Comet 67P

Barry Setterfield, 17th January 2015.



Comet 67P/Churyumov–Gerasimenko, (usually just called 67P), has been in the news recently. Originally it came from the outer debris belt in our solar system called the Kuiper Belt, but its orbit was changed to its present one by a close encounter with Jupiter in 1959. Jupiter's strong gravitational field was responsible for altering the comet's previous path. The comet itself is 2.5 miles wide by 2.7 miles long at the maximum dimensions. It actually has a dumbbell shape with a short ”neck” as shown in the images below. The comet currently rotates once every 12.4 hours.

On 2 March, 2004, the European Space Agency launched the Rosetta spacecraft with the incredible goal of orbiting and then landing on that comet. Considering that comet 67P is travelling at about 84,000 miles per hour, that was quite a goal. And quite an accomplishment, because they did it. The Rosetta spacecraft started orbiting the comet on 10th September 2014. The lander probe Philae, finally landed on the surface of the comet, after bouncing twice, on 12th November 2014. This had never been attempted before, and it was successful.

However, astronomers did not find what they expected. Three significant problems presented themselves: the density of the comet itself, its composition, and the jets, or streamers, emanating from it (which we also see on other comets).

The Standard Model states that, since comets originate so far out in the solar system where water would be frozen, then comets must be simply dirty snowballs or dusty icebergs. Then, as they approach the sun, the heat warms them up. The solid ice becomes water vapor and mixes with the dust from the comet itself, creating an atmosphere, or coma, and a tail. (The transition from solid ice directly to vapor is known as sublimation. We can see the same thing happen with "dry ice" or carbon dioxide ice here on earth -- the solid material goes directly into vapor without going through a liquid stage.)

However, there is a problem with that model because a number of comets have dust and gas clouds, with tails forming, at great distances from the sun where the ice could not possibly have melted or become gaseous. For example, in 1991, Comet Halley flared up with a cloud of gas and dust that extended for 150,000 miles; but it was out between Saturn and Uranus, over 14 times the distance of the earth from the sun. Similar effects have been noted with comets Hale-Bopp and Hyakutake.

The density of liquid water is "1." Density is figured by dividing the mass of an object over its volume. When water becomes ice, its density does not vary by much. The density of ice is 0.916. At about -180C that its density increases slightly to 0.934. So it is safe to say that the density of water remains basically the same regardless of the temperature until and unless it become vapor. It is for this reason that the density of the comet was expected to be close to 1.

That is not what was found. As the Rosetta spacecraft and its lander, Philae, circled Comet 67P before Philae landed on it, the density was measured. First, the size of the comet was measured, giving the volume. The orbing Rosetta was then also able to measure the gravitational attraction of the comet. This gave us the mass. When this mass was divided by the volume, the resulting figure was nowhere near the density of water. It was less than half of what was expected, and the resulting number was 0.4. More precise measurements in January, 2015, showed that the density must be somewhere between 0.425 and 0.475.

possible landing sites

above are mapped the possible landing sites for Philae

In response to this problem, the common suggestion is that the comet must be composed of extremely porous ice. Porous ice would be relatively soft, that is true, but if the comet were composed of porous ice, it would have to be 80% porous (open spaces). The lander had harpoons which were designed to anchor it to the comet, yet they only penetrated a few millimeters into the actual surface material after going through the dust. It had already bounced off the comet twice before coming to rest on it.

So the suggestion was that the ice in the comet must be 'sintered ice.' Sintered ice means compacted molecules, which can happen under heat or pressure or both. These forms of sintering do increase density significantly, as shown in studies on earth with polar ice.The other method of sintering is in extreme cold when the molecules are simply glued together where they touch without forming any crystal lattice or other standard formation. If the sintering occurred during the formation of the comet, due to cold, then the porous structure would still be there. Thus the density could vary widely.

two bounces

The problem was with the lander bouncing on an extraordinarily hard surface. The harpoons which were designed to anchor the lander in the ice penetrated only a few millimeters into the actual surface material after going through dust.


the final landing in the shadow of a cliff

Is this surface, below the shallow dust layer, then sintered ice? If the sintered ice on the surface was from early formation of the comet, then it would be porous and the anchors would have penetrated. Photographs of the comet, however, show a hard, blocky surface which is certainly not porous.


So if the ice is sintered, it must be the result of heat from the sun when the comet passes it. Since its interaction with Jupiter in 1959, it has had a six year orbital rate. That makes eight or nine times it has passed close to the sun since then, and we have no way of knowing how many times since the beginning it passed close to the sun before.

As mentioned above, the standard model says the comet's coma and tail are the result of vaporized ice mixed with dust from the comet. What mechanism, then, would vaporize the ice at the same time it was sintering it? Passing close to the sun is said to vaporize the ice. There is no pressure to counteract that and induce sintering. This makes the idea of the comet's surface being comprised of sintered ice very doubtful.

Water ice has a hardness that varies according to temperature; the colder it is the harder it is. If the relative Mohs scale is used, water ice at zero degrees C has a hardness of 1.5. However, at minus 70 C, the hardness of water ice increases to 6 on the Mohs scale. This is the same hardness as the mineral orthoclase that is found in granite. In comparison, quartz has a hardness of 7 on the Mohs scale and diamond is 10. Very little work has been done on the hardness of ice below minus 70 C, as this is usually studied in the context of glaciers on earth, and glaciers have average minimum temperatures around minus 30 C. Since the temperature on the comet was around minus 153 C to minus 163 C, then any water ice would be expected to be extremely hard. So the hardness data are not inconsistent with water ice at minus 160 C.

So what we are left with is that even a dirty snowball does not have a density of 0.4, yet the comet has an appearance like solid rock. The appearance of the comet close-up can be seen in the above images taken from Rosetta and Philae..

Some have suggested that the density measurements may be in error because of a variety of effects. But the data seem to be fairly robust. Unless some measured component can be shown to be faulty, the basic density still stands. So the current situation is unsatisfactory on all counts. The model of a dirty iceberg is certainly called into question, but currently there is nothing suggested in the scientific literature or the ongoing discussions that can explain its low density.

Allow a possible alternative suggestion. If the planets and sun were formed from a plasma filament or filaments, then Marklund convection was operating in those filaments to sort the elements according to ionization potential. This means that the most readily ionized elements were concentrated close to the center and the less easily ionized concentrated towards the outer parts of the filaments. Since comets come from the outer solar system, we are dealing with the outer part of the filament and the elements that will collect there have a higher ionization potential than those nearer the center. The primary elements will be Hydrogen, H, with an ionization potential of 13.59 eV; Oxygen, O, with 13.61; and Nitrogen, N, with 14.53. Carbon, C, and Sulfur, S, will also be present. Although Carbon has an ionization potential of 11.26 eV, there are few prominent elements between it and Hydrogen. In addition its second ionization potential would tend to hold it in the same collection region. In addition, the second ionization potential of sulfur, would tend to hold it in the same region. So sulfur would be expected as a minority component.

The list of atoms and molecules found in the atmosphere of the comet and its near-surface regions has been given in The Rosetta Blog as: Water (H2O), Carbon monoxide (CO), Carbon dioxide (CO2), Ammonia (NH3), Methane (CH4) and Methanol (CH3OH) with secondary concentrations of Formaldehyde (CH2O), Hydrogen sulphide (H2S), Hydrogen cyanide (HCN), Sulphur dioxide (SO2) and Carbon disulphide (CS2). In other words, the elements involved are H, O, N, C and S in various proportions. This is just what Marklund convection predicts, although it has been unexpected in the standard model. Furthermore, because of the distance from the sun, the temperatures involved would allow the formation of methane and ammonia ices.

Now here is the key point: Methane that was frozen at a temperature of minus 162 Celsius (or minus 260 F), has a density of 0.422 times that of water. Ammonia ice has a density that varies even more according to formation temperature. The lower the formation temperature is, the lower is the density. The density of ammonia ice doubles to 0.817 as the formation temperature climbs from about minus 150 C to minus 80 C. This suggests that at temperatures of minus 160 or 170 C the density of both methane and ammonia ices are in the region of 0.4 times that of water. If the comet or its parent body formed at temperatures around minus 200 C or so, the density would be even lower.  Since the plasma model of solar system formation involving Marklund Convection suggests that bodies in the outer regions should be primarily composed of carbon, hydrogen and nitrogen and oxygen, it would seem logical that any fragments from those regions which came into the inner solar system would have a similar composition. Therefore it should not come as a surprise that Comet 67P has a composition 0.4. .

In the same way, any comet composed primarily of methane (and ammonia) ice would have the same approximate density without any additional assumptions. If this is the case, it may also provide the key to another puzzle. The surface of this comet is as black as coal, just as the surfaces of Halley and some other comets are. The lander results suggest that there is a dust layer about 5 to 8 inches deep over the surface. Experiments in the lab, where an electric current or a glow discharge occurs in methane at very low pressures, disassociates the methane molecules into carbon and hydrogen atoms. The result is a film of carbon atoms or graphite, with some diamonds produced if the electron voltage is finely tuned in the plasma. Some results have been published in an IOP Science article and a Journal of Materials Research article.

Let us apply these results to comets. The solar wind is effectively an electric current composed of rapidly moving protons. In addition, the possibility exists that some of the brightness of comets comes from a glow discharge in their coma or atmosphere which is in a plasma state. This conclusion is necessary, since ultra-violet light and X-rays are emitted by the comas of a number of comets, and this requires large voltage differences and currents. These X-ray emissions flicker like a fluorescent tube over a timescale of hours. Comets known to emit X-rays include Hyakutake, Lulin and Schwassmann-Wachmann 3. These facts indicate that strong electric currents must be flowing in the cometary environment, so parallel conditions exist there to those in the lab.

The action of ultraviolet light (or electric currents) on methane precipitating its carbon atoms is a similar situation to that found with the rings of Uranus and Neptune which are made of methane ice coated with dark material (presumably carbon). Also, Neptune’s moon Triton has terrain covered by nitrogen frost along with dark wind-blown deposits probably from the carbon remaining from the dissociation of methane. There may be a similar situation with the dark coating on the crater floors of Saturn’s moon Hyperion which was captured from the Kuiper Belt where some comets have come from. So this is not an unusual situation for objects from the outer solar system.

In the low pressures encountered on comet nuclei, these electric currents impinging on their surfaces might be expected to vaporize the methane molecules and dissociate them into individual atoms. This process would then precipitate the carbon dust which covers the surface. Because of the electrical nature of this process, the precipitated carbon may still hold a residual charge. As a result, the carbon dust might be expected to be electrostatically attracted to the surface, and so the whole comet would become covered in a layer of carbon. As a result, bare ice of any sort may be hard to find.

So, in the model being considered here, the electric currents associated with the comet will impinge on the surface and vaporize any ices and also dissociate the molecules, depoiting solid carbon granules. The jets (shown in the image below) are then presumed to be the sites where these currents are acting and carrying material into the coma. Whether or not these electric currents penetrate deep below the surface or not is a separate question. However, this machining of the surface may be why the features of many comets seem “softer” than similar features seen on satellites captured from the Kuiper Belt, like Saturn’s Phoebe.


Two additional pieces of evidence from Comet 67P also require explanation. First, large nodules (that some have called “dinosaur eggs”) have been discerned in the strata in some images sent back by Rosetta and Philae. These observations have some interesting implications for cosmological models. In the early universe, it can be argued that currents and voltages were higher and have decayed over time. Using the equations in Anthony Peratt’s papers published by the IEEE, and his book Physics of the Plasma Universe, it can be shown that large “chondrules” of material can form in the Marklund Convection layers in filaments before they finally agglomerate into a planet. These chondrules would be electrostatically supported in the plasma filaments against the action of gravity until they were quite large. Their actual size would depend on the original strength of the electric and magnetic fields involved. This is not in disagreement with what has been discovered on Comet 67P. In fact, the size of the chondrules may be considered to provide information about the strengths of the initial currents and voltages compared with today.

Second is the apparent preponderance of water in the coma,which is what has supported the theory that the comet itself is a "dirty ice ball." But, as previously mentioned, if water or water ice were the main component of the comet, its density would be far more than 0.4. Frozen methane, however, gives us the density of 0.4, making methane quite likely to be the primary component rather than water ice.  

The problem appears to be the oxygen molecules, atoms or ions in the coma. We expect the hydrogen, but the appearance of oxygen again gives credence to the idea of a water ice in the comet itself. They appear in the coma in such quantities and combine with the large amounts of hydrogen ions in the coma so that they give the impression that the comet is primarily water. However, it is important to remember that there is going to be a large number of oxygen atoms or molecules from the Marklund Convection process. That is beyond dispute. It is very possible that these atoms did not react to form water or water ice.That is because the temperatures involved in the original filament were about -200 C. The oxygen would have been in the solid state and not gas. Oxygen molecules, atoms, or ions would have crystallized out of the original "mix" as a solid entity and therefore would not have had any extensive reaction with the other elements. Even on earth, solid oxygen takes the beta form at room temperature when the pressure is at a suitable level.

So the proposition is that solid oxygen is locked up throughout the structure of the comet and is released by the action of electric currents, the solar wind, or heating. These oxygen atoms, molecules or ions then appear in the coma in large quantities. This gives the impression that there is a lot of water being given off simply because of the large number of hydrogen atoms also in the coma that came from the dissociation of methane by the electric currents. If these facts can be verified on 67P, then we may be on the way to a viable model for comets after all.