Comparative Planetology 101 The Rules

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 neighbors 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 differences 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 variables, 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 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 volcanoes 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 counting 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.

Image unavailable for electronic edition 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, making 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 discussing. Geological activity 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.

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 location—the second major factor controlling planetary characteristics.

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