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NASA is planning a new mission to probe mysterious ice clouds that hover around our atmosphere at the edge of space. The Aeronomy of Ice in the Mesosphere (AIM) spacecraft aims to help researchers understand how the clouds form and explain recent changes in their formation patterns.

The craft, slated to launch on 25 April, will carry three instruments for three experiments: the Solar Occultation for Ice Experiment; the Cloud Imaging and Particle Size Experiment; and the Cosmic Dust Experiment. The instruments will collect data on air pressure and temperature, moisture content, and cloud dimensions.

Polar mesospheric clouds are so-called because they form over the poles at an altitude of around 50km in the "mesosphere", the region just above the stratosphere. They can only be seen from the ground at night, a phenomenon known as noctilucence. This is because the need to be illuminated by sunlight that isn't also striking the surface of the earth.

The clouds seem to be a relatively recent phenomenon, with the first recorded report of them dating to the late 19th century, shortly after the volcanic eruption on the Indonesian island of Krakatoa.

In recent years they have been forming at lower altitude, and appear brighter. They are also more prevalent than they used to be, and some researchers suggest this might be the result of climate change, since an increase in atmospheric CO2 actually causes a cooling in the mesosphere.

"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's atmosphere," said Mary Mellott, AIM program scientist for NASA.

"The occurrence of these clouds at the edge of space and what causes them to vary is not understood," AIM principal investigator James Russell III, from Hampton University, said.

"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."

What is known is that the brightest of the clouds are formed primarily of water ice. Scientists also know that the clouds vary seasonally, controlled by the interactions of temperature, water vapour, solar activity, atmospheric chemistry, and the small "seed particles" on which the cloud crystals form.

The clouds form during polar summers, so between May and August in the north and November and March in the south. ®

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