Greenhouse planets
Venus is twice as far from the Sun as Mercury. Which one is hotter? Give each planet a blanket of gas and find out.
Earth: surface 15 degrees Celsius, in balance.
The Venus puzzle
Venus is almost twice as far from the Sun as Mercury, and its clouds reflect three quarters of its sunlight. Yet Venus averages 464 °C (867 °F) and Mercury 167 °C.
Pick Venus, take its blanket away, then give it back.
What students walk away with
- 1A planet warms until the heat it sends to space matches the sunlight it absorbs.
- 2Greenhouse gases absorb the heat leaving the ground and send part of it back down, so the surface has to get warmer to get rid of it.
- 3A thicker blanket of greenhouse gas means a hotter surface, even farther from the Sun.
Standards
- MS-ESS3-5 Causes of rising global temperatures
- HS-ESS2-4 Energy flow and changes in climate
Grades 6 to 12. Question sets for middle school, high school, and upper high school.
How the model works
Each planet absorbs sunlight S/4 × (1 − albedo), where S is 1366 W/m² at Earth's distance and falls off as 1/d². The ground warms until the heat escaping to space equals that absorbed sunlight. The atmosphere is one grey blanket of infrared optical thickness τ, and the surface temperature is the bare-planet temperature times (1 + ¾τ)^¼: the two-stream Eddington approximation.
Real atmosphere sets τ so each planet matches its measured mean: about 0.85 for Earth, 141 for Venus, and almost nothing for Mars's thin air. Mercury has no air, so its 167 °C is a day-night average of bare rock with 430 °C days and −180 °C nights.
Albedos are held fixed (Mercury 0.07, Venus 0.76, Earth 0.30, Mars 0.25) whatever the blanket does. Venus's bright sulfuric acid clouds give it the highest albedo, which is why its bare temperature is so cold. Clouds, weather, and the temperature profile of the air are left out. The blanket slider is logarithmic in τ, and ribbon widths follow the flows up to one and a half times Earth's absorbed sunlight, then are compressed so Venus fits on screen.
Run it with a class
Pick the questions, get a class code, and watch answers arrive live. Students need only a first name.
Set up an assignment