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Our friends at the Kalamazoo Valley Museum in Michigan wondered how we know the temperature of the cloud tops of Jupiter. This kind of question actually brings to mind the general question of how scientists know what they know. It is often as interesting to learn about how questions are answered as it is to learn what the answers themselves are. One of the reasons that science is so exciting is that it has managed to find many wonderfully creative ways to learn about the universe.
So how would we learn the temperatures of the clouds on Jupiter, a planet 5 times farther away than the Sun? We can see the planet easily; in fact, you can go outside and see that it is one of the brightest objects in the nighttime sky. But of course just looking at Jupiter, even with a powerful telescope, certainly won't tell you the temperature there.
When we see an object, our eyes are responding to the visible light coming from it. It seems as if our eyes pick up such a great range of colors; just think of how beautiful the world is and how many different colors you can see. Even a rainbow does not contain all the colors your eyes treat you to. You might be surprised then that there is a much much larger range of light that you and I simply can't see. You have heard of X-rays, microwaves, radio waves, and ultraviolet light, but maybe you didn't know that they are related to the light you can see. An important difference though is that when any of these reach your eyes, nothing registers. For example, you might know of the harmful effects of ultraviolet light on your skin, causing sunburn or even worse damage; but your eyes don't allow you to see ultraviolet light. Some animals can see it however, just as some animals can hear sounds you cannot hear.
Now, all objects glow with a kind of light called infrared. The infrared light that an object gives off depends upon its temperature, so if we can measure how bright the infrared light is, we can tell how warm or cold it is. Infrared is another kind of light you and I cannot see, but scientists and engineers can build devices that can pick it up. By measuring the infrared light, they can discover the temperature. If your eyes could see infrared, things would look very different from what you are used to. You would actually be able to see the temperature. This sounds strange, but you can get an idea of what this might be like, while learning more about infrared light, by viewing the Infrared Photo Album. Nurses now often use an instrument to look at the infrared coming from inside your ear to measure your body temperature instead of using a thermometer, which takes longer and is sometimes uncomfortable.
Earth's atmosphere blocks much of the infrared signals coming from space. Therefore, NASA and other space agencies have launched a number of spacecraft that could measure infrared light from above our atmosphere. The instruments on these spacecraft have allowed scientists to find out not only the temperature of the clouds of Jupiter, but also the temperature of many other objects throughout the solar system and beyond.
Scientists can even learn more about Earth itself by looking at our home planet with infrared. Many new questions can be answered when we use infrared to look at a familiar site in a new way. To have fun learning more about the world in infrared, play the infrared matching game. I'll be waiting to see you there!