Electrical Erosion (Sputtering) in Our Electric Solar System.
Recent data from the moon indicates that OH (hydroxyl radical) and/or H2O (water) may exist on or above the surface of the moon. But what does it mean and how does it relate to active processes at Mercury, Enceladus and comets? An alternative theory says that electrical interactions may be the thread that ties them all together.
This article follows up a prior article on the OH / H2O found in the atmosphere of Mercury.
It seems that the surface of Mercury is inhospitable to water (at 750 degrees Fahrenheit). Thus the discovery of water (H2O) and the assumed breakdown product hydroxyl radicals (OH) in its atmosphere was puzzling at best to mission scientists since the surface could not be the source of intact H2O molecules.
This is very interesting, because the temperature on the surface of Mercury can range to over 400 degrees Celsius [750 degrees Fahrenheit]. Water can't really sit there. This water is clearly there. The very first time we took a whiff of the planet, it was right there."
However, not to be outsmarted by the data, mission scientist Zurbuchen offered an inventive alternative explanation that does not require H2O to exist on Mercury's metaphorically fiery surface. His rejoinder to the water problem in Mercury's atmosphere? A well-known process from the laboratory: sputtering.
...the process of chemical sputtering could create water where none existed before from the ingredients of solar wind and Mercury rock, as Zurbuchen explains.
"The solar wind is highly ionized. Those are radicals -- they want to make connections with everything that they can. Imagine a solar wind hydrogen showing up and hitting the surface. It weathers whatever the mineral is, and steals an oxygen. If you do that, you get something like OH-, for example." OH-, also known as a hydroxyl group, would produce a peak at atomic mass 17 on the FIPS spectrum. "You can do it in reverse -- an oxygen from the solar wind can steal a hydrogen. The process is called chemical sputtering."
Now, it seems that there is recent, clear evidence of OH / H2O in data from several spacecraft that have surveyed the moon. They have found:
multiple detections of the spectral signal of either water or the hydroxyl group (an oxygen and hydrogen chemically bonded).
But, whence and whither? Is this evidence for large deposits of water ice at or below the surface or in shadowed craters? Many scientists hope so.
Chandrayaan-1, India's first-ever moon probe, was aimed at mapping the lunar surface and determining its mineral composition (the orbiter's mission ended 14 months prematurely in August after an abrupt malfunction). While the probe was still active, its NASA-built Moon Mineralogy Mapper (M3) detected wavelengths of light reflected off the surface that indicated the chemical bond between hydrogen and oxygen — the telltale sign of either water or hydroxyl.
Because M3 can only penetrate the top few millimeters of lunar regolith, the newly observed water seems to be at or near the lunar surface. M3's observations also showed that the water signal got stronger toward the polar regions.
Cassini, which passed by the moon in 1999 on its way to Saturn, provides confirmation of this signal with its own slightly stronger detection of the water/hydroxyl signal. The water would have to be absorbed or trapped in the glass and minerals at the lunar surface, wrote Roger Clark of the U.S. Geological Survey in the study detailing Cassini's findings.
The Cassini data shows a global distribution of the water signal, though it also appears stronger near the poles (and low in the lunar maria).
Finally, the Deep Impact spacecraft, as part of its extended EPOXI mission and at the request of the M3 team, made infrared detections of water and hydroxyl as part of a calibration exercise during several close approaches of the Earth-Moon system en route to its planned flyby of comet 103P/Hartley 2 in November 2010.
Deep Impact detected the signal at all latitudes above 10 degrees N, though once again, the poles showed the strongest signals.
But is there an alternative explanation for the detection of OH / H2O? We come again to sputtering.
Combined, the findings show that not only is the moon hydrated, the process that makes it so is a dynamic one that is driven by the daily changes in solar radiation hitting any given spot on the surface.
The rocks and regolith that make up the lunar surface are about 45 percent oxygen (combined with other elements as mostly silicate minerals). The solar wind — the constant stream of charged particles emitted by the sun — are mostly protons, or positively charged hydrogen atoms.
If the charged hydrogens, which are traveling at one-third the speed of light, hit the lunar surface with enough force, they break apart oxygen bonds in soil materials, Taylor, the M3 team member suspects. Where free oxygen and hydrogen exist, there is a high chance that trace amounts of water will form.
The foregoing echoes Zurbuchen's assessment of H2O / OH production in Mercury's atmosphere by way of hydrogen in the solar wind scavenging oxygen knocked loose from surface rocks or vice versa.
Is sputtering a major form of H2O / OH production on rocky bodies in the solar system? Moreover, can the detection of H2O / OH around such bodies give use a false sense the bodies are composed of watery constituents when they are in fact composed largely of rock?
In a curious twist, the recent discoveries of H2O / OH in Mercury's atmosphere and on or near the lunar surface, and their proposed physical genesis, echo Australian physicist Wal Thornhill's assessment of H2O / OH production in the comas of comets.
The flaw in the conventional approach is that only gas-phase chemical reactions and reactions induced by solar radiation (photolysis) are considered. The far more energetic molecular and atomic reactions due to plasma discharge sputtering of an electrically charged comet nucleus are not even contemplated [see below]. Yet this model solves many comet mysteries that are seldom mentioned.
The hydroxyl radical, OH, is the most abundant cometary radical. It is detected in the coma at some distance from the comet nucleus, where it is assumed that water (H2O) is broken down by solar UV radiation to form OH, H and O. It is chiefly the presence of this radical that leads to estimates of the amount of water ice sublimating from the comet nucleus. The comas of O and OH are far less extensive than the H coma but have comparable density.
The negatively charged oxygen atom, or negative oxygen ion, has been detected close to cometary nuclei. And the spectrum of neutral oxygen (O) shows a "forbidden line" indicative of the presence of an "intense" electric field. The discovery at comet Halley of negative ions puzzled investigators because they are easily destroyed by solar radiation.
The electric field near the comet nucleus is expected if a comet is a highly negatively charged body, relative to the solar wind. Cathode sputtering of the comet nucleus will strip atoms and molecules directly from solid rock and charge them negatively. So the presence of negative oxygen and other ions close to the comet nucleus is to be expected. Negative oxygen ions will be accelerated away from the comet in the cathode jets and combine with protons from the solar wind to form the observed OH radical at some distance from the nucleus.
The activity in a comet coma should be viewed in terms of low-pressure gas discharge phenomena, not simply gas-phase chemistry and photolysis. If more evidence were needed that this is so, we need only remember the baffled surprise of astronomers when an orbiting x-ray telescope was accidentally pointed at a comet and strong x-ray emissions found. An explanation was cobbled together after the discovery. It involved the recombination of solar wind ions with electrons from the comet. But that constitutes an electric current between the comet and the solar wind.
Thus it seems that the new observations must be considered alongside existing observations. What's good for the goose is good for the gander. In other words, if sputtering is a non-trivial source of H2O and OH in the atmosphere of Mercury and near the surface of the Moon, then might not this additional understanding inform our assessment of existing theories? Is the theory sound with respect to comets as it appears to be at Mercury and the Moon? There is little reason to believe that it is not, other than the inertia of prior thought.
Moreover, it may also inform our understanding of the "cometary plumes" of Enceladus. If one assumes, for the sake of argument, that similar results follow from similar causes.
"A completely unexpected surprise is that the chemistry of Enceladus, what's coming out from inside, resembles that of a comet," said Hunter Waite, principal investigator for the Cassini Ion and Neutral Mass Spectrometer at the Southwest Research Institute in San Antonio. "To have primordial material coming out from inside a Saturn moon raises many questions on the formation of the Saturn system."
Despite the similarity, the mission team investigating Enceladus was quick to point out that "Enceladus is by no means a comet" in the traditional sense and that existing theory attributes the production of similar molecular profiles to completely disparate processes.
"Enceladus is by no means a comet. Comets have tails and orbit the sun, and Enceladus's activity is powered by internal heat while comet activity is powered by sunlight. Enceladus's brew is like carbonated water with an essence of natural gas," said Waite.
But does the accumulating evidence point to "sputtering" a unifying explanation for the production of similar materials from similar sources?
Thornhill certainly seems to think so:
On 14 July 2005 Cassini was lowered to a close-approach distance of 168 km. The encounter produced unequivocal evidence of a plume of water vapor and small icy particles emanating from the south polar region of Enceladus. Surprisingly, the Ion and Neutral Mass Spectrometer (INMS) found that if the mass-28 species is CO rather than N2, then the outgassing observed from the plume would have a composition that is remarkably close to that of comets. Also the very narrow size distribution of particles fed to the E-ring by Enceladus is remarkably close to that of comets. This finding favors the electric discharge sputtering mechanism. It is precisely that mechanism that operates on comet nuclei to produce jets and that produced expressions of surprise when the fineness and limited size distribution of comet dust was first measured. Dr. Torrence Johnson, imaging team member from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, had the answer intuitively when he said in December 2005, "In some ways, Enceladus resembles a huge comet." But then cognitive dissonance took over, "Only, in the case of Enceladus, the energy source for the geyser-like activity is believed to be due to internal heating by perhaps radioactivity and tides rather than the sunlight which causes cometary jets." On the contrary, using Ockham’s razor, one simple model should explain them all.
It is tempting to prefer a single, simple, unified explanation over the many disparate ad hoc explanations that currently exist. However, such unification many bring some growing pains to the sciences, as Thornhill's proposal requires closer attention to electrical processes than has perhaps been afforded by space scientists in the past. Thornhill's model takes as an assumption that the Sun is not a nuclear furnace but rather the anode of a spherical low-pressure plasma discharge, with comets, moons and planetary bodies acting as minor cathodes in the heliosphere-scale discharge.
It will be interesting to see whether space scientists are willing to put aside existing ideas, for the sake of argument, and consider the possibility of a unified explanation, even if it may require discarding a number of existing ideas. Hopefully scientists are honest and open-minded enough to take such an introspective look at existing theories and the potential for a newer more simplified theory that may overturn the tables and replace them.
Upon a little more digging and a note from a colleague, I came across an additional article on the topic of the recent Moon findings. It is additionally relevant insofar as it provides significant multimedia (images) to clarify some aspects of the finding. Several images have been added to the story relating to the moon specifically.
Three images in particular spurred additional thought processes along two lines of inquiry. 1) The graphic of the lunar temperatures versus the H2O / OH abundances. 2) The schematic of the daily water cycle. 3) The lunar mineral map.
1 and 2 above jogged a memory about a previous set of articles relating to "moon fountains" and moon storms. 3 above may provide clues to the disparity between the H2O / OH disparity between the lunar maria and other regions, as their surface composition appears to be significantly different.
With regard to the lunar mineral map, item 3 above, it seems that the lunar maria are regions of iron-rich rock not unlike volcanic lava flows here on Earth (and may have lower proportions of silicates). On the other hand, non-maria regions are indicated to be largely composed of plagioclase, which is apparently a silicate-bearing material (thus a good source of bound Oxygen).
One wonders whether the differing surface composition tends to influence the detection of H2O / OH. If so, might that be an indicator of preferential local production of the molecules through the process of sputtering (Oxygen atoms from the silicates in which they bound being knocked loose by charged particles from the solar wind, then recombining with hydrogen ions to form local OH and H2O molecules), based upon the geology of the region?
Alternatively, the albedo (reflectivity) of the maria appear to be considerably lower than other regions as well. If more energy is absorbed by the surface it may end up being considerably warmer than higher albedo regions where more light is reflected. In that case, it may be that regions with lower albedo are less hospitable to H2O / OH retention, thus the reduced detection signal in those regions.
It's unclear which possibility, if either, is more likely or whether they both may play a role.
Mission scientists have offered an image comparing H2O / OH detection abundances to the daily temperature profile of the Moon.
There is certainly an interesting correlation. The "water" (or hydroxyl) signal seems to be lowest in the region corresponding to the warmest part of the lunar daytime. But appears to be highest in the region of lunar dusk / dawn.
In another image, they offer a brief diagram showing that in the morning and evening there appears to be the greatest level of hydration (keeping in mind this is only in the upper millimeter or so of the surface, so no great volume of water), while during mid-day the signal is weakest. They infer some loss during the day due to solar bombardment with light and charged particles. To which there may be some truth, certainly.
But a curious thing to consider, if only for the sake of argument, is the prior articles on what would be inherently electrical Moon "storms" and "fountains."
The dust is electrostatically charged by the Sun in two different ways: by sunlight itself and by charged particles flowing out from the Sun (the solar wind).
On the daylit side of the Moon, solar ultraviolet and X-ray radiation is so energetic that it knocks electrons out of atoms and molecules in the lunar soil. Positive charges build up until the tiniest particles of lunar dust (measuring 1 micron and smaller) are repelled from the surface and lofted anywhere from meters to kilometers high, with the smallest particles reaching the highest altitudes, Stubbs explains. Eventually they fall back toward the surface where the process is repeated over and over again.
If that's what happens on the day side of the Moon, the natural question then becomes, what happens on the night side? The dust there, Stubbs believes, is negatively charged. This charge comes from electrons in the solar wind, which flows around the Moon onto the night side. Indeed, the fountain model suggests that the night side would charge up to higher voltages than the day side, possibly launching dust particles to higher velocities and altitudes.
Day side: positive. Night side: negative. What, then, happens at the Moon's terminator--the moving line of sunrise or sunset between day and night?
There could be "significant horizontal electric fields forming between the day and night areas, so there might be horizontal dust transport," Stubbs speculates. "Dust would get sucked across the terminator sideways."
The next time you see the moon, trace your finger along the terminator, the dividing line between lunar night and day. That's where the storm is. It's a long and skinny dust storm, stretching all the way from the north pole to the south pole, swirling across the surface, following the terminator as sunrise ceaselessly sweeps around the moon.
"To everyone's surprise," says Olhoeft, "LEAM saw a large number of particles every morning, mostly coming from the east or west--rather than above or below--and mostly slower than speeds expected for lunar ejecta."
What could cause this? Stubbs has an idea: "The dayside of the moon is positively charged; the nightside is negatively charged." At the interface between night and day, he explains, "electrostatically charged dust would be pushed across the terminator sideways," by horizontal electric fields.
Even more surprising, Olhoeft continues, a few hours after every lunar sunrise, the experiment's temperature rocketed so high--near that of boiling water--that "LEAM had to be turned off because it was overheating."
Those strange observations could mean that "electrically-charged moondust was sticking to LEAM, darkening its surface so the experiment package absorbed rather than reflected sunlight," speculates Olhoeft.
A more recent article reiterates the theme of electrical fountains and winds.
The ground, meanwhile, might leap into the sky. There’s growing evidence that fine particles of moondust might actually float, ejected from the lunar surface by electrostatic repulsion. This could create a temporary nighttime atmosphere of dust ready to blacken spacesuits, clog machinery, scratch faceplates (moondust is very abrasive) and generally make life difficult for astronauts.
Stranger still, moondust might gather itself into a sort of diaphanous wind. Drawn by differences in global charge accumulation, floating dust would naturally fly from the strongly-negative nightside to the weakly-negative dayside. This “dust storm” effect would be strongest at the moon’s terminator, the dividing line between day and night.
The best direct evidence comes from NASA’s Lunar Prospector spacecraft, which orbited the moon in 1998-99 and monitored many magnetotail crossings. During some crossings, the spacecraft sensed big changes in the lunar nightside voltage, jumping “typically from -200 V to -1000 V,” says Jasper Halekas of UC Berkeley who has been studying the decade-old data.
While it's all a bit anecdotal at this point, one wonders whether the proposed horizontal electric fields along the night-day and day-night terminators of the lunar day might play a role in production or migration of H2O or OH molecules, as has been speculated to occur with lunar dust. One wonders if the method by which lunar dust is said to become charged and leap from the surface may not be in some way similar to the sputtering of atoms from surface materials. If so, might not the electric fields of the terminators between night and day on the moon play a potential role in local production of the H2O and OH species detected by various remote sensing devices?
The similarities between the proposed daily behavior of moon dust and of the lunar daily water cycle are fascinating and may be worth investigating further. Should any pioneering investigator wish to tread lightly down the garden path a few paces.