Blue with Cold

A scientific riddle, like a detective mystery, often exhibits strange coincidences. One of these is the connection between Earth's future climate and the colours of the giant gas planets. Take the four giant planets. Jupiter, Saturn Uranus and Neptune may be of different sizes, but they are principally made up of the same gases: hydrogen, helium and methane. One might therefore expect that in appearance they should be similar if not identical. But instead their colours are vastly different. Jupiter is a cocktail of reds, browns and white, and Saturn is a subdued creamy yellow, while Uranus is a pale greeny blue and Neptune is a much deeper blue. What is the reason for all this?

Arithmetic comes to the rescue. Uranus is almost three times as far from the Sun as Saturn, and Neptune is 56 per cent further away than Uranus. The further from the Sun, the colder a planet is. Although the chemistry is somewhat complicated, essentially the atmospheres of Uranus and Neptune appear to be frozen blue because they are cold enough for red-light absorbing methane-ice clouds to exist in their atmospheres. These worlds are literally "blue with cold.'' Being on a planet circling the Sun is like sitting beside a campfire on a freezing night. Those closer to the fire will be warm, while those further off will be progressively colder. This can also be seen with the inner planets. Venus is 26 million miles closer to the Sun than we are, with the result that its surface is hot enough to melt lead. The surface of Mars on the other hand (until such time as we get round to terraforming it), barely rises above freezing point in summer at noon . The Sun itself will not cause temperatures to alter on the planets dramatically for hundreds of millions of years, until the Sun slowly starts to expand in its first step to becoming a red giant. Is there any danger that climate change could suddenly make Earth as hot as Venus, which has endured a runaway greenhouse effect and is a sweltering 400 degrees Celsius at the surface? To do this, Earth would need to change its orbit and more closer to the Sun. Such a movement cannot of course happen, being prevented by Newton's First Law, which forbids a world from changing its orbit unless "acted on by a force.'' I cannot imagine any mechanism that would provide such a force. I have heard it said: "Suppose we were by hit by an steroid the size of Texas .'' Even that would not be nearly enough force, but that is something we don't have to worry about. There are no asteroids the size of Texas . The outer gas giants and particularly their moons, such as Titan, can tell us many things about the relationship between climate and distance from the Sun. It is often said that our fate lies in the stars. It can be seen more clearly in our neighbouring planets.