Digital ACT Reading Practice Question #1608

On Earth, what two gases coexist in chemical disequilibrium as an unmistakable biosignature, according to the passage? Passage Excerpt (Natural Science): With the deployment of space observatories like the James Webb Space Telescope (JWST), the search for extraterrestrial life has shifted from listening for radio signals to analyzing exoplanet atmospheric transmission spectra. When an exoplanet transits across the face of its host star, a tiny fraction of starlight filters through the planet's atmospheric envelope. Gases in the atmosphere absorb specific wavelengths of light, leaving characteristic dark absorption lines in the starlight's spectrum. Astrobiologists search for 'chemical disequilibrium'—the simultaneous presence of gases that react violently with each other and would rapidly disappear unless continuously replenished by living biological metabolisms. On Earth, the coexistence of oxygen (O2) and methane (CH4) is an unmistakable biosignature: without photosynthesis and methanogenic microbes, oxygen would oxidize all atmospheric methane into carbon dioxide and water within a decade.