1) Atmosphere. A greenhouse gas is a gas that absorbs infrared radiation emitted by a planet carrying away energy, but allows visible light carrying energy from the Sun, such as carbon dioxide, to pass through. (CO2, H2O, O3, CH4, CFC).b. Because the Sun has a surface temperature of about 5,800 Kelvin, it emits most of its energy as visible light. Because Earth has a much lower average surface temperature of about 288 Kelvin, it emits most of its energy as infrared light. Greenhouse gases are transparent to visible light, but partially block infrared light. Greenhouse gases act like one-way valves, because they allow the Sun's energy to reach the surface as visible light, but prevent the planet's energy from exiting as infrared radiation.c. The distance from the sun influences the temperature of a planet because the closer a planet is to the sun the higher the temperature. Planet reflectivity affects temperature because the more reflective a planet's surface is, the less solar radiation will be absorbed by the planet. The thicker the atmosphere, the warmer the planet's temperature will be because the more it retains greenhouse gases, raising the temperature. The higher the pressure in the atmosphere, the higher the temperature of the planet. As seen in physics, the greater the pressure that is exerted on an object, the more the temperature will increase.2) Cool Venusa. Because Venus is completely covered in clouds and reflects most of the solar radiation back into space. Without the greenhouse effect, Venus would be colder than Earth even though it is closer to the sun.b. If Venus had no greenhouse gases or clouds, the temperature would be much lower than now. This high temperature is due to the dense atmosphere of clouds and greenhouse gases, where if neither existed the temperature would drop dramatically.c. The reason for such thick and heavy clouds is directly related to volcanic activity. High volcanic activity leads to a higher concentration of clouds and therefore a heavier atmosphere. If the cloudy atmosphere were to disappear there would no longer be high volcanic activity, in fact the most likely outcome of the clouds disappearing would be that the volcanoes on Venus had become dormant. 3) Evolution of Mars' climate. So far we still have no clear scientific evidence of the presence of organisms on Mars.