Nov. 6th - Terrestrial & Jovian Interiors

Question 1

What processes can reshape Earth’s surface?

Answer 1

• Volcanoes
• Earthquakes
• Erosion
• Water flow
• Wind

Question 2

First step - planetary interiors

We cannot see inside Earth or any other terrestrial world, but a variety of clues tell us about their internal structures.

For Earth, the most direct data come from…

Answer 2

seismic waves: vibrations that travel both through the interior and along the surface after an earthquake

In much the same way that shaking a gift box offers clues about what’s inside, seismic vibrations offer clues about what’s inside Earth. We also have seismic data for the Moon

Question 3

Our studies have shown that all the terrestrial worlds have layered interiors.

We divide these layers by density into three major categories:

Answer 3

1. Core
2. Mantle
3. Crust

Question 4

Layering by density:

Core

Answer 4

The highest-density material, consisting primarily of metals such as nickel and iron, resides in a central core.

Although not shown in the figure, Earth’s metallic core actually consists of two distinct regions:
1. A solid inner core
2. A molten (liquid) outer core.

Question 5

Layering by density:

Mantle

Answer 5

Rocky material of moderate density—mostly minerals that contain silicon, oxygen, and other elements—forms a thick mantle that surrounds the core.

Question 6

Layering by density:

Crust

Answer 6

The lowest-density rock, which includes the familiar rocks of Earth’s surface, forms a thin crust, essentially representing the world’s outer skin.

Question 7

We can understand why the interiors are layered by thinking about what happens in a mixture of oil and water:

Answer 7

Gravity pulls the denser water to the bottom, driving the less dense oil to the top. This process is called differentiation, because it results in layers made of different materials.
* For differentiation to occur, the planet must have a molten interior

Question 8

Comparing the interiors of the terrestrial worlds provides important clues about their early histories.

Answer 8

Models indicate that the relative proportions of metal and rock should have been similar throughout the inner solar system at the time the terrestrial planets formed, which means we should expect smaller worlds to have correspondingly smaller metal cores

Question 9

Layering by Rock Strength

How can rock vary in strength?

Answer 9

• The idea that rock can vary in strength may seem surprising, since we often think of rock as the very definition of strength. However, like all matter built of atoms, rock is mostly empty space; its apparent solidity arises from electrical bonds between its atoms and molecules
• Although these bonds are strong, they can still break and re-form when subjected to heat or sustained stress, which means that even solid rock can slowly deform and flow over millions and billions of years.

Question 10

Planet’s outer layer

Answer 10

• In terms of rock strength, a planet’s outer layer consists of relatively cool and rigid rock, called the lithosphere (lithos is Greek for “stone”), that essentially “floats” on warmer, softer rock beneath
• Notice that lithospheric thickness is closely related to a world’s size: Smaller worlds tend to have thicker lithospheres

Question 11

Eruptions

+ their dependence on the lithosphere

Answer 11

• The thickness of the lithosphere is very important to geology. A thin lithosphere is brittle and can crack easily. A thick lithosphere is much stronger and inhibits the passage of molten rock from below, making volcanic eruptions and the formation of mountain ranges less likely.

Question 12

Why are big worlds round?

Answer 12

• The fact that rock can deform and flow also explains why large worlds are spherical while small moons and asteroids are “potato-shaped.”
• The weak gravity of a small object is unable to overcome the rigidity of its rocky material, so the object retains the shape it had when it was born. For a larger world, gravity can overcome the strength of solid rock, slowly deforming and molding it into a spherical shape

Question 13

What causes geological activity?

Answer 13

internal heat

Question 14

What causes geological activity?

Most geological activity is driven by internal heat: But what makes some planetary interiors hotter than others?

First step in thinking

Answer 14

• Must first understand what makes interiors hot in the first place

Question 15

How is internal heat produced?

Answer 15

Internal heat is a product of the planets themselves, not of the Sun. Three sources of energy explain nearly all the interior heat of the terrestrial worlds:
* Heat of accretion
* Heat from differentiation
* Heat from radioactive decay

Question 16

3 sources of internal heat

Heat of accretion

Answer 16

• Accretion deposits energy brought in from afar by colliding planetesimals.
• As a planetesimal approaches a forming planet, its gravitational potential energy is converted to kinetic energy, causing it to accelerate.
• Upon impact, much of the kinetic energy is converted to heat, adding to the thermal energy of the planet.

Question 17

3 sources of internal heat

Heat from differentation

Answer 17

• When a world undergoes differentiation, the sinking of dense material and rising of less-dense material mean that mass moves inward, losing gravitational potential energy.
• This energy is converted to thermal energy by the friction generated as materials separate by density. The same thing happens when you drop a brick into a pool: As the brick sinks to the bottom, friction with the surrounding water heats the pool—though the amount of heat from a single brick is too small to be noticed.

Question 18

3 sources of internal heat

Heat from Radioactive Decay

Answer 18

• The rock and metal that built the terrestrial worlds contained radioactive isotopes of elements such as uranium, potassium, and thorium.
• When radioactive nuclei decay, subatomic particles fly off at high speeds, colliding with neighboring atoms and heating them. In essence, this converts some of the mass-energy (E=mc2) of the radioactive nuclei to the thermal energy of the planetary interior.

Question 19

3 sources of internal heat

How long do each of the 3 sources heat up a planetary interior?

Answer 19

• Note that accretion and differentiation deposited heat into planetary interiors only when the planets were very young. In contrast, radioactive decay provides an ongoing source of heat.
• Over billions of years, the total amount of heat deposited by radioactive decay has been comparable to or greater than the amount that was deposited initially by accretion and differentiation.
• The combination of the three heat sources explains how the terrestrial interiors ended up with their core-mantle-crust structures.

Question 20

How do interiors cool off? 3 WAYS

Answer 20

1. Convection
2. Conduction
3. Radiation

Question 21

How do interiors cool off? 3 WAYS

Conduction

Answer 21

Conduction is the transfer of heat from hot material to cooler material through contact; it is operating when you touch a hot object.

Conduction occurs through the microscopic collisions of individual atoms or molecules when two objects are in close contact, because the faster-moving molecules in the hot material tend to transfer some of their energy to the slower-moving molecules of the cooler material.

Question 22

How do interiors cool off? 3 WAYS

Radiation

Answer 22

Recall that objects emit thermal radiation characteristic of their temperatures; this radiation (light) carries energy away and therefore cools an object.

Planets lose heat to space through radiation; because of their relatively low temperatures, planets radiate primarily in the infrared.

Question 23

How do interiors cool off? 3 WAYS

Convection

Answer 23

Convection is the process by which hot material expands and rises while cooler material contracts and falls, thereby transporting heat upward; it can occur whenever there is strong heating from below.

You can see convection in a pot of soup on a hot burner, and you may be familiar with it in weather: Warm air near the ground tends to rise while cool air above tends to fall.

Question 24

Creation of Convection

Answer 24

• Hot rock from deep in the mantle gradually rises, slowly cooling as it makes its way upward. By the time it reaches the top of the mantle, the rock has transferred its excess heat to its surroundings, making it cool enough that it begins to fall. This ongoing process creates individual convection cells within the mantle, shown as small circles
• Mantle convection stops at the base of the lithosphere, because the rigid rock of the lithosphere does not flow as readily as the rock deeper down. Within the lithosphere, heat continues upward primarily through conduction. (Volcanic eruptions also carry heat through the lithosphere, since hot rock from the interior erupts onto the surface.) The heat then escapes to space through radiation.

Question 25

Planetary Size Controls Geological Activity

Answer 25

• Just as a hot potato remains hot inside much longer than a hot pea, a large planet can stay hot inside much longer than a small one. You can see why size is the critical factor by picturing a large planet as a smaller planet wrapped in extra layers of rock. The extra rock acts as insulation, so it takes much longer for interior heat to reach the surface.
• Size is therefore the primary factor in determining geological activity. The relatively small sizes of the Moon and Mercury allowed their interiors to cool significantly within a billion years or so after they formed. This cooling caused their lithospheres to thicken and confined mantle convection to deeper and deeper layers until it stopped altogether. As a result, the Moon and Mercury are now essentially “dead” geologically, meaning they have little if any heat-driven geological activity.

Question 26

MATH INSIGHT 9.1

Question 27

The great differences between the terrestrial and the jovian planets are a relatively recent discovery in human history

Answer 27

• Astronomers first recognized the immense sizes of Jupiter and Saturn less than 300 years ago. Recall that we need to know both angular size and distance to calculate an object’s true size
• Although Copernicus had figured out the relative distances to the known planets in astronomical units (AU), scientists did not establish the absolute scale of the solar system until the 1760s, when they were able to measure the true length of an astronomical unit with data from a transit of Venus
• Only then could scientists calculate the true sizes of Jupiter and Saturn from their distances and angular sizes (measured through telescopes). Knowing the scale of the solar system also told scientists the distances of orbiting moons, and because the orbital periods of the moons were easy to measure, the orbital distances allowed scientists to use Newton’s version of Kepler’s third law to calculate the masses of the jovian planets
• Together, the measurements of size and mass revealed the low densities of the jovian planets

Question 28

What initiated the “real revoluation” in understanding Jovian planet systems?

Answer 28

The real revolution in understanding jovian planet systems has come from spacecraft visits to them, which began with the Pioneer 10 and Pioneer 11 spacecraft that flew past Jupiter and Saturn in the early 1970s, followed by the Voyager 1 and Voyager 2 missions that launched in 1977.

More recently, we’ve sent orbiters to both Jupiter and Saturn. For Jupiter, the orbiters to date have been the Galileospacecraft, which orbited from 1995 to 2003, and the Juno mission, which has been in Jupiter orbit since 2016. For Saturn, Cassini was in orbit from 2004 until 2017, when the spacecraft was sent plunging into Saturn’s atmosphere.

Question 29

Are all jovian planets all alike?

Jovian Sizes

Answer 29

all four of the jovian planets are immense in size compared to Earth, the largest of the terrestrial planets.

Question 30

Are all jovian planets all alike?

Jovian Densities

Answer 30

All four also have much lower average densities than any of the terrestrial planets, and they all share the fact that their compositions are dominated by hydrogen, helium, and hydrogen compounds, all of which are relatively rare on the terrestrial planets.

Question 31

Are all jovian planets all alike?

Jovian Rotation Rates

Answer 31

• The jovian planets also all share rotation rates that are rapid compared to those of the terrestrial worlds, though their lack of solid surfaces can make these rotation rates difficult to measure precisely.
• For example, we can try to measure their rotation rates by observing their clouds, but this requires care because clouds can appear to move because of winds as well as rotation.

Question 32

Are all jovian planets all alike?

How are Jovian Rotation Rates measured, past their clouds?

Answer 32

• We usually measure jovian planet rotation rates by tracking emissions from charged particles trapped in their magnetic fields. Because these fields are generated deep in the planets’ interiors, these measurements tell us how fast their interiors are rotating
• The results show that jovian “days” range from about 10 hours on Jupiter and Saturn to 16–17 hours on Uranus and Neptune.
• Moreover, the large radii of these planets mean that their surface rotation speeds are much greater compared to Earth’s than the periods alone would suggest. Interestingly, cloud observations indicate that jovian planet rotation rates vary with latitude: Equatorial regions complete each rotation in less time than polar regions.
• Gravity alone would make these planets into perfect spheres, but rotation makes material bulge outward in the same way you feel yourself flung outward when you ride on a merry-go-round
• The size of the equatorial bulge depends on the balance between the inward pull of gravity and the outward push of rotation.

Question 33

Are all jovian planets all alike?

Jovian DIFFERENCES

Answer 33

Despite their basic similarities, there are also important differences between the jovian planets, particularly when we contrast Jupiter and Saturn with Uranus and Neptune.

The most obvious of these differences is that Jupiter and Saturn are significantly larger and more massive than Uranus and Neptune, but other differences are equally important.

Question 34

Jovian Composition Differences

Jupiter & Saturn Compositions

Answer 34

Jupiter and Saturn are made mostly of hydrogen and helium, giving them compositions much more similar to that of the Sun than to that of the terrestrial planets.

• Some people even call Jupiter a “failed star,” because it has a starlike composition but lacks the nuclear fusion needed to make it shine.
• This is a consequence of its size: Although Jupiter is large for a planet, it is about 1/80 as massive as the lowest-mass stars.
• As a result, its gravity is too weak to compress its interior to the extreme temperatures and densities needed for nuclear fusion.

Question 35

Jovian Composition Differences

Uranus & Neptune

Answer 35

Uranus and Neptune are much smaller than Jupiter and Saturn, though still much larger than Earth.

They also contain significant amounts of hydrogen and helium, but the overall proportions of these materials are much lower than for Jupiter and Saturn.

Instead, they are made primarily of hydrogen compounds such as water (H2O), methane (CH4), and ammonia (NH3), along with smaller amounts of metal and rock.

Question 36

Density Differences:

Saturn

Answer 36

• Saturn is considerably less dense than Uranus or Neptune.
• This should make sense, because the hydrogen compounds, rock, and metal that make up Uranus and Neptune are normally much more dense than hydrogen or helium gas.

Question 37

Density Differences:

Jupiter

Answer 37

• By the same logic, we’d expect Jupiter to be even less dense than Saturn—but it’s not.
• We can understand Jupiter’s surprisingly high density by thinking about how massive planets are affected by their own gravity.
• Building a planet of hydrogen and helium is a bit like making one out of fluffy pillows.
• Imagine assembling a planet pillow by pillow. As each new pillow is added, those on the bottom are compressed more by those above.
• As the lower layers are forced closer together, their mutual gravitational attraction increases, compressing them even further.
  At first the stack grows substantially with each pillow, but eventually the growth slows until adding pillows barely increases the height of the stack

Question 38

Why did the Jovian planets end up with different compositions?

Jupiter & Saturn

Answer 38

• Jupiter and Saturn captured so much hydrogen and helium gas that these gases now make up the vast majority of their masses.
• The ice-rich planetesimals from which they grew now represent only about 3% of Jupiter’s mass and about 10% of Saturn’s mass.

Question 39

Why did the Jovian planets end up with different compositions?

Uranus & Neptune

Answer 39

• consist mostly of material from their original ice-rich planetesimals: hydrogen compounds mixed with smaller amounts of rock and metal

Question 40

Why did the Jovian planets end up with different compositions?

Why did the different planets capture different amounts of gas?

Answer 40

• The answer probably lies in their distances from the Sun as they formed. The solid particles that condensed farther from the Sun should have been more widely spread out than those that condensed nearer to the Sun, which means it would have taken longer for them to accrete into large, icy planetesimals.
• Because all the planets stopped accreting gas at the same time—when the solar wind blew all the remaining gas into interstellar space—the more distant planets had less time to capture gas and ended up smaller in size

Question 41

What are jovian planets like on the inside?

The jovian planets are often called “gas giants,” making it sound as if they are entirely gaseous like air on Earth - why is this misleading?

Answer 41

While it is true that they lack solid surfaces and are made largely from gas they collected from the solar nebula, their large masses create such strong gravity that most of their internal “gas” is compressed into other phases unlike anything we’re familiar with in everyday life.

Question 42

Where was a majority of Jupiter’s data collected from/during?

Answer 42

The best data for Jupiter has come from the Juno mission, which has made very precise measurements of how Jupiter’s gravity affects its orbit. This has allowed scientists to refine their models of Jupiter’s interior layering

Question 43

Jupiter’s Interior

Outer layer

Answer 43

• Gaseous hydrogen, mixed with helium

Question 44

Jupiter’s Interior

Second layer

Answer 44

• Liquid hydrogen

Question 45

Jupiter’s Interior

Inner Layer

Answer 45

• The pressure at 2 million times that on Earth’s surface. This extreme pressure forces hydrogen into a compact, metallic form.
• Just as is the case with everyday metals, electrons are free to move around in this metallic hydrogen, so it conducts electricity quite well.
• This layer extends through most of the rest of Jupiter’s interior and where Jupiter’s magnetic field is generated.

Question 46

Jupiter’s Interior

Juno’s found mixing of the densest materials through much of Jupiter’s interior (surprise because we might expect all of those materials to have sunk to the central core)

What 2 hypotheses could explain?

Answer 46

• The first hypothesis suggests that the formation of Jupiter’s core from icy planetesimals proceeded more slowly than scientists had predicted, so that Jupiter began to accrete hydrogen and helium gas from the solar nebula *while the core was still accreting. *
• The second hypothesis suggests that Jupiter’s core formed fairly quickly as expected, but a truly giant impact later disrupted the core, dispersing its dense materials throughout the inner half of the planet.

Question 47

Comparing the Jovian Interiors

Saturn

Low mass, weak gravity

Answer 47

• Saturn has the same basic layering as Jupiter, just as we should expect given its similar size and composition, but its lower mass and weaker gravity make the weight of the overlying layers less than on Jupiter.
• As a result, we must look deeper into Saturn to find each level where pressure changes hydrogen from one phase to another. That is, Saturn has thicker layers of gaseous and liquid hydrogen and a thinner and more deeply buried layer of metallic hydrogen above a roughly Earth-size core. The temperatures of these interior layers are also somewhat lower on Saturn than on Jupiter

Question 48

Comparing the Jovian Interiors

Uranus & Neptune

Answer 48

• Uranus and Neptune have somewhat different layering because their internal temperatures and pressures never become high enough to form liquid or metallic hydrogen, so they have only a thick layer of gaseous hydrogen surrounding their cores of hydrogen compounds, rock, and metal.
• This core material is quite hot—probably above about 5000 K, which is nearly as hot as the surface of the Sun—but the high pressure may mean it is in liquid phase, making for very odd “oceans” buried deep inside Uranus and Neptune.
• The cores of Uranus and Neptune are larger in radius than the cores of Jupiter and Saturn, even though they have about the same mass, because they are less compressed by their lighter-weight overlying layers.
• The less extreme interior conditions also allowed Uranus’s and Neptune’s cores to differentiate, so hydrogen compounds reside in a layer around a center of rock and metal.

Question 49

SPECIAL TOPIC: How were Uranus, Neptune, and Pluto Discovered?

Why was Neptune’s discovery important for Newton’s universal law of gravitation?

(COVERED IN TUTORIAL 3)

Answer 49

Careful observations of Uranus had shown its orbit to be slightly inconsistent with that predicted by Newton’s law—at least if it were being influenced only by the Sun and the other known planets

John Adams - grad student who proposed that the inconsistency could be explained if there were an unseen “eighth planet” orbiting the Sun beyond Uranus.

Question 50

How was the “ninth planet” discovered?

Answer 50

• Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, and its discovery story at first seemed similar to Neptune’s.
• After accounting for the gravitational tug of Neptune, astronomers found small, lingering discrepancies between observations and predictions for the orbit of Uranus.
• This seemed to suggest the existence of an undiscovered “ninth planet,” and Tombaugh found Pluto just 6° from this planet’s predicted position in the sky.