Thread: Mars vs Earth air speed,

Hello all, I hope I've found the right place for a question about air.

I was talking with my Son about Mars - I'd read somewhere that Mars had about 5% air that the Earth had and sometimes had 200mph winds - my Son said you couldn't walk out in wind like that. To which my ears pricked up. I said that as the air pressure was 5% of the Earths air pressure then the wind hitting your skin (assuming you could) would feel like a 10mph breeze on Earth.

My thinking here is the kenetic force an object can carry, in this case the air molecules. As the number of molecules would be 20 times less than on earth then the force delivered would be lower....

anyhow, as kids do my Son then went and told his Astronomy teacher who said I was wrong?

OK so folks, am I wrong?

I'd read somewhere that Mars had about 5% air that the Earth had

About .4% actually.

and sometimes had 200mph winds - my Son said you couldn't walk out in wind like that. To which my ears pricked up. I said that as the air pressure was 5% of the Earths air pressure then the wind hitting your skin (assuming you could) would feel like a 10mph breeze on Earth.

Less than that, actually, due to the much lower density. However, a grain of dust that is being blown by that wind has significant kinetic energy. Thus you would not be knocked backwards or anything, but if the wind was carrying any dust, a lot of sandblasting would be going on.

Well, you, as a human, can be suspected to be 2 meters tall, and your son might be only like 1.5 meters. The size of Earth's atmosphere is about 40 km big (no exact boundary), Mars should have even taller one.

The thing is, most of the air (on Earth as on Mars too) is concentrated at the bottom few meters-kilometers, and there is very little air above that - exponentially decreasing. Your 2 meter height does not make a difference here.

What am I saying is the air would be around the same dense in the first few kilometers, regardless of the composition. The 5% (or .4%) difference comes in after that,

For the sake of this argument lets assume that Mars has 5% of the Earths air pressure and Earths air pressure at sea level is 15psi -
If Mars has 5% air pressure of the Earth, and the air is travelling at 200mph then when this hits the skin would it feel the same as a 10mph breeze on Earth.

Originally Posted by MrScabs

For the sake of this argument lets assume that Mars has 5% of the Earths air pressure and Earths air pressure at sea level is 15psi -
If Mars has 5% air pressure of the Earth, and the air is travelling at 200mph then when this hits the skin would it feel the same as a 10mph breeze on Earth.

No. You seem to not take into account the exponential decrease of the air's mass.

For a better estimation, we shall integrate the barometric air pressure formula from 0 to 50000 meters, then take the result, multiply by 0.05 or whatever you choose, set up another air pressure formula for Mars (accounting for the difference in gravitational force), then by differentiation of the Earth formula, you can get air pressure at Mars at any given height.

http://quest.arc.nasa.gov/mars/ask/a...nd_on_Mars.txt

The part your missing is the Kinetic energy has the squared term, such that K=1/2 * V^2

If you plug them in you'll see:

For Earth at 10mph wind (~5 m/s) and density of about 1 kg / m^3 the KE is going to be ~25 J m^-3
For Mars at 200mph wind (~100m/s) and density of about 0.005 kg m^-3, the KE is ~50 J m^-3

The break even point for a wind pressure on Mars would be about 150 mph to equal that of a 10 mph wind on Earth.

--
PS. Some winds on Earth go over well over 200mph (e.g., tornado's and jetstreams)

The part I was wrong at is I assumed the TOTAL density of Mars's air is 5% of the TOTAL density of Earth's air. If it were the case, I'd been right.

What you - and NASA - are basicially saying is that the SURFACE density is 5% of Earth's. If this is the case, then I was wrong

Originally Posted by MrScabs

Hello all, I hope I've found the right place for a question about air.

I was talking with my Son about Mars - I'd read somewhere that Mars had about 5% air that the Earth had and sometimes had 200mph winds - my Son said you couldn't walk out in wind like that. To which my ears pricked up. I said that as the air pressure was 5% of the Earths air pressure then the wind hitting your skin (assuming you could) would feel like a 10mph breeze on Earth.

My thinking here is the kenetic force an object can carry, in this case the air molecules. As the number of molecules would be 20 times less than on earth then the force delivered would be lower....

anyhow, as kids do my Son then went and told his Astronomy teacher who said I was wrong?

OK so folks, am I wrong?

yes, you are wrong. The force is a result of the speed of the air squared, and the mass of the air hitting you. The mass is directly proportional to density, so we can use that as an analogue to compare the two planet's atmospheres. At the surface, the density of Earth's atmosphere is ~1.217 where as Mar's atmosphere is ~0.020 which is a proportional difference of about meaning that a wind on Mars would feel about the same as a wind on earth about 7.75 times slower (remember, we're looking at the speed squared for energy). So, a 200mph wind on Mars would feel like a 25.8mph wind here. And, if we were to use the 1/20th metric originally proposed, the answer would be 4.47 times slower, a 200mph wind on Mars would feel like a 44.72mph wind on Earth

Just going through this thread, it appears there is some wildly conflicting data out there on the density of Mar's atmosphere at the surface of the planet, which is why I will be adding in my sources

sources:
Mars Fact Sheet
Earth Fact Sheet

To be fair, Mars air density changes by quite a bit depending on huge surface elevations differences, as well as season sublimation/melting and return of Northern carbon dioxide/water ice cap. Regardless even at the lowest point during NH summers the pressure is never more than about 10 mb, or less than 1% of Earth's surface pressure--with density showing a similar tiny %.