An oxygen mystery

Print edition : July 21, 2017

The comet 67P/Churyumov–Gerasimenko (C-G). Photo: European Space Agency

An artist’s impression of the European Space Agency’s Rosetta cometary probe, which was launched in March 2004. Rosetta, along with its lander module Philae, performed a detailed study of the comet 67P/C-G. The spacecraft carried 11 science instruments to probe the comet’s nucleus and map its surface in fine detail. Data from the Rosetta mission are providing clues to the physical and chemical processes at work during the formation of planets, beginning 4.6 billion years ago. On September 30, 2016, the spacecraft ended its mission by hard-landing on the comet. Photo: European Space Agency

The Caltech laboratory where experiments with high-speed ions are done to study reaction dynamics at surfaces, the results of which can be used to improve methods to fabricate semiconductor devices. Water ions are generated in a plasma reactor, seen at left, which glows to produce white-blue light. Photo: Caltech

Konstantinos Giapis of Caltech. Using laboratory experiments, he has shown how molecular oxygen may be produced on the surface of comets. He and his postdoctoral scholar Yunxi Yao fired high-speed water molecules (in the background) at oxidised silicon and iron surfaces and observed the production of a plume that included molecular oxygen. Oxygen atoms are red, and hydrogen, blue. Giapis says similar conditions exist on the comet 67P/C-G (this is shown schematically in Figure 2). Photo: Caltech

Another view of 67P/C-G. Photo: European Space Agency

Figure 1: The mass spectra measurement in units of mass-charge ratio by an on-board instrument, the Double Focussing Mass Spectrometer. The black labels indicate the three major species found in the coma of 67P/C-G; the oxygen peak is the highest. The green labels and the green line identify contamination peaks from satellite thrusters firings, showing that their contributions to the oxygen peak are very low. The light blue, dark blue and purple represent lines taken at different distances from the comet nucleus. Photo: A. Bieler et al., Nature, October 29, 2015.

Figure 2: A schematic showing the chemical pathway involved in the Eley-Rideal reaction. Oxygen atoms are red and hydrogen, blue. Photo: Caltech

Figure 3: Jan Deca and colleagues of the University of Colorado, Boulder, have simulated in 3D the rich dynamics of particles around a comet. Photo: American Physical Society/Physical Review Letters

Two Caltech chemical engineers, on the basis of their experience with semiconductor fabrication technology, have come up with a novel theory that might explain the Rosetta orbiter-cum-lander spacecraft’s detection of abundant molecular oxygen outgassing from the nucleus of the comet 67P/Churyumov-Gerasimenko, which has surprised astronomers.
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