PLEASE NOTE:
*
CCNet, 26 August 1999
---------------------
(1) ASTEROIDS AS POROUS RUBBLE PILES
Larry Klaes <lklaes@bbn.com>
(2) OBSERVATIONS OF CENTAUR 1997 CU26
N. McBride, UNIVERSITY OF KENT
(3) NUMEROUS WEAK RESONANCES DRIVE ASTEROIDS TOWARD TERRESTRIAL
PLANET
ORBITS
A. Morbidelli) et al., OBSERVATORY COTE AZUR
(4) RADAR OBSERVATIONS OF ASTEROID 2063 BACCUS
L.A.M. Benner et al., CALTECH,JET PROP
LAB,4800 OAK GROVE
(5) DECONSTRUCTING CASTALIA
E. Asphaug, D.J. Scheeres, UNIVERSITY OF CALIF
SANTA CRUZ
(6) DYNAMICAL EVOLUTION OF 1036 GANYMED, THE LARGEST NEA
P. Michel, et al., OBSERVATORY COTE AZUR
(7) OBSERVATIONS OF ASTEROID 1998 KY26
S.J. Ostro et al., CALTECH
(8) WATER ICE ON KUIPER BELT OBJECT 1996 TO66
R.H. Brown et al., UNIVERSITY OF ARIZONA
(9) CLOSE APPROACHES OF ASTEROID 1999 AN10
A. Milani, UNIVERSITY OF PISA
(10) 4000 BP CLIMATIC UPHEAVAL
Doug Keenan <doug.keenan@virgin.net>
(11) CLIMATIC & SOCIAL CATASTROPHES AROUND 4000 BP
Steve Drury <S.A.Drury@open.ac.uk>
================
(1) ASTEROIDS AS POROUS RUBBLE PILES
From Larry Klaes <lklaes@bbn.com>
August Hot Idea:
Asteroids have lower densities than expected, probably because
they have
been disrupted and then reassembled into porous rubble
piles.
http://www.soest.hawaii.edu/PSRdiscoveries/Aug99/asteroidDensity.html
Honeycombed Asteroids
Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
Asteroids seem to have lower densities than the rocks scientists
believe compose them. This implies that there is quite a bit of
empty
space inside the typical asteroid or small moon. Lionel Wilson
(Lancaster University, UK), Klaus Keil (University of Hawaii),
and
Stanley Love (astronaut candidate, Johnson Space Center)
investigated
two possibilities for producing the high percentage of pore
space. In
one case they calculated that if an asteroid were broken apart
and then
reassembled, the resulting rubble pile would have a porosity of
20 to
40%, hence a density 20 to 40% lower than it had to begin with.
They
also calculated how fractures would form on bodies that contained
water
ice that was heated to steam, concluding that the fractures would
be
pervasive and, hence, decrease the density of the object.
Reference:
Wilson, L., K. Keil, and S. J. Love, 1999, The Internal
Structures and
Densities of Asteroids, Meteoritics and Planetary Science, vol.
34, p.
479-483.
Asteroid and Meteorite Densities
We have fairly accurate measurements of the densities of four
asteroid-sized bodies that have been visited by spacecraft:
Phobos and
Deimos, the small moons of Mars, and asteroids 243 Ida and 253
Mathilde. Each has been visited close enough by a spacecraft to
allow
calculation of its mass from the way it deflected the spacecraft
from
its course. Also, each of these bodies has been photographed well
enough to be able to determine its volume. Because density is
mass
divided by volume, these data allow us to determine the densities
of
the objects. See the table below (Adapted from Wilson et al.,
1999.)
Density is a useful parameter. An asteroid made of pure
iron-nickel
metal, for example, would have a density of about 7.9 grams per
cubic
centimeter, so if we found an asteroid with such a high density
we
could conclude it was made of metallic iron. Of course, things
are
never that simple. An asteroid might have empty spaces in it
(called
pore spaces) which would lower the density in proportion to the
percentage of pore spaces. Most planetary surface materials have
pore
spaces. Sand, for example, consists typically of about 40% pore
spaces.
(To get an idea of how much pore space a material can have, dump
some
sand into a volumetric beaker, up to say the 500 milliliters
level.
Then pour water from another calibrated container into the sand
until
you finally begin to get a puddle on top. You may be able to pour
200
milliliters of water into the 500 milliliters of sand.)
Even individual meteorites have some pore spaces. Guy Consolmagno
SJ
(Vatican Observatory) and Daniel Britt (University of Tennessee)
have
measured the density and porosity (the percentage of pore spaces
is
called porosity) of hundreds of meteorites and assembled a
database of
measurements made by other investigators [see PSRD supplement on:
measuring density and porosity].
They find that the porosity of meteorites ranges from 5 to 35%.
The
porosity is in the form of cracks and spaces between mineral
grains.
Carbonaceous chondrites, which are dark rocks that contain carbon
compounds (including organic chemicals) and water-bearing
minerals,
have porosities between 15% to 35%. In contrast, ordinary
chondrites,
which do not contain carbon compounds or water, have porosities
of
about 10%. Stony-iron meteorites have porosities of only about 5%
and
iron meteorites have essentially no porosity.
To compare the meteorite data with a given asteroid, we need to
know
what type of meteorite could have come from it. Wilson and
coworkers
did that by abopting a classification scheme used by astronomers.
the
classification is based on the amount and color of the light
reflected
off an asteroid, and on comparisons with the way powdered
meteorite
samples reflect light. Astronomers measuring the reflected light
have
classified 253 Mathilde as a C type asteroid, thought to be
similar to
carbonaceous chondrites. So, Wilson and colleagues assume 253
Mathilde
is a carbonaceous chondrite, with a porosity of about 15 to 35%
(see
table above). Phobos and Deimos are classified as D or P types,
which
are darker than even the darkest carbonaceous chondrite, and do
not
correspond to any known type of meteorite. Nevertheless, Wilson
assumes
that the Martian moons are more likely to be similar to CI or CM
carbonaceous chondrites, with porosity of 15-35%. 243 Ida is
classified
as an S type asteroid. What type of meteorite these correspond to
depends on who one talks to: the issue is hotly contested. They
could
be either like ordinary chondrites or stony-iron meteorites.
Wilson and
colleagues assume 243 Ida is an ordinary chondrite.
Putting all this together, the two asteroids and two moons of
Mars have
somewhat lower densities, hence higher porosities, than do the
meteorites to which they appear to be most closely related (see
table
above). This indicates that the asteroids and small moons have
more
empty spaces inside them than do small rocks removed from them.
In
other words, they appear to be gravitationally bound piles of
rocks
with empty spaces between the rocks.
Breaking Up Asteroids
The idea that asteroids are rubble piles is not new. Theoretical
studies of the evolution of asteroids by impacts indicate that
for a
large range in impact energies, the bodies break up, but the
fragments
do not move apart fast enough to escape each other. Their mutual
gravity causes the fragments to promptly reassemble into a
mixed-up,
fragmented body; see graphic below.
Chondrite meteorites provide proof that this process actually
happens.
Some chondrites are called "regolith breccias." These
are rocks
reworked by impacts on the surfaces of asteroids. We know they
formed
in the uppermost part of the asteroid because regolith breccias
contain
gases implanted from the solar wind, which penetrate only
micrometers
into rock surfaces. These breccias also contain grains of
metallic
iron-nickel. It is possible to determine how fast these metallic
particles cooled by measuring their compositions and sizes.
Numerous analyses indicate that in a given regolith breccia, the
metallic particles cooled at rates ranging from 1 to 1000 degrees
Celsius per million years. Using the laws of heat conductivity,
we can
calculate how deep a rock must be buried to cool at a given rate.
The
range of cooling rates of the particles in regolith breccias
indicates
original burial depths of a few kilometers (those cooling at 1000
degrees per million years) to 100 kilometers (those cooling at 1
degree
per million years). Clearly, there has been a lot of scrambling
of the
asteroids on which these breccias formed!
At first glance, one would think that craters of different sizes
could
dig up rocks from a large range of depths and deposit them onto
the
surface where they could be incorporated into regolith breccias.
It is
not so easy, however. In 1987 my colleagues and I calculated that
craters large enough to excavate rock from a depth of 60
kilometers
would demolish asteroids smaller than 500 kilometers in diameter
(see
graph below). The easier way to deposit rocks from great depths
and to
mix them with rocks from shallow depths is to bust up the
asteroid and
reassemble it into a rubble pile.
Increasing Porosity by Fragmentation and Reassembly
Lionel Wilson and his colleagues wondered if the high porosities
(hence, low densities) of 243 Ida, 253 Mathilde, and the Martian
moons,
could be due to break up and reassembly of the bodies. To test
this
idea, Wilson wrote a computer program that followed the paths of
numerous asteroid fragments after an impact large enough to
demolish
the asteroid. A critical piece of information he needed was the
sizes
and shapes of the fragments created by the disruption of an
asteroid.
For that information, Wilson used data from a series of
experiments by
Akiko Nakamura and Akira Fujiwara (Kyoto University, Japan). They
broke
up rock spheres by whacking them with projectiles moving at 3-4
kilometers per hour (appropriate for velocities in the asteroid
belt),
and examined the fragments produced.
Using Nakamura and Fujiwara's data as a guide, Wilson assumed
that the
asteroid broke up into elongated pieces with one large fragment,
4
smaller fragments, 32 still smaller, and 128 even smaller. About
half
the object breaks into even smaller pieces but most of them do
not fall
back onto the reassembled object. (A real asteroid, of course,
would
break up into millions of fragments, but computers can only keep
track
of a small number of particles. Too many fragments and the
program
will take forever to run.) The results are shown in the
illustration
below.
The blocks in this diagram, which vaguely resembles a cubist
painting,
represent the relative abundances and sizes of the fragments
produced
by a catastrophic impact. (Physicists frequently depict objects
as
recognize that natural objects are not all cubes and spheres!)
Orange
is a single large fragment, blue the 4 smaller ones, green the 32
next
smallest, purple the 128 next smallest, and red the smallest. The
captions in the left corners show the time elapsed since the
impact,
which takes place at the asterisk by a projectile approaching
from the
direction of the arrow. Very few of the smallest fragments
closest to
the point of impact fall back to the shattered asteroid. The
largest
fragments, because they are accelerated the least, fall back
first,
followed by progressively smaller fragments, and most of the
object is
reassembled in about half an hour. Of course, it is missing a lot
of
mass - the red region that was fragmented into small pieces that
were
accelerated too fast by the impact to return to the scene of the
crime.
Wilson and colleagues needed to determine the porosity of a
reconstructed object. No elaborate computer program or
cubist
illustrations were required for this. Wilson simply cut up a
chunk of
polystyrene (StyrofoamŪ) into a range of sizes, with the number
of
fragments increasing as the size decreased (as in his computer
calculations and in Nakamura and Fujiwara's experiments). He
dumped the
pieces into a beaker, putting the largest particles in first,
then the
smaller ones, and finally the smallest ones, to simulate the way
his
calculations indicated fragments fell onto the reassembling
asteroid in
the calculations. After adding the fragments, he gave the beaker
a
shake, then looked at the calibrations on the side of the beaker
to
determine the volume of the reassembled polystyrene
"asteroid." Because
he knew how much polystyrene he had used in making the fragments,
the
volume of empty space (the volume of pores) was simply the
difference
between the volume of the beaker taken up by the pile of
fragments
minus the volume of polystyrene particles. He repeated the
measurement
numerous times, and found that the porosity varied between 20%
and 40%.
This is about what Wilson and his co-authors estimated for the
porosity
of 243 Ida, 253 Mathilde, and the Martian moons.
Double Bubble, Toil and Trouble
Wilson and his colleagues also examined another way to increase
the
porosity of a small asteroid that originally contained lots of
water
ice mixed with rock. When such a mixture is heated (by decay of
radioactive elements, for example), the water reacts with the
rocky
grains and forms hydrated minerals, releasing hydrogen gas,
and
possibly other gases as well. Wilson's calculations indicate that
these
gases would build up faster than they could escape because the
outer
parts of the asteroid may have had solid ice layers and the
interior
was compacted.. As a result, the pressure built up, eventually
exceeding
the strength of the surrounding rock, causing cracks to form.
This
process may have caused pervasive fracturing of the interiors of
asteroids, greatly increasing their porosities. Wilson estimates
that
this cracking mechanism could have operated in asteroids smaller
than
about 50 kilometers. Higher pressure inside larger asteroids
prevented
fracturing.
Rubble Piles
These latest results, which build on a great deal of work during
the
past 15-20 years, suggest that most asteroids are gravitationally
bound
rubble piles. This may make future mining of asteroids easier
than if
they were solid rock. It might also make it more difficult to
predict
the trajectories of fragments if earthlings try to blow up an
asteroid
headed for our planet.
We will soon be able to do another test of the rubble-pile
hypothesis.
The NEAR spacecraft will visit the asteroid 433 Eros early in
2000.
Sensors onboard NEAR will try to determine the mineralogy and
chemical
composition of Eros, from which we will be able to estimate the
asteroid's density if it were solid rock. The rubble-pile idea
predicts
that Eros will have a density much lower than the rock it is
composed
of.
Asteroids: from The Nine Planets by Bill Arnett.
Asteroids: from the National Space Science Data Center.
Consolmagno, G. J. and D. T. Britt, 1998, The Density and
Porosity of
Meteorites from the Vatican Collection, Meteoritics and Planetary
Science, vol. 33, p. 1231-1241.
Galileo Mission homepage.
Nakamura, A. and A. Fujiwara, 1991, Velocity Distribution of
Fragments
Formed in a Simulated Collisional Disruption, Icarus, vol. 92, p.
132-146.
NASA Solar System Exploration.
Near Earth Object Program homepage.
NEAR Mission homepage.
Taylor, G. J., P. Maggiore, E. R. D. Scott, A. E. Rubin, and K.
Keil,
1987, Original Structures, and Fragmentation and Reassembly
Histories
of Asteroids: Evidence from Meteorites, Icarus, vol. 69, p. 1-13.
Wilson, L., K. Keil, and S. J. Love, 1999, The Internal
Structures and
Densities of Asteroids, Meteoritics and Planetary Science, vol.
34, p.
479-483.
===============
(2) OBSERVATIONS OF CENTAUR 1997 CU26
N. McBride*), J.K. Davies, S.F. Green, M.J. Foster: Optical and
infrared observations of the Centaur 1997 CU26. MONTHLY NOTICES
OF THE
ROYAL ASTRONOMICAL SOCIETY, 1999, Vol.306, No.4, pp.799-805
*) UNIVERSITY OF KENT,PHYS LAB,UNIT SPACE SCI &
ASTROPHYS,CANTERBURY CT2
7NR,KENT,ENGLAND
Minor planet 1997 CU26 is a Centaur, and is probably undergoing
dynamical evolution inwards from the Kuiper Belt. We present
optical
and infrared (VRIJHK) photometry which gives mean colours of V -
R =
0.46 +/- 0.02, V - I = 1.02 +/- 0.02, V - J = 1.74 +/- 0.02, V -
H =
2.15 +/- 0.02 and V - K = 2.25 +/- 0.02. The resulting relative
reflectance spectrum lies between those of Chiron and Pholus
(although
closer to that of Chiron). A 1.6-2.6 mu m spectrum confirms the
broad
absorption feature at 2.05 mu m associated with water ice
reported by
Brown et al.1997 CU26 displays no significant light curve
variation and
(unlike Chiron) has no observable coma. We place an upper limit
to the
dust production rate of 1.5 kg s(-1). J-band data taken at phase
angles
of 1.degrees 7 to 4.degrees 0 give a phase parameter of G(J) =
0.36 +/-
0.1, and are consistent with a phase parameter of G = 0.15 in the
V
band (a value often assigned to low-albedo objects when no other
information is available) if we assume a phase reddening of 0.017
mag
deg(-1) in the J band. We find V(1; alpha = 4.degrees 1) = 7.022
+/-
0.02, from which we deduce, by assuming G = 0.15 +/- 0.1, an
absolute
visual magnitude of H-V = 6.64 +/- 0.04. Copyright 1999,
Institute for
Scientific Information Inc.
================
(3) NUMEROUS WEAK RESONANCES DRIVE ASTEROIDS TOWARD TERRESTRIAL
PLANET
ORBITS
A. Morbidelli*), D. Nesvorny: Numerous weak resonances drive
asteroids
toward terrestrial planets orbits. ICARUS, 1999, Vol.139, No.2,
pp.295-308
*) OBSERVATORY COTE AZUR,BP 4229,F-06304 NICE 4,FRANCE
A systematic exploration of the chaotic structure of the asteroid
belt
is presented, first taking into account only the perturbations
provided
by the four giant planets and then including also the effects of
the
inner planets. We find that both the inner belt (a < 2.5 AU)
and the
outer part of the main belt (a > 2.8 AU) are mostly chaotic.
In the
outer part of the belt, chaos is due to the presence of numerous
mean-motion resonances with Jupiter and three-body resonances,
Jupiter-
Saturn-asteroid. In the inner belt; chaos is generated by mean
motion
resonances with Mars and three-body resonances,
Mars-Jupiter-asteroid.
Due to the chaoticity of the belt, asteroids tend to slowly
migrate in
eccentricity. This phenomenon of 'chaotic diffusion' allows many
bodies
in the inner belt to become Mars-crossers. The number of
asteroids
leaking out from the inner belt is large enough to keep the
population
of Mars-crossing asteroids in steady state, despite of the short
dynamical lifetime of the latter (similar to 25 Myr). We
speculate that
chaotic diffusion could have substantially eroded the
high-eccentricity
part of the asteroid belt, thus providing the impactors
responsible for
the Late Heavy Bombardment phase of the early Solar System. (C)
1999
Academic Press.
===========
(4) RADAR OBSERVATIONS OF ASTEROID 2063 BACCUS
L.A.M. Benner*), R.S. Hudson, S.J. Ostro, K.D. Rosema, J.D.
Giorgini,
D.K. Yeomans, R.F. Jurgens, D.L. Mitchell, R. Winkler, R. Rose,
M.A.
Slade, M.L. Thomas, P. Pravec: Radar observations of asteroid
2063
Bacchus. ICARUS, 1999, Vol.139, No.2, pp.309-327
*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91109
We report Doppler-only (cw) and delay-Doppler radar observations
of
Bacchus obtained at Goldstone at a transmitter frequency of 8510
MHz
(3.5 cm) on 1996 March 22, 24, and 29. Weighted, optimally
filtered
sums of cw and delay-Doppler echoes achieve signal-to-noise
ratios of
similar to 80 and similar to 25, respectively, and cower about
180
degrees of rotation phase (period = 14.90 h; Pravec et al, 1998),
Our
cw observations place up to four 2-Hz-resolution cells on Bacchus
at
echo powers greater than two standard deviations of the noise.
Delay-Doppler observations typically place about ten 0.5-mu s
(75-m)x
1-Hz cells on Bacchus above the same threshold. A weighted sum of
all
cw spectra gives an OC radar cross section of 0.12(-0.02)(+0.06)
km(2)
and a circular polarization ratio of 0.21 +/- 0.01. The
dispersion of
the echoes in time delay indicates a lower bound on Bacchus'
maximum
pole-on breadth of 0.6 km that is consistent with the echo
bandwidth
(6+/-2 Hz) and rotation period. Echo spectra on March 22 and
delay-Doppler images on all three days show a central deficit of
echo
power that provides strong evidence for a bifurcation in the
shape.
Inversion of delay-Doppler images, cw spectra, and optical
lightcurves
obtained at Ondrejov Observatory yields single-lobe and two-lobe
models
that define lower and upper bounds on the degree of bifurcation.
Both
shape models have a prominent central concavity, modestly
asymmetric
shapes, and similar physical dimensions, spin vectors, and radar
and
optical geometric albedos, We adopt the more conservative
single-lobe
shape model as our working model and explore its implications. It
has a
radar-derived sidereal rotation period P-sid = 15.0 +/- 0.2 h and
a
north pole within a few tens of degrees of ecliptic longitude
lambda =
24 degrees and ecliptic latitude beta = -26 degrees; retrograde
rotation is likely, It has dimensions of 1.11 x 0.53 x 0.50 km,
an
effective diameter (the diameter of a sphere with the same volume
as
the model) D-eff = 0.63(-0.06)(+0.13) km, and radar and optical
geometric albedos <(sigma)over cap> = 0.33(-0.11)(+0.25)
and p(v) =
0.56(-0.18)(+0.12), respectively, that are larger than most
estimated
for other asteroids. Bacchus' low circular polarization ratio and
high
radar albedo are consistent with nearly regolith-free ordinary
chondrite and basaltic achondrite compositions, but its high
optical
geometric albedo seems inconsistent with an ordinary chondrite
composition and may favor a V-class composition. Bacchus has less
structural complexity at centimeter-to-decimeter spatial scales
and its
near-surface is more dense (either more metal, lower porosity, or
both)
than the average radar-detected near-Earth asteroid. (C) 1999
Academic
Press.
===================
(5) DECONSTRUCTING CASTALIA
E. Asphaug*), D..J. Scheeres: Deconstructing Castalia: Evaluating
a
postimpact state. ICARUS, 1999, Vol.139, No.2, pp.383-386
*) UNIVERSITY OF CALIF SANTA CRUZ,DEPT EARTH SCI,SANTA
CRUZ,CA,95064
We describe the rotational and translational states of the
fragments of
a disrupted asteroid. All the fragments are found to be in
complex
rotation and have widely varying rotation periods. Although this
is
only one example of a possible collision, the numerical results
for
small fragment rotation rates are consistent with recent
observations,
including the small asteroid 1998KY26, and they lend insight into
the
latter stages of impact disruption. In particular, fragment
'memory' of
parental rotation appears to be negligible. This is the most
comprehensive description to date of the state of an asteroid or
comet
immediately following a disruptive collision. (C) 1999 Academic
Press.
===============
(6) DYNAMICAL EVOLUTION OF 1036 GANYMED, THE LARGEST NEA
P. Michel*), R. Gonczi, P. Farinella, C. Froeschle: Dynamical
evolution
of 1036 Ganymed, the largest near-Earth asteroid. ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.347, No.2, pp.711-719
*) OBSERVATORY COTE AZUR,BP 4229,F-06304 NICE,FRANCE
We have studied numerically the dynamical evolution of 1036
Ganymed,
the largest near-Earth asteroid, by integrating the orbits of
tens of
'clone' particles with similar initial conditions. Typically, the
orbit
initially undergoes large, coupled oscillations of the
eccentricity and
inclination; then, Mars encounters random-walk the semimajor axis
until
it reaches a strong Jovian resonance; and eventually, resonant
effects
pump up the eccentricity until the orbit becomes Sun-grazing or
hyperbolic (after encountering Jupiter). The median dynamical
lifetime
is of about 10 Myr. Most orbits become Earth-crossing within 10
Myr of
evolution. The origin of Ganymed and a few other sizable
Mars-crossing
asteroids with similar orbital elements is an open problem, since
the
main-belt asteroid population in the neighbouring
lower-eccentricity
portion of the phase space is quite sparse. Although Ganymed's
reflectance spectrum has some similarity to those of the ordinary
chondrites, the Earth delivery efficiency from bodies with this
type of
orbits is low, because they are short-lived after they
become
Earth-crossing. Copyright 1999, Institute for Scientific
Information
Inc.
=================
(7) OBSERVATIONS OF ASTEROID 1998 KY26
S.J. Ostro*) et al.: Radar and optical observations of asteroid
1998
KY26. SCIENCE, 1999, Vol.285, No.5427, pp.557-559
*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91109
Observations of near-Earth asteroid 1998 KY26 shortly after its
discovery reveal a slightly elongated spheroid with a diameter of
about
30 meters, a composition analogous to carbonaceous chondritic
meteorites, and a rotation period of 10.7 minutes. which is an
order of
magnitude shorter than that measured for any other solar system
object.
The rotation is too rapid for 1998 KY26 to consist of multiple
components bound together just by their mutual gravitational
attraction. This monolithic object probably is a fragment derived
from
cratering or collisional destruction of a much larger asteroid.
Copyright 1999, Institute for Scientific Information Inc.
=================
(8) WATER ICE ON KUIPER BELT OBJECT 1996 TO66
R.H. Brown*), D.P. Cruikshank, Y. Pendleton: Water ice on Kuiper
Belt
object 1996 TO66. ASTROPHYSICAL JOURNAL, 1999, Vol.519, No.1 Pt2,
pp.L101-L104
*)UNIVERSITY OF ARIZONA,LUNAR & PLANETARY LAB,TUCSON,AZ,85721
The 1.40-2.45 mu m spectrum of Kuiper Belt object 1996 TO66 was
measured at the Keck Observatory in 1998 September. Its spectrum
shows
the strong absorptions near 1.5 and 2.0 mu m that are
characteristic of
water ice-the first such detection on a Kuiper Belt object. The
depth
of the absorption bands and the continuum reflectance of 1996
TO66
suggest the presence of a black- to slightly blue-colored,
spectrally
featureless particulate material as a minority component mixed
with the
water ice. In addition, there is evidence that the intensity of
the
water bands in the spectrum of 1996 TO66 varies with rotational
phase,
suggesting a ''patchy'' surface. Copyright 1999, Institute for
Scientific Information Inc.
====================
(9) CLOSE APPROACHES OF ASTEROID 1999 AN10
A. Milani*), S.R. Chesley, G.B. Valsecchi: Close approaches of
asteroid
1999 AN(10): Resonant and non-resonant returns. ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.346, No.3, pp.L65-L68
*) UNIVERSITY OF PISA,DIPARTIMENTO MATEMAT,VIA BUONARROTI
2,I-56127
PISA,ITALY
The Earth passes very close to the orbit of the asteroid 1999
AN(10)
twice per year, but whether or not this asteroid can have a close
approach depends upon the timing of its passage across the
ecliptic
plane. Among the possible orbits there are some with a close
approach
in 2027. The period of the asteroid may be perturbed in such a
way that
it returns to an approach to the Earth at either of the possible
encounter points. We have developed a theory which successfully
predicts the 25 possible such returns up to 2040. We have also
identified 6 more close approaches resulting from the cascade of
successive returns. Because of this extremely chaotic behaviour
there
is no way to predict all possible approaches for more than a few
decades after any close encounter, but the orbit will remain
dangerously close to the orbit of the Earth for about 600 years.
Copyright 1999, Institute for Scientific Information Inc.
================
(10) 4000 BP CLIMATIC UPHEAVAL
From Doug Keenan <doug.keenan@virgin.net>
Hi Benny,
Regarding your posting yesterday on the cause of the 4000 BP
collapse
of civilisations: the cause was drought. The point is that people
need
water to live--without it, they must migrate or die.
Perhaps Egypt illustrates this best. Palaeoecological evidence
for
greatly reduced Nile flows at the time is conclusive. And
Egyptian
records tell of the Nile being so low that people could walk
across it;
even if exaggerated, it is clear that Nile flows were far from
adequate
for many decades, possibly over a century. Without the
Nile, the
Egyptians could not live (Egyptian records even tell of people
eating
their own children). So the people either left or died.
The main topic over which there can be debate is the cause of the
droughts. But, the palaeodata from Europe, North Africa, and the
Atlantic fits the pattern of a specific climatic state--a high
phase of
the North Atlantic Oscillation (with accompanying changes in NADW
production). Also, there is a likely mechanism by which an
eruption
could have pushed the NAO into that high phase, and ample
evidence for
a large eruption. By contrast, no one has suggested how a cosmic
impact
could trigger a high NAO, and there is no direct evidence of such
an
impact. The paper is on my web site.
Cheers,
Doug Keenan
http://freespace.virgin.net/doug.keenan
===============
(11) CLIMATIC & SOCIAL CATASTROPHES AROUND 4000 BP
From Steve Drury < S.A.Drury@open.ac.uk
>
Dear Benny
The conjecture of coincident collapse of Early Bronze Age
cultures in
association with volcanic- or impact-induced climate change could
be
tested (if it has not already been) from Greenland and Antarctic
ice-core records and pollen analysis of Holocene sediments.
The former
would, through layer counting, give precise ages for any
detectable
event, either in ice-cap air temperatures or the dust records. If
such
studies have been conducted I would be interested in hearing of
any
published articles.
Steve Drury
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