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(meteorobs) Excerpts from "CCNet 29/2001 - 23 February 2001"
Sorry, folks, for the delay in forwarding items from last week...
I have forwarded this number of CCNet Digest to BOTH 'meteorobs'
and 'imo-news', as I thought it of significant general interest.
A fascinating issue, full of questions (and a few answers) raised
by the NEAR-Shoemaker mission! Note in particular the fascinating
idea (re?)proposed by Prof. Jeffrey Bell of University of Hawai'i!
I can't help but wonder to what extent the mechanism described by
Prof. Bell may be of significance for dust stream evolution models
currently being tested and developed for cometary objects?
Also note the exciting reference from Dr. Fred Singer on his long-
ago published predictions, regarding electrostatic dust transport
in the region of small bodies. I wonder to what extent these will
also be a significant factor for the much more complex comae which
surround active comet nuclei? If so, then that effect should also
have an important role in dust ejection models. Perhaps Rob, Peter
and/or Esko would care to comment on whether they already account
for this effect in their respective dust stream models?
It was only slightly disappointing Prof. Bell chose to ignore the
preferred nomenclature for meteorites vs. meteoroids - or is the
usage maybe different for some reason among planetary scientists?
Either way, both his ideas and those of Dr. Singer challenge the
meteorical community in TWO intriguing ways - one, in the devel-
opment of theoretical models. And two, perhaps of more interest
to amateur 'meteorobs' readers, they challenge us to imagine and
design experiments (that is, observational programs in meteors)
which may conceivably help test these ideas indirectly!
Clear skies,
Lew Gramer
------- Forwarded Message
From: Benny Peiser <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 23 February 2001
Date: Fri, 23 Feb 2001 14:56:18 -0000
CCNet 29/2001 - 23 February 2001
--------------------------------
[...]
(4) SONIC BOOM SHAKES ENGLAND'S EAST COAST
Ron Baalke <baalke@jpl.nasadot gov>
(5) SOHO ANALYSIES KAMIKAZE COMET
ESA News, 23 February 2001
(6) NEAR MISSION EXTENDED THROUGH FEBRUARY
Ron Baalke <baalke@jpl.nasadot gov>
(7) LANDING ON EROS
Ron Baalke <baalke@jpl.nasadot gov>
(8) JEFF BELL'S EROS THESIS
David Morrison <dmorrison@arc.nasadot gov>
(9) SEVEN ENCOUNTERS WITH MINOR BODIES IN THE NEXT 5 YEARS
Daniel Fischer <dfischer@astro.uni-bonndot de>
[...]
(11) DUST ON EROS
S. Fred Singer <singer@sepp.org>
[...]
========================================================================
(4) SONIC BOOM SHAKES ENGLAND'S EAST COAST
>From Ron Baalke <baalke@jpl.nasadot gov>
http://www.mirror.codot uk/shtml/NEWS/P23S5.shtml
COAST HIT BY BOOM
The Mirror (London)
February 22, 2001
A SONIC boom has been blamed for tremors which shook buildings and
rattled windows along a 25-mile stretch of coastline yesterday.
Residents from Scarborough to Whitby in Yorkshire told of a "bang, a
boom and a rumble" for two seconds at 11.40am.
Experts ruled out an earthquake, saying it was probably a sonic boom
caused by an aircraft or even a meteor.
Seismologist Glenn Ford said: "All the evidence points to a sonic boom,
which can travel hundreds of miles if not impeded."
Investigators have contacted military officials.
========================================================================
(5) SOHO ANALYSIES KAMIKAZE COMET
>From ESA News, 23 February 2001=20
http://sci.esa.int/content/news/index.cfm?aid=3D1&cid=3D1&oid=3D26188
A comet that fell into the Sun on 7 February was tracked by two different
instruments on the ESA-NASA SOHO spacecraft, enabling scientists to
characterize it quite precisely. This was just one of nearly 300 comets
discovered by SOHO since 1996, thanks mainly to the privileged view of the
sky around the Sun given by the visible-light coronagraph LASCO. On this
occasion SOHO's ultraviolet coronagraph UVCS also observed the comet
repeatedly. It gave valuable additional information, both about the comet
and about the solar wind close to the Sun.
More at: =
http://sci.esa.int/content/news/index.cfm?aid=3D1&cid=3D1&oid=3D26188
========================================================================
(6) NEAR MISSION EXTENDED THROUGH FEBRUARY
>From Ron Baalke <baalke@jpl.nasadot gov>
http://near.jhuapldot edu/news/flash/01feb22_1.html
NEAR Extended Through February
February 22, 2001
NASA has given the go-ahead for the NEAR mission to
collect data from the surface of Eros through Feb. 28,
tacking four days onto an extension granted after the
NEAR Shoemaker spacecraft's historic landing on the
asteroid last week.
The extension gives NEAR Shoemaker's gamma-ray
spectrometer additional time to observe the elemental
composition on and below Eros' surface, and the NEAR
team at least two more opportunities to download this
information through NASA's heavily used Deep Space
Network of antennas.
"This allows us to build a much better sample," says
Jacob Trombka, of NASA's Goddard Space Flight Center,
team leader for NEAR Shoemaker's X-ray/Gamma-ray
Spectrometer. "The longer you accumulate data the more
you can reduce the uncertainty of your results. When you
look at a little bit of data you see clues, but when you
get more data down you can be a bit more definitive."
Touching down on Eros certainly hasn't kept NEAR
Shoemaker from touching base with NEAR scientists. The
spacecraft has returned readings from its magnetometer,
and today mission team members are downloading the
latest information from the gamma-ray spectrometer.
The gamma-ray instrument can measure elemental
composition to a depth of about 4 inches, and is much
more sensitive on the surface than it was in orbit.
Mission engineers fine-tuned the device last week to
account for things it hadn't encountered in orbit; it
operates at a higher temperature near the surface, for
example, because it can no longer radiate heat into
space.
"We optimized the instrument for collecting science in
its new environment," says John Goldsten, of the Johns
Hopkins Applied Physics Laboratory (APL), lead engineer
for the gamma-ray spectrometer. "Now that we know how
well it's operating . . . the data we expect from here
on is prime science data."
While Trombka says they'll need months to interpret that
data, it won't take nearly as long for mission
scientists to get a clearer picture of the asteroid's
magnetic properties - or lack thereof. NEAR Shoemaker's
magnetometer found no evidence of magnetism at its
landing site. Having returned data from the surface, the
instrument's work is done and it has been turned off.
"We already knew there was no global magnetic field, but
measuring this close dramatically increased our ability
to see if there was evidence for localized 'hot spots'
on the surface," says Brian Anderson, magnetometer
instrument scientist at APL. "The landing site shows no
evidence for an intrinsic magnetic field. Since the
sensor is only two meters above the surface this is a
pretty definitive measurement."
========================================================================
(7) LANDING ON EROS
>From Ron Baalke <baalke@jpl.nasadot gov>
http://near.jhuapldot edu/news/sci_updates/01feb20.html
NEAR Shoemaker Science Update
Landing on Eros
February 20, 2001
On Monday, 12 February 2001, the NEAR spacecraft touched
down on asteroid Eros, after transmitting 69 close-up
images of the surface during its final descent. Watching
that event was the most exciting experience of my life.
I was asked immediately afterwards how I felt, and I
mumbled something about being tired and happy, but I
missed the point. I realized afterward what I should
have said: it was like watching Michael Jordan on the
basketball court, when the game is on the line and he is
in the groove. One miracle after another unfolds, and we
are left stunned and speechless. When we learned that
the spacecraft had not only landed on the surface, but
was still operational, we hardly knew what to think.
Over the past week, we have started to come to
our senses again and to appreciate how
fortunate we are. The final weeks of low altitude
operations revealed bizarre and surprising aspects of
surface structures on Eros, including one type of
feature we noticed for the first time in the very last
image taken by the spacecraft (the incomplete image
taken from a height of 120 meters, 2001 Feb 12F ). As we
discussed previously, there are markedly fewer small,
fresh craters on Eros than we would expect from our
experience at the Moon, and an amazing profusion of
boulders, likewise more than we expected. We do not know
just what is happening on the surface of Eros to cover
and/or obliterate craters while making and/or uncovering
boulders. We have seen many examples of mass motion on
Eros - loose material sliding downhill - and that is no
doubt part of the story, but maybe not all of it. We
also believe that at least some of the bouldery debris
found on Eros is comprised of ejecta from impacts on
Eros; some of these ejecta do not escape but fall back
to the surface.
Some of the strange features we are beginning to think
about can be seen in the low altitude images obtained
during the past few weeks. The new type of feature seen
in the last image returned ( 2001 Feb 12F ) can be
found, for example, at the bottom of the image (just
above the vertical streaks indicating loss of signal),
to the left of center. It appears to be a collapse
feature, formed when support is removed from below the
surface, and it is about the size of one's hand. Other
strange sights are clusters of boulders (e.g., the upper
right of 2001 Feb 12E ) - are these cases of
disintegration in place? - and extremely flat, sharply
delineated areas in the bottoms of some craters (e.g.,
the two left panels of 2001 Jan 31 ). The mere existence
of sharp boundaries, called "contacts", is surprising in
itself, especially if the entire surface of the asteroid
is thought to have been blanketed by debris from
impacts. These boundaries can be incredibly sharp on
Eros, as evidenced by the last frame, 2001 Feb 12F
(compare the upper right and lower left of the image).
The images tell us a tale whose outcome we don't yet
know, but there is more: the story of Eros's composition
is likewise still emerging. Our orbital data from the
x-ray spectrometer showed that the abundances of key
elements on Eros are very similar to those in the
undifferentiated meteorites called ordinary chondrites,
but there was a discrepancy. The abundance of the
volatile element sulfur is less than we would expect
from an ordinary chondrite. However, the x-ray spectra
tell us only about the uppermost hundred microns of the
surface, and we do not know if the sulfur depletion
occurs only in a thin surface layer or throughout the
bulk of the asteroid.
Fortunately, the spacecraft is now in a position to help
answer the question (on the surface, that is). The gamma
ray spectrometer measures the composition to a depth of
about ten centimeters, and it is much more sensitive on
the surface than it was in orbit. We are now in the
process of trying to obtain our best yet gamma ray
spectrum of Eros. We will try to determine the
abundances of the volatile element potassium and the
major element iron from this spectrum, to look harder at
the match between the compositions of Eros and the
ordinary chondrites, and to look for evidence for bulk
depletion of volatiles. The latter would suggest that
Eros has undergone significant heating (a geologist
would call it "metamorphism").
It is sad for me to say, but the gamma ray measurement
will be the last from NEAR - one more miracle is what we
ask of this little spacecraft. Its job is almost done,
but ours is just beginning.
Andrew Cheng
NEAR Project Scientist =20
========================================================================
(8) JEFF BELL'S EROS THESIS
>From David Morrison <dmorrison@arc.nasadot gov>
NEO News (02/20/01) Jeff Bell's Eros Thesis
Dear Friends and Students of NEOs:
This is an unusual edition of NEO News. The NEAR-Shoemaker mission=20
has been a tremendous success, and the spacecraft continues to=20
collect data on the surface of the asteroid. We now know that this=20
asteroid is a monolithic (or nearly so) rock with relatively=20
primitive composition, with a surface sculpted by innumerable=20
impacts. But the size distribution of the craters on Eros is=20
different from anything we have seen before, with a remarkable=20
deficit of craters with diameters below 100 m, as well as a great=20
many rocks and boulders on the surface. Most of the interpretations=20
discussed by the NEAR science team involve the filling in of the=20
small craters by surface dust (a mobile regolith). This is the=20
"orthodox" opinion. What is printed below is an unorthodox new=20
interpretation by Jeff Bell of the University of Hawaii, submitted to=20
the Lunar and Planetary Science Conference (LPSC) to be held in=20
Houston next month. I certainly would not claim that Jeff is right,=20
but I do think his ideas are provocative and interesting, and that=20
they should contribute to a spirited technical debate about the=20
geological history of both Near Earth and Main Belt Asteroids.
David Morrison
========================================================================
========================================================================
EROS: A COMPREHENSIVE MODEL. Jeffrey F. Bell,
Hawaii Institute of Geophysics and Planetology, Univ. of
Hawaii, 2525 Correa Rd., Honolulu HI 96822
(bell@pgd.hawaiidot edu).
Introduction: The NEAR spacecraft has provided a
variety of information about the planet-crossing S-class
asteroid Eros. However, most interpretations to date
have relied heavily on earlier experience on the Moon
and asteroids which were viewed only distantly during
spacecraft flybys. The close approach of NEAR to Eros
in October 2000 revealed surprising new facts that
suggest that impact processes and regolith evolution on
asteroids are very different from any other planetary
objects.
Craters and Boulders: Early crater abundance
curves [1] based on low-resolution NEAR images
suggested a "normal" crater curve in the 1000m to
100m diameter range, i.e. a high crater density, rising
toward saturation levels at the smaller sizes. Later
NEAR images from closer distances have revealed an
extraordinary crater distribution: the crater density
declines sharply below 100m until at 4m diameter the
craters are about 200 times less abundant than expected.
Post-cratering modification seems inadequate to explain
this situation. In particular, any geological process such
as regolith migration that obscures craters should also
obscure boulders; yet the size spectrum of boulders on
Eros shows an inverse correlation with the craters,
being highly biased toward smaller sizes. It seems much
more likely that this "anomalous" crater distribution
reflects the actual size spectrum of incoming projectiles.
This must be very different from the normal inner solar
system projectile size spectrum we see reflected in the
crater records of the Moon, Mercury, and Mars. I
propose that Eros exhibits the normal cratering function
on main-belt asteroids, previously concealed from us by
the low resolution of the images obtained in distant
flybys of Gaspra, Ida, and Mathilde.
Yarkovsky to the Rescue: The Yarkovsky Effect
[2] is a mechanism for orbital evolution due to asymmetric
emission of thermal IR photons from a rotating
object that is warmer on the "afternoon" region than on
the "morning" region. This effect is strongest for objects
around a few meters in size (the force/mass ratio is too
small for larger objects, while smaller ones cannot
maintain the necessary morning/afternoon thermal
assymetry due to internal conduction). It has been
proposed [3,4] that the Yarkovsky Effect provides an
efficient mechanism for moving meteorites from any
location in the asteroid belt to the narrow Jupiter
resonance zones, from which they are rapidly perturbed
out of the belt. If so, the asteroid belt should be strongly
depleted in objects smaller than a few meters, which
would naturally produce a strong depletion in craters
smaller than about 100m on all main-belt objects. Since
the collisional cascade in the belt is prematurely cut off
by the Yarkovsky Effect, even particles too small to be
directly affected will be underabundant. Since there is
a population of dust derived from asteroids (e.g. the dust
bands associated with the Hirayama families), there
must be some mechanism for limited replenishment of
very small particles, probably direct generation of dust
from larger objects.
The underabundance of small impactors also provides
a natural explanation for the size spectrum of
boulders on Eros. Boulders are created by ejection from
larger impacts and gradually eroded by smaller ones.
This can be easily seen on the Moon, where fields of
jagged boulders are seen around fresh craters. The
absence of smaller impactors allowed boulders to
accumulate on Eros without being broken up or eroded.
The fact that this unique signature of the main-belt
environment is still visible on Eros implies that it has
undergone little cratering since its orbital evolution
decoupled it from the asteroid belt. In its current orbit,
it should be experiencing a roughly lunar-like bombar-
dment environment. There is no trace of this late phase
of Eros history on its surface.
Elemental Abundances: The increasing abundance
of boulders visible in the close-approach images suggests
that at sizes somewhat smaller than the current
resolution limit, the surface may be mostly covered with
rocks. This implies that the published X-ray data [5]
(which samples material to ~100 microns depth) is
mostly sampling the outer layers of large rocks, not
fine-grained weathered regolith as expected before the
mission [6]. (In meteoritical terms, the data mostly
samples clasts instead of matrix) The XGRS team has
suggested that impact volatilization of sulfur in the
weathered regolith could account for the grossly
nonchondritic level of sulfur (<1%) observed in the X-
ray data. It appears more likely that their alternate
hypothesis of early partial differentiation (in which
sulphur is mobilized early) is the correct explanation.
Gamma-ray data, with a sampling depth of ~10cm,
should almost entirely represent weathering-free rock
interiors and resolve this ambiguity.
Color and Spectrum: A striking "anomaly" on
Eros is the close similarity of all areas on the surface in
color. While there are many regions of higher albedo in
exactly the locations we expect to find fresh,
unweathered material (steep slopes where downslope motion is
likely), the spectra curves of these areas are very similar
to those of darker (presumably older) areas. In particular,
there is no region that looks at all like ordinary
chondrites, the most publicized meteorite analog for
Eros. The advocates of a primitive Eros have been
forced to argue that "space weathering" is so rapid there
that even the most recent crater interiors, ejected blocks,
and landslides have been weathered almost to maturity.
Little attention has been given to the alternate hypothesis:
that even the oldest surfaces are so young and
immature that they are almost identical to the youngest
craters and slumps. This is the logical result of a
Yarkovsky-controlled main-belt bombardment
environment: an intense (~1000 times lunar) bombardment of
large, low-velocity (~6 km/sec) projectiles constantly
excavating fresh bedrock, and completely swamping the
weathering (impact melting and volatilization) effects
produced by the small component of high-velocity dust
entering the belt from other sources (mostly long-period
comets).
Of course, in the time since Eros became decoupled
from the asteroid belt, it has been in a more familiar,
approximately lunar, bombardment environment. But
spectral weathering effects on the moon simply do not
occur fast enough to account for the absence of any
chondrite-like areas on Eros. Planet-crossing asteroids
are ephemeral phenomena. On average, their survival
time against sun impact or ejection by Jupiter encounters
is 5-10My [7]. From the limited orbital modeling
that has been done for Eros, it appears that it may
currently be in a special dynamical environment that
would lengthen its lifetime to perhaps 50-100My [8].
Even this is about a factor of 10-20 too short. On the
Moon, we know that crater Tycho (age 110My) is almost
pristine spectroscopically, while Copernicus (age
~800My) is well along toward maturity. Unless Eros
has been specially preserved for ~1By in some "cosmic
lockbox", it has not been out of the belt long enough for
its spectral properties to reflect its new environment.
Summary: The history of Eros may be summarized as:
1) Condensation and accretion of a larger
parent body; 2) Heating and limited partial melting; 3)
Migration of the sulfur-rich melt to another region of
the parent body; 4) Progressive collisional fragmentation
of the parent body in which the current shape and
surface of Eros was produced; 5) Intense cratering in
the asteroid belt by a projectile population strongly
depleted in small objects by the Yarkovsky Effect; 6)
Recent perturbation onto a planet-crossing orbit; 7)
Short exposure to more lunar-like bombardment and
solar-wind environments which have had insufficient
time to significantly alter the surface.
References:
[1] Veverka J. et al. (2000) Science, 289, 2088-2097.
[2] Opik E. J. (1951) Proc. R. Irish Acad., 54, 165.
[3] Hartmann W. K. et al. (1997) LPS XXVIII, 517.
[4] Farinella P. et al. (1998) Icarus, 132, 378.
[5] Trombka J. I. et al. (2000) Science, 289, 2101.
[6] Bell J. F. (1997) LPS XXVIII, 83-84.
[7] Gladman et al. (1997) Science, 277, 197.
[8] Michel et al. (1998) Astron. J., 116, 2023.
========================================================================
ADDED COMMENTS AFTER NEAR LANDING. (Jeffrey F. Bell, Univ. of Hawaii)
The model for Eros described in my LPSC abstract (above) has=20
been fully confirmed by the high resolution images acquired during=20
the "landing phase" of the NEAR mission on Feb. 12. These reveal a=20
surface dominated by rocks, so many in some areas that the surface is=20
nearly saturated with cm- to m- sized rocks. There is no sign of=20
small craters or "zap pits" on the rocks, and most of them appear=20
angular and un-eroded, indicating that the population of mm- to cm-=20
sized projectiles in the asteroid belt is still depleted well below=20
the 1 to 100 meter size range directly affected by the Yarkovsky=20
Effect. This suggests that "filling-in" of the size spectrum below=20
the Yarkovsky Gap by further collisional evolution is negligible.=20
Furthermore, it is now clear that a large fraction of the X-ray=20
photons detected by the XGRS instrument came from solid rock=20
surfaces, not fine-grained "weathered" regolith. The >90% depletion=20
of sulfur abundance relative to chondrites observed by this=20
instrument cannot be explained by weathering processes and must be=20
intrinsic to the bedrock of Eros, implying a significant degree of=20
partial melting and migration of S (and Fe?) to some other part of=20
the Eros Parent Body.
========================================================================
(9) SEVEN ENCOUNTERS WITH MINOR BODIES IN THE NEXT 5 YEARS
From: Daniel Fischer <dfischer@astro.uni-bonndot de>
Dear Benny,
CCNet readers bracing for the first science results from the first asteroid
lander might appreciate the following (not that) little list: In the next 5
or so years there will be, if everything works out, no fewer than seven
encounters of spacecraft with comets and asteroids.
All the following missions are fully funded, though only 2 of the 6 have
already been launched (the others will follow in 2002 to 2004):
2001 Sept. 22 Comet Borrelly Deep Space One (simple flyby)
2003 Nov. 12 Comet Encke CONTOUR (simple flyby)
2004 Jan. 1 Comet Wild 2 Stardust (coma sample return)
2005 July 3 Comet Tempel 1 Deep Impact (big mass impact)
2005 Sept. XX Asteroid 1998 SF36 Muses-C (sample return)
2006 June 18 Comet S.-W. 3 CONTOUR (simple flyby)
2006 July 11 Asteroid Otawara Rosetta (simple flyby)
All dates are from the respective mission homepages or press releases
(in http://www.astro.uni-bonndot de/~dfischer/mirror/219.html you can find
links to all of them in a sidebar of the NEAR stories).
There are more scheduled flybys in 2008 (CONTOUR & Rosetta again) - and
in 2011 we'll then have Rosetta as the first comet orbiter and eventually
its RoLand as the first comet lander (though both might well be beaten by
a clever Discovery mission - it's still 10 years to go).
All the missions listed above are funded by civilian space agencies (NASA,
ISAS and ESA) - but there were also two NEA missions under consideration
in the 1990's, Clementine 2 by the BMDO and NEAP by the company SpaceDev.
The former seems to have disappeared completely after the 1997 death of
its chief-scientist-to-be Gene Shoemaker, and the latter is apparently
in limbo:
NEAP will be launched "within the next 3-5 years", according to:
http://www.spacedev.com/missions/neap.htm
Daniel Fischer
========================================================================
* LETTERS TO THE MODERATOR *
========================================================================
(11) DUST ON EROS
>From S. Fred Singer <singer@sepp.org>
Dear Benny
I have read informal reports quoting Joe Veverka, a leading planetary
researcher at Cornell, that there is something strange about the
distribution of dust accumulation in craters on Eros.
Now, I haven't examined any data or seen any pictures yet. But I would
speculate that we may be dealing here with an interesting phenomenon of
electrostatic dust transport. I discussed it in papers in Icarus
around 1960 +/-- a year or two. [I don't have the papers at hand.]
The idea is the following: An airless body in interplanetary space will
carry an electric charge. [It was originally thought to be negative--
because of ambipolar diffusion (Spitzer, Whipple)--but I determined
that the photoelectric effect from solar UV was important enough to make
it positive.]
A dust particle sitting on the body's surface would have the same potential
but carry a negligible charge. However, once it is kicked up (by an impact)
and moves beyond the Debye shielding distance, its electric charge increases
and affects its motion in the body's gravity field.
Since the ratio of electrostatic to gravity force depends strongly on size
of the dust particle, there will be a strong fractionation effect. Particles
in a certain size interval will be able to hop over large distances before
returning to the surface. The tiniest particles will escape.
In addition, there will be interesting effects if the particle enters the
body's shadow and loses its positive charge. On the Moon, there are
regions in perpetual shadow. I don't know the situation on Eros, but
we should look for the effect.
Of course, once we land astronauts on Phobos and Deimos, we will be
able to carry out the detailed measurements that are required.
On to Mars!
Fred
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