Geological Processes and the Circumstellar Habitable Zone
Jerome Scelza,
jrs755@nyu.edu NYU Polytechnical School of Engineering
Abstract
The circumstellar habitable zone is the distance a planet is required to be from its primary star in order to sustain life. This distance is determined primarily by the ability of the planet to maintain liquid water on its surface (Kasting, 2014). If the planet is too close to its sun, the water will evaporate and the planet will freeze. The current consensus among astrophysi- cists is that surficial water is related solely to the availability of heat energy that is emitted by the planet’s sequence star, one that is fusing hydrogen to form helium (Kopparapu, 2013). However, Earth’s geologic processes indicate that other factors contribute to or may be even more important to the presence of water than simple stellar distance. Volcanism, atmospheric processes, tectonic plate activity, magnetic field intensity, and surface area play a distinct and well-defined role in the ability of the Earth to preserve and cycle its liquid water. These and other variables contribute to the ability of a planet to host water, and it may be too scientifically confining to suggest that the Habitable Zone is determined solely by the temperature gradient of a planet relative to its star. By taking a closer look at which of these factors contribute to the habitability of Earth (i.e., water and its associated climatic implications), we may develop a more sophisticated and meaningful understanding of the scale and radial distance of the habitable zone of other worlds.
Estimating the Habitable Zone
Liquid water is used as the defining characteristic of the circumstellar habitable zone because it drives biochemical reactions and is crucial to cellular function (Rushby, 2013). The distance from a planet’s primary star (for us, the Sun) is a criti- cal factor effecting temperatures associated with freezing and vaporizing water. This effect can be quantified by measuring the amount of radiant flux or insolation (solar radiation) that is emitted by the host star in the form of electromagnetic energy or instantaneous luminosity. The habitable zone (Figure 1) is quantified by three factors: (a) the proportion of the luminosity of a sequence star; (b) the distance between the sequence star and the planet; and (c) the mass of the planet under study.
Mathematically, this relationship has been established efficiently as follows (Dobos, 2013):
Figure 1. The inner and outer radii of the Habitable Zone as described by Kasting et al., 2014.
0.99-1.65 Astronomical Units (AU). An AU is the distance between Earth and the Sun, roughly 150 million kilometers. This approach serves as the basis for remote-life detection by most outer space navigating organizations, including NASA (Kasting, 2014).
The first verified exoplanet was not discovered until 1995 and was found orbiting a G-type (yellow) star (Mayor and Queloz, 2009). The search for new exoplanets exploded after this confirmed discovery, as did the race to find the first signs of extraterrestrial life. The Kepler advanced photometer space observatory was launched in 2007. This instrument is capable of monitoring the brightness of more than 145,000 main sequence stars (Dooling, 2014). Kepler allows the direct observation of exoplanets, and by utilizing Kasting’s habitable zone concepts, astrophysicists began to identify planets that potentially could support life.
Volcanism, the Atmosphere, and the Habitable Zone
Where: L = Luminosity of the start in solar units and Sinner
and Souter can be calculated by; Sinner = 1.296 2.139104 T 4.1910A8 T 2 Souter = 0.234 1.319105 T 6.19109 T 2 where T = Effective Temperature of the star in Kelvin. This equation defines the current Habitable Zone (HZ) of the Sun at
The solar flux that is received by the Earth from the Sun is too intense for modern or past forms of biological organisms to survive (Guo, 2010). Life began and endures on Earth in large part because of the shielding capabilities of the atmosphere. This thin combination of inert and highly reactive gases pro- tects the Earth’s surface from inhospitable amounts of solar energy while allowing for radiant heat to provide a relatively constant temperature. Carbon dioxide, water vapor, and other greenhouse gases that make up the atmosphere reflect and re-radiate substantial amounts of solar energy that is directed at Earth, while maintaining sufficient energy for biological interaction. It is this interaction, along with various forms of photosynthesis, which recycles these valuable gases and preserves the ongoing, beneficial effects.
While biological contributions to atmospheric gases are suf- ficient to maintain the greenhouse effect, it certainly was not enough to form the current atmosphere, especially considering the volatility of the early Earth environment and the initial lack of biological activity. There is substantial evidence that suggests a specific geological process – volcanism – played a
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