Glowing, silvery blue clouds that have been spreading around the world and brightening mysteriously in recent years will soon be studied in unprecedented detail by a NASA spacecraft.
The Aeronomy of Ice in the Mesosphere (AIM) mission will be the first satellite dedicated to studying this enigmatic phenomenon. Due to launch in late 2006, it should reveal whether the clouds are caused by global warming, as many scientists believe.
"Noctilucent" clouds, which glow at night, form in the upper atmosphere, at an altitude of about 80 kilometres, and their glow can be seen just after sunset or just before sunrise.
"Even though the Sun's gone down and you're in darkness, the clouds are so high up, the Sun is still illuminating them," explains AIM principal investigator James Russell at Hampton University in Virginia, US. Russell described the mission on Thursday at a meeting of the American Geophysical Union in Baltimore, Maryland, US.
Bigger and Brighter
The clouds were first observed above polar regions in 1885 - suggesting they may have been caused by the eruption of Krakatoa two years before. But they have spread to latitudes as low as 40° in recent years. "They're also getting brighter, and each year there are more of them than in the previous year," Russell told New Scientist.
Many researchers believe this proliferation is down to human activities. "You need three things for clouds to form: particles that water can condense onto; water; and cold temperatures," says Russell. He says pollution and global warming are thought to be responsible for two of those factors.
Atmospheric water may be boosted by livestock farming and the burning of fossil fuels, which spew methane into the atmosphere: sunlight breaks down the methane, releasing hydrogen that can bond with oxygen to form water.
And greenhouse gases such as carbon dioxide actually help to cool the upper atmosphere, where the clouds form. That is because the atmospheric density is so low at that altitude that the gases cannot trap heat as they do closer to the Earth's surface, and the heat is simply radiated into space.
As yet, it is not clear what the source of the particles that "seed" the clouds is. The clouds form during the local summer months, when the pole is bathed in perpetual sunlight. So one possibility is that warm air rising above the pole could carry dust upwards from lower atmospheric altitudes, onto which water can condense.
But the dust could also have a cosmic source, dropping into the atmosphere from space. "It may be there's a constant supply of particles but a changing temperature and water environment makes the conditions right to grow ice particles," says Russell.
AIM will use three instruments to study the clouds. One is a suite of four cameras that will provide panoramic views of the poles and clouds. Another, called the Solar Occultation for Ice Experiment (SOFIE), will study the chemistry of the ice particles and clouds - measuring molecules such as methane. It will also observe the Sun through the atmosphere to measure how much sunlight is dimmed by dust in the atmosphere.
The third instrument, called the Cosmic Dust Experiment, is a plastic film that sits on top of the spacecraft. It will record every "hit" from a dust particle that rains down on it from space.
"We want to know why the clouds form and why they vary," says Russell. "If there is a human connection, it'll tell us that we're doing something to the atmosphere and that we need to determine what the long-term consequences are."
Some scientists speculate that the clouds might actually help mitigate global warming, says Russell. "If these clouds were to continue to grow and cover broad areas of Earth, they would form something like a thin, semi-transparent umbrella," he told New Scientist. "They would reduce the amount of solar rays making it to the ground, so they could actually reduce the effects of global warming."
The AIM satellite will launch into a polar orbit from California's Vandenburg Air Force Base. Russell says it may lift off in December, but its exact launch date has not been set because mission planners are still working to minimise vibration forces on the spacecraft due to its Pegasus XL launch rocket.