Uranus and Neptune may harbor a strange molecule that affects their magnetic fields

Skoltech scientists and their Chinese colleagues have determined the conditions that allow a very special ion to exist. Called aquodiium, it can be conceptualized as an ordinary neutral water molecule with two additional protons attached to it, resulting in a net double positive charge.

The team suggests that the ion could be stable inside the ice giants Uranus and Neptune, and if so, it must play a role in the mechanism that gives rise to the planets’ unusual magnetic fields. The study is published in Physical overview B

A special magnetism

We don’t understand the magnetic fields of Uranus and Neptune as well as we do Jupiter and Saturn—or our own planet.

In the Earth’s interior, the circulation of an electronically conductive liquid alloy of iron and nickel creates magnetism. Deep inside Jupiter and Saturn, hydrogen is thought to be compressed into a metallic state and create magnetic fields in much the same way.

In contrast, the magnetic fields of Uranus and Neptune are thought to originate from the circulation of ionically conductive media, where the core ions are themselves charge carriers, rather than merely a support structure to allow the flow of electrons.

If planetary scientists knew exactly which ions were involved and in what proportions, they might be able to figure out why the ice giants’ magnetospheres are so strange: they are misaligned with the planets’ rotations and offset from their physical centers.

Skoltech professor Artem R. Oganov, who co-authored the paper, explains how ionic and electronic conductivities differ and where the newly predicted ion fits in: “Hydrogen surrounding Jupiter’s rocky core under these conditions is a liquid metal: it can flow, the way molten the iron in the Earth’s interior flows, and its electrical conductivity is due to the free electrons shared by all the hydrogen atoms squeezed together.

“In Uranus, we think of the hydrogen ions themselves—that is, the protons—as free charge carriers. Not necessarily as individual H⁺ ions, but possibly in the form of hydronium H₃O⁺ammonium NH₄⁺and a number of other ions. Our study adds another possibility, H₄O²⁺ an ion that is extremely interesting from a chemical point of view.”

Missing link

In chemistry, there is a concept of sp³ hybridization, which refers to the way electron orbitals combine with each other, and provides something of a natural template for creating plausible molecules and ions. Under sp³ hybridization, the nucleus of an atom—e.g., carbon, nitrogen, or oxygen—occupies the center of an imaginary tetrahedron.

Each of the four peaks hosts either a valence electron or two paired electrons that are not available to form bonds with other atoms. The simplest example would be a carbon atom with four unpaired electrons in the four vertices – add four hydrogen atoms and you get a methane molecule: CH₄.

For an oxygen atom that has two lone pairs in its outer shell along with two valence electrons, sp³ hybridization would mean that only two of the vertices could host a covalent hydrogen bond, with the remaining two occupied by electron pairs, giving H₂Oh, water.

If you attach a hydrogen ion (proton) to one of the electron pairs, you get a hydronium ion H₃O⁺and that’s actually what you get in an acidic solution because acids donate H protons⁺ into solution and the lone protons are immediately attracted to the electron pairs.

Pressure and acid

“However, the question was: Can you add another proton to the hydronium ion to fill in the missing part? Such a configuration is energetically very unfavorable under normal conditions, but our calculations show that there are two things that can cause this,” says Professor Xiao Dong from China of Nankai University, whose original idea is the basis of this research.

“First, very high pressure forces matter to shrink in volume, and sharing a previously unused electron pair of oxygen with a hydrogen ion (proton) is an elegant way to do this: like a covalent bond with hydrogen, except for both electrons in the pair. Second, you need lots of protons available, and that means an acidic environment, because that’s what acids do—they supply protons.”

The team used advanced computational tools to predict what would happen to hydrofluoric acid and water under extreme conditions. The result: At a pressure of about 1.5 million atmospheres and a temperature of about 3,000 degrees Celsius, well-separated aquodiium H₄O²⁺ ions appear in the simulation.

The researchers believe that their newly discovered ion should play an important role in the behavior and properties of water-based media, specifically those under pressure and containing acid.

This roughly corresponds to the conditions on Uranus and Neptune, where an extremely deep ocean of liquid water creates extremely high pressures and a certain amount of acid can be expected. If so, aquodium ions will form and, by participating in ocean circulation, will contribute to the magnetic fields and other properties of these planets in a way different from other ions.

Perhaps aquodiium could even form previously unknown minerals under these extreme conditions.