Noctilucent Emissions

There's a new study suggesting that "noctilucent clouds" are caused by the plumes of space shuttle launches, which is somewhat ironic for me. For those that don't know, noctilucent clouds are the highest clouds found in the Earth's atmosphere. They are ice crystals floating high in the mesosphere, where large amounts of moisture are not typically found, and are usually only seen in twilight when the setting sun illuminates them against the dark sky. There is also evidence that these clouds are a modern phenomenon, within the past century, and so they may be related to human activity, or perhaps climate change.

Noctilucent clouds over Lake Saimaa. Photograph taken by Mika Yrjölä. Permission by Creative Commons Attribution 2.0 License (from Wikipedia).

Early studies have suggested that noctilucent clouds were caused by space shuttle launches. The space shuttle exhaust plume is composed mostly of water vapor. As the shuttle launches into orbit, it can dump significant amounts of water vapor into the upper atmosphere as it passes through it. More recent studies by Dr. Michael Kelley have added credence to that idea: shuttle launches in 2003 and 2007 produced corresponding noctilucent clouds.

NASA launched a satellite called AIM in 2007 to study noctilucent clouds. AIM stands for Aeronomy of Ice in the Mesosphere. AIM continues to study the phenomenon using several instruments, including two imagers and a meteoric dust measuring device.

The part I find ironic is that NASA launched a satellite to study a phenomenon caused... by the launch of NASA satellites!

OK, there are far more noctilucent clouds than can be entirely explained by shuttle launches, so this is not a complete exercise in navel-gazing. The shuttle-noctilucent connection was known before AIM was selected by NASA.

Update 2009-07-30: Added noctilucent cloud image from Wikipedia.