Barcodes of Life: Spectroscopy for Life Detection

If chemical elements and molecules absorb or emit specific wavelengths of light, and if we can split starlight into a spectrum after it passes through an atmosphere, then we can identify the specific chemical composition of that world from Earth.

If an atom absorbs a photon, the energy must exactly match the gap between electron shells; if every element has different shell spacings, then every element will produce a unique pattern of lines in a spectrum.

If Oxygen is an oxidizer and Methane is a reducer, then they should chemically react to form water and CO2; if we see both in an atmosphere at once, then an active process like life must be continuously replenishing them.

If we need to see the individual colors or wavelengths to find missing gaps, and if a prism or grating is used to disperse light, then the electronic device recording this is a spectrograph.

If terrestrial life is built from organic molecules, and if those molecules are primarily composed of CHONP, then these are the specific chemical markers scientists look for in a planet's spectral data.

If plants use chlorophyll to absorb visible light for photosynthesis but reflect near-infrared light to avoid overheating, then a spectrum of a planet with forests will show a sudden, steep jump in reflectance near 700 nm.

If a telescope is large enough to collect a sufficient number of photons, and if the light maintains its spectral pattern over distance, then we can analyze atmospheres of planets hundreds or even thousands of light-years away.

If a planet passes in front of its star, a tiny fraction of starlight passes through the gas surrounding the planet; if that gas absorbs specific colors, then those "missing" wavelengths reveal the atmosphere's chemistry to our telescopes.

If a gas cloud or atmosphere sits between the light source and the observer, and if the atoms in that gas "soak up" specific photons, then the resulting spectrum will contain black gaps known as absorption lines.

If a biosignature is a substance specifically tied to biological processes, then O3 (from O2), N2O (from bacteria), and Chlorophyll (from plants) qualify; CO2 and H2O are necessary for life but can also exist on dead planets.

If molecules vibrate and rotate at energy levels that correspond to infrared wavelengths, then the most distinct and identifiable absorption patterns for greenhouse gases and biosignatures will be found in that part of the spectrum.

If a planet's atmosphere is in equilibrium, it is "chemically dead"; if life is present, it will continuously inject reactive gases that keep the atmosphere in a state of imbalance, which we can detect via spectroscopy.

If light behaves as a wave, and if color is determined by the physical spacing of the wave cycles, then that spacing is defined as the wavelength.

If we want to know what light was absorbed, we must first know what the light looked like originally; if the star provides a smooth, full rainbow of light, then that baseline is called the continuum.

If clouds are opaque, we can't see the gases below them; if the signal is weak or Earth's own air gets in the way, we cannot accurately measure the tiny chemical signatures from distant planets.

If light is energy, and if an electron in an atom gains that energy, then it must move to a higher "excited" state; if the photon energy is not an exact match for a jump, the atom cannot absorb it.

If molecular oxygen (O2) is hit by ultraviolet light from a star, it breaks and reforms into O3; if O3 is much easier to see in the infrared than O2, then astronomers look for Ozone to prove Oxygen is present.

If the field combines astronomy (the stars) and biology (life), and if it seeks to understand life's origin and distribution in the cosmos, then the name is astrobiology.

If a photo shows where an object is and what color it looks, but a spectrum breaks that color down into thousands of specific data points, then the spectrum reveals the actual atomic and molecular "recipe" of the object.

If a mission is equipped with infrared or optical spectrographs and stays in space to avoid Earth's air, then it can perform exoplanet spectroscopy; Voyager and Curiosity are probes that do not look at distant exoplanet atmospheres.