QUOTE (Julius @ Nov 30 2007, 05:57 AM)
I am no planetary sceintist but just a humble enthusiast!I have a few queries maybe some of you could answer:
Why should Venus have a thicker atmosphere than Earth provided it lacks a magnetic field and hence being in direct contact with the solar wind ,which should produce higher rates of atmospheric loss.This seems to be the explanation for mars!?
The moon is said to be acting as a friction brake on Earths rotation and thus is slowing it down.Has anyone explained clearly why Venus is such a slow rotator in the absence of a moon!Could a previous collision of a cosmic body with Venus help to explain this . Venus was once(i think its about 500-800 million years ago) entirely resurfaced by hot magma .Could this have happened when this theorised collision took place?
Why should Venus lack a magnetic field given its the same radius and almost same density like Earth?Could it be that its dry compared to Earth ,simply explain for this significant difference?
My knowledge is fairly basic, but here is how I understand it.
Most, if not all, of your questions center around water. Venus had, presumable, similar amounts to Earth, but lost it. Initially it was probably in oceans. These dried up due to the heat as the Sun warmed. Water is a much better greenhouse gas than CO2, so it increased the temperature even more. As the atmosphere got saturated with water, more and more of the water was photodissociated to hydrogren and oxygen by the UV from the sun. The hydrogen escaped, leaving the oxygen. Over time, most of the water disappeared.
As the climate heated up, the rocks themselves lost their water and CO2. On earth, there is a great quantity on CO2 stored in rocks as carbonates etc. So even though there is more loss due to solar wind at Venus, there is just so much more atmosphere because all the stored CO2 has been released. Plus, Venus gravity is much higher than Mars. Also, for Mars there is a theory that oblique asteroid impacts "threw" parts of the atmosphere off of the planet.
As for the slow rotation, there are multiple theories there. The one I've heard is that the atmosphere itself could have cause the rotation to slow down and switch directions over eons, with solar tides playing a big factor. A previous collision is also possible, but would be hard to prove without evidence (like a Moon!
The resurfacing is, again, related to the lack of water. The current theory is that without water, the crust is less flexible, and much thicker. Because of that, the heat generated internally builds up to high temperatures before it gets hot enough to escape to the surface. When it does, I imagine Venus, under the atmosphere, would be covered with as many volcanoes as Io.
The lack of a detectable magnetic field is due to the lack of rotation. Assuming a similar composition to Earth, it would have a molten core with the appropriate convection currents. But because the planet doesn't rotate very fast, it cannot generate a magnetic field using a dynamo effect. How I visualize the internal dynamos is to think of spider silk floating in the air (these are the magnetic field lines). Imagine grabbing it with your hand, and start spinning your hand until it's covered in the silk. The closer the silk is to itself, the stronger the field lines. In the core the field lines are close, since the conductive molten iron grabs the lines tighter than the nonconductive rest of the earth. Because Venus doesn't spin very fast, it doesn't "catch" enough field lines to make a significant magnetic field. It's probable it does generate a very small magnetic field, just that it's too weak to get very far from the core.
I'm sure Emily can give us the straight answer on all these, being the Venus expert she is!