Evidence is mounting that, on earth, the “greenhouse effect” is bringing about significant changes in the climate. In The Greenhouse Effect Harold Bernard describes the dynamic as it works on earth. As a result of the burning of fossil fuels carbon dioxide builds up in the atmosphere. This gas is transparent to solar radiation, but opaque to thermal radiation. As a result it lets energy in from the sun, allowing it to heat the earth, but does not allow the heat generated to flow back into space. This causes global warming with all the political-economic consequences that that implies. Today there is a growing conviction in the scientific community that this is indeed happening (though the degree to which it is happening is something much argued over) and there is a growing concern that the results of this could have profound, even catastrophic, effects on ocean levels, agriculture, and many areas of economic concern to human beings. We tend to think of the greenhouse effect as an earthly phenomenon. However, one of our near neighbors in the solar system, Venus, has a pronounced greenhouse effect, and another neighbor, Mars, may have had a pronounced greenhouse effect in the past and still exhibits a very slight one today.
Mars was once much different from what it is today. The contemporary surface of the planet shows features that reveal that there was once much running water there. Close studies of images of Mars reveal not only gross geological features that suggest that there was once flowing water on the surface, but also the presence of minerals, iron oxides, that are associated with a water rich environment. One way in which this is explained is that volcanism released carbon dioxide into the atmosphere thus creating a greenhouse effect that raised the temperature and allowed water to flow freely. This, however, was at a very ancient period. The peaks of fluvial activity on Mars was about 3.5 billion years ago . Some, such as Brandenburg and Paxson, have argued that this period may have been terminated by an asteroid impact , however less exotic theories postulate that the water may have evaporated into space or seeped underground, or was stored in the polar ice caps . The loss of vapor from the water would, of course, have thinned the atmosphere.