Section 4 - The Terrestrial Planets

Question 1

Compare the radii of the terrestrial planets.

Answer 1

• Earth and Venus similar in size

- Mars and Mercury significantly smaller, and Earth’s moon comparable.

Question 2

Compare the density of the terrestrial planets.

Answer 2

• General trend of increasing density with mass
• This might be expected for similar compositions and hydrostatic equilibrium.
• Mercury does not follow this pattern and is significantly over dense.

Question 3

Compare the spin period of the terrestrial planets.

Answer 3

• Planets closest to the Sun have longer spin periods, as does Earth’s moon.
• Suggests tidal interactions are important.

Question 4

Compare the magnetic fields of the terrestrial planets.

Answer 4

• Wide range of magnetic field strengths
• Earth is unique in having a strong field
• Mercury is only other terrestrial planet with significant magnetic field.

Question 5

Describe the spectrum of a planet.

Answer 5

• Separate components of reflected sunlight and thermal emission (absorbed and re-emitted Solar energy)
• Spectrum imprinted with absorption features of atmospheric molecules.

Question 6

Compare Earth’s thermal infrared spectrum to Venus and Mars.

Answer 6

Blackbody temperatures are similar, with Venus slightly cooler than Earth, and Mars slightly cooler again.
All show CO_2 absorption. Earth shows water and ozone as well.

Question 7

Mercury:

• Describe the surface of Mercury.
• Density _____ to Earth, and ______ than Mars or the Moon. Unexpectedly has a ________ _____.
• Low albedo, similar to the Moon.
• ______ rotation due to _____ ____ and lack of _________ results in poor heat distribution and large contrast between ….
• No significant atmosphere due to …

Answer 7

• Bare, rocky, heavily-cratered surface, similar to Moon. Implies geologically inactive since soon after formation, with little resurfacing.
• Similar, greater, magnetic field, slow, Solar tides, atmosphere, day and night temperatures.
• Low surface gravity.

Question 8

Venus

• Why is Venus’ albedo ( amount of light a planet reflects off) very high, and what does this cause?
• How does Venus’ equilibrium temperature compare to its surface temperature, and why?
• Why does Venus have little day/night temperature contrast despite slow rotation?
• What do the cloud structures of Venus reveal?
• What did radar mapping of Venus reveal about it’s surface?
• Describe the composition of the atmosphere of Venus

Answer 8

• Has highly reflective high-altitude clouds of sulphuric acid, this causes a low equilibrium temperature, cooler than Earth.
• Surface temperature very high due to thick CO_2 atmosphere driving a strong greenhouse effect.
• Thermal lag of atmosphere provides efficient heat redistribution.
• Super-rotation of the thick atmosphere driven by Solar heating.
• Thousands of Volcanoes including some large shield volcanoes, some impact craters but resurfaced 1 Gyr ago.
• 96% carbon dioxide , 4% nitrogen.

Question 9

Earth

• How does Earth’s mass, radius, and density, and atmosphere compare to Venus?
• Describe the atmosphere of Earth, water, continents, and clouds.
• Is Earth’s albedo high, intermediate, or low?
• How does Earth’s surface temperature compare to its equilibrium temperature and why?
• Describe the effects of tidal interactions on Earth’s rotation.
• Describe the surface of the Earth and why.

Answer 9

• Mass, radius, density similar to Venus, but much thinner atmosphere.
• Atmosphere dominated by nitrogen and oxygen, liquid water in low-lying basins, rocky continents, reflective clouds of water caused by H20 at sea level.
• Intermediate
• It is higher due to moderate Greenhouse effect.
• Weak Solar tides allows rapid rotation, gradually slowing due to tidal interaction with the Moon.
• Some impact craters, implying ongoing reprocessing of surface caused by geological activity and water erosion, ongoing volcanic activity, implying hot interior. Only planet with plate tectonics.

Question 10

Mars

• Compare Mars’ density with Earth’s, Venus’, Mercury’s.
• Describe the surface
• Describe the atmosphere.
• Mars has a similar albedo to Earth, but heat distribution is ___ despite rapid ______, because…
• Compare Mars’ axial tilt to Earth;s, and what does this cause?
• Liquid water?

Answer 10

• Lower
• Rocky surface covered with iron-oxide rich dust. Impact craters mean surface is old, but they are relatively rare compared to Mercury and the Moon, showing resurfacing continued until around 2 Gyr ago. Large shield volanoes (e.g. Olympus Mons, largest volcano in solar system) and rift valley shows Mars was geologically active in the past.
• Very thin CO_2 atmosphere.
• Poor, rotation, atmosphere is thin and soil has low thermal inertia.
• Similar to Earth (25 deg), leads to distinct seasons.
• Strong evidence for abundant liquid water in the past, and some today.

Question 11

Earth is differentiated into distinct layers of different densities. Describe these layers.

Answer 11

• Inner core - Solid iron and nickel
• Outer core - Liquid iron and nickel
• Mantle - Magnesium- iron silicate. Solid but flows and convents on geological timescales.

Question 12

• What are P-waves and S-waves, and where can they propagate?
• What does the liquid outer core do to these waves?
• What does the solid inner core do these waves?
• How are inner core s-waves detected?

Answer 12

• P-waves are compressional and propagate in solids and liquids.
• S-waves are transverse and propagate only in solids.
• Reflects and refracts p-waves and cannot transmit s-waves.
• Transmits s-waves excited by p-waves.
• Indirectly via perturbations of p-waves.

Question 13

Describe the Earth’s composition.

What does this imply about Earth’s formation?

Answer 13

• Bulk Earth composed primarily of Fe, O, Si, Mg
• Smaller proportions of Ni, Ca, Al, S
• Strongly deficient in volatile elements, with low melting/boiling temperatures e.g. H, He, C, N
• Shows Earth formed in a warm environment where ices were not present.

Question 14

• Describe the cosmic abundance of different elements.

Answer 14

• H and He are by far the most common but vulnerable to evaporation.
• O is 3rd most common - can form water with H which is volatile, but can also form refractory compounds which have high melting/boiling temperatures.
• N and C a lot less common - primarily from ammonia and methane with H which are volatile.

Question 15

• Why is the Earth’s interior still hot?

- Where did the magnetic field of the Earth originate from and what is required for this?

Answer 15

• Liquid core demonstrates interior is very hot, but heat flux at surface is much smaller than the external heating from the Sun. This means that an ongoing heat source is required, tidal heating is not sufficient, now understood to be radioactive decay.
• A magnetic dynamo in the molten iron core. This requires a conducting medium, rotation, and convection.

Question 16

• What drives convection in the Earth’s mantle?
• Describe briefly plate tectonics on Earth.
• What does the new crust formed at mid-ocean ridges tell us about the magnetic polarity of the Earth?

Answer 16

• Temperature gradient
• The crust and upper mantle (the lithosphere) fractures into plates that move on top of convection cells in the deeper mantle (the asthenosphere). New crust forms at mid-ocean ridges as plates move apart. Ocean crust is subducted and continents built where plates collide.
• Polarity reversals on a 0.5 Myr timescale.

Question 17

• What do we expect the interiors of the other terrestrial planets to be like?
• Why are smaller bodies such as the Moon, Mercury and Mars geologically inactive?
• Describe the seismology of the Moon and what it tells us about its interior?

Answer 17

• Differentiated, hot at formation and molten. All heated by radioactive decay and Mercury by tides.
• They cool more effectively due to ratio of surface area to volume t_cool directly proportional to 4piR^2/4/3piR^3 which is directly proportional to 1/R. So they are expected to form a thick lithosphere that does not fracture into plates and is geologically inactive.
• Has Moonquakes which are both shallow and deep. Shallow due to tidal strain and meteorite impacts. Deep at boundary between thick lithosphere and plastic asthenosphere. S-wave shadow shows small liquid core, but no magnetic field so no dynamo.

Question 18

• What do Marsquakes tell us about Mars’ interior?

- Describe the geological activity and magnetic field of Mercury. Why is this the case?

Answer 18

• Marsquakes orignate from thermal contraction. Reflected S-waves show core is liquid and has larger radius than expected, suggesting low density. S-wave speeds suggest thick lithosphere. Lack of global magnetic field shows no dynamo now.
• Mercury smaller than Mars and not geologically active, so expected to have thick lithosphere, but does have a magnetic field, suggesting a magnetic dynamo might still operate. Has anomalously high density, implying over-sized iron core. This combines with heating from Solar tides may allow the dynamo to continue for longer than expected

Question 19

• Describe the geological activity of Venus.

- How is the Moon believed to have been formed?

Answer 19

• Venus should have a very similar interior to Earth. Continuing geological activity shows hot interior. No sign of plate tectonics, possibly due to lack of liquid water, and no global magnetic field. Maybe no magnetic dynamo due to slow rotation.
• From Earth’s mantle following a collision with a Mars-sized proto-planet. Explains Moon’s similarities to Earth’s mantle including precise isotope ratios.

Question 20

Describe the surfaces of the near-side and far-sides of the moon.
What effect have tidal forces had on the moon?
What is the lunar surface covered in?

Answer 20

Nearside- Surface heavily cratered with impact craters.
Far-side - The crust is thicker and less prone to penetration by impacts.
Tidal forces have synchronised the spin of the Moon with the denser side facing the Earth.
Lunar regolith (soil)

Question 21

Describe the Greenhouse effect on Earth.

Answer 21

• Heating by Solar spectrum, dominated by optical light, where atmosphere relatively transparent.
• Cooling by thermal emission in the infrared where atmosphere relatively opaque due to absorption.
• Heat trapped in lower atmosphere results in higher surface temperatures.
• Carbon dioxide controls greenhouse effect on Earth.

Question 22

Why does Venus have such a high surface temperature compared to its equilibrium temperature, and what has this caused?

Answer 22

• Venus has an extremely high atmospheric opacity and is so optically thick that the surface temperature rises until the blackbody spectrum of the planet can access bluer wavelengths that are relatively transparent.

Question 23

Compare and contrast the atmospheric composition of Venus and the Earth, and
explain how these differences are thought to have arisen.

Answer 23

• The atmosphere of Venus is dominated by carbon dioxide, Earth’s is mostly nitrogen. [1]
  -The pressure of the atmosphere of Venus is much higher than for Earth. [1]
• The Earth’s atmosphere also includes significant water vapour, whereas
  Venus has very little water (only in high altitude clouds of sulphuric acid). [1]
  -These differences are thought to have arisen due to Venus suffering a runaway
  greenhouse effect [1]
• A feedback loop in which evaporating water increases the greenhouse effect which raises temperatures and drives more water evaporation until all water is in the gas phase. [1]
• Liquid water is needed for the carbonate-silicate cycle, which removes carbon-dioxide from the Earth’s atmosphere and deposits as ocean sediments. [1]
  -Without this, carbon dioxide levels on Venus rose to the present high levels due to volcanism [1].
  -In its vapour form water is vulnerable to photolysis by ultraviolet light and the hydrogen escaped from Venus due to X-ray heating and the solar wind. [1]

Question 24

What conditions are required for the runaway greenhouse effect?
Where has the water gone on Venus?
Why has this not happened on Earth?

Answer 24

• Requires water and strong solar heating.
• Runaway greenhouse, water vapour extends to high altitudes where it is vulnerable to photolysis by ultraviolet photons. The hydrogen is then vulnerable to atmospheric escape due to heating by Solar X-rays and the solar wind.
• Water is protected by the cold trap at the tropopause, where it condenses into clouds, and by ozone in the stratosphere that absorbs the ultraviolet.

Question 25

What are the two regimes of atmospheric escape? Describe each one.

Answer 25

• Jeans escape - individual atoms in the tail of the Maxwell- Boltzmann velocity distribution have thermal velocities above the escape velocity.
• Hydrodynamic escape - mass loss rate is high enough for planetary wind to act as a fluid, which can entrain heavier elements that would not have high enough thermal velocities to escape individually.
• Mass loss rate limited by Solar X-rays and/or the solar wind.

Question 26

Describe the differences in deuterium to hydrogen ratios in the terrestrial planets.
What could this be due to?

Answer 26

• Venus has extremely high D/H ratio compared to Earth, indicating catastrophic water loss from Venus.
• Mars also has anomalously high D/H ratio compared with Earth.
• Earth is shielded by its magnetic field from atmospheric escape, whereas Venus and Mars are not.

Question 27

Describe the orbits of the terrestrial planets.

Answer 27

• Apart from Mercury, the terrestrial planets have orbits that are approximately circular and co-planar. Mercury has a more perturbed orbit as it is less massive.
• At any instant, orbits of the planets are very close to the Keplerian one-body system, but over time mutual perturbations in the N-body system lead to variations in orbital parameters.

Question 28

What are secular resonances?
Is the Solar system stable?
What other than eccentricity varies due to N-body interactions?

Answer 28

Long-term variations of eccentricity and inclination of orbits due to relatively weak mutual perturbations.
Yes, apart from Mercury, where eccentricity could grow large enough for a close encounter with Venus.
Orientation of the orbit. For Mercury the perihelion position advances at a rate of 574 arcsec per century.

Question 29

Describe the rotation of the terrestrial planets.

Answer 29

• Earth and Mars rotate rapidly.
• Moon rotates slowly due to tidal interactions with the Earth.
• Venus and Mercury rotate slowly due to tidal interaction with the Sun.
• Venus has retrograde motion.

Question 30

• Why is the rotation of the Earth slowing?
• For Earth and Mars compare their Solar day with their sidereal day (true rotation period with respect to stars). Why does this occur?
• What is the equation for the Solar day?

Answer 30

• Due to tidal interactions with the Moon.
• Solar day slightly longer than the sidereal day because after one rotation they need to rotate slightly more to bring Sun to same altitude.
• 1/P(solar) = 1/P(spin) - 1/P(orb)

Question 31

• Venus’ rotation is very _____ and _______.
• Sidereal day is ______ than the year.
• Solar day lasts _____ a year.
• Solar heating leads to ________ morning and evening temperatures.
• Pressure difference leads to _________ in mass _______ of the thick atmosphere and atmospheric ______.
• Motion probably causes by equilibrium between __________ and _________ tides.

Answer 31

• Slow and retrograde
• Longer
• Half
• Asymmetric
• Asymmetry, distribution, tides
• Gravitational and atmospheric.

Question 32

• Rotation of Mercury not _______ with its orbital period.
• Locked in a ___:__ resonance with its orbit
• Leads to solar day that is _____ the year.
• Orbit is _____ so tidal interaction dominated by ______.
• Mercury also has small permanent _________, and the current resonance maintains the alignment of the _______ axis at every perihelion passage.

Answer 32

• Synchronised
• 3:2
• Twice
• Eccentric, perihelion
• Deformation, enlongated

Question 33

• What is obliquity?

- Compare the spin obliquities of the terrestrial planets.

Answer 33

• The angle between the spin axis and orbital axis.
• Spin obliquity of Mercury and Venus is very small as rotation is dominated by tides. In contrast, Earth and Mars have spin obliquities of 23-35 degrees.

Question 34

• Why is the Earth oblate, what does this mean, and what drives precession?
• Contrast the variations of obliquity of Mars and Earth, and why?

Answer 34

• Because the Earth is rapidly rotating, means Earth has a bulge around the equator, and the gravity of the Sun applies a slight torque, driving precession.
• The obliquity of Mars varies strongly on timescales of Myr, but Earth has a much more stable obliquity, varying by only 1 or 2 degrees. Obliquity of Earth is stabilised by gravitational interactions with the Moon.

Question 35

Moons:

• How many Moons do the terrestrial planets have?
• Describe the moons of Mars.
• How do tidal interactions affect the Moon and the Earth?
• Why do Mercury and Venus have no Moons?

Answer 35

• Earth is only one with a massive Moon, Mercury and Venus have no moons, and Mars has two very small moons.
• Phobos is larger than Deimos, both thought to be captured asteroids. Phobos orbits more rapidly than the rotation of Mars and so tidal synchronisation is transferring angular momentum from its orbit to the spin of the planet. This is leading to the in-spiral of Phobos. Deimos orbits more slowly than Mars’ spin and so is migrating to wider orbital separations.
• Earth rotates faster than the orbit of the Moon, so tidal bulge leads the Moon which accelerates the Moon, and slows the spin of the Earth.
• The Hill spheres of the planets are very small, and the slow rotation drives inward migration of any moons to the Roche limit, so leaves little or no space for stable Moons.