goldiloddddddddddddddddddddddddddddcks.pdf

I’ve become acutely aware of
climate since moving to Louisiana.
The summers here are steamy and the
winters fickle. Because Louisiana
houses often lack sufficient in-
sulation (due to some combination of
poverty and optimism) thermostats
work hard to moderate the
temperature, running either the air
conditioner or the heater con-
stantly—sometimes both in the same
day. Of course, compared to other
locales, such as Alaska or the Sahara,
Louisiana has a very mild climate.
Compared to other cosmic
locales, the Earth itself has a very
hospitable climate. Our neighboring
planets, Venus and Mars, range the
extremes in temperatures: the surface
of Venus, at a temperature of 900º F,
is hot enough to melt lead, while
during Martian nights the tempera-
ture drops to 220º F below zero.
Earth’s temperature is comfortably
in-between: “just right,” like the
bowl of porridge that Goldilocks ate.
Thus planetary astronomers talk
about the “Goldilocks problem”: is
Earth’s favorable climate an ac-
cident, or maybe an act of God? Is
Venus just hot because it’s closer to
the Sun, and Mars cold because it’s
farther away?
The answer is no. The more you
study any science, such as astron-
omy, the more you find that the
universe is far from random. There
are very deep reasons for the
differences in climate among the
terrestrial, or Earth-like, planets (as
opposed to the Jupiter-like gas giants
in the outer solar system), and I mean
that literally. Beauty is only skin
deep, but climate is not.
There are three layers to the answer
to the Goldilocks problem. The first
layer is like insulation, which keeps
planets warm (and would keep my
townhouse warm if it had any): the
infamous greenhouse effect.
From all the debate about the
greenhouse effect, it’s easy to get the
impression that the greenhouse effect
is a great evil. But that’s not true. The
greenhouse effect makes Earth cozy.
Without the greenhouse effect the
Earth would be frozen, more than
60º F colder. I could use my cross-
country skis in Louisiana, but that
would be a high price to pay. The
great bugaboo that has everyone
worried, global warming, is actually
a very small enhancement to the
Earth’s overall greenhouse effect.
(We should worry about global
warming, but let me save that for
later.)
We receive energy from our Sun
in the form of light. The Sun is a ball
of very hot gas, heated by ther-
monuclear reactions at its core. All
hot objects emit electromagnetic ra-
diation, which includes radio waves,
visible light, and X-rays, as well as
ultraviolet and infrared radiation.
The kind of radiation emitted de-
pends on the temperature (the higher
the temperature, the shorter the
wavelength). For example, a piece of
hot metal glows different colors at
different temperatures. At “moder-
ate” temperatures, metal glows a dull
red; then, as it gets hotter, it will be
orange in color, and then a brilliant
yellow-white—about the same as the
surface of the Sun, roughly 9000º F.
If you could heat the metal even
hotter, it would glow a bluish white,
and eventually it would emit energy
primarily in the ultraviolet.
But all objects emit electro-
magnetic radiation, even relatively
cold objects like rocks and people.
Objects at room temperature emit
infrared radiation, which is how
some night-vision scopes see.
So the greenhouse effect works
like this: the Sun, being very hot,
emits visible light. The light from the
Sun passes through the Earth’s
atmosphere, which is transparent to
visible light (that’s why our eyes
evolved to be sensitive to this kind
of electromagnetic radiation), and
warms the surface of the Earth,
which in turn reradiates the energy,
now as infrared radiation, because
the Earth’s surface isn’t as hot as
the Sun.
Is Earth’s favorable
climate an accident?
Lesson 5: The Living Earth
Activity 1: The Greenhouse Effect
Goldilocks and the Three Planets 2 0
Goldilocks and the Three Planets
By CALVIN W. JOHNSON, Ph.D., Assistant Professor, Department of Physics, San Diego State University
Reprinted with the author’s permission from: http://physics.sdsu.edu/~johnson/writing/climate.html
View of Africa and Saudi Arabia from
Apollo 17. NASA

But the Earth’s atmosphere,
while transparent to visible light, is
not transparent to infrared light. So
the heat energy, in the form of
infrared radiation, stays trapped in
the Earth’s atmosphere, and the Earth
is much warmer than it would be
without the greenhouse effect.
Only a few types of gases in the
atmosphere, the greenhouse gases,
are opaque to infrared radiation. For
example, nitrogen and oxygen, the
major components of our atmos-
phere, are not greenhouse gases. The
most important greenhouse gases are
water vapor, methane, and carbon
dioxide (CO2). It’s the CO2 that
worries people when it comes to
global warming. Carbon dioxide
makes up only about 330 parts per
million of our atmosphere, but that
has nearly doubled since the be-
ginning of the industrial revolution
because of burning fossil fuels. The
whole debate about global warming
is exactly how much carbon dioxide
one has to add in order to change
world temperatures; but no one
disputes that, eventually, too much
carbon dioxide will increase the
temperature.
What about Venus and Mars?
Unlike Earth’s atmosphere, their
atmospheres are about 95% CO2. But
it’s not the percentage of CO2, it’s the
total amount in the atmosphere.
Venus has a very thick atmosphere: at
the surface, the atmospheric pressure
is ninety times that at Earth’s surface!
You’d have to dive nearly three
thousand feet into the ocean to feel
the same kind of pressure. So the
Venusian atmosphere is set into
overdrive as far as the greenhouse
effect is concerned, cooking its
surface. The Martian atmosphere, in
contrast, although nearly pure CO2, is
very thin: less than 1% the pressure
of Earth’s. Mars barely has any
greenhouse effect—it’s only warmed
about 10° F above what it would be
without an atmosphere. Earth, of
course, with a moderate greenhouse
effect, is “just right.”
So the first step in understanding
the climates of Venus, Earth, and
Mars, is the relative amounts of
greenhouse gases in their at-
mospheres: Venus has too much,
Mars too little, and Earth just the
right amount (in fact, water vapor,
which is one percent of our at-
mosphere, dominates most of our
greenhouse effect—but additional
CO2 can dramatically change the
amount of greenhouse effect).
The climates of the terrestrial
planets are different because of
dramatic differences in their at-
mospheric composition. But why do
Venus, Earth, and Mars have such
different atmospheres? In particular,
why are there such huge differences
in the amount of CO2 in their
atmospheres? To answer that, we
must look at something called the
carbon cycle. The carbon cycle acts
as a huge, planetary thermostat:
when working properly, it cools a
planet when it gets too warm, and
heats it up when it gets too cold.
On Earth, atmospheric CO2 is
absorbed by precipitation—by rain—
and forms a very weak solution of
carbonic acid, a very mild form of
acid rain. This acid rain falls on
surface rocks, many of which contain
calcium, and the carbonic acid
dissolves a tiny bit of the calcium.
Eventually the water, containing both
carbonic acid and calcium ions,
washes down to the ocean. In our
oceans tiny plants and animals,
plankton, incorporate the calcium
and carbonic acid into shells of
calcium carbonate. When the animals
die, their calcium carbonate exo-
skeletons drift to the ocean floor.
When enough of these carbonate
deposits build up, they form
carbonate rocks, such as limestone,
which are composed of the skeletons
of trillions of dead plankton. In short,
the action of water removes CO2
from the atmosphere and puts it into
the crust of the Earth. The Earth has
roughly the same amount of CO2 as
does Venus, but it is nearly all locked
up in the crust as carbonate
sediments.
(The fact that plankton play a
role in precipitating carbonates out of
water is used to bolster the so-called
Gaia hypothesis, the idea that life is
an integral part of Earth’s climate.
Other scientists dispute the necessity
of life to the carbon cycle, for even
without plankton, calcium carbonate
at sufficiently high concentration
would precipitate out of ocean
water.)
While most of the Earth’s CO2 is
locked in her crust, it doesn’t stay
there forever. The action of plate
tectonics, the motion of the Earth’s
surface, can subduct carbonate
Life may be an
integral part of
Earth’s climate.
2 1 Goldilocks and the Three Planets
Hemispheric view of Venus produced by
Magellan. NASA

sediments; that is, as chunks of the
Earth’s crust gets pushed together,
some of the rocks get pushed deeper
into the interior, where they are
subjected to heat and pressure. Such
heat and pressure initially changes
limestone to marble. But under even
greater heat and pressure, the CO2 is
released from the rock, and makes it
way back to the surface where it is
emitted into the atmosphere through
volcanic action. Hence volcanoes are
a source of CO2.
This is the complete carbon
cycle: rainwater removes CO2 from
the atmosphere and puts it in the
crust, and volcanic action releases
CO2 from the crust and puts it back in
the atmosphere.
What happens on Venus? Venus
has no water! Early in its history
Venus may have had water, but it is
too close to the Sun to retain it. When
water molecules rise high in an at-
mosphere, ultraviolet radiation splits
the water molecules into their
component gases, oxygen and hy-
drogen, and the lighter hydrogen
molecules escape into space. While
Earth’s lower atmosphere is about
one percent water vapor (although it
seems much higher in the humid
Louisiana summers), the upper at-
mosphere, where ultraviolet radiation
can penetrate, is very dry: a cold trap,
a combination of pressure and
temperature, prevents water vapor
from rising high in the Earth’s
atmosphere. Venus has a cold trap,
too, but because Venus is closer to
the Sun its cold trap is much higher
in the atmosphere and Venusian
water molecules rise high enough to
be broken apart by ultraviolet
radiation.
Therefore the carbon cycle is
incomplete on Venus: without water,
CO2 cannot be removed from the
atmosphere. Venus does have
volcanoes, however. Radar mappings
of Venus by interplanetary probes
indicate volcano-like mountains, and
there is other evidence for volcanoes
as well. The atmosphere of Venus is
full of sulfur dioxide and sulfur
particulates. Sulfur and sulfur
dioxide is highly reactive and cannot
remain long in an atmosphere;
therefore something (volcanoes)
must be regularly replenishing the
sulfur. This theory is bolstered by
data from interplanetary probes,
which have detected large fluctua-
tions in the sulfur content of the
Venusian atmosphere, as well as
radio signals reminiscent of
lightning—and lightning is often
found in volcanic plumes.
And Mars? The carbon cycle is
also broken on Mars, but opposite to
Venus. Mars has no active volcanoes
to replenish the CO2 in its at-
mosphere. We know Mars once had
running water—we can still see
billion-year-old river beds where
water once ran—and the water may
still be there, locked up in the ice
caps and in permafrost beneath the
surface. And it seems likely that
Mars has CO2 still locked up in its
crust, deposited there billions of
years ago by the action of water. If
you could release that CO2 you could
warm up Mars again. Indeed, this is a
major premise of science fiction
stories about terraforming Mars; an
excellent example is Kim Stanley
Robinson’s Mars trilogy.
A fully active carbon cycle acts
as a thermostat, regulating a planet’s
climate. In your thermostat at home,
two strips of dissimilar metals bend
one way or the other depending on
the temperature. If it gets cold in
your house, the metal strip bends one
way and switches on the heater; if it
gets warm, it bends in the opposite
direction and switches on the air
conditioning. The carbon cycle has
similar negative feedback. Suppose
the Earth gets too warm. Then more
water will evaporate from the oceans,
and the additional precipitation will
remove CO2 from the atmosphere,
moderating the greenhouse effect and
cooling the planet. If the planet cools
too much, less water will evaporate
and there will be less precipitation to
remove CO2; the CO2 will build up,
warming the planet.
This carbon cycle thermostat
helps to explain a mystery about
Earth’s long term climate. Computer
models of our Sun show that it has
gotten progressively brighter over its
five-billion-year lifetime, by about
twenty-five percent. Since a mere
two percent change in the Sun’s
luminosity would (all other things
being equal) plunge the Earth into a
deep ice age, one might expect the
surface to have only recently
defrosted. But two hundred million
years ago the Earth was in fact
warmer than it is today. So all things
were not equal. (For one thing,
continental drift affects climate; the
formation of deep oceans tends to
cool the planet, whereas shallow
oceans warm it.) The carbon cycle
explains how the Earth’s climate can
compensate for changes in the Sun’s
luminosity. It seems likely that the
Goldilocks and the Three Planets 2 2
Late spring on Mars (centered on roughly
305 degrees longitude). NASA

Earth’s atmosphere had somewhat
more CO2 half a billion years ago
than today; as the Sun slowly grew
brighter, the carbon cycle deposited
more CO2 in the crust, keeping the
temperature “just right.”
The carbon cycle is a crude and
slow thermostat, however. It takes
millions of years to compensate,
enough for slow changes in the Sun,
but not enough to immediately
correct for humans dumping tons of
extra CO2 every year. So don’t be
complacent about global warming—
the carbon cycle won’t protect us!
The story is not quite finished.
We’ve learned that Venus is too hot
because it has a runaway greenhouse
effect, caused by a broken carbon
cycle, from too little water; Mars is
too cold because its carbon cycle is
also broken, lacking active vol-
canoes, and therefore it has too small
a greenhouse effect. Earth is lucky,
with a fully functioning carbon cycle
and therefore a moderate and
moderated greenhouse effect.
But why do Earth and Venus
have active volcanoes, and Mars
none? We know that Mars once had
volcanoes. In fact Olympus Mons on
Mars is the largest volcano, albeit
extinct for billions of years, in the
solar system. You could fit the entire
state of Rhode Island in the caldera
of Olympus Mons. But no Martian
volcanoes are active today. To
understand why, we must delve deep
into the interior of the planets.
The interior of the Earth is hot.
Part of this heat is generated by the
natural decay of radioactive elements
in the rocks, and part of the heat is
left over from the formation of the
Earth five billion years ago—when
gravity pulled together bits of gas
and dust to form our planet. And
there are three ways a planet can lose
heat from its interior. The first is
simple conduction. If you stick a
poker into a fire and hang onto it,
heat will slowly travel up the poker
to your hand and burn you. This is
the main way small planets and
moons lose heat, for the smaller the
object the faster it cools. Imagine
baking potatoes of different sizes.
When you take them out, the bigger
one cools much slower than the small
one. (This is the physics error in the
original Goldilocks tale: you’d
expect Papa Bear’s porridge to be too
hot, but Baby’s would be too cool
and Mama’s, in between in size,
would be just right.) And so this is
what happened to Mars. With only
about one-eighth the mass of the
Earth, Mars is a small potato and
cooled so rapidly it lost its heat to
power volcanoes. Jupiter, on the
other hand, as the largest planet in the
solar system, is still immensely hot in
its gaseous interior: in fact Jupiter
radiates 1.7 times as much thermal
energy as it receives from the Sun.
There are two other ways to
transport heat from the interior of a
planet to its surface, both of which
can produce volcanoes. One is
convection. You can, with care,
produce convection on your stove
top. Take some soup—thick tomato
or some sort of cream soup might
work well—and heat it slowly. Don’t
bring it to a boil; but if you get the
temperature right, you will see soup
upwelling from the bottom. Hot soup
expands and becomes lighter, and
floats to the top, while cold material
on the top is denser and sinks,
forming a cyclic pattern.
Convection drives plate tec-
tonics: the source of continental drift,
building of mountain ranges, earth-
quakes, and some kinds of vol-
canoes. If you drive up interstate
highway 5 from California to British
Columbia, you’ll see a string of
volcanoes: Lassen, Shasta, Hood, St.
Helens, Rainier, Baker. Convective
patterns driven by heat transport in
the interior of the Earth push one
crustal plate under another (sub-
duction) and the friction heats up
the magma necessary for volcanic
action—which in turn releases CO2
into the atmosphere.
Not all volcanoes arise from
plate tectonics. Sometimes a little
“worm” of hot rock will force its way
up through the crust, forming what is
known as a shield volcano, with long,
gentle slopes. The Hawaiian Islands
are formed from shield volcanoes.
This process is called advection. The
volcanoes on Venus, and the ex-
volcanoes on Mars, were all formed
by advection.
Venus is the same size as Earth,
and so its interior stays hot enough to
support vulcanism. Exactly why
Venus lacks plate tectonics is not
clear. Perhaps its crust is too thick
and brittle; or perhaps water is
necessary to “lubricate” the action of
plate tectonics. But in contrast, Mars,
being so much smaller than Venus
and Earth, lost its interior heat too
quickly to support volcanoes for
long.
So now we can solve the
Goldilocks problem. Venus is too
close to the Sun to retain water,
which breaks its carbon cycle,
leading to an overabundance of
greenhouse gases and a well-cooked
surface. Mars is too small to have a
hot interior, and thus no volcanoes to
pump greenhouse gases into its
atmosphere, and so is cold due to
lack of insulation. We Earthlings are
lucky: our planet has water and
volcanoes and a regulated climate.
If only the summers in Louisiana
were a little more regulated....
Calvin W. Johnson is Assistant
Professor at San Diego State
University.
2 3 Goldilocks and the Three Planets

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