PLEASE NOTE:
*
CCNet DIGEST, 14 September 1998
------------------------------
Food of the Day:
"Has anyone seen the new Nestle candy? It is called the
Armageddon Asteroid, and is advertised as a "milk chocolate
asteroid with meteorite candy inside". It is actually a
hollow
chocolate sphere filled with pop rocks."
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
(1) IMPACT RESEARCH
CURRENT ACTIVITIES/PROGRESS, June-August, 1998
Andrew Gilson <andrew.glikson@anu.edu.au>
(2) PERSPECTIVES ON THE ASTRONOMICAL CONNECTION OF TERRESTRIAL
EVOLUTION
Andrew Glikson <andrew.glikson@anu.edu.au>
(3) PHOBOS' SURFACE POUNDED INTO POWDER BY EONS OF COSMIC IMPACTS
NASA News <NASANews@hq.nasa.gov>
(4) METEOR REPORTS FROM SOUTHERN CALIFORNIA
http://www.channel2000.com/news/stories/news-980911-103045.html
===============
IMPACT RESEARCH DOWN UNDER
Dear Dr Peiser
On the suggestion of Duncan Steel I enclose a report on current
impact research at ANU as well as a recent article, which you may
like to communicate through your bulletin - I hope you will find
the material of interest.
My best regards
Andrew Glikson
12.9.98
--------
Research School of Earth Science
Institute of Advanced Studies
Australian National University
Tel: 61 2 6249 0593
Fax: 61 2 6279 8253
Canberra, ACT 0200
Australia
andrew.glikson@anu.edu.au
copy to: aygsearch@cos.com.au
---------------------------------
(1) IMPACT RESEARCH
CURRENT ACTIVITIES/PROGRESS, June-August, 1998
From Andrew Gilson <andrew.glikson@anu.edu.au>
1. Woodleigh structure, Gascoyne Platform, Carnarvon Basin.
The Woodleigh structure, Carnarvon Basin, Western Australia, is
buried by flat-lying Cretaceous strata of the Gascoyne Platform,
but is manifested by a multi-ring Bouguer anomaly approximately
120 km in diameter that coincides with an overall magnetic
'low'.
The gravity data shows a central peak indicative of basement
uplift, a peak ring, and two ring synclines, the outer one of
which consists of several radial sectors. The structure abuts
sharply against the older fault-bounded Ajana and Wandagee
ridges,
and its eeastern part is cut by major faults. A shallow basin,
approximately 20 km across over the centre of the structure,
contains Lower Jurassic lacustrine shale that defines the upper
age limit of the structure. Drilling on the central peak
intersected indurated quartzo-feldspathic rock, at a depth of
approximately 200 metres, displaying Planar Deformation Features
(PDF) diagnostic of shock metamorphism. The multi-ring nature of
the structure, its abrupt discordance relative to the regional
structure, the possible radial faults and the PDF are consistent
with the characteristic features of proven impact structures, for
example the Shoemaker impact structure (formerly Teague
ring).
Additionally, the seismic line across the structure displays
chaotic reflections, consistent with extensive faulting,
fracturing and brecciation typical of impact structures.
Pending
further studies, it is suggested that the Woodleigh structure is
the largest impact structure identified to date on the Australian
plate.
2. Fohn-1 structure, Timor Sea.
The following is an abstract of a paper submitted by J. Gorter
(Hardy Petroleum) and A.Y. Glikson (RSES): A breccia lens
intersected by the Fohn-1 exploration well under the Timor Sea,
northern Australia, displaying high PGE and high gamma ray
spectrometric counts, is interpreted as a buried impact crater
formed during the late Eocene to Burdigalian. Such an origin is
corroborated by down-hole log profile correlations, seismic
sections, micropalaeontology, petrological and trace elements
geochemical data. Fohn-1 includes a 350 meter thick section
displaying PGE contamination of probable meteoritic origin.
The
relatively inert PGE (Ir, Ru) are enriched in the breccia by
factors of 5-25 above common background values, whereas the more
mobile PGE (Os, Pt, Pd) show terrestrial-type values. Opposite
vertical stratigraphic trends pertain to the Ir-Ru group and to
Os
- the first group shows an overall increase downward whereas the
second group shows an overall decrease downward. It is suggested
that the mobile PGE have been diagenetically leached and in part
transported upward within the breccia pile. The least altered PGE
patterns are observed at lower levels of the breccia section,
showing near-chondritic PGE ratios Ir and Ru. This conclusion is
corroborated by Ni/Cr, Co/Cr, Ni/Ir, Ni/Pt, and Cu/Pd ratios,
which show the most departures from chondritic ratios at high
levels of the breccia lens, and least departures at lower
levels.
Sulphur shows high (~1%) levels at high levels of the breccia
lens, but the lack of systematic relations with metal abundances
does not support their concentration in sulfides. Stratigraphic
and palaeontological evidence suggests that the impact occurred
between 34 Ma or 37.5 Ma, possibly near-coincidental with the
Popigai crater (Siberia), Chesapeake Bay (off-shore Virginia) and
Toms Canyon impact craters (USA), and with the North American
microtektite swarms and associated shocked quartz and an iridium
anomalies. The Fohn-1 impact thus represents a new member of a
global cluster of impact craters formed about the late Eocene.
3. Shoemaker Impact Structure (formerly Teague ring), Nabberu
Basin, Western Australia.
The SIS is currently under investigation by GSWA (F. Pirajno, M.
Freeman), and plans for drilling are proposed by NASA Ames
Center,
Moffet Field, in connection with their Mars exobiology
studies..
The Institute of Planetology, Wilhelms University, Muenster, is
planning to study the SIS structure in further detail. A
paper on
SIS is being prepared by Pirajno, Glikson and Freeman - my role
in
this paper is concerned with the interpretation of Landsat-5-TM
and geophysical data. The recent mapping indicates that
shock
basement occurs as far as the rim of the structure at about 30 km
from the center.
4. Mingar Dome, Pilbara Craton, Western Australia
Field relations, Landsat-5-Thematic Mapper multispectral images
and geophysical data were examined in connection with the
near-perfectly circular 80 km-diameter large Mingar Dome, western
Pilbara Craton, north western Western Australia, with the aim of
elucidating its origin. The dome consists of Fortescue Group
(2.76
Ga) mafic volcanics and minor felsic volcanics and sediments
unconformably overlying core inliers of granite (3.24 Ga),
migmatite and amphibolite exposed by the Fortescue River at the
center of the structure. Local pseudotachylite microbreccia vein
systems occur immediately below the basal unconformity of the
Fortescue Group on the Fortescue River, consisting of angular
quartz grains and deformed feldspar and containing granitic
fragments from the surrounding basement. No penetrative
deformation lamella diagnostic of high velocity shock were
observed, and the veins were possibly triggered by earthquakes.
No
direct evidence has been observed for a meteoritic impact
connection of the Mingar Dome. A spin-off of the present study
has
been the Landsat-5-TM correlation and identification of a range
of
Fortescue Group and Hamersley Group volcanic and sedimentary rock
types, allowing spectral fingerprinting of a range of
lithological
types, including separation of carbonates, shales, iron
formations
and a range of volcanic rock types using band ratios and
principal
components.
5. Search for extraterrestrial impact signatures in high-Ir
carbonaceous shales, Hamersley Basin, Western Australia.
Carbonaceous shales of the Mount McRae Shale (c. 2.56 Ga) and the
Jeerinah Formation (2.63-2.68 Ga) are being investigated for
possible extraterrestrial components, in view of (1) high Iridium
levels (<1.6 ppb) about an order of magnitude above
background,
and (2) high Ni/Cr ratios, up to about 5.0, as contrasted to
background values below 1.0. As the shales are interlayered
with
banded iron formation, their geochemistry is also of interest
regarding the origin of the latter. The material has been studied
by reflective light microscopy, TEM and SEM/EDS at the University
of Queensland (Dr M. Glikson), with the purpose of identifying
the
nature and origin of their organic matter, and are in some
instances characterized by low reflectance Ro levels. It is
planned to conduct further SEM/EDS studies, precise PGE element
abundance analysis, and carbon and sulphur isotopic analysis with
the aim of elucidating the distribution and possibly the carrying
phases of the PGE, as well as identifying their depositional and
thermal history. It is proposed to conduct similar studies
of
spherule condensate units in the Upper Jeerinah (2.63 Ga), Bee
Gorge Member (upper Wittenoom Formation)(2.56 Ga) and Dales Gorge
Member (2.49 Ga) in collaboration with B.M. Simonson (Oberlin
College, Ohio).
6. Correlation of mega-impacts with tectonic/igneous episodes
Lunar mare and terrestrial crater counts and cumulative crater
size-frequency relations imply that >450 craters of
Dc>=100 km,
>50 craters of Dc>=300 km, and >19 craters of Dc>=500
km formed on
earth since the end of the Late Heavy Bombardment (LHB) ~3.8 Ga.
A
high degree of presevation of craters Dc>=300 km on the
continental crust is observed, including 3 craters (Vredefort,
Sudbury, Morokweng) out of the 11 Dc>=300 craters predicted
from
the mean of counted impact flux of 5.23x10-15km-2yr-1 and the
cumulative frequency-crater frequency relation NaDc-1.8. (N =
cumulative number of craters of diameter >Dc). As geochemical
and
isotopic constraints require that oceanic crust occupied a
time-integrated area of >80 percent since the LHB, the
occurrence
of 3 continental craters Dc>=300 km sets a lower limit of 12
oceanic craters of similar size scale, as compared to flux-based
prediction of 42 craters Dc>=300. Oceanic impacts by
projectiles
Dp>=5 km result in excavation of lithosphere and adiabatic
melting
on scales comparable to oceanic Large Igneous Provinces
(LIP).
The largest impacts may be capable of localising and accelerating
mantle plumes and ocean spreading centres, and the injection of
shock energies >>108 TNT-equivalent by Dp>>10
km-large projectiles
may, in principle, result in propagating fractures and rift
networks, thermal perturbations and ensuing igneous activity.
Examination of the geological record for correlated impact and
magmatic figerprints of such events remains inconclusive in view
of isotopic age uncertainties. Potential but unsubstantiated
candidates for such relations are represented by the (1) K-T
boundary (~65 Ma) mega-impact/s, onset of the Carlsberg Ridge
spreading, Deccan volcanism, and onset of the Hawaiian island
chain; (2) J-K boundary (~145 Ma) mega-impacts, Gondwana
breakdown, onset of fault troughs and mafic volcanism, including
precursors of the east African rift structures; (3)
Permian-Triassic boundary (~245 Ma) impact/s, Siberian Norilsk
traps and earliest Triassic rifting; (4) global tectonic-igneous
episodes outlined by isotopic age peaks in several Precambrian
shields. In view of the subduction of pre-200 Ma oceanic crust,
tests of this theory require precise age determinations of distal
ejecta, impact spherule condensates, meteoritic geochemical
anomalies and possible contemporaneous igneous units, notably
mafic dyke swarms.
Relevant publications
Glikson, A.Y., 1993. Asteroids and early Precambrian crustal
evolution.
Earth Science Reviews, 35, 285–319.
Glikson, A.Y., 1995. Asteroid/comet mega-impacts may have
triggered major
episodes of crustal evolution. Eos, February, 1995,
49-55.
Glikson, A.Y., 1996. Mega-impacts and mantle melting episodes:
tests of
possible correlations. Australian Geological Survey
Organization Journal, 16/4, 587-608.
Glikson, A.Y., 1997. Re-emergence of the catastrophic
paradigm in geology.
Journal of the China University of Geoscience.
Glikson, A.Y., 1998. Eugene Shoemaker and the impact
paradigm in Earth
science. Proceedings of the Kyoto Conference on the
dynamics of comets and planetary impacts.
Pirajno, F. and Glikson, A.Y., 1998. The Shoemaker impact
structure
(formerly Teague), Naberru Basin, Western Australia. The
Australian Geologist, March, 1998.
Glikson, A.Y. and Kojan, C.J., 1998. Origin of the Mingar
Dome, western
Pilbara Craton, Western Australia: Field, multispectral
and geophysical evidence, with reference to the search
for Australian meteoritic impact structures. Geological
Survey of Western Australian Annual Review (submitted).
Glikson, A.Y., 1998. The astronomical connection of
terrestrial evolution.
The Australian Geologist (in press).
Mori, A.J, Iaski, R., and Glikson, A.Y., 1998. The Woodley
structure,
Carnarvon Basin, Western Australia: a probable multi-ring
impact structure of 120 km diameter (in preparation).
-----------------------------
(2) PERSPECTIVES ON THE ASTRONOMICAL CONNECTION OF TERRESTRIAL
EVOLUTION
In memory of Gene Shoemaker on the anniversary
of his passing
- 18 July, 1997.
by Andrew Glikson
In Deep Impact - the recent Hollywood show - in the background of
a plethora of affairs with a Titanic-like flavour, a cosmic drama
is staged when a species is trying to deflect a comet from its
home planet. Never mind the science fiction-like scenarios of
space ships dodging flying boulders and astronauts trying to
blast
the comet away - the future is already here. The build-up of
astronomical and geological evidence on the probabilities and
consequences of impact by comets and asteroids (Sagan and Druyan,
1989; Chapman and Morrison, 1994; Shoemaker and Wolfe, 1994;
Verschuur, 1997; Alvarez, 1997) requires reconsideration of the
implications of this evidence to the natural and human sciences -
in particular Earth Science.
September, 1997: We are making our way through fused fragmented
rocks of the Black Onaping breccia within the huge (over 140 km
across) 1850 million years old Sudbury impact crater, about 400
km
north of Toronto, Ontario. The black colour of the rocks
represents heavy concentration of carbon of light isotopic
composition - diagnostic of organic origin. The carbon is
interpreted as representing the activity of thermophile (heat
seeking) bacteria, which derive energy by reducing the carbon
dioxide streaming through the crater into elemental carbon. The
impact melt rocks indicate that temperatures in the crater have
reached several hundred degrees Celsius - the bacteria have
triumphed over the most adverse possible conditions.
May, 1998: We are standing in front of the Cretaceous-Tertiary
extinction boundary at Boticcione Gorge, not far from the
medieval
city of Gubbio in the scenic Apennine mountains. A thin clay
horizon signifies the demise of the dinosaurs and the end of the
Mesozoic world 65 million years ago. Only barren red limestone -
the "Scaglia Rosa" - and dead reef debris remain above
the
boundary. My attention is drawn to unfamiliar spiralling
disc-like
structures just beneath the boundary. My companion, Alessandro
Montanari - the student and colleague of Walter Alvarez who
discovered the Iridium anomaly and the impact connection of the
Cretaceous-Tertiary extinction - tells me these are the traces of
Zoophyton earth worms. In the wake of the catastrophe these
survivors were burrowing into the calcareous mud in systematic
circular patterns, consuming every trace of the remaining
organic
matter - such is the endurance of life.
It is only a few days since I climbed the church tower at
Nordlingen, Swabian Germany, located in the midth and built of
fused breccia derived from the 26 km-large Ries impact crater.
The
crumbling walls exude an eerie feeling. In my imagination I hear
the screams of burnt witches in this walled perfectly preserved
medieval city known for its history of Catholic inquisition. Well
preserved too is the crater from which swarms of tektites - fused
glass beads - spread all around the Earth some 15 million years
ago. It only takes an asteroid slightly larger than 1 km
across
to excavate a crater of this size.
Earth is still being accreted, albeit slowly. It is
estimated
that every year some 30000 metric tons of cosmic dust are being
added. The annually accreted volume of larger extraterrestrial
bodies is more difficult to estimate. Astronomical observations
of
near-Earth asteroids (NEA) suggest that the chances of impact by
a
body about 1 km-large are about 1 in 200 000 to 300 000
years.
The probability of a collision on the scale which ended the
Mesozoic world is 1 in several tens of million years. An even
bigger potential event is represented by the comet Swift Tuttle -
a 24 km behemoth twice the size of the dinosaur-killer - which
crossed the Earth orbit in A.D. 1737, 1862 and 1992 and is
calculated to pass within about 14 days flight from the planet in
A.D. 2126. A more immediate peril is represented by the swarms of
smaller near-Earth (Amor class) and Earth-crossing (Apollo class)
asteroids - some 15000 of which are estimated to exist. The
probability of impact by NEA is in the range of 1 in every 10000
to 20000 years. As confirmed by medium size (about 10 km-large)
craters younger than about 5 million years (Bosumtwi, Ghana;
Zamanshin, Russia; Elgygytgyn, Russia), our hominoid ancestors
have witnessed such events several times since their appearance
on
the planet.
These were small nomadic clans of hunters and gatherers,
adaptable
to environmental changes as many other mammals, for example by
hiding in caves. By contrast, populations of agricultural
civilisations depend critically on optimal climate conditions for
the cultivation of crops - sunshine, temperature and rainfall. In
addition, urban populations depend on physical structures
vulnerable to earthquakes, fire and floods. The relatively small
size of the hundred meters scale NEA is misleading - under
typical
approach velocities of 30 km per second an impact by bolides
100-500 meter-large will result in the release of energy in the
range of 100 to 30000 Megaton TNT equivalent, respectively.
The
c. 500 meters-large asteroid 1989FC which missed Earth by 650 000
km or about 6 hours flight, was of a civilisation-destroying
potential. If this is of any doubt, don't even think about the
consequences of an impact by the 1.6 km-large asteroid 1997 XF11,
with a calculated near-earth pass in the year 2028,
On collision, 1989FC would create a crater about 10 km-large,
release energy almost equivalent to the world's total nuclear
arsenal, trigger an earthquake magnitude 9 to 10, spread a hot
plume burning tens of thousands of square kilometres around the
crater, eject billions of tons of rocks and dust, cloud the
stratosphere with consequent lowering of temperatures and partial
blocking of photosynthesis for several years, induce acid rain
due
to atmospheric reaction of nitrogen and oxygen, and deplete the
ozone. Ironically, some of these consequences resemble the
effects civilisation itself has on the environment. At the rate
technological societies are devastating the natural environment,
who needs cometary impacts?
It is estimated that since Hominoids emerged on the scene between
200 and 500 extraterrestrial bodies several hundred of
meters-large impacted on Earth. About two thirds of these fell in
the oceans, where a deep water wave front will spread,
whereas
the explosion of the projectile upon penetration of the sea floor
will create a second and more powerful wave front. Accentuation
of
the amplitude of deep water waves over shallow waters will result
in a tsunami at least an order of magnitude higher. The records
of
tsunami waves several hundred meters high - consequent on
earthquakes, volcanic activity and/or impacts - remain to be
investigated. Gerrit L. Verschuur, a US astronomer, observes that
historically few population centres existed around the Atlantic
or
Pacific shores. By contrast, the cradles of civilisation emerged
in relatively sheltered river valleys around smaller seas - the
Mediterranean, Red Sea and Persian Gulf, or in mountain regions
such as the Andes and the south Indian highlands. Could this, in
part, resulted from the devastating effects of tsunamis in
circum-oceanic regions?
Answers to these question may lie in the legends and myths of the
ancients. Traditionally, the difficulty in discriminating between
fact and fiction, between echoes of the real past and dreams, has
discouraged historians and scientists from undertaking such a
task. Yet, where common threads appear between ancient lores,
where falling stars, sky fire and floods are portrayed in
stories,
such as by the Washo Indians of California, the aborigines of
South Australia, the Gilgamesh myth, Peruvian legends, the
revelation of Saint John, the Biblical deluge and the story of
creation itself - have the ancients only been dreaming?
Victor Clube of Oxford University and William Napier of the Royal
Observatory of Edinburgh [now at the Armagh Observatory] did not
think so. They identified consistent patterns in ancient
writings,
which they interpreted in terms of meteoritic showers during
3000-6000 years ago (Clube, 1989). Were wrathful gods pouring
fires on Earth? Hence the emergence of astrology as a fundamental
discipline among the ancients. From an exhaustive study of
historical records Edith Tollmann and Alexander Tollmann of the
University of Vienna interpret these lores in terms of cometary
crushes in several parts of the world about 9500 years ago
(Tollmann and Tollmann, 1994). It remains for geologists to test
these suggestions through a search for tsunami deposits. More
recently, Duncan Steel and Peter Snow referred to Maori stories
of
huge fires in the Tapanui region, South Island, New Zealand -
where a large crater-like feature may suggest a cometary
atmospheric explosion. No such doubts exist with regard to the
1908 Tunguska (Siberia) explosion, caused by a comet a few tens
of
meter in size, which left the trees over one thousand of square
kilometres of Taiga flattened as well as a telltale Iridium
anomaly on the ground.
It remains possible that very large impacts on Earth early in its
history have repeatedly totally eroded the atmosphere and
extinguished life, perhaps by analogy with later events on
Mars.
In this scenario, life would have been repeatedly destroyed and
re-emerged on the planet during the first billion years or
so.
The Panspermia school of thought suggests that life was seeded on
Earth by organic matter, viruses and bacteria transported by
comets from other life havens within and outside the solar
system.
The rationale underlying this suggestion is reminiscent of the
motivation underlying Von Daniken's Chariots of the Gods and of
UFO suggestions. According to such cargo cult mentality Earth is
neither capable of being an original cradle for life nor for the
origin of civilisation - which need to be imported from somewhere
else in space. The relationship between this philosophy and
monotheistic God-in-the-sky religions, as contrasted to
Pantheistic Gaea-centred philosophy, is evident.
It follows from Panspermia that comets are messengers of both
life
and death - resulting in both seeding and extinction of life.
However, while organic molecules abound in comets and
interstellar
space, no viruses or bacteria have been identified in meteorites.
Nor is there any intrinsic reasons to preclude their emergence on
Earth itself. There is another way, however, by which comets
could
have been the bearers of life - extinction may be followed by
biological radiation. We humans are the very examples for this
pattern - mammals would not have evolved all the way into
hominoids had the dinosaurs still been roaming about...
The raison d'etre for early views of the world can be traced to
pre-Copernicus geo-centric sentiments. Newtonian and Lyellian
uniformitarianism imply long term, if not permanent, stability -
which is why it is subjectively attractive. Not until the
seventies has the reality of plate tectonics and large meteoritic
impacts been realised - the latter largely through the efforts of
Eugene Shoemaker and Robert Dietz. However, Charles Lyell's and
James Hutton's paradigm "the present is the key to the
past" is
still being applied indiscriminately to Precambrian tectonic
models. Overwhelming evidence now exists for major
catastrophes
during geological, and quite likely historical, times. The
precise
nature of the latter will only be revealed when the scientific
method is applied to test hypotheses arising from human
prehistory, including the study of impact and tsunami deposits.
Back to Deep Impact. Humankind is fast developing the means of
deflecting asteroids and comets from Earth. The Stardust mission
in 1999 is planned to reach and collect material from comet
Wild-2
in 2004. The Deep Space (DS4) mission is planned to reach
comet
Temple-1, drill a sample and return to Earth in 2010. A
US-Japan
mission named MUSES-C due to be launched in 2002 will intercept
and fire pellets into the asteroid 4660-Neurus (Science News,
1998, 153:267). Whether the stable political and technical
structures required for maintenance of the present Spacewatch
program and planned spaceguard programs will survive in the long
term - only the future can tell. Whether, by the time a comet
arrives, the effects of an environmentally destructive technical
civilisation do not exceed those of a potential impact is a mute
point.
References
Alvarez, W., 1997. T. Rex And the Crater of Doom.
Princeton University
Press, Princeton, New Jersey.
Carl Sagan and Ann Druyan, 1985. Comet. Random House Inc.,
New York.
Chapman, C.R. and Morrison, D., 1994. Impacts on the Earth
by asteroids
and comets - assessing the hazard. Nature, 367, 33.
Clube, S.V.M., 1989. Catastrophes and Evolution:
Astronomical Foundations.
Cambridge University Press, Cambridge.
Shoemaker, E.M and Wolfe, R.F., 1994. Mass extinctions,
crater ages and
comet showers. In: The Galaxy and the Solar System,
University of Arizona Press, 338-386.
Tollmann, E. and Tollmann, A., 1994. De Zondvloed: Van
mthye tot
historische werklijkheid. Baarn, Tirion.
Verschuur, G.L., 1996. Impact - The Threat of Comets and
Asteroids.
Oxford University Press, Oxford.
==================
(3) PHOBOS' SURFACE POUNDED INTO POWDER BY EONS OF COSMIC IMPACTS
From NASA News <NASANews@hq.nasa.gov>
Douglas Isbell
Headquarters, Washington,
DC
September 11, 1998
(Phone: 202/358-1547)
Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)
RELEASE: 98-164
MARTIAN MOON PHOBOS HIP-DEEP IN POWDER
New temperature data and close-up images of the Martian moon
Phobos gathered by NASA's Mars Global Surveyor indicate the
surface of this small body has been pounded into powder by eons
of
meteoroid impacts, some of which started landslides that left
dark
trails marking the steep slopes of giant craters.
New temperature measurements show the surface must be
composed largely of finely ground powder at least three feet (one
meter) thick, according to scientists studying infrared data from
the Thermal Emission Spectrometer instrument on the
spacecraft.
Measurements of the day and night sides of Phobos show such
extreme temperature variations that the sunlit side of the moon
rivals a pleasant winter day in Chicago, while only a few
kilometers away, on the dark side of the moon, the climate is
more
harsh than a night in Antarctica. High temperatures for
Phobos
were measured at 25 degrees Fahrenheit (-4 degrees Celsius) and
lows at -170 degrees Fahrenheit (-112 degrees Celsius).
The extremely fast heat loss from day to night as Phobos
turns in its seven-hour rotation can be explained if hip-deep
dust
covers its surface, said Dr. Philip Christensen of Arizona State
University, Tempe, principal investigator for the experiment on
the Mars Global Surveyor spacecraft.
"The infrared data tells us that Phobos, which does not have
an atmosphere to hold heat in during the night, probably has a
surface composed of very small particles that lose their heat
rapidly once the Sun has set," Christensen said.
"This has to be
an incredibly fine powder formed from impacts over millions of
years, and it looks like the whole surface is made up of fine
dust."
New images from the spacecraft's Mars Orbiter Camera show
many never-before seen features on Phobos, the innermost and
larger of the planet's two moons, and are among the highest
resolution pictures ever obtained of the rocky Martian
satellites.
A six-mile (10-kilometer) diameter crater called Stickney, which
is almost half the size of Phobos itself, shows light and dark
streaks trailing down the slopes of the bowl, illustrating that
even with a gravity field only about 1/1,000th that of the
Earth's, debris still tumbles downhill. Large boulders
appear to
be partly buried in the surface material.
Infrared measurements of Phobos were made on August 7, 19 and
31 from distances ranging between 648-890 miles (1,045-1,435
kilometers), far enough away to capture global views of the
Martian moon in a single spectrum. The instrument has been
able
to obtain the first global-scale infrared spectra of Earth and
Mars in addition to the new Phobos data, bringing new insights
about the composition of these three very different worlds.
"Of the three, Earth has the most complex infrared spectra,
primarily due to the presence of carbon dioxide, ozone and water
vapor in its atmosphere," Christensen said.
"Mars, which is much
colder than Earth because of its distance from the Sun, is less
complex and shows only significant amounts of carbon
dioxide. The
spectrum of Phobos, however, has little structure because it has
no atmosphere and the energy it emits is coming entirely from its
surface."
The new Phobos images and thermal spectrometer measurements
are available on the Internet at: http://www.jpl.nasa.gov
,
http://photojournal.jpl.nasa.gov/
,
http://www.msss.com/ and at http://emma.la.asu.edu
On Monday, Sept. 14, Mars Global Surveyor begins its second
phase of aerobraking, using the friction from repeated passes
through Mars' atmosphere to lower and circularize the
spacecraft's
orbit. Over the next four-and-a-half months, the
spacecraft's
flight path will be lowered from the current 11.6-hour elliptical
orbit to a two-hour, nearly circular orbit over the Martian polar
caps. The magnetometer and thermal spectrometer will be
turned on
through December to gather data each time the spacecraft passes
closest to Mars' surface. In addition, the radio science
team
will be conducting gravity field experiments by measuring small
shifts in the spacecraft's velocity as it passes behind the
planet
or is blocked from view by the Sun. The spacecraft team at
NASA's
Jet Propulsion Laboratory (JPL), Pasadena, CA, and Lockheed
Martin
Astronautics, Denver, is continuing to study possible options for
deployment of the spacecraft's high-gain antenna once it has
reached its low-altitude mapping orbit next spring.
Mars Global Surveyor is part of a sustained program of Mars
exploration, managed by JPL for NASA's Office of Space Science,
Washington, DC. Lockheed Martin Astronautics, Denver, CO,
which
built and operates the spacecraft, is JPL's industrial partner in
the mission. JPL is a division of the California Institute
of
Technology, Pasadena, CA.
===============
(4) METEOR REPORTS FROM SOUTHERN CALIFORNIA
From
<http://www.channel2000.com/news/stories/news-980911-103045.html>
Bird? Plane? Nope ... A Meteor!
SoCal Residents Spot Meteor Over Region
LOS ANGELES, Posted 8:45 a.m. September 11, 1998 -- The sighting
of
a meteor over Southern California last night prompted phone calls
to
the Griffith Park Observatory and to some sheriff's departments.
Calls began about 7:20 p.m. Thursday.
"It was early in the evening and a lot of people were
outside.
That's why it was kind of a big deal," said observatory
astronomer
James Somers.
Somers said the meteor could have been as small as a baseball or
as
large as a basketball. He estimated the meteor was traveling up
to
70 miles per second.
Deputies in Malibu fielded several calls from anxious
residents who
mistook the meteor for a flaming airplane. A sheriff's helicopter
determined the calls were due to the meteor, said Los Angeles
County
Sheriff's Department Sgt. Peter Charbonneau.
Somers said about three or four meteors break through the
atmosphere
over Los Angeles each year. He said the meteor probably was
spotted
by Southern Californians within a 100 mile radius.
It's not clear what path the meteor took, but the Observatory
will
attempt to plot its path, Somers said.
"It's done its thing," Somers said. "More than
likely it's burned
out by now."
Somers said meteors begin as part of an asteroid belt usually
orbiting the sun between Mars and Jupiter. Sometimes asteroids
collide and parts break out of orbit and fall toward Earth.
Copyright 1998 by The Associated Press
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