Space.com: Based on data from the Solar Occultation for Ice Experiment, NASA researchers have determined that 3% of each ice crystal in noctilucent clouds consists of dust from meteors. Noctilucent clouds, which are bluish clouds that can be seen just after twilight, form at an altitude of around 80 km. Their origin has been a mystery since the clouds were first discovered in 1885. James Russell, the lead researcher, believes that this new evidence is the answer. Because meteors burn as they pass through the atmosphere, they leave behind dust, which combines with water vapor to form ice crystals only 20 to 70 nm across. Those crystals are much smaller than the crystals formed lower in the atmosphere that create cirrus clouds. And the small size also explains noctilucent clouds’ bluish color, because the smaller crystals scatter the shorter, blue wavelengths. The researchers are still interested in determining why noctilucent clouds’ zone of visibility is spreading from higher latitudes toward the equator, though they believe it may have to do with increased methane levels in the atmosphere.