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The influx of small comets into Earth’s atmosphere may help explain the source of water needed to form noctilucent clouds. These strange and quite beautiful clouds can be seen over the polar regions during the summer months. They are thin clouds, wavy or banded, colored silver or bluish white. They form at an altitude of about 55 miles, in the coldest part of the upper atmosphere, a relatively unexplored boundary known as the mesopause. No other cloud occurs so high in the sky. They are called noctilucent clouds because they can only be seen against a dark sky when illuminated by the setting sun. These clouds require considerably more water vapor than can be expected at that altitude from ocean evaporation. No one thoroughly understands why these clouds exist. But rocket-borne experiments sent up by aeronomers–those who explore the upper atmosphere–to probe these clouds have shown that the clouds are composed of ice crystals formed around meteoric dust particles–a finding that suggests small comets might indeed be responsible.

Excerpted from ‘The Big Splash’ by Louis Frank and Patrick Huyghe