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(meteorobs) Excerpts from "CCNet DIGEST, 23 April 1999"


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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 23 April 1999
Date: Fri, 23 Apr 1999 11:51:00 -0400 (EDT)

CCNet DIGEST, 23 April 1999
---------------------------

(1) SPORADIC BACKGROUND COMPONENT IN METEOR ACITIVY
    Jonathan Shanklin <jdsh@mail.nerc-bas.acdot uk> 

[..]

(5) CRATER COUNT REVEAL SIGNIFICANT DIFFERENCES FOR IMPACTS ON     
    EARTH & MARS 
    J. Caers et al., STANFORD UNIVERSITY

[...]

(9) SECULAR INTERACTIONS OF COORBITING OBJECTS
    F. Namouni, UNIVERSITY OF LONDON QUEEN MARY & WESTFIELD COLL 

(10) EXTERNAL SOURCES OF WATER FOR MERCURY'S PUTATIVE ICE DEPOSIT
     J.I. Moses et al., LUNAR & PLANETARY INSTITUTE

(11) ORIGIN OF POLYMICT BRECCIAS ON ASTEROIDS
     K. Saiki et al., AKITA UNIVERSITY


===========
(1) SPORADIC BACKGROUND COMPONENT IN METEOR ACITIVY
 
>From Jonathan Shanklin <jdsh@mail.nerc-bas.acdot uk> 

Re: CCNet Digest, 22 April 1999
 
The NASA article at 
http://science.nasadot gov/newhome/headlines/ast21apr99_1.htm
is quite informative, however NASA seem to be unaware that there is a 
sporadic background component to meteor activity and that there are 
also a number of minor showers active during the first four months of 
the year. Meteor enthusiasts will actually see 5 - 10 meteors an hour 
most clear mornings from the sporadic component. The Virginid minor 
shower is active throughout March and April and in total this will 
produce more meteors than the much shorter lived Lyrid shower, though 
the peak rate is only around 5 meteors an hour. The only exception to 
this is when the Lyrids produce their occasional outbursts and I was 
fortunate enough to see the 1982 outburst from the South Atlantic. This 
produced a ZHR of around 100 but the observed rate was barely half 
this. I've seen 8 Lyrids in one watch so far in April but it looks as 
if cloud will obscure the maximum.  UK observers can start looking any 
time after midnight, but the moon will interfere a bit until around 03 
BST and it begins to get light around 04 BST.
  
Jon Shanklin
j.shanklin@bas.acdot uk
British Antarctic Survey, Cambridge, England
http://www.nbs.acdot uk/public/icd/jds

==================
(5) CRATER COUNT REVEAL SIGNIFICANT DIFFERENCES FOR IMPACTS ON     
    EARTH & MARS 
 
J. Caers*), J. Beirlant, M.A. Maes: Statistics for modeling heavy 
tailed distributions in geology: Part II. Applications. MATHEMATICAL 
GEOLOGY, 1999, Vol.31, No.4, pp.411-434
 
*) STANFORD UNIVERSITY,STANFORD,CA,94305
 
In this paper we present three diverse types of applications of extreme 
value statistics in geology, namely: earthquakes magnitudes, diamond 
values, and impact crater size distribution on terrestrial planets. 
Each of these applications has a different perspective toward tail 
modeling, yet many of these phenomena exhibit heavy or long tails which 
can be modeled by power laws. It is shown that the estimation of 
important tail characteristics, such as the extreme value index, is 
directly linked to the interpretation of the underlying geological 
process. Only the most extreme data are useful for studying such 
phenomena so thresholds must be selected above which the data become 
power laws. In the case of earthquake magnitudes, we investigate the 
use of extreme value statistics in predicting large events on the 
global scale and for shallow intracontinental earthquakes in Asia. 
Large differences are found between estimates obtained from extreme 
value statistics and the usually applied standard statistical 
techniques. In the case of diamond deposits, we investigate the impact 
of the most precious stones in the global valuation of primary 
deposits. It is shown that in the case of Pareto-type behavior the 
expected value of few extreme stones in the entire deposit has 
considerable influence on the global valuation. In the case of impact 
crater distributions, we study the difference between craters 
distributions on Earth and Mars and distributions occurring on other 
planets or satellites within the solar system. A striking result is 
that all planets display the same distributional tail except for Earth 
and Mars. In a concluding account, we demonstrate the apparent 
loghyperbolic variation in all of the above-mentioned examples.
Copyright 1999, Institute for Scientific Information Inc.

===============

(9) SECULAR INTERACTIONS OF COORBITING OBJECTS

F. Namouni: Secular interactions of coorbiting objects. ICARUS, 1999, 
Vol.137, No.2, pp.293-314

UNIVERSITY OF LONDON QUEEN MARY & WESTFIELD COLL, ASTRONOMY UNIT,
MILE END RD, LONDON E1 4NS, ENGLAND

We study the dynamics of the three-body problem when two objects are in 
the 1:1 mean motion commensurability and can experience close 
encounters. Three orbit families are relevant to the dynamics: 
horseshoe orbits, passing orbits, and retrograde satellite orbits. The 
fatter two families are present only when the relative eccentricity of 
the two objects is large in comparison to the width of their coorbital 
region. In the planar case, the three families are disjoint because the
guiding center of the motion is stationary with respect to secular 
perturbations; as a result, collisional orbits are unstable. Orbit 
transitions between different families occur in three dimensions owing 
to the secular variations of the eccentricity and argument of 
pericenter omega(r). Potentially colliding horseshoe orbits transit via 
eccentric retrograde satellite orbits in a secular mechanism where 
omega(r) librates around +/-90 degrees, Coorbital passing orbits are 
located at the Kozai resonances with 2 omega(r) librating around either 
0 degrees or 180 degrees, The former resonance is a dominant 
feature of the dynamics once passing orbits exist. The latter is found 
only at large values of the Jacobi constant. Large amplitude librations 
around the Kozai resonances involve the transition from a passing orbit 
to an association of horseshoe and retrograde satellite orbits and the 
secular periodic reversal of the relative semimajor axis of the passing 
orbit. The stability of these structures supports the existence of 
coorbiting material with the planets, especially Mercury, and favors a 
coorbital origin of the retrograde natural satellites.
(C) 1999 Academic Press.

=============== 
(10) EXTERNAL SOURCES OF WATER FOR MERCURY'S PUTATIVE ICE DEPOSIT

J.I. Moses*), K. Rawlins, K. Zahnle, L. Dones: External sources of 
water for Mercury's putative ice deposits. ICARUS, 1999, Vol.137, No.2, 
pp.197-221

*) LUNAR & PLANETARY INST,3600 BAY AREA BLVD,HOUSTON,TX,77058

Radar images have revealed the possible presence of ice deposits in 
Mercury's polar regions. Although thermal models indicate that watts 
ice can be stable in permanently shaded regions near Mercury's poles, 
the ultimate source of the water remains unclear. We use stochastic  
models and other theoretical methods to investigate the role of 
external sources in supplying Mercury with the requisite amount of 
water. By extrapolating the current terrestrial influx of 
interplanetary dust particles to that at Mercury, we find that 
continual micrometeoritic bombardment of Mercury over the last 3.5 byr 
could have resulted in the delivery of (3-60) x 10(16) g of water ice 
to tb permanently shaded regions at Mercury's poles (equivalent to an 
average ice thickness of 0.8-20 m), Erosion by micrometeoritic impact 
on exposed ice deposits could reduce the above value by about a half. 
For comparison, the current ice deposits on Mercury are believed to be 
somewhere between similar to 2 and 20 m thick. Using a Monte Carlo 
model to simulate the impact history of Mercury, we find that asteroids 
and comets can also deliver an amount of water consistent with the 
observations. Impacts from Jupiter-family comets over the last 3.5 
billion years can supply (0.1-200) x 10(16) g of water to Mercury's 
polar regions (corresponding to ice deposits 0.05-60 m thick), 
Halley-type comets can supply (0.2-20) x 10(16) g of water to the poles 

(0.07-7 m of ice), and asteroids can provide (0.4-20) x 10(16) g of 
water to the poles (0.1-8 m of ice). Although all these external 
sources are nominally sufficient to explain the estimated amount of ice 
currently at Mercury's poles, impacts by a few large comets and/or 
asteroids seem to provide the best explanation for both the amount and 
cleanliness of the ice deposits on Mercury. Despite their low 
population estimates in the inner solar system, Jupiter-family comets 
are particularly promising candidates for delivering water to Mercury 
because they have a larger volatile content than asteroids and more 
favorable orbital and impact characteristics than Halley-type comets. 
(C) 1999 Academic Press.

=========
(11) ORIGIN OF POLYMICT BRECCIAS ON ASTEROIDS

K. Saiki*), H. Takeda: Origin of polymict breccias on asteroids deduced 
from their pyroxene fragments. METEORITICS & PLANETARY SCIENCE, 1999, 
Vol.34, No.2, pp.271-283

*) AKITA UNIV,FAC ENGN & MAT SCI,RES INST MAT & RESOURCES,1-1 
   TEGATA GAKUEN MACHI,AKITA 0108502,JAPAN

Cumulate eucrite, noncumulate eucrite, and diogenite meteorites are 
considered to have come from the crust of one (or similar) parent 
asteroid. Howardites are regarded as regolith breccias of eucrites and 
diogenites, and polymict eucrites are regarded as polymict breccias of 
eucrites. These polymict breccias show many textural and chemical 
features. In order to gain a better understanding of the origin of 
polymict breccias and the origin of their components, we investigated 
four polymict breccias, Yamato (Y)-791439, Y-791192, Y-82009, and 
Y-82049 with a scanning electron microscope (SEM) equipped with a 
chemical mapping system, and by electron probe microanalysis (EPMA). We 
analyzed all pyroxene grains with chemical maps, classified them by 
chemical composition, and observed their chemistry and mineralogy in 
detail. The characteristics of pyroxenes suggest that the polymict 
breccias were generated by gathering locally ordinary eucrites and 
cumulate eucrites. The chemical-evolutionary features of the pyroxenes 
(such as homogenization, chemical zoning, and exsolution lamellae) 
suggest that there were at least two long annealing events and one 
short (or low-temperature) annealing event, separated by mixing events. 
Local heterogeneity on the asteroidal crust is also suggested. 
Copyright property of the Meteoritical Society, 1999.

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