What happened to Mars' Atmosphere?

Several reasons Mars has lost much of its atmosphere discussed at this website. Scroll almost to the bottom of the page.

http://www.astronomynotes.com/solarsys/s10.htm

The Star that we call the sun emits, along with heat and light, charged particles that are called the solar wind. To use an earthbound analogy, These particles act on the atmosphere (or anything else, for that matter) much the same way as sand blown by Earth's the wind acts on rock, gradually eroding it away. Earth is protected by it's magnetic field, which deflects the particle stream around the Earth. Some of the particles are trapped by the field in bands called the Van Allen belts, and follow the magnetic lines of force to the north and south poles, where they penetrate and ionize the gasses of the upper atmosphere in a display we call Aurora Borealis. Now, at some point in the distant past Mars lost its magnetic field. Not only did every compass on Mars stop working, but with the protective field gone the solar wind gradually stripped the planet of it's atmosphere.

I'm avoiding the whole discussion of Venus' atmosphere compared to Earth's for now. It's a more complex question, and I'm not fully prepared to answer it. For simplicity, though, we don't have a lot of sulphur dioxide and carbon dioxide. They're very heavy gases and not likely to go anywhere. Venus has lots of those two.

You're right about Mars' mass. But.....that doesn't matter. Mars has much less gravity than us. That's what matters. Furthermore, you're expecting a direct proportional relationship between the amount of atmosphere and the mass. That wouldn't be the case. It's going to be a geometric relationship, or something close to that, not an arithmetic one. Gravity is an inverse square relationship, for starters. I would expect a planet with a tenth of the mass to have about a hundredth of the atmosphere, not a tenth.

Mars' weak gravitational pull would cause it to leak away atmospheric gases to the solar wind much more so than on Earth. It's gravity is too weak to hold in a thick atmosphere.

It is a well known fact that if all of the carbonate rock on Earth were converted to CO2, then Earth would have an atmosphere at least 2/3 that of Venus, mostly CO2. (i.e. about 65 atmospheres!! talk about global warming!) The CO2 is locked up in rock on Earth, and fortunately for us, it should stay there. Some authorities feel there may be as much as 2 (Earth) atmospheres of CO2 frozen into the permafrost on Mars. This could be liberated to Mars's atmosphere simply by warming the planet up a bit, making it possible for humans to walk around on Mars with nothing more than warm clothing and scuba gear. Furthermore, most higher plants could live in such an environment, allowing forests of ponderosa pine to spread across the planet's surface, as an example. Humans could live in huge inflatable habitats that would need no structural support other than a slightly higher internal pressure to keep them inflated with an Earth-like breathable atmosphere. If you are curious to read more, I highly recommend the book:"The Case for Mars" by Dr. Robert Zubrin, PhD, available on Amazon and in many local bookstores.

Mars molten core cooled and became fairly solid. Once that happens a planet loses it's magnetic field and solar storms, instead of being deflected as they are on Earth, slowly strip away the atmosphere until very little is left.

When the Mars family became older and less interested in the family business the atmosphere between senoir executives became soured to some extent

the co2 won,t be a problem. the atmosphere is so thin, your blood would boil. surface pressure on mars is equivalent to an altitude of 35 miles on earth. that,s why anyone one the surface has to wear a space suit.

Low gravity..

No volcanic activity to dump gasses in to the air...

Cold...

Distance from the sun...

All contributing factors..

Same thing that's gonna happen to ours if we don't do something about global warming. There were prolly martians who stupidly wasted their resources away like we are doing now.