(Sen) - Skywatchers have been enjoying the peak of one of the year's best meteor showers this weekend as the Perseids rain down over the Earth providing a natural firework display.
Meteors burn up high in the atmosphere throughout the year as "shooting stars", and despite each being only the size of a grain of sand, together they deposit hundreds of tons of dust in the process.
Now, thanks to a space mission called AIM, scientists have discovered that this dust directly contributes to a mysterious phenomenon that until now has not been terribly well understood - noctilucent, or night-shining, clouds (NLC).
These electric-blue wisps, which produce spectacular rippling patterns at certain times of year, are quite unlike normal weather clouds because they form at a height of around 80km (50 miles) in a zone of the atmosphere call the mesosphere. From northern latitudes they are particularly visible during the sumer months.
The reason they shine is because at this height they still catch the light of the Sun even when it is the middle of the night. Similar displays have been seen from southern hemisphere countries during their summer in the early months of the year.
They have even been observed and photographed from above by astronauts on the International Space Station.
Noctilucent clouds - sometimes called space clouds - were first noticed in modern times in 1885 following the eruption of Indonesian volcano Krakatoa two years earlier. That eruption sent masses of dust and ash high into the atmosphere.
The connection between meteor dust and noctilucent clouds was revealed by NASA's Aeronomy of Ice in the Mesosphere satellite mission (AIM) which was air-launched aboard a Pegasus XL rocket from California in April 2007.
In the following months, the satellite returned data that showed that the ice crystals forming the clouds contained meteoric dust. Mark Hervig, who led the discovery team, said: "Using AIM's Solar Occultation for Ice Experiment (SOFIE), we found that about 3 per cent of each ice crystal in a noctilucent cloud is meteoritic."
NASA video about the AIM discovery
James Russell, of Hampton University, and principal investigator of NASA's AIM mission, added: "We've detected bits of 'meteor smoke' embedded in noctilucent clouds. This discovery supports the theory that meteor dust is the nucleating agent around which NLCs form."
This showed that the tiny particles in meteor dust attract molecules of water which assemble as ice crystals, a process called "nucleation". It is a common process in the lower atmosphere, using wind-blown dust to produce rain and snow but would hardly happen without the contribution from the vaporising meteors at the higher altitudes.
In recent years, noctilucent clouds have been seen more frequently and at latitudes further from the poles. It has been suggested that pollution, and in particular the greenhouse gas methane, might be contributing to their appearance as a side-effect of climate change.