Satellite to study source of 'night shining' clouds

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21:01 12 April 2007
NewScientist.com news service
Maggie McKee


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Noctilucent clouds were first observed in 1885, two years after the Krakatoa eruption. But most volcanic eruptions do not spew material high enough in the atmosphere to seed the formation of the icy clouds (Image: Hampton University)
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Iridescent, silvery blue clouds at the edge of space that may be connected to global warming will be studied by a NASA spacecraft set to launch on 25 April.
The Aeronomy of Ice in the Mesosphere (AIM) mission will be the first satellite dedicated to studying the enigmatic phenomenon of "noctilucent", or night-shining clouds.
The shimmering clouds can be seen glowing just before sunrise or just after sunset because they are so high up – forming at an altitude of about 80 kilometres – that the Sun illuminates them from below the horizon. Watch a video of the clouds shifting in the sky (courtesy of Hampton University).
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 in recent years they have spread to latitudes as low as 40°, while also growing in number and getting brighter.
"We don't know why this is happening," NASA mission programme executive Vicki Elsbernd said at a briefing with reporters on Wednesday. "Many scientists suspect it may be related to global warming."
Greenhouse gases
Researchers say humans may be responsible for two of the three factors required to make the icy clouds. These factors are water, cold temperatures and 'seed' particles onto which the water can condense.
Atmospheric water may be boosted by livestock farming and the production 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 carbon dioxide, like methane and water, is an efficient radiator of energy – both downwards, towards the Earth, and upwards, out to space.
Solar cycle
"By the Earth's surface, CO2 can cause the atmosphere to warm," explains AIM principal investigator James Russell at Hampton University in Virginia, US. "But at 50 miles above the surface, the opposite occurs – the radiation trapped is released to space and the atmosphere cools."
Natural factors also affect the clouds. Because water molecules can be destroyed by radiation from the Sun, scientists predicted that the abundance of water at high atmospheric altitudes would be at a minimum during periods of high solar activity in the 11-year sunspot cycle.
But the water level is actually observed to drop to a minimum about a year after the solar maximum, says Russell. "There's a time lag we don't understand."
The source of the particles that seed the clouds is also a mystery. Since the clouds form during the local summer months, when the pole is bathed in perpetual sunlight, one possibility is that warm air rising above the pole could carry dust upwards from lower atmospheric altitudes. The dust could also have a cosmic source, however, dropping into the atmosphere from space.
Hemispheric differences
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.
The AIM satellite will launch from California's Vandenburg Air Force Base. After being dropped from an aircraft at an altitude of about 12 kilometres, its Pegasus XL launch rocket will ignite, ultimately putting the satellite into a polar orbit. Watch an animation of the launch and satellite deployment (courtesy of NASA).
The $140 million mission is expected to operate for two years, which will allow it to observe two summers in each hemisphere. That will help researchers understand why the clouds appear brighter in the northern hemisphere and at higher altitudes in the southern hemisphere