Lonely Planets

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by David Grinspoon


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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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