ASTR 330: The Solar System
Announcements
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Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 14:
The Earth
Image: METEOSAT
“Mostly Harmless” –
The Hitch-Hikers Guide to the Galaxy, by Douglas Adams.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Earth As A Planet
• Most of the fields of human study, including all the humanities, social
sciences, geography, even biology and much of geology are
concerned with the thin layer of activity which occurs at the Earth’s
surface.
• In this we will examine the Earth from a whole-planet perspective,
including every layer from the core to the top of the atmosphere.
• There is far too much information to sum up what we know about the
Earth in a single lecture, so we will focus on processes which we want to
compare between the Earth and its hot and cold cousins: Mars and
Venus, especially:
• volcanism and geology
• impact cratering
• greenhouse warming in the atmosphere.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Major Concentric Zones of Earth
1. MAGNETOSPHERE: region of charged particles above the
atmosphere, from 200 km to 100,000 km above the surface.
2. ATMOSPHERE: gas layer, from surface to 200 km altitude.
3. HYDROSPHERE: or ocean, including ice caps, which covers 2/3 of
the surface.
4. CRUST: the solid surface of the Earth, about 10-30 km thick.
5. MANTLE: the solid but plastic rock layer extending to 2900 km below
the surface.
6. CORE: metal sphere, divided into outer (liquid) and inner (solid).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Important Earth Facts
• From the perspective of the planetary system, we need to take note
of the following:
HYDROSPHERE AND ATMOSPHERE:
• Earth is the only planet with liquid water on the surface.
• The Earth is partially obscured by clouds, the extent of which
varies considerably.
• Polar caps exist, which change their coverage seasonally.
• The atmosphere is 21% oxygen, a highly reactive gas not
abundant elsewhere in the planetary system.
• CO2 is a minor fraction, 1% of the Earth’s atmosphere.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Facts: contd.
SURFACE
• About 1/3 of the surface is covered by land, which has some high
mountain ranges (9 km). The seabed can be 11 km below sea
level.
• There is geologic activity: volcanoes, earthquakes, hot springs.
MAGNETOSPHERE
• Electrons and protons are trapped in magnetic fields surrounding
the Earth.
CORE
• Due to its high density, the core is probably metal.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Seismology as a probe of the interior
•
Seismic waves are very similar to sound waves, and are produced by
earthquakes.
• These low-frequency waves (several km
wavelength) occur in two types:
1. Compression (longitudinal) – can pass
through solids and liquids.
2. Shear (transverse) – transmitted by
solids only.
• Can be demonstrated with a spring.
•
Seismic waves can be detected with
seismometer.
Figure credit: Univ. Tenn. Knoxville
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Crust
• The crust is the layer of solid igneous rock between the mantle beneath
and the hydrosphere/atmosphere above.
• The ocean crust and continental crust are fundamentally different:
• Ocean crust (55% of crust):
• thin crust (6 km) composed mainly of denser basalts.
• young (200 Myr or less), somewhat analogous to lunar maria.
• formed by plate movements: where plates move apart magma rises
and solidifies, forming new crust.
• Continental crust (45%):
• mainly granites: also igneous but formed at depth and pressure.
• thickness is 20 km to 70 km.
• includes some sedimentary, metamorphic rocks.
• rocks can be much older than ocean crust: up to several Gyr.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Mantle
• Below the crust lies the mantle, which is composed of higher-density
materials. There are three distinct layers of the mantle:
Figure credit: Univ. Tenn. Knoxville
•
Upper Mantle (0-100 km depth): This is
attached to the crust, and the two move
together and are called the lithosphere.
Below the upper mantle is a plastic
boundary layer called the aesthenosphere.
•
Convective Mantle (100-700 km depth):
Heat is transported upwards from the core
by slow overturning motions. Same
composition as the upper mantle, but
different mechanical properties.
•
Lower Mantle (700-2900 km depth): solid,
no convection.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Core
• The mantle-core boundary marks a major change in composition: from
rock, to iron, with traces of nickel, sulfur and some other elements.
• The boundary occurs at a pressure of 1.3 million bars (1 bar is about the
pressure at sea level) and a temperature of 4500 K.
• Iron is liquid at these conditions: so the change in composition also
marks a transition from solid to liquid phase. Turbulent flows in this liquid
generate the Earth’s magnetic field, which can change over time.
• The core is 7000 km across: bigger than the whole of Mercury! And
accounts for 1/3 of the Earth’s mass.
• At 5200 km depth, the core itself changes from liquid to solid, due to the
massive pressure of 3.2 million bars.
• At the very center, the pressure reaches 4 million bars, and T=5000 K.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Earth-Moon Comparison
• It is useful to emphasize here that most of the ‘action’ on the Earth
occurs after the Moon had more or less reached its final form.
• On the Earth, the oldest rocks are from 3.8 Gyr ago, but much younger
than the lunar highlands. Indeed, the youngest lunar basins were formed
then!
• The mare volcanism on the Moon ceased about 3.2 Gyr ago, long before
the first single-celled life forms arose on Earth.
• At the time the last major lunar impact occurred (Copernicus), 1 Gyr ago,
dinosaurs had not yet walked the Earth!
• Indeed the dinosaurs could have seen the Tycho impact on the Moon,
100 million years ago, long before mammals dominated the Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volcanism
•
Volcanism (named after the Roman god of the forge, Vulcan) occurs
when heat within the mantle drives magma through the lithosphere to
the surface, where it may erupt in several ways.
1. Lava plains: vast outflows similar to the lunar maria.
2. A volcano, i.e. a volcanic mountain, which may be either:
i. A shield volcano: a gently-sloping mountain caused by repeat
outflow of fluid lava from a vent, or
ii. A composite volcano: a steep-sided, conical mountain,
caused by the eruption of more viscous lava.
•
•
Geologists have many more types!
Let’s look at some examples.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lava Plains
• Lava plains are vast outflows similar to the lunar maria. Examples on
Earth are the Columbia River Basalts and Deccan plains of India. Both
occurred about 60 million years ago, much younger than the maria.
Figure and picture: North Dakota State Univ; Thor Thordarson
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Shield Volcanoes
• Mauna Loa in Hawaii is the largest volcano
on the planet, rising 9 km from the sea bed.
• At the top is a large, shallow crater called a
caldera, produced by subsidence. Typical
slope is about 10°.
Pictures: R.W. Decker, J.D. Griggs (USGS)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Composite Volcanoes
• Also called stratovolcanoes, after the strata or layers of material which
are built up by successive eruptions, which can be very destructive (e.g.
Vesuvius destroyed Pompeii and Herculaneum in AD 79).
• Examples include Mt St Helens, Mt Fuji, Mt Rainier. Mt. Shishaldin
(below right) is the largest volcano in the Aleutian Islands of Alaska, and
very active.
Figure credit: Michael Ritter Image: R. McGimsey
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Other Types Of Geologic Activity
•
Other important processes on the Earth which may change the
landscape and eradicate the geologic record include:
1. Erosion: the mechanical process of wearing down the landscape,
by glaciers, wind, rain, etc. This tends to reduce the height of
mountain ranges over time, and carve features such as the Grand
Canyon of Arizona, or the fjords of Norway.
2. Weathering: the chemical destruction of the landscape, by acid
rain, dissolution, etc.
3. Sedimentation: particles which are transported by erosion (e.g.
river silts) eventually settle out (e.g. river deltas) and begin to form
layered deposits. This is the process by which sedimentary rocks
ultimately are formed.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
What About Impacts?
• The geological record of the Earth is conspicuously lacking in large
impact craters, such as we see on the Moon and Mercury.
• We might think that the atmosphere has some part to play in protecting
us. In fact, we are only protected from objects with masses less than
50,000 tons, but anything bigger will not be substantially slowed.
• Until recently, we did not worry much about large impacts on the Earth,
partly due to the lack of crater evidence.
• On the Earth, the geologic processes we have just described, not to
mention the growth of vegetation, quickly mask impact craters.
• Only now we have planes and spacecraft can we spot the blurred
outlines of ancient impacts from above.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tunguska
• The largest impact of recent times was the Tunguska event in Siberia,
on June 30th, 1908. Witnessed by very few people, over 1000 km2 of
forest was flattened, and hundreds of animals died.
• The diameter of the stony projectile was probably about 60m, about the
size of an office block. The explosive force was equal to 10-20 megatons,
the same as a modern hydrogen bomb.
• It has been estimated that the same
size of explosion over a European city
of the time could have killed 500,000
people, and burned a city to the ground.
Figure credit: Dennis J Ramsey, tmeg.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Chicxulub
• The 180-km Chicxulub
impact crater of the Yucatan
peninsula of Mexico is
believed to be the site of the
event which caused the
massive K-T extinction 65 Myr
ago, including the dinosaurs.
• The impact is believed to
have been a roughly 10-km
size asteroid, with a blast of
100 million megatons (5
million H-bombs).
• The blast deposited a global
layer of the rare metal iridium.
Gravity Map: Alan Hildebrand, Geological Survey of Canada
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Continental Drift
• Until the 1960s, geologists mostly believed that the continents had
remained in the same positions since the Earth formed.
• The first serious challenge to this notion came from Alfred Wegener
(1880-1930) who noticed reports of identical fossils found on either side of
oceans: such as Africa and South America.
• Wegener’s 1915 book postulated
that 300 million years ago, the
continents had all been joined
together in a super-continent, which
he called Pangaea.
• However his theory did not gain
general acceptance at the time: he did
not know the mechanism by which the
drifts occurred, and his rates were
much too fast.
Photo: UCMP Berkeley
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Plate Tectonics
• The mechanism for continental drift is now understood by the theory of
plate tectonics.
• We find that the Earth is divided into six major and ten smaller sections
(plates).
• The plates are the
lithosphere: they
float on the
aesthenosphere.
• Convection
currents in the
mantle force the
plates apart: new
material forms in
the gap or rift.
Figure credit: Michael Pidwirny, Okanagan Univ. Coll.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tectonic Processes
•
Plate movements give rise to four types of boundaries:
1. Rift Zone: this is an area where two plates are moving apart, such
as the mid-Atlantic ridge.
2. Subduction Zone: occurs where one plate slides under another
(typically the ocean crust goes under the continental crust), e.g. the
deep trenches off the coast of Asia.
3. Fold Mountain Range: e.g. the Himalayas. Occurs where two
plates collide and neither one goes under, typically two continental
plates.
4. Fault line: where two plates scrape against each other in a parallel
direction, e.g. the San Andreas fault of California.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary of Tectonic Processes
• This figure shows most of the tectonic processes.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lifetime of Oceanic Crust
• There are 60,000 km of active rift zones in the ocean, where new
basaltic crust is forming by outpouring magma.
• The average rate of spreading is 2-3 cm/year (Wegener guessed 100
cm/year, much too fast).
• By multiplication we find the about 2 km2 of new crust is created each
year.
• As the ocean has about 260 million km2 of crust, we can divide to find
that the lifetime of a piece of ocean crust is about 100 million years.
• Oceanic crust is destroyed at subduction zones, where is it pulled under
the continental crust, and eventually melts at 200-300 km depth.
Volcanoes can form along this line, e.g. Pacific ‘arc of fire’.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
A Super-Continent
• Wegener noticed that the shapes of the continents fit together like a
giant jigsaw puzzle. North America to Eurasia, South America to Africa.
•The original Pangaea super-continent broke apart 200 Myr ago in the
Jurassic period, into two parts: Gondwanaland and Laurasia.
Diagram: wikipedia Animation: UCMP Berkeley
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Hydrosphere
• The term hydrosphere means all the water on the surface of the Earth,
including freshwater. However, most of the mass is salt water.
• The salt in the ocean is mostly sodium chloride (NaCl), but there are
other salts. The six elements Na, Mg, Cl, Ca, S, K make up 90% of salts.
• Calcium provides a lesson about cycles. 108 tons of Ca enter the oceans
every year, yet there are only 1014 tons in the ocean, a million years worth
of inflow. What has happened to all the Ca?
• The Ca combines with CO2 and O2 to form CaCO3: limestone rock.
• Oxygen and carbon dioxide are dissolved in the oceans as well as
present in the air; 60 times more CO2 in the oceans than atmosphere!
• The average temperature of the oceans is just 4°C, although the surface
temperature can vary from 30°C to less than zero. Far below the surface,
the temperature is almost constant.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ocean Currents
• Warm surface waters and cooler sub-surface waters form a giant
circulation pattern of current, transporting nutrients and salts round the
world.
• In this figure,
warm near-surface
currents are in red,
while colder deep
currents are in blue
Figure credit: Michael Pidwirny, Okanagan Univ. Coll.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Composition of the Atmosphere
Nitrogen
N2
78.08%
Oxygen
O2
20.95%
*Water
H2O
0 to 4%
Argon
Ar
0.9300%
CO2
0.0360%
Ne
He
1.8000E-05
5.0000E-06
*Methane
CH4
1.7000E-06
Hydrogen
H2
5.0000E-07
N2O
3.0000E-07
O3
4.0000E-08
*Carbon Dioxide
Neon
Helium
*Nitrous Oxide
*Ozone
Table: Michael Pidwirny, Okanagan Univ. Coll.
• The atmosphere is mostly
composed of two gases:
nitrogen and oxygen.
• Water, CO2, CH4 and the
other gases marked with a
‘*’ are quite variable.
• Note that the atmospheric
pressure, or weight, at sea
level is 1 bar, the same
weight as a 10-m thick layer
of water. Pressure
decreases as we ascend a
mountain or in a plane.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Production and Loss of Gases
• Nitrogen and argon are fairly un-reactive, and because they cannot
escape, they tend to accumulate over geologic time.
• In contrast, oxygen and carbon dioxide are highly reactive chemically,
so their current abundances reflect a balance between production and
loss mechanisms.
• For example, CO2 is absorbed by plants, which release O2 after
photosynthesis. When plant material is burned or consumed as food,
CO2 is released again.
• We also note that if the all the water in the oceans was to evaporate,
then we would have an atmosphere of 300 bars of water vapor. Water
would be the primary constituent of the atmosphere.
• In that case, all the oceanic CO2 would also be released into the air.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Carbon Cycle
Figure credit: Michael Pidwirny, Okanagan Univ. Coll.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Biological Effects On Atmosphere
• In the absence of life, we would expect the Earth’s atmosphere to be
mainly carbon dioxide, nitrogen and argon: like Mars and Venus.
• CO2 would the main atmospheric constituent, with a surface pressure
of several tens of bars.
• Instead we find much more O2, due to life. This transition must have
taken place in the past, an estimated 2 billion years ago.
• At this time, plants were able to remove much of the carbon from the air
and lock it up in sediments. The atmosphere became oxidizing.
• We believe that life could not have arisen in a reactive oxidizing
environment, but once life had formed, it was able to adapt to the
changing conditions, and evolve the metabolism of modern animals,
using oxygen as a fuel.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lower Atmosphere: Troposphere
• The atmospheric temperature initially decreases as we ascend, until
about 11 km altitude.
• This region (0-11 km), called the troposphere (or sphere of ‘turning’), is
dominated by convection.
• Surface air is heated by the Sun, becomes warmer and less dense.
• Due to buoyancy, the warm air rises, while denser cooler air descends
elsewhere.
• Convection maintains a lapse rate (temperature drop-off rate) of about 6
degrees Celsius per kilometer.
• All our weather conditions and most of the mass is contained in the
troposphere.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric
Structure
• Above the troposphere,
temperature reaches a
minimum at the tropopause,
and then ascends through
the stratosphere.
• The warming is due to
ozone (O3) which reaches a
maximum between near 50
km altitude, the stratopause.
• The stratosphere is thin
and dry compared to the
lower atmosphere.
Figure credit: Michael Pidwirny, Okanagan Univ. Coll.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Stratosphere and Ozone
• Ozone absorbs solar UV radiation which would otherwise be harmful to
surface life.
• The energy is re-radiated in the infrared as heat, creating the
stratospheric warming.
• The ozone layer was being damaged by man-made gases called CFCs
– chlorofluorocarbons, used in refrigeration and packing materials.
• Although harmless to humans, CFCs are very long-lived (100s of years),
and make their way to the stratosphere where they attack ozone.
• This effect was discovered by researchers who noticed a ‘hole’
appearing in the ozone concentration annually over Antarctica.
• CFCs have now been phased out, and will slowly break down in the
atmosphere.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ozone Hole
• This figure shows
the record-breaking
ozone hole over
Antarctica of
Sepember 8th, 2000:
28.3 million km2.
• The ‘hole’ covers an
area three times
larger than the
continental US.
• The hole has now
stabilized and does
not seem to be
growing further.
Figure credit: Richard McPeters, NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mesosphere and Thermosphere
• The mesosphere (50-80
km altitude) is a region of
decreasing temperatures, as
the ozone levels fall.
• The temperature reaches a
minimum at the mesopause,
80 km and 190-180 K.
• The temperature then
gradually rises again due to
absorption of solar radiation
by the small number of O2
molecules. The temperature
eventually reaches 1000 K,
although the molecules are
now far apart.
Figure credit: NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary-Quiz (contd)
1. What the three major layers of the solid Earth?
2. What are the two type of seismic waves, and which can travel
through the core?
3. What the main differences between the continental and oceanic
crusts?
4. Describe the three layers of the mantle.
5. What are the main types of volcanic outflows? How does viscosity
determine which one occurs? Give an example of each type.
6. Describe n major impact event on the Earth. Why does the Earth
show less impact record than the Moon and Mercury?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary-Quiz (contd)
7. What was the evidence for continental drift, and what were the
problems?
8. How was continental drift finally explained?
9. What types of boundaries can occur when plates meet?
10. What is the hydrosphere, and why does calcium not accumulate
further?
11. What are the main variable constituents in the atmosphere?
12. What sort of atmosphere would we have now without plant life?
13. Describe the temperature structure of the atmosphere.
14. Why do we have an ozone hole?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Sunlight At The Earth
• In an earlier lecture we calculated the amount of sunlight which
intercepts a square meter of area at the Earth’s distance from the Sun:
about 1420 W/m2.
• Of course, the Earth is not a flat wall facing the Sun, it is a ball, and
hence only the center of the dayside receives this amount: other parts of
the dayside receive less. The night side receives none!
• About 30% of sunlight reaching the Earth is reflected, either by clouds,
or by the surface (ice caps, snow etc).
• The other 70% is absorbed in the atmosphere or at the surface.
• The average amount of energy absorbed by the Earth, averaged over
all latitudes is 240 W/m2: 100 billion megawatts for the whole planet!
Dr Conor Nixon Fall 2006
ASTR 330: The
Solar System
Figure credit: NASA LARC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Greenhouse Effect
• The atmosphere is mostly
transparent to visible sunlight.
• When sunlight strikes the
surface, it is absorbed, and reemitted again as heat: infrared
radiation.
• However, the atmosphere is
much more opaque to IR light:
it can’t very easily escape. It
tends to get trapped in the
atmosphere.
• The planet warms up more
than it would have without an
atmosphere: this is called the
‘greenhouse effect’.
Figure credit: IPCC 1990
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Greenhouse Gases: Effectiveness
• The greenhouse effect (you can see the analogy to a gardener’s
greenhouse) warms the planet up by about 25°C, making the difference
between a chilly -15°C average temperature, and our actual +10°C !
• Without the greenhouse effect, we’d be in a major ice-age, there would
probably never have been seas, and life may not have arisen.
Carbon Dioxide
Global
Warming
Potential
Symbol (CO2-e)
CO
1
Methane
CH
21
10
Nitrous Oxide
NO
206
150
Chlorofluorocarbons
CFCs
130-650
12,400-15,800
Hydrofluorocarbons
HFCs
3,000-5,000
15-40
Sulphur Hexafluoride
SF
23,900
-
Greenhouse
Gases
2
4
2
Table: seda.nsw.gov.au
6
Atmospheric
Lifetime
(years)
50-200
• Different gases have
different greenhouse
effectivenesses: i.e.
how much they
contribute to warming.
• Another factor is the
lifetime of these gases
in the atmosphere: how
long they stick around
after they are released.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Carbon Dioxide Emissions
• CO2 is the most important greenhouse gas: even at 0.04% of the
atmosphere it has a huge effect.
• Since the industrial revolution of the 19th century, we have been burning
fossil fuels (oil, gas, coal) at ever increasing rates, pumping more and
more CO2 into the atmosphere.
• We estimate that this has already caused global temperatures to rise
by 2°C: and temperatures are expected to rise several more degrees by
the middle of the century.
• If we do not curb our emissions, the ice caps may melt, and eventually
many coastal cities may be flooded.
• Not all scientists agree on the amount of global warming, and whether it
is an immediate threat or not.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Greenhouse Effect
• This graph shows the increasing emissions of CO2 to the atmosphere,
from various sources.
Graph: Woods Hole Oceanographic Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Greenhouse Gases:
Changes Over Time
Greenhouse
Gas
Carbon
Dioxide
Methane
Nitrous
Oxide
Chlorofluoro
carbons
(CFCs)
Ozone
Concentration Concentration
1750
280 ppm
0.70 ppm
280 ppb
0
Unknown
Percent Change Natural and Anthropogenic Sources
1995
360 ppm
1.70 ppm
310 ppb
900 ppt
Varies with
latitude and
altitude in the
atmosphere
29%
Organic decay; Forest fires; Volcanoes; Burning fossil
fuels; Deforestation; Land-use change
143%
Wetlands; Organic decay; Termites; Natural gas & oil
extraction; Biomass burning; Rice cultivation; Cattle;
Refuse landfills
11%
Forests; Grasslands; Oceans; Soils; Soil cultivation;
Fertilizers; Biomass burning; Burning of fossil fuels
Not Applicable
Refrigerators; Aerosol spray propellants; Cleaning
solvents
Global levels have:
decreased in the
stratosphere;
Created naturally by the action of sunlight on molecular
increased near the oxygen and artificially through photochemical smog
Earth's surface production
Table: Michael Pidwirny, Okanagan Univ. Coll.
Dr Conor Nixon Fall 2006