Air and the greenhouse: Venus, Earth and Mars

Venus, Earth and Mars are neighbours, and all three are made of rock. But you could only live on one of them.

Venus is hot enough to melt lead. Mars is colder than Antarctica. The Earth is in between.

Distance from the Sun is part of the reason, but only part. The bigger reason is air.

Some gases in the air work like a blanket. They let sunshine in and stop some of the warmth from getting back out. This is called the greenhouse effect.

Venus has a blanket that is far too thick. Mars has almost none. The Earth’s is just thick enough to keep the oceans from freezing.

How the blanket works. Sunlight arrives mostly as visible light, and air is transparent to that. The ground soaks it up, warms, and gives the energy back out as infrared, which is heat you cannot see. Some gases, carbon dioxide and water vapour above all, absorb infrared. They catch the heat on its way out and send part of it back down. The ground ends up warmer than it would be with no air.

Three planets compared.

	Venus	Earth	Mars
Distance from the Sun (Earth = 1)	0.72	1	1.52
Air pressure at the ground (Earth = 1)	92	1	0.006
The air is mostly	Carbon dioxide, 96%	Nitrogen 78%, oxygen 21%	Carbon dioxide, 95%
Average temperature at the ground	About 464 °C	About 15 °C	About −65 °C
Warming from the greenhouse effect	About 500 degrees	About 33 degrees	About 5 degrees

Venus is the extreme. Its clouds are so bright that it soaks up less sunlight than the Earth does, even though it is closer to the Sun. With no greenhouse effect it would be colder than we are. Instead its crushing carbon dioxide air holds the surface at about 464 °C, day and night, pole to equator. That is hotter than Mercury, which is much nearer the Sun.

The Earth without its greenhouse gases would average about −18 °C, and the oceans would freeze over. The natural effect is what makes the planet liveable. The concern today is about adding to it: burning coal, oil and gas has raised the carbon dioxide in the air by about half since the 1700s.

Mars has air that is mostly carbon dioxide too, but so little of it that it barely helps. Dry riverbeds and lake deposits show that Mars once had thicker air and running water.

Why they ended up so different. All three probably began with similar ingredients.

• Venus was a little too close to the Sun. Any early oceans evaporated, and water vapour is itself a greenhouse gas, so warming fed more warming. With no water, carbon dioxide from volcanoes had nowhere to go, and it stayed in the air.
• Earth kept its oceans. Rain washes carbon dioxide out of the air, and it ends up locked in limestone and other rock. There is about as much carbon in the Earth’s rocks as in the whole atmosphere of Venus.
• Mars is small, with a tenth of the Earth’s mass. Its weak gravity holds air poorly, its interior cooled, and it lost the magnetic field that had sheltered its air from the solar wind. Most of its atmosphere leaked away to space.

How much water Venus ever had, and for how long, is still an open question. No spacecraft has yet settled it.

A wider lesson. Astronomers talk about a habitable zone round a star, where a planet could have liquid water. Venus and Mars both sit near the edges of the Sun’s. They show that being in the zone is only the start: what a planet’s air does matters as much as where the planet is.