Air
If you shrank Earth down to the size of a basketball, then by comparison its atmosphere would have roughly the thickness of a sheet of plastic wrap. All that is essential to us in our everyday lives is contained in this relatively thin sheet. Without its "wrapper," our world would be as cold and barren as the moon. Yet with it, the Earth is blanketed in all the essential ingredients-air, water, nutrients, and heat-needed to sustain the vast web of life. Over billions of years, the atmosphere has evolved into a complex system in which life and climate are intricately interwoven. For instance, we know that biological systems, as well as many of the physical processes such as temperature changes, are driven by the amount of sunlight that reaches the Earth's surface. Yet, when we measure the amount of incoming sunlight at the top of Earth's atmosphere, and compare this number with the amount of sunlight received at the surface, we find that 8 percent of the sunlight is missing! This number varies depending upon whether it is a clear, hazy, or cloudy day. Scientists aren't sure what percentage of the missing sunlight is due to the absorption and reflection of clouds; what percentage is being scattered by aerosols-trillions of tiny solid and liquid molecules-suspended in the atmosphere; or some combination of both clouds and aerosols. EOS Terra will enable scientists to find the missing sunlight and, subsequently, greatly improve the accuracy of their global climate models.

Now picture the atmosphere as a solid block of ice. With no impurities contained within it, the ice would appear translucent like glass, allowing light to shine through it. But if there were many tiny bubbles trapped within the ice, it would appear less clear and whitish, depending upon the number and density of the bubbles. This is because the bubbles would absorb and scatter light, rather than allowing it to pass through. Likewise, aerosol particles absorb and scatter sunlight in the atmosphere, thereby having a net cooling effect at the Earth's surface. Moreover, when they mingle with clouds, aerosols modify clouds' droplets, enhancing their ability to scatter and absorb more sunlight. Scientists have observed that clouds polluted with aerosols generally appear whiter and brighter than those unpolluted. We know that due to their interactions with Earth's radiant energy, clouds and aerosols play important roles in ongoing climate changes, yet the full extent of their contributions is not fully understood. The mystery lies partly in the fact that the physical and chemical structures of both clouds and aerosols change rapidly through time and space. So, one of the major challenges of the EOS Terra mission is to not only observe how and why clouds and aerosols change, but to also simultaneously measure their effects on the Earth's total radiant energy.

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