As I wrote about earlier, scientists are now scrutinizing methane emissions on Mars to determine whether they were caused by microorganisms or some unusual geologic phenomenon. But the astute reader might wonder; how did astronomers on Earth discover Martian methane in the first place? Because of its orbit, Mars can be anywhere between 55 million kilometers and 400 million kilometers away from Earth. Not to mention methane is clear, how did astronomers “see” it? What gives?
Of course, physics has an answer. One of the most powerful tools in an observatory is called the spectrometer and astronomers use it to tell what elements make up objects very far away. It’s a brilliant system that needs nothing more than visible light to work.
When light hits an object, some wavelengths are absorbed by the object, while others are reflected off, giving the object its color. Spectroscopy uses this reflected light to create a sort of fingerprint of the elements in the object.
Atoms are the fundamental building blocks of every element in the universe. They’re composed of a number of electrons orbiting around a dense nucleus of protons and neutrons, the number of which is unique to each element. Because of the strange laws of quantum mechanics, these electrons can only orbit at very specific, discrete distances away from the nucleus, and the distances are unique for every element. When light hits the cloud of electrons they’ll absorb some of the energy of the light, and jump from their ground state (their closest orbit) to their excited state (an orbit farther away). The energy they absorb is at a very specific energy wavelength on the electromagnetic spectrum.
As a result, when light reflects off of any material, it’s missing thin wavelengths of energy. When this light is shown through a prism, it spreads out from longer, low energy wavelengths (red) to the smaller, high energy wavelengths (violet) in the form of a rainbow. The missing packets of energy show up a pattern of small dark bands at certain colors, each pattern unique for every element.
That’s for reflected light, like sunlight off of Mars. The process works in reverse when you have a star or other bright object giving off light of its own like a star. When the object’s electrons jump from their excited states to their ground states, they give off specific energies of light, at exactly the same frequencies that they absorbed them. If you look at a star through a spectrometer you’ll see a dark bar with thin bands of color where on the bars would be if the light had been reflected.
Using these techniques, astronomers can find out what elements make up far away objects, like the methane on Mars. They don’t stop there. Spectroscopy can be used over tremendous distances, like when carbon dioxide and water vapor was found in the atmospheres of planets many light years away.
Since it was first used on stars by Father Angelo Secchi in the mid 19th century, spectroscopy has allowed astronomers to probe the composition of the universe over great distances with extraordinary accuracy. Without it we would have to physically travel to Mars to find methane.