Astronomy 161 An Introduction to Solar System Astronomy Prof. Scott Gaudi

Lecture 36:
The Terrestrial Planets in Comparison:

Key Ideas:

Surfaces & Interiors depend on size:

• Small bodies (Mercury, Mars, & Moon) have older surfaces & colder interiors
• Large bodies (Venus & Earth) have younger surfaces & hotter interiors

• All start with substantial atmospheres
• Evolution driven by the Greenhouse Effect

The Terrestrial Planets

• Earth
• Venus (0.95 R_Earth, 0.82 M_Earth)

Small Bodies:

• Mars (0.53 R_Earth, 0.11 M_Earth)
• Mercury (0.38 R_Earth, 0.055 M_Earth)
• Moon (0.27 R_Earth, 0.012 M_Earth)

Evolution of Planetary Surfaces

• Impact cratering
• Volcanism
• Tectonism (crustal movement)

• Depends on whether the interior hot enough for volcanism

Small Planet Surfaces

• Old, heavily cratered surfaces >3 Gyr old
• Single, continuous crust (no plates)
• Vertical Tectonism dominates, primarily stationary upwelling from below.

• Primary crust: shaped by impacts (highlands)
• Secondary crust: shaped by volcanism (hot-spot volcanoes on Mars, Maria on the Moon, lava plains on Mercury).

Large Planet Surfaces

Younger Surfaces:

• Earth: most of the surface is <200 Myr old
• Venus: surface ~500 Myr old

Active "tertiary" crusts:

• Earth: plate tectonics (subduction, sea-floor spreading, upthrust) rebuild crust constantly.
• Venus: volcanoes over mantle upwelling, and compression over mantle downwelling (Venus has a one-plate crust).

Evolution of Planetary Interiors

First Stage: Differentiation (heat of formation)

• Dense molten metals sink into the core.
• Lighter silicate rocks float to the crust.

Second stage: Volcanism

• Mantle is still molten, primarily from internal heating (radioactive decay) and heavy impacts.
• Magmas rise to the surface as volcanoes.

Terrestrial Planet Interiors

• Smaller bodies have cool rapidly
• Mantles solidify, ending tectonic activity
• Thick, cool, rigid crusts

Large Bodies:

• Hot interiors are kept hot by radioactive decay.
• Semi-molten mantles with convective motions
• Drives tectonism, and active (tertiary) resurfacing of the crust

Terrestrial Planet Atmospheres

Mercury & Moon:

No atmospheres

Gravity is too low to retain all but trace gases

 

Venus:

Hot, heavy, dry CO₂ atmosphere

 

Earth:

Warm, light, moist, N₂ & O₂ atmosphere

 

Mars:

Cold, thin, dry CO₂ atmosphere

Atmosphere Formation

• Fewer volatiles close to the Sun (too hot)
• Get more volatiles moving outwards (cooler)

Primordial Atmosphere formation:

• Outgassing from primordial volcanoes
• Comet impacts delivering frozen volatiles
• Substantial atmospheres of CO₂, H₂O, and N₂
• H & He quickly lost (gravity is too low)

Evolution of Planetary Atmospheres

Their subsequent evolution was driven by a number of factors:

• Greenhouse Effect, which affects solar heating and the atmospheric cooling balance
• Planetary Gravity, which affects the planet's ability to retain hot atoms and molecules.
• Chemistry, particularly CO₂ and H₂O chemistry, which affects the CO₂ content.

The Greenhouse Effect

	Without Atmosphere	With Atmosphere
Earth	255 K	285 K
Venus	280 K	750 K
Mars	214 K	220 K

Mercury's Atmosphere

• Shuts down CO₂ and H₂O chemistry
• Sets up a runaway greenhouse effect

Mercury has a low surface gravity:

• Too small to hold onto its hot atmosphere
• Would have lost all volatiles into space after only ~1 Gyr.

Result is no atmosphere to speak of in the current epoch.

Venus' Atmosphere

• Might have had hot oceans very early that evaporated (still very speculative)
• Get a runaway greenhouse effect

Venus is big enough to retain an atmosphere:

• Hot, heavy CO₂ and N₂ atmosphere
• H₂ and O lost to UV photolysis, H₂ escapes & the O reacts with other gasses.

Result is a very dry, super hot, heavy atmosphere.

Earth's Atmosphere

• H₂O condenses into massive, deep oceans
• Setup a cycle of evaporation and precipitation in the atmosphere.

CO₂ chemistry in water:

• Locks CO₂ into carbonaceous rocks
• Plant life thrives in water, converting gaseous CO₂ into O₂, boosting the O₂ content.
• A mild greenhouse effect keeps water liquid.

Result is a light, warm, moist N₂ & O₂ atmosphere.

Mars' Atmosphere

• CO₂ gets locked into carbonaceous rocks? This is a controversial idea.
• Evidence of liquid water in the past from the Mars Rovers

As Mars cools:

• H₂O freezes out (most may already have been frozen into saturated rocks)
• Much of the remaining CO₂ and N₂ escapes the low gravity of Mars

Result is a thin, cold, dry, CO₂ and N₂ atmosphere.

Present-day Atmospheres

	Earth	Venus	Mars
CO2	0.035%	96%	95%
N2	77%	3.5%	2.7%
H2O	1%	0.01%	0.007%
Ar	0.93%	0.007%	1.6%
O2	21%	trace	trace

Lessons for a Future Earth

In ~1 Gyr, the Sun will be ~10% brighter:

• Will trigger a "moist greenhouse effect"
• H₂O vapor is lost to space, drying out the atmosphere

In ~3.5 Gyr, the Sun will be ~40% brighter:

• Extra sunlight will trigger a "runaway greenhouse effect"
• Oceans evaporate, making the Earth like Venus