The absorption lines in the spectrum of a planet or satellite do not necessarily indicate the composition of the planet or satellite's atmosphere. Why not?

When exploring the mysteries of celestial bodies, the composition of their atmospheres provides us with clues about their nature and potential for supporting life. The atmosphere of a planet or satellite is made up of various gases, each contributing its unique qualities and interactions.
For instance, Earth's atmosphere contains nitrogen, oxygen, and a small portion of other gases like carbon dioxide and argon. The atmosphere protects us from harmful solar radiation, insulates the planet, and allows life to thrive.
Understanding atmospheric composition is crucial in the field of astrobiology. It helps scientists predict the potential habitability of exoplanets. However, analyzing another world's atmosphere isn't straightforward. Many gases are transparent to visible light, and only specific instruments can detect their presence. Through the technique called spectroscopy, researchers dissect the light from a planet or satellite to determine which gases are present based on absorption lines — dark lines in the light spectrum due to particles in the atmosphere absorbing specific wavelengths.

Spectral lines are like fingerprints for elements and molecules found within a gaseous medium, such as a planetary atmosphere. When light traverses through an atmosphere, certain wavelengths get absorbed by the gas. This creates dark lines—known as absorption lines—in the otherwise continuous spectrum.
Each element absorbs light uniquely due to its electron structure, resulting in a specific set of spectral lines. For example, hydrogen has prominent spectral lines in the visible region of the spectrum, which are known as the Balmer series.
The study of these spectral lines is essential in astrophysics, allowing scientists to deduce the chemical composition and physical conditions (like temperature and pressure) of celestial bodies. However, the task is complex because the spectral lines from different elements can overlap, and various atmospheric layers may contribute different spectral lines, adding to the interpretive challenge.

Planetary atmospheres can be thought of as the delicate shrouds that envelop planets, affecting their climates, geology, and potential for supporting life. Understanding the atmospheres of planets within our Solar System and beyond is a key objective in planetary science.
Planetary atmospheres are made up of layers, each having varying temperatures, pressures, and compositions. These factors can significantly influence the absorption spectra, complicating the process of analyzing the atmospheric makeup. Notably, the thickness of the atmosphere and the gravitational pull of the planet also play roles in determining the distribution of gases.
The study of planetary atmospheres extends our knowledge of climate dynamics, guides the search for extraterrestrial life, and can even offer insights into Earth's past and future environmental changes. As we develop more sophisticated technologies like space telescopes equipped with spectrometers, our understanding of these complex systems continues to deepen, unlocking many of the secrets held by distant worlds.