Structures of some simple carbon compounds

A comparison of the atmospheric compositions of the Earth and its neighbours, Mars and Venus, which were most likely formed at the same time, is given in Table 1.2. This shows that the main differences relate to the concentrations of carbon dioxide in the atmospheres of Mars and Venus when compared to the atmospheric composition of the Earth, and the relative abundance of oxygen and nitrogen in the atmosphere of the latter. In view of a likely common origin of all three planets, these differences are surprising. However, Earth has had life forms almost since its ineeption. 

There have been various times in Earth history when the extent of ice has been considerably greater than it is now. There were five such periods in which great ice ages occurred, as listed in Table 1.3. All were characterized by extensive polar ice sheets and cooler global temperatures than at present. 

The causes of the long-term past and recent ice ages are foimded on two factors. First, there are the astronomical factors which relate to the way the Earth revolves aroimd the Sim and, second, there are factors involving the carbon cycle. The important astronomical factors are now considered to 

---

Figure 1.3. Structures of some simple carbon compounds...

---