How did we detect gases on Venus?

 We have found Phosphine gas on Venus. But did we find composition of these gases from Earth? Answer is Spectroscopy. It tells how matter interacts with electromagnetic radiation. 

Atoms or molecules absorb and emit discrete wavelengths of EM radiation. Light coming from the stars, planets or any other bodies is separated into components. There is a distinct pattern in the spectrum which is different for every element.

They serve as fingerprint for the elements and enables astrophysicist for find chemical element composition of chemical element. This method can be used to find:

1. Chemical element.

2. Temperature.

3. Density.

4. Mass.

5. Luminosity etc .

Of celestial bodies.