verse, Earth.In the beginning, impacts of very large objects were very common, some as big as Mars or half the diameter of Earth. Collision of large bodies orbiting Earth played a role in its initial tilt of spin axis, the length of its day, direction of spin, and the thermal state of the interior. This violent bombardment continued for 3.9 billion years. Final composition of Earth had several crucial structural effects. Enough metal was present early on to allow formation of an iron and nickel rich core that is partially liquid. This enables a magnetic field that deflects some harmful radiation from reaching the surface. Enough radioactive elements are also present in the core to maintain long term heating which drives plate tectonics. At 4.5 billion years ago, Earth separated into different layers: an inner core (made of iron and nickel), a land layer of lower density material, and an early atmosphere of carbon dioxide and steam. At 3.9 billion years ago, surface temperatures dropped to a range where liquid water could be maintained. Liquid water makes up approximately 75% of the planet’s surface today which is roughly what it was then. The most important requirement for life as we know it is the presence of liquid water. This is the one substance that can serve as a universal solvent - that is, it can dissolve and transport minerals and nutrients from both the ground and from the atmosphere and can allow the mixing of these materials. No other substance can perform this function that is integral to the appearance and continuation of life as we know it. Therefore, within even our limited understanding, it seems that liquid water is essential to the formation of life.1 A planet having water in the liquid state in conjunction with land masses seems to be an important factor in the creation of life that is not seen on many other worlds. The formation of land was due to volcanism and plate tectonics. Land can remain a constant on th...
