When we send astronomical satellites out to explore the solar system, we also take the opportunity to set them to look back on Earth. This tradition dates back to the time of the late 1970s when astronomer Carl Sagan requested a view of Earth from the Voyager 1 satellite from vantage point of Saturn.
In that famous photo, the entire Earth occupies a scant couple of pixels on the image. Since that time, many pictures of Earth have ben taken at still higher spatial resolution. As the level of our ability to see fine details grew, we started noticed something curious about the images.
In addition to seeing the beauty of our tiny marble-colored planet, we also saw brief surges of light appear on the surface. The source of such “twinkles” went undiscovered from their first occurrence in 1993 until 2016.
At first planetary scientists attributed these mysterious twinkles to reflections off of the water, except that the twinkling took place both on water and on land. Then lightning was cited as the cause, except that the events did not take place all over the planet as would be expected of electrical storms. Instead, the events showed a restriction in latitudes. It was this clue led us to the most likely solution, that the twinkling is caused by sunlight reflecting off of the Earth.
With just the right angle between the Sun, Earth and satellite, such a twinkle can be generated by ice crystals. This study was conducted based on analyzing 860 twinkles appearing over a span of 23 years. Note the satellites that produced the images of the Earth from space were positioned typically one million miles from Earth (yes one million), yet the ice crystals are smaller than the size of a single poppy seed (50 – 100 micrometers).
If ice crystals are really so bright and shiny, then in principle, one can apply a similar technique to make the first discoveries of ice crystals (and potentially water) on planets orbiting other stars.