In dark interstellar clouds where new stars are born, chemistry starts with a tiny particle — the trihydrogen ion (H₃⁺). It collides with other atoms and sets off a cascade of reactions that build ever more complex molecules. For a long time, it was thought that the main reaction producing it, H₂⁺ colliding with H₂, only happens in the gas phase. Whether it could occur right on the surface of cosmic dust grains had never been tested.
A team of researchers took silica nanoparticles covered with water and blasted them with powerful X-ray pulses at an energy of 1.88 keV. They used three techniques simultaneously: ion velocity map imaging, electron time-of-flight spectroscopy, and diffraction imaging of individual particles. This allowed them to observe the chemistry on each grain separately. They found that the X-ray pulse creates an electric field of staggering strength at the surface — on the order of a volt per nanometer. For comparison, if such a field acted over a distance of a millimeter, the potential difference would be a million volts.
Using quantum calculations, the scientists traced the entire charge transfer pathway. The mechanism turned out to be surprisingly familiar: it exactly mirrors the band bending in semiconductor photoelectrodes. Thus, cosmic chemistry on dust grains and terrestrial catalysis on surfaces have converged into a single picture. Now it is clear that the surface field is a universal conductor, orchestrating the birth of the first molecules both in the interstellar medium and in our laboratories.
The H₃⁺ ion is the most abundant molecular ion in the universe after H₂, but on Earth it is almost never found in natural conditions due to its high reactivity.
In Carl Sagan's novel 'Contact,' the alien signal begins with prime numbers, but in reality, the first 'hello' from distant worlds might be the spectral lines of H₃⁺ in exoplanet atmospheres.