NASA picked the 100 greatest images of Saturn from its Cassini mission - here are some of the best

After 13 years of orbiting Saturn and its moons, NASA's bus-size Cassini probe is now a puff of radioactive vapor in the planet's swirling clouds.

Space agency leaders knew this day would come since 2010, when they decided to empty Cassini's tanks to continue exploring Saturn as long as possible.

Without a means of controlling the probe, they reasoned, Cassini had to be destroyed. This would prevent it crashing into Saturn's icy moons Enceladus and Titan - which hide vast, salty, global oceans that may be habitable to alien life - since Cassini left Earth contaminated with small amounts of bacteria.

To honor the more than 453,000 photos Cassini took of Saturn, its rings, moons, and other objects, several NASA divisions worked together to select its 100 greatest visuals. Those have been turned into a 110-page ebook, titled "The Saturn System Through the Eyes of Cassini", which is free to download for iBooks, Kindles, and other ebook readers (and just as a PDF).

"While these images represent the tip of the iceberg - each telling a story about Saturn and its mysterious moons - our hope is that the mission will inspire future artists and explorers," NASA wrote in a press release. "The sheer beauty of these images is surpassed only by the science and discoveries they represent."

Jim Green, NASA's head of planetary science, wrote the forward to the collection of pictures.

"This book is the first chapter of what I predict will be the greatest story ever told: how humans reached for the stars and discovered life beyond Earth," Green said.

Here's a small collection of the best images and what they reveal.