When is Mercury Like a Comet?

First FIPS spectrum of ions in Mercury's exosphere
MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) scoops up ions from Mercury's exosphere and measures their composition as a ratio of their mass to their charge. Each atomic or molecular ion shows up as a distinct peak in this spectrum. Most ions are singly charged (missing one or having one extra electron) so are visible as a peak at the atomic mass of the element (for example, sodium at 23). A few are present as multiply charged ions, for instance doubly chaged oxygen at a peak of 8. The surprising result is the detection of water-related ions like O+, OH-, and H2O+. Credit: NASA / JHUAPL / U. Michigan

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."

How could there be water on Mercury? Zurbuchen listed three possibilities, which are not mutually exclusive. Firstly, it has long been theorized (but not yet proved) from Earth-based radar observations that there may be reservoirs of water ice in small areas of Mercury's poles where local topography creates permanently shadowed spots in crater walls that might trap water over the age of the solar system. Second, the water could come from comets.

Third, 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."

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. 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.

The evidence suggests that comets are highly negatively charged with respect to the Sun. As they rush toward the Sun, the voltage increases until at some point the comet nucleus begins to discharge. Electrons are stripped from a few points on the comet surface where the electric field is strongest. These “spark discharges” finely machine rocky material from the surface to form a “cathode jet” of negatively charged dust together with surface matter that has been torn apart to release ionized atoms and molecules, including oxygen.

Under the conventional model there is no reason for the high density of negative ions discovered near the comet nucleus. Negative ions are difficult to produce by solar heating and are quickly destroyed by solar radiation. Nevertheless, in March 1986 when the Giotto spacecraft flew within 600km of Comet Halley, an abundance of negatively charged atoms was discovered in the inner coma—direct evidence that a comet is the cathode in an electric exchange with the Sun. A few years later, scientists discovered an unexpected “forbidden oxygen” line at 1128Å in the spectrum of Comet Austin. That line is consistent with the presence of an intense electric field and/or densities in the coma many orders of magnitude higher than those predicted from standard cometary theory.

There is reason to believe that the positively charged ions from the solar wind react preferentially with the negatively charged oxygen from the nucleus to generate the water observed surrounding comets. The probe Vega 2 found the H2O (water) production by comet Halley was one fifth of the OH production. But scientists had supposed that OH was formed by photo-dissociation of H2O at some distance from the nucleus. The report in Nature in May 1986 reads: "only indirect and sometimes ambiguous evidence in favor of water has been found; indeed, some facts appear to contradict this hypothesis." Thus, the authors suggest, "This problem requires further analysis and may indicate the existence of parents of OH other than H2O."

Such a discovery is most simply explained if the parents of OH were a combination of solar protons (hydrogen) and negative oxygen ions electrically removed from silicates and other minerals in the nucleus. The greater abundance of OH would then be expected. It then becomes clear that the water we see is being produced through electrical exchange: Negatively charged oxygen from the comet nucleus combines with the positively charged hydrogen ions from the Sun, via the solar wind.

When Comet Linear disintegrated in front of their eyes, astronomers were not just shocked by the event (a comet exploding many millions of miles from the Sun), they were astonished to find virtually no water in the immediate debris.

The absences of detectable water on comet nuclei had produced a crisis in comet theory well before Deep Impact. And the mission did nothing to rescue the theory. The Harvard-Smithsonian Center for Astrophysics summarized the early findings with the headline, “Deep Impact Was a Dust-up, Not a Gusher.” Smithsonian astronomers reported the detection of “only weak emission from water vapor and a host of other gases that were expected to erupt from the impact site. The most conspicuous feature of the blast was brightening due to sunlight scattered by the ejected dust.”

The results of the Deep Impact mission were published in the journal Science. Team members reported that they found only a smattering of water ice on the surface of Tempel 1. In fact, to account for the water supposedly emitted into the coma of Tempel 1, the investigators needed 200 times more exposed water-ice than they could find.

But a much different vantage point on the water question is possible. When astronomers view the comas of comets spectroscopically, what they actually see is the hydroxyl radical (OH), which they assume to be a residue of water (H2O) broken down by the ultraviolet light of the Sun (photolysis). This assumption is not only unwarranted, it requires a speed of “processing” by solar radiation beyond anything that can be demonstrated experimentally.

The mysteries find direct answers electrically—in the transaction between a negatively charged comet nucleus and the Sun. In the electric model, negative oxygen ions are accelerated away from the comet in energetic jets, then combine preferentially with protons from the solar wind to form the observed OH radical and the neutral hydrogen gathered around the coma in vast concentric bubbles. These abundances simply confirm the energetic charge exchange between the nucleus and the Sun.

The electric model thus resolves two problems for the standard theory: 1) Cometologists have never verified that the assumed photolysis is feasible on the super-efficient scale their “explanation” requires; 2) Neutral hydrogen is far too plentiful in the coma to be the “leftover” of the hypothesized conversion of water into OH. But if the negatively charged nucleus provides the electrons in a charge exchange with the solar wind, the dilemma is resolved and the vast hydrogen envelope is a predictable effect.

Only a source of internal heat has been considered on Enceladus. The crucial discovery is the “completely unexpected surprise” of the similarity of the chemistry of the jets of comets and Enceladus. It should not have been a surprise. The jets of both are an electric discharge phenomenon, heating the surface. The matter in the jets is coming from the surface and not the interior. And the chemistry of the jets is comparable because comets are born from the same parent bodies and under the same electrical conditions as rocky and icy planets and moons. The concept of ‘primordial material’ has no basis outside the nebular theory. The only difference between Enceladus and a comet is their orbits. If Enceladus were to follow a cometary orbit it would suffer electrical interactions with the solar plasma and appear a giant comet.

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.

The “other stuff” found in some of the ice particles may be composed of atoms formed by nuclear modification of hydrogen, oxygen, carbon and nitrogen, detected by INMS. Even CO may be a product of a nitrogen molecule, N2, under these conditions. If the “other stuff” can be analyzed, it may provide the identity of the blue coloration of the tiger stripes.

Electric discharge machining of planetary surfaces is the most powerful sculpting force in Nature. Until planetary scientists recognize this fact they will continue to be surprised and puzzled by images and data returned from other bodies in the solar system.