Answers to BRIEF REVIEW Questions

Answers to BRIEF REVIEW Questions

Essentials 6th

Chapter 1: THE ORIGIN OF THE OCEAN

1. Why did I write that there’s one world ocean? What about the Pacific and Atlantic oceans, the “Seven Seas?”

Traditionally, we have divided the ocean into artificial compartments called oceans and seas, using the boundaries of continents and imaginary lines such as the equator. In fact there are few dependable natural divisions, only one great mass of water. Because of the movement of continents and ocean floors (about which you’ll learn more in Chapter 3) the Pacific and AtlanticOceans and the Mediterranean and BalticSeas, so named for our convenience, are in reality only temporary features of a single world ocean. In this book we refer to the world ocean, or simply the ocean, as a single entity, with subtly different characteristics at different locations but with very few natural partitions. This view emphasizes the interdependence of ocean and land, life and water, atmospheric and oceanic circulation, and natural and man-made environments.

2. Which is greater: the average depth of the ocean or the average height of the continents above sea level?

If Earth's contours were leveled to a smooth ball, the ocean would cover it to a depth of 2,686 meters (8,810 feet). The volume of the world ocean is presently 11 times the volume of land above sea level -- average land elevation is only 840 meters (2,772 feet), but average ocean depth is 4½ times greater!

3. Is most of Earth’s water in the ocean?

Most of Earth’s water lies in the crust. It’s not present in great hollow reservoirs, but rather is bound to the rocky material. About 1/40th of a cubic mile of this water escapes each year as steam from volcanic vents, deep-ocean seeps, and other places.

4. Can the scientific method be applied to speculations about the natural world that are not subject to test or observation?

Science is a systematic process of asking questions about the observable world, and testing the answers to those questions. The scientific method is the orderly process by which theories are verified or rejected. It is based on the assumption that nature "plays fair" -- that the answers to our questions about nature are ultimately knowable as our powers of questioning and observing improve.

By its very nature, the scientific method depends on the application of specific tests to bits and pieces of the natural world, and explaining, by virtue of these tests, how the natural world will react in a given situation. Hypotheses and theories are devised to explain the outcomes. The tests must be repeatable -- that is, other researches at other sites must be able to replicate the experiments (tests) with similar results. If replication is impossible, or if other outcomes are observed, the hypotheses and theories are discarded and replaced with new ones. Figure 1.4 shows the process.

Can these methods be applied to speculations about the natural world that are not subject to test or observation? By definition, they cannot.

5. What is the nature of “truth” in science? Can anything be proven absolutely true?

Nothing is ever proven absolutely true by the scientific method. Hypotheses and theories may change as our knowledge and powers of observation change; thus all scientific understanding is tentative. The conclusions about the natural world that we reach by the process of science maynot always be popular or immediately embraced, but if those conclusions consistently match observations, they may be considered true.

6. What if, at the moment you shake the keys, the wires under the hood are jostled by a breeze and fall back into place? What if the car starts when you try it again? Can you see how superstition might arise?

For an interesting perspective on this question, find some back issues of Skeptical Inquirer magazine. Also, author Michael Shermer has written extensively on the issue of irrational superstition and the alarming rise of illogical and superstitious thought in American society.

7. Can scientific inquiry probe further back in time than the “Big Bang?”

No. Spacetime began at the origin of the universe, so the concept of “before” is meaningless when applied to the period preceding that astonishing moment.

8. What element makes up most of the detectable mass in the universe?

Hydrogen

9. Outline the main points in the condensation theory of star and planet formation.

The life of a star begins when a diffuse area of a spinning nebula begins to shrink and heat up under the influence of its own weak gravity. Gradually, the cloudlike sphere flattens and condenses at the center into a knot of gases called a protostar. The fusion process begins – the star “turns on.” Much of the outer material eventually became planets, the smaller bodies that orbit a star and do not shine by their own light.

10. Trace the life of a typical star.

After fusion reactions begin, the star becomes stable— neither shrinking nor expanding, and burning its hydrogen fuel at a steady rate. Over a long and productive life, the star converts a large percentage of its hydrogen to atoms as heavy as carbon or oxygen. This stable phase does not last forever, though. The life history and death of a star depend on its initial mass. When a medium-mass star (like our sun) begins to consume carbon and oxygen atoms, its energy output slowly rises and its body swells to a stage aptly named red giant by astronomers. The dying giant slowly pulsates, incinerating its planets and throwing off concentric shells of light gas enriched with these heavy elements. But most of the harvest of carbon and oxygen is forever trapped in the cooling ember at the star’s heart.

11. How are the heaviest elements (uranium or gold) thought to be formed?

From the deaths of ancient stars.

After a long and productive life, an average star converts a large percentage of its hydrogen to atoms as heavy as carbon or oxygen. When a medium-mass star begins to consume these heavier atoms, its energy output slowly rises and its body swells to a stage aptly named red giant by astronomers. The dying giant slowly pulsates, throwing off concentric shells of light gas enriched with these heavy elements. But most of the harvest of carbon and oxygen is forever trapped in the cooling ember at the star’s heart.

The dying phase of a massive star is more interesting. The end begins when its depleted core collapses in on itself. This rapid compression causes the star’s internal temperature to soar. When the infalling material can no longer be compressed, the energy of the inward fall is converted to cataclysmic expansion called a supernova. The explosive release of energy in a supernova is so sudden that the star is blown to bits, and its shattered mass is accelerated outward at nearly the speed of light. The explosion lasts only about 10 seconds, but in that short time the nuclear forces holding apart individual atomic nuclei are overcome—and atoms heavier than iron are formed. The gold of your rings, the mercury in a thermometer, and the uranium in nuclear power plants were all created during such a brief and stupendous flash. The atoms produced by a star through millions of years of orderly fusion, and the heavy atoms generated in a few moments of unimaginable chaos, are sprayed into space.

Every chemical element heavier than hydrogen—most of the atoms that make up the planets, the oceans, and living creatures—was manufactured by the stars.

12. What is density stratification?

Density is mass per unit of volume. Early in its formation, the still-fluid Earth was sorted by density -- heavy elements and compounds were driven by gravity towards its center, lighter gases rose to the outside. The resulting layers (strata) are arranged with the densest at and near the Earth's center, the least dense as the atmosphere. The process of density stratification lasted perhaps 100 million years, and ended 4.6 billion years ago with the formation of Earth's first solid crust. For a preview of the result, see Figure 3.6.

13. How old is Earth?

Many lines of evidence interlock to suggest Earth is about 4.6 billion (4,600 million) years old.

Radiometric dating (described in an appendix) is a powerful technique based on the discovery that unstable, naturally radioactive elements lose particles from their nuclei and ultimately change into new stable elements. The radioactive decay occurs at a predictable rate, and measuring the ratio of radioactive to stable atoms in a sample provides its age. Using radiometric dating, researchers have identified small zircon grains from western Australian sandstone that are 4.2 billion years old. The zircons were probably eroded from nearby continental rocks and deposited by rivers. (Older crust is now unidentifiable, having been altered and converted into other rocks by geological processes.)

Observing the rates of mountain-building and erosion also provides clues. If we assume the processes we observe occur now at rates similar to rates in the past, we can extrapolate suggestions of age. Even the rate at which heat leaks from within Earth can provide data.

One of the most interesting methods of age-dating searches for cosmic-ray traces in metallic meteorites that have fallen from space. It is likely these objects are remnants of the cores of failed planets, or from material ejected from Earth that ended up in our moon. The density of the traces suggests how long it has been since the objects formed.

The moon itself has provided clues. Its angular momentum and orbital shape can tell of its early formation and subsequent movements, and samples brought back by Apollo astronauts have confirmed an age similar to (but slightly younger than) Earth.

14. How was the moon formed?

About 30 million years after its initial formation, a planetary body somewhat larger than Mars smashed into the young Earth and broke apart. The metallic core fell into Earth’s core and joined with it, while the rocky mantle was ejected to form a ring of debris around Earth. The debris began condensing soon after and became our moon. The newly formed moon, still glowing from heat generated by the kinetic energy of infalling objects, is depicted in Figure 1.13.

15. Is the world ocean a comparatively new feature of Earth, or has it been around for most of Earth’s history?

A few million years after the moon-forming impact, Earth cooled enough to allow the upper clouds to form droplets. Hot rains fell toward Earth, only to boil back into the clouds again. As the surface became cooler, water collected in basins and began to dissolve minerals from the rocks. Some of the water evaporated, cooled, and fell again, but the minerals remained behind. The salty world ocean was gradually accumulating.

These heavy rains may have lasted about 20 million years. Large amounts of water vapor and other gases continued to escape through volcanic vents during that time and for millions of years thereafter. The ocean grew deeper. Evidence suggests that Earth’s crust grew thicker as well, perhaps in part from chemical reaction with oceanic compounds. Although most of the ocean was in place about 4 billion years ago, ocean formation continues very slowly even today.

16. Is Earth’s present atmosphere similar to or different from its first atmosphere?

In a sense, there have been three atmospheres: the original atmosphere blown away by the ignition shock of the sun, the reducing atmosphere that outgassed from within the Earth, and the oxidizing atmosphere that has resulted from the work of photosynthesizing plants and plant-like organisms.

The volcanic venting of volatile substances including water vapor -- outgassing -- gave rise to the present ocean. As hot water vapor rose, it condensed into clouds in the cool upper atmosphere. Recent research suggests that millions of tiny icy comets colliding with the Earth may also have contributed to the accumulating mass of water vapor, this ocean-to-be.

17. Are the atoms and basic molecules that compose living things different from the molecules that make up nonliving things? Where were the atoms in living things formed?

No, atoms are atoms. An atom of carbon in a living object is indistinguishable from an atom of carbon in a rock. Other than primordial hydrogen and some helium, all the elements were made in and by stars. As Carl Sagan was fond of saying, “We are, all of us, bits of stardust.”

18. How old is the oldest evidence for life on Earth? On what are those estimates based?

The oldest fossils yet found, from northwestern Australia, are between 3.4 billion and 3.5 billion years old (Figure 1.16). They are remnants of fairly complex bacteria-like organisms, indicating that life must have originated even earlier, probably only a few hundred million years after a stable ocean formed. Evidence of an even more ancient beginning has been found in the form of carbon-based residues in some of the oldest rocks on Earth, from Akilia Island near Greenland. These 3.85-billion-year-old specks of carbon bear a chemical fingerprint that many researchers feel could only have come from a living organism. Life and Earth have grown old together; each has greatly influenced the other.

19. Was Earth’s atmosphere rich in oxygen when life originated here?

No. The present oxygen-rich atmosphere has resulted from the work of photosynthesizing plants and plant-like organisms. Life (as we know it) could not have evolved in a highly oxidizing atmosphere.

20. The particles that make up the atoms of your body have existed for nearly all of the age of the universe. Look again at Figure 1.9. What could be next?

The ultimate recycling, that’s what! It is arresting to think that the atoms you are using to comprehend this sentence were formed in titanic explosions, have cycled through space and time, and will – eventually – be ejected back into space to condense into some new object.

21. Where would you look for water in our solar system?

Water has been found on Mars (as ice), and there are tremendous quantities of water in the atmospheres of Jupiter and Saturn.

22. If we encounter life elsewhere, would we expect its chemistry and appearance to resemble life on Earth?

For starters, let's look at stars. Most stars visible to us are members of multiple-star systems. If the Earth were in orbit around a typical multiple-star system, we would be close to at least one of the host stars at certain places in our orbit, and too far away at others. Also, not all stars -- in single or multiple systems -- are as stable and steady in energy output as our sun. If we were in orbit around a star that grew hotter and cooler at intervals, our situation would be radically different than it is at the moment.

Next, let's look at orbital characteristics. Our Earth is in a nearly circular orbit at just the right distance from the sun to allow liquid water to exist over most of the surface through most of the year.

Next, consider our planet's cargo of elements. We picked these up during the accretion phase. At our area of orbit there was an unusually large amount of water (or chemical materials that would led to the formation of water).

So, with a stable star, a pleasant circular orbit that is well placed, and suitable and abundant raw materials, we are a water planet. This marvelous combination is probably not found in many places in the galaxy.

The course of evolution of life on any planet depends on the materials and energy available. Because water planets are probably very rare, I shouldn’t expect alien life to resemble what we see on Earth – indeed, we may have difficulty recognizing truly foreign life forms if or when we encounter them.

Chapter 2: A HISTORY OF MARINE SCIENCE

1. What advantages would a culture gain if it could use the ocean as a source of transport and resources?

Any coastal culture skilled at raft building or small boat navigation would have economic and nutritional advantages over less skilled competitors. From the earliest period of human history, understanding and appreciating the ocean and its life-forms benefited coastal civilizations.

2. How was the culture of the Library of Alexandria unique for its time? How was the size and shape of Earth calculated there?

The great Library at Alexandria constituted history's greatest accumulation of ancient writings. As we have seen, the characteristics of nations, trade, natural wonders, artistic achievements, tourist sights, investment opportunities, and other items of interest to seafarers were catalogued and filed in its stacks. Manuscripts describing the Mediterranean coast were of great interest.