AS101 Chapter 13 - Comparative Planetology of the Terrestrial Planets

Crust of a Terrestrial planet

• Topmost low-density rocky layer

  • Where living on this right now

Mantle of a Terrestrial planet

Between the crust and core, composed of dense rock

Core of a Terrestrial planet

High-density, metallic composition

Why do magnetic fields exists?

Because of the High-density, metallic composition of the core

Describe the atmosphere of Venus

Very thick atmosphere

Describe the atmosphere of Mars

Thin atmosphere

Describe the atmosphere of Moon and Mercury

Have no atmosphere

What do atmospheres consist of?

Consists of only gas; oxygen, CO2, nitrogen gas

____ surfaces are much more heavily cratered than ___ surfaces

Old; younger

Heat

– Remnant of planet formation or generated by radioactive decay

– Flows outward and causes a number of processes

____ terrestrial worlds cool off quicker and become geologically dead sooner than ___ planets

Small; larger

 youngest to oldest average surface age of terrestrial worlds

Earth, Mars, Venus and Moon

What does the average density of Earth indicate?

about 5.5 g/cm3

indicates that the interior is made up of material denser than rock

Earth’s interior consists of a

metallic core - inner solid and outer liquid core

a dense rocky mantle, and a thin, low-density crust

How does the liquid outer core of the Earth generates its magnetic field?

Through the dynamo effect; A mechanism by which a celestial body such as earth or a star generates a magnetic field

The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales

Define the Dynamo Effect

a geophysical theory that explains the origin of the Earth’s main magnetic field in terms of a self-exciting (or self-sustaining) dynamo

Seismic Waves - How do we know what’s inside a planet?

Vibrations that travel through Earth’s interior tell us what Earth is like on the inside

What are the three important points about Earth’s crust?

1. The motion of crustal plates produces much of the geological activity on Earth. The molten rock that emerges from volcanoes come from the upper mantle

2. The continents on Earth’s surface have moved and changed over periods of hundreds of millions of years. The youngest crust is near mid ocean ridges and spreading zones

3. Most of the geological features on Earth are recent products of Earth’s active surface

What did the first primary atmosphere of Earth consist of?

was rich in carbon dioxide, nitrogen, water vapour, hydrogen and methane

What happened as earth cooled?

 carbon dioxide was scrubbed out by the oceans, leaving the atmosphere rich in other gases, such as nitrogen and oxygen (due to photosynthesis)

What is the secondary atmosphere?

The atmosphere we breathe today

An atmosphere rich in carbon dioxide (CO2) can trap heat by a process called

the greenhouse effect

How did oxygen come to be in Earth’s atmosphere

• When Earth was young, its atmosphere had no free oxygen

• About 3–2.5 billion years ago, plants began to rapidly produce oxygen via photosynthesis

• Carbon dioxide was scrubbed out by the oceans, leaving the atmosphere rich in other gases, such as nitrogen and oxygen (due to photosynthesis)

Geological History of Earth 

1. Differentiation – The separation of each planet’s material into layers according to density
   

2. Cratering and giant basin formation – Could not begin until a solid surface formed

3. Slow surface evolution – Has continued for at least the past 3.5 billion years.

What is a common misconception regarding Earth?

There is life on Earth because of oxygen

What are the truths of the common misconception regarding Earth?

Oxygen was not part of Earth’s primeval atmosphere 

There is oxygen in Earth’s atmosphere because of life

Photosynthesis by plants is what keeps steady supply of oxygen in Earth’s atmosphere

Why is the ozone layer essential?

protects Earth’s surface from UV radiation

What is tidal coupling

Earth’s gravitation has produced tidal bulges on the Moon.

Tidal forces have slowed the rotation down to the same period as the orbital 

What are the three failed hypotheses of the origin of the Earth’s Moon

The fission hypothesis, The condensation hypothesis, The capture hypothesis

All these hypothesis failed to go with evidence.

The fission hypothesis

The Moon broke off from a rapidly spinning proto-Earth

The condensation hypothesis

Earth and its Moon condensed from the same cloud of matter in the solar nebula

The capture hypothesis

The Moon formed elsewhere in the solar nebula and was later captured by Earth

The large-impact hypothesis

• suggests that the Moon formed when a planetesimal(small celestial bodies), estimated to have been at least as large as Mars, grazed the proto-Earth

  • The disk of ejected material coalesced to form the Moon.

  • This hypothesis concerning the formation of the Moon makes predictions that best fit the observed lunar data, therefore it is likely the correct one

Magma ocean

The Moon formed in a mostly molten state

Cratering and basin formation

Began as soon as the crust solidified. Multiringed basins also formed

The increased cratering rate of the moon occurred during the ______

late heavy bombardment

Why doesn’t the moon have an atmosphere?

Due to its small size and cooling rapidly

Maria of the moon

– Lunar lowlands filled by successive flows of dark lava.

low albedo.(It reflects only 6% of the light)

Albedo

The ratio of the amount of light reflected from an object to the amount of light received by the object

Highlands of the moon

Heavily cratered, lightercoloured regions, composed of low-density rock (e.g., anorthosite), older than maria.

Many rocks found on the Moon are ____

breccias

Why has the Moon become geologically dead?

because it has lost its internal heat

How is mercury similar to Earth’s moon?

• Small; no atmosphere 

• lowlands flooded by ancient lava flows 

• heavily cratered surfaces 

Rotation and Revolution of Mercury

Like Earth’s moon (tidally locked to revolution around Earth), Mercury’ s rotation has been altered by the sun’s tidal forces, but not completely tidally locked

• Revolution period = 3/2 times rotation period 

• Revolution: ≈ 88 days 

• Rotation: ≈ 59 days

Why cannot we live on Mercury

its too hot to survive there, and won’t be enough light (absence of light for 30 days, way too cold at times)

The surface of mercury

Very similar to Earth’ s moon: Heavily battered with craters, including some large basins

The interior of mercury

Large, metallic core and over 60% denser than Earth’ s moon 

Magnetic field only ~ 0.5 % of Earth’ s magnetic field 

Difficult to explain at present: Liquid metallic core should produce larger magnetic field

History of Mercury

• Innermost planet in the Solar system → Only heavy elements could condense out 

• Later bombardment removed more of the lighter rocks from Mercury’s surface 

• No atmosphere → Heavy bombardment left many impact craters 

• Stronger gravity than the moon → Secondary impacts less spread out over the surface 

• Cooling interior contracted → crust broke to form lobate scarps

Venus’s diameter is ___ that of Earth’s and it is ___ closer to the Sun?

95%, 30%

Why is the surface of venus hidden below thick clouds

Due to a runaway greenhouse effect (when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving) on Venus

What metal is Venus’s hot surface enough to melt?

Lead

A solar day of Venus is

116.75 Earth days long

The atmosphere of Venus composes of

96% CO2 

The rest is mostly nitrogen, with some argon, sulphur dioxide, and small amounts of sulphuric acid, hydrochloric acid, and hydrofluoric acid

The history of Venus

• Venus may have had oceans when it was young but it was warmer, and CO2 in the atmosphere created a runaway greenhouse effect that made the planet even warmer

  • Volcanism appears to dominate the geology of Venus

• There is no plate tectonics, but there is evidence that convection currents below the crust are deforming the crust to make coronae, push up mountains, and create some folded mountains.

The surface of venus - what are used to map the surface?

Since the atmosphere is opaque to visible and infrared, radio waves are used to map the surface

Radar maps reveal many smaller volcanoes, faults, and sunken regions produced when magma below the surface drained away. 

Coronae

The large, round geological faults in the crust caused by the intrusion of magma (hot fluid ) below the crust.

The average age of the surface of Venus is estimated to be

roughly half a billion years

The atmosphere of Mars

• 95% CO2 , 3% nitrogen, and 2% argon. There is no ozone layer.

• Only about 1% as dense (very thin) as Earth’s atmosphere. 

  • Reason being, it has smaller escape velocity which let most of its atmosphere to escape

  • Whatever water is present on Mars is frozen either within the polar caps or as permafrost in the soil

The surface of mars

shows dichotomy – heavily cratered southern hemisphere and younger lowland northern hemisphere. 

Evidence of volcanism (Tharsis region).

All volcanoes are shield volcanoes (Olympus Mons is the largest)

Rift valleys (Valles Marineris (great canyon)is the largest).

Water on mars?

• once existed in large quantities on Mars

•  Rovers have found clear evidence that liquid water flowed over the surface in some place

• Outflow channels and valley networks show evidence of water flow in the past. 

• A radar study has found frozen water extending at least a kilometre beneath both polar caps. 

Why does mars no longer have a magnetic field?

Mars once had a global magnetic field, like Earth’s, but the iron core dynamo that generated it shut down billions of years ago

leaving behind only patches of magnetism due to magnetized minerals in the Martian crust

History of Mars

Differentiation into crust, mantle, and core, the core cooled quickly and shut off the dynamo

Cratering may have broken or at least weakened the crust, triggering lava flows that flooded some basins

Volcanoes may still occasionally erupt

The age of liquid water must have ended more than 3 billion years ago.

What are the moons of Mars

Phobos and Deimos - These two irregularly shaped moons are likely captured asteroids.

Is Mars the only planet in our Solar System we could colonize?

Yes. If you eliminate the gas giants, Earth and Mars - you’re left with:

• Pluto - too far, too cold, not enough sunlight. The Moon is MUCH more friendly

• Venus - insanely high atmospheric pressure and temperatures - and a “day” and “night” duration of 100 earth-days each…making solar power tough. It rains concentrated acid. 

• Mercury - too hot and again, slow rotation period resulting in alternately boiling heat and profound cold