Free Astronomy Magazine July-August 2025

5 JULY-AUGUST 2025 ASTRO PUBLISHING Dimethyl sulfide as a biosignature Dimethyl sulfide (CH 3 SCH 3 , or DMS) is a volatile organic compound of sulfur, produced primarily on Earth by the degradation of metabolites of phytoplankton and bacteria. DMS has a characteristic unpleasant smell that can resemble brackish water, boiled broccoli, or rotten eggs. On our planet, DMS is con- P hytoplankton populations perhaps similar to the one pictured here may be re- sponsible for a biosignature called dimethyl sulfide, traces of which have been detected in the transmission spectrum of the exoplanet K2-18 b, according to re- searchers at the University of Cambridge. [Getty image] stantly released from the surface of oceans and seas up into the atmos- phere, but constitutes only about a billionth of its total mass. Depending on the altitude reached in the atmosphere and the amount of radiation from space, the life of DMS molecules can last from a few hours to a few days before ultravi- olet rays and interactions with other gases destroy them. If the Earth were devoid of life, most likely there would be no trace of DMS in its at- mosphere. Given the speed with which its chemical bonds break, there would equally be no trace of DMS if it were not being constantly synthesized.