RAISE YOUR hand if you've ever seen a TV show in which humans land on an alien planet, leap out of their spaceship, and immediately breathe the atmosphere with no apparent problem.

We can't see you, but we suspect that most everyone's hand is in the air. Media science fiction abounds with planets with atmosphere just like the one that surrounds our planet — which is convenient for both fictional space travelers and those responsible for creating sf TV shows. Consider how the faceplate of a protective suit might obscure the face of the lead actor thus depriving (or should we say sparing?) the audience of some memorable emoting (or should we say acting?). And what producer would want to spend special effects money on special costuming if it could be avoided? (If you doubt the importance of the role of budget in TV production, we suggest that you do an online search for "Kirk's Rock" and learn how many Star Trek episodes featured a particular rock formation in the Vasquez Rocks Natural Area Park of Los Angeles County.)

Unfortunately, reality isn't as convenient as fiction. In our solar system, Earth is the only planet with an atmosphere we can breathe. As Arnold Schwarzenegger demonstrates in the first movie version of Total Recall, a person without a spacesuit on the surface of Mars gasps for oxygen and begins to swell up as the liquids in his body start to boil. On the surface of Venus, that same unprotected person would be crushed. On the surface of Jupiter—well, actually, you'd have a hard time even finding the surface of Jupiter.

Many factors affect whether a planet has an atmosphere, how thick that atmosphere is, what gases compose it, which gases react with the surface, and how quickly those gases are escaping the planet.

At this particular point in time, Earth has a wonderfully breathable atmosphere. In this column, we will provide a service to pessimists, explaining how and why this fortunate situation won't last forever: the Earth's atmosphere is leaking off into space. As a service to would-be interplanetary travelers, we also offer information on the atmospheres of Mars, Venus, and Jupiter. And, as a service to those who harbor affection for shellfish, we will explain why clams are your friends.
 

About 4.5 billion years ago (give or take a hundred million years), volcanoes started belching out other gases: water vapor, carbon dioxide, ammonia, and more. As the Earth cooled, the water vapor condensed and some of the carbon dioxide dissolved in the liquid water.

Around 2.5 billion years back, life evolved. Simple organisms, known as cyanobacteria or blue-green algae, lived on energy from the sun and carbon dioxide from the water and produced oxygen as a waste product, which is very convenient for us.

You could think of this as the greatest pollution event in history: earthbound organisms produced exhaust gases that changed the atmosphere of the planet. The main exhaust gas, oxygen, was poisonous to the anaerobic bacteria of the planet. As oxygen levels in the atmosphere increased, thousands of organisms went extinct because they could not adapt to the changing conditions. (Even humans die when exposed to too much oxygen. Two atmospheres of pressure of pure oxygen is a toxic dose.) Evidence of the ancient cyanobacteria colonies that changed the Earth's atmosphere has been preserved in the form of stromatolites, the columnar structures that are among Earth's oldest fossils. These rocky structures were created in layers by mats of cyanobacteria.

The initial pollution of Earth's atmosphere with oxygen happened around two and a half billion years ago. It took another billion years (a period sometimes known as the "boring billion") for levels of oxygen to rise high enough to support the evolution of animals. The reason it took so long was that oxygen is a highly reactive gas. It quickly reacted with iron in surface rocks, turning them rusty red/brown. Only after the surface rocks had been oxidized could oxygen accumulate in the atmosphere.
 

The Earth's atmosphere sticks around the planet for the same reason you do. Gravity attracts you to the planet and it attracts air as well. After all, air has weight. Exactly how much a cubic foot of air weighs depends on its density, which depends in turn on its temperature and pressure.

If you are skeptical about the weight of air, Paul suggests you get the largest balloon you can — one at least a meter in diameter. Trying to feel the weight of air in the balloon by supporting it on your hand is useless because of the buoyancy of the surrounding air. But there is a way to experience the mass of the air-filled balloon, which correlates with its weight. Have a friend throw the air-filled balloon at you. The air in a one-meter-diameter balloon has a mass that is almost half a kilogram or about a pound, so you will definitely experience the mass of the air. (Of course, you should also do the control experiment: have your friend throw an empty balloon and feel the difference.)

Since air has weight, gravity pulls it toward the Earth. But we have a bit of bad news for anyone fond of breathing. Despite that gravitational attraction, our planetary atmosphere (and the atmosphere of any planet) is doomed to escape slowly into space. Don't panic yet — the escape of an atmosphere can take a long time. After 4.5 billion years, the Earth still has a substantial atmosphere. Hydrogen gas is currently escaping the Earth at a rate of 3 Kg per second, not much but it does add up after billions of years. Other gases do not escape as quickly.

To see why atmosphere leaves a planet, we'll begin by considering a rocky planet in space with no atmosphere — a planet that's a lot like our Moon. This planet is near a star so that starlight shines on its surface. The surface is dark gray like our Moon. (The Moon looks white in comparison with the much blacker background of space only because it's reflecting the bright light of the sun.) This rocky gray surface reflects a part of the light and absorbs the rest, turning the light's energy into heat that warms the rock.

If you've been reading our columns for a while, you know how an increase in temperature affects atoms. The temperature of the planet's rocky surface is related to how fast its atoms or molecules are vibrating. (Yes, even though the rock looks perfectly stationary, its atoms are bouncing and twisting all over the place. As a fantasy and sf reader, you'll have no trouble accepting that apparent contradiction. The higher the temperature, the greater the vibration.

Now let's drop a single gas atom onto the rocky surface from a height of a few centimeters — perhaps an atom of argon, since it doesn't react with much of anything. The argon atom falls under the gravitational pull of the planet until it hits the surface. When it hits the surface, there's a good chance it will receive a rude surprise — if it hits a vibrating atom, it'll get a "kick in the pants" that launches it back up into space at a fairly high speed. In fact if the surface of the rock were only 0°C, the argon could be launched upward at a speed of 275 meters per second — that's 615 mph, almost mach 1, the speed of sound on Earth.

Remember the famous line "what goes up must come down?" You probably know that in the space age that line must be amended to, "What goes up at less than escape velocity must come down." (Escape velocity, for those fantasy readers in the audience, is the minimum speed an object has to be traveling to continue to move away from the planet, rather than settling into orbit.) If escape velocity from this planet is greater than 275 m/s, then the gas atom will rise up from the rocky surface, slow down due to the gravitational pull, and fall back to the surface once more, where it will be batted upward again.

On this second impact, the atom could rebound faster or slower than its initial launch speed, depending on exactly how it hits the moving surface atom. If the speed of our lonely atom is greater than escape velocity, it will leave the surface and not return.

Of course with real atmospheres containing many gas atoms and molecules, things get a little more complicated. When you have many gas atoms, they receive different kicks when they collide with the vibrating atoms of the surface rock, and bounce away at different speeds. You can picture the surface of a planet as covered by jumping atomic fleas — always jumping, but never quite escaping their gravitational prison.

Even without the kick in the pants from the planet's surface, the atoms in the gas are traveling at many speeds. In 1866, James Clerk Maxwell figured out the distribution of speeds that gas atoms would have at a given temperature, known as the Maxwell distribution. Essentially a gas in contact with a surface at 0°C would itself achieve a temperature of 0°C. At this temperature, a few atoms of the gas would be traveling slowly, few would be traveling very rapidly, and many would be traveling near some in-between speed near 275 m/s.

So even if the most probable speed for a gas atom is 275 m/s, some atoms will have a much higher speed, perhaps greater than escape velocity. If these fastest atoms don't hit any other atoms as they leave the surface, then they will escape. The remaining atoms will continue to collide with the surface. In time, some of them will also achieve escape velocity. Eventually, all the gas atoms will leave.
 

That's what happens at the surface. Higher in the atmosphere, the pressure and density of the air decreases. At the summit of Mount Whitney — at almost 4500 meters (14,800 feet) above sea level — the atmospheric pressure and density is only half what it is at sea level. If you climb up from sea level in one day, you will feel like someone removed one of your lungs, because each breath contains only half the oxygen as at sea level.

Every increase in elevation of 5.6 km or so reduces the atmospheric pressure by half. At above 500 kilometers, when the density of the atmosphere has been cut in half ninety times, the atmosphere is so thin that a gas atom has only a fifty percent chance of colliding with one other molecule on its way to interplanetary space. This level, called the exosphere, is the level from which atoms can escape from the Earth's atmosphere.

You can think of it this way: Pretend you are standing in the middle of a large, ancient forest. The trees grow in a random pattern and there is no underbrush. Beyond the forest are open meadows. When you are standing in the middle of the forest, you can't see the meadows because a tree blocks every sightline. If you start running in a random direction in a straight line, you'll slam right into a tree. In this situation, you are like the atom trying to escape the Earth's surface and the trees are all the other atoms around you.

Now suppose you walk toward the edge of the forest. (Yes, now you can walk around the trees; take your time). Finally you see light from beyond the trees coming through between the trunks. Now if you start running in a straight line in a random direction there is a small chance you will make it out of the forest. At some point you approach so close to the edge that there is a fifty percent chance that you will exit the forest, and a fifty percent chance you will hit a tree. When you are at this point, your situation is like that of an atom at the base of the exosphere, a layer known as the exobase.
 

The Earth's atmosphere is made of a mixture of gases, including nitrogen, oxygen, argon, carbon dioxide, and water vapor, along with tiny amounts of helium, hydrogen, and other gases. You might think that the atoms of different gases would travel at the same speed if they were all at the same temperature—but that's not the case. They all have the same kinetic energy (the energy of motion), but the kinetic energy of a particle depends on both its speed and its mass.

Here Paul cites what he calls a "familiar equation." The equation is KE = 1/2mv², where KE stand for kinetic energy, m for mass, and v for velocity. If you are savvy when it comes to equations, you will immediately see that the velocity associated with a given amount of kinetic energy varies with the mass. So a heavier atom will be traveling slower than a lighter atom with the same kinetic energy.

Pat prefers to leave equations out of it and goes straight for a comparison. Push a toddler on a swing, and the kid goes soaring at an impressive speed with little effort on your part. Put a heavyweight sumo wrestler on the same swing and use the same effort. The wrestler will barely budge. The same energy makes the lighter particle (the toddler) move faster than the heavy one (the wrestler).

Whether you go by the equation or the comparison, it's clear that mass makes a difference. In the exosphere, the atoms with the lowest masses are traveling faster than more massive atoms—and are more likely to achieve escape velocity. Because they are the lightest atoms, hydrogen and helium atoms are the fastest moving and the first to leave an atmosphere. The most likely speed for hydrogen atoms (the lightest atom) is 5 km/s. The fastest hydrogen atoms in the Maxwell distribution at around 1,000°C exceed the escape velocity from Earth of 11.2 km/s.

The Earth loses its supply of helium continuously as this gas leaks away into space. Paul notes that this is why it is terrible that we release the helium we mine as a byproduct of natural gas production into the atmosphere where it will be diluted and lost. Users of helium—whether they are research physicists using liquid helium or children with party balloons—will pay more for our precious helium supplies.
 

Mars is right next to the cosmic bowling alley known as the asteroid belt. Asteroids hit Mars far more frequently than they hit Earth. Each asteroid strike not only excavates a crater with a radius ten times the diameter of the asteroid, it also blows off a chunk of atmosphere into space.

Shortly after the birth of the solar system, between 4.1 and 3.8 billion years ago, many large asteroids collided with the already formed planets. Scientists call this period the "Late Heavy Bombardment" or the lunar cataclysm. Traces of the craters made by this bombardment are obvious on our Moon, but the bombardment also affected Earth, Venus, and Mars.

In what seems like a cosmic case of injustice, the thinner a planet's atmosphere, the more atmosphere each asteroid strike ejects. The asteroid impact that brought about the demise of the dinosaurs blasted some of the Earth's atmosphere into space. A comparable impact on Mars would have ejected more than twice as much gas.

Scientists have calculated that when Mars was young, it would be hit by enough large impacts to remove its atmosphere completely in a mere 100 million years. The Late Heavy Bombardment lasted three times this long, so it's a mystery that Mars has any atmosphere at all. Scientists are not sure, but they suspect that Martian volcanoes replenished the atmosphere.
 

Why hasn't Venus lost its atmosphere? To start with, Venus has an escape velocity of 10 km/s, almost the same escape velocity as Earth. But the real reason is much weirder — and it is the reason we all owe a debt of gratitude to clams, oysters, corals, and other organisms that make shells and other structures from calcium carbonate.

Earth's atmosphere is thinner than that of Venus because Earth lost atmosphere at the bottom, rather than the top. Remember our description of what happened as Earth's atmosphere formed? Water molecules condensed into liquid water oceans. Carbon dioxide, a large component of Earth's atmosphere, dissolved in the water.

But wait—there's more. Carbon dioxide dissolved in the oceans met up with calcium from eroded rocks and formed calcium carbonate or limestone. Organisms—like clams and oysters and coral— evolved to take advantage of this combination to create rock-like shells to protect their soft bodies from predators. When these organisms died, their shells were buried, removing the carbon dioxide from the atmosphere and adding it to the crust of the Earth. This never happened on Venus, leaving this planet with an atmosphere that's more than ninety-six percent carbon dioxide. Hot as Venus is, it is still not hot enough to lose the heavy molecules of carbon dioxide rapidly.
 

That atmosphere will be around for a long time even though it is made of hydrogen. Jupiter is much farther from the Sun than the Earth, and so receives less heating of its exosphere. Jupiter also has an escape velocity of 59.5 km/s, almost six times higher than Earth.

In 1995, scientists learned about Jupiter's atmosphere by sending a probe from the Galileo mission plunging into Jupiter to measure the atmosphere. If you were to drop a spacecraft probe into the atmosphere of one of the terrestrial planets, it would fall through ever-thickening gas until it struck a rock or a liquid surface with a sudden crash or splash—just like the whale in freefall in The Hitchhiker's Guide to the Galaxy. That does not happen on Jupiter because there is no solid or liquid surface.

On Jupiter, the tops of clouds appear to be the planet's surface. But like Earth clouds, Jupiter's clouds are simply droplets or ice crystals suspended in a gas. The probe passed through the clouds, which had an atmospheric pressure of 1/2 of the Earth's atmospheric pressure.

As the Galileo probe fell deeper into Jupiter, the gas got denser and hotter. The gas soon was as dense as water on Earth, and yet it was warm enough that it did not liquefy. That may sound a little odd, but here's the difference between a liquid and a gas: if you release a liquid ball into a pressurized space capsule far from gravitational effects, it remains a liquid ball except for evaporation from the surface. On the other hand, if you release a gas ball inside the same spacecraft, it immediately expands. Even when compressed to the density of a liquid, the atmosphere of Jupiter was still a gas. Grab a ball of it, keep the density and temperature the same, and then release it into a spacecraft and — whammo — the atoms all flee the ball. It is, by definition, a gas.

The Galileo probe stopped taking data when it had descended to a region with a pressure over twenty-three times that of Earth's atmosphere, at a temperature of 150°C. Scientists speculated that the instruments inside the probe had cooked to death. As the dead spacecraft continued to descend into Jupiter's atmosphere, its nylon parachute melted, then its aluminum parts melted. Finally, at a temperature of 1660°C, the titanium of the spacecraft would have melted. As the molten drops descended, they would eventually be heated to the point where they would vaporize and be mixed with the Jupiter atmosphere. The atmosphere continues to increase in density with depth, eventually reaching the density of water, and even rock, and yet it remains gas-like, behaving as what scientists call a supercritical fluid. There is no surface on Jupiter.
 

We've written in the past about NASA's Kepler mission, which is searching for Earth-sized planets that could support life. Such planets would be located in the Goldilocks zone—not too far and not too close to the star they orbit. To find an atmosphere like that of our current planet, explorers will have to arrive at a planet in the Goldilocks zone at just the right period during the planet's history. Changes in the atmosphere aren't quick—but as the Earth's history shows, they are constant — and they aren't over yet. Humans have been pouring carbon dioxide into the atmosphere, which has affected the planet's retention of heat—but there are plenty of other changes to come that have nothing to do with us.

The sun has been brightening by about ten percent with each passing billion years. Over the next billion years, the sun will heat the ocean's surface, evaporating more water. When this water vapor reaches the exosphere, ultraviolet light will break it apart into its component atoms: hydrogen and oxygen. The hydrogen will escape, leaving behind the oxygen. After a couple of billion years, the oceans will have leaked into space, leaving our planet dry. And after five billion years or so, the Sun will expand into a red giant, swallowing the Earth, heating and removing the remaining atmosphere.

But don't worry. All of this is so far off that even the most dedicated pessimist would be hard-pressed to fret about it.