Probably you are all wondering how Mars has lost it's Earth-kind atmosphere.
Story looks to be pretty simple:
Process of global warming has caused a stretching of Marsian atmosphere up. Upper atmosphere became under lower influence of planet's gravity and solar wind has done a rest.
The same looks to be going on now on Earth. In far future, Earth will be looking like Mars today, and Mars will be looking like Mercury or our Moon (no atmosphere at all).
What do you think about my theory ?