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Evidence of Electrical Activity on Comet 67P: Towards an Electrochemical Framework for Cometary Phenomena

Thunderblog -- Comet 67P 400X299

Evidence of Electrical Activity on Comet 67P: Towards an Electrochemical Framework for Cometary Phenomena
By Franklin Anariba, PhD

Episode 1 – Charge Separation in the Plasma Environment of Comet 67P

Let me provide a context for these episode series on comet 67P. The Rosetta mission is providing us with a plethora of scientific reports filled very valuable information on the composition of the coma and nucleus of comet 67P/Churyumov-Gerasimenko, hereafter referred to as 67P. In this episode series, I intend to pinpoint relevant findings from different published scientific articles that are relevant from the perspective of the Electric Universe. It is a critical review that recognizes the significant contributions of all authors where no misrepresentation is intended and which overall purpose is to inform the general audience of an alternative interpretation of the reported findings.

This is accomplished by commenting on recent findings reported using various instruments and techniques, including the Rosetta Plasma Consortium Ion and Electron Sensor (RPC-IES), Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), which is a mass spectrometer capable of detecting positive ions or cations, and the Rosetta Orbiter Spectrometer for Neutral Analysis Cometary Pressure Sensor (ROSINA-COPS). My analysis is derived from observations made on comet 67P at heliocentric distances of 2 to 4 astronomical units (AU). At such heliocentric distances, comet 67P displays low cometary activity as it is far from reaching perihelion, but this fact might be decisive in elucidating phenomena that otherwise will be convoluted and difficult to discern. My commentary is based on reported data and interpretation of same by the authors, which on occasion I re-interpret from the perspective of someone with an electrochemical background.

As can be ascertained by reading most of the reports published on scientific journals on comet 67P, the main paradigm is the one proposed by Whipple some time ago which can be called the Condensation-Sublimation model. I am not here to refute this model, which I recognize to be of importance near perihelion distances if comets were to contain large amounts of volatiles ices on their surface or in their subsurface. Instead, my intention is bring to everyone’s attention that there could be other mechanisms at work for the origin of water and other chemical species in the cometary coma. One mechanism is proton implantation, which has been recalled to explain water ice presence on craters in the moon, can be evoked to explain the presence of small amounts of water ices on the surface of comets with tenuous ion density in their comas. Another mechanism, mostly unknown to the astronomical community, is based on electrochemical principles which I have named electron-stripping. This mechanism can be dominant at larger heliocentric distances, away from perihelion, when the voltage differential between the comet and the surrounding plasma sheath of the sun can be largest. This mechanism can also be recalled to explain the presence of water vapour in the coma of comets. The invocation of these two other mechanisms is necessary because so far no significant amount of water ices have been identified on the surface and in the subsurface of the nucleus of comet 67P. Furthermore, the recent findings of molecular oxygen on the coma of comet 67P, along with its link with origin and presence to water vapour, points to the presence of an electrochemical method of production.

Before I continue on to discuss the various findings, it is important to clarify and define important terminology used in the current condensation-sublimation model for cometary origin, and its relationship to the plasma environment near the nucleus of comets. In this model, the solar wind-associated interplanetary magnetic field (IMF) interacts with comet’s coma, creating a bow shock due to the pile up of magnetic fields on the sunward direction of the comet. According to the current proposed mechanism, an induced magnetosphere forms at the interface of the solar wind and comet’s plasma sheath, which is a partially ionized and electrically conducting. This phenomenon is mostly observed near perihelion when comets in general display high cometary activity. Equally important to this model, neutral gas molecules in the coma are thought to sublimate from the nucleus and then undergo photoionization by extreme ultraviolet (EUV) radiation emanating from the sun, resulting in charge separation and the formation of a plasma sheath surrounding the nucleus.

In addition, the solar wind, which is comprised of cations, such as protons (H+) and alpha particles (He2+), and electrons, is thought to play an important role in ion gas production via sputtering of the surface of the nucleus and ensuing charge exchange mechanisms. Mass loading is thought to occur concurrently when newly formed gas ions initially traveling at low speed (~1 km.s-1) are picked up or accelerated by the solar wind-associated interplanetary electric field to its mean speed of ~ 450 km.s-1. Mass loading becomes significant at distances < 1000 km from the nucleus.

A typical mean solar wind speed ranges from 300 to 600 km.s-1. This kinetic energy can be translated into potential energy by equating both energies through the following equation:

Potential energy (PE) = Kinetic energy (KE)
eV = ½(Mass of proton) × (velocity of proton)2

We can use this equality to realize that a solar wind speed of 300 to 600 km.s-1 is equivalent to 470.4 to 1881.7 eV. For reference, the ionization potential of a gas hydrogen

(H) atom (H(gas) → H+(gas) + e-) is 13.6 eV

The Lorentz force is responsible for accelerating new born ions orthogonally to the solar wind direction and the interplanetary magnetic field and causes the ions to gyrate around and along interplanetary magnetic field lines forming a ring distribution. One of the consequences of this mass loading or pick up action is the deceleration of the solar wind through the conservation of momentum, and the excitation of cyclotron wave activity. The gyration of the ions around the solar wind can be characterized via the Larmor cyclotron parameters, such as:

Radius of gyration (Rg) = mass of the ion × velocity of the ion / (electric charge × solar wind magnetic field)

Period of gyration (tg) = ( 2π × Rg ) / velocity of the ion
Cyclotron or gyrofrequency (fg) = 1 / tg

Angular cyclotron frequency (ωg) = (electric charge × solar wind magnetic field) / mass of the ion

With these general concepts in mind, I would like to take a look at two important recent reports on the dynamics of the comet’s plasma environment. In the manuscript titled Observations of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko” by Richter et al [1] which measured the plasma environment in the inner coma, as close as 10 km, the general observations reveal a significant ion and electron activity within the inner coma of the comet. The relevant findings are summarized below:

1. The solar wind (SW) carries a detectable magnetic field of ~2 nT, while the magnetic field in the inner coma of 67P showed a value of ~4 nT, indicating that the magnetic field energy density was higher closer to the nucleus.
2. Magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus and showed no preferred propagation direction.
3. An electric current density of 4.8 × 10-9 A.m2 was determined.

The findings are of particular interest and intriguing. First, electrical activity near the nucleus of the comet is discerned. Second, most of the ions detected in the cometary coma of 67P are thought to be water cations (H2O+), which display a cyclotron frequency in the range of 0.8 to 3.5 mHz, much different from the dominant 40 mHz detected. This result is unexpected, indicating a new mechanism must be in action. The authors add that “[i]t should be noted that the suggested wave source is not co-moving with the solar wind flow…[T]he wave source is almost fixed in the nucleus frame of reference.” The authors discard the classical pick up ion instabilities as the source of the magnetic oscillations and instead propose that photoionized ions move transverse to the magnetic field but along the solar wind, which flows in the direction of the electric field, in what is called a cross-field electric current density. In other words, the electric fields control cometary ion motion but in a different way from the known mass loading action.

In the report by Nilsson et al [2] titled Evolution of the ion environment of comet 67P/Churyumov-Gerasimenkowhereby the plasma environment is measured using the ion composition analyzer mass spectrometer, capable of measuring positive ions, the following findings are of attention:

1. Cometary ions detected are mostly water cations with two distinct potential energies. Cold water ions contain potential energies of ~10 to 50 eV and traveling from an area between the sun and the nucleus, while the accelerated water ions contain potential energies of ~120 to 5000 eV and are detected coming from the direction of the sun.
2. Water ion flux increased by 4-orders of magnitude when the heliocentric distance decreased from 3.6 to 2.0 AU and the solar flux only increased by ~ a factor of 2.
3. The solar wind did not slow down noticeably, suggesting mass loading is negligible.

Interestingly, the authors add: “A simple scenario would be that the pick-up ions initially move along the undisturbed solar wind electric field, while the newly created electrons . . . drift, causing a charge separation [emphasis added]. The electric field of that charge separation would initially have a component opposite to the solar wind electric field and a component in the anti-sunward direction. This is consistent with both the flow of the water ions and the observed deflection of the solar wind.” The implications are three-fold:

a. Charge separation is due to an electric field, which is an indication of the presence of voltage differential.
b. If the accelerated water cations (H2O+) are traveling from the direction of the sun, electrons must be traveling in the direction towards the sun.
c. The high water cation flux production rate may be an indication that another process is at work besides the traditionally proposed photoionization due to ultraviolet radiation.

To summarize episode 1, taken together the overall findings paint a picture of a highly dynamical plasma environment in the vicinity of the nucleus with potential consequences such as charge separation of cations and anions due to a voltage differential.

Episode 2 – Fast Moving Electrons and Electric Fields in the Plasma Environment of Comet 67P

In episode 2 of this series, I expand on the presence of electrical activity in the vicinity of the nucleus of comet 67P. This is done by commenting on two important recent reports. In the manuscript titled The Rosetta ion and electron sensor (IES) measurement of the development of pick up ions from comet 67P/Churyumov-Gerasimenko” by Goldstein et al [3], the plasma environment is scrutinized at a heliocentric distance of ~3.5 AU and at 28 km from the nucleus of the comet. In summary the authors report:

1. Detection of low energy water cation populations for the first time in a weakly “out-gassing” comet far from the sun.
2. Water cation populations detected display time-varying distributions.
3. No angular distribution is observed for the water ions populations, suggesting that these ions are created locally and have not been accelerated by the solar wind electric field as expected with mass loading.
4. Interestingly, mass-loaded ions and solar wind protons move in opposite direction, while electrons remain unaffected.

The implications of these findings reside on the assumptions that mass-loading plays an important role at such large heliocentric distances on a comet with a tenuous coma density. For instance, it has been reported that the solar wind ions are deflected but not slowed down as expected during a mass-loading process [4], suggesting that mass loading is negligible. Therefore, these findings instead can be interpret to indicate a complex and dynamical electron-rich environment near the nucleus at such large heliocentric distances where mass loading is negligible, suggesting another ion motion mechanism is dominant.

In this line of thinking, of utmost interest is the paper by Clark et al titled “Suprathermal electron environment of comet 67P/ Churyumov-Gerasimenko: observations from the Rosetta ion and electron sensor” [5]. The term “suprathermal” implies highly heated electrons possessing high kinetic energy. In the manuscript, according to the authors, suprathermal or fast-moving electrons are “accelerated by an unknown mechanism from a few eV upward to 100 s of eV and play an important role in the electron-neutral chemistry as well as in dust grain charging.” Moreover, previous studies showed that these accelerated electrons are ubiquitous, but could not agree on a particular mechanism. Several accelerating electron mechanisms have been put forth:

1. “Suprathermal” electrons are likely associated with upstream cyclotron wave fluctuations convecting tailward. In other words, the accelerating energy is provided by mass-loaded ions traveling from the direction of the sun towards the tail of the coma.
2. “Suprathermal” electrons are accelerated in a similar manner as electrons behind interplanetary collision-less bow shocks.
3. The Active Magnetospheric Particle Tracer Explorer (AMPTE) satellite showed that lower “hybrid” cyclotron waves from mass-loaded ions were responsible for the electrons. “hybrid” refers to cyclotron frequencies in between a proton and an electron.
4. Some studies ascribe “suprathermal” electron acceleration to photoionization, while other studies indicate the role of photoionization to be negligible.

It is then clear that there is no agreement on the dominant mechanism behind suprathermal electron acceleration, more so for comet 67P, which has a 100 times more tenuous coma density than comet Halley. In this context, the authors inform:

1. A highly dynamical “suprathermal” electron cloud consistent with observations of peak comet activity near 1 AU. The authors add: “It is surprising that a weakly out-gassing comet at a large heliocentric distance produces a dynamic and energetic suprathermal electron environment.”
2. Electron densities near the nucleus are in the range of 10 to 100 cm3.

Taking into consideration that comet 67P was ~3 AU at the time of the observations and displays a tenuous coma, which contradicts all previous expectations, it is very likely that another mechanism for accelerating electrons near the nucleus is in action.

The report continues on considering potential mechanisms for “which processes account for the density (and flux) enhancements that IES observes are not obvious.” For instance:

1. Photoelectrons. Play a role by ionizing molecules and creating energetic secondary ions via photoelectron impact dissociation.
2. Ambipolar electric field. The authors comment that “[i]f a well-structured electric field were present, then a well-defined peak would be expected. Again, this points to fine structure in the plasma environment.” The implications are that the electric field is filamentary in nature.
3. Acceleration caused by cross-shock potentials. Interesting enough, a cross-shock potential, which has been observed to accelerate electrons, can be thought of as a double layer in an electrochemical experiment. However, there is no evidence for a bow shock or a reverse bow shock at a heliocentric distance of 3.5 AU.
4. Magnetic field compression. However, there was no data on the direction and magnitude of the solar wind-associated magnetic field.
5. Lower hybrid cyclotron waves due to mass-loaded ions. The authors settled for this explanation to explain their data.

Surprisingly, the authors conclude that the “suprathermal” electrons are being accelerated by “an admixture of photoelectrons and [cyclotron] waves produced by pick-up [or mass-loaded] ion instability.” Their conclusion is contradictory because in the text the authors report on a rich and dynamical electron cloud, indicating a high density of electrons in the vicinity of the nucleus, even though comet 67P displays a tenuous coma density. Hence, the question is: Where are the electrons coming from? Another contradiction is that mass loading is associated with a solar wind mean speed deceleration.

However, the authors state that “the ion data do not show a significant deceleration of the solar wind,” suggesting that ion mass loading by the solar wind is negligible.” In addition, the author write that “[u]nfortunately, there is no instrument onboard Rosetta that can measure electromagnetic waves at this frequency,” referring to lower hybrid cyclotron frequencies. These observations might indicate that their conclusion is premature. From an electrical perspective, electron acceleration is due to the point charged sitting on an electric field, which is an indication of the presence of a voltage differential.

Episode 3 – Solar Wind Sputtering and Its Relation to Water Observation in Comet 67P

In episode 3, I address the potential relationship between solar wind sputtering on the surface, the detection of metal ions, and water production. Currently, there are 5 processes thought to be responsible for extracting alkali atoms from the surface of comets:

1. Thermal desorption
2. Photon stimulated desorption
3. Solar wind sputtering
4. Micro-meteroid vaporization
5. Photo-dissociation of parent molecules [6]

In the manuscript titled Solar wind sputtering of dust on the surface of 67P/Churyumov-Gerasimenko” by Wurz et al [7], a double focusing mass spectrometer is used to detect neutral volatiles in the coma, which are not expected to be sublimated or partially sublimated, such a Na, K, Si, Ca, and S. Their findings can be summarized as follows:

1. Silicon (Si) is the most dominant species observed by the mass spectrometer, which is reasonable since it has been shown that comet 67P is comprised of refractory minerals, such as olivine.
2. Most of the species are detected from above less illuminated areas in the nucleus of the comet; that is, the winter hemisphere.
3. In comparison to carbonaceous chondrites, comet 67P shows Na abundance, Ca depletion, and K excess.

The summary is that detected neutral metal fluxes are the result of solar wind sputtering on the surface of the nucleus from the winter hemisphere, indicating that areas of high erosion activity, such as the neck region, appear unaffected. Solar wind sputtering can explain dust jets and water ice formation in less illuminated areas of the comet, but not those areas facing the sun as the solar wind does not penetrate through the plasma sheath of the comet. These findings suggest another mechanism must be at work along solar wind sputtering to generate all of the dust found in the coma. A corollary of this study is that proton ion implantation is more likely to occur in areas of less illumination.

Let’s keep in mind that the Microwave Instrument for Rosetta Orbiter (MIRO) [8] , months ago, did not find significant water ice on the surface of comet 67P. In the article titled The diurnal cycle of water ice on comet 67P/Churyumov-Gerasimenko” by De Sanctis et al [9] a water ice cycle is proposed based on measurements using infrared spectroscopy (VIRTIS). In this study, the infrared reflectance spectra taken from various levels of illuminated regions of the surface of comet 67P reveals:

1. A broad absorption band at 2.8 to 3.6 μm, attributed to organic compounds.
2. The absence of pure water ice absorption bands indicates an upper limit of about 1% by volume of water ice at VIRTIS resolution.
3. A “flattening of the continuum slope and a reduced thermal emission as the 3-μm band depth increases,” suggesting exposed water ice in addition to organic material.

On the basis of these observations, the authors use scattering theory to model the infrared reflectance spectra as an intimate mixture of water ice and organics to derive water ice abundance maps. In order to fit the data the model required for relative water ice abundance in the mixture to be 10-15%. This kind of particular analysis of modelling empirical observation is quite acceptable in the scientific community and the authors here make reasonable assumptions to fit the data. However, the strength of a model is measured by the assumptions made. Subsequently, from the modelling, the authors extract water ice abundance maps and propose a “direct condensation of gas sublimation from the subsurface under appropriate thermodynamic conditions.” By thermodynamic condition, the authors imply a temperature inversion mechanism, which works more or less like this:

Water ice in the water/organic mixture sublimates from the uppermost surface layers. When this surface is not illuminated by the sun, such as shadows or night time, a temperature inversion occurs between the now colder top surface layer and interior layers, a few centimeters deep. Water vapour in the warmer interior layers diffuses upwards and condenses in the uppermost colder layer, where it sits stably until it is illuminated again, upon which the water ice undergoes sublimation. Then, the cycle repeats again.

It is beneficial to place the study in a wider context. Although based on the title of the manuscript, one might think that there is water ice on the surface of the nucleus of comet 67P, the authors clearly state that:

a. “It must be mentioned that the contribution to the total out-gassing from these surface layers is limited in time.”
b. “The amount of water flux coming from the superficial ice documented by VIRTIS represents ~3% of the total water flux measured by MIRO.” VIRTIS uses infrared emission spectroscopy while MIRO uses microwave spectroscopy.

In other words, this mechanism represents a very small percentage of the total water presence on the surface of the nucleus. A quick analysis of these findings and the observations reported on solar sputtering mentioned earlier leads me to think that solar sputtering can play a role in the water ice observations for the following reasons:

a. According to the derive water ice abundance maps, only small amounts of water ice are detected on the surface.
b. Water ice seems to be replenished in regions of lesser illumination, such as shadows or at night time.
c. Solar sputtering has been shown to be effective in areas of lesser illumination.

Taken together, it is reasonable to attribute the observed water ice formed in less illuminated areas to proton (H+) implantation into refractory silicates where they interact with oxygen atoms located in the refractory silicate lattice structures. This is not unusual as it has already been demonstrated to occur in laboratory experiments and thought to be the dominant mechanism for water ice found on crater rims on the moon.

Episode 4 – Nature of Ejected Charged Nanograins From Comets

In episode 4, I address the nature and charge of detected nanograins in the coma of comets. An interesting article put forth is that of Gambosi et al titled Negatively charged nano-dust grains at 67P/Churyumov-Gerasimenko” [10]. The main findings in this manuscript are quite interesting and summarized as follows:

1. The ion and electron sensor (IES) detected nanograins with nearly monoenergetic beams in the 200 to 500 eV range.
2. These nanograins, which authors attribute to water nano-grains 3 to 4 nm in diameter, were negative-charged and were traveling away from the nucleus of the comet in the sun-ward direction.
3. Moreover, no signal for positive-charged nanograins was detected by the sensor.

The authors explained the observation via the folding umbrella pattern of dust nanograins. In this reasoning, water nanograins are dragged out by gas outflow, which are then pushed back in the direction of the nucleus by radiation pressure.

But this attribution is problematic and to some degree contradictory for the following reasons:

a. The folding umbrella pattern would indicate dust nanograins travel towards the nucleus, thus contradicting the observation that the negative-charged dust nanograins were moving from the direction of the nucleus when detected.
b. The lack of detection of positive-charged nanograins is attributed to photoionization near the nucleus of the comet to form negative-charged particles. However, even if photoelectron impact dissociation ionization mechanism is considered as previously published in Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with Alice far-ultraviolet spectrograph on Rosetta,” [11] positive ions would have been generated and should have been detected.
c. Finally, the authors attribute the lack of signal for positive-charged dust nanograins to an instrumental failure.

In a wider context, the explanation given is unsatisfactorily. A different but feasible interpretation is put forth: the dust nanograins are being charged from electrons originating from the surface of the nucleus because as I have presented in previous episodes there are plenty of accounted electrons in the vicinity of the nucleus not attributed to photoionization. Instead, I propose these electrons are a consequence of an electro-stripping process driven by a voltage differential between the nucleus and the comet’s surroundings.

Further support for this theoretical framework is revealed in a study of comet Pan-STARRS, which was analyzed using white light when the comet was at a heliocentric distance of 0.3 AU, close to perihelion. In the report by Raouafi et al “Dynamics of high-velocity evanescent clumps [HVECs] emitted from comet C/2011 L4 as observed by STEREO,” [12] highly dynamical ejecta traveling in near-radial anti-sunward direction is observed. The traditional view has been that along with volatile species, dust particles with varying sizes are ejected into the cometary coma and subjected to a wide range of physical processes, such as gravity, radiation pressure, fragmentation, charging, and collisions. The ejected clumps are found to have an initial speed of 200 to 400 km.s-1, which is then accelerated to 600 km.s-1. Furthermore, STEREO images morphology suggests that particles < 1 μm are swept radially in the anti-sunward direction regardless of their ejection times and locations along the comet’s orbit, while particles > 1 μm flow into the dust tail forming striae. And although solar radiation plays a role in size sorting of the grains into dust, ion, and neutral tails, it cannot accelerate to velocities > 200 km.s-1 in the observed time interval and distance. For instance, for a 1 μm cluster to be accelerated by radiation pressure to a velocity of 350 km.s-1 in a distance of 108 km, requiring ~ 2,700,000 seconds. Instead, a faster acceleration is seen, needing only ~ 20,000 seconds. As a result, the authors suggest that radiation pressure plays no role in the acceleration of the ejecta with sizes < 1 μm, but instead the ejecta is charged by photoionization and picked-up through mass loading by the heliocentric electric fields. After a long and careful analysis, the authors conclude that:

1. Nanograins are comprised of positive-charge (cations) species containing low ionization metallic ions (Na+, K+) and neutral (Na, K, Li, Ca) species.
2. Provided the nanograins were singly-charged, a cluster of 5 olivine molecules [Mg2SiO4] with a radius of 1.5 nm is able to explain the observations.
3. Subject to confirmation, the one-to-one relationship between the clumps and striae may indicate the presence of filamentary electric fields in the vicinity of the comet’s tail.

These findings are of particular importance since similar coma/tail conformation has been observed in comet 1P/ Halley in 1910, and 1986, comet C/1995 01 Hale-Bopp, comet C/2006 P1 McNaught, and comet C/2001 V1 NEAT. In addition, ejecta have been observed in comet C/2011 L4 at distances beyond the water-ice sublimation zone, which is 5 to 6 AU. Such observations have been unsatisfactorily explained by:

1. Latent heat release from amorphous-crystalline water-ice transitions.
2. Sublimation of frozen super-volatiles
3. Comet fragmentation

Of course, the question is what is the driving force for comet activity at such large heliocentric distances? In my theoretical framework of electron-stripping, the driving force is a voltage differential between the comet its surrounding plasma sheath. In this episode we have seen two different articles reporting on negative-charged nanograins traveling from the nucleus in a sunward direction, and positive-charged nanograins traveling from the nucleus in the anti-sunward direction. We have also come across reports describing increase electron beams and fluxes in the vicinity of the nucleus. The overall implication of these observations is that there is charge separation, hence the presence of a voltage differential as illustrated by electric fields accelerating charged particles. There is enough voltage differential for low ionization metals to be ionized while residing in the lattice structures of minerals and silicates for the electrons to be stripped away of the lattice and accelerated.

The results are electrons appreciably accelerated away from the surface of the nucleus, which we detect as ion beams and fluxes, causing ion impact dissociation on other volatiles, mainly because the associated electric fields accelerating the charge particles are filamentary in nature. Now, as the electrons are accelerated away, the metals in the lattice structure become positive-charged and smaller in size, destabilizing the lattice structure to the point of collapse, upon which fragmentation becomes a dominant mechanism. This electron-stripping process can be a source of both electrons and positive-charged nanograins. In addition, because of the complex electrical environment charge separation occurs, and in conjunction with charge exchange, negative-charged nanograins can be also expected to travel towards the sun-ward direction, as previously discussed.

Episode 5 – Implications of the Detection of Molecular Oxygen in Comet 67P

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.

References
[1] Richter I. et al. Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko. 2015, Ann. Geophys., 33, 1036.
[2] Nilsson H. et al. Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: observations between 3.6 and 2.0 AU. 2015, Astronomy and Astrophysics, 583, A20.
[3] Goldstein R. et al. The Rosetta Ion and Electron Sensor (IES) measurement of the development of pickup ions from comet 67P/Churyumov-Gerasimenko. 2014, Geophys. Res. Lett., 2, 3093.
[4] Broiles, T. W. et al. Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko. 2015, Astronomy and Astrophysics, 583, A21.
[5] Clark, G. Suprathermal electron environment of comet 67P/Churyumov-Gerasimenko: Observations from the Rosetta Ion and Electron Sensor. 2015, Astronomy and Astrophysics, 583, A24.
[6] Fulle, M. et al. Potassium detection and lithium depletion in comets C/2011 L4 (Panstarrs) and C/1965 S1 (Ikeya-Seki). 2013, Astrophysical J. Lett., 771, L21.
[7] Wurz, P et al. Solar wind sputtering of dust on the surface of 67P/Churyumov-Gerasimenko. 2015, Astronomy and Astrophysics, 583, A22.
[8] Biver, N. et al. Distribution of water around the nucleus of comet 67P/Churyumov-Gerasimenko at 3.4 AU from the sun as seen by the MIRO instrument on Rosetta. 2015, Astronomy and Astrophysics, 583, A3.
[9] De Sanctis, M. C. et al. The diurnal cycle of water ice on comet 67P/Churyumov-Gerasimenko. 2015, Nature, 525, 500.
[10] Gombosi, T. I. et al. Negatively charged nano-grains at 67P/Churyumov-Gerasimenko. 2015, Astronomy and Astrophysics, 583, A23.
[11] Feldman, P. D. et al. Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with Alice far-ultraviolet spectrograph on Rosetta. 2015, Astronomy and Astrophysics, 583, A8.
[12] Raouafi, N.-E. et al. Dynamics of high-velocity evanescent clumps [HVECs] emitted from comet C/2011 L4 as observed by STEREO. 2015, J. Geophysical Res. (Space Phys.) 120, 5329.
[13] Bieler, A. et al. Abundant molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko. 2015, Nature, 526, 678.
[14] Biver, N. et al. Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy). 2015, Sci. Adv. 1, e1500863.


EU2013 Franklin Anariba Photo -1Dr. Franklin Anariba is currently a lecturer at Singapore University of Technology and Design where he teaches chemistry and carries out research in areas of electrochemistry and biosensing for biomedical applications. He received a BA in Chemistry from Rutgers University, obtained a MSc in Analytical Chemistry and PhD in molecular electronics from The Ohio State University. His professional experience includes positions at Merck & Co, Center for Nanoscale Science and Engineering (CNSE), and California Polytechnic State University (Cal Poly) in the US, the Institute of Bioengineering and Nanotechnology (IBN) and Nanyang Technological University (NTU) in Singapore.

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