Figure 4-3 RIVUXG Analyzing a Moon’s Atmosphere through Its Spectrum (a) Titan is the only moon in the solar system with a substantial atmosphere. (b) The dips in the spectrum of sunlight reflected from Titan are due to absorption by hydrogen atoms (H), oxygen molecules (O2), and methane molecules (CH4. Of these, only methane is actually present in Titan’s atmosphere. (c) This illustration shows the path of the light that reaches us from Titan. To interpret the spectrum of this light as shown in (b), astronomers must account for the absorption that takes place in the atmospheres of the Sun and Earth.