The AstroTour illustrates what the surface temperatures of Venus, Earth, and Mars would be without atmospheres, and it also shows the current value for each planet, with their actual atmospheres.

Venus: without atmosphere, - 46°C; actual average temperature, 477°C Earth: without atmosphere, - 18°C; actual average temperature, 15°C Mars: without atmosphere, - 57°C; actual average temperature, - 47°C

Explanation:

The greenhouse effect is responsible for trapping heat in the atmosphere of a planet. Some of the more important greenhouse gases are carbon dioxide and methane. The strength of the greenhouse effect varies by planet depending on the thickness of the atmosphere. Venus has an extremely dense atmosphere and so experiences a runaway greenhouse effect. Mars on the other hand, only sees it's temperature raised slightly due to it's thin atmosphere.