Mars atmosphere is rich in CO2, and is much thinner than our atmosphere. But at the same time thick enough to harbour storms. It is believed that the martian atmosphere has lost much of its gases ever since (4. 5 billion years ago) it formed. It seems like some special processes are acting on the atmosphere which preferentially looses N (abundant in our atmosphere). Mars’ low gravitational attraction and energetic ultraviolet radiation from the Sun are to be blamed for this.

Water on Mars: It is now believed that earl in its history, Mars had an ocean and running water like we have on our planet! Water seems to exist as frozen layer beneath in surface. They indeed find martian clay and mud in some of these meteorites. I guess you’re familiar with the discovery of “remains of martian micro-organism” in one of them too. But I also learnt that may be it’s too early to either prove or disprove the idea that we might have our martian counterparts.

Interior: These meteorites are similar to rocks we get from our mantle, and have told many things about the interior of Mars. They find some differences (compared to ours) in the chemical (and isotopic) composition of Mars, which may be either because of different types of “precursor” materials (planets form from dusts in the early solar system by accretion) or because of different way it grew up. But at the same time they find some similarities too, which indicate some common properties.