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Abstracts on impacts from 29th LPSC

Subject: Papers on Impact at LPSC
See http://cass.jsc.nasa.gov/meetings/LPSC98/pdf/sess42.pdf
for abstracts of papers to be presented at the 29th Lunar & Planetary
Science Conference, March 1998 (Adobe Acrobat required)
Some of the more interesting papers on impact phenomena include:

Impacts of meteroids on the moon produce a light flash that was modeled to
estimate duration, spectra, and shape; for objects of different velocities,
sizes, and compositions.  Impacts release vapor, and if that vapor
condenses, the temperature of the impact plume remains constant for an
extended time as evaporation produces a release of energy.  If there is no
condensation, adiabatic cooling causes rapid temperature drop.  The vapor
will be opaque or transparent, depending on the composition of the soil and
the meteoroid.  IR detectors would be much better than visual light
detectors, because often only 10-20% of the radiation is at visible
frequencies.  (Nemtchinov)

Two very intriguing papers concern lunar chondrules, crystalline spherules
under 1 mm in diameter, found in samples returned by 4 lunar missions.
These chondrules have various textures & mineralogies, including relict
grains and sulfide rims.  Most seem to have crystallized from a melted
state while in in free flight.  There is evidence that these might have
been formed as impact melt.  These lunar chondrules have compositional
similarities to lunar highland samples.  There is no evidence for a
meteoritic origin, since they differ in mineralogy and composition.  Their
crystalline qualities seem to be related more to cooling rates than
composition.  An important conclusion is the possibility that some
chondrules in meteorites formed from impact events.  (Ruzika)

Impact craters on the moon give us a view into the layers of basalts that
underlie the mare.  The Clementine mission provided images at 11
wavelengths.  Four craters, 18 to 28 km. in diameter and over 2 km. deep,
were mapped, charting iron content as an indicator of mare thickness.  The
crater ejecta was also measured.  Evidence was gained concerning the
thermal and resurfacing history of the moon.  (Thomson)

Verification of the comprehensive test ban treaty (forbidding underground
nuclear bomb tests) includes seismic detectors that could be triggered by a
meteorite impact.  50 of these detectors, across the globe, can detect
seismic magnitudes of 3 or 4.  This has already been an issue of concern;
in 1993, a seismic event of mag 3.6 in W. Australia occurred just north of
“where members of the Aum Shinrikyo terrorist cult had been trying to mine
uranium, and carrying out tests with chemical weapons.”  The conclusion was
that the seismic event was probably an iron meteorite, creating a crater
over 100 meters in diameter [no mention of discovery of such a crater].
Ablation & deceleration in the atmosphere, frequency of iron & stony
meteorites, water surface of the earth, and other factors were considered.
It is concluded that meteorite impacts that could trigger the seismic
detectors occur on a time scale of decades.  The existing seismic records
probably include meteorite impacts misidentified as earthquakes.  (Chyba)

165 impact craters on Mars were studied from Mariner and Viking
photographs.  ‘Unique styles of ejecta’ were noted, volume of cavity
estimated, later erosion measured,  The shapes of the craters were
mathematically analyzed.  They are conical and paraboloidal, and as
diameter increases, they become more spherical.  (Garvin)

Unrelated to impact but worthy of mention are some predictions on Martian
caves.  Whether formed from underground water flow or tectonic activity,
caves on Mars might be a likely place to look for biological activity.  The
types of caves that might exist, and the varieties of sediment they might
contain, are discussed.  There are dozens of papers on Mars, and there
appears to have developed a terminology for geological eras on Mars; as
this article mentions the “Late Amazonian”.  (Grin)

_______________________________________
Peter Abrahams   telscope@europa.com
the history of the telescope, the microscope,
    and the prism binocular
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