Despite continual habitability headlines, the debate over whether Mars ever had or currently has life still rages. At every new planetary conference, someone presents data that often for a few weeks at least appears to change the calculus of Martian astrobiology. But for well over two decades now, one of the biggest mysteries remains --- the origin of the putative methane detections made by the Tunable Laser Spectrometer (TLS) on NASA's Curiosity rover which has repeatedly reported several detections of methane.

This methane flap actually began with the European Space Agency’s Mars Express spacecraft which started orbital science operations in 2004. During Mars Express’ first year of observations and analysis, a small spectrometer on the spacecraft detected a very low-resolution spectral bump which indicated the tenuous infrared absorption of methane in the red planet’s atmosphere.

Yet the detections were at the limits of the instrument’s (the Planetary Fourier Spectrometer) sensitivity, ESA notes.

It's not that convincing, but at the very same time, two different independent research groups using ground-based telescopes were also claiming that they could see Mars methane through Earth's atmosphere, Kevin Olsen, a planetary scientist at the University of Oxford and a co-investigator of the ExoMars Trace Gas Orbiter's Atmospheric Chemistry suite instrument, told me in the U.K.

Habitability is the key.

The most important thing people care about is whether Mars has been or is habitable, and whether there's microbial life, and in the atmosphere the big thing is methane, which we don't see, says Olsen.

As for the ExoMars TGO?
The Trace Gas Orbiter primary objective is to definitively detect trace gasses such as compounds with sulfur, chlorine, and methane.

If methane is coming from a biosphere, then either living things are producing methane as they go about their daily life and use energy, says Olsen. Or maybe they're not living anymore, and there remains an ancient source of biologically created methane.

If methane is not coming from a biological source; then the most likely thing is active rock metamorphosis, he says.

This is where rock changes from one type into another, which would happen deep underground, at high pressure, with lots of heat, water and chemical energy. But after eight years of observations with TGO’s instruments working at nominal levels, the ESA team still has not found evidence for methane.

We have a thousand times better sensitivity than the Mars sample analysis suite on NASA’s Curiosity rover, but we do not see any sign of the absorption of methane in the Martian atmosphere, says Olsen.