Date sent: Wed, 25 Feb 1998 09:15:03 -0500 (EST)
From: Benny J Peiser
Subject: CC DIGEST, 25/02/98
Priority: NORMAL


Ron Baalke

Bob Kobres


From: Ron Baalke

ESO Education and Public Relations Dept.

Text with all links is available on the ESO Website at URL:

This is a provisional overview of some of the discussions that took
place at the First International Hale-Bopp Conference at Tenerife in
February 1998. It was prepared by R. M. West (ESO, email:

I. Introduction

Ten months after the perihelion passage, the First International
Meeting about Comet Hale-Bopp was held at the Conference Centre in
Puerto de la Cruz (Tenerife, Canary Islands, Spain) on February 2-5,
1998. Nearly 150 specialists from all major comet research groups in
the world participated. During 4 days of intensive debates and with the
presentation of approximately 150 papers, the participants surveyed the
current status of the many research programmes related to this most
unusual comet.

The Local Organising Committee, headed by Mark Kidger and Monica
Murphy (IAC) had done a great job and the frame was excellent. The
conference provided a good opportunity for a discussion about some of
the fundamental issues connected to this spectacular astronomical
event. For instance: why was this comet so bright and in which
respect(s) did it differ from other comets observed with modern
equipment? Although many new results were presented and some main lines
can be perceived, those present were left with the impression that
there are still many open questions. There is no doubt that the
associated research will continue for some time. It is also obvious
that further meetings on these subjects will be held in due time.

The Hale-Bopp event provided observers with a long lead time, thanks to
the early discovery in July 1995 by Alan Hale and Tom Bopp who were
both at the conference. Thus, it was possible for the scientists to
obtain a substantial amount of observing time at the world's major
observational facilities and to prepare their runs well. Moreover, the
Comet was visible in the sky for an extremely long period. It was very
bright and in the end, a large number of telescopes and instruments
were used at all wavelengths from X-ray to radio. It is therefore no
surprise that all the work by so many scientists during the past months
has resulted in important new knowledge, as exposed at this meeting.

In what follows, some of the highlights of the conference will be
reviewed. They are arranged roughly in the order they were presented at
the meeting. Kindly note that not all contributions mentioned here are
attributed to individual speakers and various information by others has
been left out in order to keep this survey within a reasonable size.
However, a complete version of the conference summary, with full
references and more details, will ultimately appear in the Conference

II. Motion and Early Observations

The meeting began with some basic information about the comet.

The Orbit

Based on more than 2600 astrometric observations from 1993-98, Brian
Marsden has calculated a new and improved orbit, now taking into
account non-gravitational forces arising from the jet effect associated
with the Comet's vigorous activity. He found that the original period
was 4211 years and that the future period will be 2392 years with a
formal uncertainty of a few months only. However, the limited knowledge
about the future development of the Comet's activity may still change
this period somewhat.

Had it arrived about four months earlier this time, it would have
passed the Earth nearly as close as did Comet Hyakutake one year
earlier. In that case it would have been an incredible view.
Interestingly, it appears that Comet Hale-Bopp may have passed very
close to Jupiter on June 7, 2216 BC. In view of the rather unstable
orbit, it is unlikely that there have been more than a few earlier,
close perihelion passages.

Early Observations

Alan Fitzsimmons reviewed the various signs of very early activity
which are typical for this comet. In particular, investigations of
early images of the dust tail by Hermann Boehnhardt and Marco Fulle
have shown that the Comet most probably was active already 4-5 years
before discovery, that is at pre-perihelion distance 18-20 AU. In
addition to a UK Schmidt pre-discovery image obtained in April 1993, an
image of the Comet may possibly be present on another photographic
plate taken with the same telescope in September 1991; this will now be

III. The Nucleus


Harold Weaver and Philippe Lamy surveyed 7 different methods which
have led to reasonably consistent estimates of the size of Comet
Hale-Bopp's nucleus. Most of these lie in the interval between 20 and
40km radius (i.e., 40 and 80 km diameter), but a few are somewhat
larger. There is also a possible indication that the nucleus may have
elongated shape. Particularly impressive among these observations were
those performed at radio wavelengths with the VLA in New Mexico and
which lasted more than 6 days -- they pointed towards a diameter of
approximately 50 km.

Interestingly, there may be more than one component of the nucleus. By
very careful analysis of high-resolution HST images obtained in 1996,
Zdenek Sekanina believes that the primary nucleus may have a lesser
companion of approximately half the size. This issue is still somewhat
controversial, but observations with the Adonis adaptive optics camera
at the ESO 3.6 m telescope in November 1997 and January 1998 by
three ESO astronomers also appear to show a double nucleus. More
observations with this facility in the coming months and/or with the
HST scheduled for later this month are expected to clarify this issue.


In a review talk on this subject, Dave Jewitt listed the observational
possibilities for measuring the rotation period of Comet Hale-Bopp's
nucleus. With the nucleus hidden inside the coma already at the moment
of discovery, they include periodic fluctuations of that part of the
light at the centre of the coma which supposedly comes from the nucleus
itself, and also periodic changes in the coma structure (orientation of
jets, outward motion of shells, etc.).

Many such observations are available; the longest series was apparently
obtained by Mark Kidger and his group at the Teide Observatory on
Tenerife, right above the site of the conference. At this moment, there
is good agreement among the values published by 8 different groups and
the true rotation period of the nucleus must be close to 11.34 +/- 0.03

Although there were originally some signs of precession (wobbling of
the rotation axis), this is now less sure. The direction of the polar
axis has also not been unambiguously determined yet, but this may
become possible after further analyses.

Composition and Structure

Dominique Bockelee-Morvan and Hans Rickman surveyed the many new
observations which will ultimately allow a better `look' into the still
unknown interior of a cometary nucleus. This is first of all due to the
very extensive observations which were made of the production rates of
various molecules, as the comet came closer to the Sun. These
observations show that not all of these species emerge in parallel and
there seem to be certain `transitory' periods during which changes in
the production rates can be observed. They are indicative of the
composition and structure of the upper layers of the nucleus.

For instance, a slowing down of the rate of increase of CO production
was observed at about the time when the water production started at a
heliocentric distance of approximately 3.5 AU. The production rates of
some, less abundant molecules, showed a very steep dependence on
heliocentric distance. All in all, the observed behaviour seems to
follow quite well what is predicted by the models which have been put
forward and which were described at the meeting by Dina Prialnik --
they include in particular heat release by sub-surface cristallization
of amorphous ice in the nucleus.

It is also well established that the unusually great activity of
Hale-Bopp which was observed while it was still far from the Sun is
mostly caused by the outgassing of CO from its interior; this process
pushed large amounts of dust into space. The more dust there is around
the nucleus, the more sunlight is reflected and the brighter will the
comet appear.

IV. The Gas Phase

Many gaseous molecules and atoms were observed in the coma. Some
of these are electrically uncharged (neutrals), others have lost one or
more electrons (ions). Sodium, a neutral atom observed extensively in
Hale-Bopp, plays a particular role and was discussed in a special


Didier Despois reviewed extensive radio observations which have led to
the discovery of a total of 8 new molecules never seen before in a
Observations of isotopes (now also including DCN, HC15N and C34S for
the first time) indicate that this comet is similar to Comet Halley and
that it was formed in the solar system. In particular, the HDO/H2O
ratio was found to be twice that measured in the Earth's oceans, and 10
times larger than the protosolar value.

Thanks to great technological advances, it has now become possible to
produce detailed maps of the distribution of individual molecules in
the coma. This has led to very interesting research which will
ultimately help to understand the extremely complex chemistry of a
cometary coma. In particular, this may allow to determine which of the
molecules observed really come from the nucleus itself (as parents) and
which are secondary products (daughters).

Jacques Crovisier reported equally exciting new observations in the
infrared spectral region, from the ground with several of the largest
infrared telescopes and from space (ISO). This includes hydrocarbons
(organic molecules) and also water for which the ortho-to-para ratio
was equal to that measured at Halley and indicates the very low spin
temperature of 25 K. It is not clear whether this is also the
temperature of formation.

Unfortunately, at least for this type of research, the very large
dust-to-gas ratio observed in Comet Hale-Bopp made observations of
spectral emission lines difficult since they were recorded on top of a
very strong continuum spectrum of solar light reflected from the dust
in the coma. Partly for this reason, it appears that it has not been
possible to gain new knowledge about the interesting emission lines
from organic molecules seen in the 3.2 - 3.6 micron band. Nevertheless,
many new mineral bands were seen in the infrared region (see below).

Many spectral observations in the optical region were reported by
Claude Arpigny. They generally show that Hale-Bopp is similar to other
long-period comets. Several groups have reported detailed, very
high-resolution spectroscopic monitoring of the various emission lines
in this wavelength region. There is obviously still much work to be
done on all of these high-dispersion spectra.

In the ultraviolet spectral region observations were made with a number
of spacecraft and also with several sounding rockets. Paul Feldman
described the spectra obtained with HST and the IUE Space
Observatories which include many atomic lines. Further towards shorter
wavelengths, a line of singly ionized oxygen (O+) has been detected by
the EUVE satellite at 538 A, but unexpectedly, neon (Ne) was not
etected in the same spectral region. This points to a very low
neon-to-oxygen ratio in this comet, at least 25 times less than the
solar value.

An enormous Lyman-alpha halo of hydrogen, about 150 million km
diameter, that is the distance from the Sun to the Earth, was observed
by the SOHO Observatory when the comet was near perihelion. It was also
possible to view the comet in the ultraviolet light of various atoms;
when compared to images obtained at other spectral wavelengths, they
will contribute to the understanding of the processes in the coma.


Heike Rauer reported that most of the ions known in earlier comets have
also been observed in Comet Hale-Bopp. Strangely, emission from CO+
was first detected quite late (at a heliocentric distance of 3.6 AU);
the reason for this is still unclear. Very complex coma and tail
structures were observed by Steve Larson and others in the light of CO+
and other selected ions, indicating an exceedingly complex interaction
between the solar wind and the cometary ions (streamers, sunward arcs,
etc.). In this respect, the detailed mapping of the spatial
distribution in the coma and the corresponding velocity field of HCO+
by groups in Europe and the USA provided very valuable observational

There has clearly been tremendous progress in the modelling of the
solar wind/comet interaction in recent years. Tamas Gombosi showed that
new and very complex computer software running on the fastest machines
available now make it possible to reproduce in quite some detail the
observed structure (distribution of ions, magnetic field lines,
cavities, sheets, etc.). In this context, the discovery by the Ulysses
Spacecraft that the solar wind moves faster at high ecliptic latitudes
and therefore interacts stronger with the comet when it is far from the
ecliptic plane, has provided an important breakthrough in this field.


While sodium has been seen since 1910 in comets that come close to the
Sun, the first signs of a sodium tail was reported in 1957 from an
objective prism spectrum obtained of the unusual Comet Mrkos.. However,
it was in mid-April 1997 that the now famous third cometary tail of
neutral sodium atoms and measuring more than 50 million km was
discovered by Gabriele Cremonese and his colleagues of the European
Comet Hale-Bopp Team. Already at that time, the correct interpretation
was brought forward, that is fluorescence acceleration of sodium atoms
released in the coma.

Meanwhile, this and other groups have also reported the presence of
neutral sodium in the normal dust tail, demonstrating that these atoms
are also released from the dust in this tail. It is still unclear,
however, from where the sodium in the inner coma comes. Interestingly,
no NaOH (soda) or NaCl (salt) was found in gaseous form in the coma
(but may still be present in the dust grains).

V. Dust Phase

Observations of new minerals

Klaus Jockers reported on extensive observations of the dust in Comet
Hale-Bopp. These concern direct imaging, the distribution of colours
within the coma and the tail and also the polarization. This comet had
a somewhat higher degree of polarization when observed at large phase
angles than other comets, indicating differences in the dust component.

A true breakthrough has occurred in the field of remote observing of
cometary minerals, as discussed by Martha Hanner and others.
Ground-based and space-based observations of the detailed infrared
spectrum of Comet Hale-Bopp have revealed for the first time many new
spectral features which can be assigned to particular minerals with a
great degree of certainty. They include above all cristalline olivines,
in particular the magnesium-rich forsterite, and also pyroxene-rich

In fact, it seems that the composition of some of the grains observed
in Comet Hale-Bopp are very similar to those of two main types of
interplanetary dust particles which have been collected in the Earth's
atmosphere and subsequently analysed in great detail in terrestrial

Dust production

The dust production of Comet Hale-Bopp was enormous, especially when
compared to other comets, for instance 100 times more than in Comet
Halley. Similarly, the dust-to-gas ratio was very high, from most
measurements estimated as between 2 and 5. The dust production at the
maximum reached about 400 tonnes/sec, but since the nucleus is so
large, the entire mass loss at this passage is probably still less than
0.1 percent of its total mass.

Similarities with circumstellar dust

It is also very interesting to compare the infrared spectra of Comet
Hale-Bopp obtained with the ISO Observatory with spectra of stars which
are surrounded by circumstellar dust. As Christoffel Waelkens pointed
out, there are great similarities, but also some differences. For
instance, the spectrum of the star HD 100546 also displays the minerals
mentioned above, as well as cristalline water, but contrary to the
Comet, it also has strong spectral features of organic components in
the 3.5 micron band.

There may thus be a close relationship between comets like Hale-Bopp
and the material observed in circumstellar disks, e.g. around the
southern star Beta Pictoris. All of this may provide very valuable new
information about the formation of the cometary reservoirs in the solar
system (Kuiper Belt and Oort Cloud).

VI. Dust-Gas Interactions

As mentioned above, the gas chemistry of cometary comae is extremely
complicated and when the dust component is also taken into account,
everything becomes even more complex. Thus, it is most promising to see
that it has now become possible to model in significantly greater
detail what is going on in a cometary coma by means of very elaborate
three-dimensional computer models. The report by Mike Combi and
others proved that, when taken together with the new observations which
have become available and which have been mentioned above, we may
expect to reach a much better understanding of the various interactions
in cometary comae in the future.


An intensive debate is still raging about the origin of the soft X-ray
emission that has now been observed in a total of 10 comets, including
Hale-Bopp. No less than 5 different explanations (models) have been put
forward and none has yet been ruled out. It now seems that two of these
may both play particularly important roles. The first is based on a
charge exchange between heavy ions in the solar wind and light atoms in
the cometary coma. By excitation of the inner atomic levels of these
atoms, X-rays are released from these. The other is based on the
presence of large numbers of extremely fine dust grains (so-called
"atto-dust" seen in Comet Halley) which reflect the solar X-rays. It is
obvious that more observations of more comets are needed before this
controversy can be resolved.

VII. Some Conclusions

Comet Hale-Bopp has indeed proven to be a bonanza for researchers in
this field. Never has a comet been observed so extensively at such
large heliocentric distances and not even in the case of Comet Halley
was it possible to obtain such detailed information about the
progressive changes that took place in the coma of Comet Hale-Bopp as
it approached the Sun.

This gives substantial hope that it will now be possible to understand
better the structure and composition of cometary nuclei, before the
first cometary space missions perform in-situ measurements. The newly
found, clear similarities between the cometary dust and the dust around
certain stars also promise to give new insights into the origin and
formation of the comets in the solar system.

Some participants in this very successful meeting expressed that the
appearance of Comet Hale-Bopp was such an important event in the
history of cometary research that it may later be considered almost a
par with the Halley encounter in 1986.

Observations of Comet Hale-Bopp will continue for quite some time. On
the spectroscopic front, astronomers with access to large telescopes
will follow the steady decrease of gas production which, with the
exception of CO and CO2, is likely to cease during the next years.
Images will be made which will show structural changes in the coma and
allow to study the decreasing dust activity. Perhaps it will later be
possible to observe directly the naked nucleus and to get an accurate
understanding of its spin state. Continued astrometric observations
will gradually improve the orbit so that very accurate predictions can
be made for the Comet's next return, some 24 centuries from now.

But work will also continue on other fronts. Much of the enormous
amount of data has not yet been thoroughly studied and there important
new information may still be uncovered. At the same time, it is obvious
that the modelling of the Comet's coma, the processes therein and the
interaction with the solar wind will advance greatly in the coming
years. Progress is also likely for the modelling of the cometary
nucleus itself.


From: Bob Kobres

I still remember well the looks I got about a half-decade back when I
casually mentioned the possible but unknown role of giant beavers in
the genesis of the Carolina Bays. "That's Bob — You can’t take him
anywhere…" Anyway, we (a geomorphologist, archaeologist, ecologist, and
I) were on a ‘just happened to work out’ tour of Carolina Bays in and
near the Savannah River Nuclear Plant in South Carolina. I didn’t dwell
on the subject at the time though I did later send Mark, the
archaeologist, a newspaper clipping that reported the problems that
beavers were beginning to cause for watershed engineers in the nearby
Appalachian mountains. Since that time beavers have been diligently
producing more beavers and lately they have been taking back noticeable
areas of the coastal plain that their larger, now extinct, Pleistocene
cousins and their own earlier ancestors once occupied.

As a result of this recent activity it now seems a bit more credible to
skeptical researchers not familiar with the potential ecological
influence of beavers that these workaholic rodents — particularly the
now gone bear size ones — could easily produce half a million or more
ponds along the Atlantic coastal plain of North America. So I think
that one of the riddles regarding the origin of Carolina Bays — Why are
there so many of them? — can be attributed to an earlier long term
residency of this keystone species. The big question, however,
remains — Why are these features so similar to one another? Did beavers
just form their ponds in a way that ensured the regular appearance
observed today or was there another agent involved?

My hunch is that there is something more than beaver business and wind
involved in the formation of the features visible today. I think these
elliptical structures got their present shape due to violent steam
explosions that were caused principally by their exposed water
content’s efficient absorption of radiant energy, likely aided by large
abrupt atmospheric pressure changes. Both the intense heat and
air-pressure excursions were caused by hyper-velocity material entering
Earth’s atmosphere with the most likely source of such material being a
large (Hale-Bopp size?)comet coming close enough to Earth to lose some
of itself in passing.

There are a number of clues suggesting that this might indeed be the
case. First, there is the mega-fauna extinction that is most pronounced
on the North American continent. This extinction event, which included
the giant beavers, is contemporary with the Younger Dryas cold interval
that is now known to be a globally felt rapid climate downturn dating
around 11,500 years ago — a date often estimated for Carolina Bay
formation. Also there are Native American legends that describe the
loss of better times and large animals due to a mighty snake that
flooded and burnt the land as well as making it colder, the great man in
the sky hurling bolts from the top of the Alleghenies, and in one more
recently collected story, a comet scorching Earth with its tail! In
addition, the geomorphic structure of the Carolina Bays is very close to
that of some maars, which have occurred in similar soil conditions as
the Bays, and maars are produced by volcanically induced steam
explosions. Also suggestive is the recent recognition of the potential
for the North Atlantic deep-sea current to change its flow
characteristics rapidly due to the input of a large volume of
melt-water from the North American continent. A blistering pulse of
sun-surface-like heat would have wasted some ice and perhaps tripped
that trigger.

I know ... So what — Where is the proof?

What I suggest is that this scenario could be treated as a heuristic
model that could better show us what to look for in the way of hard
evidence. In other words, we’ve learned a great deal about both the
Younger Dryas time period and comet Hale-Bopp. The Carolina Bays are a
geographically defined feature, so it might be a fruitful exercise to
try to produce a modeled Younger Dryas episode with a Hale-Bopp
featured nucleus in a Taurid-object type orbit with the constraint of
producing steam-explosions from ponds where we find Carolina Bays
today. If the physics of this can be made to work, then we will have a
much improved chance of defining both the types and quantities of
exotic materials we should expect to detect within preserved strata of
that time period. At minimum such an effort might demonstrate whether
it is in fact possible to produce maar like features with fire from
above. Any takers?

A daunting proposition I know, but if something similar to this did
happen at the close of the Pleistocene there will probably be no impact
craters or easy to discern exotics so, how else do we go about learning
whether an ancient atmospheric impact event really occurred at that

Rather than ramble on, I invite interested readers to try the new
search tool on my Web resource at:

Query terms such as steam, beaver, Pleistocene, clay, bay, extinction,
fauna, dryas, mammoth, mastodon snake, etc. will quickly find various
references related to the above topic.

A synopsis of the recently collected (and so perhaps less original)
Ojibwa story mentioned above can read at:

A good collection of abstracts related to the Younger Dryas may be
found among:

Also, there is a comprehensive collection of papers that focus on the
102, number C12, November 30, 1997. Particularly pertinent to the
above discussion are: ‘Mineralogy of atmospheric microparticles
deposited along the Greenland Ice Core Project ice core,’ by V. Maggi,
pages 26,725-26,734, and ‘Continental biogenic species in the Greenland
Ice Core Project ice core: Tracing back the biomass history of the
North American continent,’ by Katrin Fuhrer, and Michel Legrand, pages

Up to date findings on comet Hale-Bopp can be gathered at:

Still searching.
Bob Kobres
Main Library
University of Georgia
Athens, GA 30602

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