Chapter 4: Space and Time

Question 1

Define ‘Asteroid’.

Answer 1

A sub-planetary object orbiting the Sun. The orbits of most asteroids lie between the orbits of Mars and Jupiter.

Question 2

Define ‘Astronomical unit’.

Answer 2

Unit of measure used by astronomers; it is the average distance from Earth to the Sun; 149,600,000 kilometers.

Question 3

Define ‘Comet’.

Answer 3

Small solar system body com- posed primarily of ice with some dust and rock particles, which orbits the Sun in a highly elliptical orbit.

Question 4

Define ‘Galaxy’.

Answer 4

A cluster of a billion or more stars, plus gas and dust, that is held together by gravity.

Question 5

Define ‘Geological column’.

Answer 5

A composite diagram combining in chronological order the succession of known strata, fitted together on the basis of their fossils or other evidence of relative or actual age.

Question 6

Define ‘Half-life’.

Answer 6

The time needed for the number of parent atoms of a radioactive isotope to be reduced by one-half.

Question 7

Define ‘Jovian planets’.

Answer 7

Giant planets in the outer regions of the solar system that are characterized by great masses, low densities, and thick atmospheres consisting primarily of hydrogen and helium.

Question 8

Define ‘Main sequence’.

Answer 8

The principal series of stars in the Hertzsprung-Russell diagram, which includes stars that are converting hydrogen to helium.

Question 9

Define ‘Meteorite’. vs meteoroid and meteor?

Answer 9

Piece of natural debris that falls to Earth.

Question 10

Define ‘Moon’.

Answer 10

A natural object in a regular orbit around a planet.

Question 11

Define ‘Nebular hypothesis’.

Answer 11

The proposition that the Sun and planets formed from a huge, swirling cloud of cosmic gas and dust. Elements heavier than hydrogen and helium came from the remains of an older star that exploded in a supernova.

Question 12

Define ‘Numerical age’.

Answer 12

The time in years when a specific event happened or a specific material formed or was deposited.

Question 13

Define ‘Planet’.

Answer 13

A natural body in orbit around a star that is massive enough to be spherical and to have cleared its orbital path of other objects.

Question 14

Define ‘Planetary accretion’.

Answer 14

The process by which bits of condensed solid matter were gathered to form the planets.

Question 15

Define ‘Planetary differentiation’.

Answer 15

The process of chemical segregation by which a planet separates into a core of dense matter surrounded by one or more layers of less dense rocky matter.

Question 16

Define ‘Primary (primordial) atmosphere’.

Answer 16

The original envelopes of hydrogen and helium which surrounded the terrestrial planets early in the history of the solar system.

Question 17

Define ‘Principle of uniformitarianism’.

Answer 17

The same external and internal processes we recognize in action today have been operating unchanged, though at different rates, throughout most of the Earth’s history.

Question 18

Define ‘Radiometric dating’.

Answer 18

Determination of the time in years since the formation of a rock or other natural object using the contained radioactive isotopes.

Question 19

Define ‘Relative age’.

Answer 19

The age of an object, material, or event, as determined by comparison to an older or younger object or event.

Question 20

Define ‘Secondary atmosphere’.

Answer 20

The envelope of gaseous volatile elements that leaked from the interior of a terrestrial planet via volcanoes and was trapped by the planet’s gravity.

Question 21

Define ‘Solar nebula’.

Answer 21

A flattened rotating disc of gas and dust surrounding the Sun.

Question 22

Define ‘Solar system’.

Answer 22

The group of planets, moons, asteroids, comets, and other natural objects in orbit around the Sun.

Question 23

Define ‘Terrestrial planets’.

Answer 23

The innermost planets of the solar system (Mercury, Venus, Earth, and Mars), which have high densities and rocky compositions.

Question 24

The Sun is a star, the central body of our solar system. It is part of the Milky Way galaxy, one of about 100 billion galaxies, each of which contains between 200 and 400 billion stars. Though an ordinary star, the Sun overwhelmingly dominates our solar system, containing ___ percent of its mass—mostly hydrogen and helium, with small amounts of the heavier elements.

Answer 24

99.8%

Question 25

How is the blanketing layer on Earth formed?

Answer 25

Radiatively active gases in the lower part of the atmosphere absorb outgoing longer-wavelength terrestrial radiation, forming a “blanketing” layer that raises the aver- age surface temperature, making life possible on this planet.

Question 26

We experience seasons because Earth’s rotational axis is tilted at ___ with respect to the ecliptic. This means that at certain times the northern hemisphere points toward the Sun (June) and at other times of the year the southern hemisphere points toward the Sun, thus receiving the most direct solar energy (Dec).

Answer 26

23.5 degrees.

Question 27

The solar system consists of the Sun, ___ planets, at least ___ dwarf planets, a vast number of small rocky bodies called asteroids, millions of comets, innumerable small fragments of rock and dust called meteoroids, and 170 known moons, all of which travel along trajectories determined by gravity. The planets, asteroids, comets, and meteoroids orbit the Sun, whereas the moons orbit the planets.

Answer 27

8 planets, and 5 dwarf planets.

Question 28

What features are characteristic of the terrestrial planets?

Answer 28

Mercury, Venus, Earth, and Mars—are small, dense, and rocky and metallic in composition.

Question 29

What features are characteristic of the jovian planets?

Answer 29

Jupiter, Saturn, Uranus, and Neptune—are much larger and more massive, though much less dense than the terrestrial plan- ets since they are composed dominantly of hydrogen and helium.

Question 30

Almost everything in the solar system—planets, moons, and the Sun—revolves and rotates in the same direction and in the same plane. What is a notable exception?

Answer 30

Venus, which spins in retrograde direction.

Question 31

Condensation in the solar nebula produced a cosmic “snow” of high-temperature, rocky condensates in the ___ part and low-temperature, volatile, icy condensates in the ___ part,

Answer 31

Inner and outer.

Question 32

What ae the main factors that have influenced the subsequent evolution of the terrestrial planets? Although we cannot see the surfaces of the jovian planets through their thick atmospheres, it is reasonable to conclude that these factors also influenced their evolution as planets.

Answer 32

• Impact cratering and resultant partial melting
• Volcanism
• Distance from or proximity to the Sun
• Absence or presence of a biosphere.

Question 33

What processes/features did all of the terrestrial planets share?

Answer 33

The terrestrial planets underwent partial melting and differentiation into low-density, rocky crusts; rocky, intermediate-density mantles; and metallic, high-density cores. All terrestrial planets have experienced volcanism and intense meteorite collisions. All of the terrestrial planets lost their primary atmospheres, but Earth, Mars, and Venus evolved and retained secondary atmospheres.

Question 34

What is unique to Earth?

Answer 34

Plate tectonics may be unique to Earth, which also has a stronger magnetic field than the other terrestrial planets.

Question 35

What is the composition of the gas giants?

Answer 35

The outer planets are gas giants. Jupiter and Saturn are so massive that their atmospheric gases have not escaped—even the lightest gases, hydrogen and helium. Thus, they have compositions very similar to that of the solar nebula from which they formed. Huge storms, high- speed winds, and lightning are common in their deep atmospheres. They probably have small rocky cores mantled by ice. Deep in their interiors, pressures are so intense that hydrogen condenses, becoming a liquid or even a metal inside Jupiter and Saturn.

Question 36

How did Earth’s moon form?

Answer 36

Earth’s Moon originated as a result of a massive collision (with Theia) that tilted Earth’s axis of rotation.

Question 37

Where do asteroids orbit in the solar system? Comets?

Answer 37

Asteroid belt.

Kuiper belt or Oort cloud.

Question 38

Where did much of Earth’s water come from?

Answer 38

Likely from impacts with comets.

Question 39

What are stellar classes based on?

Answer 39

Luminosity and colour.

Question 40

What determines a star’s size (and also its main sequence lifetime)?

Answer 40

The balance of inward gravitational forces and outward radiation forces inside a star.

Question 41

How do stars end their lives?

Answer 41

When a small (1 S) star ends its main-sequence lifetime, it undergoes core contraction and shell fusion of hydrogen, becoming a red giant and then a white dwarf. Very massive stars (100 S) have very high temperatures and luminosities, quickly deplete their fuel, and have short lives on the main sequence. Once off the main sequence, massive stars go though burnout, core contraction, and shell fusion of hydrogen and helium. Eventually, fusion begins to form elements heavier than carbon, releasing so much energy that the star blows up in a supernova.

Question 42

___ to ___ percent of the stars in the Milky Way have characteristics similar to those of our Sun.

Answer 42

5-10%

Question 43

The widely accepted value for the age of the universe is …?

Answer 43

13.7 billion years.

Question 44

How has the age of the solar system (4.56 billion years) been determined?

Answer 44

By direct measurement of the ages of the oldest and most primitive known objects in our solar system, carbonaceous chondrites.

Question 45

The Sun probably existed as a young star for about 100 million years prior to the formation of the solar system; hence its age is about …?

Answer 45

4.7 billion years.

Question 46

The rate of decay of radioactive materials is not changed by geologic processes, so it can be used to determine ___ ___.

Answer 46

Numerical age.