Sky watchers and even astronauts in orbit have long observed wispy electric-blue clouds glowing on the fringes of space. Theories abounded over the origin of these noctilucent clouds (NLCs), but a Nasa mission has found a clue: traces of extraterrestrial smoke. Researchers with the Aeronomy of Ice in the Mesosphere (AIM) mission can determine the chemical composition of NLCs by the way light reacts to the particles. “We’ve detected bits of ‘meteor smoke’ embedded in noctilucent clouds,” reports AIM principal investigator James Russell. Ice crystals grow around tiny bits of cosmic matter left behind when meteors burn up in the atmosphere; when sunlight strikes the haze, it scatters in short wavelengths, making the clouds appear to glow blue. – Joe Pappalardo
Glossary: Noctilucent clouds, also known as polar mesospheric clouds (PMCs) over Wismar, Germany. These clouds shine brightly even during the night.
Related content: Click here to view a stunning image of noctilucent clouds taken by the crew on the International Space Station as they passed over the Tibetan Plateau.