Free Astronomy Magazine November-December 2020

T he atmosphere is the calling card of the Earth. To the eyes of a distant observer, the periods in which it appears most polluted may be evident. Today it is very much pol- luted, and one of the most worrying pollutants is methane (CH 4 ), the molecular geome- try of which is rep- resented alongside. [Hamish Weir] NOVEMBER-DECEMBER 2020 ATMOSPHERIC POLLUTION 25 ene as messenger advanced civilization on a particular planet and if they are at some level of develop- ment similar to some level of ours over the course of our history. It is understandable that atmospheres polluted over a longer period of time will be recognizable at greater distances from the Earth compared to those in which pollution has only re- cently begun, as the light that carries traces of gases of non- natural origin will have trav- eled for longer. Observations of this type will be clearly limited by the resolving power of the i n s t r u m e n t s