Consider a scenario in which biological warfare rapidly wiped out a planet’s population. Microorganisms that cause decomposition would gorge themselves on alien corpses. In doing so, they would excrete chemical compounds, dramatically increasing the levels of methane and ethane in the atmosphere.

If the population size of the alien world were comparable to that of Earth, the methane and ethane gases would dissipate in about a year, so there would be only a short window of opportunity to detect the cataclysm.

However, if the biological arsenal included a genetically modified virus capable of jumping species, then the planet’s casualties might also include its animal life. In that case, the telltale signs of catastrophic biowarfare could be visible for several years.