Lecture 22

Terrestrial Planets

What are they like? Why?

Mercury EarthVenus Mars

Terrestrial planets are mostly made of rocky materials (with some metals) that can deform and flow over time.

Likewise, the larger moons of the Jovian planets are made largely of icy materials (with some rocks and metals) that can deform and flow.

The ability to deform and flow leads every object exceeding approximately 500 km in diameter to become spherical under the influence of gravity.

Early in their existence, the Terrestrial planets and the large moons had an extended period when they were mostly molten.

The heating that led to this condition was caused by impacts, where the kinetic energy of the impacting material was converted to thermal energy.

Today, the interiors of planets are heated mainly by radioactive decay.

The heating to a molten state, and subsequent cooling, had important repercussions for:

interior structure of the planet, surface features, atmosphere, magnetic fields, presence/absence of water.

Differentiation – the process by which gravity separates materials according to their densities

Denser materials sink, less dense material “float” towards top

Differentiation during the molten phase resulted in the formation of three distinct density zones within each terrestrial world:

Core - contains metals (e.g., iron, nickel)

Mantle – intermediate layer with rocky material (sometimes partially semi-molten)

Crust – lowest-density rocks (surface)

Terrestrial planets have metallic cores (which may or may not be molten) & rocky mantles

Earth (solid inner, molten outer core)

Mercury (solid core)Earth’s interior structure

Differentiated Jovian moons have rocky cores & icy

mantles

Europa Io

CallistoGanymeade

The Lithosphere…

Layer of rigid rock (crust plus upper mantle) that floats on softer (mantle) rock below

While interior rock is mostly solid, high pressures and stresses can cause rock to deform and flow (think of silly putty)

This is why we have spherical planets/moons

The interiors of the terrestrial planets slowly cool as their heat escapes.

This cooling gradually makes the lithosphere thicker and moves molten rocks deeper.

Larger planets take longer to cool, and thus larger planets:

1) retain molten cores longer

2) have thinner, and thus weaker, lithospheres

The stronger (thicker) the lithosphere, the less geological activity the planet exhibits.

Planets with cooler interiors have thicker lithospheres.

lithospheres of the Terrestrial planets:

Stresses in the lithosphere lead to “geological activity” (e.g., volcanoes, mountains, earthquakes, rifts, …) and, through out-gassing, leads to the formation and maintenance of atmospheres.

Cooling of planetary interiors (energy transported from the planetary interior to the surface) creates these stresses

Convection - the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e.g., boiling water).

Convection is the main cooling process for planets with warm interiors.

Larger planets stay hot longer.

Earth and Venus (larger) have continued to cool over the lifetime of the solar system thin lithosphere, lots of geological activity

Mercury, Mars and Moon (smaller) have cooled earlier thicker lithospheres, little to no geological activity

Under what circumstances can differentiation occur in a planet?

red) The planet must have a molten interior.

blue) The planet must be made of both metal and rock.

orange) The planet must be geologically active, that is, have volcanoes, planet-quakes, and erosion from weather.

green) The planet must have a rocky surface.

Under what circumstances can differentiation occur in a planet?

red) The planet must have a molten interior.

Which internal energy source is the most important in continuing to heat the terrestrial planets today?

red) differentiation

blue) tidal heating

orange) accretion

green) radioactivity

Which internal energy source is the most important in continuing to heat the terrestrial planets today?

green) radioactivity

Heat escapes from the planet's surface into space by thermal radiation. Planets radiate almost entirely in the wavelength range of the

red) infrared. blue) visible. yellow) radio. green) ultraviolet. orange) none of the above

Heat escapes from the planet's surface into space by thermal radiation. Planets radiate almost entirely in the wavelength range of the

red) infrared.

Side effect of hot interiors - global planetary magnetic fields

Requirements:

1. Interior region of electrically conducting fluid (e.g., molten iron)

2. Convection in this fluid layer

3. “rapid” rotation

Earth fits requirements

Venus rotates too slowly

Mercury, Mars & the Moon lack molten metallic cores

Sun has strong field

Planetary Surfaces4 major processes affect planetary surfaces:

Impact cratering – from collisions with asteroids and comets

Volcanism – eruption of molten rocks

Tectonics – disruption of a planet's surface by internal stresses

Erosion – wearing down or building up geological feature by wind, water, ice, etc.

Impact Cratering: The most common geological process shaping the surfaces of rigid objects in the solar system (Terrestrial

planets, moon, asteroids)

Volcanism

Volcanoes help erase impact craters

Volcanic outgassing: source of atmospheres and water

Erosion: the breakdown and transport of rocks and soil by an atmosphere.

Wind, rain, rivers, glaciers contribute to erosion.

Erosion can build new formations: sand dunes, river deltas, deep valleys).

Erosion is significant only on planets with substantial atmospheres.

Tectonics: the action of internal forces and stresses on the lithosphere leading to the creation of surface features & geological activity.

Tectonics can only occur on planets with convection in the mantle (Earth & Venus today, some icey Jovian moons)

Tectonics…

raises mountains

creates huge valleys (rifts) and cliffs

creates new crust

moves large segments of the lithosphere (plate tectonics)

Tectonic plates

divergent plate boundary (plates move away from each other).

Atlantic Ocean

Great Rift Valley in Africa

Valles Marineris (Mars)

Portion of Valles

Marineris on Mars

It was created by

tectonic stresses

during formation of the

Tharsis Bulge

convergent plate boundary with subduction : plates move towards each other & one slides beneath the other.

Nazca plate being subducted under the South American plate to form the Andes Mountain Chain.

Island arc system

convergent plate boundary without subduction : plates move towards each other and compress.

Formation of Himalayas.

Plates sliding past each other: earthquakes, valleys, mountain building

Tectonic plates

Half of the world’s volcanoes surround the Pacific plate