Announcement on Availability of Particles

http://www-curator.jsc.nasa.gov/cura...Skjellerup.htm

Availability of particles from collection surfaces exposed in the
Stratosphere during the 2002 Leonid Shower (comet 55P/Tempel-Tuttle),
and Earth's predicted crossing of the dust steam from comet
Grigg-Skjellerup in 2003

Johnson Space Center

The majority of stratospheric collections are made at random times.
However, ever since Dermott and Liou (1994) proposed that there should
be enhancements in certain types of asteroidal dust at certain times
of the year, we have endeavored to build up a collection of collection
surfaces targeted at different times throughout the year. Typically,
these take the form of collectors, both large and small, flown for one
calendar month only. Inspection of our holdings of collection surfaces
will reveal many of these surfaces. In the past year we have made two
targeted collections of the stratosphere in attempts to collect dust
from specific comets. We consider these efforts to be long shots,
since the enhancements of the comet dust over background
extraterrestrial material are not terribly large. Still we have made a
special effort to provide these collection surfaces to the community,
since the potential payoffs from successfully locating comet dust from
a known source are so great.

55P/Tempel-Tuttle

We directed a NASA ER-2 aircraft to fly at ~20 km over the
southwestern USA for a period of 4 days beginning on Nov 19 in an
attempt to collect Leonid storm debris from comet 55P/Tempel-Tuttle
(Rietmeijer et al., 2003). These flights began on the day of the
shower peaks, and continued for the next 3 days. The two small
collection surfaces, U2108 and U2109, each collected for a total
period of 8 hours and 3 minutes. Based upon measurements of the rather
high particle entry velocities for this shower, we would expect that
any surviving Tempel-Tuttle samples would be silicate spheres (having
melted). Preliminary inspection of the collectors showed nothing out
of the ordinary, that is ~1 (probable) large chondritic IDP per
collector per collection hour. There was one apparent black sphere on
one of the collectors; none on the other.

These collection surfaces are available for particle allocations as
part of the routine JSC Stratospheric Particle Curation Program.

Grigg-Skjellerup

This year, we also made an attempt to collect dust from comet
Grigg-Skjellerup. In a recent report (Messenger, 2003), four
short-period comets were identified that have produced Earth-crossing
dust streams in the past century with low Earth-encounter velocities.
Dust from these comets enters the atmosphere with velocities ranging
from 15 to 21 km/s, low enough that even some very large particles
could survive atmospheric entry without melting (Love and Brownlee,
1998). Among these, comet Grigg-Skjellerup was identified as the best
candidate for collection owing to its fortuitously close encounters
with Earth's orbital path for seven apparitions beginning in 1967. The
Earth encounters the Grigg-Skjellerup dust stream during April 23-24th
each year, with peak anticipated fluxes in 2003 and 2004. The
estimated number of particles that could be collected from this dust
stream are uncertain, but may reach 1 to 50 % of the large (40 um)
size IDPs.

To permit testing of this prediction, and therefore possibly collect
samples of comet Grigg-Skjellerup we flew two dedicated large area
stratospheric collectors (L2054 and L2055) on a NASA ER-2 aircraft for
a total of 7.9 hours during the period April 30-31, 2003, over the
southwestern USA. These collectors were only exposed during this
period, however despite this fact these two collectors should contain
many IDPs from many sources (not just from Grigg-Skjellerup), so one
would probably have to analyze many grains from these collectors
before Grigg-Skjellerup samples could be identified (if they are
indeed present in significant numbers). Two additional large area
collectors (L2052 and L2053) were also flown during this period,
though they had already accumulated 13 hours each from previous
flights. Thus these latter collectors might also contain
Grigg-Skjellerup grains, but should also contain many other IDPs from
other sources, and are therefore less valuable for this exercise.

These collection surfaces are available for particle allocations as
part of the routine JSC Stratospheric Particle Curation Program.

Discussion

Since the abundance of samples from the target comets' particles are
probably well below the background IDP flux to the stratospheric
collectors, it will be necessary to identify them on the basis of some
distinguishing feature. For example, very short space exposure ages
should be characteristic of these particular comet grains. Two
possible measurements of space residence time are solar flare tracks
(which shows that most IDPs have been in space for thousands of years)
and solar noble gas (which saturates the surfaces in the order of
hundreds of years). Solar noble gases can also be used to estimate the
peak temperatures experienced by particles during their descent into
the atmosphere. There may also be mineralogic or compositional
characteristics particular to these comets which could be recognized.
The size-dependant flux of the Grigg-Skjellerup dust and the diluting
effect of stratospheric residence time both favor searching for
samples from this particular comet among large (probably cluster)
IDPs, in the size range of 40 micrometers and larger. For comet
Tempel-Tuttle samples, their large atmospheric entry velocities would
make silicate spheres the most likely candidates.

Samples from these collectors are now available to any qualified
investigator, in the same manner as are all of our stratospheric
samples. All sample requests should be sent to the Cosmic Dust
Curator, Michael Zolensky ). All such
requests will then be relayed to the Cosmic Dust Working Group for
peer review.

The allocation of complete collection surfaces is not an option in
this instance, since there are two few of these (only two for each
collection opportunity). However, as in the past any qualified
investigators are welcome (and encouraged) to visit the Cosmic Dust
Lab at JSC to inspect and select potential samples for allocation.

References

Dermott S.F. and Liou J.C. (1994) Detection of asteroidal dust
particles from known families in near-Earth orbits. In Analysis of
Interplanetary Dust (Eds. M. Zolensky, T. Wilson, F. Rietmeijer and G.
Flynn), American Institute of Physics, New York, pp. 13-22.

Messenger S. (2002) Opportunities for the Stratospheric Collection of
Dust from Short-Period Comets. Meteoritics and Planetary Science 37,
1491.

Rietmeijer F., Pfeffer M., Chizmadia L., Macy B., Fischer T., Zolensky
M., Warren J., and Jenniskens P. (2003) Leonid dust spheres captured
during the 2002 storm - Lunar and Planetary Science XXXIV, The Lunar
and Planetary Institute.

Love S. and Brownlee D. (1998) Heating and Thermal Transformation of
Micrometeoroids Entering the Earth's Atmosphere, Icarus 89, 26.

[Mod. note: reformatted -- mjh]