The PLATO (PLAnetary Transits and Oscillations of stars) mission was selected along with three other M-class mission candidates for an assessment study in the framework of the ESA Cosmic Vision programme. It was short listed because of its important and high profile scientific objectives and its comparatively low technical risk. PLATO's main scientific goal is to study exoplanetary systems, by detecting and measuring exoplanet photometric transits in front of a large sample of bright stars, and by fully characterizing the planets and their host stars. In particular, this characterization will include seismic analysis of the planet host stars, providing accurate and reliable measurements of the masses, radii and ages of the host stars, hence allowing us to measure similar quantities for the detected exoplanets. Classical methods for characterizing stars, essentially based on their precise location in the HR diagram and its comparison with computed evolutionary tracks and isochrones, fail to provide the accurate and reliable stellar parameters that are needed. Masses cannot be known to better than 15%, while ages remain essentially undetermined for main sequence stars. New investigations, such as seismic analysis of exoplanet host stars, are therefore necessary to progress in this area. The current crop of missions MOST, CoRoT, Kepler (and the candidate SMEX mission TESS) are expected to bring essential results in our understanding of exoplanetary systems, but suffer limitations in terms of number of stars monitored, minimum planet size, maximum orbital period, precision in frequency measurements for asteroseismology. PLATO, as a next-generation mission after these pioneering experiments, will overcome these limitations, and provide us with a much better, much deeper investigation tool for studying the statistics of exoplanets and their evolution, as well as stellar structure and evolution. PLATO will therefore provide us with an unprecedented sample of fully characterized exoplanets, with accurate and reliable measurements of their masses, sizes and ages, from which exoplanetary system formation and evolution can be studied. Also, because PLATO will search for planets orbiting bright stars, it will provide key target lists, including earth analog systems, for further studies of planets, ideal for future ground- and space-based coronographic and/or interferometric facilities (eg ELT) and spectroscopic searches for bio-markers. This ability places PLATO central to statements in the STFC Delivery Plan/Roadmap and ESA Roadmap concerned with the occurrence of earth analogue systems and the emergence of life. In this proposal we bid for funding for the PLATO Phase A. The UK community is well represented on the PLATO Council (3 members) and are serious contenders for obtaining the PI for the Science Consortium (or at least co-PI). Hence we are influential in the design and implementation of the experiment itself. Within the UK we need to prepare both the exoplanet and asterosiesmology communities to engage with the mission as a significant level of preparation and organisation will be needed to fully exploit the datasets. On the technology front our efforts will concentrate on areas that we can already claim some leadership ie CCD procurement and characterisation, the design and construction of the focal plane, and data analysis and archiving. In each of these we can claim considerable relevant experience due to significant preparatory work for ESA's Eddington Mission (which had a similarly complex focal plane), the Gaia data pipeline and the Wide Angle Search for Planets (WASP) archive and transit search software.

MARS-XRD is a combined X-ray diffraction and fluorescence instrument. The aim is to better a 200 eV energy resolution at 6 keV. In addition we aim to maximise the angular resolution, which determines the ability of the instrument to distinguish between different types of minerals in diffraction spectra; the target is < 1 degree. If the detectors are operated in an 'integrating' mode to collect X-ray images, then the fluorescence signal from the sample will generate a background that can be used to determine the composition of the sample. These XRF background events can be reconstructed into an energy spectrum. In conventional systems, this XRF signal is a source of background; however, our energy discriminating instrument will enable the separation of the diffraction pattern from the fluorescence signals thus improving the accuracy of both. If the CCDs are operated in photon counting mode, then the energy of each detected X-ray event can be determined; the diffracted primary monochromatic X-rays can be assembled into a diffraction image, and later a diffractogram which does not contain fluorescence background. The energy information from all photons can be used to create an XRF spectrum of the sample. This photon counting mode is identical to the mode of operation of the CCDs in the XMM EPIC instrument and the Swift X-ray telescope and this highlights the UK heritage behind the development of this instrument. With the 2 theta range of the proposed instrument and the elemental analysis provided by XRF, it will be possible to determine the exact elemental chemistry of rock components covering minerals from clays to oxides, including silicates, carbonates, evaporates and apatites