Greenhouse planets
Venus is almost twice as far from the Sun as Mercury. Which one is hotter? Change each planet's greenhouse gas and find out.
Earth: surface 15 degrees Celsius, in balance.
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.
- 3More 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
Sim. Grades 6 to 12. Question sets for middle school, high school, and upper high school.
How the sim works
A short opening sets the puzzle: Venus is almost twice as far from the Sun as Mercury, yet it is the hottest planet. Then students explore: pick any planet and change its greenhouse gas. The Goals button opens five optional goals, each judged by the sim once the temperature settles: take away Earth's greenhouse gas (it freezes at −18 °C), give it back (15 °C), take away Venus's (it falls to −44 °C, colder than Earth, because its clouds reflect most of its sunlight), make Venus hotter than Mercury, and warm Mars to 15 °C. Hint, then Skip, moves past a goal; Explore goes back to the planets and the goal waits.
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 greenhouse gas sets the air's infrared optical thickness τ, and the surface temperature is the bare-planet temperature times (1 + ¾τ)^¼: the two-stream Eddington approximation. The slider is logarithmic in τ.
Real atmosphere sets τ so each planet matches its measured mean: about 0.85 for Earth, 141 for Venus, and 0.08 for Mars, whose thin carbon dioxide air warms it only about 3 °C (published estimates are near 5 °C; Mars uses the often-quoted −60 °C average). Mercury has no air: its 167 °C is NASA's average, while its slow-turning surface swings from about 430 °C at noon to −180 °C at night. Albedos are held fixed (Mercury 0.07, Venus 0.76, Earth 0.30, Mars 0.25) whatever the gas does.
What the picture shows, and what it simplifies
- Ribbon widths are energy per square metre, on one scale for every planet: sunlight in equals reflected plus absorbed, and once a planet settles, heat to space equals sunlight absorbed.
- Under the ground, the absorbed sunlight and the heat sent back down pour into one pool of heat, and the heat beam rises out of it. The pool runs deeper and glows brighter as the ground warms.
- The red heat beam leaves the ground and splits at the greenhouse gas: part goes on to space, part curves back down to the ground. Its width is heat from the ground = heat to space + heat sent back down. The gas absorbs the heat and sends it out again in every direction; the curve is the share that ends up going down, not a reflection. On Venus the heat sent back down is about 70 times Earth's absorbed sunlight, far too wide for the screen, so past about twice Earth's the loop keeps widening as the gas grows, but ever more slowly.
- Each planet's air is drawn at a depth that follows its real amount of air; a lot of added greenhouse gas makes it deeper. Depth is not to scale: Venus has roughly 200,000 times as much carbon dioxide above each square metre as Earth. Earth's greenhouse gas is mostly water vapor, with carbon dioxide second.
- All sunlight is absorbed at the ground, and only Venus reflects from its clouds, drawn as one deck; other clouds and weather are left out.
Run it with a class
Pick the questions, get a class code, and watch answers arrive live. Students need only a first name.
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