When in orbit, the TGO will capture breathtaking high-resolution observations of geological features possibly related to the emissions of trace gases in the Martian atmosphere. Detections of small quantities of methane in Mars' atmosphere by past and present missions (such as NASA's Curiosity rover) has excited geologists and astrobiologists alike as its presence suggests something on Mars is currently generating the gas. Methane breaks down quickly when exposed to sunlight, so unless it's being produced by volcanic water-rock interactions or, possibly, microbial life, it shouldn't be there.
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Therefore, the TGO has three instruments used to detect trace gases in the atmosphere and it will use the high-resolution camera to track down their sources.
In addition to the orbiter, the TGO has a small lander called Schiaparelli that will be used to demonstrate entry, descent and landing in the Martian atmosphere. Named after famed Italian astronomer Giovanni Schiaparelli who mapped the red planet's surface in the 19th Century, the Entry, Descent and Landing Demonstrator Module (EDM) is also itself equipped with trace gas sensors that will "sniff" out rare gases after landing. Schiaparelli will enter the atmosphere on Oct. 19, on the same day the TGO enters orbit having separated from the satellite some 3 days earlier.
During this approach phase to Mars, the other instruments have been undergoing checks and mission control will send commands to the spacecraft on July 28 to make a major course correction to put it on the correct path to orbital insertion.
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The TGO is only the first half of the ExoMars mission, however. The ExoMars rover is planned for launch in 2018 and the TGO will act as the wheeled robot's communications relay with Earth after it lands.
So next month will be a big month for planetary exploration: first, NASA's Juno probe will arrive in Jupiter orbit on July 4, then ExoMars arrives at the red planet three weeks later. Keep your calendars clear!