Trihydrogen is the most important ion you’ve never heard of

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The simplest 3-atom molecule is revolutionizing observations of outer space and illuminating the choreography of chemical bond formation

our hundred light-years from Earth in the constellation Ophiuchus—known as the snake bearer because it resembles a man grasping a serpent—floats an interstellar dust cloud. This relatively dense gathering of molecules and particles holds the makings of two future stars. Made mostly of hydrogen gas, the cloud also contains helium and frozen dust grains of carbon and silicon sometimes coated with ice. The list of ingredients making up this star nursery is interesting, but perhaps pedestrian, to chemists on Earth. That is, until you get to the part of the list that mentions trihydrogen, or H3+. This unearthly molecule consists of three protons arranged in an equilateral triangle, sharing two electrons among them.

The cloud’s temperature hovers a few tens of degrees above absolute zero. In this environment, atoms and molecules occasionally collide and then bounce apart unchanged because they don’t have enough energy to react. The highly reactive H3+, however, is primed to donate a proton to anything it stumbles into. The little molecule enriches the chemistry of the cloud by launching chains of reactions that make larger and more diverse molecules involving mostly carbon, hydrogen, and oxygen. This extreme reactivity, a boon for interstellar chemistry, also means that in a dense molecular environment, such as that found on Earth, H3+’s existence is so short lived, it’s rarely observed. As a result, it’s a relative unknown among chemists.

Astronomers, who are more familiar with the simple molecule, have exploited it as a temperature gauge and a cosmological clock, using it as a tool to understand conditions around planets in our solar system and beyond. “Every time we look at H3+, it helps us uncover some cool, crazy physics,” says James O’Donoghue, a planetary scientist at the Japan Aerospace Exploration Agency. Meanwhile, scientists here on Earth are using new technology to generate the triangular molecule and learn the atomic details of how it forms. H3+ is helping unravel the mysteries of planets, outer space, star formation, and fundamental chemical processes.



British physicist J. J. Thomson first discovered H3+ in 1911 in a plasma tube in his lab using an early form of mass spectrometry. By the 1960s, scientists speculated that H3+ might be found in space, but it was 1989 before researchers spotted its characteristic signal coming from Jupiter.

The discovery of H3+ in space hinged on a description of the molecule’s spectrum, parts of which had been defined in 1980 by the University of Chicago’s Takeshi Oka (Phys. Rev. Lett. 1980, DOI: 10.1103/physrevlett.45.531). The molecule emits infrared light at signature wavelengths that can penetrate the vast distances of space, arriving unimpeded at detectors here on Earth. Importantly, the ion unleashes its strongest emissions in a set of wavelengths rarely given off by other molecules, making it a relatively easy molecule to spot, even light-years away.



A special form of H3+ allowed scientists to estimate the age of one of these clouds in the star-forming region of the constellation Ophiuchus, shown here in an infrared photo.

Jupiter has spectacular auroras—colorful clouds of charged gas—but in the 1980s, little was known of their chemistry, says Steve Miller, a planetary scientist at University College London. So Pierre Drossart of the Paris Observatory, Miller, and their colleagues focused an infrared telescope on the auroras hovering over Jupiter’s poles. With a sensitive new spectrometer they hooked up to the telescope, they expected to see evidence of lots of hydrogen gas, H2, the most abundant molecule on the gas giant. Indeed, they did. But the spectrometer also picked up another set of unexpected IR wavelengths; Miller and colleagues realized that their predicted IR spectrum of H3+, which they had built from Oka’s work, was a perfect match for the mysterious light emissions coming from Jupiter (Nature 1989, DOI: 10.1038/340539a0). The unexpected first-time discovery of H3+ in space inspired scientists to search for it elsewhere in the universe. In the past 30 years, researchers have found H3+ nearly everywhere in outer space that they have looked. Its presence has given them a tool to directly observe processes in space that had previously been only theorized about.


“It’s not just that we can see H3+ in the upper atmospheres of planets like Jupiter, Saturn, and Uranus, but we can derive properties such as the temperature and density of H3+,” which telegraphs the temperature and density of the molecule’s surroundings, O’Donoghue says.

Out in space, when sunlight strikes H3+ or molecules bang into it, the ion absorbs energy and then releases light at particular IR wavelengths. The intensity of the energy emitted at each wavelength varies according to the molecule’s temperature, allowing H3+ to act as a virtual thermometer of outer space.

Models can also predict the amount of light that a single molecule of H3+ should emit at various temperatures. Because of this ability, measuring the light intensity that reaches their detectors enables researchers to derive the concentration of H3+ above planets’ surfaces. Knowing this allows scientists to infer the density of other molecules, such as the water in Saturn’s upper atmosphere.

Every time we look at H3+, it helps us uncover some cool, crazy physics.
James O’Donoghue, planetary scientist, Japan Aerospace Exploration Agency

These kinds of measurements allowed O’Donoghue and colleagues this year to confirm a long-held hypothesis about the rings of Saturn. The rings are made of chunks and particles of ice, held in orbit by the balance between the planet’s gravity and the spinning rings’ centrifugal force. Scientists have long suspected that sometimes these particles rain down onto the planet. They proposed that ice particles might get charged by collisions with micrometeors rocketing across space or by ultraviolet light from the sun. These charged particles could then get captured by Saturn’s magnetic field and be drawn into the planet’s upper atmosphere, where they could sublimate into gaseous, neutrally charged water vapor, the scientists hypothesized. Neutral water reduces the density of electrons in the atmosphere, which in turn prolongs the life span of H3+, so areas of the planet receiving such ring rain should have higher densities of H3+.

Studies of H3+ emissions from Saturn had observed high concentrations of the molecule encircling the planet right where water should be coming out of the rings and into the atmosphere. But a detailed analysis of temperature and density at different latitudes was missing, O’Donoghue says. After carrying out such analyses, he and his team not only confirmed that H3+ was present in patterns that backed up the ring rain theory but also calculated that the entire ring system will be gone in less than 300 million years, a blink of an eye in cosmological time, he says (Icarus 2019, DOI: 10.1016/j.icarus.2018.10.027).

The H3+ ion has also helped solve a mystery about Jupiter’s upper atmosphere. Jupiter is five times as far from the sun as Earth is, so its upper atmosphere should be extremely cold. And yet scientists have measured it to be about as warm as Earth’s upper atmosphere. Why?

Earlier modeling studies had suggested that sound waves emanating from the surface of Jupiter could be warming the upper atmosphere. Acoustic waves produced above thunderstorms are known to travel upward and heat Earth’s atmosphere. Jupiter’s famous Great Red Spot hosts the largest storm in our solar system, with winds gusting to over 600 km/h, so it would stand to reason that it might play a part in warming the planet’s atmosphere.

Using wavelengths emitted by H3+, O’Donoghue and his team reported in 2016 that they had mapped the temperature of Jupiter’s upper atmosphere for the first time, finding that the maximum temperatures occurred right over the Great Red Spot. The team determined that the pattern of planetary temperatures was consistent with researchers’ hypothesis that sound waves from the Great Red Spot are heating the atmosphere. The sound waves travel upward, breaking at the outer layer of the atmosphere like waves on a beach, causing H3+ and other molecules there to vibrate and rotate more than normal. This increased kinetic energy means a heated atmosphere (Nature2016, DOI: 10.1038/nature18940).

Such findings can help scientists understand more terrestrial matters, too. Building on these results has revealed that the low-frequency sound of ocean waves crashing into each other could be heating Earth’s upper atmosphere (Geophys. Res. Lett. 2018, DOI: 10.1029/2018gl077737).



O’Donoghue is looking to find H3+ in the atmosphere of an exoplanet, a planet outside our solar system. Seeing the characteristic light emissions of H3+ around an exoplanet would indicate the presence of an ionosphere, a layer of charged particles in its upper atmosphere. By probing the ionosphere, scientists could learn about conditions on the planet, including whether it might harbor

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