Comet Tails of the Expected

Specific predictions were made almost four years ago on this website about the possible effects to be observed in the Deep Impact experiment. Key predictions were that there would be a flash just before impact and that the outburst accompanying the event would be more energetic than expected from a mechanical collision. These predictions were quite contrary to the concern expressed by some NASA astronomers that there would be little or nothing to see.

Unusual predictions that succeed are the hallmark of a good theory. But, to this day, having a good theory considered fairly remains a huge problem if it calls into question prevailing dogma.

Comparison of several comets

Credit: New Scientist

On the 9th of September 2005, the news service published “Comet tails of the unexpected” by Stuart Clark. The author prefaces the printed article with, “There’s nothing as confusing as a comet.” In other words, following a number of close encounters, no one yet has been able to figure out these so-called “Rosetta Stones” left over from the formation of the solar system.

ON 4 JULY, the world’s TV screens were filled with high-fiving NASA astronomers celebrating the Deep Impact mission’s direct hit on comet Tempel 1. It was an extraordinary achievement, and fully merited the celebrations. A few weeks later, though, when the cameras had gone, the astronomers were left scratching their heads in confusion.

The Deep Impact team had hoped that, when the impactor spacecraft hit Tempel 1, it would kick up a relatively small cloud of dust, expose an area of pristine icy material underneath, and instigate some spectacular jet activity. This is exactly what didn’t happen. The dust cloud was more than 10 times bigger than expected, and the effect on Tempel 1’s activity was almost nil.

We have now had four close encounters with comets, and every one of them has thrown astronomers onto their back foot. This week, at the American Astronomical Society’s Division for Planetary Sciences meeting in Cambridge, UK, the Deep Impact team will report that comets are defying all attempts to understand them. “We really need to think differently,” says Peter Schultz of Brown University in Providence, Rhode Island, a member of the Deep Impact team. “They are like no other bodies in the solar system.”

Comment: The “need to think differently” has been expressed with monotonous regularity in the space sciences. But the actions are always “business as usual.” This seems to be explained by the prime motivation of any organization – its own perpetuation. To really think “outside the box” is daunting because it threatens to change the individual. And that could lead to the breakdown of the organization.

Comets have a special place in the hearts of astronomers. These balls of ice, rock and dust originated in the frozen wastes of the outer solar system, but were nudged by the gravitational fields of the giant planets – and even passing stars – into the inner solar system. Comets are thought to be related to the icy building blocks that formed the giant planets Jupiter, Saturn, Uranus and Neptune. Many of the moons of these worlds, not to mention the planet Pluto itself, can be thought of as super-sized comets because they, too, are composed mainly of ice and rock.

But unlike planets, comets are far from stable. Each time one passes close to the sun, the heat makes material such as water and carbon dioxide evaporate away into space, creating a tail of dust and gas that stretches behind it for millions of kilometres. Their surfaces also display intermittent “activity”, shooting out jets of dust and gases. “The best way to think of them is that they are in a constant state of disintegration,” says Schultz.

Comment: Thinking differently requires that we stop repeating unsubstantiated dogma about the origin and decay of comets. Astronomy indulges in too many invisible or undetectable objects in space in order to satisfy theory. A noted astronomer, R. A. Lyttleton, described the theory of the origin of comets as “a piece of trash.” And the sharp surface relief and unexplained jets have discredited the notion that comets gently sublime away in the heat of the Sun. It is an assumption to state that comets “disintegrate.” It implies a passiveness that is belied by their activity. If we are to think differently, shouldn’t we consider external machining of a comet’s surface?

But the details of that disintegration are proving ever more perplexing. Prior to the European Space Agency’s Giotto mission to study Halley’s comet in 1985, for example, astronomers believed that as sunlight fell onto a comet, its spin would mean that the heat evaporates a more or less even layer, revealing more icy material beneath. Giotto showed that this idea was hopelessly simplistic. “As soon as we saw the nucleus it was clear that activity was confined to individual jets and not coming from the whole surface,” says Giotto project scientist Gerhard Schwehm of the European Space Agency. In fact, only 15 per cent of Halley’s total surface area was expelling material at the time of the fly-by. The observation has shown astronomers that they are in the dark about even the basics. “We still do not know what drives comet activity,” says Schwehm.

Donald Brownlee of the University of Washington in Seattle goes further. “It’s a mystery to me how comets work at all,” he says. Brownlee has good reason to make this claim. He is the principal investigator on NASA’s Stardust mission, which flew past comet Wild 2 on 2 January 2004. The fly-by images showed 20 active jets spread across the comet’s sunlit side. So far, so good. Then they saw something that added a new twist to the mystery. Two of the jets were on the night side of the comet.

Astronomers had expected that the jets would simply turn off when the comet turned them away from the warming rays of the sun. For Brownlee it seems to be pointing to an inescapable conclusion. “I think that some process is allowing heat to get down below the surface of a comet and drive the activity from the inside out,” he says.

The clue might be in the dark surface layers of the comets. Though it is hardly what you would expect of icy bodies, the exteriors of both Halley and Wild 2 are as black as coal, and these dark layers absorb heat. At the time of the Stardust encounter, when the comet was almost twice as far away from the sun as the Earth, the surface of Wild 2 was a comfortable 18°C. Its interior would have been much colder, well below 0°C in fact, so heat would naturally flow inwards. That’s as far as the explanation goes at present. “I have no idea about the details of the process,” Brownlee admits.

Comment: The problems associated with the passive heating model of comet behaviour are highlighted in these comments. The burnt-black appearance of comet nuclei is the first problem for the dirty snowball model. The sharp surface relief is another. A comet losing icy material in the Sun’s heat should look like a melted ice-cream. And the loss of material in the form of jets makes no sense whatsoever in this model.

If the NASA scientists really wanted to think “differently,” the presence of jets on the dark side of the comet nucleus should have highlighted the possibility that there is an energetic process going on that is independent of solar heating. And there is a well-known process in industry that naturally gives rise to jets in the process of eroding a surface. It is known as electric discharge machining, or EDM. Myriad tiny cathode jets etch the surface. I showed in an earlier news item how the surface of Comet Wild 2 was directly comparable in appearance to an EDM etched surface.

Comet Wild 2 compared to electrically machined surface

Comet Wild 2 is shown in closeup left. Right is a microscopic view of an EDM surface. The flat floored depressions with steep scalloped walls and terracing. The small white spots on the comet can then be reasonably identified as the active cathode arcs which produce the cometary jets.

Enter Deep Impact. The NASA scientists hoped their impactor would not only eject material for them to analyse but also kick-start a new area of research by exposing an area of pristine, icy material inside the comet. And maybe that would provide a few clues to what drives comet activity. Unfortunately, things didn’t quite go according to plan. The Deep Impact team thought their 370-kilogram impactor would liberate about a month’s worth of dust, based on normal emission rates, but it now seems more likely that a whole year’s worth escaped the comet. “If I had to choose just one surprising result from this encounter, it would be the amount of material thrown up,” says Schultz.

Comment: This result was predicted in October 2001, based on the electric discharge model of comet activity. It requires that a discharging comet be strongly charged with respect to the solar plasma. The sudden encounter with the Deep Impact projectile, which is at the same potential as the solar plasma, would suddenly release considerable electrical energy.

Deep mystery

The ease with which the dust lifted into space suggests that the comet has a remarkably fragile surface, says Michael A’Hearn of the University of Maryland at College Park, Deep Impact’s principal investigator. “The surface material can have no more strength than lightly packed snow, otherwise we would not have seen that amount of dust.”

Comment: The dust was not “lifted” into space. It was jetted into space electrically. The effect is known as “cathode sputtering.” It accounts for the surprising fineness of the dust particles. (Ironically, it is a process used to coat astronomical telescope mirrors with a thin metallic reflective surface). If comets were formed by accretion we should expect a wide range of particle sizes.

Cathode sputtering can strip material, atom by atom, from a solid surface. It does not require that comet Tempel 1 be lightly packed dust or ice.

And there was another surprise in store for the team. As the impactor hurtled towards Tempel 1’s nucleus at over 10 kilometres per second, it returned pictures of two craters, each a kilometre across. Though they seem to be ubiquitous on every other solid surface in the solar system, craters have never before been seen on a comet. When Giotto flew by Halley’s comet in 1986 and returned the first ever pictures of a comet’s icy nucleus, no craters were revealed. Twenty-five years later, NASA’s Deep Space One flew past comet Borrelly and revealed another surface devoid of craters. Wild 2 did have large numbers of circular depressions on its surface, but their unusual shape suggested to astronomers that these were not created in collisions. “We had given up the hope of seeing craters on comets,” says A’Hearn.

So where did the holes in Tempel 1 come from? Well, as with Wild 2, they might not be impact craters at all. The depressions have flat floors and their walls appear like giant staircases, and this suggests that they were caused by an explosion within the comet, rather than a hit from outside, according to Laurence Soderblom of the US Geological Survey in Flagstaff, Arizona.

Comment: As explained above, the irregular craters with stepped or terraced walls are a natural feature of an EDM surface. But a strong arc will always create a neat circular crater. Here we strike a dogma that has resisted any “thinking differently.” Astronomers attribute circular craters to impacts without any observational evidence to back up the theory. No impact has ever been witnessed. And attempts to reproduce the detailed appearance of craters by impacts have not met with success.

The theory of impact cratering has persisted simply because no one was able to “think differently” enough. Yet Brian Ford first put forward experimental electrical cratering evidence matching the features on the Moon in the Journal of the British Interplanetary Society, Spaceflight, Vol VII, No. 1, January 1965. The problem that Ford and others face in proposing such ideas is that astronomers have been indoctrinated in the unshakeable belief that while “there might be electricity in space, it doesn’t do anything.” This belief, like the earlier one about a flat Earth, is destined to become the standard joke about astronomy and cosmology of the 20th century.

Brownlee believes the porous structure of the comet might allow light to penetrate beneath the surface and heat the interior. The dark layers stop heat escaping, and pressure builds up, eventually resulting in an explosion – and an unusually shaped crater. It’s a pretty vague explanation, but the Deep Impact astronomers are looking for some evidence to back it up. A’Hearn reckons the numerous jets that they saw as the spacecraft approached Tempel 1 might hold some clues, though it is proving difficult to trace them because no one knows what the features that release jets look like, or how big they are. They could be nothing more than fissures, too small to be picked out by Deep Impact’s cameras.

Comment: “A pretty vague explanation” is putting it kindly. Astronomers seem unable to “think differently.” Their training drives them to the same old mechanical approach that has dogged theorists ever since they dismissed Kristian Birkeland. Birkeland was an outstanding pioneer of the early 20th century who demonstrated by observation and experiment the electrical nature of the Sun and the solar system.

So, for the moment, the team is short on clues as to what makes a comet tick. Their detective work has been made even more difficult by the fact that Tempel 1 seems unperturbed by the impact. A week of follow-up observations using the European Southern Observatory’s Very Large Telescope in Chile revealed that after the initial outburst the comet’s activity levels remained very much as they were before the encounter. The new jet they had hoped to trigger simply did not materialise. A’Hearn believes the amount of dust ejected and the lack of follow-on activity indicate the crater might be wide but not deep, and that the impact merely blasted off the desiccated surface layers without making any serious impression on the icy material buried beneath.

Comment: The electrical discharge triggered by the Deep Impact projectile would be transient and insufficient to alter the charge on the comet to any significant degree. It would not be expected to alter the comet’s activity levels. Also, if the comet is solid rock, the impact would not have caused more than a superficial physical disturbance.

Unfortunately, the amount of dust released, combined with a focusing fault on Deep Impact’s high-resolution camera means that the images the team hoped to take of the newly formed crater may now elude them. There may be no way to confirm what happened.

If this is the case, the team will fail in the first two of its stated mission objectives: to observe how the crater forms, and to measure its depth and diameter. They partially succeeded in the third, which is to analyse the composition of the interior of the crater and its ejecta: they’ve analysed the ejecta, but can’t see the crater. Objective four, which is to determine the changes in the quantity of material ejected by the impact, has been met, even if the answer seems to be a big fat zero.

Comment: The irony is that EDM naturally achieves what Deep Impact was meant to do. The electrical discharges are cratering the comet. It is another example of a poorly designed and expensive experiment, based on false ideas about the origin and nature of comets. Furthermore, if the crater could have been seen, the interpretation would have been invalid because it was not formed solely by impact but was modified by powerful electrical activity.

It’s disappointing, but it’s not all bad news. The big cloud the impact kicked up promises a potential science first: a hint of the comet’s internal structure. “By watching the movement of the ejecta cloud with the fly-by spacecraft, we think we can determine the distribution of mass inside the comet,” says A’Hearn. Such information will show whether it is a solid object or a conglomeration of pieces, and reveal whether the rock and ice are uniformly mixed throughout the comet, or separated into distinct regions. The Deep Impact researchers are continuing to sift through the images and spectroscopic data transmitted from the spacecraft to piece together all the information they can about Tempel 1.

Comment: The comet’s internal structure will not be found by applying purely mechanical and gravitational considerations. A comet is fundamentally an electrical phenomenon. This was commonly accepted, if not understood, before the end of the nineteenth century. It was not until Sidney Chapman dogmatically rejected Birkeland’s work and the notion of electrical transactions between the Sun and the Earth that this obvious idea about comets was killed off.

The determination of the density of celestial bodies by gravitational perturbation rests on a number of unexplored assumptions. In the past it has suggested that many rocky-looking asteroids and comets are insubstantial objects. In my view the visual evidence should take precedence.

We may learn a little more about comets next January, when the Stardust mission brings dust from Wild 2 to Earth, but many astronomers are now pinning their hopes on the European Space Agency’s Rosetta mission to comet Churyumov-Gerasimenko. “Rosetta will be the key to understanding comet activity because it will not be just another snapshot of a comet, it will watch it continuously,” says Brownlee. Upon arrival in 2014, Rosetta will enter orbit around the 2-kilometre-wide nucleus and monitor the comet for two years, during which time it will make its closest approach to the sun and begin to head back out again. Once Rosetta has mapped the comet, a small lander called Philae will descend to the surface. Equipped with harpoons to anchor itself to the comet’s surface, Philae will examine the composition and structure of the surface in fine detail.

Comment: This ambitious mission has little chance of success because the electrical nature of comets has not been considered. There is a high probability of crippling plasma discharges to the spacecraft and the lander.

With so much left unknown about the nature of comets, that nine-year wait for Rosetta is going to feel like an eternity to the astronomers meeting in Cambridge this week. And it’s possible, of course, that Churyumov-Gerasimenko will throw up another set of surprises. When it comes to comets, there’s only one clear message: expect the unexpected.

Comment: Expecting the unexpected is a tacit admission that comet theory is “a piece of trash.”

Comet conundrums

1: Why do they disintegrate?

If heat from the sun can become trapped inside a comet, driving later activity, it may also explain one of the most puzzling cometary observations: why some of them simply fall to pieces when they are nowhere near the sun.

About 50 comets are known to have split up in this way. The latest was comet 2005k2 LINEAR, which split into two in June over 100 million kilometres from the sun. Others have broken up much further away. Astronomers think that trapped heat melts the comet from the inside out, increasing the pressure under the frozen surface until finally the comet explodes.

Tempel 1 could be next, if one tentative observation is confirmed. “We see a feature running across the nucleus that almost looks like a fault line. But how can that exist? Perhaps Tempel 1 was almost shattered sometime in its past and large blocks are just resting together,” says Michael A’Hearn of the University of Maryland at College Park. “That’s off the top of my head speculation,” he adds.

If Tempel 1 is really a jumble of blocks of ice and rock resting lightly on top of one another, it would not take much to force them apart. But the major puzzle is still how heat can be channelled and trapped inside a comet in the first place.

Comment: The “fault line” on Tempel 1 is probably a channel formed by a plasma discharge travelling across the surface. Such “rilles” are commonly found alongside electrical cratering. It is the kind of surface scarring that occurs in electrical transactions between two bodies in close proximity. It is the situation that is proposed to occur during the electrical “birth” of a comet from a larger body. So the tentative identification of a linear feature on the comet as a fault line is most unlikely.

A comet spends most of its time in deep interplanetary space where it comes into balance with the plasma voltage there. But when it hurtles toward the Sun, the rapidly increasing voltage difference between the comet nucleus and the solar plasma gives rise to the plasma discharge phenomenon that we call a comet. Unexpected cometary outbursts far from the Sun have been observed and correlated with solar activity. It is such sudden changes in the comet’s electrical environment that cause it to behave like a leaky capacitor, where sudden induced currents within the dielectric material of the comet may cause an explosion, rending the comet into fragments.

2: What are they made of?

IF our understanding of asteroids is anything to go by, the solid material in comets could be carbonaceous, silicaceous or metallic. But, as yet, we simply don’t know enough about comets to generalise about what they are made of. And that’s a shame because it might tell us more about their history. Donald Brownlee of the University of Washington in Seattle imagines a scenario in which large objects, perhaps as big or bigger than Pluto, formed deep in the outer solar system during the general planet-forming process. Such bodies would generate enough internal heat, by natural radioactivity, for the denser material to sink to the centre, leaving the lighter material to rise to the top. Collisions between these objects could shatter them, creating a shower of comets, all with different compositions depending on where they originated.

If Brownlee is correct it means that astronomers might need to rethink their ideas about comets. Instead of thinking of them as the raw material for new planets, perhaps comets are better described as the debris from failed ones.

Comment: The story of the formation of the solar system also requires us to “think differently” if we have left out powerful electromagnetic influences. Plasma cosmologists have shown that stars do not form by gravitational accretion. Stars form in a cosmic discharge, inside a plasma z-pinch. The dusty disks seen about some stars may not be due to gravitational accretion but are more likely to be matter expelled electrically by the central star. Electrical expulsion can also explain the formation of the observed close orbiting gas giants. In a hierarchical fashion, comets can be seen as the debris, or afterbirth, of a planet. They are not primordial.

3: Where are the impact craters?

SEISMIC tremors caused by small impacts could disturb the surface material on a comet enough to “fluff it up”, burying or even destroying any craters or other features and creating the smooth plains, suggests Laurence Soderblom of the US Geological Survey, who was a member of the Deep Space One team. “The gravity is so low on a comet that it wouldn’t take much to move the surface material around,” he says.

But if that’s the case, why do two craters survive on Tempel 1? “That’s part of the mystery that we have to solve. Perhaps they are not old but young craters,” says Soderblom.

Comment: As explained earlier, the dust from Tempel 1 is best explained as electrically sputtered rock particles. The material in the comet jets is not unaltered surface material. Past observations of the presence of negative ions of oxygen and forbidden spectral lines are both evidence for electrical activity at a comet’s surface.

The two craters strongly suggest that Tempel 1 is rocky. They also argue against crater formation by impact because in the region beyond Pluto’s orbit, where it is supposed comets are “stored,” relative velocities are very low. Yet neatly circular craters are supposed to be caused by an explosion following hypervelocity impacts.

There are many diagnostic features of electric arc cratering that cannot be matched by impacts. Electric arcs always impinge vertically on high points of a surface. That ensures circularity of the resulting crater. It explains the puzzling fact that small craters are often found neatly centered on the raised rim of a larger crater. That is also why craters are not found in the steep walls of large craters when there should be many found if craters are caused by impacts from all directions.

Comets today are not subject to the intense arcing that accompanied their birth. Instead, their surfaces are subject to a slow “spark machining” or cathode erosion. The odd white spots seen on Tempel 1 and Wild 2 are probably the focus of electrical discharges, feeding the cathode jets. Slow cathodic erosion tends to take place along the walls of existing craters, producing odd-shaped craters. That effect was seen on Io, where a line of bright cathode spots was observed strung along a “caldera” wall. The machining left a sharp “cookie-cutter” appearance to the cliff faces. When imaged closeup by the Galileo Orbiter, the arcs burnt out many pixels, which moved NASA to color them in as if they were lava fountains. Such is the power of our beliefs to color what we see.

The electric discharge nature of the plumes on Io were identified by plasma physicists, Peratt and Dessler, in 1987 following an earlier suggestion by Professor Thomas Gold in 1979. The plumes formed by cometary jets are of the same electrical nature. So it was amusing to see a description of an encounter with one of the jets from Comet Wild 2 as like being “struck by a thunderbolt.”

Wal Thornhill

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