In episode 5, I provide a review of a recent article “Abundant molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko” by Bieler et al, [13] where the detection of relative abundances of molecular oxygen is reported. The presence of molecular oxygen is surprising because it is not predicted by the current and dominant condensation-sublimation model of cometary origin and composition. The letter sent to Nature magazine is well-presented and the observations are derived through direct mass spectrometric measurements with sufficient resolution, such as the double-focused mass spectrometer from ROSINA. Here are the relevant observations:
1. Local molecular oxygen abundances range from 1 to 10% with an average of 3.8% with respect to water abundances.
2. There is a strong correlation between water and molecular oxygen relative abundances in the coma of comet 67P.
3. Molecular oxygen to water relative abundance ratios do not vary as a function of heliocentric distance and for a period of 7 months of observations.
4. Molecular oxygen was detected as close as 10 km away from the comet’s nucleus.
In the backdrop of these observations, the authors put forth a series of well-thought mechanisms to explain the origins of molecular oxygen and water vapour within the condensation-sublimation paradigm. Unfortunately, no explanations beyond this paradigm were presented. Here is a list of the considered mechanisms:
1. Photolysis – defined as ultraviolet photons capable of breaking molecular bonds. This explanation is unsatisfactorily because according to the authors “instantaneous creation of the measured O2 by radiolysis or photolysis seems, overall, unlikely, and would lead to variable O2 ratios due to different illuminated conditions.” In other words, if electromagnetic radiation were the driving force behind molecular oxygen formation, then the molecular oxygen to water vapour ratios would not remain constant, which is not supported by the data.
2. Recent Radiolysis – envisions molecular oxygen production on the nucleus from the interaction of solar energetic particles with peak energies in the few of tens of MeV. This mechanism will only be effective within a few micrometers of the surface. If it were in action, the O2/H2O ratios would have to decrease over time as the top micrometer layers will be exhausted by erosion as the comet orbits the sun. Furthermore, due to comet erosion rates, O2 production rates would have to be very high in order to be supported by the observed data.
3. Primordial O2 – molecular oxygen was formed during the formation of the solar system and accrued in the nucleus of comet 67P. This mechanism implies that higher temperatures are needed to produce significant amounts of O2, suggesting the solar system accretion model formed from unusually warm molecular clouds. In addition, the solar system accretion model does not predict high O2/H2O ratio values as observed. Finally, the solar system accretion model predicts the presence of ozone (O3), which was not detected.
4. Radiolysis before the solar system accretion – O2 is produced by radiolysis in water-ices that are then trapped in voids while somehow hydrogen diffuses out, avoiding hydrogenation of O2, increasing and stabilizing O2 concentrations which can be incorporated into the nucleus. This is an unlikely scenario due to the high O2 reactivity, and because ozone (O3) resulting from O2 radiolysis, as reported to be trapped in Ganymede’s surface, was not detected.
5. In situ O2 production in the coma – The authors argued that the coma is tenuous and therefore may not afford the necessary collision frequency to overcome the activation energy barrier associated with required chemical reactions. Subsequently, the reaction of O + OH → O2 + H remains unfeasible. This analysis holds only if we assume that chemical reactions are driven by electromagnetic radiation.
6. Long term radiolysis after the solar system accretion – O2 production from water by radiolysis produces H, O, and OH, followed by H2, HO2, and H2O2 rearrangement. However, at current erosion rates, comet 67P would require substantial O2 diffusion into the comet’s nucleus from each subsequent passage around the sun to avoid O2 earlier depletion. Moreover, galactic cosmic rays (H+) produce O2 only within tens of meters of the surface, which would have been already released.
7. O2 production via solar wind surface sputtering – This mechanism is not supported by previous findings that show preferential solar wind sputtering in the northern hemisphere. In addition, the northern hemisphere does not contain water ice from which molecular oxygen can be derived. Finally, the O2/H2O relative abundance ratios are independent of latitude and relative constant over 7 months.
8. Incorporation of primordial O2 into the cometary nucleus – Gaseous O2 incorporates into water ices in the proto nebula, which rapidly cools down from > 100 K to < 30 K, trapping O2 in water ices. The last mechanism is the one favored by the authors. However, it can be highlighted that such a mechanism needs the presence of unusually higher temperature in young proto planetary disks, which then undergoes selective rapid cooling. It is possible but difficult to evaluate since no time scale for the cooling is given. Furthermore, O2-riched grains will have to further accrete into larger bodies and then be trapped into the comet nucleus, all the while not reacting, but it is well known that O2 is rather highly reactive.
Highlighting comments made on the ESA website reveal the impact of the findings:
“We weren’t really expecting to detect O2 at the comet – and in such high abundance – because it is so chemically reactive, so it was quite a surprise,” says Kathrin Altwegg of the University of Bern, and principal investigator of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA.
“The amount of molecular oxygen detected showed a strong relationship to the amount of water measured at any given time, suggesting that their origin on the nucleus and release mechanism is linked. By contrast, the amount of O2 seen was poorly correlated with carbon monoxide and molecular nitrogen, even though they have a similar volatility to O2. In addition, no ozone was detected” says Andre Bieler of the University of Michigan and lead author of the paper describing the new results in the journal Nature this week.
The following quotation is telling, Andre Bieler goes on to say that “The instantaneous generation of O2 also seems unlikely, as that should lead to variable O2 ratios under different illumination conditions. Instead, it seems more likely that primordial O2 was somehow incorporated into the comet’s ices during its formation, and is being released with the water vapour today.” The only reason in situ O2 generation is not considered is because photons are thought to be the only driving force for chemical reactions in the coma of 67P.
Obviously, under such circumstances the phenomenon seems mysterious. “This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of Solar System evolution,” says Matt Taylor, ESA’s Rosetta project scientist.
It is clear that the correlation of relative abundances between O2 and H2O indicates a single origin and release mechanism. The question is: which mechanism? I propose a mechanism that allows for the simultaneous formation of O2 and H2O, where the driving force is a voltage differential, such as an electrochemical method. There is plenty of evidence of the presence of filamentary electric fields accelerating electrons and ions in the vicinity of the nucleus, leading to charge separation, as has been demonstrated throughout these episodes. I called this mechanism:
O2 and H2O formation by electrochemical means – In this method, O2, O2-, OH, -OH, and other chemical species are released into the coma by various mechanisms, such as “electron-stripping,” solar wind sputtering, and solar heating. In this context, O2 can absorb a negative charged through charge exchange due to demonstrated high electron densities in the vicinity of the nucleus, followed up by protonation via the solar wind. Subsequently, water can then be formed via at least two pathways linking O2 and H2O formation:
(1) O2 + H → H2O
(2) O2- + H+ → H2O
In this theoretical framework whereby the driving force is a voltage differential in the vicinity of the nucleus, the instantaneous O2 formation reaction that was deemed unfeasible becomes feasible when looked at from an electrochemical perspective as the reaction can occur directly via gas phase collisions or be mediated via silicate catalysis on nanograin surfaces. In a general perspective, a voltage-driven chemical reaction mechanism looks promising as it can explain the formation of complex chemical species, such as water, alcohols, sugar, [14] cyanide polymeric chains, amino acids, and others, with satisfaction without the thermodynamic constraint of low temperatures-driven collisions.
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!) models that "gases are released from a comet's nucleus continuously, starting at each gas's sublimation temperature". The experiment showed that this assumption was incorrect.
