Free Astronomy Magazine November-December 2020

NOVEMBER-DECEMBER 2020 atmospheres and their attribution to bio- logical processes is also nothing new, even in our own solar system. Seasonal methane variations in the Martian atmosphere are one example. Methane, CH 4 , is broken down by UV radiation. The thin Martian atmos- phere provides little protection from solar radiation, resulting in what should mean the eventual depletion of methane. That it is found at all – and changes seasonally – indicates that either some geochemical process is producing methane cyclically or that, possibly, some biological process is oc- curring that is seasonally restoring atmos- pheric methane. While the detection of methane around a rocky exoplanet would be a significant dis- covery, there is another molecule that would make all of exobiology take notice. Oxygen presently makes up 21% of Earth’s atmosphere, but this percentage would itself eventually reduce to zero if not con- stantly replenished through photosynthe- sis. In fact, the evolution of life did not begin with aerobic, or oxygen-consuming, respiration. It was the evolution of single- celled organisms capable of photosynthe- sizing CO 2 and producing O 2 as a waste product that ultimately changed the at- mosphere, and evolutionary history, of Earth. T his illustration depicts the Venusian surface and atmosphere, as well as phos- phine molecules. These molecules float in the wind- blown clouds at altitudes of 55 to 80 km, absorbing some of the mil- limeter waves that are produced at lower altitudes. [ESO/M. Korn- messer/L. Calçada]