Phys 214. Planets and Life

Dr. Cristina Buzea

Department of Physics

Room 259

E-mail: cristi@physics.queensu.ca

(Please use PHYS214 in e-mail subject)

Lecture 11.

Geology and life Part 2

(Page 124-145)

January 30

Contents

Textbook: Pages 124-145

Plate tectonics

Retention of planetary atmospheres

The magnetic field of Earth

Climate regulation and change

The carbon dioxide cycle, ice ages & snowball Earth

Acknowledgments: Images from NASA, ESA, Hubble

Plate tectonics

The Earth’s lithosphere has broken up into about dozen platesbecause of mantle convection.

Plates - moving a few cm per year (fingernail growth)

Evidence – GPS measurements, evidence of past continental drift(puzzles), seafloor spreading, difference between the nature ofcrust on the seafloors and the continents.

Plate tectonics

Seafloor crust - made of high-density igneous rock (basalt) , 5-10

km thick, quite young (average 70 millions years)

Continental crust – lower density rock (granite), much thicker 20-

70 km, wide range of ages

Seafloor spreading on the Earth is the process by which molten

lava comes out between two tectonic plates that are slowly

moving apart.

Plate tectonics

Subduction on the Earth is the process by which oceanic crust sinks below continental

crust at a tectonic plate boundary.

Plate tectonics

Two continental plates collide into each other – creatingmountains (the Himalayas)

The Andes were formed by pieces of oceanic and continental crustcolliding.

Plate tectonics

The East African rift zone was formed by two pieces of continental crust separating.

Plate boundary faults occur where plates are sliding past each other.

(San Andreas fault between the Pacific plate and North American plate). Los Angelesand San Francisco bay – 20 million years. Earthquakes several meters in a fewseconds. Not all earthquakes occur near plate boundaries.

Plate tectonics

Volcanic activity may arise where hot mantle material rises up to make a hot spot.

The Hawaiian Islands were formed when volcanic material flowed out from the interior

at a hot spot beneath the Pacific Ocean. Plate tectonics carries the pacific plate over

the hot spot, forming a chain of volcanic islands.

Continental hot spots – geysers and hot springs.

Plate tectonics through times

150 million years ago Earth’s map would be quite different because plate tectonics would havecaused a shift in the position of the continents.

Plate shift of 2 cm/year = >200 km in 100 million years

225 million years ago – a single continent - Pangea

Plate tectonics in the solar system

Earth the only planet with ongoing plate tectonics!

Moon, Mercury, and Mars do not have plate tectonics - they have cooled so rapidly

that they do not have enough internal convection to break up their thick crusts.

Even though Venus is almost the same size as the Earth it does not have plate tectonics.

Probably because due to the high temperatures at its surface, its crust has dried up

and thickened so it cannot break up into plates.

Retention of planetary atmospheres

Not in the textbook!

If gas molecules move fast enough and exceed the escape velocity – they escape into space.

Escape velocity = the minimum speed needed before a body has enough kinetic energy to escape from the

surface of a planet (overcome gravitational field).

• The escape velocity from a planet of mass M and distance R is

where G – gravitational constant of the planet

The extent of thermal escape in a planetary atmosphere is dependent on the temperature (T) and molecular

weight of the gases. The gas escapes if

kB - Boltzmann constant, T - temperature, m - molecular mass of gas

R

GMvesc

2=!

Ec = Ep , mvesc

2

2=G

mM

r

!

kBT =mv

thermal

2

2"mvesc

2

2# vesc <

2kBT

m

Retention of planetary atmospheres

Not in the textbook!

• Different gases have different

molecular masses – their

average speeds are different at a

given temperature.

• For a planetary body to retain a

particular gas in its atmosphere

for a period of time = the age of

the solar system, the average

speed of the molecules in the

gas should be less than 1/6 of

the escape velocity.

Retention of planetary atmosphere

Volcanically active planets and satellites

How to loose a planetary atmosphere?

• 1. Thermal escape

• 2. Impacts

• 3. Solar wind stripping

Smaller worlds are more prone to atmospheric loss via impactsthan large worlds because they have weaker gravities andsmaller escape velocities.

The sweeping of atmospheric gas particles into space by theSun is referred to as solar wind stripping.

The magnetic field of Earth

In order to have a global magnetic field, a planet must have:

1. an electrically conducting fluid in its interior

2. which is undergoing convection

3. and a reasonably rapid rotation.

The magnetosphere and the solar wind

The Earth’s magnetosphere is a cavity carved out in the solar wind by the Earth’s magnetic field.

The magnetosphere deflects most of solar wind particles while chaneling a few towards the poles –

auroras; charges particles trapped within are forming Van Allen belts.

The magnetosphere protects the atmosphere, preventing the solar wind from stripping it away.

Requirements for a global magnetic field

Only Earth and Mercury among terrestrial planets have magnetic fields.

Mercury –slight enigma – small & has a slow rotation (1 rotation in 59Earth days), but a very large metal core.

Mars & Moon have no magnetic field probably because of coresolidification. Mars lost much of its atmosphere when its interiorcooled.

Venus rotates too slow (one rotation in 243-day Earth days).

Climate regulation and change

• Long-term habitability – volcanism, plate tectonics, magnetic field.

• The climate on earth has been sufficiently stable to exist continually for nearly 4 billion years.

• Life on Earth needs liquid water => oceans should be at least partially liquid

• Temperature range for water to be liquid may seem wide to humans, but compared to

temperatures on other worlds -> Earth’s climate remarkably stable

• Why Earth long-term climate is stable and warm enough for water to be liquid?

Climate regulation

The right distance from the Sun is not enough for a body to have liquid water

Moon – daytime 125oC (above boiling point of water), night-time temperature -175oC.

The Sun gradually has brighten with age (30% brighter today than when Earth formed)

Earth average temperature based solely on its distance from the Sun = -1oC.

The actual global average temperature today = 15oC.

The greenhouse effect = the trapping of infrared radiation from the Earth’s surface by greenhousegases like carbon dioxide, methane, and water.

Without the greenhouse effect, the oceans on the Earth’s surface would be frozen

The greenhouse effect

Mechanism: trapping some of the

infrared light emitted by the surface

Greenhouse gases = some atmospheric

gasses can absorb infrared light;

water vapors H2O, carbon dioxide

CO2, methane CH4.

Diatom molecules like nitrogen N2,

oxygen O2 do not absorb infrared.

After a greenhouse molecule absorbs an

infrared photon -> emits a new

infrared photon in a random

direction

It slows the escape of infrared radiation

from lower atmosphere – making it

warmer – like a blanket.

Venus – the greenhouse effect

The greenhouse effect is not intrinsically bad – as portrayed in the news.

Human activity may add too much greenhouse gases in the atmosphere –global warming

The greenhouse effect on VENUS is responsible for the searing 470oCtemperature!

CO2 is less than 1% of Earth atmosphere, more than 96% of Venusatmosphere!

Venus and Earth are nearly the same size – volcanic outgassing probablyreleased the same amount of CO2

What happened with the CO2 on Earth?

What regulates Earth’s climate?

Most of the Earth’s carbon dioxide hasdissolved in the oceans and is locked upin sedimentary carbonate rocks likelimestone (containing about 170,000times as much CO2 as our atmosphere).

The carbon dioxide cycle regulates the surfacetemperature by varying the amount ofcarbon dioxide in the atmosphere.

- Atmospheric CO2 dissolves in rainwater –mild acid

- The acid rain erodes rocks, rivers carry theminerals to the oceans

- In the oceans, calcium combines withdissolved CO2 and falls to the ocean floormaking carbonate minerals – accumulateas limestone

- Plate tectonics carry carbonate rocks tosubduction zones into the mantle;

- Some melt and release CO2 throughvolcanoes

The carbon dioxide cycle as a thermostat

If the temperature of the Earth warms up - the CO2 cycle speeds up the formation ofcarbonate minerals in the oceans, thus pulling more carbon dioxide out of theatmosphere.

If the temperature of the Earth cools - the CO2 cycle slows the formation of carbonateminerals in the oceans, thus pulling less carbon dioxide out of the atmosphere.

The CO2 cycle cannot easily correct the CO2 due to industrialization because it operates fartoo slowly to correct for any short-term changes (the feedback mechanism takes about400,000 years).

How did Earth’s climate change over the time?

Ice ages = mild, short-term cooling periodsduring the Earth’s (global averagetemperature drops by a few degrees)

During the past few million years, ice ageswere MOST LIKELY caused by smallchanges in the Earth’s axis tilt.

On long time scale - the sun brightening andthe migration of continents influenced theclimate

On shorter time scale – small, cyclicalchanges in earth’s rotation and orbit –making season more or less extreme

Greater tilt = more extreme seasons, warmersummers and cooler winters,prevents icefrom building up – warmer planet

Smaller tilt = the opposite

Snowball Earth

Long and deep ice ages between 750-580 million years ago, 2.4-2.2 billion years ago

Glaciers advanced to the equator, oceans freezing worldwide

90% sunlight reflected by ice compared to 5% by water = > surface cooled more

The CO2 cycle stops -> CO2 outgassed by volcanoes – heats the Earth

Global average temperature –50oC!!

Snowball Earth = severe, long-term cooling periods during the Earth’s history.

The Earth is able to recover from a snowball phase by carbon dioxide gradually building up in theatmosphere from volcanism

The end of the last snowball Earth phase roughly coincides with the beginning of the Cambrianperiod which produced an explosion in diversity of life.

Earth’s long term habitability

Major factors that kept earth habitable for the past

4 billion years

• Volcanic outgassing (gases and water)

• Protective effect of its magnetic field

• The greenhouse effect warmed the planet for

the water to be liquid

• The moderate greenhouse effect is maintained

by the self-regulating CO2 cycle

• Earth axis tilt and rotation influences its climate

• The climate regulation is not perfect, leading

sometimes to snowball earth

The formation of the Moon

Models:

1. Moon formed along with Earth during accretion (calculations did not support this

model; the Moon average density is much lower than Earth’s)

2. Moon has been an independent planet captured into Earth’s orbit (improbable to loose

its orbital energy; only likely with small bodies – friction with gas surrounding the planet -

Mars)

3. Young molten Earth spinning so fast it split into 2 pieces (improbable to spin so fast)

4. Impact with a body the size of Mars

The formation of the Moon

• The currently accepted theory for the formation of the Moon is the giant impact

model in which material was blasted from the Earth after an impact with a body the

size of the planet Mars

• The strongest of evidence that supports the giant impact model for the formation of

the Moon is that the overall composition of Moon rocks returned from the Apollo

program is similar to the composition of Earth’s mantle material, except for the

lack of volatile elements

Next lecture

• Chapter 5. The nature of life on Earth