noctilucent clouds — 'night shining' in Latin — are shining-blue clouds that form higher than any other type of cloud, at nearly the edge of space.A scientific mystery up until now, rare
Visible in deep twilight, they are so high up that they are still able to reflect light from the Sun long after it has set. Their first documented appearance was after the eruption of Krakatoa in 1883, so at first it was thought that they were linked to volcanic dust.
However, a recently discovery has shed more light on noctilucent clouds (NLCs).
"We've detected bits of 'meteor smoke' imbedded in noctilucent clouds," says from James Russell, the principal investigator of NASA's Aeronomy of Ice in the Mesosphere (AIM) mission and Co-Director of the Center for Atmospheric Sciences at Hampton University in Hampton, Va, according to a statement released by NASA. "This discovery supports the theory that meteor dust is the nucleating agent around which NLCs form."
Clouds form by water vapour condensing onto tiny particles in the atmosphere — dust, soot, salt, etc — collectively called 'condensation nuclei', but since the air at the top of the mesosphere is so thin that it is essentially outer space (1/100,000 of sea-level pressure), where the water and condensation nuclei were coming from baffled scientists.
Using the Solar Occultation for Ice Experiment (SOFIE), which is part of the AIM mission, the researchers took readings from sunlight passing through these high-flying clouds, and found that all the ice crystals in the clouds have a tiny bit of dust in them that came from meteors burning up in the atmosphere.
The scientists have also solved another part of the mystery — where the water comes from — with a much more terrestrial explanation: Methane.
"When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor," Russell said. "This extra water vapor is then available to grow ice crystals for NLCs."
Noctilucent clouds were normally only observed near the north and south pole back in the 19th century, but have been seen further away from the poles in more recent times. The spread of these clouds away from the poles could be due to the amount of methane that is being released into the atmosphere by human activity.
These clouds are harmless, of course, so no catastrophic predictions are being made from seeing them further from the poles. However, if they are showing up more often, and further away from the poles, it gives us some insight on how methane behaves in the atmosphere and what effect all the extra methane released is having.
As Dr. Russell says: "Noctilucent clouds might look alien, but they're telling us something very important about our own planet."
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