Could Jupiter and Saturn Contain Liquid Metal Helium?

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The interiors of the two gas giants, Jupiter and Saturn, are pretty extreme places. With atmospheric pressures of around 70 million Earth atmospheres, the phases of material become a bit difficult to understand. Usually when we think of a liquid metal, we have thoughts about liquid mercury at room temperature (or the reassembling liquid metal T-1000 played by Robert Patrick in the film Terminator 2), rarely do we consider two of the most abundant elements in the Universe to be a liquid metal in certain conditions. And yet, this is what a team of physicists from UC Berkley are claiming; helium and hydrogen can mix together, forced by the massive pressures near the cores of Jupiter and Saturn, forming a liquid metal alloy, possibly changing our perception of what lies beneath those Jovian storms…

Usually planetary physicists and chemists focus most of their attention on the characteristics of the most abundant element in the Universe: hydrogen. Indeed, over 90% of both Jupiter and Saturn is hydrogen too. But within these gas giant’s atmospheres is not the simple hydrogen atom, it is the surprisingly complex diatomic hydrogen gas (i.e. molecular hydrogen, H₂). So, to understand the dynamics and nature of the insides of the most massive planets in our Solar System, researchers from UC Berkley and London are looking into a far simpler element; the second most abundant gas in the Universe: helium.

Raymond Jeanloz, a professor at UC Berkeley, and his team have uncovered an interesting characteristic of helium at the extreme pressures that can be exerted near the cores of Jupiter and Saturn. Helium will form a metallic liquid alloy when mixed with hydrogen. This state of matter was thought to be rare, but these new findings suggest liquid metal helium alloys may be more common than we previously thought.

“This is a breakthrough in terms of our understanding of materials, and that’s important because in order to understand the long-term evolution of planets, we need to know more about their properties deep down. The finding is also interesting from the point of view of understanding why materials are the way they are, and what determines their stability and their physical and chemical properties.” – Raymond Jeanloz.

Jupiter for example exerts an enormous pressure on the gases in its atmosphere. Due to it’s large mass, one can expect pressures up to 70 million Earth atmospheres (no, that isn’t enough to kick-start fusion…), creating core temperatures of between 10,000 to 20,000 K (that’s 2-4 times hotter than the Sun’s photosphere!). So helium was chosen as the element to study under these extreme conditions, a gas that makes up 5-10% of the Universe’s observable matter.

Using quantum mechanics to calculate the behaviour of helium under different extreme pressures and temperatures, the researchers found that helium will turn into a liquid metal at very high pressure. Usually, helium is thought of as a colourless and transparent gas. In Earth-atmosphere conditions this is true. However, it turns into an entirely different creature at 70 million Earth atmospheres. Rather than being an insulating gas, it turns into a conducting liquid metal substance, more like mercury, “only less reflective,” Jeanloz added.

This result comes as a surprise as it has always been thought that massive pressures make it more difficult for elements like hydrogen and helium to become metal-like. This is because the high temperatures in locations like Jupiter’s core cause increased vibrations in atoms, thus deflecting the paths of electrons trying to flow in the material. If there is no electron flow, the material becomes an insulator and cannot be called a “metal.”

However, these new findings suggest that atomic vibrations under these kinds of pressures actually have the counter-intuitive effect of creating new paths for the electrons to flow. Suddenly the liquid helium becomes conductive, meaning it is a metal.

In another twist, it is thought that the helium liquid metal could easily mix with hydrogen. Planetary physics tells us that this isn’t possible, hydrogen and helium separate like oil and water inside the gas giant bodies. But Jeanloz’s team has found that the two elements could actually mix, creating a liquid metal alloy. If this is to be the case, some serious re-thinking of planetary evolution needs to be done.

Both Jupiter and Saturn release more energy than the Sun provides meaning both planets are generating their own energy. The accepted mechanism for this is condensing helium droplets that fall from the planets’ upper atmospheres and to the core, releasing gravitational potential as the helium falls as “rain.” However, if this research is proven to be the case, the gas giant interior is likely to be a lot more homogenous than previously thought meaning there can be no helium droplets.

So the next task for Jeanloz and his team is to find an alternate power source generating heat in the cores of Jupiter and Saturn (so don’t go re-writing the textbooks quite yet…)

Source: UC Berkeley