The composition reads like an exhaust stream: 95.3% carbon dioxide, 2.7% nitrogen, 1.6% argon, and about 0.13% oxygen. Nitrogen matters more than the small number suggests. On Earth it is the buffer gas, the 78% of inert filler that makes 21% oxygen safe to breathe rather than an explosive hazard. Mars has almost none.
Pressure varies more than Earth's does, and for an unusual reason: a substantial fraction of the atmosphere freezes onto the winter pole each year and sublimes back in spring. Altitude matters too — the floor of Hellas Planitia, seven kilometres below the datum, holds roughly 1,155 pascals while the summit of Olympus Mons has almost nothing at all.
The number that governs everything is the Armstrong limit: 6.3 kPa, about 63 millibars. Below it, water boils at human body temperature. Above it, you still need oxygen but you no longer need a pressure suit — a mask and warm clothes would do. Mars today is at one tenth of that threshold, which is why every warming scheme is ultimately judged on how much pressure it delivers.
The ladder above the Armstrong limit runs: about 80–100 millibars to make masks workable in the deepest basins, 100+ to retire pressure suits entirely, and roughly 250 millibars with the CO2 fraction brought down before air becomes genuinely breathable. Getting from 6 to 250 is the whole terraforming problem stated in one line — and Mars does not appear to contain enough accessible gas to do it.
Six millibars is enough for aerodynamic braking on entry, for parachutes, for wind turbines in principle, and for a helicopter — Ingenuity flew 72 times in it.
95% CO2 means the atmosphere itself is the raw material for oxygen and for methane fuel. MOXIE demonstrated exactly that on Perseverance.
Thin as it is, the atmosphere moderates the day-night swing and absorbs a fraction of incoming particle radiation that a vacuum world would deliver in full.
At 610 Pa, exposed body fluids boil at 37 °C. Every human on the surface needs a full pressure suit, indefinitely, until pressure rises by a factor of ten.
At 2.7% of a very thin atmosphere there is no buffer gas worth the name, and no plausible local source of the 10¹⁷ kg a breathable atmosphere would need.
Raising pressure with more carbon dioxide makes the air more lethal even as it becomes thicker. Above a few percent CO2 kills regardless of oxygen availability.
MAVEN measured ongoing loss to the solar wind. Anything added to the atmosphere begins leaking the moment it arrives, absent a magnetic shield.
| To retire pressure suits | >63 mbar | ~10× the present atmosphere |
| To breathe unaided | ~250 mbar | With CO2 fraction reduced |
| Oxygen partial pressure | ~13 kPa min | Below this, hypoxia |
| Gas per millibar | 3.89 × 10¹⁵ kg | Global mean |
About 610 pascals on average — roughly 0.6% of Earth's sea-level pressure. It ranges from near zero atop Olympus Mons to about 1,155 pascals on the floor of Hellas Planitia.
Roughly 95.3% carbon dioxide, 2.7% nitrogen, 1.6% argon and 0.13% oxygen, plus traces of carbon monoxide and water vapour.
Because of the Armstrong limit. Below about 6.3 kPa, water boils at body temperature, so exposed fluids in the lungs and eyes vaporise. A mask supplies oxygen but cannot supply pressure.
Above the 63 millibar Armstrong limit a pressure suit becomes unnecessary, though a mask is still required. Genuinely breathable air needs roughly 250 millibars with the carbon dioxide fraction reduced.
