Hail Mary Science
Light, heat and climate

Infrared spectroscopy lab

Every molecule absorbs infrared light at specific wavelengths. Turn on several gases and overlay their peaks to see how they are told apart and where they overlap. It is the technique used to analyze the atmospheres of exoplanets.

Try it

How to use the simulator

Every molecule absorbs infrared light at specific wavelengths, set by the way its bonds vibrate. That pattern of peaks works like a fingerprint: analyze light that has passed through an atmosphere and you can tell which gases it holds without touching it. This is how astronomers study exoplanet atmospheres today, for instance with the James Webb Space Telescope.

Controls

  • Gas buttons: CO₂, H₂O, CH₄, NH₃ and O₂. Turn on several at once to overlay their peaks on the same axis.

Things to try

  1. Turn on CO₂ alone and find its peaks. The 4.26 and 15 μm bands are its strongest real ones; the novel adds 18.31 μm as the signal astrophages follow.
  2. Add H₂O and CH₄ and look for overlaps. When two gases absorb in the same place, telling them apart takes more resolution.
  3. Compare ammonia with water: both have peaks near 3 μm because both have hydrogen bonded to a light atom (N–H and O–H).

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Molecular fingerprints

The bonds between atoms vibrate like springs: they stretch, bend and twist at frequencies set by the atoms' masses and the bond strength. When infrared light arrives at exactly that frequency, the molecule absorbs it and vibrates harder.

The result is a pattern of absorption peaks unique to each molecule. CO₂ absorbs strongly around 4.26 and 15 microns; water near 2.7 and 6.3; methane at 3.3 and 7.7. Shine light through an unknown gas, see what is missing, and you know what it contains.

Atmospheres of other worlds

When an exoplanet passes in front of its star, a small part of the starlight crosses its atmosphere. Comparing the star's spectrum with and without the planet in front reveals the absorption lines of its gases. In 2022 the James Webb telescope made an unambiguous detection of carbon dioxide this way in the atmosphere of the planet WASP-39b.

Astrobiologists look for gas combinations that are hard to explain without life, such as oxygen alongside methane, which react with each other and shouldn't last long together unless something keeps replenishing them.

Worked example

Example: telling water from ammonia

  1. Turn on water (H₂O) alone: you will see a strong peak near 2.7–3 microns, from the O–H bond vibration.
  2. Add ammonia (NH₃): it also absorbs near 3 microns, from the N–H bond. At low resolution the two peaks blur together.
  3. Now look for a peak only one of them has: ammonia absorbs around 10.5 microns, where water barely does. That peak tells them apart.
  4. The general rule: the more independent peaks you compare, the more certain the identification.

In Project Hail Mary

In the novel, scientists study which wavelengths a fictional microorganism responds to and link those lines to those of a particular gas. Spectroscopy is the tool that makes that possible.

This tool appears in:Chapter 5The Life Cycle of Astrophages

Frequently asked questions

What is the difference between absorption and emission spectra?

An absorption spectrum shows dark lines: light a cool gas has removed from a background source. An emission spectrum shows bright lines: light a hot gas gives off. For the same gas, the lines sit in the same places.

Why do O₂ and N₂ barely absorb infrared?

Because they are molecules of two identical atoms: vibrating doesn't change their charge distribution, and without that change they don't interact with infrared. That is why they aren't greenhouse gases.

Is spectroscopy used on Earth?

All the time: in breathalyzers, food and drug testing, CO₂ sensors and pollution monitoring.

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