Lonely Planets
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doctoral thesis adviser at the University of Arizona.
When we started to understand more about the process by which
planets actually assembled themselves, it caused trouble for this theory.
It seems that planet formation is a much messier process than we once
believed. Faster computers have allowed more realistic simulations of
planet formation. We now believe that the final stage of planet making
was a time of anarchy in the solar system. The growing gravitational
power of all the young worlds caused them to throw each other
around, displacing orbits every which way. In a few million years, the
orderly pattern of chemical zones arranged with distance from the Sun,
as neatly predicted by theory, was smeared out in a wild rumpus of
giant collisions and orbital chaos. In the final stages of planetary
growth much of the beautiful order created by the laws of chemistry
got smooshed out of existence by the laws of orbital physics.
*If you make certain simplifying assumptions, you can basically calculate what molecules should be present, given a certain mix of elements, at any temperature. These are known as chemical equilibrium calculations.
†You may find it strange that we use the word predict to discuss events that happened a long time ago, before there was even an Earth. What we mean by predict in this context is
“show the logical necessity of this outcome based on prior conditions.” It may seem like a cheap psychic’s trick to predict that which has already occurred, but we can test this type of prediction by seeing what else that has not yet been observed is also predicted by the same theory, and looking for these signs to confirm or reject our theory.
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The dream of a universal theory to explain the nature of all the plan-
ets still eludes us. If such rules worked well enough, then, by under-
standing the formation of our own solar system, we would have the
tools to predict the structure and properties of other planetary systems,
from basic conditions at birth. This would allow us to say with some
confidence where life like ours might evolve. Yet, it may be that the
nature of planets is inherently resistant to such schemes, because the
planets in their formation did not follow simple rules.
Even so, when the planets were made, some larger order underlay the
chaos: the temperature-dependent segregation of materials does seem to
explain the basic, large-scale structure of our solar system. Perhaps it can’t
predict the detailed differences among the rocky planets any more than an
astrologer can tell me when I’ll win the lottery, but it does make sense of
the overall groupings of planets, the major architectural features of our
planetary system. We can explain why we have worlds of rock and metal
clustered near the Sun where these materials could stand the heat, and
why we find ice moons and gas giants roaming the more distant, frosty
regions of our system. This less ambitious application of equilibrium con-
densation remains the closest thing we have to a universal theory of plan-
etary formation. We won’t know how good our theories are until we get
to examine a number of other planetary systems in detail. If our current
theories are correct, then we would expect other planetary systems to con-
form at least to these more crude structural principles: little rock-worlds
orbiting near a star, giant gas-worlds farther out.
We used to think that the solar system was sort of like a chemistry
experiment. I always loved chemistry because, if you knew the original
conditions and the rules, then you could predict the outcome. Instead,
it seems that the solar system is more like a mix between a chem lab
and a game of craps, a chemistry experiment where certain steps of the
recipe were left up to chance, so that the amounts of certain ingredients
and the steps included in their physical handling were specified by
repeated rolls of the dice.
What we are doing with comparative planetology is examining the
final results of such an experiment, and trying to figure out how much
randomness was in the mix, and what structure we can discern beneath
the muddy waters of chance.
When discussing the nature and fates of worlds, then, we have to add
a third variable to the two, size and location, we have discussed. We
have to add the influence of luck.
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O D D B A L L
Earth’s peculiarly oxygenated atmosphere was made as pollution by
photosynthesizing life. Here, plants used sunlight to turn CO2 and
water into oxygen and organic food. Life took over the cycles of car-
bon, nitrogen, sulfur, and water that dominate the planet’s activities.
The signs of life are so clearly evident in the makeup of our atmosphere
and its differences from that of our CO2-dominated neighboring plan-
ets that it is obvious that they do not possess our kind of life. This
doesn’t mean, in itself, that these planets are lifeless. But, if we ask,
“Has life played the role on Venus and Mars that it has played on
Earth?” the answer, my friend, is blowing in the carbon dioxide wind:
an unambiguous no.
It is tempting to regard the comparative histories of Earth vs. its
neighbors as a controlled experiment showing the effects of life on a
planet, like a set of identical sterile petri dishes where living cells were
injected in only one. Especially in comparing the lives of Earth and
Venus, so similar in other respects, it sometimes seems like a case of
identical worlds, where life was added to one, and 4 billion years later,
voilà, you can observe the consequences.
If only it were that simple. The universe has not given us very many
nearby planets to study, and our expanded appreciation of the role of
accidental, large impacts and orbital chaos limits our ability to untan-
gle the complex web of causality shaping the lives of planets.
Now we’ve seen how size, location and luck affected the birth and
infancy of the worlds we know best, and sent them down their separate
paths. Venus and Mars are Earth’s only close siblings, but these three
have all have gone their separate ways. Perhaps if we can understand
the divergent life stories of these triplets then we can start to picture the
environments of Earth-like planets elsewhere in the galaxy, and to pon-
der the prospects for life in such places.
Venus and Mars
11
The Earth? Oh, the Earth will be gone in a few sec-
onds . . . I’m going to blow it up. It’s obstructing
Image unavailable for
my view of Venus.
electronic edition
—MARVIN THE MARTIAN
The cities a flood
And our love turns to rust
Image unavailable for
We’re beaten and blown by the wind
electronic edition
Trampled into dust
I’ll show you a place
High on a desert plain
Where the streets have no name
—U2, “WHERE THE STREETS
HAVE NO NAME”
H O L D I N G W A T E R
What combination of fate and circumstance left Venus a dry, scorched,
volcanic pressure cooker, Mars a frozen, windswept, barren desert, and
only Earth a warm, wet, living oasis?
Remember, Earth was born in steam. At the same time, baby Venus
and baby Mars arrived, also swaddled in thick, steamy air. They were
littermates, fresh rocky worlds coalescing out of a single swarm of tus-
sling planetesimals. Almost surely, they all began life somewhat water-
logged. How come only Earth has managed to remain so? Why did
Venus and Mars lose their water while Earth retained what seems to us
a healthy amount? The answer may lie in the different ways that each
responded to the hot, steamy birth experience.
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Recall how the young Earth had to fight to keep its water. When our
planet’s water was all puffed up in steam, it was helpless against the
various forces ripping, stripping, and blasting our atmosphere off into
space. Once the planet cooled enough, the water rained down into
oceans that were much more secure against these atmospheric assaults.
Young Venus and Mars suffered the same attacks, but each was ulti-
mately much less successful—Venus because of location, Mars because
of size—in holding off the falling rocks and solar breezes trying to steal
its water.
Because of its lower gravity, little Mars was defenseless during the
late stages of the primordial bombardment. This drubbing stripped
Mars of its earliest atmosphere. Mars had surface water during its early
history, and we know ice is locked beneath its surface today. But it
probably never had more than a small fraction of Earth’s original
bounty, due to the ease with which a small planet can lose its early
steam atmosphere.
And what of the fate of water on Venus? Our sunward sister does not
have the Martian excuse of being too small to hold water. Yet Venus,
although remarkably Earth-like in size, is seriously lacking in oceans or
even puddles. Blame it on location. Where Venus sits, sunlight is twice
as bright as on Earth. These twins may have started out nearly identi-
cal, but baby Venus got too much sun and became dehydrated. While
Earth was cooling and enjoying the first rains after eons of choking
steam, the water of Venus, heated by the nearby Sun, remained as
steam. In the longer steam phase on Venus, much more water was lost
to space. Eventually, rain came to Venus as well, and the remaining
water condensed out on the surface, but we don’t know how much
water was left at that time. All we know is that now she’s hardly got
any. Four and a half billion years of evolution have left Venus a thor-
oughly desiccated place. Given all the similarities between Earth and
Venus, and given the importance of water in determining so much of
Earth’s character and habitability, it is stunning that Earth today has
about one hundred thousand times as much water as Venus.
Venus must have had a fair amount of water left when the rains
finally came. There should have been oceans. There should have been
seas, lakes, and warm little ponds. What happened to this water? It got
hit by a runaway greenhouse. Again, location was key. The oceans of
Venus, doomed by her proximity to the Sun, met a fate that awaits our
own oceans in the distant future.
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What causes a runaway greenhouse? Picture a planet like Venus with
warm oceans, heated by twice the sunlight hitting Earth. Hot water
evaporates, adding more vapor to the air. Water vapor is one of those
floppy greenhouse molecules that absorbs infrared and helps hold in a
planet’s heat. The resulting greenhouse effect heats the surface, which
evaporates more water, and so on. It’s a positive feedback, a runaway
train, and it will just keep getting hotter until much of the water is
steam again. Then it is, once again, vulnerable to being lost to space. In
the upper atmosphere, water is broken apart by solar ultraviolet, split
into hydrogen and oxygen. Liberated from their clunky oxygen ballast,
the fast and loose hydrogen atoms stream off into space. This, we
believe, was the fate of most of Venus’s remaining water.*
We don’t yet know how long the oceans of Venus lasted. Our best
models point back to hot oceans that persisted for hundreds of millions
of years. It could have been billions. For a large portion of its history,
our neighbor planet may have looked very much as portrayed in Lucky
Starr and the Oceans of Venus.
One vital question that we can answer only through further plane-
tary exploration is “Who kept its ocean longer, Venus or Mars?” We’ll
know the answers for Mars long before we do for Venus. Mars is easier
to explore because the conditions there are less brutal on Earth-built
machines, and Mars shows its entire history on its face. It’s much
harder to find evidence for ancient bodies of water (or ancient any-
thing) on Venus, since big planets tend to eat their past. One conse-
quence of the more vigorous geological activity on larger planets is
that, above a certain size, they will always destroy their own rock trail
as thoroughly as a politician with a paper shredder. The oceans of
Venus may have been much larger and longer lasting than those of
Mars, but the more active geology of Venus has erased all obvious signs
of this watery time. One goal for future exploration of Venus is to
search for more subtle traces of the lost water.
During the long oceanic phase, Venus and Earth may have been
*“What about the oxygen?” you might ask. Good question. The flight of the hydrogen probably left Venus with an oxygen-rich atmosphere. We should keep this in mind as we study the atmospheres of planets around other stars, since oxygen is usually cited as a possible “sign of life” on another planet (see chapter 14). Eventually, some oxygen might also have escaped into space, but the rest probably reacted with surface rocks, oxidizing various minerals churned up from the interior of Venus by the vigorous geological activity of that hot, young world.
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nearly indistinguishable, only developing their individual quirks later in
life. If we don’t think that the origin of life on Earth was an unlikely
fluke, then Venus should have had life. We may never know how far
life evolved on Venus before she lost her oceans. When this catastrophic
change came, the last drops of water disappearing into vapor, did life
die out? Or did it adapt, as Earth life did repeatedly when faced with
major global environmental catastrophes? If Venusian life wanted to
survive the transition from warm oceans to hot CO2, it would have had
two choices: either find some new kind of metabolism, one that is not
based on carbon and water and can thrive at nine hundred degrees, or
migrate thirty miles up, into the cool clouds, and learn to love acid.*
V E N U S I N T E R R U P T U S
Modern planetary exploration rudely interrupted our age-old dreams
of an Earth-like Venus. The high surface temperature on Venus was the
first significant discovery ever made at another planet with a visiting
spacecraft. Venus, a metal-m
elting furnace with a corrosive atmosphere
and clouds so acidic they could etch glass, was declared off-limits to
life. The romance was over.
But (planets seem to be like this) every time we take a closer look, we
see new sides to the place. Our understanding of the environment on
Venus has again changed radically in recent years. In the 1990s we
were finally able, with Magellan’s radar eyes, to peer through the
clouds and map the entire surface in stunning detail. What we found
was a much more varied and vigorous world than we had expected—a
world where, as on Earth, the distant geological past has been con-
sumed by the roiling present. Venus may be drier than Phoenix in June,
but it’s not dead yet.†
A big surprise is that almost the whole surface seems to have formed
at roughly the same time. On most planets you can identify older areas
and younger areas from global maps of impact craters. For example, on
both Mars and the Moon you find ancient, heavily cratered highlands
and younger (but still unbelievably ancient) volcanic plains with many
fewer craters.
*I’ll return to the subject of possible cloud life on Venus in chapter 17.
†I’m not sure I believe it, but some of my friends claim that, in my first invited conference talk, as a grad student, I nervously blurted out “Phoenix” when I meant “Venus.”
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On Venus craters are spread randomly around the whole planet. It
looks as though almost the entire surface is the same age, and the total
number of craters tells you roughly what that age is. Venus is accumu-
lating a new crater about once every seven hundred thousand years.
Since about nine hundred craters are on the planet, that means the sur-
face is around 600 million years old.
We don’t know of any other planet with a surface that formed all at
once. What could have happened to suddenly wipe out all preexisting
terrain? It sounds disturbingly biblical. Was God practicing on Venus?
Is this catastrophic tale really the true story of Venus?
The evidence points to a period of massive volcanic flooding that
covered most of the planet under thick flows of lava a mere 700 million
years ago (give or take 100 million)—just last month in geologic time.
In a striking case of planetary amnesia, almost all surface memories of