PLEASE NOTE:
*
CCNet 124/2001 - 23 November 2001: LEONID METEORITES?
=====================================================
(1) WEST COAST COAL AND POLLEN SHED LIGHT ON GLOBAL CATASTROPHE
Andrew Yee <ayee@nova.astro.utoronto.ca>
(2) LEONID METEORITES?
Benny J Peiser <b.j.peiser@livjm.ac.uk>
(3) LEONID METEORITES?
Duncan Steel <D.I.Steel@salford.ac.uk>
(4) LEONIDS METEORITES?
David Dunham <dunham@erols.com>
(5) BRIGHTEST LUNAR IMPACTORS WERE PROBABLY LARGE OBJECTS
(CCNet 8 December 1999)
David Dunham <dunham@erols.com>
(6) LEONID METEORITE IMPACTS ON MOON MAY BE VISIBLE ON EARTH
Ron Baalke <baalke@ssd.jpl.nasa.gov>
3 November 1999
(7) LUNAR LEONID METEORS
IAU Circular 7320
(8) ANSMET - COLLECTING METEORITES IN THE ANTARCTIC WITH THE
INTERNET
Matthew Genge <M.Genge@nhm.ac.uk>
==========
(1) WEST COAST COAL AND POLLEN SHED LIGHT ON GLOBAL CATASTROPHE
>From Andrew Yee <ayee@nova.astro.utoronto.ca>
Institute of Geological & Nuclear Sciences
Lower Hutt, New Zealand
Contact:
Institute of Geological & Nuclear Sciences
Phone: (04) 570 1444
Fax: (04) 570 4600
NEWS RELEASE: 23 NOVEMBER 2001
WEST COAST COAL AND POLLEN SHED LIGHT ON GLOBAL CATASTROPHE
A New Zealand-led group of scientists has found the first
evidence for
global destruction of forests when a large asteroid hit the Earth
65
million years ago, killing off the dinosaurs.
Until now scientists believed that destruction of forests due to
an
"impact winter" or impact-ignited wildfires was largely
confined to the
American continent, within a radius of several thousand
kilometres of
the inferred impact site on the Yucatan Peninsula, Mexico.
The New Zealand finding of a sudden death of a mixed forest and
rapid
recolonisation by ferns, on the opposite side of the Earth to the
impact site, is compelling evidence that the asteroid impact
caused
sudden destruction of terrestrial plants worldwide.
The study by paleontologists Chris Hollis and Ian Raine of the
Institute of Geological and Nuclear Sciences Limited, and Swedish
researcher Vivi Vadja, is published in the latest issue of the
international magazine Science.
The trio focused on pollen grains preserved in exposed coal seams
in a
stream bank adjacent to the Moody Creek coal mine, north of
Greymouth.
GNS scientists have a good knowledge of the Greymouth coalfield
having
mapped it in detail over many years.
Working on a hunch that the coal might contain the evidence they
were
looking for, Dr Raine chipped off pieces of the coal seam and
brought
them to Wellington where the microscopic pollen grains were
studied.
The scientists found a mixed forest community had been abruptly
replaced by a few species of fern directly after the meteorite
impact.
The types of fern identified are known as early colonisers of
open
ground.
Geochemical analysis of the coal showed extremely high
concentrations
of the elements iridium, cobalt, and chromium. The iridium
concentration of 71 parts per billion is the highest known from
non-marine rocks anywhere in the world.
These three elements are known to be much more abundant in
meteorites
than in the Earth's crust. They have been found at high
concentrations
before in New Zealand, but only where the impact layer is
preserved in
marine sedimentary rocks in eastern Marlborough.
"Whether the forest destruction was caused by prolonged
darkness and
freezing conditions associated with the impact winter, or by
global
outbreaks of wildfires, is a matter for further study by the
research
team," Dr Hollis said.
"Either way, however, it is no longer difficult to explain
the mass
extinction of large herbivorous dinosaurs and their predatory
cousins,
especially in the southern hemisphere."
The research was supported by the New Zealand Marsden Fund, and
the
Swedish Wenner-Gren Foundation and Royal Physiographic Society.
-ENDS-
Note: Sixty-five million years ago there were at least four types
of
dinosaur living in New Zealand. They included a sauropod,
theropod,
hypsilophodont, and ankylosaurid. There were also numerous marine
reptiles. Bones of these creatures have been found in a stream
north
of Napier. Sixty-five million years ago New Zealand was about
1000km
closer to the South Pole than it is today, and several degrees
warmer
than today. The Hawke's Bay stream where the remains of New
Zealand's
dinosaurs have been found is one of four sites in the southern
hemisphere where the so-called "polar dinosaurs" have
been found. The
other locations are in Australia, the Beardmore Glacier
Antarctica,
and the Antarctic Peninsula.
=================
(2) LEONID METEORITES?
>From Benny J Peiser <b.j.peiser@livjm.ac.uk>
Dear David, Jay and Duncan
After all the fuss regarding recent claims in the US about
meteorite
falls during this year's Leonids, I was wondering whether you
would like
to comment on the question whether or not meteor streams may
include
'larger' pieces of debris. I plan to remind CCNet subscribers
about the
1999 lunar impacts related to the Leonids and whether this has
any
implications for the terrestrial influx. I would much appreciate
your
views.
Kind regards, Benny
===============
(3) LEONID METEORITES?
>From Duncan Steel <D.I.Steel@salford.ac.uk>
Dear Benny,
You have asked me to comment on the basic idea of the Leonid
meteor
shower dropping meteorites on the Earth's surface (as per recent
reports) in comparison with reports in 1999 of flashes observed
on the lunar surface coincident with the shower in that year. As
you note, the latter indicates large-mass meteoroids in the
stream.
However, there is a fundamental fallacy here in drawing a link
between
the two.
Let me start with the lunar flashes. I accept the excellent
observational evidence for these, presented by several groups, as
being
impacts by large Leonid meteoroids. In a message back in 1999 Jay
Melosh
wrote: "a half meter diameter projectile would mass about
500 kg", and I
would differ there. Jay has assumed a stony-type density, whereas
for a
comet-derived meteoroid we might anticipate a far lower density,
and a
fluffy structure. Working backwards from a required mass of 500
kg to
explain the flash intensity (and noting that there are a host of
assumed
parameters in that conversion), I would expect the meteoroids to
be a few
metres across (car-sized, if you like) but very low density, and
very dark.
They would be composed of the non-volatile material remaining
from a
chunk broken off the comet's surface: small silicate fragments
held
together by a heavy organic "glue" after all the water,
other inorganic
volatiles and lighter organics had sublimated away in
interplanetary
space due to solar heating. As they hit the lunar surface, with
no
atmosphere to speak of, of course they make a flash, impacting at
around 70 km/sec.
The presence of large meteoroids in the Leonid stream is
irrelevant,
though, from the perspective of whether *meteorites* (on Earth)
may be
deposited by such a shower. This is the fallacy I mentioned
earlier.
For a meteoroid to survive atmospheric entry and have a
substantial
remnant mass decelerated until it drops vertically at the
free-fall speed (just like it had fallen from an aeroplane) there
is
a stringent set of conditions that must be met (with some
interdependence between them):
(a) The arrival speed must be low: the minimum possible at the
top of the
atmosphere is 11 km/sec (terrestrial escape speed) and certainly
those
having speeds below 15 km/sec are much favoured. This implies
prograde,
low-inclination, low-eccentricity heliocentric orbits.
(The Leonids arrive at 71 km/sec because they are in quite the
opposite
orbits!)
(b) The entry angle must be low: zenith angles of the radiant of
circa 80
degrees. Any shallower an angle (>85 degrees) and the object
may "bounce
off" the top of the atmosphere and not enter at all (cf. the
object filmed
over the Grand Tetons in 1972). At a steeper angle the object
will be
totally ablated away. (I'll note why this is the case below, in
terms of
entry physics.)
(c) The object must be strong. Thus iron meteorites occur
disproportionally
in collections, compared to their fractional population in space.
(Of course
there is also their greater likelihood of being noticed on the
ground.) Next
in terms of strength are stony meteoroids. Pretty much last come
the fragile
carbonaceous chondrites. For one of those to survive, it must
have a very
low speed and just the right arrival angle. My bet would be that
carbonaceous
chondrites in fact represent a large fraction of the very small
meteoroids in space
(see my penultimate paragraph below).
Now let's look at the entry physics. The meteoroid reaches the
top of
the atmosphere with a very substantial kinetic energy: even at
the lowest
feasible arrival speed of 11 km/sec, it has about 14 times more
KE than
the chemical energy of the same mass of TNT. If any mass is to
survive
entry (and so produce a meteorite) then it must dump in essence
all of
that KE. How does it do it? The KE is dissipated in three ways:
(i) Heating the meteoroid from its initial temperature (ca. 250
K) to
the temperature at which its vapour pressure is substantial and
so
sublimation starts in earnest. The temperature involved may be
between (say)
500 K for tarry organics through to 1500 K plus for stony and
metallic
constituents. The energy involved in this heating is only a small
fraction
of the initial KE.
(ii) Radiating away energy due to the meteoroid being heated (a
term
that looks like A epsilon sigma T^4, where A is the surface area,
epsilon
its emissivity, sigma the Stefan-Boltzmann constant and T the
temperature).
This is a heat loss *rate* (in watts, or joules per second).
(iii) Ablating away the meteoroidal material: this is the
sublimation
term, which becomes large once the temperature has grown large
enough for
the vapour pressure to take off (cf. item i above). (Note that I
have
ignored energy losses through other minor mechanisms - cracking
the object
apart, generating sound etc. - as these consume very
small fractions of the KE.)
Now think about what actually happens, if a meteorite is to be
dropped
intact (some solid mass survives entry). First, you want the KE
to be as
small as possible, and thus the speed as low as possible (and of
course
the KE rises as the square of the arrival speed). As it enters
the upper
atmosphere, drag slows the meteoroid down and the KE it is losing
initially goes to heating the meteoroid (term i above dominates,
and terms
ii and iii are essentially zero). Once the meteoroid reaches a
sufficiently
high temperature, term i disappears (no more heating occurs, just
as a
boiling kettle never exceeds 100 deg C) and the KE being lost as
the
meteoroid is decelerated goes to terms ii and iii.
(This would be the case for a very small meteoroid, which may be
considered to be isothermal. For larger meteoroids (bigger than
millimetres)
the thermal inertia is large enough such that it takes a long
time, compared
to the atmospheric entry episode, for the heat to be conducted to
the
middle. Thus it is only a surface layer which is hot, and
ablation of that
occurs with the centre remaining cold. That's why meteorites
picked up
within minutes of a fall feel cold, not hot.)
If the entry angle is too steep, the deceleration is very great
(some
tens of g's) and the shock breaks the meteoroid apart, whereupon
all
the fragments ablate away to practically nothing (via item iii).
With
an entry angle which is slightly less steep but still substantial
(70
degrees, say), item iii leads to complete ablation: that is, the
KE
goes mostly to sublimation, and rather little is radiated away.
If the
entry angle is shallow enough, though, the duration of the entry
phase
is long enough for item ii to radiate away sufficient energy
(note ii is
a heat loss *rate* so that making the time t long results in ii
dissipating a large fraction of the KE: it gets hot enough to
radiate away
energy but not hot enough for complete sublimation to happen)
such that
comparatively little goes to iii, and so the meteoroid does *not*
completely
ablate away.
The above really is a quite simple set of arguments. Of course it
is
precisely what is involved in controlled re-entries of
spacecraft: the
need to hit the "entry corridor". I remember Christmas
1968, when Apollo
8 was on its way back from the Moon, and TV programmes were
discussing how
if it came in too shallow then the capsule would bounce off the
top of
the atmosphere, whereas if it came in too steep it would break
apart and
burn up. (The Apollo capsules had ablation shields which did
indeed ablate,
but as they did so their rate of heat dissipation through
radiation was 65
MW/m2, and that's where most of the KE went; and they were coming
in at
close to the minimum speed, around 13 km/sec.)
OK, now back to the Leonids. Even those meteoroids coming in at
shallow
angles do not survive because of their extreme speeds, coupled
with
their weakness. It's easy to do numerical experiments along those
lines, as
I have. Even a steel ball-bearing entering at 71 km/sec at a
shallow angle
does not survive: it sublimates away to essentially nothing (you
may
get a tiny metallic sphere left which will take days to fall out,
and
these are found - remnants of iron meteoroids - all over the
globe;
first identified in deep-ocean samples collected on the
Challenger
expedition in the 1870s).
Is it possible *at all* for the Leonids to produce a meteorite?
The only
way I could imagine would be multiple bounces off the upper
atmosphere
(through very shallow approaches) which cause it to gradually
slow down
(as is being done now with the Odyssey satellite at Mars) and
eventually
enter at 11 km/sec. The trouble with that idea is that the Leonid
meteoroid would be moving so fast after its bounce that it would
not be
trapped in a geocentric orbit, but would be off again into a
(slightly
changed) heliocentric orbit. One could imagine other peculiar
mechanisms
involving the terrestrial and lunar gravitational fields
conjointly causing
some slow-down, but again these are inadequate to cause any
substantial drop
from the high Leonid speed.
Getting back to lunar impacts, to conclude, one might ask: why
don't we
see tremendously bright fireballs on Earth due to the same sorts
of objects
entering the atmosphere? My answer to that comes from the
structure and
composition of Leonid meteoroids I mentioned above. I believe
they are
fluffy, fragile, and composed largely of heavy organics.
Therefore they
would disintegrate at extreme altitudes (as recent optical Leonid
observations show: altitudes above 150 km), and largely ablate
away at low
temperatures such that there is comparatively little optical
emission.
A variety of radar observations of meteors in general, including
my own
at HF and VHF in Australia but also by others at UHF (Arecibo and
EISCAT),
have shown that indeed there is a very large meteoroidal
population that
ablates too high to be stony or metallic. They're tar-balls. Not
like
meteorites at all, in fact.
Do the Leonids produce meteorites? My answer is no, of course
not. There
will be micrometeorites right now falling from extreme altitudes
that
are tiny (tens of micron-sized) remnant masses from the Leonids,
but
those can take years to reach the surface. If we could identify
which
were Leonid-derived that would be wonderful for science. But even
then,
as with all meteorites of all sizes, the processes taking place
during
atmospheric entry mean that what reaches the ground intact is a
highly-
selective minor portion of what started out on the tortuous
voyage from
the parent object.
Regards,
Duncan Steel
================
(4) LEONIDS METEORITES?
>From David Dunham <dunham@erols.com>
Benny,
- The quote from below was an early estimate that I passed on
soon
after my announcement of the discovery of confirmed
impacts. It is now
generally thought that the brightest lunar Leonid impact flashes
were by objects with a mass of a few kilograms and perhaps 20 cm
in
diameter. At 71 km/sec, such objects would surely burn up
in the
Earth's atmosphere, probably with a brightness approaching that
of
the full Moon. From what I've heard about this year's
shower in
the U.S.A., the brightest meteors were around mag. -5 or -6, not
as
bright as many of the meteors during the 1998 fireball
shower. I
observed the shower myself, but unfortunately through varying
amounts
of fog, so that I saw only a few dozen, rather than the many
hundreds
that others with clear sky saw. The brightest that I saw
was about
mag. -5. I haven't seen reports of Leonid meteorite claims
this year,
but from the above, I would tend to discount them. There is
still a
rather large uncertainty in knowledge of the sizes of the lunar
Leonid impactors due to the uncertainty in the luminous
efficiency
at velocities far above anything achievable in laboratories, or
even in low-Earth orbit. I'm copying this to others besides
Jay
Melosh who might have other thoughts on this.
This year, I recorded the dark side of
the Moon with cameras more
sensitive than I had in 1999, but haven't had time to review
them.
I've received a few reports of lunar Leonids from others, but no
confirmations yet.
David
_____________________________________________________________
- my early quote below:
>BRIGHTEST LUNAR IMPACTORS WERE PROBABLY LARGE OBJECTS
>
>From Joan and David Dunham <dunham@erols.com>
>
>The objects that caused the brightest flashes that we
observed were
>probably a few hundred kilograms, according to the messages
below,
>since very little of the impact energy is converted into
light. I've
>added a few comments about the observations in the messages,
using
>" - " to preface my remarks.
==============
(5) BRIGHTEST LUNAR IMPACTORS WERE PROBABLY LARGE OBJECTS
(from CCNet 8 December 1999)
>From Joan and David Dunham <dunham@erols.com>
The objects that caused the brightest flashes that we observed
were
probably a few hundred kilograms, according to the messages
below,
since very little of the impact energy is converted into
light. I've
added a few comments about the observations in the messages,
using
" - " to preface my remarks.
There was a suggestion that sunglints from artificial satellites
might
be involved, but this is unlikely since the observations were
made late
at night local time when most of these would be deep in the
Earth's
shadow. Also, with six events simultaneously recorded at two or
more
separated locations, the chances are much greater that they are
lunar
phenomena than something closer. In the cases where lunar
location
information is available in the separate video records, there is
also
good agreement.
Brian Cudnik reports that he observed from the Houston
Astronomical
Society's site near Columbus, TX, at long. 96 deg. 39' 50"
W., lat. 29
deg. 37' 07" N., h 98m, about 100 km west of downtown
Houston.
There are many previous observations of probable lunar impacts,
although none of them apparently were confirmed. Many of these
were
published in a NASA Technical Report on transient lunar phenomena
that
is on the Web at http://www.mufor.org/tlp/lunar.html
One can also find there a link to a modern (about one year old)
effort
to videorecord TLP's simultaneously from different locations, a
"lunascan" project, which has other useful links, but
so far they don't
have news of the Nov. 18th lunar impacts. Apparently their
project has
concentrated more on the terminator and sunlit side of the
Moon. Also,
published in the Proceedings of the 48th convention of the
Association
of Lunar and Planetary Observers (Las Cruces, NM, June 25-29,
1997) is
a good paper by John Westfall on "Worthy of Resurrection:
Two Past ALPO
Lunar Projects", including one on "Lunar Meteor
Search" that includes a
table of meteor size, frequency, flash magnitude, and crater
diameter
that is in rather good agreement with the messages below, as well
as a
good history of efforts up to 1997.
Does anyone know of a Web (or other) reference to an account of
the
large impact observed by Canterbury monks in 1178 that apparently
caused the near-far-side crater Bruno? That's probably the first
observation of a lunar impact, although not confirmed from
observations elsewhere.
David Dunham, 1999 Dec. 7
_________________________________________________________
Date: Mon, 6 Dec 1999 11:30:36 -0700
To: Joan and David Dunham <dunham@erols.com>
From: Jay Melosh <jmelosh@LPL.Arizona.EDU>
Subject: References & calcs. of lunar meteor impacts
Cc: pweissman@issac.jpl.nasa.gov
Dear Joan and David:
I just heard from Paul Weissman that he estimates that your m = 3
flashes must have been made by an object "about half a
meter" in diameter. I
have to agree with this estimate--which implies masses *much*
larger
than you have mentioned! (a half meter diameter projectile would
mass
about 500 kg). The problem is that the luminous efficiency of an
impact
onto a solid surface is *much* lower than the ca. 10% Mike Mazur
estimates for a bolide. This is discussed in detail in the
Nemtchinov
paper I mentioned in my last email, but let me work out the
consequences using Mazur's estimates for the energy released by
the
various flashes on the moon (i.e. L_obj=10^[(m-26.98)/-2.5] J/s,
and an
estimated duration of 33 milliseconds). I use Nemtchinov et al.'s
luminous efficiency estimate of
10^-4:
impactor mass,kg crater
diameter,m luminous energy,J
Magnitude,m
100
9.8
2.5e7
4.8
300
13.
7.6e7
3.6
500
15.
1.3e8
3.0
I used my web program for computing crater sizes at
www.lpl.arizona.edu/tekton/crater.html
for the crater size
computations, assuming a projectile density of 1000 kg/m^3,
impact
angle of 45 degrees, impact velocity of 71 km/sec and a target of
loose
sand (lunar regolith) with a mean density of 2500 kg/m^3.
A potentially serious problem in these estimates is the duration
of the
flash. Nemtchinov and I computed that most of the light is
emitted in a
single millisecond for a 1 m radius impactor--much shorter than
the 1/30
sec Mazur estimates! Smaller objects will produce
correspondingly
shorter
flashes. However, I presume that your video camera (is it a
CCD?)
- Yes
integrates the light emitted over the duration of one frame (1/30
second?)
- Actually, with interlaced video, the even lines
are scanned in
1/60th of a second, then the odd lines
are scanned in the next
1/60th of a second to form a
1/30th-second frame. But some VCR's,
including the ones we used, can work
with the half-frames to
achieve 1/60th second time resolution.
so Mazur's estimates for total energy emitted may be correct--but
this
has to be verified before these estimates can be accepted.
If the
actual integration time was much smaller than assumed, that will
reduce
the mass of the projectile fragment accordingly.
- No, the integration time per half-frame is close
to 1/60th second;
as I understand, there is very little
"dead time" between scans
but there is some. The E flash is
curious in that I think it
peaked between two scans in my tape,
where it is almost equally
bright, but rather faint, around 7th
mag., on two successive
half-frames. But it must have been
brighter, around 5th mag.,
on a single half-frame for it to show up
so well in Sada's tape.
The other uncertainty is that Nemtchinov et al. assumed an impact
velocity of 30 km/sec. It is possible that the luminous
efficiency is
higher at 70 km/sec, and this is something that should be looked
into,
but it seems unlikely it will be off by as much as a factor of 4.
I hope this is of help to you.
- Yes, certainly, many thanks.
Sincerely, Jay Melosh
########################################################################
###
Jay
Melosh
Tel: (520) 621-2806
Professor of Planetary
Science
Fax: (520) 621-4933
Lunar and Planetary
Lab
email: jmelosh@lpl.arizona.edu
University of Arizona
Tucson AZ 85721-0092
_________________________________________________________
Date: Mon, 06 Dec 1999 20:00:35 -0800
From: R Clark <rclark99a@earthlink.net>
To: dunham@erols.com
Subject: size of lunar leonid impacts
Hello Dr. Dunham,
I was very excited to hear that several impacts on the lunar
surface
had been detected during the Leonids. The possibility of such
observations has been examined several times over the years, and
generally ruled out as a difficult project with little likelihood
of a
quick success. However the high efficiency and capabilities of
modern
sensors and their widespread use by amateurs has now made the
observation a reality.
In the discussion of the observations you mention questions about
the
size of the impactors that produced these flashes. You mention
size
estimates ranging from >1000 kg to ~100 grams. I am curious
about how
the latter figure reached you.
- The lower estimates were from
well-intentioned astrophysical
calculations by others who,
however, did not realize the
very low fraction of energy that
is transformed into visible
light during these impacts.
For the size of the objects that produced these flashes, I have
to
agree with the earlier figure... even though I am probably the
source
of the latter. In my thesis at the University of Arizona I
studied the
detectability of a different feature associated with lunar
impacts.
High velocity lunar impacts produce several phenomena that may be
observed. The impact produces shockwaves in the target that may
be
detected as seismic energy. The Apollo missions left a network of
seismometers which detected numerous impacts between 1969 and
1977 when
they, and the remaining active Apollo surface instruments were
foolishly shut down. (the old story about spending $40 billion to
plant
a flag but not being able to afford the $50K/yr to receive and
archive
the low but unending volume of science data still being returned)
Impacts of objects down to a few kg were detected with this
networ
- It sure would have been nice to have had ALSEP
observations
of the Nov. 18th impacts! Someone
should have thought to try
to look for flashes from separate
observatories before shutting
down the network. Of course, the
widespread availability of
inexpensive sensitive CCD video cameras
was key to this effort
(the cameras we used only cost $80), and
these didn't exist
in 1977.
Another impact phenomenon, probably the most obvious thing to
look for,
is the flash produced by the impact fireball. At velocities above
~12
km/sec (virtually all impacts of asteroidal or cometary material
at
Earth) the impacting object and some ammount of target material
(increasing with higher velocities) will be vaporized to
incandescent
temperatures. The radiation from this fireball will have its peak
intensity at visible or UV wavelengths, quickly dropping into the
IR as
the gasses disperse and cool. The fraction of the impact energy
partitioned into the initial fireball is generally at most 10%.
Only a
small fraction of this energy is released as 'visible' radiation
while
the fireball gas is still hot and dense enough to radiate
efficiently.
This has now been observed!
A very large fraction of the total impact energy (~60%) ends up
as
thermal energy in the immediate vicinity of the newly formed
impact
crater. This is what I was studying. After modeling cooling
craters to
determine their radiative characteristics, I considered how to
detect
them against the background of the cold lunar nightside with
groundbased, LEO, and lunar orbiting sensors. For space based
sensors
the optimum wavelength range is in the 1-6 micron range. In the
case of
a lunar orbiting sensor I concluded that an impact <100gm may
be
detetable. For groundbased observations most of this wavelength
range
is unavailable, although the 2 micron window might allow impacts
of a
few kg or less to be detected, depending on scattering of light
from
the sunlit portion of the disk and skyglow.
I am very pleased, and more than a little surprised, at how
quickly the
(groundbased!) detection of any lunar impact events has come
within the
grasp of modern instruments and sensors.
Richard Clark
rclark@lpl.arizona.edu
Joan and David Dunham
7006 Megan Lane
Greenbelt, MD 20770
(301) 474-4722
dunham@erols.com
============
(6) LEONID METEORITE IMPACTS ON MOON MAY BE VISIBLE ON EARTH
>From Ron Baalke <baalke@ssd.jpl.nasa.gov>
Leonids on the Moon
Marshall Space Flight Center
http://science.nasa.gov/newhome/headlines/ast03nov99_1.htm
Leonid meteorite impacts on the Moon might be visible from Earth
and
provide a means for long-distance lunar prospecting.
Nov. 3, 1999: When the Leonid meteor shower strikes on the
morning of
November 18, 1999, our planet won't be the only place in the
cross
hairs. The Moon will also pass very close to the debris stream of
comet
Tempel-Tuttle. Here on Earth, space-borne meteoroids will plummet
into
the atmosphere and burn up, creating streaks of light called
meteors.
The vast majority of meteoroids will burn and disintegrate well
before
they hit the ground. The situation on the Moon, where there is no
appreciable atmosphere, is different. Every bit of comet debris
that
rains down on our satellite will hit its surface. Some meteor
enthusiasts hope that will create a different sort of display.
Rather
than streaks of light in lunar skies, there could be flashes of
light
on the Moon's surface each time a sizable meteoroid hits the
ground.
Last year, during the 1998 Leonid meteor shower, the phase of the
moon was new. It was so close to the sun in the sky that
observing
faint lunar meteorite flashes was impossible. This year is
different.
During the 1999 Leonid shower the phase of the Moon will be just
2
days past first quarter. That means the moon will visible in the
night sky during the early evening on November 17, and
approximately
35% of the lunar disk as seen from Earth will not be illuminated
by
sunlight. There will be plenty of dark lunar terrain where
flashes
might be visible.
Is it possible to observe such flashes?
Maybe, say researchers. It depends a great deal on the mass
spectrum
of particles in the Tempel-Tuttle debris stream and how
efficiently
kinetic energy is converted into optical light as a result of the
impacts. Both factors are poorly known. Although flashes are
unlikely
to be seen with the naked eye, they may be detectable through
amateur
telescopes.
"The impact of a one gram particle would generate of the
order of
1023 to 1024 photons in the peak sensitivity range of the human
eye,"
says Dr. Bo Gustafson of the University of Florida Laboratory for
Astrophysics. "Given the distance to the Moon, we could
expect a few
times 106 photons per square meter at the Earth. This should be
barely detectable using a small telescope."
In June 1999, Ciel & Espace reported that a Spanish team of
astronomers led by J.L. Ortiz had reached similar conclusions:
Watching meteorites fall on the moon ...
is within reach of
(modest) amateur telescopes. Because the
Moon doesn't have a
substantial atmosphere, meteorite
impacts there are much more
violent than here on Earth liberating
much more energy: 20 million
joules for a 1-kg block. As seen from
the Earth, this would
produce a flash of magnitude 9 to 15.
From Ciel & Espace, No. 349
- Juin 1999, p. 17: Si, c'est possible!
(Translation courtesy
Bernd Pauli HD).
"The Leonid debris stream is in a retrograde orbit, and it's
inclined
just 22 degrees from the plane of Earth's orbit around the
sun," says
Professor George Lebo of the University of Florida Department of
Astronomy. "That's why the Leonids enter the atmosphere with
such a
high velocity [72 km/s]. The Earth and the Leonids hit head-on,
like
a head-on collision between two speeding automobiles."
"If you put yourself in the reference frame of the Earth
it's pretty
easy to figure out where these meteoroids will hit the Moon,
"continued Lebo. "On November 18, at 0h UT the lunar
sub-Leonid point
[the spot where Leonid meteoroids rain directly down on the
Moon's
surface] will be 9.4 degrees north of the lunar equator and 9.5
degrees sun ward of the day-night terminator. In other words, the
greatest flux of Leonids are going to hit nearly dead center on
the
lunar disk as seen from Earth, just over the terminator on the
sunlit
side."
It won't be possible to see flashes on the Moon's sunlit surface,
so
amateurs will have to look where the terrain is dark. The best
approach will be to train a telescope -- higher powers are best
for
discerning faint flashes -- at a spot near the lunar equator on
the
night side of the terminator, keeping the sunlit side of the moon
completely out of the field of view. Flashes observed with the
naked
eye would certainly be exciting, but might have little scientific
value. Instead, experienced observers suggest using a low-light
astronomical CCD video camera to make a permanent record.
The Leonids radiant, in the constellation Leo, rises above the
horizon at mid-northern latitudes around midnight on November
17/18.
That's about the same time that the Moon sets. It's an ideal
situation for observers who can monitor the Moon for the first
half
of the night and then enjoy the Leonid meteor shower from
midnight
until dawn.
Leonid Lunar Prospecting
Although optical flashes were not observed on the moon during
last
year's meteor shower, a team of scientists from the Boston
University
Center for Space Physics discovered indirect evidence for Leonid
impacts.
The Moon has an extremely tenuous atmosphere that contains, among
other things, sodium atoms. Just above the Moon's surface the
density
of sodium is 50 atoms per cubic centimeter. For comparison, the
sodium density in Earth's lower atmosphere is 1019/cc! Although
the
Moon's atmosphere is incredibly thin, researchers at Boston
University's space physics lab have built sensitive cameras that
can
trace its sodium component out to several lunar radii.
In mid-November 1998 the Boston University group were using their
sodium camera to monitor Earth's atmosphere for changes due to
Leonid
meteors. To their surprise they detected a bright sodium spot on
November 17 that grew in brightness, peaked on November 19, and
then
faded away. The spot was almost 180 degrees away from the new
Moon in
the night sky. Nevertheless, the source of the sodium was
apparently
Earth's satellite. When Leonid meteoroids crashed into the Moon's
dusty soil they kicked up an extra helping of sodium atoms,
increasing the density of the Moon's thin atmosphere. A long
lunar
sodium tail formed (much like the tail of a comet) which swept by
our
planet two days later.
The Boston University experiment showed for the first time that
intense meteor showers might be one way of "lunar
prospecting" from a
distance -- by looking at materials blasted off the surface as
meteoroids strike. A team of scientists from the University of
Texas
and NASA tried something similar earlier this year when they
crashed
NASA's Lunar Prospector spacecraft into the Moon. The probe was
sent
hurtling into a south polar crater on July 31 in hopes that the
impact would vaporize shadowed water-ice and send a cloud of
water
vapor and OH flying over the lunar limb. Telescopes, including
the
Hubble Space Telescope, looked near the impact site after the
crash,
but failed to detect evidence for water. That doesn't mean
there's no
water on the moon, say scientists. Lunar Prospector may simply
have
hit a dry spot, or perhaps the water vapor didn't rise high
enough to
see.
Dr. David Goldstein, a professor at the University of Texas who
proposed the Lunar Prospector impact experiment, is wondering if
the
Leonids might succeed where the Lunar Prospector crash failed.
Data
from Lunar Prospector's neutron spectrometer indicate that
water-ice
on the moon is concentrated around the Moon's poles where
shadowed
areas would allow pockets of water to remain frozen (see the
figure
below). The 1999 Leonids won't reach the Moon's south pole, but
many
meteoroids should strike the north pole.
"The Leonids will be coming in from above the ecliptic
plane," says
Goldstein. "Given the Earth-moon geometry on November 18th
that means
that the lunar north pole will be exposed, but not the south
pole.
That's unfortunate because there's thought to be more water
around
the south pole where we crashed Lunar Prospector. There's no
chance
of a Leonid meteoroid hitting the crater where Prospector
crashed.
Near the north pole the meteoroids will be coming in at several
degrees above the horizon -- very similar to the Lunar Prospector
trajectory."
"Compared to Lunar Prospector, Leonid meteoroids are light
weight and
tiny, but they move a lot faster," Goldstein continued.
"The mass of
Lunar Prospector was 160 kg and it was moving 1.7 km/s when it
hit
the moon on July 31. Leonid particles are going about 72 km/s.
That
means that a Leonid the mass of a golf ball (about 0.1 kg) would
deliver the same kinetic energy as the Lunar Prospector
crash."
"If a Leonid meteoroid did hit a spot near the north pole
with frozen
water, it's not clear what we would see. The Lunar Prospector
collision was like a car crash -- it was moving at relatively
slow
speed. When it hit, we hoped it would kick up water vapor that
would
be dissociated into OH by ultraviolet sunlight. In theory we
would
then see the OH by looking above the sunlit lunar limb with
appropriate spectrometers. A Leonid crash would be much more
violent.
Instead of water vapor gently wafting above the lunar limb, we
might
see ionized, hot plasma. It's possible that we would also get
some
warm water vapor that didn't sustain such a damaging shock wave,
but
it's really hard to say. We haven't done the high speed
simulations
yet."
Goldstein says that he and his colleagues may not have time to
organize a search for signs of water kicked up by Leonids this
year,
following so closely on the heels of the Lunar Prospector
experiment.
However, with some experts predicting significant Leonid activity
into the next millennium, there will be time to arrange an
observing
campaign for next year and beyond.
==========
(7) LUNAR LEONID METEORS
>From the IAU Circular 7320
http://cfa-www.harvard.edu/iauc/07300/07320.html#Item1
On Nov. 19 D. W. Dunham, Applied Physics Laboratory, Johns
Hopkins
University, reported the visual observation by B. Cudnik
(Houston, TX,
0.36-m telescope) on Nov. 18 of a brief flash near the center of
the
moon's dark limb, at least as bright as psi1 Aqr nearby.
This event,
1'.7 from the moon's edge, was apparently confirmed by Dunham
(Mount
Airy, MD, 0.13-m telescope) on two half-frames of a videotape
that
showed fading by about 5 mag during the intervening 1/60
second. On
Nov. 23 and 24 Dunham reported his confirmation of two lunar
flashes
videorecorded by P. V. Sada (Monterrey, Mexico, 0.13-m telescope)
half
an hour after Cudnik's observation, as well as of two lunar
flashes
videorecorded by D. Palmer (Greenbelt, MD) up to an hour or so
earlier;
there was also a probable untimed additional visual confirmation
of the
Cudnik event by S. Hendrix (Cameron, MO, 0.11-m telescope).
Dunham has
summarized his own measurements of the five Nov. 18 events as
follows:
Disc.
UT
m1 m2 lambda beta Lunar location
h
m s
s
deg deg
Palmer 3 49 40.5 +/- 0.4
3 7 48 W 1
N 175 km SW of Kepler
Palmer 4 08 04.1 +/- 0.6
5 8 70 W 15 S 175
km S of Grimaldi
Cudnik 4 46 15.2 +/- 0.1
3 8 71 W 14
N 50 km ENE of Cardanus
Sada 5 14 12.93 +/- 0.05
7 8 58 W 15 N 200
km WNW of Marius
Sada 5 15 20.23 +/- 0.05
4 7 59 W 21
N 75 km S of Schiaparelli
The magnitude m1 is that on the first frame showing the event, m2
that
on the following half-frame; the first event listed also seems to
be
present on a third half-frame at mag 9. The selenographic
coordinates
(longitude lambda and latitude beta) and lunar location for the
first
two events are uncertain by 5 deg or more, but the others should
be
accurate to within about 2 deg (50 km). Following Dunham's
suggestion
that the flashes resulted from Leonid impacts on the moon, D. J.
Asher,
Armagh Observatory, computed that the center of the 1899 dust
trail
that evidently produced the 1999 Nov. 18 Leonid activity (cf.
IAUC
7311) by nominally passing 0.0007 AU from the geocenter would
have
passed 0.0002 AU from the selenocenter around 4h49m UT.
DD CIRCINI
A. C. Gilmore provides further photometry of DD Cir = Nova Cir
1999,
obtained as before (see IAUC 7249): Sept. 3.415 UT, V = 10.42,
U-B =
-0.43, B-V = +0.38, V-R = +2.01, V-I = +1.65, airmass = 1.50;
4.389,
10.38, -0.43, +0.35, +2.00, +1.57, 1.42; 13.368, 10.98, -0.46,
+0.14,
+1.82, +1.05, 1.43. Standard deviations are 0.01 mag or less.
(C) Copyright 1999 CBAT
1999 November
26
(7320)
Brian G. Marsden
Reproduced by permission.
===============
(8) ANSMET - COLLECTING METEORITES IN THE ANTARCTIC WITH THE
INTERNET
>From Matthew Genge <M.Genge@nhm.ac.uk>
Dear Benny,
I'm off to the Antarctic to collect meteorites on Monday and
thought
that CCNet might be interested in the following.
The internet, it seems, gets everywhere. Web cameras capture
virtually
every human endeavor from DIY to sky diving. Now the internet has
finally reached even the remotest region of the Earth's surface.
Over
the next two months the Antarctic Search for Meteorites
expedition to
the frozen continent will be covered by http://www.webexpeditions.com.
You'll be able to see images of our daily hunting escapades and
read our
journal entries. If you've ever wondered how cold meteoriticists
have to get
before they stop talking about meteorites then this is your
chance to
find out.
Minus 30 ought to do it.
Matthew Genge
____________________
Dr Matthew J. Genge
Researcher (Meteoritics)
Department of Mineralogy, The Natural History Museum
Cromwell Road, London SW7 5BD, UK.
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