The first mission in a joint European-Russian campaign to look for life on Mars is scheduled for launch on Monday.
The primary satellite, called Trace Gas Orbiter, or TGO, is designed to circle Mars looking for telltale chemicals in the planet's atmosphere. Of particular interest is methane, which on Earth is mostly tied to biological activity.
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Methane on Mars, which has appeared in inexplicable spurts, is a more complicated story, one that may or may not have to do with indigenous populations of past or living microorganisms.
Easily broken apart by ultraviolet light from the sun, methane should have a fairly short lifetime in the Martian atmosphere so its occasional appearance and puzzling disappearance over particular locations at particular times is one of the biggest mysteries of current-day Mars.
The methane could be a byproduct of subsurface, olivine-rich rocks interacting with water, a geological process known as serpentinization. Another option is that methane was produced long ago and is trapped in ice-like, crystal structures called clathrates, which occasionally melt or break apart, releasing the gas.
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Most alluring is the prospect that methane-producing microbes living deep beneath the planet's radiation-blasted surface are producing the gas today.
With four science instruments, TGO is designed to very precisely map locations and times when methane and other trace gases appear in the atmosphere. That information should help scientists pin down the source.
TGO sets the stage for a more ambitious mission to directly search for life. The ExoMars rover, scheduled for launch in 2018, will be able to tunnel down into the planet's surface and hunt for past and present-day microbes.
TGO is carrying a demonstration lander called Schiaparelli that will collect data during its six-minute descent through the Martian atmosphere.
"It's basically a technology demonstration practice, which we exercise the full process of entry, descent and landing," said ExoMars 2016 project scientist Hakan Svedhem, with the European Space Agency.
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Once on the planet's surface, Schiaparelli's batteries should have enough power for humidity, pressure, temperature and other sensors to operate for a few days.
NASA, which has two rovers currently operating on Mars, has taken a different tact in the quest to learn if the planet most like Earth in the solar system has or ever had life. After discovering many signs of past water on Mars, NASA dispatched the Curiosity rover to see if Mars had the chemical ingredients and environments suitable for terrestrial-like life to exist.
With its first sample analysis, Curiosity scientists were able to answer that question with a definitive yes. Now they are trying to learn what conditions on Mars might best preserve the organic chemistry that would be proof of life. With NASA's next rover, scheduled to fly in 2020, scientists want to not only have a better idea of where to look for life, but also use the rover to cache samples for an eventual return to Earth.
TGO and Schiaparelli are due to launch aboard a Russian Proton rocket at 5:31 a.m. EDT Monday (Daylight Savings Time starts on Sunday) from the Baikonur Cosmodrome in Kazakhstan.
The trip to Mars will take seven months, with TGO putting itself into orbit and Schiaparelli landing on the surface on Oct. 19. TGO will spend about a year skimming the Martian atmosphere to get itself into position for its science mission, which is scheduled to begin at the end of 2017.