In the depths of our solar system—a realm where chemistry meets speculation—scientists have reported the possible existence of a molecule known as aquadium, an elusive cousin of the ammonium ion. If true, this could explain the oddities in the magnetic fields of Neptune and Uranus.
This is a big deal because stable aquadium, which consists of four hydrogen atoms and one oxygen atom (H4O2+), has never been observed before due to the high energy barrier associated with adding a second proton to the hydronium molecule (H3O+), so it must to form an aquadium. However, Hydronium is a little easier to create. It is formed by the basic process of adding a proton to water. The jump from hydronium to aquadium is the hard part.
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However, using advanced computer models, researchers have pinpointed a potential habitat for aquadium: the extreme pressures found in the cores of the ice giants Uranus and Neptune. And importantly, its presence in this intense, icy environment may help explain the planets’ unusual magnetic fields—strangely, both are highly tilted relative to their spin axis and significantly offset from the planets’ centers.
Could the aquadium be to blame?
Strange chemistry for strange worlds
Because of their similar size and mass, the cores of Neptune and Uranus are nearly identical. Both have rocky cores like Jupiter and Saturn, but unlike their larger neighbors, their internal pressures are not sufficient to turn molecular hydrogen into an electrically conductive liquid metal. Instead, a large mantle of icy water and ammonia forms about 12,427 miles (20,000 kilometers) below the surface of these worlds.
Here’s where things get interesting: the study authors suggest that the planets’ unusual magnetic fields may be generated by ions acting as charge carriers. Ions are atoms or molecules with a net electrical charge resulting from the loss or gain of one or more electrons. These ions related to the magnetic fields of Uranus and Neptune need not exist only as single protons, but may also include hydronium, ammonium, and aquadium.
In chemistry, a molecule usually exists in its lowest energy form, known as the ground state. This is because nature tends to follow the path of least resistance and the ground state minimizes factors such as bond stress, which means the atoms in the molecule are bonded at less than ideal angles, and electrostatic repulsion where charged atoms or groups within molecules repel each other. The challenge in forming aquadium (H4O2+) lies in the increased electrostatic repulsion and tension that occurs when a second proton is added to the hydronium ion—it’s like trying to bring two positively charged magnets together.
When a proton is added to water to form hydronium, these two factors are more easily overcome; the resulting molecule has a positive charge localized on only one of the oxygen atoms, with the hydrogen atoms arranged in a stable geometry around the central oxygen atom. To move from this situation to aquadium, you would need to add a second proton to the structure – but this would increase the amount of positive charge in the molecule, causing significant electrostatic repulsion between the positively charged protons and disrupting the existing hydronium molecular structure, creating tension.
Under normal conditions, these factors prevent the formation of a stable aquadium. The only way this would be possible is if enough energy is present in the reaction strength molecule to come together anyway, amid all the tension, repulsion, and other complications not even discussed. We don’t have that kind of energy on Earth. But in the extreme conditions of Uranus and Neptune, there may indeed be enough energy.
The scientists report that aquadium seems a plausible outcome in their simulations, particularly because the very high pressure found on these worlds promotes the binding of oxygen and hydrogen ions so that aquadium can be stabilized. And if these planets harbor stable aquadium, well, we may finally be on the way to decoding where they get their strange magnetic fields.
The study was published in May in the diary Physical examination B.
