The infrared spectrum of ${\mathrm{H}}_3^{+}$ in laboratory and space plasmas

Protonated hydrogen, ${\mathrm{H}}_3^{+}$, composed of three protons and two electrons, is the simplest stable polyatomic system. It is the most abundant ionic species in molecular hydrogen plasmas, and it is assumed to play a central role in the chemical evolution of molecular clouds, the birthplace of stars. The existence of this species has been known since its discovery by J. J. Thomson in 1912, but its first spectrum was observed only in 1980. The infrared spectrum has since been extended to higher vibrational and rotational states. The existence of a large amount of ${\mathrm{H}}_3^{+}$ in Nature was first demonstrated in the serendipitous discovery of its infrared emission bands in the polar auroral regions of the Jupiter ionosphere. The spectrum is extremely intense and pure and can be used to monitor the morphology and temporal variation of the dynamic Jovian plasmas. Early this year, an identification of ${\mathrm{H}}_3^{+}$ emission was claimed in Supernova 1987A. In April strong ${\mathrm{H}}_3^{+}$ infrared emission was detected in Uranus. These developments are reviewed and the future is projected.