Chapter 4 - Origin of the Solar System

Odd Rotation

Giant impacts might also explain the different rotation axes of some planets.

What do we mean by the period of heavy bombardment in the context of the history of our solar system?

the first few hundred million years after the planets formed, which is when most impact craters were formed

What do we mean by accretion in the context of planet formation?

the growth of planetesimals from smaller solid particles that collided and stuck together

In the context of the formation of planets in the solar nebula, the frost line marks the distance from the Sun beyond which __________.

the temperature was low enough for hydrogen compounds to condense into ices

Radioactive Decay

• Some isotopes decay into other nuclei. • A half-life is the time for half the nuclei in a substance to decay.

Two Major Planet Types

• Terrestrial planets are rocky, relatively small, and close to the Sun. • Jovian planets are gaseous, larger, and farther from the Sun.

Nebular Theory

• The nebular theory states that our solar system formed from the gravitational collapse of a giant interstellar gas cloud—the solar nebula. - (Nebula is the Latin word for cloud.) • Kant and Laplace proposed the nebular hypothesis over two centuries ago. • A large amount of evidence now supports this idea.

What does the solar system look like?

• There are eight major planets with nearly circular orbits. • Dwarf planets are smaller than the major planets and some have quite elliptical orbits. • Planets all orbit in same direction and nearly in same plane.

While we may never be able to prove that our Moon really formed in a giant impact, which of the following would give scientists greater confidence that the model is correct? Select all that apply.

- Finding evidence that other giant impacts occurred in our solar system, with a total number of impacts consistent with what models of solar system formation lead us to expect. - Observing evidence of recent giant impacts in other solar systems that are currently in the process of formation. - Creating more sophisticated models of the giant impact that correctly predict many detailed characteristics of the Earth and Moon.

Now consider the second major feature (two types of planets). Which of the following statements are true for the terrestrial and jovian planets in our solar system?

- Jovian planets are larger in mass than terrestrial planets. - Jovian planets are larger in size than terrestrial planets. - Jovian planets have more moons than terrestrial planets. - Jovian planets orbit farther from the Sun than terrestrial planets.

For the purposes of seeking a theory to explain the formation of the solar system, we identify four major features of our solar system. Which of the following represent these four major features?

- Large bodies in the solar system have orderly motions. - Several exceptions to the general trends stand out. - Swarms of asteroids and comets populate the solar system. - Planets fall into two major categories (terrestrial and jovian).

Consider the first major feature (orderly motions). Which of the following correctly describe patterns of motion in our solar system?

- Planets have nearly circular orbits. - All the planets orbit the Sun in nearly the same plane. - The Sun and most of the planets rotate in the same direction that the planets orbit.

All the following statements are true. Which of them are considered to be "exceptions" to the general trends described by the first three major features of the solar system?

- Venus rotates in a direction opposite to the rotation of the other terrestrial planets. - Our Moon has a diameter more than 1/4 the diameter of Earth. - Uranus rotates with an axis tilt that lies nearly in the ecliptic plane.

How do we explain the existence of our Moon?

- our moon is too big to be considered a captured moon - we cannot think they were formed simultaneously with earth because in this case both the earth and the moon should have the same composition - Today we know the composition of the moon is not the same as the earth.

The solar system contains vast numbers of small bodies, which we call asteroids when they are rocky and comets when they are icy. These small bodies are concentrated in the region(s) of the solar system that we call __________.

- the asteroid belt - the Oort cloud - the Kuiper belt

What properties of our solar system must a formation theory explain?

1. Patterns of motion of the large bodies - Orbit in same direction and plane 2. Existence of two types of planets - Terrestrial and jovian 3. Existence of smaller bodies - Asteroids and comets 4. Notable exceptions to usual patterns - Rotation of Uranus, Earth's Moon, etc.

Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 7 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?

3.75 billion years ago

_______ allows us to determine the age of a solid rock.

Radiometric dating

Suppose you have a rock that, when it solidifies, contains 1 microgram of a radioactive isotope. How much of this isotope remains after five half-lives?

1/32 microgram

Suppose you find a rock and measure that 12.5% of the original uranium-235 still remains it, while the other 87.5% has decayed into lead-207. About how old is the rock?

2.1 billion years

About how old is the Solar System?

4.5 billion years

Approximately what is the half-life of uranium-235?

700 million years

The verdict at the end of the Extraordinary Claims box says "Likely correct, though may never be possible to prove definitively." In the context of models, which of the following best explains this verdict?

A model that precisely reproduces major characteristics of the Moon would seem likely correct, but we can't go back in time to see that it actually happened that way.

When did the objects that we now see as asteroids and comets form in the solar system?

After solid particles condensed from the gas but before the planets finished forming.

The Nebula Theory is support by which bit of data?

All the planets orbit in the same direction.

As discussed in the Extraordinary Claims box, when the giant impact idea was first proposed, it was generally thought to be so unlikely that it wasn't seriously considered until decades later, after the Apollo missions to the Moon. What key piece of scientific understanding was missing that made it seem so unlikely when first proposed?

An understanding of the number and size of leftover planetesimals in the early solar system.

Consider the three items from Part A that are not relevant to the giant impact hypothesis. As you'll learn in later chapters, the lack of volcanoes is a result of the Moon's relatively small size. But what explains the other two items — the Moon's synchronous rotation and gradually increasing distance from Earth?

Both are results of tidal interactions between the Moon and Earth.

The materials that made up the solar nebula can be categorized into the four general types as follows. Rank these materials from left to right based on their abundance in the solar nebula, from highest to lowest.

Hydrogen and helium gas, hydrogen compounds, rock, metals

As you've learned from Part B, hydrogen and helium gas never condense under conditions found in the solar nebula. The remaining three categories of material in the solar nebula are shown again here. Rank these materials from left to right based on the distance from the Sun at which they could condense into a solid in the solar nebula, from farthest to closest.

Hydrogen compounds, rock, metals

According to our theory of solar system formation, what three major changes occurred in the solar nebula as it shrank in size?

It got hotter, its rate of rotation increased, and it flattened into a disk.

Suppose you found a star with the same mass as the Sun moving back and forth with a period of 8 years. What could you conclude?

It has a planet orbiting at exactly 4 AU.

Which of the following statements was true beyond the frost line (but not interior to it)?

It was cold enough for hydrogen compounds to condense into ices.

The following images show four planets in our solar system. Rank these planets from left to right based on the number of moons that orbit them, from highest to lowest. (Not to scale.)

Jupiter, Mars, Earth, Mercury

The jovian planets in our solar system are __________.

Jupiter, Saturn, Uranus, and Neptune

Heavy Bombardment

Leftover planetesimals bombarded other objects in the late stages of solar system formation.

Accretion of Planetesimals

Many smaller objects collected into just a few large ones.

The following images show five planets in our solar system. Rank these planets from left to right based on their average surface (or cloud-top) temperature, from highest to lowest. (Not to scale.)

Mercury, Earth, Mars, Jupiter, Neptune

The materials that made up the solar nebula can be categorized into these four general types. Rank these materials from left to right based on the temperature at which each would condense into a solid, from highest to lowest. Note: For a substance that does not condense at all, rank it as very low temperature.

Metals, rock, hydrogen compounds, hydrogen and helium gas

The following images show five planets in our solar system. Rank these planets from left to right based on the amount of time it takes them to orbit the Sun, from longest to shortest. (Not to scale.)

Neptune, Jupiter, Mars, Earth, Mercury

Based on what the nebular theory tells us about the formation of our own solar system, what does the theory predict for the possibility of other planetary systems?

Planetary systems should be common.

Which of the following discoveries provides strong evidence in favor of the nebular theory?

Planets are common around other stars.

Major features of the Sun and planets

Planets are very tiny compared to distances between them.

The following images show six objects in our solar system. Rank the objects from left to right based on their average distance from the Sun, from farthest to closest. (Not to scale.)

Pluto, Saturn, Jupiter, Mars, Earth, Mercury

The following images show six objects in our solar system. Rank these objects from left to right based on their mass, from highest to lowest. (Not to scale.)

Sun, Jupiter, Earth, Mars, Mercury, Pluto

The images below show six objects in our solar system. Rank these objects by size (average equatorial radius), from largest to smallest. (Not to scale.)

Sun, Jupiter, Earth, Mars, Mercury, Pluto

The statements below are all true. Some of them represent important reasons why the giant impact hypothesis for the Moon's formation is taken seriously, and some of them are not relevant to deciding between this and other hypotheses. Sort the statements into the correct bin according to whether or not they provide important support to the giant impact hypothesis.

Supports giant impact hypothesis: - Moon's composition is similar to Earth's mantle - Moon has a low proportion of easily vaporized ingredients - Models indicate early solar system had Mars-size leftover planetesimals True but not relevant to giant impact hypothesis: - Moon's distance is gradually increasing - Moon has synchronous rotation (keeps same face to Earth) - Moon lacks active volcanoes

Listed following are statements that, based on our current theory of solar system formation, apply either to the formation of terrestrial planets or of jovian planets, but not both. Match these to the appropriate category.

Terrestrial Planets: -- accreted from planetismals of rock and metal -- surfaces dramatically altered during the heavy bombardment Jovian Planets: -- formed in regions cold enough for water to freeze -- large moons formed in surrounding disks of material -- formed in a region of the solar system with lower orbital speeds --accreted from icy planetismals -- ejected icy planetismals that are now Oort cloud comets

Listed following are characteristics that can identify a planet as either terrestrial or jovian. Match these to the appropriate category.

Terrestrial planets - Solid, rocky surface - Located within the inner solar system - Small Size Jovian Planets - Primarily composed of hydrogen, helium, and hydrogen compounds - Numerous orbiting moons - Low average density - Extensive ring systems

What is the giant impact hypothesis for the origin of the Moon?

The Moon formed from material blasted out of the Earth's mantle and crust by the impact of a Mars-size object.

How do we explain "exceptions to the rules"?

The bombardment of newly formed planets by planetesimals may explain the exceptions. Material torn from Earth's crust by a giant impact formed the Moon.

Suppose the solar wind had cleared away the solar nebula before the seeds of the jovian planets could gravitationally draw in hydrogen and helium gas. How would the planets of the outer solar system be different? Would they still have many moons? Explain your answer in a few sentences.

The lack of hydrogen and helium used to make their gaseous atmospheres would cause the mass of these planets to be significantly smaller overall. There would be fewer moons since a lack of mass equates to a weaker gravitational pull, resulting in fewer moons a planet is able to hold onto in orbit.

The frost line is:

The line where past this hydrogen compounds could form ices

What do we mean when we say that a nucleus undergoes radioactive decay?

The number of protons or neutrons (or both) in the nucleus changes.

Consider only the first major feature, which concerns observed patterns of motion in the solar system. Scientifically, which of the following possible conclusions is justified from the patterns of motion alone?

The planets were not each born in a separate, random event.

The terrestrial planets are made almost entirely of elements heavier than hydrogen and helium. According to modern science, where did the elements heavier than hydrogen and helium come from?

They were produced by stars that lived and died before our solar system was born.

Which one of the following is not one of the four major features of the solar system?

The solar system contains eight planets plus dwarf planets (such as Ceres, Pluto, and Eris).

Where did asteroids and comets come from?

They are leftover planetesimals, according to the nebular theory.

Captured Moons

Unusual moons of some planets may be captured planetesimals.

The planet in our solar system with the highest average surface temperature is __________.

Venus

Origin of Earth's Water

Water may have come to Earth by way of icy planetesimals.

How would the solar system be different if the solar nebula had cooled, with a temperature half its actual value? a) Jovian planets would have formed closer to Sun. b) There would be no asteroids. c) There would be no comets. d) Terrestrial planets would be larger.

a) Jovian planets would have formed closer to Sun.

Mars was formed by the _________ of smaller objects.

accretion

How many of the planets orbit the Sun in the same direction as Earth does?

all

Where would you expect terrestrial planets to form in the solar nebula?

anywhere between the innermost regions (within about the inner 0.3 AU) and the frost line

What process created the elements from which the terrestrial planets were made? a) the Big Bang b) nuclear fusion in stars c) chemical processes in interstellar clouds d) their origin is unknown.

b) nuclear fusion in stars

How does the Earth-Sun distance compare with the Sun's radius? a) It's about 10 times larger. b) It's about 50 times larger. c) It's about 200 times larger. d) It's about 1000 times larger.

c) It's about 200 times larger.

When we say that jovian planets contain significant amounts of hydrogen compounds, we mean all the following chemicals except __________.

carbon dioxide

Ice can form from a gas through the process of

condensation

Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form? a) 1.25 billion years ago b) 2.5 billion years ago c) 3.75 billion years ago d) 5 billion years ago

d) 5 billion years ago

Which of these facts is NOT explained by the nebular theory? a) There are two main types of planets: terrestrial and jovian. b) Planets orbit in same direction and plane. c) Existence of asteroids and comets. d) Number of planets of each type (four terrestrial and four jovian).

d) Number of planets of each type (four terrestrial and four jovian).

Planetary orbits are

fairly circular and in the same plane.

Hydrogen compounds in the solar system can condense into ices only beyond the

frost line

Today, scientists have a theory (the nebular theory) that explains all the major characteristics of the solar system. In science, we expect a theory like this not only to explain the observed characteristics of our solar system but also to predict __________.

general characteristics of other solar systems

Which of the following is thought to explain many of the "exceptions to the rules" of our solar system?

giant impacts

The first few hundred million years of the solar system's history were the time of the _______, during which Earth suffered many large impacts.

heavy bombardment

When a spinning ice skater pulls in his arms, he spins faster because __________.

his angular momentum must be conserved, so reducing his radius must increase his speed of rotation

Which of the following types of material can condense into what we call ice at low temperatures?

hydrogen compounds

Which have more moons on average?

jovian planets

Provided following are stages that occurred during the formation of our solar system. Rank these stages from left to right based on when they occurred, from first to last.

large cloud of gas and dust>>contraction of solar nebula>>condensation of solid particles>>accretion of planetesimals>>clearing the solar nebula

According to our theory of solar system formation, what are asteroids and comets?

leftover planetesimals that never accreted into planets

The frost line of the solar nebula refers to the location beyond which

metal, rock, and ice could condense into solid form.

What substances existed as solid flakes within the inner 0.3 AU of the solar system before planets began to form?

none

What substances existed as solid flakes within the innermost regions (within about the inner 0.3 AU) of the solar system before planets began to form?

none

In essence, the nebular theory holds that __________.

our solar system formed from the collapse of an interstellar cloud of gas and dust

Now consider why the observed patterns of motion lead to the conclusion that the planets were not born in separate, random events. The reason for this conclusion is that, if the planets had been born in separate, random events, we would expect that __________.

planetary orbits would have many different orientations and directions, rather than all being in the same direction and in the same plane

Our Moon was most likely formed by a collision between Earth and a Mars-sized

planetesimal

The jovian planets are thought to have formed as gravity drew hydrogen and helium gas around planetesimals made of __________.

rocks, metals, and ices

The jovian planets are thought to have formed as gravity drew hydrogen and helium gas around planetesimals made of ____________.

rocks, metals, and ices

What substances were found in the innermost regions (within about the inner 0.3 AU) of the solar system before planets began to form?

rocks, metals, hydrogen compounds, hydrogen, and helium, all in gaseous form

What substances were found within the inner 0.3 AU of the solar system before planets began to form?

rocks, metals, hydrogen compounds, hydrogen, and helium, all in gaseous form

Our solar system was created by the gravitational collapse of the

solar nebula

The era of planet formation ended when the remaining hydrogen and helium gas of the solar nebula was swept into interstellar space by the

solar wind

In the context of studying major bodies of our solar system, what category of object does our Moon best fit?

terrestrial world

Suppose that two objects collide. Which of the following is not necessarily the same both before and after the collision? (In other words, which quantity is not conserved according to the laws of physics?)

the total temperature of the objects

What features of our solar system provide clues to how it formed? Motion of Large Bodies

• All large bodies in the solar system orbit in the same direction and in nearly the same plane. • Most also rotate in that direction.

Earth

• An oasis of life (average surface temperature 290K) • Mainly made of rocks and metals • The only surface liquid water in the solar system • One surprisingly large moon • Average distance from the Sun: 1.00 AU

Transit of Venus

• Apparent position of Venus on Sun during transit depends on distances in solar system and your position on Earth.

Why are there two major types of planets? Conservation of Energy

• As gravity causes cloud to contract, it heats up. • Inner parts of disk are hotter than outer parts. • Rock can be solid at much greater temperatures than ice. • Inside the frost line: too hot for hydrogen compounds to form ices • Outside the frost line: cold enough for ices to form

Flattening

• Collisions between particles in the cloud caused it to flatten into a disk. • Collisions between gas particles in cloud gradually reduce random motions. • Collisions between gas particles also reduce up and down motions. • Spinning cloud flattens as it shrinks.

Where did the solar system come from? Galactic Recycling

• Elements that formed planets were made in stars and then recycled through interstellar space. • We can see stars forming in other interstellar gas clouds, lending support to the nebular theory.

Was our solar system destined to be?

• Formation of planets in the solar nebula seems inevitable. • But details of individual planets could have been different.

How did we arrive at a theory of solar system formation?

• Four major features needed to be explained • Several reasonable hypotheses were explored -Close encounter hypothesis -Nebular hypothesis

Saturn

• Giant and gaseous like Jupiter • Spectacular rings • Many moons (at least 62), including cloudy Titan • Average distance from the Sun: 9.54 AU • Radius about 9.4 times larger than the radius of Earth • Cloud-top temperature 95K • Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. • Cassiniprobe arrived July 2004 (launched in 1997).

How did the jovian planets form?

• Ice could also form small particles outside the frost line. • Larger planetesimals and planets were able to form. • Gravity of these larger planets was able to draw in surrounding H and He gases. • Gravity of rock and ice in jovian planets draws in H and He gases. • Moons of jovianplanets form in miniature disks. How did the jovian planets form? • A combination of photons and the solar wind—outflowing matter from the Sun—blew away the leftover gases.

Solar Rotation

• In nebular theory, young Sun rotated much faster than now. • Friction between solar magnetic field and solar nebular probably slowed the rotation over time.

Asteroids and Comets

• Leftovers from the accretion process • Rocky asteroids inside frost line • Icy comets outside frost line

Mars

• Looks almost Earth-like, but don't go without a spacesuit! • Giant volcanoes, a huge canyon, polar caps, more • Water flowed in distant past; could there have been life? • Made of rocks and metals • Average surface temperature 220K • 2 small moons (Phobos and Deimos) • Average distance from the Sun: 1.52 AU) • Curiosity rover landed in August 2012.

Mercury

• Made of metal and rock; large iron core • Desolate, cratered; long, tall, steep cliffs • Very hot, very cold: 425°C (day), -170°C (night) • Average distance from the Sun: 0.39 AU • No moons

Swarms of Smaller Bodies

• Many rocky asteroids and icy comets populate the solar system.

Jupiter

• Much farther from Sun than inner planets - Average distance from the Sun: 5.20 AU (past the asteroid belt) • Mostly H/He; no solid surface • 300 times more massive than Earth • Radius about 11.2 times larger than the radius of Earth • Many moons (at least 67), rings • Cloud-top temperature: 125K - Jupiter's moons can be as interesting as planets themselves, especially Jupiter's four Galilean moons. Jupiter • Io (shown here): active volcanoes all over • Europa: possible subsurface ocean • Ganymede: largest moon in solar system • Callisto: a large, cratered "ice ball"

Dwarf Planets: Pluto, Eris, and more

• Much smaller than major planets • Icy, comet-like composition • Pluto has 5 moons • Pluto's main moon (Charon) is of similar size • Pluto was thought of as "ninth planet" for 75 years • Largest asteroid of the asteroid belt is Ceres

Venus

• Nearly identical in size to Earth; surface hidden by clouds • Made of rocks and metals • Hellish conditions due to an extreme greenhouse effect • Even hotter than Mercury: 470°C (740K), day and night • Average distance from the Sun: 0.72 AU • No moons

Disks around Other Stars

• Observations of disks around other stars support the nebular hypothesis.

Sun

• Over 99.9% of solar system's mass (333,000 times larger than the mass of Earth) • Made mostly of H/He gas (plasma) (98%) • Converts 4 million tons of mass into energy each second • Surface temperature 5800K

Close Encounter Hypothesis

• Proposed that the planets formed from debris torn off the Sun by a close encounter with another star. • That hypothesis could not explain observed motions and types of planets.

How do we know the age of the solar system?

• Radiometric dating tells us that oldest moon rocks are 4.4 billion years old. • Oldest meteorites are 4.55 billion years old. • Planets probably formed 4.5 billion years ago.

What caused the orderly patterns of motion in our solar system? Conservation of Angular Momentum

• Rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted. • Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

Notable Exceptions

• Several exceptions to the normal patterns need to be explained - Uranus: axial tilt and Triton's retrograde orbit - Venus: retrograde (clockwise) rotation on its axis - Earth: relatively large moon

Neptune

• Similar to Uranus (except for axis tilt) • Many moons (at least 13), including Triton • Average distance from the Sun: 30.1 AU • Cloud-top temperature: 60K

How did the terrestrial planets form?

• Small particles of rock and metal were present inside the frost line. • Planetesimals of rock and metal built up as these particles collided. • Gravity eventually assembled these planetesimals into terrestrial planets. • Tiny solid particles stick to form planetesimals. • Gravity draws planetesimals together to form planets. • This process of assembly is called accretion.

Uranus

• Smaller than Jupiter/Saturn; much larger than Earth - Radius about 4 times larger than the radius of Earth • Made of H/He gas and hydrogen compounds(H2O, NH3(ammonia), CH4 (methane)) • Extreme axis tilt (rotating "on its side") • Moons (at least 27) and rings • Cloud-top temperature 60K • Average distance from the Sun: 19.2 AU)

Comparative Planetology

• We can learn more about a world like our Earth by studying it in context with other worlds in the solar system. • Stay focused on processes common to multiple worlds instead of individual facts specific to a particular world. • Comparing the planets reveals patterns among them. • Those patterns provide insights that help us understand our own planet.