Accretion and Shifts of the Levels of O2 and CO2 in the Biosphere

Abstract

In the solar system and perhaps beyond, when many aspects are considered, Earth is a unique planet. Often called the twin planet to Earth because of its close proximity, its comparable radius/size, and its similar mass and density, the atmosphere of Venus is very different from that of Earth. It comprises 97% carbon dioxide (CO2), 2% nitrogen (N2), and less than 1% molecular oxygen (O2), water (vapor) (H2O), and methane (CH4) (e.g., Ingersoll 2007; Svedhem et al. 2007). Among the planets of the solar system, while the atmosphere of Earth now contains only a small amount of CO2, those of Venus and Mars contain ~96.5 and ~98% of it, respectively. The atmospheres of Jupiter and Saturn, two of the four solar system’s gas giant (also called Jovian) planets, consist mostly of hydrogen (H2) and helium (He) (e.g., Lissauer and Stevenson 2006), while Mercury has a very thin and highly variable atmosphere containing H2, He, O2, Sodium (Na), calcium (Ca), potassium (K), and water vapor with a combined pressure level of ~10−14 bar (1 nPa) (Domingue et al. 2007; McClintock et al. 2008). The most important factors that sanctioned the realization of the so-called “carbon-based life” on Earth include (a) an atmosphere rich in O2, (b) a biological range of temperature and the presence of water (the much acclaimed “universal solvent”) in the three states of matter – solid (ice), liquid (water), and gas (water vapor), and (c) a magnitude of gravity sufficient to prevent the loss of most of the atmospheric gases to the outer space, including hydrogen (H), the smallest atom, without wielding too much pressure on delicate biological life.