Space Science, February 12, 2003 "It looks like it's hanging out in space," said astronaut Don Pettit onboard the International Space Station last month. Out the window was an electric-blue cloud, faint with feathered edges, hovering high above Earth's limb. "Over the past few weeks we've been enjoying outstanding views of these clouds over the southern hemisphere," he added during a NASA TV broadcast last month. Ordinary clouds like stratus or cumulus clouds are ground huggers. They float less than 5 miles high. What Pettit saw was 50 miles above Earth's surface. Looking across Earth's limb, Pettit said the clouds were "twice the thickness of the atmosphere above our horizon ... literally on the fringes of space." Sky watchers on Earth have seen these strange clouds before. They're called noctilucent ("night-shining") clouds, or NLCs for short, and they're a big mystery. "These clouds were first observed in 1884," says Gary Thomas, a professor at the University of Colorado who studies NLCs. Krakatoa had erupted the year before, hurling enormous plumes of volcanic dust into Earth's atmosphere. Sunsets in those days were spectacular; they attracted many people outdoors to watch the evening sky. One sky watcher, T.W. Backhouse of Britain, noticed something odd. On some nights after the sun set, colorful filaments would appear, glowing electric blue and yellow against the black night sky. No one had ever seen anything like them. 19th-century scientists figured it was some strange side-effect of volcanic ash. But there was a problem with that idea: Eventually the ash settled, the vivid sunsets of Krakatoa faded ... yet noctilucent clouds remained. Indeed, more than a century later, they seem to be multiplying. "Since the 1960s we've seen more of these clouds than ever before," says Thomas.
In the 19th century, noctilucent clouds were confined to latitudes above 50 degrees. You could spot them from places like Scandinavia, Russian and Britian, but nowhere else. "For the past three years in a row, however, NLCs have been sighted as far south as Utah and Colorado," says Thomas. What are these strange clouds? Here's what scientists have learned so far: Although NLCs look like they're in space, they're really inside Earth's atmosphere, in a layer called the mesosphere ranging from 50 to 85 km in altitude. The mesosphere is not only very cold (-125 C), but also very dry. A cubic centimeter of air in the upper mesosphere would contain only 7 water molecules--a billion times dryer than the Sahara desert. Nevertheless, NLCs are made of water ... "in the form of ice crystals," says Thomas, "about the size of particles in cigarette smoke." Like tiny prisms, these ice crystals reflect and refract sunlight--hence the vivid colors. The glowing clouds are too dim to see in broad daylight, but for a while after sunset, when the ground is dark and high clouds are still sunlit, noctilucent clouds can take your breath away. How tiny ice crystals form in the mesosphere is both the essential mystery of noctilucent clouds and, possibly, their link to something big: global warming. "We need two things to make ice crystals," explains Thomas. "Water molecules and something for them to stick to--like dust." Water gathering on dust to form droplets or ice is a process called nucleation. It happens all the time in ordinary clouds. But there's not a lot of dust or water in the mesosphere. Krakatoa may have provided some dust in the late 19th century, but that doesn't explain NLCs now. Ordinary volcanoes and desert dust storms don't waft much dust to the edge of space. "Perhaps," Thomas speculates, "the source is space itself." Every day Earth sweeps up tons of meteoroids--tiny bits of debris from comets and asteroids. Most are just the right size to seed noctilucent clouds. Water vapor is perhaps the bigger mystery. "Upwelling winds in the summertime carry water vapor from the moist lower atmosphere toward the mesosphere," says Thomas. (This is why NLCs are observed during summer, not winter.) Yet these winds were present long before NLCs appeared in the 19th century. What makes them more effective now? It could be the greenhouse effect. While greenhouse gases warm Earth's surface, they actually lower temperatures in the high atmosphere--strange but true. "Extremely cold temperatures are required to form ice in a dry environment like the mesosphere," says Thomas. The greenhouse effect, by cooling the mesosphere, promotes noctilucent clouds. One greenhouse gas in particular, methane (CH4), directly boosts the water content of the mesosphere. Methane is a light gas that rises to high altitudes where it's broken apart by solar ultraviolet radiation. Hydrogen from methane then reacts with atmospheric oxygen to form the water vapor required by NLCs. Thomas notes that the first sightings of noctilucent clouds coincided with the Industrial Revolution -- a time when greenhouse gas production increased. The Space Age might have something to do with it, too. The main engines of the space shuttle produce water vapor--lots of it. "The shuttle pumps hundreds of tons of water into the atmosphere every time it goes to orbit," says Thomas. Those water molecules circulating through the upper atmosphere could become raw materials for high-altitude clouds. Are NLCs a lovely side effect of space travel? A troublesome sign of global warming? Or something else entirely? A NASA satellite scheduled for launch in 2006 will take data scientists need to find the answers. The Aeronomy of Ice in the Mesosphere satellite, or AIM for short, will be controlled from the University of Colorado as it orbits Earth 300 miles high. Although it's a small satellite, says Thomas, there are many sensors on board. AIM will take wide angle pictures of NLCs, measure their temperatures and chemical abundances, monitor dusty aerosols, and count meteoroids raining down on Earth. "For the first time we'll be able to monitor all the crucial factors at once." Meanwhile NLCs are spreading so you might be able to see some yourself. The best time to look is summer nights when the Sun is about 6 degrees below the horizon--the optimum geometry for NLC illumination. Just don't forget your camera. As NLC enthusiast Don Pettit remarked from the ISS, "you can never have too many pictures of noctilucent clouds." | 