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EXPOSE facilities were ESA multiuser facilities mounted outside the International Space Station for astrobiology experiments. Between 2008 and 2016, three series of experiments were conducted involving chemical and biological samples to test their resistance and evolution in the space environment in low Earth orbit. In this Astrobiology special collection, results from two experiments of the EXPOSE-R2 campaign (2014–2016) are presented: Biofilm Organisms Surfing Space (BOSS) relating to biology and Photochemistry on the Space Station (PSS) dealing with astrochemistry. [ABSTRACT FROM AUTHOR]

We present a photostability study of two nucleobases, guanine and uracil. For the first time, the photoabsorption cross-section spectra of these molecules in the solid phase were measured in the VUV and mid-UV domain (115≤λ≤300 nm). They show a quite similar absorption level throughout this wavelength range, highlighting the importance of considering the whole VUV and UV domain during photolysis experiments in the laboratory. Their photolysis constant ( J) can be estimated from those measurements as follows: 2.2×10−2 s−1±11% for guanine and 5.3×10−2 s−1±14% for uracil. This work shows that (i) measuring kinetic constants from a direct and 'traditional' photolysis of a thin sample in the laboratory suffers strong limitations and (ii) achieving this measurement requires comprehensive modeling of the radiative transfer that occurs in any sample not optically thin ( i.e.,≤2 nm). Moreover, this work has provided other data of interest: the refractive index of solid guanine and of uracil at 650 nm are 1.52 (±0.01) and 1.39 (±0.02), respectively, and the integrated IR band strengths ( A) of solid guanine between 3700 and 2120 cm−1 (3.4×10−16 cm·molecule−1±13%) and of solid uracil between 3400 and 1890 cm−1 (2.1×10−16 cm·molecule−1±21%). Key words: Nucleobases-Photochemistry-VUV spectrometry-Absorption cross sections-Astrobiology. Astrobiology 15, 268-282. [ABSTRACT FROM AUTHOR]

The PROCESS (PRebiotic Organic ChEmistry on the Space Station) experiment was part of the EXPOSE-E payload outside the European Columbus module of the International Space Station from February 2008 to August 2009. During this interval, organic samples were exposed to space conditions to simulate their evolution in various astrophysical environments. The samples used represent organic species related to the evolution of organic matter on the small bodies of the Solar System (carbonaceous asteroids and comets), the photolysis of methane in the atmosphere of Titan, and the search for organic matter at the surface of Mars. This paper describes the hardware developed for this experiment as well as the results for the glycine solid-phase samples and the gas-phase samples that were used with regard to the atmosphere of Titan. Lessons learned from this experiment are also presented for future low-Earth orbit astrochemistry investigations. Key Words: Astrochemistry-Astrobiology-Low-Earth orbit-EXPOSE-Photochemistry-Hardware. Astrobiology 12, 412-425. [ABSTRACT FROM AUTHOR]