Lesson 7

7 :: “The Evolution of Everything”

In this unit we discuss the ever-changing face of our universe.

Planet Formation

In recent years, astronomers have found numerous planets around sun-like stars. These extra-solar worlds often defy all our expectations of what solar systems should look like. To learn more about the search for these new planets and what we know about them so far, click to the NASA Origins program at:

http://origins.jpl.nasa.gov/

Our Place in Space

Man has existed on Earth for a few tens of thousands of years. Earth itself is only five billion years old in a 15-billion-year-old universe. We live on a small planet in the backwaters of an average galaxy that is just one galaxy among millions that dot our universe. Our existence might seem insignificant in context, but it took the evolution of the entire universe to allow us to ponder the heavens.

As you learned in Lesson 3, the big bang produced the majority of helium in the universe, while the stars and supernovae produced all heavier atoms. You learned in Lesson 6 that if the amount of dark matter in the universe had been any higher or lower, our galaxy would not have formed as it did. In this lesson we consider additional factors that affected the universe’s ability to produce a small blue-white world capable of supporting life.

What goes into forming a world? Our planet is mixture of metals; organic compounds that include atoms such as carbon, nitrogen, and oxygen; and deposits of exotic materials, such as uranium, platinum, and gold. Generations of stars needed to live, create heavy atoms in their cores, and die in order to recycle these materials into space. Our sun is at least a third-generation star. In astronomical terms, it belongs to population I — the youngest, most metal-rich stars that fill the disk and bulge of the Milky Way. These stars were formed from metal-enriched gas created in previous stellar generations. Population II stars are older and fill the halo and globular clusters. They are made primarily of the big bang gas mixture, seasoned with some material from the first generation of stars. The rare population III stars are the oldest of all and the few still burning are also in the galaxy’s halo. Population III stars are made strictly from primordial hydrogen, helium, and lithium.

Our solar system started as a cloud of gas that was shocked by something — possibly a supernova — that caused it to collapse. At it collapsed, it began to spin and the cloud flattened into a disk. Because our sun is not part of a binary star system and not located in a particularly dense region of space, this disk was able to form without external disturbances. There is a narrow region in the disk of the Milky Way in which planets can form. If a star is at a much greater distance from the galactic center, there are not enough metals to form a rocky planet. If a star is much closer to the center, there are too many other stars around to allow a stable planetary disk to form.

Exactly what happened after the cloud of gas collapsed into a spinning disk is somewhat unclear. Since Timothy Ferris wrote The Whole Shebang, the discovery of Jupiter-like planets in Mercury-like orbits around sun-like stars have made us realize that our understanding of planet formation is — to say the least — incomplete.

We do know for certain that the early universe was a violent place, where proto-planets and proto-moons careened into one another to create broken moons, the asteroid belt, and larger systems like the Earth and her moon. Scientists believe that our moon was formed from a Mercury-sized body hitting the young Earth. The impact of this second body enlarged the Earth and splashed a large portion of the Earth’s mantle (the stuff continents are made of) into space, where it coalesced into the Moon.

The young Earth was a molten ball irradiated by a young, fiery proto-sun. The high heat burned off much of the gas and water that was then part of the Earth’s atmosphere. As the sun settled into its current state and cooled, volcanoes deep in the Earth replenished these gases. Comet crashes also brought gas and water to Earth, along with the amino acids necessary to form DNA.

The early universe was hot and violent; the violence was necessary to form our moon and to bring us the water and amino acids that we require to exist. At the most basic level, humans are bags of water that came from space. Out of the furnace of the early solar system life was able to emerge and proliferate.

Tunguska Explosion

The explosion of a mystery object over the remote Siberian forests in 1908 has aroused the curiosity of the public and scientific community for almost a century. To learn more, explore these Web sites:

Eyewitness Accounts:

http://www.psi.edu/projects/siberia/siberia.html

Southworth Planetarium Meteorite Expeditions:

http://www.usm.maine.edu/~planet/gallery.html

And for a collection of wild theories:

The Great Siberian Explosion:

http://unmuseum.mus.pa.us/siberia.htm

Do We Need to Worry?

For a statistical analysis of our likely doom (the sky isn’t too likely to crash down any time soon), please explore these sites:

Near-Earth Object Program:

http://neo.jpl.nasa.gov/torino_scale.html

Asteroid and Comet Impact Hazard:

http://impact.arc.nasa.gov/

Earth Defense Initiative (an article with a radical bent):

http://abob.libs.uga.edu/bobk/meteor.html

Protective Bodies

Life requires more than just simple atoms to thrive; it needs time to evolve. Although the worst of the celestial barrage has ended, violence in the solar system continues. The majority of the Earthbound comets were swept up in the first millennia of the solar system, but new comets continue to emerge from the distant edges of the solar system and small rocky bodies — asteroids on rogue orbits — continue to cross the Earth’s orbit.

For life as we know it to have evolved, a planet needs protection. In the case of our Earth, that protection is Jupiter and the Moon. In 1994, comet Shoemaker-Levy 9 (SL9) hit Jupiter. On its first observed pass through the solar system, Jupiter’s gravitational pull radically changed SL9’s orbit and broke SL9 into multiple pieces. On its second observed pass, SL9 was sucked into Jupiter, permanently preventing one large piece of space junk from hitting the Earth or anything else in the solar system.

The cosmological idea that we do not live in a special time or place means that scientists don’t believe that this collision was a particularly rare event. Rather, it is likely that Jupiter’s gravitational influence has removed a lot of debris from our solar system. A larger number of comets would bombard the Earth without a large planet like Jupiter orbiting our sun at a greater distance than the Earth.

Similarly, our moon absorbs many impacts. During the Apollo missions, astronauts took pictures of the dark side of the Moon — the side opposite from the Earth. The Moon rotates at the same rate that it orbits. This keeps the same side of the Moon constantly facing the Earth (see the animation below). The only way to see the other side of the Moon is to go and look. When astronauts went and looked, they found that the far side of the Moon is significantly more cratered than the side we see (see Figure 7-2). This cratering has continued into recent times. Everything that has hit the Moon could have hit the Earth; without the Moon, living on Earth would be a lot more dangerous. As the Moon orbits, it keeps the same face towards the Earth all the time.

image037

Figure 7-2: The two sides of the Moon have very different cratering patterns. These images were obtained with NASA’s Clementine Satellite (http://wwwflag.wr.usgs.gov/USGSFlag/Space/clementine/clementine.html).

Even with these guardian worlds the Earth is not impact-free. In 1908, an object from space (now believed to have been an asteroid) raced across the Siberian skies, exploding near Tunguska, Siberia. Another meteor crashed into the desert of Saudi Arabia less than 150 years ago. Neither of these impacts affected life outside of a few tens of square kilometers; however, an asteroid of this size hitting a significant population center would have devastating effects. These extraterrestrial missiles explode with the force of small nuclear weapons and, like volcanoes, spew dust and debris into the atmosphere.

And these asteroids have much bigger brothers.

Massive, life-destroying asteroids have hit the Earth more than once. There is clear evidence that the asteroid that left a huge crater along Mexico’s Yucatan Peninsula brought a violent end to the Cretaceous era — the era of the dinosaurs. There is some evidence that comet or asteroid impacts have caused many other mass extinctions, roughly every 26 million years.

Currently there are several international collaborations watching the skies for the next great impact. At this time, there are no known objects with orbits that will intersect with Earth. This does not mean that we are safe.

Is Our World Dying?

Many biologists believe that the Earth is currently undergoing its sixth great mass extinction. The exact cause is unclear. Perhaps global warming is at its root, or perhaps segmenting the “wild” into island parks and nature reserves is to blame. Maybe the answer is as simple as overhunting and pollution. No matter what the leading cause might be, it is clear that humanity is the root cause. To learn more, go to these scientific sites:

National Geographic: The Sixth Extinction

http://www.nationalgeographic.com/ngm/9902/fngm/index.html

World Resources Institute: A History of Extinction

http://www.igc.apc.org/wri/biodiv/b03-koa.html

American Museum of Natural History: The Sixth Extinction

http://www.amnh.org/exhibitions/hall_tour/extinct.html

Extinction Is Good?

Each major terrestrial impact not only caused mass extinctions, but also brought mankind one step closer to existing. One of evolution’s major tenets is that plants and animals evolve to more effectively fill ecological niches. Once something develops to fill a niche, it can be displaced only by something more efficient. Under normal conditions this is very rare because the lesser animals (or plants) are kept in their place by the larger animals (or plants). Modern man somehow replaced Neanderthals; more typically, changes in the food chain have required human intervention. The introduction of cats to the Hawaiian Islands, for instance, wiped out many reptile and bird species because the cats more effectively hunted the same birds as the reptiles and (adding insult to injury) also killed the reptiles.

Mass extinctions are usually caused by, or accompany, radical changes in the environment. If a species is too specialized, it cannot adapt to these changes. Although the giant panda of China is the ideal bamboo eater, it is unable to survive in an environment where there is no bamboo. Asia’s bamboo blight is slowly destroying this majestic animal, the largest herbivore in its area of the world.

Mass extinctions have allowed some animals to climb the food chain while others tumbled into the fossil record. In general, smaller animals are best at adapting to environmental changes. The human animal is perhaps an exception. (Although pound for pound we aren’t exceptionally large; mastiff dogs and lions, tigers and bears are all significantly larger.) The common European swallow has spread with sea ships to every non-Arctic corner of the globe, learning to find new edible seeds and to forage in every environment from Africa to Newfoundland.

That asteroid impact reduced the dinosaurs to a few surviving fish — such as the coelacanth. The surviving rodents and marsupials evolved to fill the environmental spaces dinosaurs left behind. When the animals at the top of the food chain die off, animals at the bottom readily evolve into their ecological spaces. The small furry animals of the Cretaceous period were able to adapt to the drop in temperature and change in plant diversity. The dinosaurs, too specialized and too large, simply died. Humanity was able to evolve from some of those mammal survivors and become the dominant carnivore on Earth.

The current manmade mass extinction might not be so kind to mankind. What has taken the universe 15 billion years to create, modern man is managing to destroy in the cosmic blink of an eye. Many scientists hope that if humans better understand the amazing chain of events that produced human beings, perhaps they will learn to take better heed of what they are doing to destroy the Earth.

Human existence required a big bang with certain characteristics, including shepherd bodies — our Moon and Jupiter — to protect us from comet and asteroid impacts. Our existence also required mass extinctions to encourage evolution. Humans exist; our future is in our own hands. When asked why I study astronomy, I reply that it is the one field of science that encourages humanity to look beyond Earth-bound existence and dream of greater things. By looking to the stars, perhaps we can find a better future here on Earth.

In the next and final lesson, I consider some questions I have so far avoided: the questions of origins, of God, and of the existence of little green men.

Assignment: The Impact Effect

Read Chapter 7 of The Whole Shebang: A State-of-the-Universe(s) Report by Timothy Ferris. In science fiction movies and books, we often find ourselves face-to-face with worlds orbiting multiple suns (Tatooine in the Star Wars saga), and life on moons (Endor, again in Star Wars). In this lesson and reading we learned that it is important for evolving life to have long periods of stability, with occasional evolution-inducing interruptions, such as asteroid impacts. If you were searching for life, would you consider moons and planets in multiple-star systems as likely places to find it?

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