GEOLOGICAL PROCESSES
major role in the development of the atmosphere (Sekhar and King, 2014).
The Earth and its surface were too hot for water vapor to condense during the early stages of its formation. However, water vapor alone is not a significant enough greenhouse gas to insulate the surface against life-damaging solar flux. The Earth’s primordial, non-protective atmosphere remained in place until large quantities of gases were emitted from the upper mantle via volcanoes. These gases, which include car- bon dioxide methane, nitrogen, and carbon monoxide, were vital to establishing today’s modern atmosphere. Without this fundamental change in the atmosphere caused by intense geodynamic activity, organic life forms would be unprotected from damaging solar radiation.
Volcanism also needs to be considered a modifying factor in HZ calculations. The heat received and retained through solar flux is not directly proportional to a planet’s ability to support life. A planet may be warm enough to contain liquid water and at the same time be absorbing too much radiation. Conversely, it may be receiving less than optimal amounts of solar flux but due to the greenhouse gases (enhanced by volcanic activity), a planet may be fully capable of supporting a thriving ecosystem. It is therefore not prudent to judge the HZ solely based on the distance of a planet from its star and its projected surface temperature.
Plate Tectonics and the Habitable Zone
Plate tectonics is a geological process that plays a crucial role in the maintenance and development of the Earth’s, and perhaps other planets’, habitability (Noak, 2014). Subduction zones are created by the overlapping of tectonic plates caused primarily by compressive forces, and result in the movement of one side of either plate into the mantle (Sekhar and King, 2014). Thermal activity within the mantle melts the subducted rock and it is this relationship that allows recycling of the crust, which maintains the thickness of the lithosphere.
An appropriate lithospheric thickness is important in the ability of a planet to sustain its temperature (Kiefer, 2009). Mars is located in the HZ (1.52 AU from the Sun) but as of the date of this paper, liquid water has not been detected on or below its surface. The Martian environment is substan- tially colder than Earth’s; -87 to 0°C versus an Earth average temperature of 15°C. This temperature difference may be related primarily to its thin atmosphere and therefore lack of greenhouse effect. However, plate tectonic movement is not apparent on Mars (Sekhar and King, 2014) and its crust appears to be a single sheet of lithospheric material, although similar in composition to the Earth’s. This lack of tectonic activity suggests that Mars lacks the ability to recycle its crust in a significant manner.
Without crustal recycling via subduction, the thickness of the Martian lithosphere continued to grow and this pro- cess served as a self-insulating mechanism (Morschhauser, 2011) (Figure 2). The inner core temperatures of the planet suppressed and eventually disabled geothermally-dynamic processes. These processes, in combination with its thin, volcanically-deprived atmosphere, could have contributed to planetary cooling and helped create the below-freezing, appar- ently lifeless environment currently being explored today, despite the location of Mars well within the Habitable Zone. uted to planetary cooling and helped create the below-freezing, apparently lifeless environment currently being explored today, despite Mars presence well within the Habitable Zone.
Figure 2. An illustration of Mars and its thickening Lithosphere (Calvin J. Hamilton).
Magnetic Flux and the Habitable Zone
Another geological feature that is a necessity in the early formation of a future habitable planet is magnetic flux. An electromagnetic field surrounds the Earth and deflects spikes of solar radiation (solar flares) that would be damaging to life. The field is generated by a collaboration between the solid metallic (nickel-iron) inner core and liquid outer core (Hulot, 2010). Differences in temperatures and pressures between the inner and outer cores, in combination with the Earth’s rotation, generate an electrical current and thus a magnetic field (Figure 3). Because of this geological feature, the Earth is able to sustain life at a substantially closer distance to the Sun than might otherwise be possible. With this protective shield, the Earth could be as close as 0.65 AU to the Sun and still protect developing life forms from excessive solar radiation.
Figure 3. Earth’s protective magnetic field defending it against a massive solar flare (NASA/SDO).
Water to Land Fraction and the HZ
A recently identified geologic concept associated with the Habitable Zone is the water surface to land fraction (Abbot, 2012). An ocean planet or water world (those with very little or no subaerial land masses) is not capable of sustaining life or even its own atmosphere. Such conditions would drastically alter or eliminate the scope of its Habitable Zone. The continen- tal weathering and associated photosynthetic and biological degradation that take place on land are substantially greater than the hydrothermal weathering (carbon dioxide recycling) that takes place in the basaltic oceanic crust at the seafloor (Abbot, 2012). Therefore, a planet that has a higher fraction of water on its surface would have far greater difficulty recy-
cling CO2 through its atmosphere. The increased production or buildup of carbon dioxide in the atmosphere would result in enhanced greenhouse effects and a considerably warmer climate. The importance of the water surface to land fraction is that a completely or largely water-covered planet would not be
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