New Discoveries - Lecture 25 - Recreating the Ice Inside Uranus and Neptune

Greetings [snorts] and welcome to the introduction to astronomy. In this episode of new discoveries in astronomy, we will talk about how scientists recreated the extreme conditions inside ice giant planets and discovered a new form of superionic ice. So, let's get started. What form does ice take deep inside an ice giant? That might like a simple question, but inside planets such as Uranus and Neptune, the word ice means something very different from the frozen water we encounter on Earth. Temperatures can reach thousands of degrees, while pressures rise into millions of times Earth's atmospheric pressure. And because we cannot travel into those deep planetary interiors and collect a sample, scientists have to investigate them another way. They recreate those extreme conditions on Earth. Deep inside an ice giant, water may enter an unusual state known as superionic ice. In this state, the oxygen atoms occupy an ordered solid structure or lattice, but the hydrogen nuclei are much more mobile, moving through that oxygen framework. So, superionic ice has properties of both a solid and a fluid. And because those mobile charged particles allow it to conduct electricity, its behavior could be important for understanding the interiors and potentially the unusual magnetic fields of Uranus and Neptune. But there has been an important unanswered question. What crystal structure does the oxygen actually form under the most extreme conditions? Scientists had already identified a structure known as face-centered cubic or FCC under superionic conditions. But theoretical work suggested that water might rearrange into other structures as pressure and temperature increased. Testing that prediction is extremely difficult. We cannot simply reproduce the interior of Neptune in an ordinary laboratory chamber. Instead, researchers need to create enormous pressures in an incredibly small sample and then determine what has happened to its atoms while it remains under those conditions. That is exactly what the new experiment was designed to do. The researchers placed a tiny sample of water between the tips of two diamonds in a device called a diamond anvil cell. Diamonds are extraordinarily hard, so concentrating force onto the tiny sample can generate tremendous pressure. In these experiments, pressures reached up to about 230 gigapascals, more than 2 million times atmospheric pressure at Earth's surface. The researchers then used lasers to heat the sample to temperatures above 1800 K. But creating these conditions was only half the experiment. The team also needed to determine the structure of the material they had produced. For that, they used X-ray diffraction at the European Synchrotron Radiation Facility. The way X-rays scatter from an ordered material provide information about how its atoms are arranged. And the diffraction pattern revealed something new. Under the most extreme conditions investigated, the oxygen atoms formed a hexagonal close-packed structure, usually abbreviated HCP. Above roughly 200 gigapascals and 1800 Kelvin, this HCP structure became the dominant superionic phase observed in the experiment, replacing the face-centered cubic structure. The distinction is important. The researchers did not discover superionic ice itself. That phase was already known. What they directly observed was a new crystal structure within the superionic regime, an HCP arrangement of the oxygen atoms, while the hydrogen remained mobile. Why should the arrangement of oxygen atoms matter for astronomy? Because the microscopic structure of material influences its larger-scale physical properties. A hexagonal oxygen lattice may behave differently from a cubic one, including potentially having different electrical and mechanical properties. And those properties matter when scientists construct models of planetary interiors. If superionic water takes different forms at different pressures and temperatures, then models of Uranus and Neptune need to account for those transitions. That could eventually help scientists better understand how material is arranged and behaves deep inside these planets. It may also have implications for models attempting to explain their unusual magnetic fields. This experiment demonstrates something fundamental about planetary science. We can see Neptune's atmosphere from hundreds of millions of kilometers away. But it's deepest interior remains inaccessible to us. So scientists bring a tiny piece of that problem into the laboratory. They squeeze water between diamonds, heat it with lasers, illuminate it with x-rays, and use the resulting pattern to infer how matter behaves under conditions found on distant worlds. To explore a world we cannot enter, sometimes we recreate its conditions here on Earth. So that concludes our discussion on recreating the ice inside Uranus and Neptune. We'll be back again next time for another new discovery in astronomy. So until then, have a great day everyone, and I will see you in class.