COMPARISONS between planets in the Solar System, which have distinct atmospheres, especially Mercury, Venus, Earth and Mars, cast light on the massive interactions that determine the evolution of climates.

Mercury, the planet closest to the Sun, has an extremely tenuous atmosphere, consisting mainly of hydrogen, helium and nitrogen, which are continually becoming charged and escaping from the planet. As a result, Mercury's atmosphere has a life span of a few days or less. All the gases present in the atmosphere at any point of time are quickly lost and have to be replenished from other sources. This takes place mainly by emission of gases from the planet's interior (degassing) or through the impact of asteroids.

Venus is thought to have had an early atmosphere similar to that of the Earth, with a large amount of water, which is often referred to as an ocean although it may never have been in liquid form. Because Venus receives twice as much heat from the Sun as the Earth, the high level of humidity would have caused a large greenhouse effect.

There is a critical value of the solar energy received by a planet above which oceans evaporate completely into the atmosphere because the processes of evaporation and greenhouse warming reinforce each other. As water vaporises, the higher humidity intensifies the greenhouse warming, raising the humidity further.

Such a "runaway greenhouse" process is thought to have been important in the early history of Venus. Even today, Venus exhibits a huge greenhouse warming, sustained by the carbon dioxide in its atmosphere, which raises the surface temperature to about 500* Celsius.

Mars' atmosphere, like that of Venus, is mostly carbon dioxide. The planet is today too cold to sustain water in liquid form, and it is thought to contain a large amount of ice underground.

Victor Baker and his colleagues at the University of Arizona have recently suggested that Mars had several episodes of high atmospheric pressure, warm conditions and heavy rain and snow, with a north polar ocean and southern glaciers.

These episodes are thought to have been triggered by an abrupt increase in volcanic activity and each would have lasted several million years. Baker's hypothesis, though still controversial, explains the odd assortment of surface features -- huge channels, basins and canyons -- visible in images sent back by the Viking spacecraft in the late 1970s. Scientists are now waiting for photographs from the Mars Observer spacecraft, which will have 10 times the resolution of the Viking pictures. The Observer is due to reach Mars in August.