https://www.scientificamerican.com/article/mars-methane-hunt-comes-up-empty-flummoxing-scientists/

A spacecraft that was supposed to solve the mystery of methane on Mars has instead compounded scientists’ confusion. The European–Russian Trace Gas Orbiter (TGO), which began looking for the gas last year, has yet to find any whiffs of it in Mars’s atmosphere, says a study published on 10 April in Nature.

“It’s a huge surprise,” says Dorothy Oehler, a planetary geologist at the Planetary Science Institute in Houston, Texas.