visiting a planet is its atmosphere. The "proto-planets" that were forming before the sun ignited looked very different compared to today's planets - especially the first four. Keeping in mind that the entire solar system formed from the same source of materials, those planets would have had much larger and thicker atmospheres, mainly composed of hydrogen and helium (just like the more distant planets).
Jupiter was the closest of the planets to becoming a star, but it came in a distant second place to the sun; the "proto-star" that became the sun was growing much faster, and much larger, than the other clumps around it. When the sun grew large enough to undergo nuclear fusion, the energy it released blew away much of the atmospheres around the first four planets - their small masses did not have enough gravity to cling to the lighter elements. Hydrogen and helium were stripped from these planets, leaving only heavier elements - nitrogen, oxygen, argon, etc. - and a few compounds - carbon dioxide and water - behind.
If you refer back to the scale map that you made in the previous unit, the above explanation will begin to make more sense. Imagine that the planets are dandelions and that you are the sun. Scale them in inches; the first four dandelions are within 4 inches of your mouth, but the next one is over a foot away, and the remaining dandelions are even further.
Take a deep breath and try to blow all of the seeds off and you'll understand why the terrestrial planets have (relatively) little to no atmosphere, while the gas giants still maintain the atmospheres that formed before the sun ignited.
At first glance, the terrestrial planets have very different atmospheres. Keep in mind that they would have originally had large atmospheres of hydrogen and helium, but those gases were stripped away, and what is present today are the heavier elements and compounds that the planets' gravitational pulls were able to retain. Mercury, the smallest planet, lost its entire atmosphere due to its weak gravity and its close proximity to the sun. Venus and Mars have very similar atmospheric compositions - mainly carbon dioxide, with a small amount of nitrogen and other trace gases. It is thought that Earth's original atmosphere would have been more similar to Venus' and Mars' atmospheres - mainly nitrogen and CO2 - but it has undergone some unique changes in the last couple of billion years...
Complete the atmosphere review activity below.
IMAGES CREATED BY GAVS OR OPENSOURCE