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Venus has a greenhouse affect because its atmosphere is 96 percent carbon dioxide. However, although mercury is closer to the sun it does not have the greenhouse effect because it has no atmosphere and only an exosphere. I was wondering why doesn't mars have a greenhouse effect like Venus? They both have around 95 percent carbon dioxide in their atmosphere and consist of an atmosphere. In addition, I was also wondering about what are the requirements to have a greenhouse effect on a planet?

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For there to be a significant greenhouse effect there has to be a lot of greenhouse gas in the atmosphere. Not just a large percentage, but a large amount of gas

There is a greenhouse effect on Mars, however, as its atmosphere is very thin. The surface pressure on Mars is only 0.6% of the pressure on Earth. As such there very little of anything to trap heat. Even though a large percentage of the atmosphere is CO2, the greenhouse effect only raises temperatures by about 5 Kelvin. http://nova.stanford.edu/projects/mod-x/id-green.html

It wasn't always this way. During a dust storm, more heat can be trapped, and temporarily raise temperatures. Early in Mars's past the atmosphere was thicker at temperatures were higher. It is even possible that at one point, a runaway greenhouse effect heated Mars, until the atmosphere thinned and the planet cooled http://www.astrobio.net/mars/did-ancient-mars-have-a-runaway-greenhouse/

The only requirement for a planet to be warmed by the greenhouse effect is for its atmosphere to contain greenhouse gases. Mars does have a greenhouse effect, as other bodies with atmospheres: Earth, Titan and Venus. https://www.nasa.gov/home/hqnews/1991/91-143.txt Other rocky bodies don't have a thick enough atmosphere for a greenhouse effect to be measured.

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