Aptly named Jupiter is the king of the solar system. At 318 times the mass of Earth, Jupiter is the largest planet in orbit around the Sun. Because it is so massive, its gravity determines the dynamics of the solar system, harassing comets and bouncing asteroids.
Considering how big and powerful Jupiter is, you might think that science has already figured out all the basic facts about its structure. But you would be wrong – as wrong, in fact, as the scientists who thought for many decades that they had these basic facts. Thanks to recent data from the Juno space probe and from experiments conducted at the world’s largest laser fusion laboratories, we now know that there is a mystery at the core of the great gas giant.
Rock Science
Your high school Earth science class may have included a module on the solar system that describes the architecture of its planets. You would learn that the inner part of the system contains the small and rocky earthly worlds — Mercury, Venus, Earth and Mars. The outer solar system is the land of the giants. There are gas giants Jupiter and Saturn and ice giants Uranus and Neptune. At the center of all these larger worlds, there is supposed to be a rocky core. As for Jupiter, your high school textbook might have an illustrated cross-section of the gas giant showing thick outer layers of hydrogen and helium surrounding a compact chunk of rocky material nestled deep beneath.
The story of the rocky core was the gospel of planetary science for decades. However, for scientists studying Jupiter, the details of this core have always been somewhat elusive. The planet is so large that the physics of its interior push our understanding of matter to its limits. The pressure inside Jupiter can reach millions of times that of Earth’s surface, with core temperatures rising to 36,000°F (~20,000°C).
Because these conditions are so extreme, scientists must turn to extreme instruments to probe Jovian’s interior. The only way Earth-bound researchers can reach Jupiter-like pressures is by using the world’s most powerful lasers to compress samples of material. That’s exactly the mission of the Center for Matter at Atomic Pressure, or CMAP, a National Science Foundation-funded research consortium based at the University of Rochester. (I am a member of the CMAP team.)
Rochester is the perfect place to host this effort because it is the site of Laser Energy Laboratory, one of the leading laser fusion facilities in the world. With the Laboratory’s 60-beam laser system, CMAP scientists use light to knock samples of hydrogen, helium, or rock material to the same temperatures and pressures that nature produces in Jupiter’s core using only gravity.
The other tool scientists use to study Jupiter is the one that led to the crisis in understanding its nature: the Juno spacecraft. Juno is orbiting the gas giant right now, diving so close that it can snap pictures of Jupiter’s cloud tops. By tracking Juno’s orbital motions with extreme precision, scientists can track the gravitational signature created by Jupiter’s internal distribution of matter.
This is how the mystery in Jupiter’s core first became known.
Jupiter’s equation of state
As the Juno data came in, it soon became clear that Jupiter’s interior structure differed greatly from what long-standing models had predicted. The probe’s gravity measurements revealed that the classic picture of Jupiter’s large rocky core is incorrect.
It is not easy to build a better model to fit the data. The laws of physics determine how the different layers of a planet like Jupiter can be organized. In particular, something called equation of state determines how dense a material becomes as pressure and temperature increase as you fall through these layers to the core. But the equation of state is in turn determined by things like the mix of elements in each layer as well as their physical characteristics.
Any attempt to change models of Jupiter’s structure must conform to the physics of the equation of state, and this is where CMAP steps in. The laser experiments are key to understanding Jupiter’s equation of state, placing constraints on the physics that form the models.
The crux of the problem
Now at last we have approached the heart of the matter. Based on detailed studies led by CMAP member Burkard Militzer, the idea of a compact rocky core for Jupiter may have to be a thing of the past. Using computer models, Militzer and his team explored different configurations of the planet’s internal structure. Their findings point to a rarefied core — one with rock material extending 63% of the way to the surface.
A core that spans nearly two-thirds of the planet’s radius is not a core at all, and certainly not the compact core that the old models predicted. In fact, deep in these inner regions, Militzer’s team found that only 18% of the material is rocky (that is, made of material like silicon that passes into rocks).
If these results are correct, then there may be a lot wrong with our prevailing ideas about how giant planets form and how their early evolution proceeds. But there is still much work to be done to confirm the models and their interpretation. Laser experiments will help with this task by probing the structure of matter under the conditions predicted by these new models. More modeling will also be needed. But if all goes well, Juno will have done her job in showing us how naive our ideas about the king of the planets were. This is all for the good. Overturning old concepts can be the most exciting thing in science.
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