Expert: Philip Stahl - 6/10/2004

Question
I understand that Mars' atmosphere is very thin to the point that the air pressure is 1/100 that of Earth at sea level.  There also seems to be a large amount of evidence that water may have flowed on the surface of Mars in the distant past.  In order for water to flow, the atmosphere would have to be much more dense.  What could have happened in Mars' past that could have led to Mars losing so much of its atmosphere?

Answer
Hello.

In all likelihood, what happened is that Mars' atmosphere was "outgassed", e.g. the particles - molecules escaped.

Note also, that it isn't necessary for the mean molecular speed of a gas (say made up mostly of oxygen or O2) to be equal or exceed the escape velocity of a planet to be lost into space.

Calculations show, in fact, that if the mean molecular speed is as much as one-third the velocity of escape (or 1.7 km/sec for Mars) the planet will lose one half of its atmospheric gas in only a few weeks.

If the mean molecular speed is even one fifth of escape velocity (1.02 km/sec for Mars) the gas will disperse into space in a few hundred million years.

To hold a gas of sufficient atmospheric density to allow standing water- for billions of years - would necessitate a velocity of escape 6-8 times the mean molecular speed of the gas in question.  This is simply not the case for Mars, where one can easily work out the mean molecular speed of oxygen - say from:

v =  SQRT(3 k T/m)

where k is Boltmann's constant (1.38 x 10^-23 J/K), T is the absolute temperature applicable in degrees K, and m is the mass of a single gas molecule.

and compare it to Mars' escape velocity of ~ 5.1 km/sec.