A better understanding of the Earth's climate requires a better understanding of the interaction between the planets’ geophysical processes and the dynamics of the Solar System as a whole. Earth is placed between two worlds (Mars and Venus) that have been devastated by climate catastrophes. To understand our own global warming, our neighbours can provide valuable insights into the way climate catastrophes affect planets. "It seems both Mars and Venus started out much more like Earth, both had water and then changed. They both hold priceless climate information for Earth," says Grinspoon, a climate modeller at Denver Museum of Nature and Science, and one of Venus Express’ interdisciplinary scientists.
Sun's energy heats up the Earth's surface (Fig. 1b) and the planet radiates energy at different wavelengths back into the space. However, the atmospheric gases trap some of the outgoing energy and it is retained as heat. This is termed as greenhouse effect, and it occurs on Mars, Earth and Venus. In fact, Venus undergoes the most powerful greenhouse effect in the Solar System. Greenhouse gases on the Venus include water vapour, carbon dioxide, sulphuric acid and aerosols. This causes 80% of the incoming solar radiation to be reflected back to the space by the cloud layer, about 10% to be absorbed by the atmosphere and only 10% manages to get through it and heat the surface. This results in a temperature difference of ~450oC between the surface and the cloud top (fig. 1a).
CLIMATE OF PLANETS
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 greenhouse 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 (UNEP GRID 2008). The main atmospheric constituents of terrestrial planets are given in Table 1.
Venus has almost 97% of its thick atmosphere composed of greenhouse gases, while Earth has only about 1.04% of its atmosphere composed of these gases. Mars, with its thin atmosphere, has about 96% of its atmosphere made of greenhouse gases. So, while the percent composition of the atmosphere is important, the amount of atmosphere does seem to play a role. Venus, with an atmospheric composition similar to that of Mars, has a much larger greenhouse effect. For the outer planets, mainly Jupiter, the climate is fairly warm. No solid surface exists. Instead, there is only a gradual transition to liquid. Jupiter's atmosphere contains trace amounts of water, ammonia, methane and other organic (carbon) compounds. |
