It all started when NASA scientist Alexander Marshak spotted a pale blue twinkling dot on images sent back from a weather satellite launched in 2015. Long term observation revealed hundreds of instances of the flashing light over a period of a year, many noted by eagle-eyed amateurs. Now space boffins think they have uncovered the cause...

Every hour since 2015, a high performance camera on board the National Oceanic and Atmospheric Administration’s Deep Space Climate Observatory (DSCOVER) has been taking snaps of the earth. Whilst skimming through these images, Marshak noticed sparkling lights over the oceans. But this was not the first time these lights had been recorded. Carl Sagan first spotted them in 1993 when Jupiter probe Galileo momentarily turned its lens towards Earth.

At the time, Sagan put the flashes down to oceanic reflections. However, many of Galileo’s images showed the flashes over land, which could not be explained in the same way. Marshak writes: "When I first saw it, I thought maybe there was some water there, or a lake the sun reflects off. But the glint is pretty big, so it wasn't that.” After exhausting a range of possibilities, Marshak eventually noticed that the glints occurred only when thin, wispy cirrus clouds were present. Cirrus clouds are composed of super cooled crystal, which has led Marshak and his NASA team to the theory that the flashes are “most likely solar reflection off horizontally oriented particles”.

Sources:

1. CBS News:
   Mystery over Earth’s light flashes may be solved.

2. LiveScience:
   Mysterious Light Flashes First Discovered by Carl Sagan Explained.