1. A planet's climate is decided by its mass, its distance from the sun and the composition of its atmosphere. Mars is too small to keep a thick atmosphere. Its atmosphere consists mainly of carbon dioxide, but the atmosphere is very thin. The atmosphere of the Earth is a hundred times thicker. Most of Mars' carbon dioxide is frozen in the ground. Mars' average surface temperature is about -50°C. Venus has almost the same mass as Earth but a thicker atmosphere, composed of 96% carbon dioxide. The surface temperature on Venus is +460°C. Earth's atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. Carbon dioxide accounts for just 0.03 - 0.04%. Water vapour, varying in amount from 0 to 2%, carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the surface and emit radiation from much higher and colder levels out to space. These radiatively active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effect of a greenhouse. This blanketing is known as the natural greenhous effect. Without the greenhouse gases, Earth's average temperature would be roughly -20°C. The climates on Mars and Venus are very different, but very stable and highly predictable. The Earth's climate is unstable and rather unpredictable as compared with that of the other two planets.