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(meteorobs) Excerpts from "CCNet 69/2001 - 18 May 2001"
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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 69/2001 - 18 May 2001
Date: Fri, 18 May 2001 10:13:25 +0100
CCNet 69/2001 - 18 May 2001
----------------------------
"A comet that shattered on its approach to the Sun breathed new life
into the theory that comet impacts provided most of the water in Earth's
oceans. The same NASA observations of the comet, designated C/1999 S4
LINEAR (LINEAR), also support the idea that comet impacts furnished a
significant amount of the organic molecules used in life that later
arose on Earth."
Mark Hess, Goddard Space Flight Center, 17 May 2001
[...]
(2) ASTRONOMERS PUZZLED OVER COMET LINEAR'S MISSING PIECES
Andrew Yee <ayee@nova.astro.utorontodot ca>
(3) DYING COMET'S KIN MAY HAVE NOURISHED LIFE ON EARTH
Mark Hess <mhess@pop100.gsfc.nasadot gov>
[...]
(5) SOHO'S UNIQUE VIEW OF A COMET THAT FELL TO PIECES
ESA Science News, 18 May 2001
[...]
======================================================================
(2) ASTRONOMERS PUZZLED OVER COMET LINEAR'S MISSING PIECES
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
CONTACT:
Donna Weaver
Space Telescope Science Institute, Baltimore, MD 21218
(Phone: 410-338-4493, E-mail: dweaver@stscidot edu)
Hal Weaver
The Johns Hopkins University,6-4251, E-mail: weaver@pha.jhudot edu)
EMBARGOED UNTIL: 2:00 p.m. (EDT) Thursday, May 17, 2001
PRESS RELEASE NO.: STScI-PR01-14
ASTRONOMERS PUZZLED OVER COMET LINEAR'S MISSING PIECES
Astronomers analyzing debris from a comet that broke apart last summer spied
pieces as small as smoke-sized particles and as large as
football-field-sized fragments. But it's the material they didn't see that
has aroused their curiosity.
Tracking the doomed comet, named C/1999 S4 (LINEAR), NASA's Hubble Space
Telescope's Wide Field and Planetary Camera 2 found tiny particles that made
up the 62,000-mile-long (100,000-kilometer-long) dust tail and 16 large
fragments, some as wide as 330 feet (100 meters). Hubble detected the small
particles in the dust tail because, together, they occupy a large surface
area, which makes them stand out in reflected sunlight. However, the
estimated mass of the observed debris doesn't match up to the comet's bulk
before it cracked up.
"The mass of the original, intact nucleus is estimated to be about 660
billion pounds (300 billion kilograms), according to some ground-based
observers who were measuring its gas output," says Hal Weaver, an astronomer
at the Johns Hopkins University in Baltimore, Md., who studied the comet
with the Hubble telescope, the European Southern Observatory's Very Large
Telescope (VLT) in Chile, and other ground-based telescopes.
"However, the total mass in the largest fragments measured by the Hubble
telescope and the VLT is only about 6.6 billion pounds (3 billion
kilograms), and the dust tail has an even smaller mass of about 0.7 billion
pounds (0.3 billion kilograms). In other words, the total mass measured
following the breakup is about 100 times less than the estimated total mass
prior to the
breakup." Weaver's results will be published in a special May 18 issue of
Science devoted to the transitory comet.
So where is the rest of the comet's fractured nucleus? Perhaps, suggest
Weaver and other investigators, most of the comet's bulk after the breakup
was contained in pieces between about 0.1 inches (2.5 millimeters) and 160
feet (50 meters) across. These pebble-sized to house-sized fragments cannot
be seen by visible-light telescopes because they do not have enough surface
area to make them stand out in reflected sunlight. Comets are leftover
debris from the creation of the solar system 4.6 million years ago. They're
made up of a combination of solid rock and frozen gases held together by
gravity.
If the midsized cometary fragments exist, then the fundamental building
blocks that comprised LINEAR's nucleus may be somewhat smaller than what
current "rubble pile" theories of the solar system's formation suggest.
These theories generally favor football-field-sized fragments, like the ones
observed by the VLT and the Hubble telescope. The analysis of LINEAR's
fragments indicates that the "rubble" comprising cometary nuclei may be
somewhat smaller than previously thought.
Another puzzling question is why the comet broke apart between June and July
of last year as it made its closest approach to the Sun.
"We still don't know what triggered the comet's demise," Weaver says. "But
we do know that carbon monoxide (CO) ice probably did not contribute to the
breakup."
Hubble's Space Telescope Imaging Spectrograph detected low levels of this
volatile material, about 50 times less than was observed in comets Hale-Bopp
and Hyakutake. Carbon monoxide ice sublimates [changes directly from a solid
to a vapor] vigorously, even at the cold temperatures in a comet's interior.
This activity could lead to a buildup of pressure within the core that might
cause the nucleus to fragment.
"The scarcity of carbon monoxide in LINEAR's nucleus is problematic for any
theory that attempts to invoke it as the trigger for the comet's demise,"
Weaver says.
An armada of observatories, including the Hubble telescope, watched the
dazzling end to the transitory comet. Hubble was the first observatory to
witness LINEAR breaking apart, spying in early July a small piece of the
nucleus flowing down the doomed comet's tail. LINEAR completely
disintegrated in late July as it made its closest approach to the Sun, at a
cozy 71 million miles. Again, the Hubble telescope tracked the comet,
finding at least 16 fragments that resembled "mini-comets" with tails. Now
LINEAR is little more than a trail of debris orbiting the Sun. The comet is
believed to have wandered into the inner solar system from its home in the
Oort Cloud, a reservoir of space debris on the outskirts of the solar
system.
"We were witnessing a rare view of a comet falling to pieces," Weaver says.
"These observations are important because, by watching comet LINEAR unravel,
we are essentially seeing its formation in reverse. The nucleus was put
together 4.6 billion years ago when the Earth and other planets were
forming, so by watching the breakup we are looking backwards in time and
learning about conditions during the birth of the solar system."
Weaver notes, however, that astronomers may have witnessed an "oddball"
comet break apart.
"I've never seen anything like this," he says. "I know of no other example
of a comet falling to pieces like this. Comet Shoemaker-Levy 9 fell apart,
but tidal forces from Jupiter caused that disintegration. LINEAR didn't come
close to any other large object. Comet Tabur (C/1996 Q1) also seemed to
vanish without a trace, but it already was the fragment of another comet
nucleus [C/1988 A1 (Liller)]. Some investigators concluded that Tabur did
not even break up but rather, became 'invisible' only because the icy area
on its surface was no longer in sunlight, and its activity shut down as a
result."
Comet LINEAR was named for the observatory that first spotted it, the
Lincoln Near Earth Asteroid Research (LINEAR) program.
The Space Telescope Science Institute (STScI) is operated by the Association
of Universities for Research in Astronomy, Inc. (AURA), for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble
Space Telescope is a project of international cooperation between NASA and
the European Space Agency (ESA).
There are no new Hubble pictures, but previously released Hubble Space
Telescope images of Comet LINEAR's breakup are available on the Web at:
http://oposite.stscidot edu/pubinfo/pr/2000/26/index.html
and
http://oposite.stscidot edu/pubinfo/pr/2000/27/index.html
======================================================================
(3) DYING COMET'S KIN MAY HAVE NOURISHED LIFE ON EARTH
>From Mark Hess < mhess@pop100.gsfc.nasadot gov >
William Steigerwald EMBARGOED FOR RELEASE
William.A.Steigerwald.1@gsfc.nasadot gov May 17, 2001 at 2:00 p.m.
EDT
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5017)
Release No. 01-46
DYING COMET'S KIN MAY HAVE NOURISHED LIFE ON EARTH
A comet that shattered on its approach to the Sun breathed new life into the
theory that comet impacts provided most of the water in Earth's oceans. The
same NASA observations of the comet, designated C/1999 S4 LINEAR (LINEAR),
also support the idea that comet impacts furnished a significant amount of
the organic molecules used in life that later arose on Earth.
LINEAR was the first comet with a chemistry that indicated its water had the
same isotopic composition as the water actually found on Earth.
"The idea that comets seeded life on Earth with water and essential
molecular building blocks is hotly debated, and for the first time, we have
seen a comet with the right composition to do the job," said Dr. Michael
Mumma of NASA's Goddard Space Flight Center in Greenbelt, Md. Mumma is lead
author of a paper about this research to appear in the May 18 issue of
Science.
A separate announcement, also to appear in the May 18 Science, is a unique
observation that reveals just how much water comets of this type can carry.
LINEAR, with a nucleus estimated at 2,500 to 3,300 feet (about 750 to 1,000
meters) in diameter, carried about 3.6 million tons (3.3 billion kilograms)
of water within its bulk, according to astronomers who used the Solar Wind
Anisotropies instrument on the Solar and Heliospheric Observatory spacecraft
to observe water vapor released from the comet as it fragmented.
Using telescopes sensitive to infrared light, Mumma and a team of
astronomers studied comet LINEAR before its dramatic breakup last July and
determined that its unusual chemistry points to an origin near Jupiter's
orbit. Comets that formed in this region are expected to have the same ratio
of normal water to "heavy" water as found in Earth's oceans.
Although it would appear that all water molecules are identical -- two atoms
of hydrogen joined to one oxygen atom -- this isn't the case. Hydrogen comes
in different types (isotopes) that behave the same way chemically but are
heavier due to an extra component (one or more neutrons) in their nuclei.
One such heavy cousin of hydrogen is called deuterium (one extra neutron).
Based on very low-temperature experiments of gas chemical reactions, water
ice incorporated in comets that formed far from the Sun (near Neptune's
orbit, for example) should have a greater deuterium to hydrogen (D to H)
ratio than the water found on Earth.
Recent observations of comets Halley, Hyakutake, and Hale-Bopp confirm this,
leading researchers to believe that these comets formed further from the Sun
than LINEAR. Pinpointing the origin of these comets was remarkable, but it
provided no support for the cometary origin of water on Earth.
The chemistry of LINEAR, however, indicated that it formed in warmer regions
closer to the Sun. For example, it had much less carbon monoxide (CO),
methane (CH4), ethane (C2H6), and acetylene (C2H2) than typical
remote-origin comets like Halley. These volatile organic molecules freeze at
extremely cold temperatures, so it appears that LINEAR formed in a place
where it was too warm to incorporate a great deal of these volatile
molecules into its ices.
However, the same low-temperature experiments that successfully predicted
the correct D to H ratio in remote-origin comets predict that a comet
forming in a warmer Jupiter orbit region should have the same D to H ratio
as Earth's water. LINEAR broke up before this could be confirmed, but its
low amount of volatile organic molecules provides a strong indication that
it carried the same kind of water that comprises terrestrial seas.
LINEAR is believed to have arrived from the Oort cloud, a vast comet swarm
surrounding the frigid distant regions of the solar system, trillions of
miles from the Sun. According to theories of the solar system's formation,
these comets formed from the same gas and dust cloud that gave rise to the
planets and the Sun. They accumulated in the colder regions where the gas
giant planets are found today (Jupiter - Neptune). Gravity from the gas
giants kicked the comets out of the solar system, either to interstellar
space or to the Oort cloud region. Occasionally, the Oort cloud is
perturbed, perhaps by the gravity of a passing star, returning some comets
to the inner solar system. The amount of various molecules incorporated into
a comet's ices depends on temperature, so determining a comet's chemistry
reveals where in the gas giant region the comet formed.
As the most massive planet in the solar system, Jupiter's gravity was so
powerful that it shoved most comets near it into interstellar space, while
the lesser gravity from the smaller gas giants gave comets near them a
gentler push, landing a greater portion in the Oort cloud.
Consequently, comets that formed near Jupiter are rare today, but they would
have been in the majority during the solar system's formation, simply
because the Jupiter orbit region had most of the material in the
pre-planetary gas and dust cloud. Therefore, scientists expect that the
primordial Earth would have intercepted more comets formed near Jupiter's
region than those formed elsewhere.
Because Jupiter's region was closer to the Sun than the other gas giant
planets, it received more light and was warmer, so more reactions occurred
in the gas. Thus, greater amounts of complex organic molecules were
available to wind up in a comet. Also, Jupiter's powerful gravity kept
collision speeds between comets near it high, preventing them from growing
very large. Both factors may have given a boost to life on Earth.
"It's like being hit by a snowball instead of an iceberg," said Mumma. "The
smaller comets from Jupiter's region impacted Earth relatively gently,
shattering high in the atmosphere and delivering most of their organic
molecules intact. Also, these comets would have had a greater portion of
life's building blocks -- the complex organic molecules -- to begin with.
This means life on Earth did not have to start completely from scratch.
Instead, it was delivered in kit form from space."
The team used infrared-sensitive instruments on telescopes at the W. M. Keck
Observatory and the NASA Infrared Telescope Facility, both on Mauna Kea,
Hawaii, to make the observations. Heat and light from the Sun caused
material from LINEAR to evaporate into space and form a gas cloud around the
comet as it entered the solar system. Sunlight energized molecules in the
gas cloud surrounding LINEAR, allowing the team to identify the comet's
chemistry by the unique types of infrared light emitted by its various
molecular components. Comet LINEAR was named for the observatory that first
spotted it, the Lincoln Near Earth Asteroid Research (LINEAR) program.
For more information and pictures, refer to:
http://www.gsfc.nasadot gov/GSFC/SpaceSci/origins/linearwater/linearwater.htm
======================================================================
(5) SOHO'S UNIQUE VIEW OF A COMET THAT FELL TO PIECES
>From ESA Science News
http://sci.esa.int
18 May 2001
SOHO's unique view of a comet that fell to pieces
When Spain's Instituto de Astrofisica de Canarias reported on 28 July 2000
that an ordinary-looking comet was breaking up, some of the world's top
telescopes watched its subsequent disintegration till nothing was left. The
French-Finnish SWAN instrument on the SOHO spacecraft had already been
observing Comet LINEAR by ultraviolet light for two months, and continued to
watch it till the remnants faded from view in mid-August. Today the SWAN
team reports, in the journal Science, that their observations showed four
major outbursts in June and July.
The fragmentation seen by SWAN began on 21 July, almost a week before
observers on the ground noticed it. Between 25 May and 12 August, the dying
comet released altogether 3.3 million tonnes of water vapour into space, as
its ice evaporated in the warmth of the Sun. The data also suggest that the
density of Comet LINEAR was extremely low.
"Only SWAN on SOHO saw the entire drama of this self-destroying object,"
comments Teemu Mdkinen of the Finnish Meteorological Institute, lead author
of the report in Science. "The ice on the surface of the comet's nucleus did
not simply vaporize as in a normal comet, but came away in large chunks. We
saw 90 per cent of the ice falling off before the complete fragmentation of
the remainder began."
Comet LINEAR, known more formally as Comet 1999 S4, was discovered by the
LINEAR asteroid-hunting telescope in the USA, and may have been making its
first visit to the Sun. It disappointed amateur astronomers by not becoming
bright enough to see with the naked eye. The break-up occurred near the time
of the comet's closest approach to the Sun on 26 July, when it was moving
across the sky from Ursa Major towards Leo.
In early August the NASA-ESA Hubble Space Telescope and the European
Southern Observatory's Very Large Telescope in Chile both saw about 16
fragments in the form of mini-comets, which faded away by the middle of the
month. These observations by visible light indicated that the pieces were
about 100 metres in diameter. A prominent dust tail still visible in early
August corresponded with the onset of fragmentation seen by SWAN on 21 July.
SWAN's unique capability in observing comets comes from its continuous
scanning of the whole sky, at just the right ultraviolet wavelength to see
the cloud of hydrogen atoms that surrounds every moderately active comet.
The hydrogen comes from the break-up of water molecules released from the
comet by the Sun's warmth. SWAN also benefits from its location on the
ESA-NASA SOHO spacecraft 1.5 million kilometres from the Earth, well clear
of a hydrogen cloud that surrounds the Earth itself.
"Our primary aim is to study the interaction of the solar wind with
interstellar hydrogen," explains Jean-Loup Bertaux of France's Service
d'Aironomie, the principal investigator for SWAN. "But we always knew that
we'd have an excellent view of comets too. They are quite often traceable in
our records even before their formal discovery by others."
Lessons from the SWAN song of Comet LINEAR
Complete fragmentation provides a rare opportunity for scientists to learn
about the internal make-up of a comet. Members of the SWAN team believe that
their newly published results compel them and their fellow scientists to
think afresh about Comet LINEAR's construction, and to consider that
different parts of the young Solar System may have produced comets of
different sorts.
"Comets do not usually blow themselves to smithereens," says lead author
Mdkinen. "So we should not be surprised if Comet LINEAR was peculiar in
composition and structure compared with other comets."
The character of the comet did not change throughout the months of
observation by SWAN, even when deep layers inside the nucleus were being
laid bare. Comet scientists usually have to consider the possibility that
the surface of the nucleus is different in composition from the interior.
One lesson from the 'SWAN song' of Comet LINEAR seems to be that, in this
case at least, the surface exposed at the outset was representative of the
whole nucleus.
The SWAN team also suspects that Comet LINEAR was as flimsy and light as the
expanded polystyrene used for packing fragile equipment. The density of its
water ice may have been as low as 15 kilograms per cubic meter, compared
with 917 kg/m3 for familiar non-porous ice on the Earth. Even allowing for a
possibly equal mass of dust grains within the comet, a total density of 30
kg/m3 would be far less than the 500 kg/m3 often assumed by comet
scientists. By this reckoning, the initial diameter of Comet LINEAR on its
approach to the Sun was about 750 metres.
"Our opinion about the low density is tentative and controversial," says
Jean-Loup Bertaux. "We expect plenty of arguments with our colleagues when
we put all the observations of Comet LINEAR together. But we start with the
advantage of having seen the whole course of events, which no
one else did."
The break-up of Comet LINEAR gave a small-scale impression of the
disintegration, many centuries ago, of a far larger comet into an enormous
swarm of mini-comets. LASCO, another instrument on SOHO, has observed
hundreds of the fragments from that event falling into the Sun.
For more information please contact:
Dr. Paal Brekke, ESA-SOHO Deputy Project Scientist
Tel: +1 301 286 6983 / +1 301 996 9028
Fax: +1 301 286 0264
Email:pbrekke@esa.nascom.nasadot gov
Dr. Teemu Mdkinen, SWAN scientist, Finnish Meteorological Institute
Tel: +358-9-1929-4647
Fax: +358-9-1929-4603
Email:teemu.makinen@fmi.fi
Dr. Jean-Loup Bertaux, Service d'Aeronomie du CNRS
Tel : 33-(0)1-64 47 42 51
Fax : 33-(0)1-69 20 29 99
Email:bertaux@aerov.jussieu.fr
USEFUL LINKS FOR THIS STORY
* SWAN home page (Finnish Meteorological Institute)
http://www.fmi.fi/research_space/space_7.html
* SWAN home page (Service d'Aeronomie, France)
http://www.aero.jussieu.fr/~jgoutail/
* SOHO home page (at the ESA Science web site)
http://sci.esa.int/soho/
* SOHO mission
http://sohowww.estec.esadot nl/
* SOHO's comet discoveries
http://sci.esa.int/content/news/index.cfm?aid=14&cid=2097&oid=12363
* SOHO analyses a kamikaze comet
http://sci.esa.int/content/news/index.cfm?aid=14&cid=2097&oid=26188
* SOHO sees two comets plunge into Sun
http://sci.esa.int/content/news/index.cfm?aid=14&cid=2097&oid=12206
IMAGE CAPTIONS:
[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=2708
8&ooid=27113]
The hydrogen cloud around Comet LINEAR as observed by the SWAN instrument on
SOHO on 26 June 2000, almost a month before the comet disintegrated. The
field of view is 21 million kilometres wide. Credit: SOHO/SWAN (ESA & NASA)
& J.T.T. Mdkinen et al.
[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=2708
8&ooid=27114]
Fragments of Comet LINEAR seen as mini-comets by the Hubble Space Telescope
on 5 August 2000. Part of a dust tail is visible at top right. Credit:
HST/WFPC2 (NASA & ESA) & H.A. Weaver et al.
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