Four worlds, three origins of air
Venus, Earth and Mars are rocky planets that grew from the same inner region of the solar nebula. Titan is a rock-and-ice moon that grew in the disk around Saturn. Their air also has different histories: some gases came from the original material, some from volcanic outgassing, and some from impacts. That is why this is not a simple story of four planets on one scale.
The useful comparison is not just “which gas is most common”. It is how much air each world has, how strong its gravity is, how hot its surface is, and whether water or another liquid can stay stable there. Those details explain why similar starting materials ended up as a 92-bar greenhouse, a breathable nitrogen–oxygen mix, a thin desert atmosphere and a cold methane cycle.
Why pressure matters as much as percentage
Mars and Venus are both dominated by carbon dioxide, yet their surfaces are wildly different. Venus has a surface pressure of about 92 bar and a mean temperature of about 464 °C / 867 °F / 737 K. Mars has a surface pressure of about 6 millibars, less than 1% of Earth's, and a mean temperature of about -63 °C / -81 °F / 210 K.
A gas can be the main ingredient and still be too sparse to trap much heat. The greenhouse effect depends on the total column of absorbing gas, not only its share of the mix. Mars also lost much of its early atmosphere to space. NASA's MAVEN mission measured about 100 grams per second being stripped away by the solar wind, or roughly 6 kilograms per minute. That loss is slow today, but it adds up over billions of years.
Venus: a thick CO₂ atmosphere and a wet past
What the air is made of
Venus's atmosphere is about 96.5% carbon dioxide and 3.5% nitrogen, with sulfur dioxide, argon and water vapour in trace amounts. The nitrogen inventory is roughly four times Earth's by mass, because Venus has so much more air overall. The surface pressure is about 92 bar and the mean temperature is about 464 °C / 867 °F / 737 K. See the full Venus weather guide for the cloud decks and winds.
What happened to the water?
Venus is dry today, but its deuterium-to-hydrogen ratio is about 120 times Earth's. That is strong evidence that Venus lost a large amount of water: lighter hydrogen escaped to space more easily than heavier deuterium. Whether Venus had long-lived oceans is still debated. Some climate models find a wet early phase; others point to a drier interior and a shorter wet period. The honest summary is that Venus probably had water, but the size and lifetime of its oceans are not settled.
Weather: sulfuric acid clouds and fast winds
Venus is wrapped in clouds of sulfuric acid. At the cloud tops, winds circle the planet in about four days, far faster than the planet rotates. Near the surface the air is much slower, and the sulfuric acid rain evaporates before it reaches the ground. The result is a hot, high-pressure surface under a fast-moving cloud deck.
Earth: nitrogen and oxygen, kept in balance
What the air is made of
Earth's dry air is about 78.08% nitrogen, 20.95% oxygen, 0.934% argon and 0.042% carbon dioxide, with methane at about 1.9 ppm. Water vapour varies from place to place and is a major part of the greenhouse effect. The surface pressure is about 1.013 bar and the mean temperature is about 15 °C / 59 °F / 288 K. The Earth weather guide covers the circulation and the 3D globe.
The carbonate–silicate thermostat
Earth keeps its carbon dioxide in check over millions of years. Rain dissolves CO₂ and weathers silicate rock. Rivers carry the dissolved carbon to the ocean, where it is stored as limestone. Plate tectonics eventually returns some carbon to the atmosphere through volcanoes. Warmer conditions speed up weathering, which removes CO₂ and cools the planet. This is a negative feedback, not a quick fix: it works on geological timescales.
Weather: water clouds, rain and wind
Earth's weather is driven by solar heating, water vapour and the planet's rotation. Water evaporates, condenses into clouds, falls as rain or snow, and returns to the ocean or the land. Large-scale winds are bent by the Coriolis effect; you can explore that in the Coriolis effect simulator. Earth is the only one of these four worlds with a water cycle at the surface.
Mars: thin CO₂ air and a cold surface
What the air is made of
Mars's atmosphere is about 95.3% carbon dioxide, 2.6% nitrogen, 1.9% argon and 0.17% oxygen, with methane at about 0.4 ppb in the background. The oxygen figure comes from Curiosity's Sample Analysis at Mars instrument; NASA's summary rounds it to 0.16%. The surface pressure is only about 6 millibars, and the mean temperature is about -63 °C / -81 °F / 210 K. The Mars atmosphere and weather guide has more on the seasons and dust.
Why 95% CO₂ is still cold
The percentage is high, but the total amount of air is tiny. Mars has less than 1% of Earth's sea-level pressure, so there are far fewer CO₂ molecules above each square metre to absorb and re-radiate heat. The thin air also has little heat capacity, so the surface cools quickly at night. A high mixing ratio is not a thick blanket.
Weather: dust storms and CO₂ frost
Mars has planet-wide dust storms, dust devils and seasonal polar caps. In winter, carbon dioxide freezes out of the atmosphere onto the poles as dry-ice frost, and the pressure changes through the year. The air is too thin for liquid water to stay stable at the surface for long, although brines might exist in small amounts.
Titan: nitrogen air and methane weather
What the air is made of
Titan's atmosphere is about 94.2% nitrogen near the surface, with methane at about 5.65%. The methane mixing ratio falls to about 1.4% in the stratosphere. There is also about 0.1% hydrogen and ppm-level ethane, acetylene and hydrogen cyanide. Titan is the only world besides Earth known to have stable liquid on its surface; Venus has a thicker atmosphere, but no surface lakes. NASA's Titan fact page is a good starting point.
Why methane can form lakes
At about -179 °C / -290 °F / 94 K and 1.45 bar, methane is close to its liquid range. The lakes are not pure methane. They are methane-rich mixtures with ethane and dissolved nitrogen, and that mixture stays liquid over a wider range of conditions than pure methane would. Cassini radar altimetry found depths of up to about 160 metres in Ligeia Mare along one track. The measurement was made with the radar altimeter, not a bistatic radar experiment.
Weather: methane rain and organic haze
Titan has clouds, rain, rivers and lakes of liquid methane and ethane. Sunlight breaks methane apart and builds complex organic particles, the tholins that give Titan its orange haze. Because methane is destroyed over tens of millions of years, something must resupply it. Scientists think the methane comes from Titan's interior, possibly through cryovolcanism or clathrate breakdown, but the source is unresolved.
Comparison to Earth
The table below puts the same numbers in a simpler frame. Pressure is shown relative to Earth; temperatures are mean surface values, not ratios of habitability.
| World | Pressure vs Earth | Mean temp | Main gas | Stable surface liquid? |
|---|---|---|---|---|
| Venus | about 91× | 464 °C / 867 °F / 737 K | CO₂ | No stable surface water |
| Earth | 1× | 15 °C / 59 °F / 288 K | N₂ / O₂ | Water |
| Mars | about 0.006× | -63 °C / -81 °F / 210 K | CO₂ | No known stable water; brines may exist |
| Titan | about 1.43× | -179 °C / -290 °F / 94 K | N₂ / CH₄ | Methane and ethane |
Earth's mean temperature is about 15 °C / 59 °F / 288 K. Differences are rounded. Mars may have transient brines, but no known stable surface water.
What this means for habitable worlds
A planet can sit in the habitable zone and still lose its air. It can be rich in carbon dioxide and still be cold. It can have a thick atmosphere and still lack liquid water. Pressure, gravity, magnetic protection and the history of volatile delivery all matter, and the four worlds here show different combinations of those factors.
For exoplanet searches, the practical lesson is to treat atmospheric composition and atmospheric mass as separate measurements. A spectrum that shows a strong CO₂ signal does not tell you whether the planet has a thin atmosphere like Mars or a thick one like Venus. You need the surface pressure, or at least a good estimate of the planet's radius and mass, to know which regime you are looking at.