sentence: these included strong absorption of light at the red end of the visible spectrum ( especially over continents ) which was caused by absorption by chlorophyll in photosynthesizing plants , absorption bands of molecular oxygen which is also a result of plant activity , infrared absorption bands caused by the ~ 1 micromole per mole ( µmol / mol ) of methane in earth 's atmosphere ( a gas which must be replenished by either volcanic or biological activity ) , and modulated narrowband radio wave transmissions uncharacteristic of any known natural source .