Drastic environmental change and its effects on a planetary biosphere Dirk Schulze-Makuch a,⇑ , Louis N. Irwin b , Alberto G. Fairén c a School of the Environment, Washington State University, Pullman, WA 99164, USA b Dept. of Biological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA c Dept. of Astronomy, Cornell University, Ithaca, NY 14853, USA article info Article history: Received 8 January 2013 Revised 26 March 2013 Accepted 2 May 2013 Available online 14 May 2013 Keywords: Earth Venus Mars Exobiology abstract Environmental conditions can change drastically and rapidly during the natural history of a planetary body. These changes affect the biosphere and can spur evolution via the mechanism of directional selec- tion leading to the innovation of new processes and forms of life, or alternatively leading to the extinction of certain life forms. Based on the natural history of Earth, the effect on a planet’s biosphere depends on three factors: (1) the nature and time scale of change, (2) the composition of the biosphere prior to change, and (3) the nature of the environment following the change. Though Earth has undergone various periods of drastic environmental change, life has shown an enormous resiliency and became more diverse and complex as a consequence of these events. Mars and Venus have undergone even larger envi- ronmental changes, both from habitable conditions under which the origin of life (or transfer of life from Earth) seem plausible, to a dry and cold planet punctuated by wetter conditions, and a hyperthermic greenhouse, respectively. Given its planetary history, life on Mars could have retreated to a psychrophilic lifestyle in the deep subsurface or to environmental near-surface niches, such as hydrothermal regions and caves. Further, strong directional selection could have pushed putative martian life to evolve alter- nating cycles between active and dormant forms, as well as the innovation of new traits adapted to chal- lenging near-surface conditions. Life in the subsurface or on the surface of Venus seems impossible today, but microorganisms may have adapted to thrive in the lower cloud layer, possibly using a biochemical strategy analogous to Photosystem I and chemoautotrophic sulfur metabolism, and employing cycloocta sulfur for UV protection. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction The planetary environment determines whether a given planet or even a locality on that planet is habitable. Habitability is here defined as the environmental conditions in a planetary setting that are conducive to life, i.e., physical and chemical parameters of that environment that fall within the known constraints of life (Schu- lze-Makuch and Irwin, 2008). This in general assumes that the pla- net or at least part of the planet can provide the necessary resources for life. Although the physical and chemical limits of habitability are imprecise, there are certain environmental param- eters that clearly favor habitability, including the presence of an atmosphere, a liquid solvent, an energy source, organic chemicals, and a particular thermal range (Schulze-Makuch et al., 2011). Dras- tic environmental change is not only prevalent on Earth (e.g., Snowball Earth events; K/T (Cretaceous/Tertiary) boundary and other time-period transitions during the Phanerozoic triggered by environmental alterations due to different causes), but also on our neighboring planets Venus and Mars, and some of the moons of the outer Solar System such as Europa and Titan. The onset of rapid and drastic environmental changes seems often to be trig- gered by exogenic events such as asteroid impacts or supernova explosions; others are triggered by endogenic processes in the pla- net or moon itself (e.g., supervolcano eruptions, cessation of inner core dynamos or plate tectonics). The question to be explored here is how these events affect the habitability of the planet and in par- ticular what effects they are expected to have on any existing biosphere. 2. Environmental change and the biosphere: lessons from Earth Environmental change can be extremely rapid and drastic, espe- cially when triggered by astronomical or geological events. In the extreme case it may even prohibit the origin of life, or be so devas- tating that existing life would not be able to recover, being all but extinguished from a planet (Ryder, 2003). If the proposed cataclys- mic event presumed to have created our Moon (Hartmann et al., 1986) would have occurred later in time, when life was already present on Earth, it would surely have extinguished all life by pushing the planet beyond any reasonable habitability constraints. Fortunately, most catastrophes are not that cataclysmic, but may 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2013.05.001 ⇑ Corresponding author. E-mail address: dirksm@wsu.edu (D. Schulze-Makuch). Icarus 225 (2013) 775–780 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus