ABSTRACT
Today, Venus has a thick carbon dioxide-nitrogen atmosphere with little water, and the planet has no water in any form on the surface. Venus is gravitationally locked into near synchrony with the Sun, with one rotation of its axis taking longer than one orbit around the Sun. Nevertheless, surface temperatures on the planet’s daytime and nighttime are nearly identical, because of the high heat conductance of an atmosphere many times denser than Earth’s. e upper atmosphere is super rotating, circling the planet in 4 days with wind speeds up to 100 m/s (Svedhem et al., 2007). Circulating vortices deep within the planet’s atmosphere, with dynamical and morphological similarities to terrestrial hurricanes, appear to maintain the superrotation (Limaye et al., 2009). H2S and SO2 have been found together in the Venusian atmosphere. Since these two compounds react with each other, there must be a yet unidentied mechanism producing them. Venus Express results also conrm latitudinal variation of another sulfur compound, COS, at concentrations of about 2-4 ppm in the troposphere that are anticorrelated to CO concentrations (Marcq et al., 2013). One surprising recent result was the detection of an ozone layer at an altitude of about 100 km (Montmessin et al., 2011), hundreds of times less dense than on Earth, but possibly indicating that some of the same key chemical reactions occurring in Earth’s stratosphere may also operate on Venus. e most earthlike conditions that can be found today on Venus are in the lower cloud layer of the atmosphere. If water exists in the subsurface, it has to be in a supercritical state (Schulze-Makuch and Irwin, 2002) incompatible with the structural stability required of macromolecules in living systems. In the lower cloud layer, however, where temperatures range from about 300 to 350 K, the pH is approximately 0, and the pressure is about 1 bar; water vapor concentrations of up to several hundred ppm are found (Montmessin et al., 2011). Due to the thickness and superrotation of the Venusian atmosphere, particles of micrometer dimensions have much longer residence times than in Earth’s atmosphere, on the order of months compared to days (Schulze-Makuch et al., 2004). If microbial life on Venus ever gained a foothold, either by independent origin or by panspermia from Earth, thermoacidophiles could have evolved as the surface waters turned warmer and more acidic and then retreated to the large liquid droplets of the lower cloud layer, where they might still oat as microbial extremophiles
today (Schulze-Makuch and Irwin, 2008), provided that the changes occurred slowly enough for microbial life to adapt to an airborne state. It is important to understand the climate history of Venus to elucidate what the original water endowment of Venus was, as this has a profound implication on how benign the environmental conditions of Venus were in the past and whether they were consistent with the requirements of life (Taylor and Grinspoon, 2009).
