Our current understanding of the Martian atmosphere is limited by a lack of measurement data. We will make daily composition measurements of known atmospheric species with spectra between 250-2500 nm (including water, ozone, oxygen, carbon dioxide, methane and aerosol) and search for undetected species. We will also obtain meteorology data, information on how key atmospheric species are partitioned and altitude-profile-composition information. Our lander-based instrumentation will allow us to observe directly the atmospheric surface environment below the boundary layer, which is unlikely to be observed by remote spacecraft sensing. We shall also be able to measure directly the surface-radiation flux and separate direct and diffuse flux components. We will use this data to improve our current understanding of the Martian atmospheric environment, its chemistry and variation. We will look for atmospheric gases that would support the indirect evidence of recent volcanism, and indicate the presence of life. We will also be able to provide a ground-based validation point to compare results from orbiting atmospheric instruments and from other landers deployed during our mission.