QUESTION:
If a 35 mph wind is blowing on Mars, what would it "feel" like to an
observer? I heard a commentator say that the atmosphere of Mars was 1/10
that of Earth's. I assume that means 1/10 as dense. Would the corresponding
wind "feel" 1/10 as strong? Can the wind on Mars move rocks?

ANSWER from Mary Urquhart on July 17, 1997:
Actually, the density of the Mars atmosphere is only about 1/100 of the
Earth's.  How a wind "feels" to an object in its path is dependent on the
density of the wind (the number of molecules per unit volume) and the
velocity of the wind.  The answer to your question depends in part on what
you mean by how the wind would "feel".

The momentum of the wind is the density (really the mass of the molecules in
the wind added together) times the velocity of the wind. So, an object on
Mars will encounter wind with 1/100 the momentum of wind on the Earth
traveling at the same velocity.  That means the wind on Mars has to be
traveling a 100 times faster than the wind on Earth to have the same
momentum.

However, the kinetic energy content of the wind is dependent on the velocity
of the wind squared (or K.E. = (M x V x V)/2 ), so the wind on Mars only has
to travel about 10 times as fast to make up for the difference in density.

Mary Urquhart                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder       

ANSWER from Jim Murphy on July 15, 1997:
The wind on Mars is not "strong" enough to move rocks on the surface. Even
though winds on Mars can probably reach large speeds, the atmospheric
density is so low, that the force the wind can impose on a rock is quite
small. For instance, a wind of 10 meters per second (about 20 miles per
hour) here on Earth produces a force which is four times stronger than does
a 50 meter per second wind (a bit more than 100 miles per hour) on the
surface of Mars. So, since a 20 mile per hour wind here on Earth does not
generally move rocks about on the surface (though it does raise dust), the
winds on Mars don't move rocks on the surface either.

Your question is a terrific one, and offers a great chance to show the
differences between Earth and Mars.

Jim Murphy
Mars Pathfinder ASI/MET Science Team

ANSWER from Mike Mellon on November 19, 1997:
The force of drag caused by wind, that an object the size of a person would
experience, is proportional to the gas density and the wind velocity squared
for both Mars or Earth atmospheres. While the pressure at the surface Mars
is 168 times smaller than at Earth's surface, the density of the atmosphere
at the surface of Mars is only 81 times smaller than Earth's. Therefore, to
"feel" a breeze on Mars of the same force as on Earth, we would require 9
times the wind velocity (square root of 81). For example, to "feel" a light
breeze of about 10 miles/hr on Earth, would require hurricane speed winds on
Mars of 90 miles/hr (but not hurricane force).