Ensuing Universe

January 25, 2011

Areios orbits Hemera somewhere between where Mercury and Venus would be in our Solar System; at first, Areios lies just within its habitability zone where liquid water could exist on the surface, and because Hemera initially output less light when Areios was forming, it seems unlikely that life could arise on this frigid planet right from the start. On Earth, life may have formed just as soon as the crust solidified and the oceans condensed from the atmosphere, yet our planet would have been cooler than it is now were it not for the greenhouse effect. This early arrival for life suggests that life could be common in the universe if it can be spawned on a habitable planet so early in its formation. Because Areios is on the edge of the habitable zone, it may take some time to warm before it can be habitable for life. The habitability zone for life will change over time as Hemera evolves; Areios is positioned in the very outer habitable zone near the beginning of Hemera’s life and by the end of its main sequence stage, Areios is only just tucked inside the inner edge of that habitable zone. For the 38 billion years or so that Hemera is in the main sequence stage, Areios is within the habitable zone for 30 billion of those years. For the first and last four billion years of Hemera’s evolution, Areios will be sterilized of all life, first because of freezing temperatures early on and then because of the boiling temperatures near the end of Hemera’s life. As Areios’ surface temperature gets pushed hotter, Areios will have to shed more and more of its thick atmosphere like a jacket to cool off until its atmosphere is too thin to support liquid water on its surface and the oceans boil away. But more on the evolution of a habitable planet later…

Areios formed from the collision of planetesimals billions of years ago; these violent interactions also created two out of three of its moons and a third moon was captured later during a period of asteroid and comet bombardment. When the cataclysms of the planet formation ended, Areios was a still-molten ball of rock with a swirling ring of debris that would later become its moons. Areios is more massive than Earth and contains a bigger mantle and a thinner crust. The importance of this will be revealed later on, but this distinction is not trivial when it comes to the potential for life on Areios. Because of Areios’ girth, the planet would take longer to cool and the internal portions of the planet would stay hotter for longer because the core is wrapped in a much thicker insulating blanket of mantle. This early planet would soon cool on the outside, though, and a process called differentiation would occur; Areios was at first a well-mixed sphere or magma, but it began to cool and settle. The crust formed like a skin like a bowl of soup left to cool; this outermost layer is only a few kilometers thick, but covers the entire planet and serves as the palette for the thin veneer of life that is to come. The crust covers the mantle of the planet, which makes up the bulk of Areios’ mass. The mantle is a made of melted rock kept solid by the intense pressure coming down on it; unlike Jules Verne’s Journey to the Center of the Earth, there are no caverns or caves in the mantle because this solid rock can flow like a liquid and would quickly fill any void beneath the planet, despite the fact that pressures make the molten material behave like solid rock.

This is a depiction of what a planet like Areios would look like early in its formation.

As Areios formed, there were three processes that kept generating the internal heat of the mantle; the kinetic energy of impacts, differentiation, and radiogenic heating. During the formation of Earth, there was a period called the Late Heavy Bombardment where more comet and asteroid impacts struck the planet, boiling the oceans and melting the crust for a couple hundred million years before the rain of fire subsided. When these objects struck the Earth, their gravitational potential energy as they fell to the earth was converted to kinetic energy in the form of heat. Areios experienced a similar event to the late heavy bombardment for a couple of hundred million years after the planet formed and for a while the kinetic energy from those impacts kept heating the planet, but once those impacts subsided, the planet cooled enough to form crust and the oceans. Areios started out as a homogeneous ball of magma, but slowly the heavier metals started to settle in the core of the planet. As these denser materials sank into the mantle, their potential energy was converted into kinetic energy until the planet differentiated into the three distinct layers of the crust, mantle and core. Once these three layers were fully formed, the planet no longer generated heat by differentiation. The final and only ongoing way the interior of the planet generates heat is through radiogenic heating. When the planet formed, it incorporated some heavier unstable elements like thorium and uranium. Over time, these elements would decay into lighter elements like potassium or lead; this radioactive decay would release energy in the form of heat that keeps the internal parts of Areios hot. This Late Heavy Bombardment era for Areios would deliver water to the planet and later determined how much of Areios will be covered in lakes and oceans.

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