Surface Sputtering Confirmed as Source of Moon Water.
Recently it was announced that several probes had unequivocally shown that OH (hydroxyl radical) and H2O (water) were present at the surface of the moon in minute quantities. However, it was unclear how it got there. It has now been confirmed that electrical sputtering and recombination is the source.
In a prior story it was stated that clear evidence for surface OH (hydroxyl radical) and H2O (water) was corroborated in the data from several probes to have observed the moon.
Since man first touched the moon and brought pieces of it back to Earth, scientists have thought that the lunar surface was bone dry.
When Apollo astronauts returned from the moon 40 years ago, they brought back several samples of lunar rocks.
The moon rocks were analyzed for signs of water bound to minerals present in the rocks; while trace amounts of water were detected, these were assumed to be contamination from Earth, because the containers the rocks came back in had leaked.
"The isotopes of oxygen that exist on the moon are the same as those that exist on Earth, so it was difficult if not impossible to tell the difference between water from the moon and water from Earth," said Larry Taylor of the University of Tennessee, Knoxville ...
But new observations from three different spacecraft have put this notion to rest with what has been called "unambiguous evidence" of water across the surface of the moon.
The moon remains drier than any desert on Earth, but the water is said to exist on the moon in very small quantities. One ton of the top layer of the lunar surface would hold about 32 ounces of water, researchers said.
The findings of all three spacecraft "provide unambiguous evidence for the presence of hydroxyl or water," said Paul Lucey of the University of Hawaii in an opinion essay accompanying the three studies.
However, it was unclear where the water came from and how it persisted or was replenished. The leading theories were that the OH and H2O were either from deposits by icy comets or were locally generated in situ by a process known as sputtering.
There are potentially two types of water on the moon: that brought from outside sources, such as water-bearing comets striking the surface, or that that originates on the moon.
This second, endogenic, source is thought to possibly come from the interaction of the solar wind with moon rocks and soils.
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.
It is the latter explanation, technically called 'sputtering,' that has apparently been confirmed by looking through additional data.
The Moon is a big sponge that absorbs electrically charged particles given out by the Sun. These particles interact with the oxygen present in some dust grains on the lunar surface, producing water.
The SARA [Sub-KeV Atom Reflecting Analyser] results confirm that solar hydrogen nuclei are indeed being absorbed by the lunar regolith but also highlight a mystery: not every proton is absorbed. One out of every five rebounds into space. In the process, the proton joins with an electron to become an atom of hydrogen. "We didn't expect to see this at all," says Stas Barabash, Swedish Institute of Space Physics ...
The incoming protons are part of the solar wind, a constant stream of particles given off by the Sun. They collide with every celestial object in the Solar System but are usually stopped by the body’s atmosphere. On bodies without such a natural shield, for example asteroids or the planet Mercury, the solar wind reaches the ground. The SARA team expects that these objects too will reflect many of the incoming protons back into space as hydrogen atoms.
In fact, the SARA results seem to be in line with recent IBEX results, when it was switched on, and the moon happened to pass through it's field of view. Surprisingly, this tidit seems to have been overlooked.
"Just after we got IBEX-Hi turned on, the moon happened to pass right through its field of view, and there they were," says Dr. David J. McComas, IBEX principal investigator and assistant vice president of the SwRI Space Science and Engineering Division. "The instrument lit up with a clear signal of the neutral atoms being detected as they backscattered from the moon."
From its vantage point in space, IBEX sees about half of the moon -- one quarter of it is dark and faces the nightside (away from the sun), while the other quarter faces the dayside (toward the sun). Solar wind particles impact only the dayside, where most of them are embedded in the lunar surface, while some scatter off in different directions. The scattered ones mostly become neutral atoms in this reflection process by picking up electrons from the lunar surface.
The IBEX team estimates that only about 10 percent of the solar wind ions reflect off the sunward side of the moon as neutral atoms, while the remaining 90 percent are embedded in the lunar surface.
The results confirmed by SARA are not especially surprising considering a recent news release on OH / H2O found where they shouldn't be: in Mercury's atmosphere (Mercury's surface is too hot for water to exist).
...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 [signal, however] is clearly there. The very first time we took a whiff of the planet, it was right there.
The resolution of the mystery came from the application of the laboratory electrical / chemical process known as 'sputtering.'
...the process of chemical sputtering could create water where none existed before from the ingredients of solar wind and Mercury rock, as Zurburchen 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."
Recently, Nancy Atkinson of Universe Today speculated that the water signal recently discovered in an asteroid's spectra may also come from a similar source.
Another option is that an action similar to the recent findings of water on the Moon, where solar wind interacts with a rocky body without an atmosphere to create H2O and OH molecules. Without an atmosphere, the body is exposed to solar wind, which includes hydrogen ions. The hydrogen is able to interact with oxygen in surface of the asteroid to create water molecules.
This seems like a reasonable suggestion.
However, it also beg the question of whether comets produce the OH / H2O abundances found in their comas in the same manner. That is to say, whether it's a local production due to cathode sputtering of oxygen atoms from surface silicates, then a recombination with hydrogen from the solar wind.
Perhaps comets aren't the "dirty snowballs" (loose agglomerations of dust and rubble held together by water ice) envisioned by pioneering astronomer Fred Whipple, et al.
[Fred Whipple] revolutionized the study of comets when in 1950-51 he proposed that they were not "sandbags" but small bodies made of rock, dust and ice.
Whipple knew that some comets have been orbiting the Sun a thousand times and more. If they were nothing but sand, they would have broken up.
In 1950, he published a paper suggesting they were "icy conglomerates"; what the media later called "dirty snowballs."
As the comet gets closer to the Sun, the ice vaporises and forms a spectacular coma, or tail.
But rather, they may be rocky bodies discharging electrically as suggested by plasma physicist Wal Thornhill, et al. Evidenced by the "forbidden lines" in cometary comas indicative of strong electric fields.
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 ... 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. They wrote, "an efficient production mechanism, so far unidentified, is required to account for the observed densities." And the intense electric field near the comet nucleus is inexplicable if it is merely an inert body ploughing through the solar wind.
Let's see how the electrical model of comets explains these mysteries. 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 important point is that the OH does not need to come from water ice on, or in, the comet.
The recent suggestions, from independent researchers, of the sputtering of atoms from surface materials on various bodies with little or no atmosphere (Mercury, the Moon and asteroids) and recombination with atoms from the solar wind to produce OH / H2O abundances, appears to lend credence to Thornhill's contention, despite its controversial implications.
Thornhill's electrical theory posits that comets are largely rocky bodies holding an electric charge and discharging under influence of a radial electric field within the heliosphere, centered on the sun. Their brilliant displays are then considered to be an electrical phenomenon (a plasma [Langmuir] sheath).
Perhaps it's time to candidly and objectively reassess the nature of comets and with them the nature and function of the solar plasma environment. If a unifying thread can be found that ties together and explains multiple phenomena under the same auspices, is it not the duty of responsible scientists to pursue all avenues of investigation, even if they may run counter to popularly held opinion?
See prior story:
Electrical Erosion (Sputtering) in Our Electric Solar System.