How ExoMars studies the atmosphere
The graphic shows a simple representation (not to scale) of the three observing modes that will be used. In nadir mode (left) the spacecraft looks directly at the sunlight reflected from the surface and atmosphere of Mars. In limb mode (centre) it looks across the martian horizon at emission from the atmosphere. In solar occultation mode (right), the instruments point through the atmosphere toward the Sun and observe how different atmospheric ingredients absorb the Sun's light.
Since different chemicals have distinctive fingerprints, these observations provide a detailed inventory of the atmosphere's composition. These observations are critical to detect atmospheric gases that exist in tiny amounts, but which play an important role in determining if Mars is active today – either geologically or biologically speaking.
Note that the orientation of the spacecraft is not true: it is shown here with the instrument panel facing the viewer for illustrative purposes only. ACS is represented by the yellow square shape at the front of the orbiter; NOMAD is the grey box also at the front. Click here for a labelled diagram.