# Feather Falling on Mars

1. Jun 3, 2007

### Worzo

Firstly, this isn't my homework question. I was trying to answer another, broader question for a student, and it boiled down to this one. There's quite a subtle point here, I think, but I just can't grasp it.

Consider stable atmospheric conditions on Mars and Earth. A feather is dropped from a great height on both planents. Which planet gives the feather the higher terminal velocity?

Data given is:
- Mars gravity = (1/3)g
- Earth atmosphere: 1000mbar
- Mars atmosphere: 10mbar

So terminal velocity goes as square root of gravitational force and inverse square root of viscosity. I can't work out how the viscosity changes with temperature and pressure.

Gut feeling tells you that the weaker gravity (a third of Earth's) contributes to lowering the terminal velocity. However, doesn't the fact that the pressure is 100 times smaller contribute to the viscosity somehow?

I remember proving in kinetic theory that viscosity is independent of pressure (except for high pressures), but does that hold here? I can't help thinking temperature has something to do with it as well.

Any explanation/calculation of Terrestrial/Martian atmospheric viscosity would be most appreciated.

2. Jun 4, 2007

3. Jun 4, 2007

### DaveC426913

AFAIK, Mars' atmo is more akin to vacuum than it is to a real atmo.

4. Jun 5, 2007

### Worzo

That's what I thought, but I can't find any expression for how the viscosity changes at low pressure.