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Comet LINEAR (C/1999 S4) surprised astronomers during the summer of
2000 by pulling a disappearance act: its nucleus completely disrupted
into a trail of debris on or about July 22nd. The principal scientific
results are published in the
18 May 2001 issue of Science magazine.
For your convenience, we provide here a
listing
of those papers, their authors, and a brief description
of the observations presented by each.
Below we discuss some of the principal scientific questions
posed by the demise of C/LINEAR.
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Did C/LINEAR completely disintegrate?
Yes, the nucleus, which was originally a single object about a
kilometer (0.62 miles) across, was reduced to a shower of fine dust
particles (micrometers in size) and to some football field sized
fragments (roughly 100 meters or 100 yards across) that were picked up
by the Earth-orbiting Hubble Space Telescope (HST) and the Very Large
Telescope (VLT) in Chile. But the amount of material in the dust and
large fragments is about 100 times less massive than the original
nucleus, which creates a
missing mass problem.
Apparently, much of the material in the original nucleus ended
up in debris that was between about 1 cm and 50 meters in size,
which we could not see directly with telescopes because the objects
in that size range didn't have very much surface area to scatter
sunlight. The tiny dust particles from C/LINEAR outshined everything
else because they have so much surface area (think about shattering
something to bits and comparing the surface area of the bits to
the surface area of the original object; the bits win hands down),
but they contain very little of the mass.
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Are the fragments created by the breakup of C/LINEAR the
primordial planetesimals that came together to form its nucleus?
Possibly, but it's difficult to say for sure. The evidence from the
breakup suggests that C/LINEAR fell apart rather gently, thereby
preserving the signature of its original ``rubble pile'' structure. In
other words, it's not as if someone stuck a piece of dynamite in the
nucleus and blew it to bits, which would have wiped out any evidence of
the formation process. In the case of C/LINEAR, we think that
observing the unraveling of its nucleus is similar to watching its
formation 4.6 billion years ago in reverse. Understanding how cometary
nuclei formed is critical for understanding how the rest of the Solar
System formed because comets are the basic building blocks for the
cores of the Giant Planets (Jupiter, Saturn, Uranus, and Neptune).
The results on the breakup of C/LINEAR will help to refine the
models being developed to explain the formation of the Solar System.
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What triggered the comet's fragmentation?
We don't know for sure. We do know that tidal disruption, which was
the mechanism responsible for tearing apart Comet Shoemaker-Levy 9
when it passed close to Jupiter in 1992, was not
responsible for the demise of C/LINEAR because C/LINEAR didn't
pass close enough to any planet. Also, C/LINEAR's nucleus had very little
supervolatile carbon monoxide (CO) ice, so pressure
buildup from the vigorous sublimation (going directly from the solid to vapor phase) of CO ice in the interior of the nucleus was probably not
responsible for the demise either. Some contributing factors to
C/LINEAR's breakup may have included:
- centrifugal forces due to fast rotation (if the comet's
rotational period was as fast as 2 to 3 hours, as suggested
by some unpublished observations),
- lack of icy ``glue'' (icy material sticks together better
than rocky material and C/LINEAR seems to have been depleted
in icy material compared to other comets),
- a combination of increased heating as the comet got
closer to the Sun, a sunward-pointing rotational axis (which
concentrates the heating at the pole), and internal pressure
buildup from the sublimation of supervolatiles
(even though carbon monoxide was depleted, maybe carbon dioxide
was not).
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What are the implications for the physical structure of
cometary nuclei?
Since the forces that disrupted C/LINEAR were apparently very weak
(you could easily pull the nucleus of C/LINEAR apart with your
bare hands), the observations of C/LINEAR have reinforced the
idea that at least some comets are ``rubble piles'' having
high porosity and held together by little more than the
mutual gravity of the individual pieces comprising the
nucleus. Some detailed models for rubble pile nuclei, for
example that of Stuart Weidenschilling of the Planetary
Science Institute in Tucson, Arizona, suggest that the
largest components of the rubble pile should be about
100 meters across, i.e., similar in size to the largest
C/LINEAR fragments detected by the HST and VLT observations.
However, unlike the model, the observations of C/LINEAR
suggest that most of the mass in the debris created after
the breakup ended up in objects between about 1 cm and
50 meters in size. So the principal building blocks of
cometary nuclei might be somewhat smaller than originally
envisaged and there may have to be some fine-tuning
of the current models for cometary formation.
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Where did C/LINEAR form?
There are two pieces of evidence suggesting that C/LINEAR formed
relatively close to the Sun, compared to other comets, possibly near
the orbit of Jupiter. First, infrared observations by a team led by
Mike Mumma showed that C/LINEAR was strongly depleted of
``supervolatiles'', such as carbon monoxide, methane, and ethane.
Secondly, by comparing the total amount of ice measured by the
SWAN/SOHO instrument (by Teemu Makinen and his team) to the mass
estimated for the original nucleus (by Tony Farnham and his team), one
concludes that C/LINEAR had much more meteoritic material than icy
material (which makes C/LINEAR more akin to a ``snowy dirtball'' than a
``dirty snowball''). Both pieces of evidence point towards a formation
of C/LINEAR's nucleus relatively close to the Sun. Since most of the
mass in the dust disk from which the planets formed 4.6 billion years
ago was near Jupiter, and since the temperatures in that region were
warm enough to prevent supervolatile retention, while cold enough to
allow some ice accumulation, a formation site near Jupiter seems
reasonable.
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Was C/LINEAR homogeneous in composition?
One of the nice things about having a comet breakup completely is that
we get a rare opportunity to examine the innards of the nucleus. Radio
observations (by Dominique Bockelee-Morvan and her team) of the gases
evolved from the debris after the breakup show that the chemical
composition of the interior of C/LINEAR is essentially identical to the
composition of its surface, which is what we were sampling prior to
breakup. So it appears, at least for ``new'' comets like C/LINEAR that
are making their first passage through the inner Solar System, that
cometary nuclei do not show strong differentiation
with depth.
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