Targeted Sample Collections
Particles collected in the stratosphere during the 2002 Leonid Shower and the 2003 crossing of comet Grigg-Skjellerup
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, 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.
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, 2002 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 particles 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.
We also made an attempt to collect dust from comet Grigg-Skjellerup. In a 2003 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), and two small collectors (U2120 and U2121) 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 four 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.
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 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 too few of these. 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.
- 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.
- Love S. and Brownlee D. (1998) Heating and Thermal Transformation of Micrometeoroids Entering the Earth's Atmosphere, Icarus 89, 26.
- 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.