Ancient Atmosphere

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme Ultraviolet light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See Atmospheric loss for more information.

Mars' geologic history is grouped into three main periods: the Noachian, the Hesperian, and the Amazonian.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' First Atmosphere

About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean. During this time, Mars formed its core, mantle, and crust. The atmosphere was largely composed of steam, with significant amounts of hydrogen, nitrogen, and carbon dioxide. Tho speculative, Mars may have started with an atmosphere of 11 bar, tho this rapidly thinned.

Hydrodynamic Gas Escape

Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss, ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.1 Ga, but would be strongest early in that period.