Friday Night Science: What's Inside Jupiter?

The interior of all rocky planets or terrestrial planets, viz., Earth, Mars, Venus, Mercury, and even their (rocky) moons, more or less identically differentiates into an (outer) crust, a (middle) mantle, and an (inner) core. Of course, there are further particularities, such as when the planet is small, over billions of years, its interior loses heat, its geo-dynamo stops, its magnetic field vanishes, whatever plate tectonics it previously had comes to a halt, the outer surface begins to crack and turn wrinkly as the entire planet ''freezes'' into a cold lump of rock. Of all the rocky planets and moons in the solar system, Earth is the only candidate with active plate tectonics. 

You're already familiar with the interior of rocky planets. Now imagine you're to slice through Jupiter — a gas giant. With dozens of storms spiraling in fractals around the poles, sometimes coming together in perfect geometrical patterns, hexagons or octagons, with a four-hundred-year-old anticyclonic storm stuck between turbulent tracks of roaring trade winds, to an outside observer, the great planet presents an ever-dynamic landscape. Even after several decades of surveys, flyby and orbiter missions, and theoretical modeling, what happens under the colored bands of atmospheric turbulence remains a mystery. But one time, we dared to throw in a probe into the land of the eternal tempests. The Galileo atmospheric descent probe of the Galileo missions (terminated in 2003) added a wealth of knowledge and refined some of our previous theories regarding the structure of Jupiter's atmosphere. 

Timeline of Galileo probe's atmospheric entry and termination of signal. Galileo
descent probe transmitted nearly 58 minutes of valuable data before succumbing to intense pressure.  
 Credit: NASA

Being a gas giant, Jupiter has no solid surface to land on. Like Earth's atmosphere, Jupiter's atmosphere has four layers: troposphere, stratosphere, thermosphere, and exosphere. Since the planet has no surface like terrestrial planets, the base of the troposphere lies 90 km below the point where the pressure is equal to 1 bar. The temperature and pressure at the base of the troposphere or the so-called ''surface'' are 340 K and 10 bars, respectively. Jupiter's atmosphere extends vertically upwards for 5000 km until it merges into the interplanetary medium or space. The temperature gradient in Jupiter's atmosphere mimics Earth - temperature from the base of the troposphere decreases with increasing height, maintains a constant value throughout the stratosphere, and sharply increases in the thermosphere. 

Jupiter's clouds expected and atmospheric data
returned by Galileo descent probe. 
Credit: NASA

Below the base of the troposphere, things turn interesting. As the pressure increases, the gaseous (hydrogen) atmosphere turns into a fluid. Deeper still, where the pressure crosses a million bars, fluid molecular hydrogen can transition into an even stranger phase of matter. Under normal atmospheric pressure (1 bar at mean sea level), hydrogen is a gas. However, when its temperature goes below 33 K, gaseous hydrogen transforms into a liquid, and for it to remain in that state, the temperature must be further reduced to 20 K. At or below the critical temperature, if pressure increases to 13 bars, liquid hydrogen becomes supercritical hydrogen. In this state, hydrogen molecules are so close together that the sharp distinction between gas and liquid phases of matter disappears, and the supercritical fluid displays the dual nature of gas and liquid. If the pressure is increased enormously, upwards of 4 million bars, the hydrogen molecules are compressed even further. A phase change occurs during which the previously compressed liquid hydrogen molecules arrange themselves in a specific crystal lattice, take on the characteristics of a metal and conduct electricity. 

However, the metallic state of hydrogen remains an elusive quest. The possibility of hydrogen turning into a liquid metal at ultrahigh pressures was investigated theoretically as early as 1935 by physicists Eugene Wigner and Hillard Bell Huntington. So far, no laboratory has been able to synthesize it. However, scientists speculate that the highly compressed interior of gas giant planets might offer the right conditions for hydrogen gas to turn into a queer batch of liquid metal. Such speculations aren't unjust. The origins of Jupiter's strong magnetic field can be understood in terms of electric currents churning throughout the mantle of metallic hydrogen. 

Jupiter's interior lies in the realm of speculation. Based on some probable hypotheses, this diagram shows the great planet's interior divided into a solid core of rock and iron, within the larger envelope of metallic hydrogen overlaid with an outer envelope of liquid molecular hydrogen. Actual conditions could be different. 
Credit: Public Domain, via Wikimedia Commons. 

Planetary scientists aren't sure if Jupiter has a solid core or if the planet is entirely volatile all the way down. And as always, there are two competing theories. One theory argues that the great planet was the first to condense out of the solar nebula beyond the snow-line, a minimum distance away from the young proto-sun, where temperatures were low enough for volatiles like liquid water, and gases like ammonia, methane, carbon dioxide, and others to condense into tiny grains and eventually grow into planetesimals (precursor to planets) and then planets. Proponents of this theory assume that Jupiter started as a small lump of ice and rock, which then attracted hydrogen and other volatiles and continued growing into a massive giant. Jupiter was almost on the verge of initiating nuclear fusion and turning into a second Sun. Alternatively, the other theory suggests that Jupiter condensed from the solar nebula where deep below the planet is volatile, with exotic states of matter. Nonetheless, whether there's a core or not, scientists speculate that central temperatures can be more than 30,000 K and pressures in terms of tens of millions of Earth atmospheres.

In summary, scientists don't know for sure what Jupiter's interior might look like. They can only guess. And perhaps in the upcoming future, we'll be able to answer more of our questions.