Final Formation
4 BYA, the Sun brightened to 70% of its current light level, while the intense solar ultraviolet output dropped dramatically: by more than 30 times.
By absorbing more of the Sun’s energy, Earth failed to ice over when the Sun was dimmer. Earth’s surface was darker. The continents were much smaller, so the oceans, which are typically much darker than land masses, absorbed more heat.
Earth’s early atmosphere was a brew of greenhouse gases that helped stabilize global temperature. Carbon dioxide (CO2) and methane (CH4) prevented the planet from freezing and triggered synthesis of a rich variety of organic molecules via ultraviolet radiation in the upper atmosphere.
Bombardment from space continued after Earth was moonstruck, cratering both Moon and Earth. The celestial siege of Earth eased somewhat after practically sterilizing the planet’s surface, but bringing water, hydrogen, nitrogen, and a wealth of minerals and organic compounds that would transform the planet.
Jupiter was instrumental in both seeding Earth and in sweeping up errant projectiles, some of which formed the array of moons and asteroids coming under Jupiter’s sway.
Discount the “Jupiter as shield” concept. Jupiter was responsible for the vast majority of the encounters that “kicked” outer planet material into the terrestrial planet region, delivering the volatile-laden material required for the formation of life. Saturn assisted in the process far more than has previously been acknowledged. ~ American planetary physicist Kevin Grazier
To this day, Jupiter is Janus-faced toward Earth. While it does vacuum some debris, it also sometimes hurls objects Earth’s way.
In 1770, a large comet whizzed by, missing Earth by a mere million miles. The comet had come into the outer solar system 3 years earlier, its path determinedly far from Earth. But the comet passed close to Jupiter, which diverted it to a new course: a cosmic whisker away from collision with the blue planet that Jupiter only sometimes protects.
The comet made 2 passes around the Sun before heading out. In 1779, the comet again passed close to Jupiter, which summarily slung it out of the solar system.
Though still subject to upheaval, Earth’s crust was complete within 100 million years after its birth: a solid but deformable shell.
Nearly half of the crust’s mass is made up of oxygen and over 27% silicon. Both are major components of rocks. Metals used in manufacture, such as iron and aluminum, comprise ~18% of the crust.
The lithosphere sorted itself into continents above sea level, resulting in land surface. Volcanism accomplished this. Frequent eruptions subducted hot surface materials, eventuating in a cool, thick crust by altering convection dynamics between Earth’s crust and mantle.
The mantle’s viscosity is extremely dependent on its temperature. ~ Australian geophysicist Craig O’Neill et al
An abrupt transition to tectonics began ~3 BYA, once the lithosphere had sufficiently cooled. Before that, the upper mantle was too hot to convey rock without melting it.
Despite voluminous bombardment, early Earth mineral variety was quite limited. Of the 4,500 chemical species on Earth today, up to 2/3rds are attributable to biological activity. The earliest life engendered mineral evolution.
Late arrivals from space added to land mass. Meteorite impacts shifted mantle convection patterns, triggering plumes that heated the crust from below. Continents evolved.
In chewing rocks for sustenance, the earliest microbes were instrumental in creating the continents. Their waste products – sedimentation – acted as a viscous lubricant for tectonic plate subduction, thereby facilitating the rise of vast land masses. Without the lubricating sediment, Earth might have been a water world, dotted with small volcanic islands.
Earth’s oldest rocks were volcanic artifacts (igneous). As the surface cooled, torrential storms ensued, begetting erosion. From surface debris emerged the 2nd great family of rocks (sedimentary).
The heat and crushing turmoil of tectonics led to melting and recrystallization of older rocks, producing a 3rd rock family (metamorphic). From these mountains were made.
The atmosphere in the late Hadean eon comprised gases released by volcanic activity, primarily large volumes of carbon gases (CO, CO2, CH4) which helped keep the surface warm.
Water vapor was in the air, as oceans had already formed; but free oxygen of any form (O2, O3 (ozone)) was entirely absent.
