Download Phys 214. Planets and Life

Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(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 plates
because 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 of
crust 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 2070 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 – creating
mountains (the Himalayas)
The Andes were formed by pieces of oceanic and continental crust
colliding.
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 Angeles
and San Francisco bay – 20 million years. Earthquakes several meters in a few
seconds. 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 have
caused 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).
2
mvesc
mM
Ec = E p ,
=G
2
r
•
The escape velocity from a planet of mass M and distance R is
!
vesc
2GM
=
R
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
k BT =
2
mvthermal
2
2
mvesc
2k BT
"
# vesc <
2
m
kB - Boltzmann constant, T - temperature, m - molecular mass of gas
!
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 impacts
than large worlds because they have weaker gravities and
smaller escape velocities.
The sweeping of atmospheric gas particles into space by the
Sun 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 59
Earth days), but a very large metal core.
Mars & Moon have no magnetic field probably because of core
solidification. Mars lost much of its atmosphere when its interior
cooled.
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 greenhouse
gases 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 470oC
temperature!
CO2 is less than 1% of Earth atmosphere, more than 96% of Venus
atmosphere!
Venus and Earth are nearly the same size – volcanic outgassing probably
released the same amount of CO2
What happened with the CO2 on Earth?
What regulates Earth’s climate?
Most of the Earth’s carbon dioxide has
dissolved in the oceans and is locked up
in sedimentary carbonate rocks like
limestone (containing about 170,000
times as much CO2 as our atmosphere).
The carbon dioxide cycle regulates the surface
temperature by varying the amount of
carbon dioxide in the atmosphere.
- Atmospheric CO2 dissolves in rainwater –
mild acid
- The acid rain erodes rocks, rivers carry the
minerals to the oceans
- In the oceans, calcium combines with
dissolved CO2 and falls to the ocean floor
making carbonate minerals – accumulate
as limestone
- Plate tectonics carry carbonate rocks to
subduction zones into the mantle;
- Some melt and release CO2 through
volcanoes
The carbon dioxide cycle as a thermostat
If the temperature of the Earth warms up - the CO2 cycle speeds up the formation of
carbonate minerals in the oceans, thus pulling more carbon dioxide out of the
atmosphere.
If the temperature of the Earth cools - the CO2 cycle slows the formation of carbonate
minerals 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 far
too slowly to correct for any short-term changes (the feedback mechanism takes about
400,000 years).
How did Earth’s climate change over the time?
Ice ages = mild, short-term cooling periods
during the Earth’s (global average
temperature drops by a few degrees)
During the past few million years, ice ages
were MOST LIKELY caused by small
changes in the Earth’s axis tilt.
On long time scale - the sun brightening and
the migration of continents influenced the
climate
On shorter time scale – small, cyclical
changes in earth’s rotation and orbit –
making season more or less extreme
Greater tilt = more extreme seasons, warmer
summers and cooler winters,prevents ice
from 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 the
atmosphere from volcanism
The end of the last snowball Earth phase roughly coincides with the beginning of the Cambrian
period 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