10.IV: Sun, Earth, and Moon

Planet Earth under formation during the "heavy bombardment," four billion years ago.

Planet Earth under formation during the “heavy bombardment,” over four billion years ago.  See image credit below.

 

A. Formation of the Sun and Solar System

B. Formation of the Earth and Moon

C. Earth’s Early Atmosphere and Oceans

D. We Come From Outer Space!

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A. Formation of the Sun and solar system

The first generation of stars probably burned out quickly (there certainly are none left that are close enough to see!) 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. With no radiation to counteract gravity, the cold molecular clouds once again began to collapse 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 life-cycle 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 when the universe was nine billion years old, about five billion years ago, a particular region of our galaxy was fairly dense with those dirty gaseous remains. This cloud of gas and dust was the pre-solar nebula. 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 other forces (perhaps a nearby supernova explosion) 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 3,000,000 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, which 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 proto-planets. Eventually, eight of these proto-planets “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 proto-planets 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 very interesting 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. Because metals and minerals are much more rare than ices, the inner rocky planets (Mercury, Venus, Earth, and Mars) did not have as much raw material, so they developed 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 great name of the Hadean Eon, named after Hades, the ancient Greek name for Hell. The Earth was very much 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 proto-planets. These collisions, and the decay of radioactive elements inside the Earth, kept the planet very 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 on top as the Earth’s crust.

According to a serious hypothesis, one of Earth’s last major collisions was with a large proto-planet called Theia. 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, and has been slowly drifting away from Earth ever since. This giant collision may also account for the prominent tilt of Earth’s axis, which results in the four seasons as the 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 very 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.

The 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 very cores. As a consequence, they are 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. From these examples, we can see that the planet owes its biological life to its geological life!

C. Earth’s early atmosphere and oceans

Earth is massive enough to hold onto a thin layer of gas, the atmosphere. Earth had an atmosphere early in its history, though it was noxious (to us), nothing like the atmosphere of today. Some of this gas was released from within the bubbling new planet, some was captured along Earth’s orbit, and some was captured from collisions with comets and planetesimals. The early atmosphere was rich in carbon dioxide, water vapor, and ammonia. 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. Ammonia, a very common compound in space and on early Earth, has the chemical formula NH_3. 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, N_2, 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 very stable. The atmosphere is still 80% N_2 today.

The history of water on Earth is especially interesting. The planet formed with some water, but not enough to account for the vast amounts here on Earth today. Recall that Earth is inside of the solar system’s frost line, too hot for ice. Therefore, most of Earth’s water was delivered here by comets and icy planetesimals that originated beyond the frost belt! Jupiter’s gravitational influence may have played a role in sending those bodies toward the inner solar system.

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. As the planet gradually cooled down, the 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 oceans also contained a great deal of iron at that time, which was coming up from the mantle below through volcanic activity. Because of all this iron, the early ocean was red or brown.

One gas was conspicuously absent from the early atmosphere: oxygen. As we will soon find out, that was a very important difference between the Earth of Hadean times and the Earth of today.

D. We come from outer space!

We have seen very clearly that our origins lie in outer space. From the fundamental particles created in the Big Bang, to the elements of life synthesized in stars, to the formation of Earth from space dust, to the influence of Jupiter and comets in seeding our planet with water, outer space has been essential to our discussion so far.

From this point forward, pretty much all that matters to us is the Earth and Sun. Very little news from outer space has had a major influence on the further development of our planet. One exception is the occasional impact of space debris such as meteors and comets. When large bodies collide with Earth, they can have devastating effects such as climate change and mass extinctions. Life on Earth has gone through five mass extinctions, and at least some of them seem to be strongly associated with space impacts.

Other than that, the far reaches of outer space have little to no bearing on the development of our world. “Far” outer space (beyond the sun-Earth system) is primarily of interest for what it teaches us about nature and our origins. It will also become a major concern to our species again in the far future when the sun changes and destroys Earth. If our descendants are still around in a billion years, they will have to complete the grand cycle, eventually returning back to the stars from which we came. For the moment, the Sun is still basically the same as when the Earth was formed. It is vital to our life, but not very interesting anymore as a historical development.

And with those grandiose thoughts addressed, the rest of this history can now focus on Planet Earth.

Image © Sdecoret | Dreamstime.comPlanet Eart Apocalypse Illustration Photo.  Royalty free license.

Return to Ch. 10

Continue to Section 10.V: Life

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