Chemical Fossils Probably Detected on Mars.
“Organosulfur compounds are organic compounds that contain sulfur…. Fossil fuels, coal, petroleum, and natural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds, the removal of which is a major focus of oil refineries.” (note 2)
Kerogen is a macromolecule that weighs more than 1,000 atomic mass units. Kerogen exists in coal and oil shale. When heated, kerogen can release crude oil, which contains organic sulfur molecules. Kerogen was formed by life only (note 4).
Note 4: See https://en.wikipedia.org/wiki/Kerogen#Formation