Planetary Geology - University of Floridafreyes/classes/ast2003/FR_CH_9.pdf · Planetary Geology •Geology is the science that deal with Earth’s physical structure, it history

Planetary Geology Earth and other terrestrial worlds

Chapter 9

Planetary Geology

• Geology is the science that deal with Earth’s physical structure, it history and processes that act on it.

• An extension of this science is Planetary geology, the extension of the study to other solid bodies in the solar system

What are the terrestrial planets like inside?

Most of the terrestrial planet interiors are divided into three layers:

• Core

Mainly consist of high density material such as iron and nickel

• Mantle

This is the layer that surrounds the core. It is rocky material that consist of mineral that contains silicon, oxygen and other elements

• Crust

This is the outer most layer. It is lowest-density rocky material such as granite and basalt ( a common form of volcanic rock)

The interior of the planets are layered because the material was melted. The heavier material sank towards the interior. Gravity was the force that drag the heavy material and left the lighter material to remain at the top.

This process is called differentiation

All the terrestrial planets went trough the process of differentiation

The Earth’s metallic core consist of two distinctive regions: A solid inner core and a molten (liquid) outer core

Interior structure of the terrestrial planets

Why is the Earth Differentiated?

• Begins by accretion

– 4.6 Billion years ago (age of Sun)

• Differentiation - The Earth melts

– 4.5 Billion years ago

• Crustal Formation

– 3.7 Billion years ago

2,000 – 2,500 K

Melt iron

1,500 K

1/3 mass of Earth falls to

the center in the form of

molten iron 5000 K

(molten Earth)

What causes differentiation and geological activity

• Heat of accretion As planetesimals collide and form a planet, the kinetic energy from the

collision is transformed in heat which add to the thermal energy of the planet

• Heat from differentiation In the process of differentiation, the dense material sink and the lighter

material rises. The gravitational potential energy is transformed into thermal energy by friction

• Heat from radioactive decay The material that built the planets contain radioactive isotopes of

elements such as uranium, potassium and thorium. These elements decay into lighter elements releasing energetic subatomic particles that collide with the material heating it.

Heating mechanisms for a terrestrial planet

Cooling mechanisms for the interior of a terrestrial planet

Convection.

Hot material expands and rises

while cooler material contract

and falls

Conduction.

Transfer of heat from hot

material to cooler material

through contact

Radiation.

A planet will lose heat to space

through thermal radiation. A

planet acts like a black body

and emit radiation (light) .

Because of their low

temperature, planets radiate

primarily in IR

How do we know what is inside a planet?

The only planet for which we have information about the interior is the Earth. But the information does not come from direct sampling of the interior of the Earth

Earth’s radius is 6,378 km. Drilling only can go not more than a few km (16 km).

It is possible to sample only a small upper layer of the crust!

For Earth, much of the information about the interior comes from seismic waves (waves generated during an earthquake)

Seismic waves have been used to study the interior of the Moon using monitoring stations left by the Apollo astronauts on the surface

Seeing inside the Earth

• Drill a hole

– Petroleum geologists usually

drill to ~ 6 km – Deepest hole: Kola Super

deep borehole (~ 12 km)

– Deepest we can go ~16 km

P (pressure) and S (shear) Wave analogy

How do we know anything about the interior of the Earth?

• Earth’s interior structure is probed by

studying how seismic waves travel

through it (we can only drill so far! – 16

km).

• Earthquakes generate seismic waves.

•P waves (pressure) can travel through

the liquid core but they are deflected by

the core

•S waves (shear) travel in the mantle

but not through the core

• Waves are reflected and refracted by

different materials and travel through

these materials at different speeds

(higher density material →faster speed).

Seismology!

• Mantle - 3000 km thick (80% of planet volume). • Crust - 15 km thick (8 km under ocean - 20-50 km under continents. • Density and temperature increase with depth. • High central density suggests the core is mostly nickel and iron. • There is a “jump” in density between the mantle and the core caused by different materials. • No jump in density between inner and outer core because material is the same and just goes from liquid to solid. • The temperature in the core is about 5,000K and the density about 12 g/cm^3 •(Reference: Density of water is 1 g/cm^3 or 1000 kg/m^3)

The Earth interior Using seismic wave and computer models it

is possible to model the interior

Earth radius = 6,378 km

The surface area-to-volume ratio Mathematical Insight 9.1

For a spherical object (planet), r is the radius of the

sphere:

Surface area = 4 π r²

Volume = 4/3 π r³

Surface area-to-volume ratio = Surface area/ volume

= (4 π r²) / (4/3 π r³)

= 3/r

The radius appears in the denominator. Larger bodies

have a lower surface area- to-volume ratios.

If two objects start with the same internal temperature, the

larger body cool off slower than the smaller body

Parameters that affect the geological history of a planet

Shaping the planetary surface

Process that shape a planetary surface:

• Impact cratering Creation of bowl-shape impact craters by asteroids and comets striking

the surface

• Volcanism Molten rocks or lava coming from the plantet’s interior in an eruption

• Tectonics The disruption of the planet’s surface by the internals stresses

• Erosion Wearing down or building up of features by effect of wind, rain, water and

ice

Barringer meteor Crater Located near Winslow,

Arizona

The best preserved impact

crater

Diameter 1.2 km

Age ~ 50,000 yr

Estimated impactor size

30-50 m

Estimated speed of

impactor ~ 26,000 miles/hr

Shaping the crust: Impacts on Earth

Shaping the crust: Volcanoes

Mt. St Helen before and after 1980 eruption

Mauna Loa – 1984

Length ~ 75 miles

Covers ~ half Big Island of Hawaii

33 eruptions since 1850s

Shaping the crust: Cordon Caulle Volcano (Chile). June, 2011

Shaping the crust: Cordon Caulle volcano (Chile). June 2011

Shaping the Crust: Tectonics

• The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated.

• Convection currents beneath the plates move the crustal plates in different directions.

• The source of heat driving the convection currents is radioactivity deep in the Earths mantle

Shaping the Crust: Erosion and Deposition

• Erosion (breaking down) and deposition (building up) require the presence of a fluid (gas or liquid)

• Water, rain, wind cause erosion

River erosion created the

Grand Canyon

Deposition from the Mississippi

river created Louisiana

wetlands.

Geology of the Moon and Mercury

Mercury’s Surface

• Surface similar to the moon, large

number of impact craters!

•Old surface

•No indication of plate tectonics

•Craters have a flat bottom and have

thinner ejecta rims than lunar craters due

to higher gravity on Mercury than the

moon

•Craters not as dense as on the moon -

filled by volcanic activity

• Not dark features like the “maria” on

the Moon

• Caloris Basin, evidence of a large

impact crater. It is the largest crater on the

planet, about 1/2 Mercury radius

Mariner 10 image (Flyby in 1974-

1975)

Mercury, an image of half of Caloris Basin Image taken by Mariner 10

Mercury’s Surface • Cliffs are seen on the surface

•This features are not seen on

the moon

•They appear to be about 4

billion years old

•They are not the result of

plate tectonics

•Probably the result of the

surface cooling, shrinking and

splitting at this time

•Some are several hundred

km long and has high as 3 km

high

•Cliffs may have formed

when tectonic forces

compressed the crust

Mercury A recent image (false colors) taken by the Messenger spacecraft

The Messenger

spacecraft is at the

present in orbit around

Mercury. It went into

orbit in 2011

It is the first and only

spacecraft to orbit this

planet

A high resolution image of Mercury taken by the Messenger mission

A detailed view of craters in high resolution image of Mercury taken by the Messenger mission

Lunar Geological History

Moon surface

Energy from the formation caused at least the outer few kilometers to melt

(deep ocean of molten rock)

This took place about 3-4 billions years ago.

Molten rocks flooded the largest impact craters. Maria (singular mare) are

large impact craters flooded by lava. The dark color comes from dense, iron

rich basalt that rose from the lunar mantle

Lunar Surface , large scale features

Lack of atmosphere and water preserves surface features Maria, singular Mare (younger) –

mantle material (maria means

“seas’)

• Maria - darker areas resulting

from earlier lava flow

• Basaltic, relatively iron rich, high

density (3,3 g/cm3).

Highlands (older) – crust material

• Elevated many km above maria

• Aluminum rich, low density (2,9

g/cm3).

Highlands - light, rough, heavily cratered

Older areas compared to maria

Heavy cratered compared with maria

Lunar Highlands

Lowlands – dark, smooth Maria

Basalt – fine grained dark igneous rock rich in iron and magnesium (stuff that sank in magma ocean)

Few hundred meters thick

Younger than the high lands

Less craters compared with highlands

Lunar Lowlands

The formation of Mare Humorum

Lunar maria and mountain ranges

← Mare

Serenitatis

Caucasus Mountain

↓

← Apollo 11

Mare Tranquillitatis

Moon Volcanism: Maria (Dark Areas)

SW Mare Imbrium Mare Imbrium

The volcanism took placeafter the impacts – most 3 – 1 billion years ago

Rilles

Aristarchus Plateau Marius Hills

Mountains

Montes Tenerife

Montes Alpes- Mons Blanc 3.6 km

The Alpine valley is the long feature

to the left of the image

Montes Appeninus

(Mons Huygens 5.5 km)

Lunar Erosion

Lunar Craters - caused by meteoroid impacts

• Pressure to the lunar surface heats the rock and deforms

the ground.

• Explosion pushes rock layers up and out.

• The ejecta blanket surrounds the crater

•It forms radial features around some craters. This are

called rayed craters

• In some cases, the material compressed bounces back

and form a crater with a central peak

• Craters can be up to 100 km in diameter

• A new 10 km crater is formed every 10 million years

• A new 1m crater is formed each month

• A new 1cm crater is formed every few minutes!

Cratering on the Moon

• The rate of cratering on the moon is determined from the known ages of the highland and

maria regions.

• The Moon (and solar system?) experienced a sharp drop in the rate of meteoritic

bombardment ~ 3.9 billion years ago.

• The rate of cratering has been roughly constant since that time.

• Only a few craters appear on the maria, showing that they are younger

•The highlands have a large concentration of craters

Rim

Central Peak

Floor

Wall

Rays Tycho Crater ~ 100 MY old

Fresh rays, young feature

85 kilometers across

Lunar Impact Basins

Imbrium Rim Orientale Basin (Far side of the Moon)

Big, frequent impacts until 3.8 billion years ago Impact events continue on all moons and planets today but at a much lower rate!

Geology of Mars

• Main geological features

• Evidence of flow of water

Major Martian

geographical feature

Tharsis Bulge •Roughly the size of North America

•Lies on the equator - 10km high

•Less heavily cratered = Young (2-3

billion yrs)

Valles Marineris

• Valley that extends one-fifth of the way

around the planet(!) at the equator

•Up to 120 km across and 7 km deep

•The Grand Canyon would fit into one of its

side "tributary" cracks

•NOT caused by water - probably due to

stretching and cracking when Tharsis bulge

formed

Map from

Mars Global

Surveyor -

2001

Valles Marineris 120 km across and at least 2,000 km long

Mars - Telescopic Exploration

Giovanni Schiaparelli (1835-1910)

•Mapped bright and dark regions

•Saw polar caps which changed with seasons

•Surface colors appeared to change - plant life?

•Identified long narrow features he called them

canali (grooves, channels). It was translated as

canals…

Percival Lowell (1855 -1916)

•Built an observatory in 1894 in Flagstaff AZ to

study Mars (now Lowell Observatory)

•He purchase and used a 24” Alvan Clark

refractor for his observations

• He thought that canals were used by a

civilization to bring water from the poles to the

equatorial desert . He argued that canal were the

work of an intelligent civilization on Mars

•Images taken by the Mariner and Viking space

craft about a century later (1960”s) prove that

there were no canals

Map of Mars by Schiaparelli (1988)

Volcanism on Mars

Olympus Mons on Tharsis slope

Its base is about 600 km across. Its

peak stands about 26 km above the

average surface level (about three

time as high as Mount Everest)

• The largest volcanoes in the solar system are found in Mars

•Shield volcanoes

•None are known to be currently active but eruptions occurred 100

million years ago

Mars has a surface gravity only 40

percent that of Earth, and its volcanoes

rise roughly 2.5 times as high because of

this.

Liquid Water on Mars?

Yes – Massive flow long ago

Some recent evidence of minor water flow

Runoff channels

•Found in southern highlands

•Extensive river systems (like Earth)

•Carried water from highland to valleys

About 4 billion years ago Mars had a thicker

atmosphere, warmer surface, and liquid

water.

Outflow channels

•Caused by flooding

•Found at the equator

•Formed about 3 billions year ago

Mars Earth

A high resolution image of a Mars runoff channel Evidence of flow of liquid water

Ridges of crater suggest the impact debris was muddy. This crater was probably made by an impact on icy ground

More evidence of flow of liquid water on Mars

Another image showing the erosion caused by flow of water in Mars.

The Oraibis crater is about 32 km across. The bottom is filled with

sediments.

Image was taken by the Mars Express in May, 2011

What happened to the water?

• Liquid water (from runoff channels) froze into permafrost (water ice just below

surface) and polar caps about 4 billion years ago

• After ~1 billion years, volcanic activity heated the surface and melted permafrost

• Flash floods created outflow channels

• Volcanic activity slowed after that and the liquid water refroze

• Mars Global Surveyor in 2000 revealed “gullies” along the insides of craters -

evidence for more recent existence of liquid water?

Evidence of salty water running down the slopes of rims of craters (gullies).

Salty water may stay in liquid phase for a short time due to its lower freezing point

Polar Lander Phoenix (Phoenix landed on Mars on May 25, 2008)

More evidence of frozen water under the surface

A trench carved by the

scoop of Phoenix

lander (about 10 cm

deep).

The white material

inside the trench is

water ice that melted

(or sublimated) soon

after being exposed

Mars most recent mission: rover Curiosity

•Curiosity landed on

Mars on August 5th,

2012 inside the Gale

crater

•The area may have

been flooded by fast-

moving water

•A good site to look for

possible organic

material which may be

indication of fossil

bacterial life

An image send by Curiosity (left) showing evidence of flowing of liquid water. Comparison with deposit of rocks in a dry river bed

on Earth

The Face on Mars In 1976, Viking orbiter 1 images revealed a mountain that looked somewhat

like a human face

•Commented on in a NASA press release

•Immediately seized by tabloids

•Taken by some as proof that an

advanced civilization existed on Mars

•More likely just how our minds find

familiar patterns

•Below is a high resolution image taken

a few years ago by the Mars Orbiter

spacecraft

Geology of Venus The thick cloud cover of the planet completely hide the surface of Venus.

No visual images from the surface of the planet are available.

All the images shown here (except for the image from Venera) are radar images .

Radar: Radio waves are emitted towards the surface , they bounce back and are received by the spacecraft

Magellan spacecraft (1990-1994) used radar to map the surface of Venus. This were the first look at the surface of the planet.

Major geological features

Impact craters Venus has only a few impact craters. Most of the ancient craters have been erased.

The few craters are large, there are no small craters suggesting that small bodies may burn in the thick atmosphere before they reach the surface.

Geology of Venus Major geological features

Volcanic and tectonic features Evidence of volcanism is clear. Several volcanoes were imaged by the radar.

There is evidence of lava flow, lava plains and volcanic mountains. The radar images do not show active volcanoes or eruptions. The presence of sulfuric acid clouds suggest that outgassing is replenishing the atmosphere of sulfur. Sulfur dioxide is removed from the atmosphere by chemical reaction with rock. This suggest that outgassing must still occur at least in geological time scales (millions of years)

Weak erosion The images returned by the Venera mission does not show evidence of erosion. Since the

temperature is high, there is no precipitation. Wind must be very slow at the surface.

No evidence of plate tectonics. All the surface seems to have the same age everywhere. Radar maps of the surface doesn’t show the types of plates we find on the Earth.

Venus’s Surface

Radar (radio waves) echoes reveal the surface topology (Magellan spacecraft)

There are no direct images of the surface. The planets is always

covered by thick clouds. Only radio waves can penetrate the clouds

•Elevated “continents” make up 8% of the surface (25% on Earth)

•Mostly rolling plains with some mountains (up to 14 km)

•No indication of plate tectonics

•Buckled and fractured crust with numerous lava flows

Venus’s Surface: Volcanoes and Craters

Images from the Magellan spacecraft (1990-1994)

•Largest volcanic structures are called

coronae - upwelling in the mantle which

causes the surface to bulge out - not a full-

fledged volcano.

•Usually surrounded by other volcanoes

•Venus is thought to still be volcanically

active today. Magellan radar images did not

show any active volcano.

•Volcanoes resurface the planet every

~300 million years

•Shield volcanoes are the most

common (like Hawaiian Islands)

• A caldera (crater) is formed at the

summit when the underlying lava

withdraws

Radar image of a volcano with lava flow taken by Magellan spacecraft

A 3-D view of three volcanoes taken by Magellan spacecraft (radar images)

Radar images taken by Magellan of “pancake” volcanoes

An image taken by the soviet spacecraft Venera. The spacecrafts

Venera landed on the surface of Venus in 1970s

• The spacecraft had a very short life time on the surface, survived only an hour

before the heat damaged the equipment.

•No other spacecraft have landed on the surface of Venus after the Venera

•Little evidence of erosion - young surface

•Rocks are basaltic and granite

• The thick cloud cover makes Venus seem like a heavily overcast day on Earth

all the time!

The unique geology of Earth

• Plate tectonics

• How was the earth surface shaped by plate tectonics?

Overview of the Earth

• Dense rocky composition

• Evolving surface < 600 Myr old

• Atmosphere mostly Nitrogen and oxygen

• Oceans of Water

• Magnetic Field

Mass 6 x 1024 kg

Radius 6400 km

Density 5.5 g/cm3

Surface gravity 9.8 m/s2

Escape Speed 11 km/s

The Earth Interior

• The Earth is very dense: 5.5 g/cm3

– Reference: water 1 g/cm3

– Normal Rocks 2-4 g/cm3

– Pure Iron 7.8 g/cm3

The Earth’s interior is

differentiated (distinct layers –

densest sink, lightest floats)

At some point in our past,

Earth melted

bombardment of debris

radioactivity

The Earth Interior

Layer Comp Type Thickness Temp

Core Iron & Nickel Solid/liquid 3500 km >6000 K

Mantle Silicates Plastic 2900 km 1300 – 3800 K

Crust Granite & Basalt Solid 10-1100 km <1000 K

Plate boundaries

The motion of the Earth surface

• The plate tectonic theory explain how the plates moves and drift as they “float” on top of the mantle (Asthenosphere)

• The theory was proposed in 1912 by the German scientist Wegener. The idea was that continents drift across the surface of the Earth.

• But at that time, nobody knew about a mechanism that will allow the continents to move.

• But today there is more evidence to back up the theory and it is widely accepted.

• The discovery of mid-ocean ridges and the seafloor spreading in the 1950’s provided more evidence.

• Today the continental motion is explained by convection, driven by heat released by the Earth interior

Seafloor spreading

Convection pushed the material up into the ridges in the ocean floor

separating the two plates

Subduction pushes denser seafloor crust material under layers of less

dense continental crust

California’s San Andreas fault

• The San Andreas fault

is the boundary where

two plates are sliding

sideways

• It is very active

• The dates mark the

most recent earthquakes

• The lines in the inset

show how far the two

sides of the fault shift

moved in an earthquake

Where do we find volcanoes? An example are the Hawaiian islands

Stratovolcanoes typically where one

plate descends beneath an adjacent

plate.

Shield volcanoes are

formed by “Hot Spots”.

As the plate moves

over the “hot spot”

multiple volcanoes are

formed.

Hawaiian Island hot spot

Geological history of the terrestrial planets

Continental Drift Past present and future of Earth’s continents

About 200 millions years ago all the continents were together in a single “supercontinent” called Pangaea

Production of magnetic fields in planets

Creation of magnetic field in planets: An interior of electrically conducting fluid (Liquid). Charged particles move with the molten or

conductive material

Convection in that layer

Rapid rotation

The Earth meet all the requirements

Venus has a very slow rotation rate (-243 days), no magnetic field

Mars may retain enough heat but not enough to have convection

Mercury is a mystery: It has some magnetic field but has a slow rotation (58.6 days). It is possible that

it has a molten core with convection (or the magnetic field could be relics of ancient magnetic field

frozen in the core?)

How are the planetary magnetic fields generated?

– metallic liquid regions plus rotation of planet

– Jupiter & Saturn: liquid metallic

– Uranus & Neptune: near surface convecting ices?

Earth Magnetic Field

• Earth’s magnetic field protect us from energetic particles in the solar wind and cosmic rays.

• These particles do not strike directly the surface, they are trapped in the magnetic field lines.

Auroras • Some charged particles from the solar wind get trapped in the Earth’s

magnetic field lines. They will spiral toward the magnetic poles and precipitate in the atmosphere releasing energy .

• That energy excite the atoms in the upper part of the atmosphere and they emit light. The emission is line emission. The colors of the aurora depends on the atoms being excited.