Follow up question about Mars atmosphere?

Question

Thanks to everyone who answered my question about liquid water on Mars. However, I believe the geologic evidence on Mars suggests that once there was lots of water on Mars. Enough to carve canyons. So if the water boiled away because the atmosphere was too thin, what made the atmosphere get thin? There must have been enough air pressure for liquid water in the distant past. What happened to Mars atmosphere? Can air just drift away into space? Is Mars gravity strong enough to hold a dense atmosphere? If so, how did the air disappear?

Answer 1

Anything that could have ripped away Mars's atmosphere must have been a violent occurance.  Well, might have, I mean.  I forgot about the possible disappearance of its magnetic field.

Take a look at this map of mars:

http://www.hjo3.net/mars_map_botha.jpg

Do you see the large circlular formation in the lower right called Hellas Planitia?  It's a giant impact basin, the second largest in the solar system.  Some time in the past it was hit by a large asteroid which may have ripped away much of its atmosphere.  Do you also see the cluster of volcanoes in the upper left of the map?  They formed at the same time as a result of the impact's shock wave travelling through the planet and focusing on the opposite side.

BTW, Titan is much smaller than Mars, and yet retains an atmosphere even thicker than Earth's.  The key to that, however, is its distance from the sun and the lack of solar wind.  I would still think Mars is capable of retaining a thicker atmosphere than it currently does.

Answer 2

Air can and does escape to space. Air is escaping to space very slowly from Earth, much faster for Mars which has only 1/3 the gravity of Earth, and REALLY fast for the Moon which as only 1/6 the gravity of Earth. But we really don't know for sure where all the air and water went or exactly how much Mars started with. Much of the water is believed to still be there, permanently frozen at the poles or buried under a layer of dust at other latitudes. If the ice is cold enough, it won't sublimate even in a vacuum.

Answer 3

I don't know if anyone has actually calculated the gravity requirements to hold an atmosphere.  Everything I've read simply assumes that Mars' gravity is too little to hold a real atmosphere for very long.

If it is too little, then slow diffusion would explain the current atmosphere.

If it isn't, I would think that magnetic effects and solar wind, catastrophic asteroid strikes, or glancing blows or near misses by some other large body, or maybe massive volcanic eruptions, would be required to explain it.  It's not impossible - the Syrtis Major region sure looks like something BIG hit there, and there's Olympic Mons, the biggest volcanic mountain known in our solar system.

We don't know.  More research data would be nice.  Say, some rock samples, atmospheric samples, and on-the-spot geological surveys (grin).

EDIT:  Eeelfins points out Hellas Planatia, which is almost definitely from an impact.  And the mention of Titan.  Excellent points.

I've read descriptions of theories of the origin of Luna that described the Earth receiving glancing blows or close approaches by a "Mars-sized body."

Could it have been not just a "Mars-sized body" but Mars ITSELF?  And Mars lost its atmosphere in the process?

Answer 4

There was, supposedly, LOTS of water on Mars some time ago in comparison to the nearly non existent water there today. The water boiled away because the air was too thin, yep. As I understand it some believe Mars's atmophere got so thin because Mars being so far away from the Sun the internal temperature of its core was much cooler than the Earth's. Mars may have had a magnetic field much like Earth's a few billion years ago but sooner or later the core cooled to the point where the convection wasn't enough to propel conductive iron at its center around fast enough to produce the magnetic field. Without a sufficiently large magnetic field solar winds began to blow away Mars's atmosphere particle by particle.

Water when hit with solar radiation sometimes splits into oxygen and hydrogen. The hydrogen floats to the top of the stratosphere where it is the first gas to get blown away (not even Earth can keep hydrogen in its atmosphere for long because if it did it would quickly grow into a gas giant). Over time the atmosphere thinned to todays levels.