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(meteorobs) Excerpts from "CCNet 53/2002 - 23 April 2002"




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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet: JPL'S "ASTRO-COPS" CHECK OUT ASTEROID 1999 GU3 
Date: Tue, 23 Apr 2002 09:59:39 +0100

CCNet 53/2002 - 23 April 2002 
-----------------------------

[...]

(3) COMET BORELLLY: DRY AND HOT
    Sky & Telescope, 22 April 2002

[...]

(6) EVIDENCE FOR YOUNG PLANETS FOUND IN DUSTY ORBITS AROUND NEARBY STAR 
    Andrew Yee <ayee@nova.astro.utorontodot ca>

[...]

(9) COMET ENCOUNTERS REVISITED
    Roy Tucker <tucker@noaodot edu>

[...]

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(3) COMET BORELLLY: DRY AND HOT

>From Sky & Telescope, 22 April 2002
http://skyandtelescope.com/news/current/article_579_1.asp

By J. Kelly Beatty

April 22, 2002 | Scientific intuition tells us that a comet's nucleus should
be a frozen mountain of ice and dust. But that's not what Deep Space 1
discovered when it flew past Comet 19P/Borrelly last year. A recently
released analysis of spacecraft spectra finds that Borrelly's "icy heart"
exhibits no trace of water ice or any water-bearing minerals. Moreover, the
nucleus is actually quite hot - ranging from 3000 to 3450 Kelvin (800 to
1600 F). 

What this means, according to Laurence Soderblom (U.S. Geological Survey),
who led the analysis team, is that virtually all of the comet's surface has
become inactive. As further evidence, Soderblom notes that gas and dust
appear to be escaping only from localized jets totaling less than 10 percent
of the surface seen by the spacecraft. Ground-based observations also show
Borrelly to be a weak producer of gas and dust, typically releasing less
than a ton of water per second. Because this comet has been trapped in a
7-year-long orbit around the Sun for at least two centuries, scientists
believe it has exhausted most of the volatile consituents needed to create
an impressive coma and tail. 

Deep Space 1's spectra weren't entirely featureless: the comet's inky black
nucleus exhibits an unexplained absorption at 3.29 microns. Soderblom
guesses that this might be the signature of polyoxymethylene (a chained
polymer of formaldehyde, H2CO, previously detected in Comet Halley) or some
other organic compound. The team's full analysis appears in the online
version of Science for April 4th; a summary was presented two weeks earlier
at the Lunar and Planetary Science Conference. 

Mission scientists are thrilled to have any spectra at all to work with.
Just as it passed 2,170 kilometers from the comet last September 22nd, Deep
Space 1 scored a direct hit with its camera-spectrometer, recording 45 scans
across the 8-km-long nucleus. And because only a handful of tightly
collimated jets were spewing into space, the spacecraft had a clear
sightline through the inner coma. The resulting images record features on
the nucleus as small as 48 meters - far more detailed than the views of
Comet Halley returned by the Giotto and Vega spacecraft in 1986. 

Copyright 2002 Sky Publishing Corp.

==================

(6) EVIDENCE FOR YOUNG PLANETS FOUND IN DUSTY ORBITS AROUND NEARBY STAR 

>From Andrew Yee <ayee@nova.astro.utorontodot ca>

W.M. Keck Observatory 
Kamuela, Hawaii 

Media Contact:
Laura K. Kraft, (808) 885-7887, lkraft@keck.hawaiidot edu

April 11, 2002

EVIDENCE FOR YOUNG PLANETS FOUND IN DUSTY ORBITS AROUND NEARBY STAR 

TUCSON, Arizona -- Two independent teams of astronomers are presenting the
discovery of new features in an edge-on disk around the nearby star Beta
Pictoris at the Gillett Symposium on "Debris Disks and the Formation of
Planets" in Tucson, Arizona. 

Infrared images from the W. M. Keck Observatory reveal an important clue in
the configuration of dust confined to a solar-system sized region close to
the star: the dust orbits in a plane that is offset by approximately 14
degrees from that of the outer disk. Moreover, the offset is in the opposite
direction from that of a larger scale warp detected previously by Hubble
Space Telescope. This double warp is believed to be due to the presence of
one or more unseen planets and constitutes one of the strongest pieces of
evidence yet which links observations of circumstellar disks to the actual
formation of planets. 

At the Keck II telescope at Mauna Kea, Hawaii, Prof. David Koerner and
graduate student Zahed Wahhaj of the University of Pennsylvania led a team
of astronomers from NASA's Jet Propulsion Laboratory (JPL), Franklin and
Marshall College, and Caltech in observations of Beta Pic with MIRLIN, a
mid-infrared camera from JPL (http://cougar.jpl.nasadot gov/mirlin.html).
Alycia Weinberger, now at the Carnegie Institution of Washington, and Eric
Becklin and Ben Zuckerman from UCLA carried out observations with the Long
Wavelength Spectrometer at Keck I (LWS)
(http://www2.keck.hawaiidot edu:3636/realpublic/inst/lws/lws.html). Both
telescopes have 10-meter (400-inch) apertures. Both MIRLIN and LWS work at
wavelengths between 8 and 20 microns. 

Prof. Koerner reported, "We've seen disk features before that could be due
to planets -- inner holes, narrow rings, and variations in azimuthal
brightness. To date, however, most of these were discovered far outside the
region where planets reside in our own solar system, and plausible
non-planetary explanations have been found for some of them. In contrast,
the distorted disk plane in Keck images occurs at Jovian-planet distances
from the star (from 5 to 30 Astronomical
Units or AU; 1 AU is the average distance between the Earth and the Sun).
Moreover, no obvious explanation exists for its origin other than the
gravitational influence of planets. The different inclinations of dust grain
orbits around Beta Pic bear a resemblance to those of planetary orbits in
our own solar system. Pluto's orbit is inclined by 17 degrees compared to
Earth's, and Mercury's differs by 7 degrees, for example. The new Keck
images may be interpreted as circumstantial evidence for a similarly
organized planetary system." 

Dr. Weinberger added, "The images show the power of large ground-based
telescopes, like Keck, to reveal disk details in the hot inner portions of
disks." In addition to imaging, Weinberger and colleagues obtained spectra
at different locations along the disk using the same Keck instrument (LWS).
Spectroscopy spreads the disk radiation into component wavelengths, much the
same way that a prism divides up visible light. The result enables
astronomers to study composition
as well as geometry. Weinberger's group found that, at the position of the
newly discovered warp, the disk is composed of small silicate particles that
are hotter than expected. Weinberger says, "It may be that as a planet warps
the disk, it also causes more collisions of rocks in its neighborhood." The
very small grains produced in collisions would tend to be hotter, at the
same distance from the star, than larger dust grains. Outside the warp, in
the outer part of the disk, the disk light appears to come either from
larger grains or from dust that is composed of something other than
silicates. 

To ensure that the observed offset was not the product of optical distortion
in either the atmosphere or telescope, Zahed Wahhaj carried out computer
modeling of the Keck image using a disk model and images of a nearby star
that were taken at the same time. His analysis provides an estimate of the
uncertainty in the measured value of the offset. "We generated millions of
different computer models of disks and used them to simulate images of Beta
Pic as
observed with the Keck telescope. Computational comparisons of the models
with the images showed that the inner disk is offset from the outer disk by
an angle somewhere between 10 and 18 degrees. This is in good agreement with
a value between 11 and 15 degrees, as determined by the other team." 

Beta Pictoris is a young star about 20 million years old that is located 63
light years away in the constellation Pictor (the painter's easel). The star
is located too far south to be visible from the continental United States,
but it can be seen in winter from Hawaii where it rises just 20 degrees
above the horizon. In 1983, astronomers discovered dust radiation, first
from Vega, and later from Beta Pictoris using the Infrared Astronomical
Satellite (IRAS). 

The Gillett Symposium commemorates Fred Gillett's role in the discovery of
the first IRAS disk detection around Vega and is being held in his memory
one year after his death. Subsequent telescope observations of Beta Pic
yielded the first image of a protoplanetary disk. Like all
observations carried out at visible wavelengths, it required a coronagraph
to block out the glare from the central star. As a consequence, the region
of the disk corresponding to our solar system was not discernible for study.
The human eye is insensitive to the infrared light collected in the new Keck
observations of Beta Pic. The contrast between star and disk radiation is
more favorable, however, so the Jovian planet region was discernible for the
first time. 

The W. M. Keck Observatory provides astronomers from associated institutions
access to two 10-meter telescopes, the world's largest. Each telescope
features a revolutionary primary mirror composed of 36 hexagonal segments
that work in concert as a single piece of reflective glass to provide
unprecedented power and precision. Each telescope stands eight stories tall
and weighs 300 tons, yet operates with nanometer precision. The observatory
is operated by the California
Association for Research in Astronomy, a partnership of the California
Institute of Technology, the University of California, and the National
Aeronautics and Space Administration (NASA), which joined the partnership in
October 1996. For more information, visit the W. M. Keck Observatory Web
site at www.keckobservatory.org or send e-mail to: www@keck.hawaiidot edu .

IMAGE CAPTIONS:

[Image 1: http://www.hep.upenndot edu/~davidk/bigpress2.gif (731KB)] Dust
around the young nearby star, Beta Pictoris. This image was made with the
Keck II 10-meter (400 inch) telescopes using an infrared camera operating at
18 microns. The inner contours are misaligned with respect to the outer
disk, and provide evidence of a newly discovered warp in the disk (labeled
as "A"). For comparison, an image of reflected light from Beta Pic is shown,
as it appears in observations
taken with the Space Telescope Imaging Spectrometer (STIS) on board the
Hubble Space Telesope (HST). The HST/STIS image is exaggerated in vertical
scale to show a warp which occure further out and in the opposite direction
from that seen in the Keck infrared image. This morphology can be reproduced
as an inner disk with radius 5 to 30 AU and an orbital inclination that is
offset 14 +/- 4 degrees from the large outer disk, and in the opposite sense
of the HST/STIS warp. "B" refers to lobes equidistant from the star that are
consistent with a 40-AU-radius ring or bright inner edge of the outer disk.
"C" is a peak that could be associated with a ring further out that is not
azimuthally symmetric (i.e., its counterpart on the other side of the star
is not very prominent).

[Image 2: http://www.hep.upenndot edu/~davidk/bigmod80.gif (94KB)
Image 3: http://www.hep.upenndot edu/~davidk/bigmod89.gif (21KB)] Examples of
computer representation of the infrared emission from Beta Pic, before the
images were blurred for comparison to Keck results. Viewing angles are 10
degrees above the disk plane (upper [Image 2]) and along the line of sight
from Earth to Beta Pic (lower [Image 3]).
  
============================
* LETTERS TO THE MODERATOR *
============================

(9) COMET ENCOUNTERS REVISITED

>From Roy Tucker <tucker@noaodot edu>

Hi Bob,

There are several important points to note when discussing the old cometary
dust issue. If one refers to the original Hoyle/Wickramasingh paper, they
note that an encounter of the earth with a "giant comet" may be expected to
occur only about once in 100 million years, certainly much more
rarely than the millennial time scales that some had speculated. Also, to
make the encounter described in my simple model as severe as possible, I
stipulated that the ENTIRE MASS OF THE COMET was converted to dust and
uniformly distributed within a spherical volume of space with a radius equal
to the orbit of the moon around the earth. My model also had the earth
making a central passage through this spherical cloud to make the encounter
last as long as possible.

Your computation of the energy produced by such a modeled encounter is
correct, the kinetic energy of the dust impacting the atmosphere would equal
about 50% the energy arriving from the sun. The worst possible case would be
if the energy was deposited upon the daytime side in summer under clear
skies. This would be a very warm day, indeed!

HOWEVER, my model of this encounter with a comet is very unrealistic. My
intent was to try to determine an absolute maximum upper limit to dust
deposition for comparison with what has been observed from volcanic events.
In reality, One would not expect to have an entire comet dispersed into a
spherical cloud of dust just before encountering the earth. A real comet
will have only a tiny amount of its mass in the form of a dusty coma at any
one time, and this dust will be very non-uniformly distributed. A more
realistic "worst-possible-case" encounter would have the comet's nucleus
passing just outside the earth's atmosphere on the sunward side. In this
particular case, tidal forces would disrupt the nucleus and a tremendous
burst of dust could be dumped into the atmosphere but the event would last
only seconds or minutes at the most. Such a close approach with a "great
comet" would be extremely rare and may indeed not have occurred during the
history of the solar system. An actual impact or more remote approach would
be more likely.

During the discussions precipitated by my original posting, it became
apparent that the greatest environmental consequence of a non-impact
encounter with a real cometary coma would be due to the introduction into
the upper atmosphere of fine dust with sizes on the order of microns and the
resultant reduction in atmospheric transparency. Even in the highly rarified
upper atmosphere, settling times may be on the order of months or years,
neglecting electrostatic effects in the atmosphere. However, the dust
density to be found in cometary comae and what fraction of this dust is of
micron size is still not well determined. It is certain to be far below my
model.

I enjoy your postings on CCNET and I thank you for the opportunity to
further discuss the cometary dust model.

				Best regards,
				  - Roy

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