A. Formation of the Sun and Solar System
The first generation of stars burned out quickly. 2 Some of them faded away into dark isolation, with their cores forever trapped inside. Others, as discussed above, released their chemically rich interiors back into the giant molecular clouds of interstellar space. Over the next few billion years, the cycle of stars’ births and deaths continued. The molecular clouds recollapsed into increasingly smaller regions called nebulae. Once again, the centers of these gassy nebulae reached critical mass and began to glow as stars. The star lifecycle goes on continuously. It would be cute to say that our sun had a “father sun” and a “grandfather sun,” but there are no clear-cut generations of stars as there are for living things. All we can say is that some stars lived and died in our galaxy for billions of years before the sun did, leaving their dirty gaseous remains behind.
So it came to be that, almost 5 BYA, our region of the galaxy formed the pre-solar nebula, a cloud of gas and dust that would become the solar system. Approximately 98% of the pre-solar nebula was hydrogen and helium left over from the big bang, while only 2% was recycled “star stuff” – new helium, heavier elements, ices, molecules, minerals, and metals. Gravity, gas pressure, and perhaps even the shockwave of a nearby supernova explosion 3 shaped the pre-solar nebula into a flat, rotating accretion disk. The center of the disk contracted into a dense sphere, achieved that critical core temperature of 10,000,000 K, and began to shine as our sun. The gas and dust throughout the rest of the accretion disk formed into planets and the various smaller solid objects in the solar system at roughly the same time.
Planet building was a gradual constructive process that continued for a few ten-million years after the sun started to shine. What started out as fine dust particles began to accrete randomly into dirt clods, rocks, asteroids and planetesimals about 10 km across. Planetesimals were large enough to attract each other gravitationally. Some of their collisions were violent enough to melt the planetesimals together into larger bodies. These were called the protoplanets. Eventually, eight of these protoplanets “cleaned out” their orbital zones and became the worlds that we recognize as planets today: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Other protoplanets ended up as satellites (like the moon) or dwarf planets (like Pluto). There are perhaps trillions of smaller bodies in the solar system, from asteroids and comets to space pebbles. Some zones of the accretion disk never did get gravitationally cleaned out by large bodies, so they remain cluttered with small debris. The largest cluttered zones are the asteroid belt between Mars and Jupiter, the Kuiper Belt just beyond the planets (Pluto’s home), and the Oort Cloud of comets 1,000 times farther out.
The heat of the early sun created a material division in the solar system, the frost line. Within the frost line, the inner solar system (out to the present-day asteroid belt, including Earth) was too hot for ices. 1 In the inner solar system, the largest bodies were formed from metals and minerals instead of ices. Large icy bodies could only survive in the outer solar system beyond the frost line, beginning at Jupiter’s orbit. Metals and minerals are much rarer than ices. Because the inner rocky planets (Mercury, Venus, Earth, and Mars) did not have as much raw material, they remained smaller than the outer icy planets. The outer planets (Jupiter, Saturn, Uranus, and Neptune) formed first as large icy cores. Once they grew massive enough, they could gravitationally attract even the lightest gases such as hydrogen and helium. Today, 99% of the hydrogen in the solar system is found in the sun, and 99% of the hydrogen outside of the sun is in Jupiter.
B. Formation of the Earth and Moon
About 4.5 billion years ago, in the first few ten-million years after the sun’s first light, our planet was “under construction” and was completely uninhabitable. This formative period in Earth’s history has been given the evocative name of the Hadean Eon, named after Hades, the ancient Greek name for Hell. Earth was truly hellish at that time. Meteorites rained down on Earth constantly, the planet exuded geothermal heat, and volcanoes must have been erupting everywhere. The Earth was exposed bare naked to cruel ultra-violet radiation and solar wind from the sun. The embryonic planet was subject to occasional collisions with other large protoplanets. These collisions, and the decay of radioactive elements inside the Earth, kept the planet hot. Temperatures ran high enough for much of the planet to persist in a molten state (liquid rock, like lava). The liquidity allowed the heaviest metallic elements such as iron and nickel to sink to the core, while the lightest compounds such as oxides and silicates (common dirt and sand) floated to the top to form Earth’s crust. According to a leading hypothesis, one of Earth’s last major collisions was with a large protoplanet. 4 The impact was so massive that both worlds liquefied and merged. A large blob was ejected into space and ended up in orbit around Earth. That blob solidified as the moon, which has been slowly drifting away from Earth ever since. This planetary collision may also account for the prominent tilt of Earth’s axis, which results in the four seasons as Earth goes around the sun.
When a planet as large as Earth is hot enough to be liquefied, it takes a long, long time to cool down. It cools from the outside in. Earth’s outer layer, the crust, was cool enough to solidify by 4 billion years ago. We can still find a few exposed outcroppings of rock that can be dated to the 4 billion year mark, the oldest known continuously solid matter on Earth. 2 The layer of Earth just beneath the crust is called the mantle. It is a thick layer of heavy rock in a quasi-molten state; it is hot enough to flow slowly but perceptibly over long periods of time. Beneath that is the Earth’s core of nickel and iron, the two heaviest elements created in abundance by stars. The metallic core is responsible for the Earth’s magnetic field, which not only allows us to navigate with a compass but also helps shield the planet from the Sun’s most harmful radiation.
Solidification of the crust was crucial to keep Earth’s geothermal heat mostly trapped inside. The Earth is unique among the rocky planets in that it retains its inner heat to this day. Smaller worlds like Mars and the moon long ago cooled down because they are smaller, and because they had huge, deep volcanoes that drew heat from their cores. As a consequence, they are now geologically dead worlds. Only Earth has an inner life, driven by heat that has been trapped there for 4 billion years and will continue to be trapped under the crust for the rest of the planet’s lifetime. Earth’s ongoing geothermal energy drives geologic activity such as volcanoes, geysers, oceanic thermal vents, and plate tectonics, movement of the crust over the mantle. These processes are crucial to life. We will discuss below that Earth’s earliest life may have originated at thermal vents. Plate tectonic movement traps carbon dioxide gas and keeps it from becoming a greenhouse gas in the atmosphere. 5 From these examples, we can see that the planet owes its biological life to its geological life.
C. Earth’s Early Atmosphere and Oceans
All planets that are about Earth’s size or larger can gravitationally secure a thin layer of gas, an atmosphere. Earth had an atmosphere early in its history, though it was noxious (to us), nothing like the atmosphere of today. The early atmosphere was rich in carbon dioxide, water vapor, and ammonia. Some of this gas was released from within the bubbling new planet, 6 some was captured along Earth’s orbit, and some was captured from collisions with comets and planetesimals.
Ammonia, a common compound in space and on early Earth, has the chemical formula NH3. When sunlight strikes ammonia in the atmosphere, it tends to break the molecule apart and release the nitrogen. Over millions of years, levels of nitrogen gas, N2, built up in the atmosphere. Nitrogen gas is inert, meaning that it does not chemically react with anything. It is neither good nor bad for life. Because it is inert, nitrogen gas is also stable. The atmosphere is still 80% N2 today.
You may wonder about the universe’s most common gases, hydrogen and helium. They are not abundant on Earth because the planet’s gravity is not strong enough to hold onto such light gases. Furthermore, hydrogen gas reacts easily with other molecules to create water and other byproducts. There is one other vital gas that was conspicuously absent from the early atmosphere: oxygen gas, O2. As we will soon discuss, that was a crucial difference between the Earth of Hadean times and the Earth of today.
Earth is the only known world that supports liquid surface water. Theoretically, every star has a habitable zone where Earthlike planets could support water. Astronomers know tens of potentially habitable planets in other solar systems, 7 although today’s technology cannot confirm liquid water that far away.
The question of how the oceans formed in the first place, however, is not straightforward. Liquids do not exist in the voids of space. Liquid requires a solid substrate below and atmospheric pressure above, or else it vaporizes. Therefore, Earth’s liquid water must be a cumulative byproduct of three processes: condensation of vapor, melting of ice, and chemical reactions. Mounting evidence suggests that most of Earth’s water was embedded as trace amounts of ice, or separately as oxygen and hydrogen, in the minerals that formed the planet. 8
The ocean basins contained some liquid water as far back as geologic records can show, perhaps as long ago as 4.3 or 4.4 billion years. 9 As the planet gradually cooled down, atmospheric water condensed into clouds and then precipitated as rain. This extended rainfall added more water to the oceans. During this process, the atmospheric water reacted with carbon dioxide to create acid rain, which was deposited in the oceans and underlying rocks. With the carbon dioxide washed out of the air, the atmosphere was then primarily nitrogen. The acidic water dissolved minerals, giving rise to the salt content of the sea. The oceans also contained a great deal of iron at that time, which was coming up from the mantle below through volcanic activity. If we could have visited Earth at that time, we would have seen orange skies and a global green ocean with a few black islands. 10
From the human perspective, the heavens and the earth seem to be utterly different realms. We have learned now that Earth is just a small part of the larger universe. From the fundamental particles created in the big bang, to the elements of life synthesized in stars, to the accretion of Earth from space dust, our origins lie in outer space. We continue to be at the whim of our place within the solar system. From this point forward, though, the stage is set. For the rest of the interesting news, we can now bring our attention down to Earth.
- Image © Sdecoret | Dreamstime.com – Planet Eart Apocalypse Illustration Photo. Royalty free license. ↩
- Umberto Maio et al., “The transition from population III to population II-I star formation”, Mon. Not. R. Astron. Soc. 407, 1003-1015 at 1003 (5/10/2010), https://academic.oup.com/mnras/article/407/2/1003/1119897 (accessed and saved 6/30/19). ↩
- Alan P. Boss and Sandra A. Keiser, “Who pulled the trigger: a supernova or an asymptotic giant branch star?” Astrophysical Journal Letters, 717:L1-L5 (7/01/2010), https://iopscience.iop.org/article/10.1088/2041-8205/717/1/L1/meta (accessed and saved 6/30/19). ↩
- Reginald A. Daly, “Origin of the Moon and Its Topography”, Proceedings of the American Philosophical Society 90(2):104-119 (May, 1946), https://courses.seas.harvard.edu/climate/eli/Courses/EPS281r/Sources/Origin-of-the-Moon/2-Daly-1946.pdf (accessed and saved 7/28/19). ↩
- Douwe G. Van Der Meer et al., “Plate tectonic controls on atmospheric CO2 levels since the Triassic”, PNAS 111(12):4380-4385 (3/25/2014), https://www.pnas.org/content/111/12/4380 (accessed and saved 7/28/19). ↩
- William W. Rubey, “Development of the hydrosphere and atmosphere, with special reference to probable composition of the early atmosphere”, In: Arie Poldervaart (ed.), Crust of the Earth: A Symposium, Geol Soc Am, Special Paper 62, 631-664 (1/01/1955), https://pubs.geoscienceworld.org/books/book/711/chapter/3809004/development-of-the-hydrosphere-and-atmosphere-with (introduction accessed and saved 7/28/19). ↩
- Abel Méndez et al., “Habitable Exoplanets Catalog”, Planetary Habitability Laboratory (6/18/2019), http://phl.upr.edu/projects/habitable-exoplanets-catalog/ (accessed and saved 6/30/19). ↩
- Adam Sarafian et al., “Early accretion of water and volatile elements to the inner Solar System: Evidence from angrites”, Science 346(6209):623-626 (10/31/2014), https://science.sciencemag.org/content/346/6209/623 (accessed and saved 7/28/19). ↩
- Simon A. Wilde et al., “Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago”, Nature 409, 175-178 (1/11/2001), https://www.nature.com/articles/35051550 (accessed and saved 7/28/19). ↩
- Martin van Kranendonk as quoted by Seth Borenstein, “Scientists find 3.7 billion-year-old fossil, oldest yet”, Phys.org (8/31/2016), https://phys.org/news/2016-08-scientists-billion-year-old-fossil-oldest.html (accessed and saved 7/28/19). ↩
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