x x x
Next Lecture Last Lecture UWS Physics Dept

Observations and the Solar System : Lecture 6

Comparisons of Atmospheres of Venus/Earth/Mars

Planet N2 O2 CO2 other surface pressure
Venus 4% trace 96% trace 70 At
Earth 77% 21% trace 2% 1 At
Mars 3% trace 95% 2% 0.006 At
It is thought that all three planets developed atmospheres from the same source -volcanic activity. Volcanoes produce gases - CO2,Nitrogen and sulphur dioxide and a little water vapour. This is essentially the atmosphere of Venus today. Despite the common original atmosphere the three planets look very different today - why?

To understand we can examine what happens on earth. There is very little CO2 -about 0.03% in the earths atmosphere however there is some - and it is very important being essential for plant life. It also has a small but rather nice effect on temperature it is estimated that the CO2's effect is about 30C. That is without the CO2 the earth would be rather colder. On earth we still have volcanoes so why does the CO2 not build up? Well, it is because we have liquid water and rain. CO2 is a fairly soluble gas - it dissolves in water reasonable well. Because of this rain washes the CO2 into the sea. Once in the sea it may become bound in other chemicals and can become deposited on the sea bed. A good example of this is in Chalk! The process of removal is fairly fast - it would only take a few thousand years for all the CO2 to be washed from the atmoshere.

Mars

The situation in mars has similarities to that on Earth -with differences.
  • Mars is further from the sun so naturly colder
  • To have liquid water Mars would an atmosphere with 1000mb or 1At of CO2
  • It is thought that early in its life this was the case
  • Mar's volcanic activity has ceased - CO2 is no longer replenished in the atmosphere
  • Temperatures fell as CO2 was washed out and the greenhouse effect ceased.
  • CO2/Water both now exist only in the frozen polar caps

    Venus

    Venus has gone the opposite way from Mars. CO2 is still being produced but since Temperatures are > 100C liquid water is not present. Hence the processes removing CO2 from the atmosphere do not take place. Thus the runaway greenhouse effect

    To summarise on earth a balance is maintained bettwen the creation of CO2 and its removal from the atmosphere. On Mars the creation process stopped. On Venus the removel process never got happening

    Life on Mars?

    There has long been speculation that life may have existed on mars. Although the planet now appears to be a harsh enviroment to sustain life there appears evidence that in previous epochs Mars was a much more positive place for the existance of Life. There is even some evidence that fossil life exists on a meteorite which originated from Mars.

    ALH84001

    There is some evidence to suggest that bacterial-like life existed on Mars. This information comes not from probes sent to mars but from evidence gathered by another sourse. This evidence come from meterites which have landed on earth and are thought to have come from Mars. These meterites have been found in antartica. . There are 12-14 meterites generally accepted are originating from Mars. Of these it is ALH84001 upon which attention has focused. The potted history of this 2kg rock is thought to be On the rock there is some evidence that it contains residues from bacteria like-life. NASA reckons this is strong evidence that like existed on Mars

    If the residues on ALH84001 are confirmed to be biological this will be one of the major scientific discoveries of the last century. It will however leave many further questions