NASA researchers recently discovered that the uppermost layer of our planet's atmosphere, called the thermosphere, recently collapsed and now is rebounding.

The collapse wasn't a surprise to researchers because it occurred during the recent solar minimum of 2008-09. The thermosphere continuously contracts and cools when solar activity is low. However, the magnitude of this recent collapse was the biggest contraction of the thermosphere in the past 43 years.

The thermosphere ranges in altitude from about 40 to 375 miles above Earth's surface. It's the domain of meteors, auroras and satellites, which skim through the thermosphere as they circle Earth. It is also our planet's first layer of defense from solar radiation.

The thermosphere captures extreme ultraviolet (EUV) photons from the sun and prevents them from reaching the surface of Earth. When solar activity is high, solar EUV warms the thermosphere, causing it to puff up like a marshmallow held over a camp fire. When solar activity is low, the thermosphere collapses.

When solar activity was very low in 2008 and 2009, scientists turned their attention to the upper atmosphere to see what effects the century-class solar minimum would have on the thermosphere.

They analyzed the decay rates of more than 5,000 satellites ranging in altitude between 125 miles and 375 miles over the past 43 years. This data provided researchers a unique sampling of the thermosphere's density, temperature and pressure during that period. Using this clever technique, researchers discovered that the thermosphere's recent collapse was not only bigger than any previous collapse, but also bigger than the sun alone could explain.

Scientists believe that low solar EUV accounts for about 30 percent of the collapse. Extra CO2 in the thermosphere accounts for at least another 10 percent. That leaves as much as 60 percent unaccounted for.

Researchers stated in a recent report that global climate could also alter the composition of the thermosphere.

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