Constraints on a potential aerial biosphere on Venus: I. Cosmic rays

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2015-09, Vol.257, p.396-405
• Main Authors: Dartnell, Lewis R., Nordheim, Tom Andre, Patel, Manish R., Mason, Jonathon P., Coates, Andrew J., Jones, Geraint H.
• Format: Article
• Language: English
• Subjects: Aerials; Altitude; Astrobiology; Atmospheres; Atmospherics; Biosphere; Cosmic rays; Exobiology; Ultraviolet radiation; Venus; Venus (planet); Venus atmosphere; Venus, atmospheres
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2015.05.006

The habitable zone high in the atmosphere of Venus, defined here by the combination of high temperature and acidity, lies between 51 and 62km altitude and overlaps with the cloud layers. Here we study the hazard posed to this potential aerial biosphere by the ionising radiation of penetrating galactic cosmic rays (GCR) or extreme solar particle events (SPE) such as the 1859 Carrington Event and 775 AD event, modelled on the spectral shapes of the February 1956, August 1972 or October 1989 SPEs. [Display omitted] •Venus aerial habitable zone defined as 62–51km altitude by acidity–heat combination.•Modelled radiation exposure from cosmic rays and extreme solar particle events of 1859 Carrington Event and 775 AD event.•Ionising radiation at top of habitable zone enhanced around 50,000× by modelled 775 AD event.•But total absorbed dose unlikely to pose survival hazard to venusian aerial life.•Companion paper models solar ultraviolet radiation penetration through habitable zone. While the present-day surface of Venus is certainly incompatible with terrestrial biology, the planet may have possessed oceans in the past and provided conditions suitable for the origin of life. Venusian life may persist today high in the atmosphere where the temperature and pH regime is tolerable to terrestrial extremophile microbes: an aerial habitable zone. Here we argue that on the basis of the combined biological hazard of high temperature and high acidity this habitable zone lies between 51km (65°C) and 62km (−20°C) altitude. Compared to Earth, this potential venusian biosphere may be exposed to substantially more comic ionising radiation: Venus has no protective magnetic field, orbits closer to the Sun, and the entire habitable region lies high in the atmosphere – if this narrow band is sterilised there is no reservoir of deeper life that can recolonise afterwards. Here we model the propagation of particle radiation through the venusian atmosphere, considering both the background flux of high-energy galactic cosmic rays and the transient but exceptionally high-fluence bursts of extreme solar particle events (SPE), such as the Carrington Event of 1859 and that inferred for AD 775. We calculate the altitude profiles of both energy deposition into the atmosphere and the absorbed radiation dose to assess this astrophysical threat to the potential high-altitude venusian biosphere. We find that at the top of the habitable zone (62km altitude; 190g/cm2 shielding depth) the radiation d