In the depths of our solar system—a region where chemistry meets speculation—scientists have announced the possible existence of a molecule known as aquodiium, an elusive cousin of the ammonium ion. If true, it could explain the peculiarities in the magnetic fields of Neptune and Uranus.
This is a big problem because stable aquodiium, 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 a hydronium molecule (H3O+), so aquodiium must form. However, Hydronium is a bit easier to create. It is created by the basic process of adding a proton to water. The jump from hydronium to aquodium is the hardest part.
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But with the help of advanced computer models, scientists pinpointed a potential location for aquodiium: 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, they are both significantly tilted relative to their spin axis and significantly offset from the planets’ centers.
Is aquodiium to blame?
Special chemistry for special worlds
Due to their similar size and mass, the cores of Neptune and Uranus are almost identical. Both have rocky cores like Jupiter and Saturn, but unlike their larger neighbors, their internal pressure is 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 surfaces of these worlds.
Here’s where it gets interesting: the study’s authors suggest that the planets’ unusual magnetic fields could 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 may not only exist as single protons, but may also include hydronium, ammonium – and aquodiium.
In chemistry, a molecule typically exists in the 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 that atoms in a molecule are held together at a less than ideal angle, and electrostatic repulsion, where charged atoms or groups within a molecule they repel each other. The challenge in creating aquodium (H4O2+) is the increased electrostatic repulsion and tension that occurs when a second proton is added to a 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 to only one of the oxygen atoms, with the hydrogen atoms arranged in a stable geometry around the central oxygen atom. To get from this situation to aquodium, you would have to add a second proton to the structure – but this would increase the amount of positive charge in the molecule, leading to significant electrostatic repulsion between the positively charged protons and disrupting the existing hydronium. molecular structure forming a strain.
Under normal conditions, these factors do not allow the formation of a stable aquodium. The only way this would be possible would be if enough energy was present in the reaction validity molecule to come together anyway, amidst all the tension, repulsion, and other complications not even discussed. We don’t have that kind of energy on Earth. However, in extreme conditions on Uranus and Neptune, energy may actually be plentiful.
The researchers said that aquodiium appeared to be a plausible outcome in their simulations, specifically because the very high pressures found on these worlds favor the binding of oxygen and hydrogen ions so that aquodiium can be stabilized. And if these planets harbor stable aquodiium, we may finally be on our way to deciphering where they get their special magnetic fields.
The study was published in May in the diary Physical overview B.