Oct 15 2021

Superionic Ice and Magnetic Fields

Published by under Astronomy
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Some planets have planetary magnetic fields, while others don’t. Mercury has a weak magnetic field, while Venus and Mars have no significant magnetic field. This was bad news for Mars (or any critters living on Mars in the past) because the lack of a significant magnetic field allowed the solar wind to slowly strip away most of its atmosphere. Life on Earth enjoys the protection of a strong planetary magnetic field, protecting us from solar radiation.

The Earth’s magnetic field is created by molten iron in the outer core. Rotating electrical charges generate magnetic fields, and iron is a conductive material. This is called the dynamo theory, with the vast momentum of the spinning iron core translating some of its energy into generating a magnetic field. In fact, the molten outer core is rotating a little faster than the rest of the Earth. This phenomenon is also likely the source of Mercury’s weak magnetic field.

The two gas giants have magnetic fields, with Jupiter having the strongest field of any planet (the sun has the strongest magnetic field in the solar system). It’s largest moon, Ganymede, also has a weak magnetic field, making it the only moon in our solar system to have one. Jupiter’s magnetic field is 20,000 times stronger than Earth’s. It’s massive and powerful. The question is – what is generating the magnetic field inside Jupiter? It’s probably not a molten iron core, like on Earth. Based on Jupiter’s mass and other features, astronomers suspect that the magnetic field is generated by liquid hydrogen in its core. Under extreme pressure, even at high temperatures, hydrogen can become a metallic liquid, capable of carrying a charge, and therefore generating a magnetic field. This is likely also the source of Saturn’s magnetic field, although it’s field is slightly weaker than Earth’s.

Uranus and Neptune are considered ice giants, with about 80% of their mass made up of icy material such as methane, ammonia, and water. They both have strange magnetic fields. Neptune’s is 27 times stronger than Earth’s, and is significantly tilted from its axis of rotation. Uranus’s is even stranger, ranging from 1/3 to 4 times the Earth’s field strength. According to NASA, “The magnetic axis is tilted nearly 60 degrees from the planet’s axis of rotation, and is also offset from the center of the planet by one-third of the planet’s radius.” Because of the shape and characteristics of their fields, astronomers do not think they derive from a liquid metallic hydrogen core. Instead they theorize that their magnetic fields derive from liquid layers just below their liquid surface.

If small rocky worlds get their magnetic fields from molten iron cores, and gas giants get their magnetic fields from liquid metallic hydrogen, where do the ice giants get their magnetic fields? That is a question astronomers are exploring, and a recent study may shed some light on the question. The hypothesis is that the rotating conductive material in ice giants generating their magnetic fields is superionic water ice. To understand what this is, take a look at the phase diagram of water above. The horizontal axis is temperature and the vertical axis is pressure. As you can see, at super high pressures even very hot water is still in an ice phase (the blue section). The question for physicists is – what are the properties of water ice at the kinds of pressures we would see inside the ice giants?

What their research suggests is that at the temperature and pressure likely present beneath the surface of Neptune and Uranus water would form what they are calling superionic ice. The oxygen atoms in the water would form a lattice, with the hydrogen ions free to flow through that lattice. Free flowing hydrogen ions would create a high electrical charge, and a moving electrical charge generates a magnetic field. They therefore might have hit upon the mechanism by which ice giants tend to generate a planetary magnetic field.

How did the researchers do it?

“So, our research team, led by the University of Chicago’s Vitali Prakapenka, set out to use multiple spectroscopic tools to map changes in ice’s structure and properties under conditions ranging up to 1.5 million times normal atmospheric pressure and about 11,200 degrees Fahrenheit,” explained Carnegie’s Alexander Goncharov.

“In order to probe the structure of this unique state of matter under very extreme conditions—heated by a laser and compressed between two diamonds—we used the Advanced Photon Source’s brilliant high-energy synchrotron x-ray beam, which was focused down to about 3 micrometers, 30 times smaller than a single human hair,” said Prakapenka, explaining the work done using the facility’s GSECARS beamline. “These experiments are so challenging that we had to run a few thousand of them over a decade to get enough high-quality data to solve the long-standing mystery of high-pressure, high-temperature behavior of ice under conditions relevant to giant planet interiors.”

More work needs to be done, but this sounds like a very plausible explanation for ice giant magnetic fields.

The universe has lots of ways of generating magnetic fields, again all you need is moving charged particles. Neutron stars have the strongest magnetic fields, with the strongest detected being about 1 billion Tesla, or about a quadrillion times that of Earth. In fact, neutron stars with the most powerful magnetic fields are called magnetars. If you ever got to within 1000 km of a magnetar the field would be so powerful it would rip apart your ions, deconstructing you at the atomic level.

Magnetic fields can be our friend, when they protect us from harmful radiation, or they can kill us instantly.

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