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Date sent: Wed, 25 Feb 1998 09:15:03 -0500 (EST)
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CAMBRIDGE-CONFERENCE DIGEST, 25 February 1998
---------------------------------------------
(1) REPORT FROM THE FIRST INTERNATIONAL HALE-BOPP CONFERENCE
Ron Baalke BAALKE@kelvin.jpl.nasa.gov
(2) BEAVER TALES AND COMET TRAILS: CONJECTURES ON THE ORIGIN OF
THE
CAROLINA BAYS
Bob Kobres bkobres@uga.edu
============================
(1) A REPORT FROM THE FIRST INTERNATIONAL HALE-BOPP CONFERNCE
From: Ron Baalke BAALKE@kelvin.jpl.nasa.gov
ESO Education and Public Relations Dept.
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Text with all links is available on the ESO Website at URL:
http://www.eso.org/outreach/info-events/hale-bopp/report-rw-hbitp98.html
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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:
rwest@eso.org).
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
Proceedings.
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
investigated.
III. The Nucleus
Size
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.
Rotation
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
hours.
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
session.
Neutrals
Didier Despois reviewed extensive radio observations which have
led to
the discovery of a total of 8 new molecules never seen before in
a
comet (SO, SO2, H2CS, HC3N, HNCO, NH2CHO, HCOOH, CH3OCHO).
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.
Ions
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
information.
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.
Sodium
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
minerals.
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
laboratories.
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.
X-rays
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.
=========================
(2) BEAVER TALES AND COMET TRAILS: CONJECTURES ON THE ORIGIN OF
THE
CAROLINA BAYS
From: Bob Kobres bkobres@uga.edu
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
time?
Rather than ramble on, I invite interested readers to try the new
search tool on my Web resource at:
http://abob.libs.uga.edu/bobk/query.htm
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:
http://medicine.wustl.edu/~kronkg/metlegends.html
A good collection of abstracts related to the Younger Dryas may
be
found among:
http://gust.sr.unh.edu/GISP2/Contri_Series/full/06.html
http://www.pd.uwa.edu.au/~dobson/im.lakes.html
http://webhost1.cerf.net/journals/epsl/jnl/embase/abstracts/html/0866989_91J-08452.html
http://earth.agu.org/revgeophys/mayews01/node6.html
http://molscat.giss.nasa.gov/research/intro/peteet.01/
http://www.ngdc.noaa.gov/paleo/pale/pt97/pt015.html
http://www-bprc.mps.ohio-state.edu/Icecore/Abstracts/TM_TDLHDBL.95.html
Also, there is a comprehensive collection of papers that focus on
the
Greenland Ice Core Project in THE JOURNAL OF GEOPHYSICAL
RESEARCH, Vol.
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
26,735-26,745.
Up to date findings on comet Hale-Bopp can be gathered at:
http://www.eso.org/outreach/info-events/hale-bopp/report-rw-hbitp98.html
Still searching.
bobk
Bob Kobres
bkobres@uga.edu
http://abob.libs.uga.edu/bobk
706-542-0583
Main Library
University of Georgia
Athens, GA 30602
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