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Our solar system contains eight planets, but life is only known from our own, Earth, which is the third planet from the Sun. You might wonder why and how so much life ended up here instead of, say, on our two planetary neighbors, Venus and Mars (Figure 14.1).

In August 2012, NASA’s wheeled rover Curiosity landed on Mars and sent back bleak images of a red, stone- and sand-strewn landscape stretching to the horizon (Figure 14.2). Those stark, lifeless views were hardly surprising because the average surface temperature is about –65°C (–85°F), a place where even a polar bear would have trouble staying warm. It still might be possible that there are microbes hidden in some icy crevice, but they would have to be highly resistant to freezing. As for Venus, life as we know it would be impossible, because it’s about as hot as a traditional wood-fired pizza oven: The surface temperature averages 464°C (867°F).

Sitting comfortably between the temperature extremes of Mars and Venus, we find Earth, with an average global temperature of about 15°C (59°F), a climate to which its inhabitants are well attuned. In their 2012 book The Goldilocks Planet: The 4 Billion Year Story of Earth’s Climate, authors Jan Zalasiewicz and Mark Williams compare Earth to the bowl of porridge in the classic fairy tale of Goldilocks and the three bears. They write that our planet was neither too cold nor too hot but “just right” for humanity. What do you think produces the differences in temperature among these planets?

Distance from the Sun explains some of the differences. However, whereas Venus is closer than Earth to the Sun, Venus is 2.8 times hotter than the planet Mercury, which is the closest planet to the Sun (Figure 14.3). Clearly, distance to the Sun is not all that accounts for how climates differ.

Earth’s atmosphere is a layer of gases that stretches from the surface of Earth to the edge of space, some 500 kilometers above the surface. Our atmosphere is made up mainly of nitrogen and oxygen, which are basically transparent to visible sunlight. On a clear day, most of the Sun’s beams penetrate the atmosphere like a glass window and hit Earth’s surface, where two things happen. Some of that light is immediately reflected back toward the sky, particularly when it has hit a bright surface like fresh, white snow. Most of it warms Earth’s surface like a parking lot on a summer day, and that energy is slowly re-emitted not as visible light, but as infrared radiation, one avenue for transmission of heat energy. Consider that even after the Sun has set on a particularly hot day and there is no more visible light, you can still feel the heat radiating from the ground.

Infrared radiation has longer wavelengths than visible light, which means it has different properties. Most of it does not pass back through the atmosphere into outer space, but is rather absorbed by clouds and gases such as carbon dioxide and water vapor, creating a warm blanket of air. This phenomenon is known as the greenhouse effect and heats Earth approximately 33°C above what it would be without an atmosphere, making life on the planet possible (Figure 14.4).

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Differences in the greenhouse effect account for difference in the climates of Earth, Mars, and Venus. Mars’s atmosphere is made up of over 95% carbon dioxide, but it is just 1% as dense as Earth’s, giving its atmosphere very little heat-trapping capacity, hence its frigid temperatures. At the other extreme, the atmosphere of Venus is also almost entirely carbon dioxide, but it is 92 times denser than Earth’s. Therefore, it traps massive amounts of heat, producing a huge greenhouse effect.