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THE TALE OF TWO METEORITES

 

 

On the evening of January 17, 2009 a brilliant fireball crossed the Baltic Sea, travelling toward the west.  The event was well observed visually, photographically, by radar and even at a seismic station, resulting in a remarkably accurate determination of the meteor’s flight path and orbit.  The nature of the fireball strongly suggested that a meteorite may have reached the ground and, although some feared that it might have plunged into the Baltic Sea, after a careful search in the area adjacent to the sea, a small fragment weighing 25.8 grams was found partially buried in soil near the Danish town of Maribo on March 4. The fragment was fractured and fell into several pieces after removal from the ground.

Analysis of the meteorite revealed it to be a member of the class of carbonaceous chondrites. This is a fascinating group for several reasons. They contain the most primitive Solar-System material known to reach Earth in macroscopic quantities and they are known to contain organic matter of remarkable complexity, including substances of biological significance. More controversially, certain organic features found within them have even been proposed as the fossilized remains of very primitive forms of living organism!

Maribo proved to be the second most “primitive” type of carbonaceous chondrite; a so-called Type-2. More specifically, it was classified as a CM2, placing it within the same subclass as such famous meteorites as Murray of 1950 and Murchison of 1969.

Intriguingly however, not all of the material found within this body appeared equally “primitive” in the sense that some of it revealed evidence of having been subjected to alteration through heating in its parent body. Oddly, material that had been “processed” to some degree was found together with material that had remained essentially unaltered, leading researchers to conclude that some of the material had been “recycled”, so to speak, from bodies that existed prior to the accretion of the true Maribo parent. This is not really surprising given the chaotic nature of the infant Solar System!

The Maribo meteorite entered our planet’s atmosphere with a velocity of around 28.5 kilometres per second; remarkably fast for a meteorite. Indeed, this was the highest velocity measured at that time for any meteoroid that survived to the ground as a meteorite. It also meant that the orbit was quite eccentric. Indeed, the calculated orbit gives an eccentricity of 0.795 with a perihelion close to the orbital distance of mercury at 0.481 Astronomical Units or about 72 million kilometres from the Sun.

Although most meteorites are fragments splintered from the myriad asteroids that populate the “Main Belt” (sometimes called the “Rocky Belt”) between the orbits of Mars and Jupiter, those of the carbonaceous variety (or some of these at least) are also suspected by certain researchers as having a cometary origin. As comets normally leave strings of particles strewn around their orbits and as these produce meteor showers if they are so located as to encounter Earth’s atmosphere, it is always interesting to see if a new meteorite – especially one of the carbonaceous chondrite variety – is associated with one of the known annual meteor showers. Of course, most “shower meteors” are far too small, too fragile and arrive at too great a velocity to drops meteorites, but larger bodies are known to exist in some meteor streams and the possibility of a meteorite from a meteor shower is not to be dismissed. It would be very interesting indeed if a specific meteorite could be identified with a specific shower; even more so if that shower could be associated with a known comet ... or a known asteroid, as some asteroids in Earth-crossing orbits are also associated with meteor showers and some are, for that reason, suspected as having once been active comets.

As it turns out, Maribo arrived on the very night of the month that the Delta Cancrid meteor shower reaches its feeble peak!

Although the first solid evidence for this stream did not come until the 1970s, reports of meteors radiating from the region around the stars Delta and Theta Cancri date back to the late 1800s and a “Theta Cancrid” orbit was given by A. Terentjeva in 1967. Although a diligent observer under excellent skies may see no more than two or three stream members during an hour’s observing at the time of maximum, the stream is nevertheless now well established and, like most showers with their radiants near the ecliptic, has a northern and a southern branch. The southern branch seems to be more or less identical with Terentjeva’s Theta Cancrids.

Though the meteor numbers from this stream are small, occasionally it produces a brilliant fireball, such as the one which floodlit The Netherlands and parts of Germany on the night of January 17, 2002. This meteor was so bright as to shine through an overcast sky like the Moon, which it equalled or exceeded in brilliance.

The direction and velocity of Maribo are also in pretty good agreement with the members of this meteor stream. But because the orbit of Maribo has been calculated, a more convincing test of membership can be applied. In 1980, J. Drummond devised a criterion, known as the D’ criterion, for matching the orbits of Solar System objects. If two orbits are compared and the value of D’ comes out at 0.105 or less, the orbits are considered matching and (other things being equal) the probability that the objects pursuing these orbits are related is high. By comparing the orbit of Maribo with the three Delta Cancrid orbits published in Gary Kronk’s  Meteor Showers: A Descriptive Catalog (1988), we find D’ values of 0.109, 0.038 and 0.066. The first lies just outside the value for an orbital match (but given the broad and diffuse nature of this stream, the agreement is still not too bad!), but the other two reveal a clear association. Comparison with Terentjeva’s orbit yields a D’ value of about 0.08.

The next step is to see if there is some known object – a comet or Apollo asteroid – that moves in an orbit similar to this meteor stream and which might be considered the parent of the meteoroids, including Maribo. One rather strong candidate is known; the asteroid 1991 AQ. Running a D’-criterion comparison of its orbit with the three Delta Cancrid orbits published by Kronk yields values of 0.08, 0.12 and 0.06. Comparison of the asteroid’s orbit and that of Maribo gives a D’ value of just 0.04.  This is pretty impressive, and one might be tempted to say that 1991 AQ was once a comet that formed the meteor stream during its active career but has since retired to the quiet life of a dormant Apollo asteroid. On the other hand, 1991 AQ may be the sibling of the Delta Cancrid meteoroids rather than their parent. In other words, it might be simply a very large member of the stream; the true parent being another object that gave rise to this asteroid together with a host of smaller meteoroidal particles, one of which landed in Denmark in 2009. We will leave this issue hanging for the present, albeit with the hint that the second alternative should not be dismissed too readily.

An important finding concerning the Maribo meteorite concerns its cosmic-ray exposure age (CRE) or the time during which it has been exposed to the radiation of space, either as a free- floating object or on the surface of a small body. This seems to be somewhere between six hundred thousand and twelve hundred thousand years. This is rather short by the standards of meteorites in general, although not very unusual for carbonaceous chondrites as these typically have shorter CRE ages than most other varieties of stony meteorite. The latter normally have CRE ages in the order of ten million years or thereabouts.

Because it typically takes several millions of years for a fragment chipped off an asteroid in the Main Belt region to migrate into an Earth-crossing orbit, the relatively short CRE age of Maribo strongly implies that it was not released from an asteroid in that region. Presumably, its parent body was already in an Earth-crossing orbit not too different from that on which Maribo approached our planet. This supports the reality of its association with a regular meteor shower.

More will be said later about the Maribo meteorite and other objects apparently associated with it. First however, let us look at the second meteorite with which we are concerned – the Sutter’s Mill fall of April 22, 2012.

 

In broad daylight on that day, residents of a large region of California and Nevada were surprised by the appearance of a great fireball between 100 and nearly 700 times more brilliant than the full Moon. The appearance of the fireball was followed by an air blast and sonic boom trailing off into a prolonged rumbling like thunder. Subsequent to the fall, some 77 meteoritic stones were recovered, the largest weighing in at 205 grams. Altogether, a total mass of 943 grams was recovered, although this is believed to have been but a tiny fraction of the total mass of the body prior to its encounter with our atmosphere (estimated at somewhere between 20,000 and 80,000 kilograms). This was clearly a more major meteoritic event than the Maribo fall and was judged to have been the most energetic since the impact of asteroid 2008 TC3 some four years earlier.

There is a degree of physical difference also in so far as this meteorite is a breccia; putting it bluntly, a conglomerate of many broken pieces, some of which (like some of the material found in Maribo) originating in other parent bodies. The meteorite seems to have been part of the deep regolith of its parent body.

In most other respects however, the Sutter’s Mill meteorite (as this fall is now officially known) closely resembled Maribo. Both are carbonaceous chondrites of the CM2 classification. Both entered the atmosphere at record-breaking velocities for surviving meteorites – 28.6 kms/sec for Sutter’s Mill. Both meteorites arrived on orbits of low inclination; 0.72 degrees for Maribo and 2.38 for Sutter’s Mill. Moreover, the eccentricities of their orbits were high. Maribo’s orbit had a calculated eccentricity of 0.795 while Sutter’s Mill had a slightly larger 0.824. Both meteorites came to perihelion near the orbit of Mercury at 0.481 AU for Maribo and a slightly smaller 0.456 for Sutter’s Mill. The main difference in their orbits concerned their orientation in space as expressed by a measurement known as the longitude of perihelion. For Maribo, this value is approximately 217 degrees whereas the longitude of perihelion of Sutter’s Mill is 111 degrees. The difference in these two figures hints at something very important, as we shall see in due course.

Another striking difference between these two meteorites concerns their CRE ages. As we have already seen, the CRE age of Maribo is short, albeit not remarkably so for this type of meteorite. However, the CRE age of Sutter’s Mill is remarkable, even for a carbonaceous chondrite. The preliminary estimate of around 50,000 years was remarkable enough, but the revised age as given by Ulrich Ott and Qing-Zhu Yin at the 44th. Lunar and Planetary Science Conference in 2013 is staggeringly short. Their estimate of the nominal CRE age is just 19,000 years. Indeed, they remark that there is no unequivocal evidence of any cosmic-ray exposure at all, at least in so far as the presence of cosmic-ray induced Ne in the sample they studied indicates. Because of the very young CRE age of Sutter’s Mill, the remarks made earlier about Maribo and the improbability of that meteorite being a fragment of a Main Belt asteroid that dynamically evolved into an Earth-impacting object applies in an even more extreme way here.

But if this meteorite also came from a parent already in an Earth-crossing orbit, there would seem to be a good chance that it likewise was part of a meteor shower. A check through Kronk’s Catalog revealed that this does appear to be the case. Early May witnesses a daytime meteor shower, observable by means of radar, for which orbits quite similar to that of Sutter’s Mill have been calculated. True, the meteorite arrived somewhat early, but the spread of diffuse meteor showers is not sharply determined and a small number of shower members may arrive significantly outside the dates usually given for the shower’s duration. Three orbits for this shower, known as the May Arietids, are given in Kronk and a D’ comparison with the Sutter’s Mill orbit yields values of  0.09, 0.12 and 0.12. The first orbit shows a rather close orbital similarity. The other two fall outside the “matching orbits” criteria, but are still quite close given the diffuse nature of the stream and the position of Sutter’s Mill as an early arrival. The D’ comparison between the meteorite orbit and the average for the shower yields a value of 0.097.

What is especially interesting however is the apparent link between this meteor stream and the Taurid shower of November, in particular, the southern Taurid stream. A D’ comparison between the average May Arietid orbit and the one calculated by Kronk for the Southern Taurids yields a value of 0.094. During November, the meteoroids encounter Earth on their way to perihelion and in May they meet us on the way out, but the two streams are really part of the one diffuse system with, presumably, the same parent object. The parent of the Southern Taurids is Comet 2P/Encke thereby relating this comet directly to the May Arietids as well. The orbits of these meteors, including Sutter’s Mill, and Encke are not all that close today, but this is a very old meteoric system, as the ages of these formations go, and much diffusion of the individual particles has taken place since they were spawned by this comet. Indeed, the Taurid system appears to be a complex of several related meteor streams and even Apollo asteroids and is thought to date back to a major disruption of a much-enlarged Comet Encke about 20,000 years ago. Note, by the way, the effective coincidence of that date with the CRE age of Sutter’s Mill!

The northern branch of the Taurid stream appears to be directly associated with the Apollo asteroid 2004 TG10 but, because both the northern and southern branches of the Taurids are so closely related, this “asteroid” is, presumably, a fragment that broke away from Encke at some time in the past and spawned the meteor stream during its career as an active comet. Presumably its lack of contemporary activity means, either, that it has exhausted its supply of volatile material or an insulating layer has formed over its entire surface and is currently shielding the remaining internal volatiles from the Sun’s heat. If the latter is true, this object is more appropriately to be thought of as a dormant, rather than an extinct, comet and may someday become active if part of the surface insulating layer is disrupted.

In October 2015, two Apollo asteroids pursuing Taurid-type orbits were discovered and each passed relatively close to Earth during the second half of the month, just as the Taurid meteors were working toward their extended annual maximum. These asteroids, designated 2015 TX24 and 2015 TD144, are associated with the southern branch of the stream and the second of the pair displays a certain orbital similarity with the May Arietids. Comparing its orbit with the three May Arietid orbits published by Kronk yield D’ values of 0.06, 0.107 and 0.1565. The D’ comparison with the average of these orbits gives 0.104 and, with the orbit of Sutter’s Mill, 0.11. Another Taurid asteroid discovered back in 1996 and designated 1996 SK follows an orbit which, when compared with that of Sutter’s Mill, yields a somewhat smaller D’ = 0.077.  Comparison with the three May Arietid orbits gives D’ values of 0.089, 0.1313 and 0.1663 and with 2015 TD144, 0.068.

The most straightforward explanation of this complex of small Apollo asteroids and meteor streams moving in orbits more or less similar to that of Comet Encke is to assume that this comet was once a far larger object than it is today and over the course of thousands – indeed, several tens of thousands – of years it has progressively lost massive numbers of fragments ranging in size from sand grains to fragments several kilometres in diameter. These latter, we might presume, initially contained enough volatile materials to maintain cometary activity for an extended period, in the process generating meteor streams within the broader complex.

Studies by D. I. Steel, D. Asher, D. Clube, W. Napier and others during the closing decades of last century yielded strong evidence supporting this model. Indeed, it was found that (to quote Steel and Asher in their paper published in The Observatory, 1994 October) “When one selects from our inventory of Earth-crossing asteroids all those with similar [semi-major axis, eccentricity and inclination] values to those of P/Encke and its related meteor showers, one finds that, in terms of orbital orientations, two distinct groups appear.” What separates these two groups is their orientation in space, quantified by a value known as the longitude of perihelion. The first group – the main Taurid complex – consists of objects having orbits with their longitudes of perihelion between 100 and 190 degrees whereas the second consists of a cluster of objects having longitudes of perihelion from around 222 - 251 degrees. These boundaries are not sharply delineated and were defined only according to the orbits of the objects of both groups known at the time. In fact, given the similarities of the various orbits in other respects and the somewhat unusual nature of these Encke-type orbits amongst small Solar-System bodies, both authors opined that these groups were not really distinct associations but, rather, simply two concentrations at the opposite ends of a single continuous association of objects having orbits ranging from in longitude of perihelion from about 100 to 251 degrees or thereabouts. If that is correct, this entire complex presumably has a common origin in the disruption of the (very large) primitive Comet Encke. The second group was referred to as the “Hephaistos complex” after the largest asteroid with an orbit typical of the group.

At this point, it will be interesting to recall the longitudes of perihelion of our meteorites. For Maribo, this value is about 217 degrees and for Sutter’s Mill, about 111 degrees. The latter clearly falls into the Taurid group while the former is a little less than the minimum 222 degrees given by Steel and Asher for the Hephaistos group. However, as already stated, the figure they gave was simply the lowest for the Hephaistos-group objects known at that time. Interestingly, the object in their list which had this value was 1991 AQ! We can therefore feel safe in assigning Maribo a position within the Hephaistos complex.

Both clusters of objects include several asteroids, meteor showers and at least one comet apiece that have orbits of a clearly Encke-type in terms of perihelion distance, inclination and eccentricity. But there are also several asteroids – some relatively large – which have similar orbits, yet not ones that strictly match the true Encke-types. Are these really members of the groups, sharing a common origin, or are they interlopers, maybe from the main asteroid belt? Opinions have always differed here, but the answer will determine how we are to think of the primitive Encke. Steel, Asher, Napier and Clube took a wide view which included a large number of objects, not all of which had strictly Encke-type orbits. If these all originated in the primitive Encke, that comet must have been Hale-Bopp sized (50-60 kilometres in diameter) or larger. If only the “core” objects – those with strictly Encke-type orbits – are the indigenous members of the complex, then the primitive Encke is reduced in size to, perhaps 15 – 20 kilometres; more like Halley than Hale-Bopp.

If the asteroids in the Taurid/Hephaistos complex are truly inactive comets, it follows that they should look similar to the nuclei of active comets, that is to say, they should be dark objects having a low reflectivity similar to C-Type asteroids and their close relatives. The apparent association of the Maribo and Sutter’s Mill meteorites with this complex supports this as both of these meteorites have the sort of reflectivity that one would expect from an inactive comet and it would seem natural to assume that, if these relatively small Taurid/Hephaistos objects were dark bodies, the larger members of the stream – the asteroids visible through our telescopes – should possess a similar reflectivity and composition. But this is where the surprises begin! Certainly, some of the asteroids fit this description, but a significant percentage of the ones named by Steel, Asher and colleagues as members of the complex do not. Most of these are relatively reflective bodies having reflectance spectra similar to the S-Type asteroids, and their near relatives, populating the inner Main Belt. This discovery has led many researchers to conclude that these objects are not “real” members of the complex but are, rather, interlopers from the asteroid belt that have been perturbed into orbits that more or less resemble the genuine articles. Even Hephaistos does not have the reflectance spectrum expected for a dormant comet and may be an interloper in the group that has been given its name!

It appears likely that many of these objects are indeed interlopers. This is especially true of the relatively large light-coloured asteroids in orbits which are not very Encke-like. If that is indeed correct, the Taurid/Hephaistos complex is not as massive as had earlier been believed and the primitive Encke need not have been as large as (for instance) Clube and Napier postulated in their writings.

However, while it would be nice to dismiss the issue of light-coloured Taurid/Hephaistos asteroids as being entirely the result of “asteroidal interlopers”, the situation is actually more complex and confusing. We have already seen that there is a good orbital match between the Maribo meteorite and asteroid 1991 AQ and between the Sutter’s Mill meteorite and 1996 SK. We also saw how both of these asteroids move within meteor streams associated with the Taurid/Hephaistos complex and (from dynamical considerations alone) may be, either, the immediate parents of their respective streams or large “meteoroids” belonging to those streams. Either way, and especially considering their close orbital association with carbonaceous meteorites, we would expect them to be dark objects of the C-Type or some close approximation thereto. Well, the reflectance spectra of both of these asteroids has indeed been observed, and the results are surprising. Asteroid 1991 AQ turns out to have a QU-Type spectrum, essentially implying that it is a light-coloured rocky object having a composition similar to that of an ordinary (non-carbonaceous) chondrite meteorite! By all degrees of logic and common sense, this object cannot produce a carbonaceous meteorite!

As for 1996 SK, this object was found to have a very inhomogeneous surface with reflectance spectra varying between Q-Type and S-Type, with the latter being the predominate one. Once again, it is a very unlikely parent for a carbonaceous meteorite and an equally unlikely candidate for a defunct comet nucleus. Nevertheless, the orbits of both of these objects with respect to each of the meteorites and to the meteor streams makes it difficult to explain away their presence as fortuitously placed interlopers.

What then, is going on here?

If we can engage in some free (but hopefully not wild!) speculation, perhaps a clue is offered by the mixture of material found within these meteorites. If their parent body, which we assume was the primitive Comet Encke, incorporated material that was once part of earlier, shattered, asteroidal bodies, maybe it also incorporated macroscopic slabs of these asteroids as well. Maybe the large comet nucleus had within it, “xenoliths” ranging in size from centimetres to hundreds of metres or even several kilometres; some of which have come adrift over time and joined the streams of more typical cometary debris that form the various meteor streams associated with the complex. Maybe some of these xenoliths also carried some of the cometary material with them for a time following their breaking away from the main mass and in that way may have acted as immediate parents for the streams in which they are now embedded. Moreover, subsequent impacts by stray meteoroids may also have chipped away particles from their surfaces and supplied a small percentage of the stream members, so the division between simple “members” of the streams and their immediate “parents” may not be absolute. If pieces broken from the xenolithic bodies do populate some of the streams, it implies that the latter consist of both carbonaceous and ordinary chondritic meteoroids, although the former should be by far the most numerous.

It may (perhaps?!) be possible that something even stranger happened. We know that certain minor Solar-System bodies are double-lobed and it seems that these were formed by the fusion of two initially separate objects. The periodic comet Tuttle is one such body. Although unlikely, it does not seem impossible that an asteroidal and cometary body might become fused together following a gentle collision during the Solar System’s formative years.

Incidentally, the primitive Encke may even have been a binary object. Perhaps a close approach to one of the terrestrial planets pulled away the secondary object, which subsequently evolved dynamically into the parent of the Hephaistos group?

Be that as it may, we suggest that the light-coloured rocky bodies moving in truly Encke-type orbits began as xenoliths within the original cometary nucleus. This suggestion would, I think, find some support (a) if an ordinary chondritic meteorite fell from a Taurid shower (although a counter argument postulating this as an interloper might be raised), (b) If an asteroidal remnant linked with one of the apparently defunct comets associated with the Hephaistos group (which we will discuss shortly) is discovered and found to have an S- (or related) – Type reflectance spectra or (c) if a future space probe to Encke finds large chondritic blocks embedded within its nucleus.

Some researchers (for example, D. Steel) have suggested that (a) above has already occurred. During the daytime hours of June 25, 1890, a meteorite fell near Farmington in Kansas. The date and circumstances of the fall were consistent (or, at least, not inconsistent) with an association with the Beta Taurid. However, what most impressed Steel was the unusually short CRE age of the meteorite. Farmington was an ordinary chondrite, the CRE ages of which normally are in the range of several millions of years. However, Farmington had a CRE age of just 25,000 years. Even for a carbonaceous chondrite, this would be short, but for an ordinary chondrite it was truly remarkable and also close to the time when the primitive Encke is thought to have begun its process of disruption. It was this last fact that seemed most significant to Steel.

An approximate orbit for this object was computed by Levin and colleagues using old newspaper reports and even an account by a 98-year-old eyewitness. The computed range of orbits was pretty rough, but indicated a rather Taurid-like perihelion distance near the orbit of Mercury. However, most of the possible orbits were otherwise not very similar to the Taurid complex. The researchers found that the most likely parent bodies of this meteorite were the asteroids Apollo, Hermes or Cerberus with Geographos and Toro being less likely possibilities. Of these, Hermes was listed by Steel and Asher as a member of the Taurid complex, although this is one of the bodies now widely thought to be an asteroidal interloper. Whatever the truth of this, the fact that this asteroid was listed as a possible Farmington parent leaves open the slight possibility of a Taurid association, however when the reflectance spectra of the possible parent asteroids is taken into consideration, Apollo emerges as the most likely candidate. This body is a Q-Type; a classification thought to be the most likely source of ordinary chondritic meteorites. The other four bodies, including Hermes, are S-types; dry rocky bodies, but not quite a match to meteorites of the Farmington variety.

 

Earlier, we mentioned two comets that might be associated with the Hephaistos complex. Encke’s is, of course, associated with the Taurid complex and, if the Hephaistos complex is ultimately an extension of the Taurid association, it follows that it is associated with this as well. However, there are two further comets which seem to have an interesting relationship with the Hephaistos group and it will be interesting to take a closer look at these.

The first is the mysterious comet Helfenzrieder observed during April 1766. It was a fine object, appearing to the unaided eye as a compact object of second or third magnitude and sporting a tail some seven degrees in length. Although its orbit is not known with high precision, it is clearly a short-period ellipse with a period between about four and five years. Perihelion lies at 0.41 from the Sun and, as Steel and Asher pointed out in 1994, the orbit is typical of an object in the Hephaistos group.

Now, it would be nice if a fine object like this turned up every five years or thereabouts, but (alas!) that is not so. Except for its spectacular display in 1766, no other observations of the comet are known. Presumably, prior to 1766 it was a faint or even dormant object that flared mightily at that one return and then faded out and possibly disintegrated. If any remnant of this comet persists to our day it must be very small and faint, although with the advance in searches for Earth-approaching bodies in recent years, it may still be possible that some day a small asteroid/large meteoroid will be found and linked with this comet. Interestingly, Terentjeva gives this comet as the possible parent object of the “Theta Cancrid” meteor stream, although the orbital match is not close enough to be convincing.

The second comet is the one discovered by Dunlop in 1833. It was observed just eleven times, and only by its discoverer, from September 30 until October 16 and the orbit remains poorly determined. Most catalogues simply list it as parabolic, although the small inclination of just over seven degrees is seen by many astronomers as possibly indicating a short-period ellipse. Indeed, as long ago as 1888 L. Schulhof found that the available positions could be satisfied by an elliptical orbit having an eccentricity of around 0.8 and a period of just 3.5 years. With a perihelion of about 0.46 AU, this orbit would fit very well within the Hephaistos group.

This comet was not as bright as Helfenzrieder, but if its orbit really was of such a short period, it must have behaved in a similar manner. That is to say, it must have been faint or even inactive for many returns prior to 1833 before experiencing a large outburst (enabling its discovery) and subsequently fading away. From Dunlop’s observations, it seems that the comet’s visibility varied from night to night which, if due to intrinsic fluctuations in the comet’s brightness, might indicate instability and possible signs of disruption. On the other hand, Dunlop’s notes also make it clear that the weather at the time was not settled and, given the low altitude of the comet and the variable atmospheric conditions, the apparent variations in visibility may have been simply due to atmospheric haze. In view, however, of the comet’s possible short-period orbit, it would be interesting to backtrack the orbit of any suspicious asteroid that might be found during future searches just in case one turns out to be an inactive remnant/fragment of this comet.

Assuming that the orbit of Dunlop really is elliptical, and with an eccentricity of 0.8, we find that D’ comparisons with the three Delta Cancrid orbits listed by Kronk give values of between 0.06 and 0.08 and, with the “Theta Cancrids” of Terentjeva’s list, a value of 0.09. Comparisons with Maribo yield 0.08 and, with 1991 AQ, 0.05. Although the hypothetical nature of the eccentricity value means that these actual numbers cannot be pressed too far, their consistently small values are, at least, very intriguing. Moreover, when the longitude of perihelion (which is independent of eccentricity) of the Dunlop orbit is considered, the results are equally interesting. This value comes out at around 225 degrees. Kronk’s three orbits for the Delta Cancrids plus that given by Terentjeva for the Theta Cancrids have ranges from 219 – 228 degrees while 1991 AQ has 222.6 and Maribo 217 degrees. Dunlop fits nicely into this grouping! It seems that a case can be made for Comet Dunlop as being the immediate parent object of the Delta Cancrids and for being intimately associated with Maribo and 1991 AQ. This case would be greatly strengthened if a remnant of this comet could be discovered and the short-period nature of its orbit thereby confirmed.

Incidentally, a D’ comparison between the Dunlop orbit (with the assumed value for eccentricity) and the second (averaged) orbit given by Kronk for the Sigma Leonid meteor stream of February yields a value of 0.08, indicating a possible association here as well. This meteor stream, by the way, is not to be confused the one active from Leo during March and sometimes listed as the “Sigma Leonids”. This stream, termed the “Rho Leonids” in Kronk, has an entirely different orbit and shows no obvious association with Comet Dunlop.

 

In conclusion, it appears that a case can be made for linking both the Maribo and Sutter’s Mill meteorites with a large (Halley-sized?) comet that entered the inner Solar System – presumably from the Kuiper Belt and following a relatively long sojourn in a Centaur orbit – several tens of thousands of years ago. This comet (which we have been calling “primitive Encke” after its remaining active fragment) progressively broke up into a debris complex of meteor streams, asteroids and comets. We suggest that some of these asteroids were rocky xenoliths freed from the original comet nucleus whereas others are inactive comets that began life as fragments of that nucleus itself. These latter may either be defunct comets (i.e. ones that have exhausted their store of volatiles) or merely dormant ones in which the activity has been shut off through the accumulation of a layer of insulating refractory material on their surfaces. These retain the potential for activation if this layer is disrupted through meteorite impacts or thermal stress resulting, either, in a long period of “rejuvenation” (as probably happened with Encke itself) or in catastrophic and possible fatal outburst as happened in the case of Helfenzrieder and, possibly, Dunlop.

As a final thought, we remark that at least one Centaur (and maybe even the prototype Chiron itself) appears to be encircled by a debris ring, probably built up over the millions of years tha it has been in a Centaur orbit and sustained impacts from meteorites. There is no reason to suppose that primitive Encke did not possess such a ring, and if it did, this could have contained mini satellites thrown up from the main body progressively over a period of hundreds of thousands or even millions of years. If we suppose that these ring bodies entered the inner planetary susyem together with their primary, they would eventually have become meteoroids within the Taurid complex. But because the ring was probaly formed by several impacts spread over a long time, its constituent objects may well have exhibited a CRE age scatter equal to the difference between the Maribo and Sutter's Mill meteorites.

 

David A. J. Seargent  MA  PhD  FRAS