On 17th April we learned that Helium Hydride – the first molecule of the Universe – has been spotted in space. Our columnist provides the background to the epic quest for this elusive molecule and describes its importance. Helium Hydride formed immediately after Big Bang is the precursor of molecular Hydrogen and richly deserves to be known as the Mother Molecule.
The Mother Molecule
As I sat down to write this month’s column, news trickled in about the detection of the first molecule of our Universe – Helium Hydride. I could not help dropping my chosen topic and turn my gaze heavenwards. There is something ethereal about this new scientific discovery. It is about the genesis of chemistry. Discovery of this Mother Molecule in outer space follows decades of perseverance and patience and pays a tribute to the human quest for truth. A fascinating story was waiting to be unraveled and here it goes.
Eons ago, approximately 13.8 billion years ago, the Big Bang happened and our Universe was born. During that dawn, Hydrogen and Helium were the only two elements in our Universe and they were completely ionized. Over several thousand years as the universe cooled, helium became stable atoms but hydrogen was still ionized. Scientists hypothesized that during this phase, 380,000 years after the Big Bang, helium atoms and hydrogen ions would have collided to form the first ever molecular bond of the universe resulting in Helium Hydride – HeH+. In fact, it was first synthesized in the laboratory as far back as 1925 by Hogness and Lunn. But not able to detect the molecule in nature cast serious doubts on the hypothesis about the formation of the primordial molecule.
Dabrowski & Herzberg were the first to propose the idea of possible detection of HeH+ in space. By calculating the vibrational and rotational energy levels of the ground electronic state of HeH+, they inferred that it could have been formed by the radiative association. At about the same time, calculation of the abundance of HeH+ together with its emission intensities highlighted the difficulty of its detection.
The quest for the elusive Helium Hydride began in full earnestness some 50 years ago with scientists identifying regions and objects in outer space where HeH+ would be present in sufficient abundance leading to its possible spotting. Mechanisms postulated to explain the formation and destruction of HeH+ in physical plasmas suggested that, in principle, HeH+ could be detected in dense molecular clouds subject to X-ray and extreme UV ionization.
The most promising sources were Planetary Nebulae, which are expanding clouds of ionized gases that are expelled during the last stages of a star’s life. A class of helium-rich stars known as White Dwarfs was another promising candidates. Other possible sources included supernovae and quasi-stellar objects, where HeH+ production could be enhanced by local high-energy processes, such as collisional excitations or X-ray excitations.
One particularly promising Planetary Nebulae was NGC 7027 located 3000 lightyears away in the Cygnus constellation and known for having one of the hottest central stars surrounded by an envelope of ionized gas that is rich in atomic hydrogen and helium. Theoretically, this would ensure an abundance of helium hydride. However, several observations spanning many years failed to yield conclusive evidence. The repeated failures caused despondency in concerned circles and even raised doubts about the hypothesis.
There were 2 major challenges in the detection of helium hydride in outer space. One of them was the technological limitations of spectrophotometry. The low-resolution spectrophotometry yielded ambiguous and inconclusive results at the best. The HeH+ rotational ground state has a wavelength of 149.137µm and faced interference due to spectroscopic features from the more commonly occurring carbon-hydrogen bonds that appear at 149.09µm and 149.39µm. Detection of HeH+ thus required very sensitive sensors and high spectral resolution. Another problem was that much of this wavelength is blocked out by ozone and water molecules in earth’s atmosphere.
To overcome these problems an instrument was specifically built. GREAT – The German Receiver for Astronomy at Terahertz Frequencies – was developed by a consortium of German institutes. GREAT is designed for far-infrared spectroscopy and its high spectral resolution in the terahertz regime enables detailed studies of planetary atmospheres, galactic interstellar medium and extragalactic and early universe processes. Further, to overcome the opacity of earth’s atmosphere, GREAT was mounted on board a NASA-modified Boeing 747 aircraft SOFIA – Stratospheric Observatory for Infrared Astronomy. A specially built instrument of high precision was thus carried above the interfering atmosphere of earth to search for the elusive helium hydride.
In May 2016, Rolf Güsten and his team from the Max Planck Institute for Radioastronomy flew three sorties of SOFIA and from a height of 14000 meters trained the supersensitive GREAT on PN NGC 7027. They were able to unambiguously detect the signature of the elusive Helium Hydride. It was a defining moment in the hunt for the Mother Molecule. Güsten and his team shared their findings with the world a few days back in a paper published in Nature on 17th April 2019.
It has to be mentioned at this stage that the helium hydride detected in NGC 7027 is not the same as those formed immediately after the Big Bang but through similar processes. The original helium hydride molecules were destroyed some 500,000 years after the Big Bang to form hydrogen molecules.
Why is this discovery important to the scientific community? The unambiguous finding of Helium Hydride in space brings down the curtain on the decades-long hunt for the first molecule of our universe. In Güsten’s own words – “The lack of evidence of HeH+ caused some doubts on whether we do understand the formation and destruction of this special molecule as well as we thought. This concern is gone now.” The discovery puts to rest doubts about our understanding of the chemistry in the early universe.
On a different level, it is a triumph of the scientific spirit and should serve as a great inspiration for the youth in these troubled doubt-ridden times. It demonstrates to the lay citizens the diligence and steadfastness of the scientific community in their unwavering pursuit of the ultimate truth. It is a great leap for pure sciences, especially chemistry. It goes a long way to re-establish chemistry as the mother of all sciences.
Usually, technological advances are an outcome of scientific discovery. This is a rare example of advanced technology facilitating scientific discovery.
It is futile to seek commercial spin-offs of this discovery; probably there aren’t any. Probably it will inspire youngsters to make a career in pure sciences. Let us revel in the delights of pure research and take pride in the supremacy of science.
Molecular hydrogen was the first neutral molecule of the universe, but it did not form directly by a radiative process since it does not have a dipole moment. Helium hydride was the precursor to hydrogen, and thus deserves the epithet of “Mother Molecule”. As the universe cooled, other elements of the Periodic Table made their appearance one by one. By spotting Helium Hydride, the birth pangs of chemistry have been revealed beyond doubt.