First, Happy New Year to all...
It has been postulated that Mars has no atmosphere because it lost its magnetic field billions of years ago. This might be because of a cataclysmic collision (again postulated) that caused the vast geological differences between the northern and southern hemispheres of the planet. It may also be because Mars cooled off faster because of its size, thereby losing its molten core and dynamo effect. And, with no atmosphere, it was not able to sustain liquid water, although the Phoenix lander has definitely proven there is water ice on Mars, and more than likely it had much more water before it lost its atmosphere.
Well, okay, I can buy all that. But now we come to Venus, the hothouse of the Solar System. It also has no magnetic field, but it definitely does have an atmosphere. It also rotates on its axis in the opposite direction from most of the other planets. My conjecture is that it was also smacked by another large planetoid early in the Solar System's development. This collision happened in such a way as to cause the planet to either start rotating the other way, or cause it to wobble on its axis so much that it eventually turned over 180 degrees, thus causing the apparent reversal of spin.
So, no magnetic field. Then why did it keep its atmosphere? I believe it's because of volcanic activity still going on within the planet that is spewing out gases, enough to keep its atmosphere. I think there may be two reasons for this inner activity. First, its proximity to the Sun. It's closer, so gets more heat, so cools off more slowly. Second, the collision I talked about above. It could have kabonged Venus so hard that it still has inner activity keeping it warmer. Venus also may still have a molten core, but it may not be spinning, so no dynamo effect. This again could have been caused by my hypothetical collision. The whack that caused it to rotate in the opposite direction may have had a deteriorating effect on the spin of the molten core.
Okay, that's my theory about Venus and Mars. Start off the new year right by knocking holes in it!
Regards, Mike Petersen
Happy New Year to one and all.
Just wondering what has the magnetic field got to do with keeping ones atmosphere about one?
Not wanting to change the subject but I have also often wondered why our atmosphere has so little hydrogen in it if it is the building block of the universe.
cheers,
Lyndon