An upper layer of Earth’s atmosphere recently shrank so much that researchers are at a loss to adequately explain it, NASA said on Thursday.

The thermosphere, which blocks harmful ultraviolet rays, expands and contracts regularly due to the sun’s activities. As carbon dioxide increases, it has a cooling effect at such high altitudes, which also contributes to the contraction.

But even these two factors aren’t fully explaining the extraordinary contraction which, though unlikely to affect the weather, can affect the movement of satellites, researchers said.

“This is the biggest contraction of the thermosphere in at least 43 years,” John Emmert of the Naval Research Lab was quoted as saying in NASA news report.

Emmert is the lead author of a paper announcing the finding in the June 19 issue of the journal Geophysical Research Letters.

“We cannot explain the abnormally low densities, which are about 30 percent lower” than from previous contractions, Emmert told CNN.com.

via Scientists baffled by unusual upper atmosphere shrinkage – CNN.com.

Under current models, the sun activity and the amount of CO2 in the atmosphere at that time doesn’t explain the extraordinary collapse in the thermosphere. There is a piece of the puzzle missing that has scientists scrambling for an explanation.