These close-up images of Comet Tempel 1, taken by the camera on the
Feb 17, 2006
The idea of an electric comet traces to scientific discussion in the second half of the nineteenth century. In 1871, professor W. Stanley Jevons, suggested (in the journal Nature) that comets might owe their “peculiar phenomena to electric action”. The following year Scientific American reported on the research of “Professor Zollner of Leipsic”, who suggested that comet tails, “which consist of very small particles, yield to the action of the free electricity of the sun”.
Ten years later the electric comet had gained momentum. The 1882 English Mechanic and World of Science reported a “rapidly growing feeling amongst physicists that both the self-light of comets and the phenomena of their tails belong to the order of electrical phenomena”. By 1896, Nature could report: “It has long been imagined that the phenomenon of comet’s tails are in some way due to a solar electrical repulsion”.
In retrospect it is clear that those envisioning electric activity of comets were limited by traditional concepts of electrostatics, concepts that have continued to breed misunderstanding into the 21st century. But experimental knowledge of the “plasma universe” began with Kristian Birkeland very early in the 20th century, not long before Irving Langmuir named “plasma” for its life-like qualities. Later, the groundbreaking work of Hannes Alfvén showed conclusively that simple electrostatic formulae were wholly inadequate to account for plasma behavior.
These things were unknown to the scientific community when Hugo Benioff published “The Present State of the Electric Theory of Comet Forms” in 1920. Benioff acknowledged that, “the outward radial motions in all directions of particles close to the nucleus are best explained as resulting from an electrical charge associated with the nucleus”. But as for the “repulsion” of comet tails, he said, this required a charge separation beyond anything that could be practically envisioned. So, to explain the behavior of comet tails, he settled on the principle of “radiation pressure”, an idea to which all of astronomy moved in the following decades.
Today, however, we know that comet tails can be influenced by solar radiation “pressure”, but they are clearly not governed by it.
Nevertheless, the electric comet faded quickly as astronomers came to envision an electrically inert, gravitationally dominated universe. Earlier investigators, despite a comparative lack of data, were more interdisciplinary. They could see certain features of comets calling for an electrical explanation. But as specialization took over, astronomers soon lost all interest in electricity, a subject eventually banished from the training of astronomers and disappearing completely from their vocabulary.
Was the progressive dismissal of electricity based on evidence, or on something else? The comet provides a good illustration of the point we’ve made many times in these pages. Theoretical assumptions can marginalize uncomfortable facts to such an extent that they are no longer noticed or remembered.
When 19th century astronomers wondered about the role of electricity in comet behavior, they could see that a cometary coma, the spherical envelope around the nucleus, could not be maintained by gravity. But well before the full flowering of modern plasma research, experiments showed that when a charged probe was placed in plasma, a sphere of oppositely charged particles would gather around the probe. The early researchers were only following the experimental evidence when they recognized electrical phenomena in the sphere of the coma.
Today our view of the comet is greatly enhanced by the technological achievements of the twentieth century; but critical thinking—the ability to question theoretical assumptions—has collapsed to the point that astronomers barely notice the incongruities in coma behavior.
Given the trivial gravity of a typical comet nucleus, the escape velocity will be something like walking speed. Take a hop and you will never return. Our visits to comets have shown material escaping from the surface of nuclei in jets, some at supersonic speeds. The jets throw material into space in all directions, at different speeds and in irregular patterns. Then what happens? A force that you cannot find in the lexicon of astronomers gathers the material into a spherical form, despite the fact that much of this material is millions of kilometers or more from the nucleus and could not possibly “see” the nucleus gravitationally. Nevertheless, in the vacuum of space, as the comet speeds around the Sun, the nucleus continues to hold in place the giant spherical cloud, up to 10 million kilometers or more in diameter.
As astronomers continued to evolve their gravitational models of the heavens, the pioneers of plasma science explored the role of the electric force, which is known to be 39 orders of magnitude (1000,000,000,000,000,000,000,000,000,000,000,000,000 times) more powerful than gravity. Their explorations took them far beyond electrostatics, to demonstrate the powerful dynamics of electric currents in plasma and in high-energy plasma discharge. They enumerated the attributes of “double layers” that gather as spherical shells around charged objects in plasma. Across the walls of such layers, they observed intense electric fields, while across the larger distances between such layers the field could be much weaker, even imperceptibly weak.
Plasma events are scalable. What occurs in the plasma laboratory can occur on a vastly larger scale in space plasma. Hence, observations of plasma behavior in the laboratory are a logical reference when considering the mysteries of cometary comas—and that includes the many enigmas that surround the identification of “water” in the comas. According to the electric theorists, electricity can accomplish the very things that have baffled the cometologists.
In their analysis of the coma, astronomers begin with the assumption that water is evaporating in the heat of the Sun, off the surface ices of the nucleus. They do not “see” the water, but call upon the effects of solar radiation (photolysis) on assumed “water” to account for the abundant hydroxyl radical OH (oxygen-hydrogen molecules) in the coma.
In our previous Picture of the Day we noted another possibility. Astronomers have not considered the energetic ionic chemical reactions that would accompany plasma discharge “sputtering” of a cathode or negatively charged object in space. Production of OH would be virtually certain if proton streams sputtered material from the surface in the fashion that the electric theorists have claimed.
When theoretical issues arise, the contrast between predicted behavior under the two vantage points becomes a distinct advantage. With this advantage in mind we offer the following summary of facts and contrasting interpretations.
1. Negatively charged nucleus. The electric view compares the behavior of negatively charged probes in plasma experiments to the behavior of comets. It therefore predicts a spacecraft moving through the coma would encounter a number of plasma sheaths or double layers as it approached the nucleus of a comet. Plasma sheaths will form between regions in which the characteristics of the plasma itself change. Across a sheath the voltage differential between the comet nucleus and the solar wind should show up most dramatically. Positive ions should "pile up" on the sunward side of the sheath in the coma’s electrical response to the solar wind. In fact, this was observed at both comets Hyakutake and Hale-Bopp and surprised researchers by its unexpected stability over "hours, days and even weeks."
The researchers were surprised because they had imagined that the concentration of ions was a mechanical “bow shock” as material in the jets encountered the solar wind. Since the jets are highly variable, the intensity of the “bow shock” should vary accordingly. However, plasma sheaths respond only to the electrical environment, which will be less variable than episodic jets, and will be most concentrated in the sunward direction, precisely as observed.
Neutral oxygen (O) near the nucleus shows a spectral line indicative of the presence of an "intense" electric field. So the electric model anticipates energetic "hot" electrons and negatively charged ions close to the nucleus, as sputtering strips atoms and molecules directly from negatively charged rock. The International Cometary Explorer (ICE) mission to comet Giacobini-Zinner found "hot electrons coming back more and more frequently." The Halley probes detected “very energetic electron populations” in the coma. And the presence of negatively charged ions surprised the investigators. They wrote, "…an efficient production mechanism, so far unidentified, is required to account for the observed densities [of negative ions]."
In fact, the intense electric field near the comet nucleus makes no sense whatsoever if a comet is merely an inert body plowing through the solar wind. Electric currents produce magnetic fields, and "magnetized cometary plasma … is much larger than was theoretically predicted" [emphasis ours], according to the 1986 Nature report on Comet Halley.
2. OH production. If one accepts the evidence that a comet is a negatively charged body moving through the weak radial electric field of a positively charged Sun, the production of OH in the coma will not look anything like the standard picture.
Taken as a whole, the facts we have already summarized (here and here), virtually preclude abundant water on the comet nucleus, while the sputtering hypothesis stands out in its consistency with all available data. In the electric model, negative oxygen ions will be 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. The reactions simply confirm the energetic charge exchange between the nucleus and Sun.
It is interesting to note that the warning signs for standard theory came very early. Even before the first visit to a comet, a 1980 report in the journal Nature outlined some of the mysteries and anomalies. It concluded: “…cometary scientists need to consider more carefully whether H20-ice really does constitute a major fraction of comet nuclei…" This cautionary note was not heeded. Later, in 1986, Nature reported that OH issues remained perplexing and "may indicate the existence of parents of OH other than H20”. But in the years that followed, despite the shocking failure of the “dirty snowball” model to predict any milestone discoveries, one of the most critical questions simply disappeared from scientific discussion.
3. Too much atomic hydrogen. Early in the 1970s, astronomers were stunned when they observed cometary comas in ultraviolet light. They discovered immense envelopes of fluorescing hydrogen atoms much larger than the visible coma. In the case of Comet Bennett the hydrogen coma was an "almost unbelievable" 15 million kilometers in diameter. That's 10 times the diameter of the Sun! Where did this immense volume of atomic hydrogen come from? The prevailing theory of OH production requires some sort of balance between OH and neutral hydrogen. Whatever the difficulties faced by the standard model explanation of the spherical coma, the difficulties can only grow in relation to a hydrogen envelope millions of kilometers in diameter.
4. Plasma sheaths and “double layers”. Many features of the electric model of the comet derive from the laboratory behavior of electrified plasma and plasma discharge. In an electrically neutral environment nothing comparable to the sheaths that occur around charged bodies in plasma will be expected. Comet researchers working with the “dirty snowball” model of a comet expected no such phenomena.
Across the wall or boundary of a plasma sheath—what plasma experts call “a double layer”—an intense electric field may occur in contrast to a weaker field between these boundaries. Variations in the energies of charged particles will contrast sharply with what would be expected in an electrically neutral environment.
This is exactly what occurred as Giotto and the two Vega spacecraft moved through Halley’s coma. The Nature reports are replete with references to unexpected variations in charged particle energy levels—
The report notes three regions of variation in ion (charged particle) characteristics. An outer region “contains pick-up ions in the solar wind". This may be interpreted electrically as the outer edge of the comet's plasma sheath. A second region inside the so-called “bow shock” stretches for several thousand kilometers, revealing the most intense fluxes and distinct intensity spikes. This may be the crossing of the double layer, where a strong radial electric field exists. A third region is characterized by lower intensities, but with sharp spikes at closest approach. Here we may be seeing the cometary plasma being disturbed by the accelerated ions and electrons from the comet jets.
In Vega 1’s close approach, narrow peaks were evident “at all energies”. The report says, “We note that this feature coincides with the occurrence of maximum magnetic field intensity and rapid changes in field direction”. Of course, the magnetic field measures the strength of the electric currents flowing near the comet. Finally, at closest approach, there was a sudden increase in highly energetic electrons. “No significant variation in this flux had been observed for several days preceding closest approach”.
At a distance of 40,000 kilometers from the nucleus, the Vega 2 craft detected a surge in cometary plasma density, “accompanied by large fluxes of suprathermal electrons with energies up to a few keV” [emphasis ours].
“The most dramatic effects were observed in the last minute before closest approach… two short bursts of ions with energies up to 400 eV were observed: During the last 45 sec before closest approach, the flux increases rapidly until the spacecraft appears to be surrounded by a dense and very hot cloud of plasma… the energies are very much higher than had been anticipated”. For these “energies of the observed ions” the researchers had no explanation.
5. X-rays. In1996, the German X-ray Roentgen Satellite (ROSAT) viewed the comet Hyakutake. The astronomers hoped to see a small smudge at best and some wondered why anyone would bother. X-rays had never been detected from a comet before and theorists could only imagine a few ways that a comet could produce any x-rays at all. So the astronomers were shocked to find x-rays up to 100 times more intense than even the most optimistic predictions! Also the emission flickered on a time scale of hours. "We were prepared to see nothing. So it was an enormous surprise when this thing was just a boomer," said a team member. A NASA report noted, "…there must be previously unsuspected 'high-energy' processes taking place in the comet…"
This was the last thing that the standard model would have anticipated, but an electric current in a near vacuum is the way we produce x-rays on earth. The flickering is characteristic of a glow discharge. An intense electric field in a cometary double layer can accelerate electrons and cometary ions so that they collide with solar wind ions and emit x-rays. It is significant that the x-rays did not come from a region expected by a “mechanical shock” model—the only model available to the surprised astronomers. They came from a crescent-shaped region in the direction of the Sun, which is where we should expect the maximum electrical stress. Following this chance discovery, researchers have become accustomed to x-rays from comets, but the uncompromising implication of an electrical transaction, or charge exchange, between the comet and the Sun has yet to sink in.
6. Flare-ups in deep freeze. In 1991, comet Halley flared up to 300 times its normal brightness between the orbits of Saturn and Uranus, 14 times further than the Earth from the Sun. The comet's surface should be at -200 ˚C and "no kind of chemistry can work that far out from the Sun." No theory could explain the outburst from the 15-kilometer nucleus, which created a cloud of dust 300,000 kilometers wide. The cloud was "made mainly of dust, with no sign of any spectral lines emitted by any gas." Significantly for the electrical model (which does not require any gas from heated ices to explain the outburst) the Sun was going through a maximum of activity that fitted the outburst of comet Halley. Astronomers could not see the significance: "…the amount of energy in the bursts is diluted as they move outward. Even the most intense burst of protons should not deliver enough energy to provoke an outburst of this size at such a distance." But comments such as this require one to exclude electrical currents from consideration. A high voltage, negatively charged comet will attract protons to the nucleus from a huge volume of surrounding space.
7. Surface erosion. In the electric model as formulated by Wallace Thornhill, “cathode sputtering” will disintegrate surface layers of the negatively charged object by bombarding it with energetic ions in an electric discharge. The discharge will be concentrated in small spots as arcs eat away a surface, giving rise to steep-walled craters and broad flat-floored valleys surrounded by sharply-defined mesas or terraces. That is the familiar look of electrical etching. A beautiful example is seen on the surface of Comet Tempel 1 above, but other examples are abundant on planets and moons. The electrically etched surface of Jupiter’s moon Io is the most striking example because the process is still underway in the electrified Jovian environment.
Significantly, a paper in the journal Science, in October 2005, noted that “shocks” caused by ion sputtering of a cathode or negatively charged surface sharpen steep surface features—a dramatic contrast to the way the evaporation of ices will attenuate surface relief. A steep “cliff” remains even as it is eaten away by an arc progressively expanding the dimensions of the valley floor.
As Thornhill has long contended, cathode arcs tend to impinge on sharp edges because of the higher electric field there—a point that reinforces the Science article. In contrast to prevailing ideas about Io’s “volcanoes”, Thornhill predicted that the electric-discharge plumes would move around the edges of the valley floors. And that is what the Galileo probe discovered—another surprise for astronomers and planetary scientists who had not expected to find “volcanoes” to be moving across the surface of Io.
Viewing the comet in similar electrical terms will allow us to answer one of the least noticed but most profound mysteries posed by Comet Tempel 1. In the best pictures of the nucleus taken from the spacecraft, numerous patches of whiteout appear, most frequently on the edges of mesa cliffs, crater walls, and other surface relief. It is clearly not a random glitch in photography. Somehow the camera, photographing a body as dark as copier toner, was selectively saturated by bright spots on the surface.
Have we ever seen such a thing before? Curiously, exactly the same thing occurred when the Galileo probe viewed the electric plumes moving across the surface of Io. The designers of the camera had not anticipated anything so bright. But that is the nature of the electric arc—it’s why arc welders wear those darkened masks!
What will it take, then, to convince NASA scientists to ask the question: Do the patches of whiteout in close-ups of Tempel 1 reveal electrical discharge activity—on a scale that would immediately invalidate the foundational assumptions of today’s cometary science?
8. Fine cometary dust. Cathode sputtering has an effect that is simply “beyond the reach” of evaporating volatiles. It can create an exceedingly fine dust down to 1 micrometer or even finer. (One micrometer is just 40 millionths of an inch). This unique capability of cathode sputtering is why the process is used in the manufacture of highly reflective mirrors for modern telescopes. So again, a comparison of practical electrical technology with the discoveries of Deep Impact is only reasonable.
This line of investigation introduces another surprise: Astronomers could not understand what occurred when the 800-pound projectile hit the comet nucleus. An enormous volume of an extraordinarily fine dust was thrown into space at high speed, creating an extremely bright cloud due to the dust’s remarkable reflectivity. NASA scientists estimated that the dust particles were only .5 to 1 micrometer in diameter.
But was the surprise justified? Almost twenty years earlier the visit to Halley had investigators wondering how “sublimating ices” could produce such fine comet dust. But that surprise, like so many others, seems to have been quickly forgotten.
Also from the report in Science, in its recent report on the Deep Impact explosion: "The brightness increase lasted at least an order of magnitude longer than the expected crater formation time of 3–6 minutes." And the "…kinetic energy of the impactor is insufficient to provide the energy required to sublimate the observed amounts of water."
Remembering that the water was estimated from OH molecules seen after the impact, we can see that another key prediction by Thornhill, made in October 2001 concerning the expected outcomes of the impact with Tempel 1, was satisfied: "the energetic effects of the encounter should exceed that of a simple physical impact, in the same way that was seen with comet Shoemaker-Levy 9 at Jupiter". In the electric view, the unexpected energies of the Deep Impact explosion, and the release of unexpectedly fine dust, are both the predictable consequence of plasma discharge.
PLEASE NOTE: In the presentation above we have combined a series of TPODs in order to present, on one “page”, a unified answer to the question of water-ice on comets. This Picture of the Day will remain posted here through February 26.
COMING FEBRUARY 27, 2006: “Comets and the Future of Science”
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