Caption: 
While Uranus (pictured, image taken from the 1998 Voyager 2 mission) and Neptune have long been classified as ice giants, the composition of their interiors is a mystery. New research led by Southwest Research Institute (SwRI) measured the planets’ atmospheric argon-40 to estimate the amount of rock found inside them, searching for clues about the planets’ origins, including where and how they formed in our solar system.