Wet metallic hydrogen

Many efforts have been made to crush hydrogen into a metallic state, so far without success. A metal is an array of positive metal ions, surrounded by a sea of mobile, delocalized electrons. In metallic hydrogen, if we could make it, the ions would presumably be simply protons.

Daedalus now points out that while hydrogen ions feature prominently in chemistry, they never, ever, exist as free protons. Always they attach themselves to some molecule or other. Thus in water, the proton “H⁺” exists as the hydroxonium ion H₃O⁺. So, he says, the search for metallic hydrogen is wrong-headed. The way forward is to exert pressure, not on pure hydrogen, but on a solution of hydrogen in water. The result would be a new metal, hydroxonium. It would be an array of H₃O⁺ ions in an electron sea. It should be far easier to make than the putative metallic hydrogen itself; with luck, it will form at quite low pressures.

Hydroxonium will be a sort of metallic ice. Once it has been compressed into existence, it will be resonance-stabilized by the delocalization of its conduction electrons. So it should stay stable if the pressure is then reduced. Daedalus even dreams that it might remain stable right down to normal pressures. Even if it does, hydroxonium will probably not be much use in engineering. Like sodium, with which it is isoelectronic, it will be too soft and reactive. Indeed, it may be so wildly reactive as to be a splendid rocket fuel. On heating, it should decompose to steam, hydrogen and a great deal of energy.

Hydroxonium might be better employed as a minor constituent of metal alloys. Even highly reactive metals (such as lithium) can form useful alloys. Diluted with stabler metals, they lose their reactivity while still conferring valuable properties, such as high tenacity and low density, on the resulting alloy.

But Daedalus reckons that nature may have got there first. Many comets, asteroids and satellites are taken to be ‘dirty snowballs’ — water ice with impurities. Hydrogen, the most abundant element in the cosmos, will be one such impurity. So the gravitationally compressed centre of the biggest dirty snowballs may consist of hydroxonium. Space visionaries have often proposed a self-supporting form of space exploration, in which each planetary body is mined for water and rocket fuel to reach the next. Hydroxonium could make it feasible.