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(meteorobs) Excerpts from "CCNet DIGEST, 14 May 1999"
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 14 May 1999
Date: Fri, 14 May 1999 12:37:07 -0400 (EDT)
CCNet DIGEST, 14 May 1999
-------------------------
[...]
(3) NEAs: REMNANTS OF LARGE ASTEROID COLLISIONS
Andrew Yee <ayee@nova.astro.utorontodot ca>
[..]
(7) HIGH TEMPERATURES IN RETURNING EJECTA FROM THE R IMPACT OF COMET SL9
S.J. Kim et al., KYUNGHEE UNIVERSITY
(8) IMPACT EROSION OF PLANETARY ATMOSPHERES
W.I. Newman et al., UNIVERSITY OF CALIFORINIA
[...]
(11) HISTORICAL OBSERVATIONS OF THE LEONID METEOR SHOWER
P. Brown, UNIVERSITY OF WESTERN ONTARIO
---------------------------------------------------
(3) NEAs: REMNANTS OF LARGE ASTEROID COLLISIONS
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
ESA Science News
http://sci.esa.int
11 May 1999
>From Near Earth Asteroids to water on Mars -- astronomers review
latest ISO results
Near-Earth Asteroids -- asteroids whose orbits bring them close
to Earth -- very likely originate from collisions between larger
asteroids that orbit the Sun between the planets Mars and Jupiter.
This result, obtained by ESA's infrared space telescope, ISO, was
presented yesterday at the workshop on ISO results on Solar System,
held at ESA's Villafranca Satellite Tracking Station in Spain. Other
findings related to the atmosphere of Mars and the giant planets --
Saturn, Jupiter, Neptune and Uranus -- were also presented during
a press conference yesterday morning.
About 250 Near-Earth Asteroids (NEA) are known so far, with sizes
ranging from one to 40 kilometres in diameter. But according to Alan
Harris, of the German Aerospace Centre (Berlin), there are more
than 2000 NEA not yet identified. Their origin is still unclear, but
scientists are working with two main ideas: many probably originate
from collisions among the larger bodies in the Main Belt of asteroids,
between Mars and Jupiter, while others might be old, 'dead' comets
that have undergone so many fly-bys of the Sun that they have lost
all their coma -- the beautiful comet tail of dust and gas.
The infrared observations made by Harris with ISO provide
information about the surface and mineralogy of the NEAs, and
confirm that many are fragments of larger asteroids. ISO sees a
rocky surface without much residual dust; a dusty surface is what
would be expected if the NEA were 'dead comets'. Harris, however,
is still analyzing the data and has not yet ruled out the possibility
of finding traces of dust in one of the asteroids observed, the orbit
of which is very similar to that of a comet.
As Harris points out, knowledge on the composition of these objects
is necessary to predict the consequences of a future impact on the
Earth. "We still know just a few of them, and although the odds that
any one will hit the Earth in the near future are very low, there's a
possibility. Therefore, the more we know about these objects, the
better", he said.
The new data about Mars refer to the water in the atmosphere: ISO
sees that it condenses and freezes close to the planet's surface.
This result will help to understand the behaviour of the Martian
atmosphere.
"We got these observations while NASA's Mars Pathfinder mission
was also studying the planet, and the results of both missions are
in agreement. ISO confirms the Mars Pathfinder data", said Therese
Encrenaz, of the Observatoire de Paris-Meudon.
Encrenaz also described the detection of new molecules in the
atmospheres of the giant planets, mainly hydrocarbons like the
radical CH3, never detected before, and benzene. Benzene is a
common molecule on Earth (in petrol for instance), but is
apparently less common on other planets.ISO's spectrometers see
it now in Saturn.
The discovery of water in the atmosphere of Saturn's largest moon,
Titan, was also reviewed by Athena Coustenis of the Observatoire
de Paris-Meudon.
USEFUL LINKS FOR THIS STORY
ISO Science Homepage
http://www.iso.vilspa.esa.es/
ISO Data Centre
http://www.iso.vilspa.esa.es/users/idc/IDC.html
[NOTE: Images supporting this article are available at
http://sci.esa.int/story.cfm?TypeID=3D1&ContentID=3D4850&Storytype=3D18]=
---------------------------------------------------
(7) HIGH TEMPERATURES IN RETURNING EJECTA FROM THE R IMPACT OF COMET SL9
S.J. Kim*), M. Ruiz, G.H. Rieke, M.J. Rieke, K. Zahnle: High=20
temperatures in returning ejecta from the R impact of comet SL9.=20
ICARUS, 1999, Vol.138, No.2, pp.164-172
*) KYUNGHEE UNIVERSITY,DEPT ASTRON & SPACE SCI,KYUNGGIDO 449701,SOUTH
KOREA
Observations from ground-based observatories and with the Galileo=20
spacecraft suggest that the flares from the SL9 impacts resulted from=20
ejecta falling back onto Jupiter in ballistic plumes. This explanation=20
is supported by comparing the plume height as a function of time in HST =
images with the flare light curve. We show that the rotational=20
temperature of CO in the shock from the R impact rose from less than=20
2000 K near the beginning of the main flare to about 5000 K at its end. =
This behavior agrees with a simple physical model of ballistic plumes=20
with a mean molecular weight indicating they are 50% or more jovian=20
air, Alternate models involving formation of molecules at the original=20
impact site, or formation of dust grains to initiate the hare, are=20
inconsistent with these measurements. The energy is emitted primarily=20
as a hot continuum, supporting the possibility that finely divided dust =
grains are heated in the reentry shock and emit to create the flare.=20
Scaling such models to the energy of the KIT event supports proposals=20
that ballistic plumes were responsible for the global disaster=20
associated with it, (C) 1999 Academic Press.
---------------------------------------------------
(8) IMPACT EROSION OF PLANETARY ATMOSPHERES
W.I. Newman*), E.M.D. Symbalisty, T.J. Ahrens, E.M. Jones: Impact=20
erosion of planetary atmospheres: Some surprising results. ICARUS,=20
1999, Vol.138, No.2, pp.224-240
*) UNIVERSITY OF CALIFORINIA LOS ANGELES,DEPT EARTH & SPACE SCI,LOS=20
ANGELES,CA,90095
We have investigated by analytical and computational means the effect=20
of Cretaceous-Tertiary (K/T) size impacts (5 x 10(30) erg, 9-km-radius=20
bolide of 10(19) g) on terrestrial atmospheres. We have extended=20
analytically the approximate solution due to A. S, Kompaneets (1960,=20
Sov. Phys, Dokl, Engl. Transl, 5, 46-48) for the blast wave obtained=20
for atmospheric nuclear explosions (idealized to isothermal=20
atmospheres) to ideal adiabatic atmospheres and to data-based models of =
the Earth's atmosphere. For the first time, we have been able to obtain =
analytically the particle trajectories in an isothermal atmosphere. The =
outcome of this nonlinear analysis is that a massive impact (without=20
the subsequent ejection of substantial mass) would only influence a=20
column of approximate to 30-km radius in the Earth's atmosphere and=20
that the shocked gas would be propelled up and against the column=20
''wall,'' but would not escape from the planet. We examined the=20
validity of ''hemispheric blowoff,'' the hypothesis that all material=20
in a hemisphere lying above a plane tangent to the point of impact=20
radially accelerated outward and, if sufficiently energetic, would also =
be ejected. We adapted and used a state-of-the-art code (CAVEAT), a=20
hybrid Los Alamos-Sandia Lagrangian-Eulerian finite difference scheme=20
for multimaterial flow problems with large distortion and internal=20
slip. In our CAVEAT calculations, the vapor cloud produced by the=20
impact produces a shock that is orders of magnitude stronger than any=20
previous use of such codes. We developed new methods to test the=20
accuracy and convergence of CAVEAT for KIT size impact events, and it=20
proved to be a robust tool. We explored a KIT size impact where the 9-
km-radius bolide was vaporized and injected into the atmosphere and=20
found no radial outflow in agreement with the analytic model but,=20
instead, a 50-km-radius vertical column formed with only a small=20
fraction of material reaching escape velocity-no more than about 7% of=20
the vaporized bolide plus atmospheric mass will escape the gravitation=20
of the Earth. (C) 1999 Academic Press.
---------------------------------------------------
(11) HISTORICAL OBSERVATIONS OF THE LEONID METEOR SHOWER
P. Brown: The Leonid meteor shower: Historical visual observations
ICARUS, 1999, Vol.138, No.2, pp.287-308
UNIVERSITY OF WESTERN ONTARIO,DEPT PHYS & ASTRON, LONDON, ON N6A 3K7,=20
CANADA
The original visual accounts of the Leonids from 1799 to 1997 are=20
examined and the times and magnitude of peak activity are established=20
for 32 Leonid returns during this two-century interval, Previous=20
secondary accounts of many of these returns are shown to differ from=20
the information contained in the original accounts due to=20
misinterpretations, typographical errors, and unsupported assumptions.=20
The strongest Leonid storms are shown to follow a Gaussian activity=20
profile and to occur after the perihelion passage and nodal longitude=20
of 55P/Tempel-Tuttle. The relationship between the Gaussian width of=20
the strongest returns and their peak activity is established, and the=20
particle density/stream width relationship is found to compare=20
favorably to that expected based on observations of IRAS cometary dust=20
trails. Variations in the width of the 1966 storm as a function of=20
meteoroid mass are shown to be consistent with that expected from=20
classical gas-drag meteoroid ejection treatments. The five largest=20
storms from 1799 to 1966 are found to peak at solar longitudes=20
systematically larger than 55P/Tempel-Tuttle's nodal longitude at the=20
same epochs, suggesting an asymmetry in the dust ejection perpendicular =
to the cometary orbital plane. The dust-distribution about=20
55P/Tempel-Tuttle is reevaluated with these new data and predictions=20
are made for the 1999-2000 showers, (C) 1999 Academic Press.
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