Of the nine planets in our solar system, the closest to Earth are Mars (about 50-percent farther from the Sun) and Venus (about 30-percent closer). Venus surface temperature is about 900 degrees Fahrenheit (482 degrees Celsius), whereas a warm day on Mars is like one of the cooler days in Antarctica. What happened to these planets to make them so inhospitable for biological organisms as we know them?

Carbon dioxide is a greenhouse gas, which means it traps warmth in the form of infrared radiation like a gaseous "blanket" over a planets atmosphere. Earths oceans contain about 60 times more carbon dioxide than its atmosphere. In fact, carbon dioxide gas makes up very little (less than 1 percent) of the Earths atmosphere. The rest is made up of about three-fourths nitrogen, one-fourth oxygen, some argon, carbon dioxide and other trace gases.

Venus is too hot to sustain life as we know it.

Continue On With SETI"s Solar System Tour

Part One : Where life is concerned, location is critical -- especially where the distance from the Sun is concerned.

Part Two : Planetary neighborhoods, more on Venus and Mars, and the source of Earth"s oceans.

However, the small amount of carbon dioxide is essential for keeping Earth warm. It works with the water vapor in the atmosphere to keep Earths oceans from freezing. By dissolving carbon dioxide and re-releasing it into the atmosphere, the oceans can be thought of as controlling the temperature of our planet (this process is, of course, much more complex). If all the carbon dioxide in our oceans were released at once, our atmosphere would become unbearably hot. The same effect would take place if humans released too much carbon dioxide into the atmosphere by polluting processes such as the internal combustion engine. (Pollution has increased the amount of carbon dioxide in our atmosphere by at least 25 percent in the last 150 years.)

Some scientists believe that Venus [more reference data] had an ocean. According to the theory, Venus was too close to the Sun to maintain it. When water vapor reached the upper atmosphere, the ultraviolet light from the Sun would break it into its components -- oxygen and hydrogen -- the latter of which escaped into space. As Venus oceans were evaporating, their carbon dioxide was released into the atmosphere, which caused increasing heat, which, in turn, caused increasing evaporation, and so on. This is called the "runaway greenhouse effect" and is the reason why Venus is so hot today.

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Mars [more reference data], on the other hand, may have been just right for habitability during the early days of the solar system. It likely had a great ocean, lakes and thermal vents. This is why so many scientists are interested in looking for fossils there. But unlike Earth, Mars does not have plate tectonics -- the recycling of the continents that is so active on Earth.

Mars, the rusty desert world

A nail left outside will rust; it will turn into iron oxide. When it rusts, it takes a bit of oxygen out of the air. Rocks do this also, forming all sorts of compounds with oxygen and other gases as constituents. Then why dont rocks deplete our air supply? Because rocks on Earth are recycled. The movement of the continents folds the rocks deep into Earths crust, re-melting and vaporizing them, thus re-releasing the oxygen and other atmospheric constituents they contain back into our atmosphere. However, this process of continental folding requires a heat source to keep it going. Our planets heat source consists of the radioactive decay of heavy elements such as uranium and thorium deep within Earth.

So what happened on Mars? Mars is about one-tenth the size (mass) of Earth and therefore cooled much faster (as smaller things do). If Mars had much plate tectonic activity it has long since stopped, as the interior of Mars cooled more rapidly than Earths did. Mars was not able to recycle its rocks and so its atmosphere is mostly "trapped" within them today. Without much of an atmosphere to keep it warm, Mars remains a very cold place -- too cold, and with too sparse an atmosphere, to allow liquid water to persist on its surface.

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Thus we have two very interesting examples of how our neighborhood planets work. Mars lets us appreciate our planets continual recycling (we can learn to build structures that withstand earthquakes, and stay away from volcanoes, but the planet still needs them to operate effectively). Venus, on the other hand, warns us to avoid carbon dioxide pollution if we want to keep our planet at the nice, cool liquid water temperature it now has. What would have happened if Venus had been located in Mars orbit? If this was the case, we might have had two habitable, liquid-water planets in our solar system today.

Our immediate planetary neighbors are thus studies in appreciating how neatly our own planet works. Are there other ways a planet-sized body might work to make a nice place to live? We will discuss this in the next essay.