Spectroscopic diagnostics in the VUV for solar and stellar plasmas

Summary The VUV emission spectra from the solar atmosphere and stellar atmospheres have been intensively studied during the past 25 years with several major space programs. In this review we discuss the spectroscopic diagnostic techniques used to study astrophysical plasmas, the atomic processes involved, the recent observations and the plans for future space missions.     

UVolution: Compared photochemistry of prebiotic organic compounds in low Earth orbit and in the laboratory

Solar UV radiation is a major source of energy for chemical evolution of organic materials in the Solar System. Therefore studies on the photostability of organic compounds in extraterrestrial environments are of prime importance for the understanding of the extraterrestrial origin of organic materials on Earth. A series of organic samples have been photolysed in Earth orbit during the ESA BIOPAN 6 mission (14-26/09/2007). Their photochemical lifetime has been measured and compared to results recorded in the laboratory using a lamp that simulates the solar radiation in the VUV domain. The half-lives at a distance of 1 AU from the Sun have been measured for glycine, xanthine, hypoxanthine, adenine, guanine, urea, carbon suboxide polymer ((C₃O₂)_n) and HCN polymer. They range from a few days to a lower limit of a few tens of days for the most photoresistant (e.g. adenine, guanine, hypoxanthine). Lifetimes measured in terrestrial orbit are very different from those derived with laboratory experiments. These measurements confirm that it is difficult to simulate the solar spectrum below 200 nm in the laboratory. Results are discussed and highlight the necessity to conduct experiments in orbit, and for longer duration. It also appears that the laboratory measurements made in VUV must be extrapolated very cautiously to the different environments they are supposed to simulate.     

Interplanetary survival probability of Aspergillus terreus spores under simulated solar vacuum ultraviolet irradiation

This work is a part of ESA/EU SURE project aiming to quantify the survival probability of fungal spores in space under solar irradiation in the vacuum ultraviolet (VUV) (110-180 nm) spectral region. The contribution and impact of VUV photons, vacuum, low temperature and their synergies on the survival probability of Aspergillus terreus spores is measured at simulated space conditions on Earth. To simulate the solar VUV irradiation, the spores are irradiated with a continuous discharge VUV hydrogen photon source and a molecular fluorine laser, at low and high photon intensities at 10¹⁵ photon m⁻² s⁻¹ and 3.9×10²⁷ photons pulse⁻¹ m⁻² s⁻¹, respectively. The survival probability of spores is independent from the intensity and the fluence of photons, within certain limits, in agreement with previous studies. The spores are shielded from a thin carbon layer, which is formed quickly on the external surface of the proteinaceous membrane at higher photon intensities at the start of the VUV irradiation. Extrapolating the results in space conditions, for an interplanetary direct transfer orbit from Mars to Earth, the spores will be irradiated with 3.3×10²¹ solar VUV photons m⁻². This photon fluence is equivalent to the irradiation of spores on Earth with 54 laser pulses with an experimental ∼92% survival probability, disregarding the contribution of space vacuum and low temperature, or to continuous solar VUV irradiation for 38 days in space near the Earth with an extrapolated ∼61% survival probability. The experimental results indicate that the damage of spores is mainly from the dehydration stress in vacuum. The high survival probability after 4 days in vacuum (∼34%) is due to the exudation of proteins on the external membrane, thus preventing further dehydration of spores. In addition, the survival probability is increasing to ∼54% at 10 K with 0.12 K/s cooling and heating rates.