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.

Impact Atmospheric Loss

Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas to impacters.

Events in the Pre-Noachian Period

The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from Solar Wind Sputtering. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid. Vulcanism was very common. There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

Losing the Magnetic Field

Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.

Without the magnetic field, the rate of atmospheric loss would increase slightly.

Mars' Secondary Atmosphere

Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO₂), nitrogen (N₂),