Satellite to study world's most mysterious clouds
Saturday, 28 April 2007
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Mysterious clouds that form at high altitudes over polar regions are to be studied for the first time by a scientific satellite that is specifically designed for the task.
Scientists hope that the wisp-like clouds - the highest in the world - will lead them to a deeper understanding of how the Earth's atmosphere is able to protect the planet from harmful solar and cosmic radiation.
The night-shining "noctilucent" clouds - meaning they can only be seen when the ground is in darkness - are one of the least-understood meteorological phenomena because they appear so rarely at heights of about 50 miles.
They were first observed in 1885 by an amateur astronomer two years after the eruption of the Krakatoa volcano in Indonesia, which sent millions of tons of water vapour into the upper atmosphere.
Since the 1960s, scientists have taken a closer look at the clouds with the help of satellites designed for other purposes but there were still many unresolved questions about how and why they formed in the air.
Now that could change with the launch of a scientific satellite dedicated to studying the clouds as they form in the mesosphere, the layer directly above the stratosphere.
The US National Aeronautics and Space Administration (Nasa) said that the Aeronomy of Ice in the Mesosphere (AIM) spacecraft will spend the next two years studying the physics and chemistry of noctilucent clouds from its orbit 370 miles above the poles.
"These clouds are indicators of conditions in the upper reaches of the Earth's atmosphere, and are an important link in the chain of processes that result in the deposition of solar energy into Earth atmosphere," said Mary Mellott, the mission's project scientist at Nasa.
"AIM will provide an understanding of how and why these clouds form, an important contribution toward the Nasa goals of understanding the fundamental physical processes of our space environment and how the habitability of planets is affected by the interaction of planetary magnetic fields and atmospheres with solar variability," she said.
Noctilucent clouds form in the coldest part of the atmosphere during the summer season. In the Arctic they appear between May and August and in the Antarctic they form between mid-November and mid-March.
The clouds form as a result of a complex interaction between temperature, water vapour, solar activity, atmospheric chemistry and small particles that allow the water droplets to crystallise into ice.
In recent years, the clouds have been spotted more frequently, for longer periods and at lower latitudes, suggesting to some scientists that this may be the result of human activity, either from global warming or an increase in dust particles.
"We have seen a definite increase in the brightness of these clouds in the past 25 years, which gives us cause for concern," said Dave Rusch of the University of Colorado at Boulder.
"This mission will give us an unprecedented look at how the mesosphere, which is a very sensitive region of the Earth atmosphere, is changing," Dr Rusch said.
Noctilucent clouds were usually confined to high latitudes near the poles but in recent years they have appeared further south in the northern hemisphere, which suggests that the cloud-forming conditions are moving.
"The occurrence of these clouds at the edge of space and what causes them to vary is not understood," said James Russell, a principle investigator at Hampton University in Virginia.
"One theory is that the cloud particles grow on "seeds" of meteoric dust or dust lofted up from below.
"AIM will provide the comprehensive data needed to test current theories for cloud formation or develop new ones, and allow researchers to build tools to predict how they will change in the future," Dr Russell concuded.
