This is a picture of Earth's moon seen through the atmosphere.

This image shows the moon, Earth’s only natural satellite, at center with the limb of Earth near the bottom transitioning into the orange-colored troposphere, the lowest and most dense portion of Earth’s atmosphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange- and blue-colored atmosphere. The silvery-blue noctilucent clouds extend far above Earth’s troposphere. Image credit: NASA

Earth’s atmosphere is critically important to all of us. In addition to providing us with air to breathe, it protects us from temperature extremes, harmful space radiation, and vast numbers of incoming meteoroids. The atmosphere is a very complex system that we are only beginning to understand. Gaining a better understanding of the atmosphere, how it protects us, and how we can protect it is in all of our interests.

In order to understand Earth’s atmosphere and how it works, it is essential to study atmospheres under a wide range of conditions beyond Earth. Examining atmospheres on other planets allows this. For example, by studying the atmosphere of Venus, we learned about the role of carbon dioxide as a greenhouse gas, and saw how it drives the temperature on Venus as high as 860 degrees Fahrenheit (460 degrees Celsius).

via NASA – Why LADEE Matters.