An Earth-sized exoplanet, TOI-700 d, is found to have an atmosphere containing methane, CH4\text{CH}_4CH4, and trace amounts of propene, C3H6\text{C}_3\text{H}_6C3H6.
Explain how methane acts as a greenhouse gas to keep the surface of TOI-700 d warmer than it would be without an atmosphere. You must refer to radiation wavelength in your answer.
Explain how an astrochemical probe landing on the surface could chemically demonstrate that the trace propene is an unsaturated hydrocarbon. Describe the test and the positive result.