Asked by: Simone Walker, London
Venus is closer to the Sun and Mars is further away, but distance alone isn’t enough to account for the different conditions on these planets. The surface temperature on Venus, for example, is higher than on Mercury, even though it receives only 25 per cent as much energy from the Sun.
The explanation can be found in the three planets’ atmospheres. Billions of years ago Venus had water, but being closer to the Sun, most or all of this would have been in the form of vapour. Water vapour is a greenhouse gas, so heat from the Sun gets trapped and the temperature rises. This bakes carbon dioxide out of carbonate rocks, which increases the greenhouse effect and the temperature spirals up.
Mars is only a tenth the mass of Earth and further away. So it struggles to hang onto a thick atmosphere and water is present only as ice. Without greenhouse gases the temperature falls and any carbon dioxide condenses as dry ice, cooling the planet still further.