Venus: Earth's troubled sister

The planet Venus is one of the most familiar sights in the night sky, shining in the west at sunset, or brightening up the early morning gloom. Venus is always close to the sun in the sky, the reason being that Venus is actually closer to the sun than the Earth. Since Venus's path can never take it outside the Earth's orbit, we never see Venus on the opposite side of the sky from the sun. Hence, the lovely planet becomes both the morning and evening star, depending on which side of the sun itís on at a given time.

Venus has a special draw on our imaginations, and not just because if its lovely glow or association with the goddess of love. Venus is the closest planet to Earth, in more than one way. Literally, Venus is the planet that comes closest to Earth in its orbit. As we swing by Venus in a close approach between our two orbits, Venus seems to become impossibly bright, and UFO sightings increase significantly.

But Venus is also the twin sister to Earth in our solar system. Venus' orbit is actually quite similar to the Earth, with Venus orbiting only 20 percent closer to the sun than we do. Venus is also almost the same size and mass as our planet, and has the most similar chemical composition. But looking toward Venus in the sky, I tend not to feel a warm camaraderie between our two worlds. On the contrary, Venus actually makes a lot of scientists distinctly uneasy.

At first, Venus was merely mysterious. From Earth-based telescopes, all we could really discover about the planet was that it was cloudy. High, pale-colored clouds blanket the entire surface. At first the clouds seemed encouraging; planetary scientists of a hundred years ago imagined oceans and forests fed by eternal rains. Perhaps Venus could support life; perhaps Venus was much like Earth. But when we were finally able to send probes to our sister planet, we got a huge shock.

As serene and lovely as Venus seems, in reality the planet is as about as close to the traditional version of hell as our imaginations allow. Underneath the clouds, Venus is the hottest planet in our solar system, with average temperatures around 900 degrees Fahrenheit. Interestingly, Venus is far hotter even than the planet Mercury, which is twice as close to the sun as Venus is. And if the heat wasn't enough, the atmospheric pressure is enormous (90 times that of sea level on Earth), strong enough to crush a spaceship like an empty soda can. The landscape is pockmarked by faults, scars, and evidence of giant magma bubbles, which rose and fell back into Venus' crust, leaving behind features resembling thousand-mile wide collapsed souffles. Venus is, of course, far too hot for liquid water to exist, but that doesn't stop it from raining – it's just that the rain is made of highly corrosive sulfuric acid.

These extreme conditions make Venus an extremely difficult place to explore. The atmosphere is completely filled with clouds made of sulfuric acid aerosols that never break open to reveal the surface. It wasn't until we were able to fly satellites to orbit Venus and bounce radar down through the clouds that we had any idea what the surface looked like at all. The Soviet Union managed to land two spacecraft on Venus in the late 1970s, both of which hurriedly transmitted a picture or two before succumbing to the brutal conditions. The view: a hot, bleak desert with acid rain falling from an orange-colored sky.

Even putting our Earth-centric notions aside, something seems to have gone terribly wrong with Venus. Why are the conditions on the planet so extreme and inhospitable? It's not as simple as Venus being closer to the sun. A 20 percent difference in distance might explain a temperature difference of a few tens of degrees, but hardly over 800 degrees! The answer lies in a phenomenon that's received plenty of media attention on Earth in the last few years: the greenhouse effect. Simply put, the greenhouse effect is the ability of an atmosphere to retain heat. Taken in moderation, the greenhouse effect is actually quite a benign thing. Our own planet, in fact, would not be the lovely warm place we know today without it. The Earth is actually far enough from the sun that without the greenhouse effect, the average temperature on our planet would be well below freezing, and we would be locked in a permanent Ice Age.

The greenhouse effect is caused by gases in our atmosphere that allow sunlight to pass through, but block infrared light (what we think of as heat) from passing out into space. Specifically, the gas that tops the list as the most efficient atmosphere-warmer is carbon dioxide. The more carbon dioxide in an atmosphere, the faster the planet will heat up – it's pretty much that simple. What appears to have happened on Venus is that the level of carbon dioxide has gotten out of control. Over billions of years, the planet has turned into a giant pressure-cooker at the hands of a runaway greenhouse effect.

Interestingly, we can't simply blame a higher amount of carbon dioxide for Venus's fate. Remember that Venus is the most similar planet to earth chemically? Pound-for-pound, Venus and Earth have about the same total amount of carbon dioxide. The difference lies in how the two planets put that gas to use. Most people have heard that plants use carbon dioxide for photosynthesis, releasing a waste gas called oxygen in return. The result is that plant life on Earth (including the incredibly significant tiny sea algaes and planktons) scrubs a massive amount of carbon dioxide out of our air.

But even before life had taken hold on our planet, carbon dioxide was under control. Earth's incredibly fragile (and unique that we know of) water cycle soaks up carbon dioxide into the sea. Rain combines with carbon dioxide as it falls into the oceans, where it is dissolved and stored safely out of our atmosphere. Even the rocks of our planet do their part, chemically combining minerals with carbon dioxide from the air. Carbonate rocks, as they are called, may be the biggest carbon dioxide storage system of them all.

Venus, on the other hand, has none of these advantages. The planet must have started out just a little too warm to form a network of oceans. With little or no liquid water, life as we know it never got started. Venus may very well have had carbonate rocks in the past, but as things started to heat up, the rocks got baked. As carbonate rocks heat up, their carbon dioxide gets baked out. When that temperature threshold was passed, perhaps billions of years ago, the rocks began to exhale carbon dioxide. The result: an atmosphere so hot and heavy that it could crush an 18-ton truck before melting it into a puddle of corroded liquid metal.

The challenge for us is to not get too comfortable sitting here with lovely rain falling on our useful rocks that happen to have carbon dioxide-breathing plants growing all over them. Yes, for the moment, Earth has struck a perfect carbon dioxide balance. Too little and our planet would freeze. But heed Venus's lesson well: too much, and we will fry.

And how much is too much? The awful answer is that we just don't know. Will pollution levels crank the greenhouse effect up over that critical level? Has it in fact already happened? When you're talking about geological time, which spans millions of years, how concerned should we be that the Earth's average temperature seems poised to rise a full 10 degrees in the next 100 years? Are we, in fact, turning ourselves into a carbon (dioxide) copy of Venus?

This very question has turned scientists' eyes back to our troubled sister planet. At the moment, we have almost no specific, quantitative information about how carbon dioxide affects our planets temperature. How fast can we expect temperatures to rise? What more might be done to moderate carbon dioxide levels? Venus may just hold some of those answers.

The more we understand about Venus's atmosphere, weather (if you count a light shower of sulfuric acid as weather), and climate, the better we'll be able to predict how our own ecosystem will change over time. In a way, we're quite fortunate to have one big, hulking example of what can happen to a mismanaged planet right in front of our eyes, hanging serenely in the evening sky.

Dr. Michelle Thaller works for NASA and teaches at the California Institute for Technology in Pasadena CA.