system
Given the following post, respond as if you're commenting on Reddit:
human
Did Mars have an atmosphere like Earths in the past?

If it did what happened to it? If not is it just because we lucked out in that department?
gpt
There are a few factors going on there.  The atmosphere is held on the planet by gravity.  Particles moving too fast escape into space.  That happens on a regular basis, but on Earth, we have plate tectonics, which causes volcanic eruptions that replenish the atmosphere.  But the larger the mass, the better able to hold an atmosphere a planet will be.  

On Mars, early in its history, it probably had a somewhat denser atmosphere, at least at the beginning.  There was almost certainly abundant liquid water at the surface of Mars, and the internal heat of the planet was much higher, allowing volcanoes to erupt and emit gasses.  

But Mars is much smaller than Earth.  The crust is proportionally much thicker, and plate tectonics does not exist on Mars.  The core of the planet does not generate as significant a magnetic field as Earth.  Volcanoes rose at hot spots, instead of at plate boundaries as on Earth.  As the planet cooled, there were fewer hot spots capable of reaching the surface and emitting their gasses there.  The atmosphere would have dissipated fairly quickly.  

It is unlikely that Mars ever had an atmosphere as thick as Earth's, because of its lower mass.  Note that it does still have an atmosphere -- just not as thick as ours.  

It might be said that we lucked out.  Venus provides a point of comparison -- it has about the same mass as Earth, and it actually has a thicker atmosphere than ours (of much different composition).  


One word about composition -- our oxygen rich atmosphere is not what we had at the beginning.  That didn't come around until about 2.5 billion years ago.  The other planets have carbon rich atmospheres, with a fairly low amount of oxygen.  Life was necessary to give us this atmosphere.  If you got in a time machine and went back 3 billion years, you would not be able to breath on Earth.