# Planetary Atmospheres

Directions: Three slider controls allow you to change the properties of a planet. Changes to the planetary properties are reflected in the window above the sliders. The resulting temperature is displayed on the thermometer on the right side.

• Distance from Sun controls the semi-major axis of the planet's orbit. A planet closer to the Sun receives more sunlight, making the planet's surface warmer.
• Albedo controls the fraction of sunlight reflected away by the planet's atmosphere and its surface. Albedo can range from zero (complete absorption) to one (complete reflection).
• Optical depth controls how well the atmosphere traps infrared radiation escaping from the planet's surface. An optical depth of zero means there is no atmosphere. A large optical depth means the atmosphere is very thick.

For reference, here are the relevant properties for Mercury, Venus, Earth, the Moon, and Mars:

 Mercury: distance = 0.39 AU albedo = 0.1 optical depth = 0 (airless) Venus: distance = 0.72 AU albedo = 0.7 optical depth = 80 Earth: distance = 1.0 AU albedo = 0.3 optical depth = 0.5 Moon: distance = 1.0 AU albedo = 0.05 optical depth = 0 (airless) Mars: distance = 1.52 AU albedo = 0.2 optical depth = 0.08

 Questions to Consider: 1. What effect does increasing the planet's distance from the Sun have on the planet's temperature? (Click for Answer) Planets farther from the Sun receive less sunlight. This means that planets farther from the Sun generally have lower surface temperatures. 2. What effect does increasing the planet's albedo have on the planet's temperature? (Click for Answer) Planets with higher albedo reflect more of the sunlight they receive back into space. This reduces the temperature at the surface. 3. What effect does increasing the planet's optical depth have on the planet's temperature? (Click for Answer) Planets with thicker atmospheres are better at trapping the energy of the sunlight they absorb. Higher optical depth means higher surface temperature. The ability of an atmosphere to trap heat is known as the greenhouse effect. 4. What are some examples of greenhouse gases? A greenhouse gas is one that is very good at absorbing infrared radiation. (Click for Answer) Carbon dioxide is a powerful greenhouse gas. Water vapor and methane are also strong greenhouse gases. Atmospheres with these gases are particularly good at trapping heat from the Sun. 5. What features of a planet raise the planet's albedo? What features of a planet lower the planet's albedo? (Click for Answer) Large amounts of clouds or ice on the surface raise a planet's albedo. Earth's albedo is also raised by the presence of the oceans. Large amounts of volcanic rock, like the surfaces of Mercury and the Moon, lower a planet's albedo.

 Created by Kevin Healy, 2007