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Any Cosmic Vision ought to be based on the weak anthropic principle, on the emergence of life. Our Universe is but one of many possible worlds. For humans to exist, a remarkable fine tuning of the laws of physics and the fundamental constants is required. For life to emerge, nucleosynthesis needs to have proceeded to enrich significantly the interstellar medium and guarantee that carbon, nitrogen, oxygen and phosphor are widespread in the Universe. Studies of the metal abundance variation up to redshift z=5 are showing that the metallicity increases steadily with the age of the Universe. However there are numerous evidences of a large scatter in the metallic properties of matter for any given z; non metal-enriched clouds have been detected and chemically processed material has been found in the voids of the Cosmic Web. Meanwhile, the star formation rate seems, to be decaying from z=1. Important clues on the metal enrichment spreading on the Universe hang on inter- galactic transport processes that are poorly studied because of the lack of high sensitivity imaging capabilities to detect the warm/hot plasma emission from galactic halos. Current information comes from ultraviolet absorption spectroscopy that it is a rather inefficient technique to map the large scales involved and requires the presence of strong background sources. Most of the emission is expected to come from filaments and chimneys that radiate strongly in the UV range. To study these structures a high sensitivity imaging capability is required with resolutions at least ten times better than those provided by the GALEX mission. Galactic halos are made of collisionless plasmas, very sensitive to fields and waves. Thus, they can be used as a good diagnostic tool for variations in the galactic gravitational field, or the presence of more subtle fields as those that might be associated with dark energy.
Metallicity is relevant for life generation not only at the DNA level but also at much earlier phases. Silicates and carbonates are the key building blocks of dust grains and planetesimals in protostellar disks. Far UV radiation is a major actor in disk evolution. It drives to the photoevaporation of the gas compound from the disk releasing the rocky planetesimals for planetary building up. Unfortunately, little is known about the FUV radiation from solar-system precursors. The measurements carried out in X-ray or softer UV bands point out that the FUV flux varied significantly during the pre-main sequence evolution. Protostellar disk are shielded from the stellar hard radiation during the early phases but about 1 million years evolution, protostellar disks transform into young planetary disks and the FUV and extreme UV (EUV) radiation from the very active young Suns, heavily irradiates them. Strong stellar winds are expected to interact with the left over particles and produce diffuse Helium and Hydrogen emission that pervades the whole young systems during planets early evolution and planetary magnetosphere formation. Around the Sun, within a modest radius of 140 pc, there are thousands of young solar-like stars in all evolutionary stages. The observation of these sources would provide a unique perspective on magnetospheres and coronal evolution, as well as on its impact in planetary formation and evolution. A single spectrum in the UV range contains information about all the physical components - atmosphere, magnetosphere, outflows (Solar-like winds, jets), accretion flow, inner disk structure, residual gas in the young planetary system - and their evolution into exoplanetary systems.
Solar system planetary research is fundamental to understand atmospheres as global systems including the Earth. For example, the link between upper and lower atmospheres is poorly known for the Earth case despite the fact it can have implications on global warming. Studying the upper atmosphere of solar system planets but also of exoplanets can help to understand the mechanism working at Earth.
The stellar or solar FUV-EUV fluxes are the main energy input at high atmospheric altitudes. Many atmospheric atoms, ions or molecules have strong electronic transitions in the UV-optical domain both in emission or absorption. This wavelength range gives access to fundamental constituents of the atmospheres. In particular, bio-markers like Ozone (O3) have very strong absorption transitions in the ultraviolet protecting complex molecules especially DNA from dissociation or ionization. As examples, we can mention, absorption of O3 through the Hartley bands between 2000-3000 A. O2 has strong absorptions in the range 1500-2000 A. Atomic oxygen presents resonance transitions at 1304 A and the famous auroral green and red lines in the visible. For other planetary cases or Earth paleocases, CO has strong absorption bands below 1800A and forbidden emission band from 2000A to 3000A. H- lyman alpha at 1216 A concerns mainly the Giant planets and hot Jupiters but it is still seen on Earth, Venus and Mars. In the Earth case the geocoronal Lyman alpha emission is a strong low atmosphere UV observatories. Hydrocarbons like methane absorb strongly the FUV radiation. These species are linked to the energetic inputs in the planetary environment and can provide information on the global atmospheric system. For this reason, the observations in the UV and optical wavelength are strongly powerful diagnostic technique to characterize the atmospheres of the planets from the solar system to Earth like exoplanets through hot Jupiters.
The Ultraviolet (UV) is a basic and vital energy domain for all fields in astrophysical research; it grants high sensitivity access to diagnostic indicators for the state of diffuse plasmas in space, from planetary atmospheres to the elusive gas in the intergalactic medium. UV- visible spectral features are mainly produced by electronic transitions in molecules, atoms and ions covering the largest possible panel of species. The UV targets a broad range of temperatures that covers most astrophysical processes. The richness of spectral tracers in the UV is unrivalled by any other spectral range. Moreover, the UV photons often characterize the non LTE processes that create conditions for complex chemistry. This allows to get large molecules but also to create compounds that absorb and thus protect the planets with atmospheres from the harsh space conditions. The UV radiation field itself is a powerful astrochemical and photoionizing agent. Moreover, UV optical devices provide the best possible spatial resolution since resolution is inversely proportional to the radiation wavelength.
The impact of UV instruments on astronomical research can be clearly traced starting from the considerable success of the International Ultraviolet Explorer (IUE) observatory and successor instruments such as the Hubble Space Telescope (HST), or the Far UV Spectroscopic Explorer (FUSE) satellite. UV instruments have contributed in an outstanding manner to the progress of astrophysics, such as the first estimates of the mass of supermassive black holes1 or the first detection of the photoevaporation of the atmosphere of an extrasolar planet2 , HD 209458b; these were achieved in the ultraviolet range. The European UV community will be able to continue to profit from the refurbished Hubble Space Telescope, a 23 years old space telescope, till the end of the mission. After that, there will not be access to the UV range unless for the Russian led WSO-UV space telescope, with only a minor European participation from Spain. Among the astronomical research lines given the maximum priority in Europe, three of them require access to the UV range:  Planets and Life,  The Solar System and  The Universe. Even for the investigation of the fundamental laws of the Universe, the UV-optical range is unmatched by any other range to measure the fine structure constant along 80% of the Universe lifetime. In this white paper, we outline the key science that will be lost if a large UV-optical facility is not available in the future.
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