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CCNet 144/2002 -  16 December 2002
--------------------------------

"Since the early 1970s, astronomers have speculated about the danger
posed to our planet by exploding stars called supernovae. Among the
negative aspects to such an event would be the sudden depletion of Earth's
protective ozone layer, the thinking goes. Left naked to space, we
might then be fried by the UV rays of our own Sun. Researchers have
suggested that one or more mass extinctions during the past few hundred
million years might have been triggered by supernovae, and that it might
happen again. But a new and detailed set of calculations shows that such
events are probably extremely rare."
--Rob Britt, Space.com, 16 December 2002


(1) LUNAR CRASH OF 1953: IMPACT CRATER IDENTIFIED
    Space.com, 14 December 2002

(2) IDENTIFICATION OF THE LUNAR FLASH OF 1953
    http://www.aas.org/publications/baas/v33n3/dps2001/463.htm

(3) EARTH'S VOLCANISM LINKED TO IMPACTS? DOUBTS ABOUT RELIABLE DATING
    New Scientist, 13 December 2002

(4) EXPLODING STARS: THREAT TO EARTH EXTREMELY REMOTE
    Space.com, 16 December 2002

(5) TSUNAMI GENERATED BY ASTEROID IMPACTS
    Michael Paine <mpaine@tpg.com.au>

(6) ASTEROID MOONS PULLED IN BY GRAVITY
    UPI News, 11 December 2002

(7) NEW MODEL FOR JOVIAN SATELLITE FORMATION
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(8) NEW THEORY ACCOUNTS FOR EXISTENCE OF BINARIES IN KUIPER BELT
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(9) AND FINALLY: MYSTERY TRAINS
    Sky & Telescope, 10 December 2002

============
(1) LUNAR CRASH OF 1953: IMPACT CRATER IDENTIFIED

>From Space.com, 14 December 2002
http://www.space.com/missionlaunches/lunar_impact_021214.html

By Leonard David
 
Just a few decades ago, Earth's Moon was on the receiving end of an
asteroid-sized body that slammed into the lunar surface.

But finding telltale evidence from the celestial smacking proved elusive.
The crater would be well below the resolution limit of Earth-based
telescopes.

A research team now believes they've spotted the lunar leftovers from the
impact, caught in images taken by a robotic lunar orbiter.

Eagle-eye scientists 
 
In 1956, an amateur astronomer -- Leon H. Stuart -- reported in the
Strolling Astronomer, that he had observed and photographed a flash a few
years earlier on the Moon. This event is the only unambiguous record of the
crash of an asteroid-sized body onto the lunar surface.

Now, decades later, a study of lunar images snapped by the Clementine
spacecraft as it orbited the Moon in 1994 has uncovered a candidate crater
formed by the impact.

Eagle-eye scientists, Bonnie Buratti of NASA's Jet Propulsion Laboratory and
Lane Johnson of Pomona College in Claremont, California have locating a near
mile across (1.5-kilometer) feature with a fresh-appearing ejecta blanket at
the location of the flash. Spectral analysis of the crater, they report,
reveals it to be bluer and fresher than other young craters.

Striking image

"We identified the crater on processed multispectral Clementine images. With
the correct processing procedure and analysis it is obvious," Buratti told
SPACE.com. The crater and resulting eject blanket of material tossed up by
the impact is small. So tiny, in fact, it's not visible from Earth or on
images taken by the Lunar Orbiter probes dispatched to the Moon in
preparation for the Apollo lunar landing expeditions, she added.

A full account of their research has been accepted for publication in a
forthcoming issue of Icarus, the prestigious professional space science
journal.

Buratti and Johnson estimate that the energy of the impact event was about
0.5 megatons, resulting in the newly found feature. The radius of the
impacting body was over 65-feet (20 meters). Such an event occurs every
10-50 years, they report.

Another result of their work suggests that the effects of space weathering
-- intense solar radiation and meteorites striking the surface -- takes
place very rapidly on the Moon, Buratti said.

The team's lunar detective work was performed in part at the Jet Propulsion
Laboratory/California Institute of Technology under contract to NASA, and
funded in part from a National Science Foundation grant.

Copyright 2002, Space.com

===========
(2) IDENTIFICATION OF THE LUNAR FLASH OF 1953

http://www.aas.org/publications/baas/v33n3/dps2001/463.htm
Identifcation of the lunar flash of 1953 with a fresh crater on the Moon's surface
L. Johnson (Pomona College), B. J. Buratti (Pomona College and JPL)

The 1953 photograph of a flash on the Moon by an amateur astronomer (Stuart,
1956) is the only unambiguous record of the rare crash of an asteroid-sized
body onto the lunar surface. We estimate that this event would have made a
1-2 km sized crater, and that the radius of the impacting body was about 170
m. Such an event would cause destruction of a large metropolitan-sized area
if it occurred on Earth. Although not detectable with ground-based
telescopes, this crater should be visible on space-based images of the Moon
obtained by the Lunar Orbiter and Clementine missions. A search of images
from the Clementine mission reveals a 2-km crater with a high-albedo, blue,
fresh-appearing ejecta blanket at the location of the flash. The crater is
not clearly visible on Lunar Orbiter images of the impact region. (The size
may be substantially overestimated, as the ejecta blanket cannot be
distinguished from the crater itself). The identification of this crater
offers an opportunity to investigate subsurface unaltered lunar soils. Our
results suggest that the effects of space weathering occur rapidly.

Funded by NASA and NSF

Stuart, L. (1956). The Strolling Astronomer. Vol 10, 42-43.

=============
(3) EARTH'S VOLCANISM LINKED TO IMPACTS? DOUBTS ABOUT RELIABLE DATING
 
>From New Scientist, 13 December 2002
http://www.newscientist.com/news/news.jsp?id=ns99993171
 
Large meteorite impacts may not just throw up huge dust clouds but also
punch right through the Earth's crust, triggering gigantic volcanic
eruptions.

The idea is controversial, but evidence is mounting that the Earth's geology
has largely been driven by such events. This would also explain why our
planet has so few impact crater remnants.

Counting the number of asteroids we see in the sky suggests that over the
past 250 million years, Earth should have been hit around 440 times by
asteroids larger than one kilometre across. But scientists have found only
38 large impact craters from this period.

Dallas Abbott from Columbia University and her colleague Ann Isley from the
State University of New York studied the timing of these 38 impacts and
found that they correlate strongly with eruptions of "mantle-plume"
volcanoes during the same period.
  
Deep impact

Most volcanoes come from small amounts of the Earth's upper mantle boiling
over, but mantle-plume volcanoes happen when hot rock from deep within the
Earth's mantle shoots straight up through the Earth's crust. The timing
suggests that these volcanoes are related to asteroid impacts, Abbott and
Isley report in Earth and Planetary Science Letters (vol 205, p 53).

Unreliable dates

Not everyone agrees. "I am not enthusiastic about the idea that impacts
systematically control Earth's activity," says Boris Ivanov from the
Institute of Geospheres Dynamics in Moscow. He has used computer models to
investigate the effect of meteorites on the Earth's crust, and says he does
not believe impacts are capable of having a significant effect on the
planet's geological processes.

And geochemist Christian Koeberl from Vienna University argues that the
dates Abbott used are not reliable. "The impacts and volcanoes can only be
correlated to within tens of millions of years," he says. "This doesn't
really prove anything."

But elsewhere, there is growing support for the idea that Earth's volcanism
may be closely entwined with meteorite impacts.

Massive surge

Adrian Jones and David Price from University College London say Abbott's
work backs up their recent computer simulations. These models suggest
meteorites bigger than about 10 kilometres across could sometimes punch
right through the Earth's crust, causing huge volcanic eruptions (Earth and
Planetary Science Letters, vol 202, p 551).

"A large impact has the ability to cause instant melting where it hits,
creating its own impact plume in the mantle and resulting in a massive surge
of lava spilling out," Jones explains.

Until now Abbott and Isley were not sure how impacts might trigger volcanic
eruptions, but the UCL model suggests a mechanism. It would also explain why
we do not see as many meteorite craters as we might expect, as the surges of
molten rock would obliterate them.

Double whammy

Jones speculates that many of the impact craters Abbott analysed could have
been created by mere fragments of bigger asteroids that hit elsewhere at the
same time and broke through the crust, ultimately leaving no trace.

For example, the 10 kilometre-wide asteroid that hit Chicxulub in Mexico 65
million years ago is widely blamed for wiping out the dinosaurs. But it
could have been a piece from a much bigger rock that hit India, triggering
the surge of volcanic activity known as the Deccan Traps.

"Many areas that exhibit extensive volcanism from the past, such as the
Deccan Traps and the Siberian Traps, may in fact be sites of gigantic
meteorite impacts," says Jones. Perhaps the dinosaurs would have survived a
meteorite impact alone, but the double whammy of a meteorite and volcanoes
pushed them to extinction.
 
Kate Ravilious

Copyright 2002, New Scientist

===========
(4) EXPLODING STARS: THREAT TO EARTH EXTREMELY REMOTE

>From Space.com, 16 December 2002
http://www.space.com/scienceastronomy/supernova_threat_021216.html

By Robert Roy Britt

There are plenty of potential perils for Earth and its inhabitants, from the
threat of global warming to the potential for a deadly asteroid strike or a
devastating nuclear disaster. So it's nice to hear about a risk that's gone
down a bit.

Since the early 1970s, astronomers have speculated about the danger posed to
our planet by exploding stars called supernovae. Among the negative aspects
to such an event would be the sudden depletion of Earth's protective ozone
layer, the thinking goes. Left naked to space, we might then be fried by the
UV rays of our own Sun.

Researchers have suggested that one or more mass extinctions during the past
few hundred million years might have been triggered by supernovae, and that
it might happen again.

But a new and detailed set of calculations shows that such events are
probably extremely rare.

The study, led by Neil [the son of Tom] Gehrels of NASA's Goddard Space
Flight Center, found that for a supernova to significantly deplete ozone it
would have to occur within 26 light-years of our planet. Other data shows
that this happens only about once in a billion years.

"This particular pathway for mass extinctions may be less important than
previously thought," Gehrels and his colleagues write in a paper to be
published in the March 10, 2003 issue of the Astrophysical Journal.

The standard supernova is not the only risk factor posed by stellar
explosions, however.

Details of the study

The researchers modeled the detailed chemical reactions that go on when a
supernova's energy reaches Earth's atmosphere, and they also examined how
much energy would actually survive a trip across space and get here.

The dangerous output of a supernova involves gamma rays, the most energetic
form of light, plus cosmic rays that arrive in the form of particles,
explains John Cannizzo, another Goddard researcher who worked on the study.
The emissions interact with nitrogen gas in Earth's atmosphere and break it
into atomic nitrogen and subsequently nitrogen oxides, which in turn break
down ozone.

"When one considers together all the ingredients which go into the complete
picture for ozone depletion from nearby supernovae, Cannizzo told SPACE.com,
"the total depletion of ozone is not all that great."

There's a bit of wiggle room in the results.

"The precise results depend sensitively on many assumptions, but it's the
best we have right now," said another member of the team, John Scalo, from
the University of Texas at Austin.

Other worries

There are other possible ill effects of supernovae and other energetic
astronomical events that were not considered in the new study. Separate
research has shown that exposure to high-energy particles from space, over
time, could cause genetic mutations. Mutations are not always a bad thing,
however. Scientists consider some mutations to be beneficial or at least
normal aspect of evolution.

Another worry are so-called hypernovae, which are related to mysterious
gamma-ray bursts in deep space. Astronomers believe these are similar to
supernovae but that a beam of concentrated energy, emanating along the
star's axis of rotation, happens to be pointed at Earth.

Though the new study did not look into the hypernovae hazard, Gehrels said
it's likely for one aimed at Earth to occur once every couple of hundred
million years somewhere in our galaxy, most of the time at a very large
distance from our planet, however.

Here's how the estimate is figured: A setup called the Burst and Transient
Source Experiment (BATSE) aboard NASA's Compton Gamma-Ray Observatory
detected gamma-ray bursts (and presumably the associated hypernovae) at the
rate of about 500 per year, or just more than one per day, in the observable
universe, out to about 14 billion light-years.

Given that there are some 100 billion galaxies in that space, this rate
translates into about one gamma ray burst from inside our galaxy beamed
toward Earth every 200 million years. Because the energy is so concentrated
compared to a normal supernova, hypernovae could potentially be harmful to
life on Earth at much greater distances than supernovae.

Copyright 2002, Space.com

=============
(5) TSUNAMI GENERATED BY ASTEROID IMPACTS

>From Michael Paine <mpaine@tpg.com.au>

Dear Benny

Two papers associated with tsunami generated by asteroid impacts have just
been published on the Science of Tsunami Hazards website at
http://www.sthjournal.org/sth3.htm. Abstracts follow

The first reminds me of my bursting balloon experiment
(http://www4.tpg.com.au/users/aoaug/tsuballn.html )

regards
Michael Paine

DYNAMICS OF WATER CAVITY GENERATION
Charles L. Mader, Los Alamos National Laboratory, Los Alamos, NM 87545 USA
Michael L. Gittings, Science Applications International Corp., Los Alamos,
NM 87545 USA

ABSTRACT
The hypervelocity impact (1.25 to 6 km/sec) of projectiles into water has
been studied at the University of Arizona by Gault and Sonett. They observed
quite different behavior of the water cavity as it expanded when the
atmospheric pressure was reduced from one to a tenth atmosphere. Above about
a third of an atmosphere, a jet of water formed above the expanding bubble
and a jet or "root" emerged below the bottom of the bubble. Similar results
were observed by Kedrinskii at the Institute of Hydrodynamics in
Novosibirsk, Russia when the water cavity was generated by exploding bridge
wires with jets and roots forming for normal atmospheric pressure and not
for reduced pressures. Earlier at the Los Alamos National Laboratory B. G.
Craig, reported observing the formation of jets and roots while the gas
cavity was expanding by bubbles generated by small spherical explosives
detonated near the water surface. During the last decade a compressible
Eulerian hydrodynamic code called SAGE has been under development by the Los
Alamos National Laboratory and Science Applications International (SAIC)
which has continuous adaptive mesh refinement (AMR) for following shocks and
contact discontinuities with a very fine grid while using a coarse grid in
smooth ow regions. A version of the SAGE code that models explosives called
NOBEL has been used to model the experimental geometries of Sonett and of
Craig. The experimental observations were reproduced as the atmospheric pressure was
varied. When the atmospheric pressure was increased the difference between the pressure
outside the ejecta plume above the water cavity and the decreasing pressure inside the water
plume and cavity as it expanded resulted in the ejecta plume converging and colliding
at the axis forming a jet of water proceeding above and back into the bubble
cavity along the axis. The jet proceeding back thru the bubble cavity
penetrates the bottom of the cavity and forms the root observed
experimentally. The complicated bubble collapse and resulting cavity descent
into deeper water was numerically reproduced. Now that a code is available
that can describe the experimentally observed features of projectile
interaction with the ocean, we have a tool that can be used to evaluate
impact landslide, projectile or asteroid interactions with the ocean and the
resulting generation of tsunami waves. Science of Tsunami Hazards, Volume
21, Number 2, page 91 (2003)

TWO- AND THREE-DIMENSIONAL SIMULATIONS OF ASTEROID OCEAN IMPACTS
Galen Gisler, Robert Weaver, Charles Mader Los Alamos National Laboratory
Los Alamos, NM, USA
Michael Gittings Science Applications International Los Alamos, NM, USA

We have performed a series of two-dimensional and three-dimensional
simulations of asteroid impacts into an ocean using the SAGE code from Los
Alamos National Laboratory and Science Applications International
Corporation. The SAGE code is a compressible Eulerian hydrodynamics code
using continuous adaptive mesh refinement for following discontinuities with
a fine grid while treating the bulk of the simulation more coarsely. We have
used realistic equations of state for the atmosphere, sea water, the oceanic
crust, and the mantle. In two dimensions, we simulated asteroid impactors
moving at 20 km/s vertically through an exponential atmosphere into a 5 km
deep ocean. The impactors were composed of mantle material (3.32 g/cc) or
iron (7.8 g/cc) with diameters from 250m to 10 km. In our three-dimensional
runs we simulated asteroids of 1 km diameter composed of iron moving at 20
km/s at angles of 45 and 60 degrees from the vertical. All impacts,
including the oblique ones, produce a large underwater cavities with nearly
vertical walls followed by a collapse starting from the bottom and
subsequent vertical jetting. Substantial amounts of water are vaporized and
lofted high into the atmosphere. In the larger impacts, significant amounts
of crustal and even mantle material are lofted as well. Tsunamis up to a
kilometer in initial height are generated by the collapse of the vertical
jet. These waves are initially complex in form, and interact strongly with
shocks propagating through the water and the crust. The tsunami waves are
followed out to 100 km from the point of impact. Their periods and
wavelengths show them to be intermediate type waves, and not (in general)
shallow-water waves. At great distances, the waves decay as the inverse of
the distance from the impact point, ignoring sea-floor topography. For all
impactors smaller than about 2 km diameter, the impacting body is highly
fragmented and its remains lofted into the stratosphere with the water vapor
and crustal material, hence very little trace of the impacting body should
be found for most oceanic impacts. In the oblique impacts, the initial
asymmetry of the transient crater and crown does not persist beyond a
tsunami propagation length of 50 km.

==============
(6) ASTEROID MOONS PULLED IN BY GRAVITY

>From UPI News, 11 December 2002
http://www.upi.com/view.cfm?StoryID=20021211-042209-2670r

By Lidia Wasowicz
UPI Senior Science Writer
>From the Science & Technology Desk

PASADENA, Calif., Dec. 11 (UPI) -- In a theory-smashing discovery,
astronomers said Wednesday they have found the pull of gravity, not a clash
of the titans, spun companion moons into asteroid orbits on the edge of the
solar system.

Since observations from the spacecraft Galileo first revealed in 1993 a
binary asteroid system -- the primeval, icy space rock Ida orbited by its
satellite Dactyl -- in the main asteroid belt between the planets Mars and
Jupiter, astronomers have observed more than a dozen pairs of such frozen
relics of the solar system's beginnings.

Scientists long have thought such twin worlds -- exemplified by Earth and
its moon -- resulted from the collision of large heavenly bodies. However,
such crashes rarely occur in the deep freeze of the outermost region of the
solar system, where asteroid pairs were revealed for the first time last
year.

There, in the area known as the Kuiper belt, which stretches from just past
the frigid, cyclone-whipped planet Neptune to beyond the farthest reaches of
the tiny misfit Pluto's highly elliptical orbit, some other forces must have
been at work.

"In the Kuiper belt today, there just aren't that many collisions between
large objects, so it's a little hard to understand how there could be as
many large binary systems formed by this mechanism as we actually observe,"
Daniel Durda of the Southwest Research Institute in Boulder, Colo., who
analyzed the findings, told United Press International.

Intrigued by the mystery, a team of space watchers from the California
Institute of Technology set out to solve the puzzle.

"Previous attempts to explain Kuiper belt binaries relied upon physical
collisions," lead study author Re'em Sari told UPI.

"However, collisions are very rare in the Kuiper belt. Moreover, when a
binary is formed by a collision, it tends to be close, i.e., the separation
between the two component bodies is only a few times larger than the bodies'
diameters," he explained. "By contrast, the separation in Kuiper belt
binaries is hundreds or even thousands of diameters. Thus, it is implausible
that Kuiper belt binaries formed through collisions."

Rather, Sari and his colleagues suggest in the Dec. 12 issue of the British
journal Nature, the double worlds might have sprung from close encounters of
the gravitational kind. Specifically, they propose the gravitational effects
during the period of runaway accretion in the early solar system could have
generated perhaps 5 percent of the binaries among Kuiper-belt objects.

"We propose that gravity alone is responsible for the formation of the
binaries," Caltech researcher Yoram Lithwick told UPI. "With the help of its
gravitational field, a body can reach out to large distances, and so it can
capture a companion that is initially quite distant."

However, the mutual gravitational attraction of two bodies passing by each
other merely will deflect them from their initial trajectories, he said. For
them to slow down sufficiently to become bound as a binary, they must
dispose of some of their energy.

"We propose ... gravity is responsible for this energy loss," Sari said.
"The two bodies can lose some energy if there is a third body nearby --
close enough to feel the two bodies' gravitational fields. In certain
configurations, the two bodies will transfer some of their energy to this
third body (or a swarm of tiny bodies), resulting in a binary."

The new results reflect the wide variety of mechanisms engaged in forming
satellites around minor planets, said Durda, who in his own research is
working out asteroid collision models of satellite formation.

"Many near-Earth asteroid binaries may form through tidal breakup when
passing near the Earth, many main-belt asteroid satellites may form through
impacts and collisions, and now we're coming to understand that many of the
binaries in the outer solar system may have formed in primordial times
through comparatively gentle gravitational encounters," he told UPI.

The solution stands to shed light on an array of topics of high interest to
Earthlings.

Many comets -- primitive snowballs that hold frozen records of solar system
origins -- and other visitors from space that occasionally stop by Earth
hail from the Kuiper belt, home to icy leftovers from the formation of the
large planets 4.5 billion years ago.

"Since the Kuiper belt binaries are relics from the early solar system, they
can teach us about this early history -- in particular, how the objects that
are presently orbiting the sun (e.g., Kuiper belt objects, planets and
moons) were built up from much smaller building blocks to their present
size," Sari explained.

Scientists' evolving understanding of asteroid satellite systems carry more
practical implications as well, Durda added.

For example, astronomers estimate some 17 percent of near-Earth asteroids
come as twosomes, while models for satellite formation indicate many of
these objects are little more than rubble piles, he said.

"That, in turn, has important implications if we ever find ourselves in a
position to need to deflect an asteroid threatening us with an impact,"
Durda noted. "These rubble pile objects may be much more difficult to push
on or disrupt than solid, monolithic mountains of rock."

Planetary scientists value the Kuiper belt -- whose existence was predicted
in 1951 by Dutch-American astronomer Gerard Kuiper and proven only a decade
ago -- both as a secure safe for holding mementoes of the birth of the solar
system and a pristine laboratory for studying planet formation and
evolution.

"When did (the outer) planets form? Why are their compositions different --
more gas and ice-rich -- than the inner planets? Did the formation of these
large (Kuiper belt) objects scatter debris into the inner solar system to
impact the terrestrial planets and the early Earth?" Durda asked.

"Lots of very interesting questions that we may help to answer in part if we
can better understand exactly when and under what circumstances the remains
of planet-forming material in the outermost solar system -- the Kuiper belt
objects -- got their satellites," he said.

Since the discovery of the first Kuiper-belt object in 1992, 617 other such
objects have been identified, helping put the seemingly anomalous Pluto into
perspective. Long regarded as a planetary misfit for its unusual orbit that
crosses Neptune's, as well as its ice-rock composition and location on the
outskirts of the solar system, Pluto seems to fit in a bit better as the
largest member of a disk of icy minor planets left over from the formation
of the outer planets, Durda said.

Since the first KBO moon was discovered in 2001, another six binary systems
have been observed with ground-based telescopes and the Hubble Space
Telescope, bringing the total to eight, including Pluto/Charon. Among the
main-belt asteroids, some nine have been found to have moons.

Astronomers expect to find many more binaries, although some of them might
be separated by such small distances they could appear as one object to an
Earth-bound onlooker.

"The discovery that even small, minor planets can have satellites of their
own makes the solar system a more complex, interesting and wondrous place to
live," Durda said. "It reminds us that we will never run out of new and
interesting worlds to explore and phenomena to understand when they continue
to pop up even here in the backyard of our own solar system."

Copyright © 2002 United Press International

===========
(7) NEW MODEL FOR JOVIAN SATELLITE FORMATION

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

Southwest Research Institute (SwRI) News

For more information, contact:

Maria Martinez, Communications
(210) 522-3305, Fax (210) 522-3547
PO Drawer 28510
San Antonio, Texas 78228-0510

December 9, 2002

SwRI® researchers present new model for jovian satellite formation

Boulder, Colorado -- A new model describing the origin of the four large
moons of Jupiter -- the so-called Galilean satellites -- can reconcile the
moons' major properties with the formation of the satellites from a disk of
gas and small particles orbiting Jupiter during the very end
stages of the planet's growth. This model may represent a breakthrough in
understanding how the large satellites of Jupiter formed.

Calculations performed by two researchers at Southwest Research Institute®
(SwRI) and funded by NASA show that a prolonged period of satellite growth
from a "gas-starved" disk accounts for several key satellite properties. The
moons' bulk compositions, the unusual internal structure of the outermost
satellite, Callisto, and the survival of the satellites against inward decay
caused by interaction with the disk are properties resolved by the new
model. The findings appeared in the December issue of The Astronomical
Journal.

Researchers have long known that the Galilean satellites have bulk densities
that decrease with distance from the planet. This trend indicates an
increasing proportion of low-density ice, with the outer two, Ganymede and
Callisto, containing approximately 50 percent rock and 50 percent ice by
mass. Spacecraft encounters by NASA's Galileo mission have more recently
suggested that the interior of Callisto is not divided into a distinct central core and outer
mantle, which means it had to have formed slowly -- over more than 100,000
years -- to stay cool enough to avoid large-scale melting. The combined mass
of the satellites indicates that the amount of gas and solids (ice plus
rock) necessary to form them was about 2 percent of Jupiter's mass.

Previous models assumed that the satellites formed from a disk orbiting
Jupiter that contained 2 percent of Jupiter's mass all at one time. The
underlying assumption was that the disk formed first, and the satellites
formed within it. However, expected temperatures in such a massive and
gas-rich disk would be too high to retain ices in the region of Ganymede and
Callisto, resulting in very short satellite formation times of only about
1,000 years. In addition, the gravitational interaction of the satellites
with such a disk would have caused them to decay inward onto the planet,
likely leading to the complete loss of the newly formed satellites on a time
scale similar to the one forming them.

The SwRI model does not require that all of the mass needed to form the
satellites be present in the disk all at once. Instead, material is
delivered to the disk slowly over a prolonged period of time. This would be
expected to occur as Jupiter itself grows, as the planet channels gas and
small particles from solar orbit into orbit around itself. This inflow would
provide an ongoing source of material for the satellite disk.

Thus in the new model, the satellites grow gradually as material is supplied
to the disk. Solids entering the disk rapidly accumulate in orbit around
Jupiter and buildup over time, while the inflowing gas spreads radially and
maintains a low density. In this "gas-starved" disk, temperatures low enough
for ice in the region of Ganymede and Callisto naturally result. The new
model predicts that the satellites also form slowly -- over 100,000 to 1
million years -- consistent with the theorized internal state of Callisto.

"The strength of this model is that it can tie together a wide variety of
compositional and dynamical properties of the Galilean satellite system with
a single, simple set of origin conditions," says the paper's lead author,
Dr. Robin M. Canup, assistant director of the SwRI Space Studies Department.
Co-author Dr. William R. Ward, an Institute scientist at SwRI, adds, "We
find that the well-known interactions between planets and gas disks, thought
to cause orbital decay and to lead to hot Jupiters in extrasolar systems,
also have important ramifications for the formation of satellites in gaseous
disks."

One example is a prediction under the new model that the satellites would
migrate inward somewhat during their formation, each at a rate proportional
to its mass. A separate work by Peale and Lee (2002, Science 298) has shown
that this inward migration can lead to the establishment of the so-called
Laplace resonance as the satellites form. The Laplace resonance is a locked
configuration that exists among the inner three Galilean satellites -- Io,
Europa, and Ganymede -- in which innermost Io completes four orbits to every
two of Europa, and every one of Ganymede. This new means of establishing the
Laplace resonance would occur on shorter timescales than previously favored
explanations that require a much later outward orbit evolution of the
satellites caused by interaction with Jupiter. The existence of the
resonance leads indirectly to internal heating of the satellites, causing
Io's extensive volcanism, for example.

The conditions for Galilean satellite formation are important not only for
understanding the compositions and thermal histories of the moons
themselves, but also for the potential constraints that such models provide
on the growth of jovian planets.

=============
(8) NEW THEORY ACCOUNTS FOR EXISTENCE OF BINARIES IN KUIPER BELT

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

Media Relations
Caltech

MEDIA CONTACT:
Robert Tindol, (626) 395-3631, tindol@caltech

December 13, 2002

New Theory Accounts for Existence of Binaries in Kuiper Belt

PASADENA, Calif. -- In the last few years, researchers have discovered more
than 500 objects in the Kuiper belt, a gigantic outer ring in the outskirts
of the solar system, beyond the orbit of Neptune. Of these, seven so far
have turned out to be binaries -- two objects that orbit each
other. The surprise is that these binaries all seem to be pairs of widely
separated objects of similar size. This is surprising because more familiar
pairings, such as the Earth/moon system, tend to be unequal in size and/or
rather close together.

To account for these oddities, scientists from the California Institute of
Technology have devised a theory of Kuiper belt binary formation. Their work
is published in the December 12 issue of the journal Nature.

According to Re'em Sari, a senior research fellow at Caltech, the theory
will be tested in the near future as additional observations of Kuiper belt
objects are obtained and additional binaries are discovered. The other
authors of the paper are Peter Goldreich, DuBridge
Professor of Astrophysics and Planetary Physics at Caltech; and Yoram
Lithwick, now a postdoc at UC Berkeley.

"The binaries we are more familiar with, like the Earth/moon system,
resulted from collisions that ejected material," says Sari. "That material
coalesced to form the smaller body. Then the interaction between the spin of
the larger body and the orbit of the smaller body caused them to move
farther and farther apart."

"This doesn't work for the Kuiper belt binaries," Sari says. "They are too
far away from each other to have ever had enough spin for this effect to
take place." The members of the seven binaries are about 100 kilometers in
radius, but 10,000 to 100,000 kilometers from each other. Thus their
separations are 100 to 1,000 times their radii. By contrast, Earth is about
400,000 kilometers from the moon, and about 6,000 kilometers in radius. Even
at a distance of 60 times the radius of Earth, the tidal mechanism works
only because the moon is so much less massive than Earth.

Sari and his colleagues think the explanation is that the Kuiper belt bodies
tend to get closer together as time goes on -- exactly the reverse of the
situation with the planets and their satellites, where the separations tend
to increase. "The Earth/moon system evolves 'inside-out',
but the Kuiper belt binaries evolved 'outside-in,'" explains Sari.

Individual objects in the Kuiper belt are thought to have formed in the
early solar system by accretion of smaller objects. The region where the
gravitational influence of a body dominates over the tidal forces of the sun
is known as its Hill sphere. For a 100-kilometer body located
in the Kuiper belt, this extends to about a million kilometers. Large bodies
can accidentally pass through one another's Hill spheres. Such encounters
last a couple of centuries and, if no additional process is involved, the
"transient binary" dissolves, and the two objects continue
on separate orbits around the sun. The transient binary must lose energy to
become bound. The researchers estimate that in about 1 in 300 encounters, a
third large body would have absorbed some of the energy and left a bound
binary. An additional mechanism for energy loss is gravitational interaction
with the sea of small bodies from which the large bodies were accreting.
This interaction slows down the large bodies. Once in every 30 encounters,
they slowed down sufficiently to become bound.

Starting with a binary of large separation a million kilometers apart,
continued interaction with the sea of small objects would have led to
additional loss of energy, tightening the binary. The time required for the
formation of individual objects is sufficient for a binary orbit to shrink
all the way to contact. Indeed, the research predicts that most binaries coalesced in this
manner or at least became very tight. But if the binary system was formed
relatively late, close to the time that accretion in the Kuiper belt ceased,
a widely separated binary would survive. These are the objects we observe
today. By this mechanism it can be predicted that about 5 percent of objects
remain with large enough separation to be observed as a binary. The
prediction is in agreement with recent surveys conducted by Caltech
associate professor of planetary astronomy Mike Brown. The majority of
objects ended up as tighter binaries. Their images cannot be distinguished
from those of isolated objects when observed from Earth using existing
instruments.

These ideas will be more thoroughly tested as additional objects are
discovered and further data is collected. Further theoretical work could
predict how the inclination of a binary orbit, relative to the plane of the
solar system, evolves as the orbit shrinks. If it increases, this would
suggest that the Pluto/Charon system, although tight, was also formed by the
'outside-in' mechanism, since it is known to have large inclination.

Related Links

* Binaries in Kuiper Belt Animations
  http://pr.caltech.edu/media/kuiper/

===============
(9) AND FINALLY: MYSTERY TRAINS

>From Sky & Telescope, 10 December 2002
http://skyandtelescope.com/news/current/article_808_1.asp

By Monica Bobra

December 10, 2002 | When the Leonid meteor shower peaked on the morning of
November 19th, bits of debris from Comet Tempel-Tuttle came shooting into
Earth's atmosphere at a speed of 71 kilometers per second. The result was a
fine shooting-star show that will remembered for years to come. But what
makes the Leonids unique is that an unusual number of them leave lingering
trains in their wake - snaking white glows that can last from a second up to
20 minutes.

Jack D. Drummond (Starfire Optical Range) and a team of astronomers have
been analyzing these trains since 1998. What he's gathered shows us just how
mysterious this phenomeon is.

Drummond's group rapidly slews a 3.5-meter telescope at the Air Force
Research Laboratory in Albuquerque, New Mexico, to a train, then illuminates
it with pulses of light from a powerful sodium laser. The team uses the
laser beam as a form of radar, or "lidar," which stands for light detection
and ranging.

Not much is known about the lingering marks meteors make. Scientists think
the trains are created by chemiluminescent reactions, which come from the
catalytic conversion of single oxygen molecules into O2. The initial
friction between the meteor and the atmosphere strips oxygen atoms of
electrons. This leaves a sea of electrons that recombine with oxygen atoms
and emit light in the process. Drummond's team speculates that most of the
light from the train comes from this process.

In order to seek more answers, Drummond's team paired with one led by John
Zinn at the Los Alamos National Laboratory in Los Alamos, New Mexico. The
two groups are looking closely at computer simulations of meteors to narrow
down the cause of the trains, says Drummond. "[A meteor] comes through the
atmosphere so fast it is like an instantaneous explosion along a line,"
explains Drummond. "The tail expands rapidly and then just stops."

Although Drummond was able to study several trains in 1998 and 1999, his
group caught only one this year, and it didn't turn up anything new.
"However, that's just a preliminary conclusion," he notes.

As scientists delve deeper into the cause of these mysterious trains, there
remain several unanswered questions. One mystery is why meteors occasionally
create parallel trains, which were previously attributed to a phenomenon
known as shell burning. Drummond's group has since disproved the theory,
though they cannot yet explain the parallel trails. Another question is: Why
the Leonids? Although the Perseids sometimes have trains, they leave far
fewer than Leonids do.

Unfortunately, scientists will have to find their answers to these questions
from the data they have on hand. The next strong Leonid shower should come
in 2098.

Copyright 2002, Sky & Telescope

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