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science audience. Since we are always having to explain our jargon to
each other, I think that planetary scientists are on average better com-
municators and educators than scientists from most fields.
Planetary science is a field born from our realized dreams of explor-
ing new worlds. In just a few decades, the planets have magically meta-
morphosed from distant objects glimpsed through telescopes into
places we’ve actually visited. We now know enough about our sibling
worlds to do meaningful comparisons of planetary functioning and
evolution. We are placing the Earth and its life in a cosmic context that
Copernicus and Galileo could scarcely have imagined. We can begin to
assess the potential of our universe to create other habitats for life.
C O M P A R A T I V E P L A N E T O L O G Y 1 0 1 : T H E R U L E S
Is our planet normal or some kind of freak of nature? We desperately
need context for the Earth story. The first thing to do is visit the neigh-
bors and see what they’re like. So what do we know about the lives of
the planets? What made them what they are today? Were their fates
preordained, bred in the bone? Or was it experience and happenstance
that gave each its own unique character? Through comparative plane-
tology we’re seeking an understanding of the similarities and differ-
ences among worlds.
What patterns emerge from close study of the busy mess of the solar
system? We want to explain why planets are the way they are, based on
“first principles”: on simple rules and conditions of birth. At first
glance, this deterministic goal is not unlike the goal of astrology—to
predict your personality from planetary positions at the time of your
birth.
I doubt that the other babies born at the Newton/Wellesley Hospital
on winter solstice 1959 all grew up to be just like me. But can a planet’s
position at birth determine its own personality and fate? It’s easier to
see how there might be a connection. In fact we do find that two vari-
ables, each set at birth, have a huge influence on planetary destiny.
These two qualities are size and location (distance from its star).
Size controls both gravity and internal heat. The larger a planet is,
the more easily it can hang on to both the matter and energy it was
born with.
The story of planets is largely the story of their “thermal evolution”:
how they acquire, store, and lose heat. All planets emerge hot from the
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oven of accretion. From the start, they are balls of excess energy left over
from the violent collisions that assembled them. It’s all downhill from
there. Like steaming, freshly poured bowls of porridge they slowly
cooled off at different rates, depending on their size. This is true about
anything that is heating or cooling—smaller things exchange heat with
their surroundings faster than big things. On a cold winter walk from
the café back to my office, a small coffee cools off faster than a grande.
The same is true for steaming young planets: larger servings cool
more slowly.* A larger planet shields its own interior from the cold of
space and so hangs on to its “energy of accretion,” its initial heat of
birth, longer than a small one. The bigger the planet, the longer it stays
hot inside.
The interior of the Earth is a giant heat-engine. As the heat locked
inside finds its way out through convective churning, it creates new
crust, sucks old crust back into the cauldron, and pulls the continents
around, building mountains and driving earthquakes and volcanoes.†
The level and type of geologic activity on Earth is a direct manifestation
of the amount of heat bubbling up from the interior. You might expect
a smaller planet with a lesser flow of heat to be a less happening place
overall. Indeed, that is what we have found.
As we look around the solar system, we see a clear relationship
between planetary size and surface age. The bigger worlds are hot and
vigorous inside, and this is reflected in surfaces that are more recently
active. Thus larger planets have younger surfaces.
The Moon is relatively tiny compared with Earth (2,200 miles in
diameter versus 7,900 for Earth), and not surprisingly, it is cold inside
and has been for billions of years. No plate tectonics or active volca-
noes on the Moon.
Mercury has a diameter of 3,000 miles, larger than the Moon but
still quite small as planets go. In appearance it is quite lunarlike—an
ancient, cracked, cratered orb. If it ever had vigorous surface activity
like Earth’s it was only for a brief time in its molten infancy.
How do we know? You can tell the age of a planetary surface by count-
ing craters: older, inactive surfaces are more pockmarked by the stray
falling rocks of space. A surface full of craters is the signature of a long-
dead world. Conversely, if you see no craters, that means the surface is
*This is also the reason why babies need more protection from the cold than linebackers.
†This is the interconnected global system we call plate tectonics.
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young, and some kind of recent activity (volcanoes, erosion, earthquakes,
etc.) has refreshed its appearance and wiped away the craters.
Mars is bigger than the Moon (diameter, 4,200 miles) but still much
smaller than Earth. Mars shows signs of a much longer geological life
than the Moon. In keeping with its larger size, after birth it remained
hot for much longer. Mars, however, is long past its prime. Whatever
vigorous geological activity it had is now a distant memory. The signs
of this illustrious past are scattered about the surface, being slowly
dented with craters, buried in dust, and scoured by the winds.
Venus and Earth are remarkably similar in size. Venus, at 7,500 miles
in diameter, weighs in as the slightly smaller twin. Until recently we
didn’t have much of a clue about the age of Venus’s surface. Now, with
Magellan images, we have counted every crater on Venus and learned
that the average age of the surface is less than 1 billion years old, mak-
ing it, next to Earth, the youngest place around. Nowhere on Venus do
you see signs of the ancient heavy bombardment that saturated large
areas of Mercury, the Moon, and Mars with craters. Venus alone stands
with Earth as having erased all signs of this traumatic past with a long
life of more varied, more recent experience.
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When you ask of a planet, “How hot is it there?” it’s not the interior
heat that concerns you, but the temperature you would experience
standing on the surface—the climate. Planets need atmospheres to keep
warm, and bigger planets generally have thicker atmospheres for two
reasons. One is a direct result of the geologic forces I’ve been dis-
cussing. Geological activ
ity doesn’t just push rocks around and make
mountains. It also gushes gases into the air. The more volcanically
active a planet, the faster it supplies itself with new air.
The other way that large planets maintain atmosphere is through
sheer brute force. Big worlds possess stronger gravity, which helps them
hang on to their atmospheres over the long haul. The gradual loss of
gas to space will always doom small planets to an airless existence.
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For both these reasons (larger planets have more active volcanism,
which supplies new air, smaller planets lose air more easily) we expect
bigger worlds to have thicker atmospheres. What we’ve seen on our
planet treks largely conforms to this. Mercury and the Moon have no
atmosphere. Mars has a wimpy one—only about one-hundredth as
thick as Earth’s, so that the surface pressure on Mars is equal to that at
an altitude of about 130,000 feet on Earth.
Going just by this logic, however, we would expect the atmosphere
on Venus to be slightly thinner than Earth’s due to its slightly smaller
size. That is most definitely not what we find there. Instead, Venus has
the thickest, heaviest atmosphere of any rocky planet around. This
shows us that there is more to planetary character than just the size-
dependent effects of gravity and internal cooling. Clearly, with the thick
atmosphere of Venus we see the influence of something other than size
coming into play.
To make sense of this difference we need to consider the role of loca-
tion—the second major factor controlling planetary characteristics.
L O C A T I O N , L O C A T I O N , L O C A T I O N
A planet’s distance from the Sun (or whatever star it happens to be
near) plays a key role in its birth, life, and death. Like people milling
about a campfire on a winter night, planets closer to the Sun are hotter
and farther ones are colder. The average surface temperatures on
Venus, Earth, and Mars, in degrees Fahrenheit, are 864, 59, and –67.
No surprises at first glance: temperature falls with distance from the
Sun. However, when you look at the data in more detail, it is not quite
that simple. The difference in temperature between Venus and Earth is
much greater than you can explain by their differing positions with
respect to the solar heat lamp. Something else is going on. It is the
dense air of Venus that keeps its surface so intensely hot. But it takes
more than just a massive atmosphere to keep a planet warm. The air
has to be made of the right mix of gases.
An airless planet absorbs the warmth of sunlight and sends it right
back into space as infrared (IR) radiation. Planets with atmospheres try
the same trick, but something gets in the way. Certain atmospheric
gases act as selective filters that, like bouncers at a snotty club, let the
svelte little visible rays of sunlight pass right on by, but block the heftier
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IR rays from passing through. These bouncers are the unfairly
maligned, infamous greenhouse gases.
What makes one molecule, such as CO2, a potent greenhouse gas,
while another, such as O2, is not? Certain molecules find light at
infrared (IR) frequencies irresistible, like music with a certain beat that
compels you to dance but leaves others cold. These molecules dance so
vigorously that the energy of the radiation is completely absorbed,
transformed into molecular vibrations. Other molecules, in the pres-
ence of the same infrared music, just sit there.
It’s all in the size and structure: how big and loose a molecule is deter-
mines how well it absorbs IR. A molecule with only two atoms trying to
absorb IR is like a rhythmically challenged person trying to dance to reg-
gae music. If you can’t shake your hips and swing your arms, then you
might as well have a straitjacket on. These uptight little molecules have
nothing to move, no “degrees of freedom” as we say in the biz. So, for
example, little diatomic (two-atom) molecules such as oxygen and nitro-
gen (O2 and N2) cannot absorb infrared. They may want to dance but
they’re not hip to the right vibrations, and the IR passes them by like the
little stiffs they are. Slightly larger molecules, with three or more atoms,
such as carbon dioxide, sulfur dioxide, methane, and water (CO2, SO2,
CH4 and H2O), have a lot more stuff to shake, and this makes them all
strong IR absorbers. Pass some IR through air that’s thick with these
willing dancers and it’s easy skankin’: they just bounce, bounce, bounce,
until the IR wave (or particle or whatever the hell it is) is history and its
energy is lost to the vibrations.
We have another word for all that bouncing about of molecules. We
call it heat. The more of the big, flippy-floppy molecules in an atmo-
sphere, the more they intercept the IR trying to escape into space. In
this way, the triatomic molecules (plus fat, wobbly methane) catch IR
and harvest its heat for the planet. Those gases that dominate Earth’s
atmosphere right now, oxygen and nitrogen, do not dance. With only
one bond each, O�O and N�N are stiffer than boards and wouldn’t
notice an infrared photon if it walked up and bonked ’em on the head.
The atmospheres of Venus and Mars, on the other hand, are made
almost entirely of CO2 and so, pound for pound, they are much better
at holding on to solar heat and warming their planets. In the case of
Mars’s wispy air, that doesn’t add up to much warming. But it keeps
Venus the hothouse of the solar system.
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As you know from the debate about global warming on Earth, when
you start to look closely at planetary climate, it gets a lot more compli-
cated and hard to predict. Gases don’t just absorb radiation, they also
make clouds. Clouds reflect sunlight into space, cooling a planet.
Clouds can also absorb infrared radiation, warming a planet. The bal-
ance of these effects, whether clouds have a net heating or cooling
effect, depends on such details as their altitude, composition, droplet
size, and extent of coverage. These things depend on the mix of gases
and atmospheric motions. Atmospheric motions depend on tempera-
ture differences, which are driven by radiation absorption in the air and
the clouds. It’s a tangled web.
Climate is complex. The reason we care so much about it is the close
relationship between climate and habitability. Life, as we know it any-
way, is based on temperature-sensitive chemical reactions, and so it can
only exist within a narrow range of climate conditions.
Astrobiologists talk about a “habitable zone,” a range of distance
from a star within which temperatures are moderate enough to main-
tain liquid water (widely taken to be the elixir of life) at the surface of
a planet. But, a planet’s climate is not determined by distance from
the sun alone. Location is cle
arly important, but any two planets at
the same distance from their sun need not have the same climate. The
boundaries of the habitable zone will shift inward and outward,
toward or away from the sun, depending on just what kind of planets
we are talking about.
We would like to have one theory that explains the climates of planets
in some predictable way, telling us which ones will be likely to support
life. Increasingly, however, it looks as though a lot depends on accidents
of birth. Size and location are both important, but we still cannot predict
a planet’s fate without knowing more details about its birth and later
life. In fact, when it comes to planets, the power of prediction may be
too much to hope for. We are challenged simply to reconstruct their his-
tories and explain how they got to be the way they are today.
R O L L I N G T H E D I C E
We generally assume that the inner, rocky planets all started life much
more alike than they are now. We do have some evidence that supports
this, but it is mostly indirect. Our belief in similar origins is largely
based on our unconfirmed theories about how the planets formed.
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Recall the story, described in chapter 5, of planets forming out of a
disk-shaped nebula around the young Sun. The pattern of temperature
variation through the original nebula led to a predictable distribution
of planet-forming materials. Rocky planets formed in the hot regions
near the Sun, and icy, gaseous planets formed farther out.
This is actually a gross simplification of a much more detailed theory
that uses chemical calculations to make precise predictions of what
each planet should be made of based on its distance from the Sun. This
theory, called equilibrium condensation, can predict exactly what met-
als, minerals, ices, and gases should be available to make planets at any
given temperature.* Then, using an educated guess about the tempera-
ture distribution of the preplanetary nebula, you can predict† exactly
what raw materials went into making planets at any position in the
solar system.
John Lewis invented this theory and worked out its implications for
planetary formation at MIT in the 1970s. Lewis became well-known
for this work while I was still in grade school. A decade later he was my
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