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Habitable Zones
"And so she stopped at the planet Venus to see if that might be the place. But when she sipped the atmosphere, she exclaimed, "Oh no! This is much too hot!"
Then she went to Mars, and once more cried out. "Here it's much too thin and cold!"
At last, however, she came to Earth, and when she tasted the sweet air she sang in delight. "Ah, now this one is just right!"
(From Earth by David Brin)
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How the Habitable Zone changes with increasing star size |
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View a Flash animation on Habitable Zones
As the mass of the star increases (on the left), the habitable zone moves further and further away from the star. With our Sun, the habitable zone covers Earth and nearly reaches to Mars and Venus. If our Sun was smaller, Venus would have been habitable, and if the Sun was larger, then Mars would have been habitable. |
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Strange though it may seem, Goldilocks tasting the three bears porridge is a good analogy to idea of there being a 'habitable zone' within our solar system, and others. A habitable zone is the region around a star in which liquid water could exist on a planet's surface, liquid water being one of the necessary ingredients for life. Stars of different luminosities (brightness) have differently sized habitable zones. For example, a dim star would have a habitable zone that was much closer to it than a bright star.
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Possible evidence of running water existing on the surface of Mars in the past. This landform looks very much like a river channel with its smoothed surface and stratified walls.
Taken from the Mars Global Surveyor. Courtesy NASA/JPL. |
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Around our sun, the habitable zone extends from after Venus to just before Mars, so the only planet within it is Earth. But we know this isn't a hard and fast rule; images taken by the Mars Global Surveyor show convincing evidence that liquid water once flowed on Mars as well. So there must be more to whether a planet can sustain life than just its distance from the sun.
The size of a planet is important in whether it can hold liquid water. A small planet, smaller than Mars, has insufficient mass to hold onto its atmosphere. Eventually, it will simply diffuse away. Conversely, if you have a planet 10 or 15 times heavier than Earth, it will have enough gravity to pull in a lot of nebular gas and hold on to it, resulting in a gas giant with a high atmospheric pressure.
Yet not even that is the whole story. 'Small' details such as the shifting of continental masses (plate tectonics) play a large role in determining the temperature of a planet, and so whether water is present as ice, liquid or vapour.
Take, for example, Mars. Mars is too small to sustain plate tectonics, and so does not experience events such as volcanoes any more, which are responsible for releasing carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas which causes a planet's temperature to rise.
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Olympus Mons, the highest mountain in the Solar System. Courtesy NASA/JPL |
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Early in Mars' history, there may have been some tectonic activity; we can see the remains of volcanoes quite clearly. Carbon dioxide was released warming up the atmosphere enough so that liquid water could exist on the surface, despite Mars' distance from the sun. However, Mars' tectonic activity began to slow down, and then stop completely. No more carbon dioxide gas was released into the atmosphere, and the carbon dioxide that was present began to decrease as it was absorbed into the Martian crust. The atmosphere dropped to just 1% of that of Earth's, and combined with its distance from the sun, Mars now has an average temperature of only -60ºC, far too low for liquid water. This is the view generally accepted by planetary scientists.
Matt Golombek, Project Scientist in the Mars Pathfinder mission, is not so sure. Mars' weak gravity may have caused the water vapour in the atmosphere to escape and diffuse away. He says it is possible that Mars may have had more atmospheric carbon dioxide in the past, but asks where has it gone now? "On Mars, the carbon dioxide would have to have been taken up in rocks on the surface, and there is little evidence for that," says Golombek. He believes that the water that once ran on the surface of Mars is now buried underground.
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The Greenhouse effect |
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When heat from the sun, in the form of light, strikes Earth, it reflects back off the surface. Some of this reflected heat escapes out of the atmosphere into space, but some is trapped by the atmosphere and is reflected back to Earth again. The amount of heat that is trapped depends on the makeup of the atmosphere; large amounts of greenhouse gases such as carbon dioxide will reflect lots of heat back to the surface. Low amounts will let more heat escape into space.
This of course doesn't just apply to Earth, it applies to all planets. One of the reasons why Venus is so hot is because it has a very thick carbon dioxide atmosphere that traps all the heat inside.
The ozone layer is not related to the Greenhouse effect. |
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Unlike Mars, Venus had too much carbon dioxide. Since Venus is closer to the Sun than Earth, it has a higher surface temperature. This caused evaporation from the planet's surface, along with a great deal of volcanoes. Huge amounts of carbon dioxide were released into the atmosphere, resulting in a runaway greenhouse effect. As more carbon dioxide was released, the temperature went up higher, and this made even more carbon dioxide get released. Venus' average temperature of 482ºC is far too high for liquid water.
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"Ah, now this one is just right! Not too hot, and not too cold."
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Earth is a perfect site for life. It is close enough to the Sun to receive enough heat and light for liquid water to exist, but not too close that things get too hot. On Earth, we have tectonic activity, which early in the planet's history caused a large amount of carbon dioxide to be released into the atmosphere. Gradually that activity lessened, and then a strange thing occurred; self-replicating molecules began to spread across the planet, and some started to absorb the carbon dioxide in the atmosphere into themselves with the aid of light. In other words, life began.
The way the level of carbon dioxide is regulated on Earth is balanced very well; photosynthesising plants use light and carbon dioxide (CO2) to grow. The carbon is used for growth, and the oxygen is excreted as a waste product. Biotrophs (organisms that do not photosynthesise) use that oxygen for energy, and in turn excrete out carbon dioxide. This means that the level of carbon dioxide never strays too far from a set level.
Of course, with the advent of industrialisation, we have begun to release large amounts of carbon dioxide into the atmosphere, from factories and power plants, and by burning forests. It is believed that now there is more carbon dioxide being released than is absorbed by plants, so the average level of carbon dioxide in the atmosphere is gradually increasing, resulting in global warming through the Greenhouse Effect.
Habitable zones assume that liquid water on the surface of a planet is essential for life - and that might not be true
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A runaway greenhouse effect isn't a bad thing all the time. It is possible that if Venus were in Mars' orbit, and Mars were in Venus' orbit, both would be habitable. Venus has a larger mass than Mars, and so might have been able to hold onto enough of its atmosphere to keep the temperature warm, and Mars, with its lower mass, would have held onto less atmosphere than Venus, possibly preventing the runaway greenhouse effect.
So while the idea of 'habitable zones' around stars is, in general, valid, there are many other factors that come into play such as the size of the planet. Other factors have not been mentioned; in our article about Europa, we discuss the possibility that life may exist in an ocean underneath its ice crust; yet Europa is not in our Solar System's Habitable Zone.
Habitable zones assume that liquid water on the surface of a planet is essential for life - this may be proved to be false in the coming years as probes are sent to Europa and other planets in the 'outer' solar system.
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