The Cooling of Early Earth
Earth formed just under 4.6 billion years ago, and for its first 50 to100 million years, it was a boiling ball of liquid rock without a permanent crust (hard outermost layer).An important event in Earth’s earliest phase is known as the differentiation event, which completely changed the initially uniform composition of the planet. it happened around 4.5 billion years ago, when the planet had grown large enough for pressure to drive temperatures in the interior above1,000℃(degrees Celsius), the point at which rocks melt. Then,denser (metal-rich) materials sank to the center of the planet, and less dense (rocky) materials rose toward the surface.The sinking dense materials formed Earth’s nickel-iron core, the planet’s inner3,500 kilometers or so.The lighter materials that rose up formed the less-dense rocky mantle, the planet’s outer 2,900 kilometers.
The formation of Earth’s core transformed conditions on Earth’s surface.This is because it created the right conditions for development of the planet’s magnetic field, which originates from movements in the outer layers of Earth’s core. It had been known fora while that the magnetic field was already operational by about 3.5billion years ago, and very recent research has brought that back to before 4 billion years ago.The magnetic field is Earth’s only real protection against the solar wind (charged particles from the Sun),which was stripping gases from the earliest atmosphere before the magnetic field had started up.Thus, the differentiation event is thought to have been critical for reducing the loss of light elements from the atmosphere to space.Without it, Earth might have ended up without hydrogen, and thus without water.And over time, many heavier gases would also have been stripped off by the solar wind.Mars is thought to have started out with a magnetic field but—being much smaller than Earth—to have cooled enough for its magnetic field to die at around 4 billion years ago. It subsequently lost almost all of its atmosphere and surface water. While this often-used explanation for retaining an atmosphere by presence of a magnetic field sounds plausible, some further thought suggests that things maybe a little more complicated. Venus has no magnetic field and is closer to the Sun yet has a very well-developed atmosphere. Venus and Earth have similar sizes and masses, while Mars is much smaller—hence, gravity may have been equally or more important for retaining an atmosphere than a magnetic field, especially when the gases concerned are heavier gases, like the dominant CO2 (carbon dioxide) on Venus.
At Earth’s position in the earliest solar system, temperatures of 250°C to 350°C would be expected, but the energy from the intense early impacting by objects from space had pushed temperatures up well above the melting temperature of rocks. Still, research has demonstrated that as early as about 4.4 billion years ago, Earth’s surface had not only cooled sufficiently to form early crust (likely below 1,000°C) but even enough to allow for the presence of liquid water, which at modern atmospheric pressure would mean that temperatures had dropped below 100°C. However, at higher pressures this value is higher, and we don’t really know how dense the early atmosphere was. So 100°C is a lower estimate; true temperatures may have been double that value if the early atmosphere was very dense.
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