An Introduction to Solar System Astronomy
Prof. Scott Gaudi
The Terrestrial Planets in Comparison:
A Comparison of the Terrestrial Planets
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
- Venus (0.95 REarth, 0.82 MEarth)
- Mars (0.53 REarth, 0.11 MEarth)
- Mercury (0.38 REarth, 0.055 MEarth)
- Moon (0.27 REarth, 0.012 MEarth)
Evolution of Planetary Surfaces
The evolution of planetary surfaces is driven by three main processes:
Volcanism & tectonism are driven by the internal structure of the
- Impact cratering
- Tectonism (crustal movement)
- Depends on whether the interior hot enough for volcanism
Small Planet Surfaces
Surfaces of the small terrestrial bodies, Mars, Mercury, & the Moon,
are distinguished from the Earth & Venus by being old and relatively
- 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
Large Planet Surfaces
The surfaces of the large terrestrial planets are younger and
active than those we see on the small terrestrial planets.
- 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
Evolution of Planetary Interiors
Internal heating & subsequent cooling drives the 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
- Hot interiors are kept hot by radioactive decay.
- Semi-molten mantles with convective motions
- Drives tectonism, and active (tertiary) resurfacing of
Terrestrial Planet Atmospheres
- Mercury & Moon:
- No atmospheres
- Gravity is too low to retain all but trace gases
- Hot, heavy, dry CO2 atmosphere
- Warm, light, moist, N2 & O2
- Cold, thin, dry CO2 atmosphere
During formation, the terrestrial planets were molten from
impacts with planetesimals.
- 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,
- H & He quickly lost (gravity is too low)
Evolution of Planetary Atmospheres
All of the terrestrial planets, including Mercury, started out with
substantial primordial atmospheres consisting primarily of
CO2 and H2O.
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
Makes surface temperatures warmer than they would be with no
Data are from Bruce Jakowsky's article Atmospheres of the
Terrestrial Planets in The New Solar System, 4th
Edition, Beatty, Peterson, & Chaikin, eds, (1999, Cambridge University
Mercury is too hot for liquid water:
- 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
Result is no atmosphere to speak of in the current epoch.
Venus is also too hot for liquid water:
- 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 is cool enough for liquid water:
- 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
Might have been warm enough for liquid water during the first
- 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
The composition of present-day terrestrial planet atmospheres can
be summarized as follows:
Lessons for a Future Earth
As the Sun ages, it slowly gets brighter.
In ~1 Gyr, the Sun will be ~10% brighter:
- Will trigger a "moist greenhouse effect"
- H2O vapor is lost to space, drying out the
In ~3.5 Gyr, the Sun will be ~40% brighter:
- Extra sunlight will trigger a "runaway greenhouse
- Oceans evaporate, making the Earth like Venus
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