An oxygen mystery

WHAT has semiconductor fabrication technology got to do with a puzzling observation about comets made two years ago? Konstantinos P. Giapis and Yunxi Yao are two Caltech chemical engineers who usually tinker around with material surfaces to develop efficient microprocessor chips. They recently demonstrated that the surface chemistry involved in such research might just answer, at least partly, the as-yet unexplained detection of abundant molecular oxygen outgassing from the nucleus of the comet 67P/Churyumov-Gerasimenko (C-G).

Close encounters The European Space Agency launched the Rosetta orbiter-cum-lander spacecraft on March 2, 2004, to make a detailed study of 67P/C-G by entering into an orbit around it. During Rosetta’s close encounters with the comet between September 2014 and March 2015, it detected the presence of significant amounts of oxygen in its molecular form (O ₂ ) in 67P/C-G’s coma—the central part of a comet’s atmosphere immediately surrounding the nucleus. The gaseous comae of most comets are known to be composed mostly (95 per cent) of water (H ₂ O), carbon monoxide (CO) and carbon dioxide (CO ₂ ), and this first-ever detection of O ₂ came as a big surprise to astronomers.

The Rosetta mission ended with the crash-landing of the spacecraft on the comet on September 30, 2016. The wealth of data Rosetta gathered about 67P/C-G is expected to keep scientists busy for decades to come. Since comets are believed to have been born out of the same material that constituted the giant proto-solar nebula from which the sun and planets of the solar system froze out about 4.6 billion years ago, the study of comets, for which Rosetta data will be valuable, is expected to provide evidence or otherwise to the current theories of the formation of the solar system.

The detection of O ₂ was unexpected because on the basis of the current understanding of the formation of the planetary system and its constituents, free oxygen molecules should just not be there at all. Although oxygen is one of the most abundant elements in the galaxy and the solar system and constitutes 20 per cent of the earth’s atmosphere, it is highly reactive and combines readily with other elemental ions such as hydrogen, carbon, silicon and iron to form water, CO, CO ₂ , silicates and metal oxides. So the presence of O ₂ is, in fact, rare in the universe. Oxygen is found only in chemical combination with other elements. The earth is an exception to this because of constant oxygen production through photosynthesis by plant life. Indeed, the presence of extraterrestrial oxygen is regarded as an important biosignature.

Oxygen in its molecular form has so far been detected only in two interstellar molecular clouds (which have solid dust grains embedded in interstellar gas): the Orion Nebula and in the dense core of the Rho Ophiuchi A cloud complex. Tiny amounts of oxygen have been detected on Jupiter’s moons and Saturn’s rings, but they are thought to be produced by the splitting of water and carbon dioxide molecules by ultraviolet (UV) light (photolysis) or higher energy radiation (radiolysis). Therefore, all the primordial oxygen molecules, which would have been there in a comet’s evolution around 4.6 billion years ago, should have disappeared by now. Since there can be no life on comets, as they just do not have the other chemical ingredients to sustain life, the Rosetta’s discovery of O ₂ in 67P/C-G is an astrophysical enigma.

Rosetta’s on-board instrument called ROSINA-DFMS (Double Focussing Mass Spectrometer) detected O ₂in situ when it analysed the spectra of over 3,000 samples. The detected amount was significant: it varied between 1 and 10 per cent relative to the water molecules detected, with an average of 3.8±0.85 per cent (Figure 1), which is an order of magnitude higher than what models predict from the chemistry of interstellar molecular clouds. Key related observations by the Rosetta team were (i) amounts of H ₂ O and O ₂ detected were strongly correlated; (ii) the O ₂ /H ₂ O ratio in the coma was same in all directions (isotropic); and, significantly, (iii) the ratio was independent of the comet’s distance from the sun. According to the authors, (i) in particular suggests that the origins of both H ₂ O and O ₂ in the nucleus are similar and their release mechanisms are linked. The authors ruled out the possible outgassing of O ₂ through photolysis and radiolysis of upper layers of the cometary surface because these sources would not be able to produce the observed near-constant O ₂ /H ₂ O ratio over both space and time.

On the basis of these observations and considerations regarding other possible sources of O ₂ , the scientists argued in their paper, which was published in Nature in October 2015, that by some mechanism as yet unclear primordial oxygen was trapped in the comet’s nucleus, though, admittedly, the amounts were much higher than the very low upper limits set by remote sensing observations on interstellar space. Also, current models of solar system formation did not support the occurrence of conditions amenable to such trapping of O ₂ .

But of the two molecular clouds in which O ₂ (most likely to be primordial) was detected, only in Rho Ophiuchi A were the gaseous abundances of H ₂ O ₂ (hydrogen peroxide) and the related chemical species HO ₂ (hydroperoxyl) also measured. It turns out that the abundance ratios H ₂ O ₂ /O ₂ and HO ₂ /O ₂ in Rho Ophiuchi A and what Rosetta’s DFMS measured on 67P/C-G are of the same order (one-thousandth), though the latter ratio is about a factor 3 larger in 67P/C-G. That is, in both Rho Ophiuchi A and 67P/C-G, the abundance of O ₂ is about a thousand times more than that of H ₂ O ₂ and HO ₂ . On the basis of this, the authors speculated that the solar nebula from which the earth’s planetary system formed must have been similar to Rho Ophiuchi A. The latter’s core is believed to have had a slightly higher temperature of 20 to 30 K (−253 °C to −243 °C) compared with about 10 K (−263 °C) of most other dense interstellar nebulae. The comet-forming outer solar nebula is also believed to have similar higher temperature conditions. “If higher temperatures are indeed needed to produce significant amounts of O ₂ , this would indicate that our solar system was formed from an unusually warm molecular cloud...,” said the October 2015 paper of the Rosetta team.

Since there is nothing special about 67P/C-G compared with other comets, scientists believe that the presence of O ₂ in the coma may well be a common feature of all comets. Indeed, a subgroup of Rosetta scientists and others have already dug into the 30-year-old data that the spacecraft Giotto gathered during its Comet Halley fly-by mission in 1986 and have claimed in a paper published in December 2015 that the spectra it measured are consistent with the abundances from 67P/C-G. But, of course, compared with Giotto’s distance of closest approach of 596 kilometres, Rosetta’s orbit around 67P/C-G lay between 30 km and 10 km from the surface of the comet’s nucleus.

However, there is an important catch in the Rosetta team’s line of argument and conclusions, which the authors themselves acknowledged in their paper. According to current models of interstellar molecular clouds, the chemistry involved suggests that significant amounts of ozone (O ₃ ) should also be produced along with O ₂ . But Rosetta’s instruments found no evidence of O ₃ . The team, in fact, set an upper limit to O ₃ abundance as being less than a millionth of H ₂ O abundance. Also, the O ₂ /H ₂ O ratio Rosetta measured is an order of magnitude higher than what models predict.

The team also has put forth an alternative scenario wherein primordial gaseous O ₂ is trapped in water ice in the proto-solar nebula. According to the scientists, formation of amorphous water ice with trapped O ₂ on dust grains could have occurred when proto-planetary discs—circumstellar discs of dense gas and dust rotating around the sun before condensing to form planets—rapidly cooled from more than 100 K (−173 °C) to less than 30 K (−243 °C), which apparently is quite a probable scenario. However, this scenario has problems because it too would produce ozone (arising from the radiolysis of O ₂ ) at levels that Rosetta should have detected, whereas none was detected.

Eley-Rideal reactions Enter Giapis and Yao. Through a series of carefully designed experiments in their laboratory, they showed that chemical reactions called Eley-Rideal (ER) reactions might be responsible for this intriguing observation on comets. They normally use these reactions to improve the performance of semiconductor chips by bombarding energetic ions (charged atoms) on semiconductor material surfaces. (ER reactions occur when energetic projectile atomic/molecular species collide with surfaces and react with adsorbed atomic/molecular species to produce new molecules formed by the chemical bonding of the two, which are then emitted from the surface (Figure 2). The process is distinct from sputtering where surface material species are physically ejected.) In the process, they also discovered that ER reactions could even be driven by energetic water molecules via an excited transient state, a process hitherto only discussed as a theoretical possibility but unknown to the world of physical chemistry. Their findings were published in a recent issue of the journal Nature Communications.

In an email to Frontline , Giapis explained: “Microprocessor chips and digital memories have millions of transistors etched in them by exposing them to plasmas. The etching process is known to involve energetic ions, which bombard the microprocessor surfaces to carve these minuscule transistors out of a flat layer. Material is being removed from the layer by the action of the ions, typically highly reactive halogen ions. For a long time, scientists believed that this process is entirely physical, that is, ions sputter the surface and eject material in exposed areas—a harsh process that may cause electronic damage to the layer. There has been speculation that actual chemical reactions, called Eley-Rideal reactions, may be taking place in addition to physical sputtering but finding such proof has been elusive until 8 years ago when we produced the first conclusive evidence that these reactions do occur. This discovery introduced a new parameter to controlling chemical reactions, namely the kinetic energy of the ion. By judiciously choosing the chemical identity of the ions and precisely tuning their kinetic energy, the etching process can be exquisitely controlled, improving the yield of functional semiconductor chips.”

Typically, ER reactions occur when projectiles are “hyperthermal”, that is, they have 10-200 electronvolt incident energies. Since ions in space are normally accelerated to 100s of eV energies, Giapis started looking for places in space where ions would be accelerated against surfaces. He began looking at the Rosetta data because he suspected that the chemical reactions happening on the comet’s surface were similar to those he had been performing in the laboratory for the past 20 years. Giapis and Yao reasoned thus: ER reactions are independent of surface temperature and hence these reactions could be important in cometary environments in active periods when interactions with the solar wind can generate energetic molecular ions.

As the authors note in the paper, the conditions that obtain on cometary surfaces such as that of 67P/C-G are appropriate to allow ER reactions to proceed: accelerated water ions, with 120 to 800 eV energy, have indeed been found in the inner coma of 67P/C-G, and when the outer crust of its nucleus facing the sun gets dehydrated, the mineral surfaces are exposed to the ions. These oxidised minerals on the surface of the nucleus become the potential sources of oxygen with which striking energetic water ions interact via ER reactions. “After looking at measurements made on Rosetta’s comet, in particular regarding the energies of the water molecules hitting the comet, it all clicked. What I’ve been studying for years is happening right here on this comet,” Giapis said.

Basic premise Thus, the basic premise that Giapis and Yao have proposed, put in simple terms, is this: Water vapour molecules are ejected from the nucleus as the comet is heated by the sun. These H ₂ O molecules get ionised, or charged, by UV light from the sun and get deflected back towards the cometary surface by the solar wind. When H ₂ O molecules hit the comet’s surface, which contains oxygen bound in materials such as sand and rust and other metal oxides, they pick up another oxygen atom from the surface to form intermediate peroxide-like states, and O ₂ is finally liberated from this intermediate by the action of light from the sun. And this happens continuously on the cometary surface because, as the authors point out, even as oxygen atoms are abstracted by ER reactions, the surface is being constantly replenished with oxygen atoms produced by other mechanisms such as splitting of water ions through collisions and physical sputtering of the surface by water ions.

In other words, their proposal implies that the O ₂ Rosetta found need not be primordial but may be produced in situ and in real time on the comet. Giapis and Yao proved this proposition by actually demonstrating its occurrence through laboratory experiments using material surfaces that are analogous, or similar, to the mineral surfaces found on the comet’s nucleus. The experiments also helped in delineating the exact chemical and energy pathway by which energetic H ₂ O ions striking the surface results in the emission of oxygen with high kinetic energy. “The O ₂ production from water ion collisions with mineral surfaces is similar [to our demonstration that ER reactions do occur during etching of microprocessor chips], though proving that a water molecule can drive such reactions is unprecedented,” Giapis said in his email message.

Since silicates and iron oxides (such as rust) are surfaces commonly found on comets, the scientists used silicon and iron as targets (natural oxidation covers them with an amorphous oxide layer) and bombarded them with negative water ions or water anions (H ₂ O ₂⁻ ). Surface scattering from these targets was found to produce oxygen and hydroperoxyl anions (O ₂⁻ and HO ₂⁻ ). The surface reactions included products of both physical sputtering and direct reactions, including ER. The production of these was found to occur only between 50 eV and 200 eV and not at higher projectile energies, which is consistent with the energies that obtain in cometary plasma environments.

To determine whether O ₂⁻ and HO ₂⁻ were products of sputtering or direct reactions such as ER, a different projectile beam (neon ions) was used, which yielded significantly lesser amounts of these products. This ruled out sputtering as the chief agent for their production. The scientists repeated the experiments with platinum targets whose surface was dosed with a different oxygen isotope (O ¹⁸ instead of the normal and more abundant O ¹⁶ ). This procedure clearly separated the sputtering and direct reaction signals from the surface and helped clearly demonstrate that O ₂⁻ and HO ₂⁻ were indeed due to direct reactions.

Having determined that the oxygen anion was produced by direct surface reactions, the scientists also proposed a mechanism based on an ER reaction driven by energetic H ₂ O ions striking oxidised surfaces. According to them, a short-lived transient state, which is a combination of H ₂ O, the surface atom and the adsorbed O atom, is formed first which promptly disintegrates and produces an energetic hydrogen peroxide-like molecule in an excited state (H ₂ O—O*).They suggest that this state is actually the elusive state called oxywater in which an O atom gets chemically bonded to water. Oxywater is elusive because it is an energetically unfavourable state. This oxywater therefore quickly splits into H ⁻ and HO ₂⁻ ions. The latter then splits further to give O ₂⁻ ions. The extra electron on the oxygen molecule (which makes it O ₂⁻ ) gets finally detached by the action of light (photodetachment) to leave behind chargeless O ₂ , which is detected as oxygen outgassing from the cometary coma.

Since the lifetime of O ₂⁻ against photodetachment is about 2.6 seconds, Giapis and Yao argued that the Rosetta instrument should also have detected O ₂⁻ ions besides neutral O ₂ molecules. That is, by this demonstration, they have actually predicted the existence of O ₂⁻ and HO ₂⁻ ions in the plasma around the comet nucleus. Comet experiments thus far, including Rosetta, have detected only hydrogen ions. But since the mechanism is generic to cometary conditions, and not specific to 67P/C-G, future cometary experiments should be able to detect these ions as well, they have pointed out.

It must, however, be emphasised that H ₂ O—O* cannot actually be detected, but the hypothesis is supported by detailed analysis of the kinetics of the reaction’s final products in the laboratory experiment. The scientists also provided a convincing explanation for how their proposed mechanism is consistent with the observed strong correlation between O ₂ and water abundance in the coma and a steady O ₂ /H ₂ O ratio that is independent of the heliocentric distance of the comet. Significantly, no ozone is produced in the proposed ER mechanism, an important consistency requirement with all cometary observations so far, including on 67P/C-G. This is a serious shortcoming in any hypothesis based on the primordial origin of oxygen in comets.

A crucial missing element in this dynamic in situ generation of oxygen through the ER mechanism Giapis and Yao proposed is that the actual efficiency of the process and the final rate of O ₂ production cannot be estimated at present, which they note. This is because of the uncertainty (by at least two orders of magnitude) involved in the reported flux measurement of accelerated H ₂ O ions in the coma of 67P/C-G striking the nucleus surface. If, however, it is taken at face value, it is too low to account for the reported oxygen abundance Rosetta measured, their paper notes.

However, they point out that there are other water-derived positive ions (cations), such as H ₂ O ⁺ , OH ⁺ and O ⁺ , which can undergo similar solar-wind-driven acceleration and participate in similar ER reactions on the surface to produce oxygen. In fact, they have even suggested the possibility of ionised molecular oxygen (positive O ₂⁺ ion) having a role in the production mechanism. The quantitative mismatch with the actual observed abundance notwithstanding, Giapis and Yao’s proposal is nevertheless an interesting and innovative idea.

Primordial oxygen theory Although the idea has generated interest among comet researchers, not all of them accept it as yet, in particular those from the Rosetta team who still seem to favour a primordial oxygen theory even as they do not have any explanation for the total absence of ozone in the coma. In an email to Frontline , Kathrin Altwegg of the University of Bern and Kenneth Hansen of the University of Michigan pointed out that the cometary ionosphere, which begins to form at a heliocentric distance of about three astronomical units (AU), or about 450 million km from the sun, would prevent the solar wind from reaching the cometary nucleus and thus would not be able to provide the necessary acceleration to water ions in the coma to initiate the proposed ER reaction, whereas the observed O ₂ abundance did not change with heliocentric distance. They also suspected that the mechanism could not account for the observed abundance. “With this explanation,” Kathrin Altwegg wrote, “we now have four possible explanations. [But] all of them have some problems: the first two [which were suggested in the Rosetta paper itself] produce ozone, which we don’t see. The third [involving radiolysis of icy grains by cosmic rays] because of the [conflict with] H ₂ O ₂ [abundance] and now the newest one involving accelerated ions hitting the surface, which is not compatible with a constant H ₂ O/O ₂ ratio over the mission. I personally prefer options 1 and 2. But O ₂ remains a little bit of a mystery.”

Giapis has reasonably convincing arguments to counter Kathrin Altwegg’s points. While these are too technical to be described here, the short point he makes is that even after the comet comes closer to the sun, to heliocentric distances less than 3 AU, the coma extends several hundred kilometres away from the nucleus, well beyond the ionosphere, where ions get formed by the action of light and get accelerated before they hit the ionosphere. After they encounter the ionosphere, a process called charge exchange reactions takes place there, which produces fast and energetic neutral molecules, and these will drive the same ER reactions to produce O ₂ just as accelerated ions did before the formation of the cometary ionosphere.

In any case, as Giapis wrote to Frontline : “We offer only a competing hypothesis to the controversial ‘primordial’ origin. We explicitly state in our paper that our proposal alone cannot explain the reported abundance. In comparison [to the three competing explanations], our mechanism is the only one that explains consistently all trends and dependencies observed for O ₂ , except for its abundance. The puzzle remains incomplete but our hypothesis provides the most discernible picture. Furthermore, several of its foundations are testable.”

Very recently, Jan Deca from the University of Colorado, Boulder, United States, and colleagues produced detailed 3D simulations of the intricate interaction between the solar wind and a comet, in particular that of 67P/C-G, and explained the dynamics involved in the interactions of ions and electrons with the comet (Figure 3). Frontline asked Deca whether the simulations provided any insight into the possibility of energetic-water-ion-induced ER reactions on the surface of the nucleus leading to outgassing of oxygen. Deca and Anders Eriksson, a co-author in the simulation work, replied by email that they did not see the proposed mechanism as very promising as the degree of ionisation in the inner coma was of the order of 1 ppm, and it was so in their simulations as well, compared with reported O ₂ abundance of several per cent, which was essentially due to the solar-wind-induced formation of a cometary magnetosphere at 3 AU heliocentric distance. “This means,” they said, “there are about 10,000 times more O ₂ molecules than there are ions of any kind, which seems quite incompatible with the suggested mechanism.”

Giapis’ counter to this was roughly along the same lines as the one he made to Kathrin Altwegg since the criticism was essentially the same, which is that ionosphere/(induced) magnetosphere would interfere with the energetic water ion flux required to produce the observed O ₂ abundance. “The [water] ion kinetic energies calculated in the Deca paper are in the correct range required for Eley-Rideal reactions, whether on the nucleus surface or on the dust grains in the coma. What everybody forgets is the ‘charge exchange’ and ‘electron recombination’ reactions can produce neutral water molecules with unaltered kinetic energies, which can drive ER reactions as well. Since they are not charged, they are not affected by the [cometary] magnetosphere or the ionosphere.”

Jury still out So, as for quantitatively explaining the O ₂ abundance in 67P/C-G, the jury is still out. Suitably designed comet missions in the future should be able to pick out the correct one from competing theories.

But regardless of whether the Giapis and Yao idea is able to fully account for the cometary oxygen mystery, the ER reactions that they have proposed are novel, unexpected and had remained unproven for over 80 years.

“The true value of O ₂ story to me,” Giapis wrote, “is the discovery that mysterious ER reactions with energetic water molecules/ions are possible. They open up a new direction in chemical physics and these reactions are important in astrophysics beyond anybody’s wildest expectation.”