This is a lesson about planetary atmospheres. Learners will interpret real spectral graphs from missions to determine what some of Earth, Venus, and Mars’ atmosphere is composed of and then mathematically compare the amount of the greenhouse gas, CO2, on the planets Venus, Earth, and Mars in order to determine which has the most. Students brainstorm to figure out what things, along with greenhouse gases, can affect a planet’s temperature. The activity is part of Project Spectra, a science and engineering program for middle-high school students, focusing on how light is used to explore the Solar System.