After sputtering through a layer of surface contamination, secondary ion mass spectrometry (SIMS) was used to create two- dimensional elemental ion intensity maps of microparticle impact sites on the IDE sensors. The element intensities in the central craters of the impacts were corrected for relative ion yields and instrumental conditions and then normalized to silicon. The results were used to classify the particles' origins as "manmade", "natural" or "indeterminate". The last classification resulted from the presence of too little impactor residue, analytical interference from high background contamination, the lack of information on silicon and aluminum residues, or a combination of these circumstances.
Several analytical "blank" discharges were induced on flight sensors by pressing down on the sensor surface with a pure silicon shard. Analyses of these blank discharges showed that the discharge energy blasts away the layer of surface contamination. Only Si and Al were detected inside the discharge zones, including the central craters, of these features.
Thus far a total of 79 randomly selected microparticle impact sites from the six primary sides of the LDEF have been analyzed: 36 from tray C-9 (Leading [ram], or East, side), 18 from tray C-3 (Trailing [wake], or West, side), 12 from tray B-12 (North side), 4 from tray D-6 (South side), 3 from tray H-11 (Space end), and 6 from tray G-10 (Earth end). Residue from manmade debris has been identified in craters on all trays. (Aluminum oxide particle residues were not detectable on the Al/Si substrates.)
These results were consistent with the IDE impact record which showed highly variable long term microparticle impact flux rates on the West, Space and Earth sides of the LDEF which could not be ascribed to astronomical variability of micrometeorite density. The IDE record also showed episodic bursts of microparticle impacts on the East, North and South sides of the satellite, denoting passage through orbital debris clouds or rings.
The objective of the chemical analysis study is to empirically determine the manmade-to-natural microparticle population ratio of impactors that struck the LDEF satellite while in orbit. The study takes advantage of the purity of the IDE substrates and their location on all six primary sides of the satellite. Data from this study will be added to the growing pool of orbital hypervelocity impact site analyses produced from studies of the Solar Max, Palapa B and LDEF satellites.
EDS analyses of microparticle impact sites on the IDE sensor were limited in scope due to the concentrations of residue (~1%) required to produce detectable signals using EDS compared to the (ppm) concentrations needed to yield semi-quantitative results using the far more sensitive SIMS techniques. EDS and Auger electron spectroscopy (AES) were used to further analyze high concentration deposits of material (residues and contaminants) that were identified in and around impact sites with SIMS.
As previously described (ref. 1), all SIMS data was collected with a Cameca IMS 3F using 16O+ or 16O- ion beams. The instrument was used in the ion microscope mode and data was recorded as two- dimensional elemental positive ion maps with lateral resolution of 1-2 micrometer. Pixel intensities were used to calculate relative element abundances. Briefly, the SIMS analytical protocol involves the following steps:
In order to gather data from a statistically significant fraction of this microparticle impact sample set, the total number of these impacts on each sensor group located on the six primary sides of LDEF was determined. Then, using 10% statistics, the total number of analyses required to achieve a significant fraction was calculated. This technique assumes that the statistics of a randomly selected group of 10% of the samples in a large, random sample set will, to first order, represent the statistics of the entire sample set.
The microparticle impact sample sets on the West, Space and Earth end IDE sensors are comprised of 290, 600 and 330 impact sites, respectively. Analyses of 122 impacts would provide 10% statistics for these three sample sets.
The above logic was iterated a second time for the extremely large sample sets represented by the impacts on IDE sensors that were located on the East, North and South sides of LDEF. Optical scanning of 3 out of 32 sensors from each of these groups provided estimates of the sample set sizes of 10,000, 4,600 and 4,400 impact sites, respectively. Clearly, the resources were not available to analyze 2,000 samples. However, since the sample sets were so large, it seemed logical to select 10% of the sensors from each location and analyze 10% of the samples on each sensor. This yielded the more practical goal of 200 analyses. Thus, analyses of a total of ~320 impact sites on IDE sensors would provide a first order statistical look at the manmade/natural microparticle population ratio.
To date 79 impact sites on IDE sensors have been analyzed with SEM/EDS and SIMS. Manmade or natural classifications have been assigned to 40 of the residues, or ~ 50%. An extensive background and blank discharge study required to establish the level of contamination and other analytical interferences has been conducted, but more work is required in this area. Although there are significant analytical interferences associated with elemental analyses of impact sites on the ultra high-purity IDE detector surfaces, most of these can be mitigated through recognition. The details and results from the contamination study will be the subject of a future paper.
Identification of natural meteoroid residues followed the guidelines of the Meteoroid and Debris SIG. Residues were labeled natural if they had elemental compositions that were similar to common components of chondritic meteorites or interplanetary dust particles (IDP's). Most IDP's of micron size have solar elemental abundance ratios for Mg, Si, Fe, S, Ca, Ni and Na in decreasing order of abundance. The atomic abundances of Mg, Si and Fe are roughly equal in most IDP's and are an order of magnitude more abundant than Ca, Ni and Na. In practice, residues were labeled as natural meteoroids if they had high Mg and Fe abundances and either lacked or contained low abundances of elements not common in primitive meteoritic materials. Relative ion sensitivity factors ,"RSF" (see below) were taken into account when estimating compositions from ion intensity maps.
Manmade particle residues were identified by the presence of relatively high concentrations (>100 ppm in most cases) of metals such as Ti, Zn, Cr, Cu or Ag. Manmade classifications were also assigned to 4 residues (out of the 79) containing only Na, K and Mg under the assumption that these were the remains of impacts with paint particles that used silicate or magnesium oxide pigments. This assumption is subject to change as more insight into possible Na, K and Mg contamination is gained. All 4 of the impacts were located on Leading edge sensors.
Indeterminate classifications were assigned to 39 residues that did not fall into either of the above categories. These included sites that contained only traces of Na and/or K. A subset of the indeterminate classification, labeled "clean", was composed of impact sites with no detectable residues. These may be the result of aluminum oxide particle impacts, a likelihood for impacts on the leading (East) and the North and South sides, or the result of impacts from very high velocity submicron interplanetary dust particles that completely vaporize, a statistical likelihood for impacts on the trailing (West) and Space end trays. As the study progresses, some of the indeterminate classifications may change.
Consideration of all the analytical data was complex and subject to interpretation. As a result, some impactor classifications may change as further insight into the analytical contamination and background issues is gained. For example, H, F and Cl were present in all central craters. This has been traced to ppm level contamination from HF and HCl during sensor fabrication. Na, K, and Mg to a lesser extent ,were present in the majority of impacts and may be from residual background contamination. The presence of these elements is reported since there is no verification of background contamination at this time, and there were many impact sites with little or no detectable Na, K or Mg, . The Cameca instrument has been retrofitted with an electron flood gun that will permit depth profile studies of the IDE sensor surfaces through the insulating SiO2 layer. These depth profiles should reveal the presence of bulk and interfacial contaminants in the SiO2 and Si.
Depth profiles through the conductive aluminum layer have already shown that this material is contaminated with ~10-100 ppm of Ca. This severely limits the ability to identify Ca in impact sites. Calcium was detected all around the areas surrounding nearly all impact discharges, but was absent from most central craters, or present at concentrations much lower than the background. The few instances where Ca was found in the impact craters at higher than background levels are reported.
Differential sputtering of the contamination layer from the highly textured impact sites was unavoidable and was considered when interpreting the SIMS data. The phenomenon results from beam shadowing effects caused by the craters, ridges, and even the smaller "hills and valleys " of the vapor deposited Al surface layer. In practice, ion images of control areas in the vicinity of the impacts, both before and after oxygen beam sputter cleaning, provided an estimate of the level of "background" concentrations for these areas. Images of impact areas (after sputter cleaning) were interpreted with these values in mind, and only after all sites on a given sensor were examined.
In order to gain insight into the distribution of the material in the surface contamination layer after an impact induced discharge occured, a series of "blank discharges" were induced on a flight sensor using an ultrapure Si shard. SIMS analyses of these "blanks" showed that the C, Na, Mg, K, Ca bearing contamination layer was blown away from the central craters and surrounding zone of vaporized Al by the discharge energy. Only Si and Al were detected within the discharge craters and vaporization zones of these analytical blank discharges.
Ion RSF Ion RSF
C+ 0.007 Cu+ 1.61
Na+ 139 Zn+ 0.054
Mg+ 18.0 Ag+ 0.694
Al+ 36.0
Si+ 1.00 H- 0.602
K+ 125 C- 0.161
Ca+ 38.5 F- 102
Ti+ 13.9 Al- 0.250
Cr+ 7.69 S- 5.10
Fe+ 1.85 Cl- 26.3
Ni+ 1.35 Au- 0.658
The reduced data have several limitations. First, RSF values for
species implanted in Si were not appropriate for species deposited in
or on aluminum or for deposits that were massive enough to form their
own matrix. They are valid only for elements implanted in Si up to
concentrations of ~1%. The assumption that the negative Si electrode
exposed in the central crater region of impacts on the IDE sensors
acted as an ion trap was the reason for selecting these RSF values.
This assumption was based on the knowledge that positive ion pairs act
as the charge carriers in the IDE discharge event and would
theoretically be implanted in the Si. It should be noted that, in
general, the RSF values for species in other matrices follow the same
relative trend.
The second major limitation of the semi-quantitative data was the result of an artifact of the Cameca 3f operational protocol. This instrument could only collect data for one mass at a time. Up to ~50Angstrom of material was sputtered away during each ion imaging step. Thus, the data for each element was from a different layer within the residue. For example, several hundred angstroms of material was removed between the imaging of Mg and Fe. This is a significant limitation on the ability to deduce impactor chemical composition from the scant residues.
Examples of semi-quantitatively reduced SIMS data are included in this report. More detailed interpretations will be presented in a future report after analysis of several ground based hypervelocity microparticle impact induced discharge features on retrieved IDE sensors is completed, and a more thorough understanding of contamination issues is gained.
Microimpactor residue classifications are listed in Table 2. Elements identified at concentrations significantly higher than the background are listed for each impact site. The term "trace" refers to less than 10 ppm concentration, relative to Si, for all elements except Na and K, where the term refers to <100 ppm. Examples of impact feature morphologies, SIMS elemental ion maps, and quantitatively reduced SIMS data are presented in Figs. 1-3.
Minimum crater size of IDE impact features is ~10 micrometer due to the electrical discharge damage caused when the capacitor sensor was triggered (ref. 1). The sensors responded to hypervelocity particles ~0.5 micrometer or larger (assumed density of ~3 g/cm3). Observed spall zone sizes for small particle impacts into the crystalline IDE sensor surfaces were typically ~3X the size of the central craters (Table 2, ref. 1). The central crater size of microimpacts in crystalline materials typically approximate the impactor size (ref. 4). Thus, it was impossible to estimate impactor size in the range of 0.5 to ~3 micrometer from the crater morphology on active IDE sensors.
The formation mechanism for craters ~10-25 micrometer in size on active IDE sensors was dominated by the impactor's kinetic energy (KE) transfer, but the discharge energy caused the entire crater and spall area to melt and fuse. "Crater" dimensions listed are for this fused area, but are more representative of spall zone size. Impactor size for these features was estimated to have been ~1/3 of the fused crater/spall size.
Formation of larger impact craters (>30 micrometer) was totally dominated by the impactor ĘKE and there were frequently shock induced spall zones around the craters. These larger impact sites had the typical morphology of hypervelocity impacts in crystalline material and impactor size was estimated to have been slightly less than the central crater size. The spall sizes listed in Table 2 are for the maximum dimension of the associated spall zones.
Impact identification Nos. in Table 2 refer to the IDE sensor number followed by a crater number (i.e. 1176-C3). Locations of all impacts were recorded for future reference. Craters were not always sequentially numbered since SIMS analyses were not performed on all craters that were labeled for further study in the initial optical scan. This was primarily due to the inability of the SIMS beam to reach every crater on a given surface without venting and reloading the sample in a different orientation, and in no way affected the randomness of the micro-impact site selections.
Considering all of the limitations described above, the impactor classifications cannot be taken as absolute, but there is moderate confidence in their accuracy within the described limitations of the study. Limits can be ascribed to the manmade/natural ratios based on the results to date. Future adjustments, resulting from better understandings of contamination issues and impactor residue deposition mechanisms, and from additional analyses of orbital impact sites, may alter the statistical distribution of manmade and natural impactor classifications deduced from currently available data. Additionally, the combination of this data set with data from other LDEF investigators should provide a more accurate assessment of the microparticle population ratio in LEO.
Figure 1a shows a SEM micrograph of impact No. 1382-C2 from tray C-3 with its 22 micrometer wide central crater and its 65 micrometer wide spall zone. SIMS ion maps for Na+, Mg+, Si+, K+, Ti+ and Zn+ are shown in Fig. 1b. The black box in these images outlines the image area that was selected for quantitative data reduction. A bar graph of the calculated concentrations, relative to Si=1.0, is shown in Fig. 1c and provides an example of the type of data that can be evaluated further as the study progresses. It should be possible to derive significant information about the chemical composition of the non- volatile components of many impactors from these type of data after contamination interferences are better understood.
The three natural micrometeorite residues on tray C-3 sensors were all identified by the presence of Mg and Fe. Two of the impacts were made by small (<~3 micrometer) particles and had Na, Mg, K, Fe and Mg, and Ca, Fe, Ni present in residues. The third impact (No. 1336-C4)had a 23 x28 micrometer central crater (as described above) with no additional spall zone. A residue containing Na, Mg, K, Ca and Fe was found in the crater. Probable impactor size was estimated to have been ~10 micrometer. Figure 2a is an optical micrograph of this impact. SIMS ion maps are shown in Fig, 2b, and Fig. 2c shows a bar graph plot of the reduced image data for the central crater region.
Ten of the 18 impact sites had insufficient debris remaining to be positively identified above the background levels. This situation could be the result of natural microimpactors that had very high encounter velocities (>>10 km/s), or impacts from aluminum oxide particles which were not detectable in the Al/Si substrate. Central craters in these impacts ranged in size from 10-25 micrometer. Figure 3a shows an example of a medium size impact in the "clean" category, No. 1359-C6. Probable impactor size was estimated at ~10 micrometer. Figures 3b shows some of the ion maps associated with this impact, and Fig 3c shows a bar graph of the reduced SIMS data for the central crater. It is anticipated that further analysis of this type of data in "clean" impact sites will lead to quantitation of background contamination levels and allow more accurate assessment of chemical compositions of the non-volatile impactor components.
If all 10 of the "clean" impact sites are assumed to have been caused by small, high speed natural particles, the manmade/natural ratio equals 5/13=0.38. This microparticle value is significantly higher than the assumed ratio of 0.1 for all impactors striking the trailing edge of satellites in low earth orbit (LEO) and could be the result of contamiantion interferences. However, the IDE data showed that the long term average flux of particles >0.5 micrometer in size varied drastically during the nearly 6 year long mission (ref. 5). Of the total of 290 impacts on these sensors, 186, or 64% occurred during the first year. The overall mission flux rate measured by the sensors matched that measured by other investigators (ref. 6). It is possible that orbital debris impacts caused the enhanced rate during the first year. (This topic is discussed in more detail in ref. 5) The chemical analysis data collected thus far, subject to the stated limitations, seem to support this scenario.
Of the 11 manmade impactors, 9 were particles that were <~3 micrometer in size and 2 were particles estimated to have been 30-40 micrometer in size. Of the 9 small particle residues, one had only Ti and a trace of Na and K, three residues contained Cu in addition to Na, K and Mg, one contained Cu along with Na, K, Mg, Fe and traces of Ti and Cr, and four residues contained only Na, K and Mg.
The two largest manmade debris impacts examined have significant amounts of impactor residue. Residue from an ~30 micrometer particle contained Na, Mg, Ti, Cr, Fe, Cu, Zn and Ag and could be from a small piece of electrical component with paint. Residue from a 40 micrometer particle contained high concentrations of H, C, Ti, Cr and Fe and could have been a piece of painted plastic or a paint particle with an organic binder. (This was the only high concentration H, C residue found in the 79 impact sites.)
All 5 of the natural impactors were identified by the presence of Mg and Fe. Only one residue had Ca above background. One particle was estimated to have been <~3micrometer in size, and the other 4 were estimated to have been ~5-10 micrometer in size.
Of the 20 "indeterminate" impactors, 16 were <~3 micrometer in size, and 4 were ~4-8 micrometer in size. This could support the assumption that most of these impactors were small aluminum oxide spheres (from solid rocket motor exhaust). Zinner, et al., have reported that 8 out of 11 small particle impact craters examined on Ge capture cells from LDEF tray E-8 (near leading edge) contained only Al and O residues (ref. 7).
Three of the 4 manmade impactors were <~3 micrometer in size and the fourth was ~6 micrometer in size. Residue from the largest impactor contained Fe, Cu and Zn along with Na, K, and Mg. One of the 3 smaller impacts contained Na, Mg, K and traces of Cu and Ag, one contained Na, Mg, K, Ti and Zn, and one contained Na, Mg, Fe and Cu.
Remaining resources for this study will be utilized for the following tasks:
Preliminary estimates of the manmade/natural microimpactor population ratio for the East and the West sides of LDEF were calculated assuming that unknown impactor residues were all manmade or all natural, respectively. The calculated ratios were 6.0 for the East and 0.38 for the West. These values are subject to change as more information on contamination interferences, and more analyses impact sites is collected. Additionally, the combination of this data set with data from other LDEF investigators should provide a more accurate assessment of the microparticle population ratio in LEO.
Quantitative analyses of impactor residue chemical composition is underway, but results will not be reported until a better understanding of contamination issues is gained.
Cu was detected in 9 out of 16 "manmade" impacts on sensors from the East, North and South sides of LDEF, but was not detected in any of the 9 "manmade" impacts on sensors from the West, Space and Earth ends of the satellite. If, after further investigation, the Cu is shown not to be a contaminant, this could point to a specific source type for this debris.
The results to date are generally consistent with the IDE impact record which showed highly variable long term microparticle impact flux rates on the West, Space and Earth sides of the LDEF which could not be ascribed to astronomical variability of micrometeorite density. The IDE record also showed episodic bursts of microparticle impacts on the East, North and South sides of the satellite, denoting passage through orbital debris clouds or rings.
Impact Size (microm.) Elements Detected Impactor Classificaton
No. Crater Spall with SIMS manmade natural indeterminate
LDEF Tray C-3 (Trailing [wake], or West side)
1300-C136x54 138 (Na, Mg, K, Ca)--(Ti, Fe) X
1300-C213x18 - clean (trace Na) X
1300-C3 12 - Na, Mg, K, Fe X
1300-C4 13 - clean X
1300-C5 11 - Na, K, Mg, Cr, Fe X
1300-C6 10 - clean X
1300-C7 12 - Na, K, Ti X
1300-C8 12 - Na, Mg, K, Ti X
1336-C1 10 - clean (trace Na) X
1336-C423x28 - Na, Mg, K, Ca, Fe X
1359-C4 10 - clean X
1359-C5 9x12 - clean (trace Na, Mg) X
1359-C618x25 42 clean X
1359-C7 12 - clean X
1382-C2 22 65 Na, Mg, K, Ti, Zn X
1382-C4 9 - (trace Mg, Ca, Fe, Ni) X
1382-C5 10 - clean X
1382-C915x20 - clean (trace Mg) X
LDEF Tray C-9 (Leading [ram], or East side)
1176-C1 23 - Na, Mg, K, Fe X
1176-C2 9 - clean (trace Na, K) X
1176-C3 9 - Na, K X
1176-C4 11 - Na, K, Cu X
1176-C5 32 212 Na, Mg, Ti, Cr, Fe, Cu, Zn, Ag X
1176-C623x37 - Na, K X
1176-C7 50 138 H, C, Ti, Cr, Fe X
1176-C8 9 - Na, Mg, K X
1176-C912x16 - Na, Mg, K--(Fe) X
1176-C10 9 - Na, Mg, K, Cu X
1176-C11 9 - Na, K X
1176-C12 10 - Na, Mg, K X
1176-C13 9 - Na, Mg, K, Cu X
1176-C14 9 - Na, K X
1176-C15 9 - Na, K X
1176-C16 10 - Na, K X
1176-C17 11 - Na, Mg, K, Fe, Cu (trace Ti, Cr) X
1176-C1822x25 - Na, Mg, K, Fe X
1176-C19 11 - Na, Mg, K X
1176-C20 11 - Na, Mg, K X
1293-C113x17 - *clean X
1293-C224x31 - *Mg, K, Fe X
1293-C3 18 - *Mg, K, Ca, Fe X
1293-C4 12 - *Mg, Fe X
LDEF Tray C-9 (Leading [ram], or East side) {continued}
1293-C522x28 - *clean X
1293-C7 12 - *clean X
1293-C8 10 - *clean X
1293-C12 12 - Ti (trace Na, K) X
1293-C13 9 - clean X
1293-C14 9 - clean X
1293-C15 11 - clean X
1293-C16 11 - clean X
1293-C17 9 - clean X
1293-C18 11 - clean X
1293-C19 13 - clean X
1293-C20 11 - clean X
LDEF Tray D-6 (South side)
1252-C310x13 - Na, Mg, K, Cu X
1252-C4 10 - clean (trace K) X
1252-C5 10 - clean (trace K) X
1252-C9 10 - clean (trace K) X
LDEF Tray B-12 (North side)
1298-C111x19 34 Na, Mg, K, Fe X
1298-C2 10 - clean X
1298-C615x20 38 Na, Mg, K, Fe X
1298-C7 10 - Na, Mg, K, Ca, Fe X
1298-C816x20 30 Na, Mg, K, Fe, Cu, Zn X
1298-C9 15 - Mg, Fe X
1298-C10a10 - Na, Mg, K (trace Cu, Ag) X
1298-C10b10 - Na, Mg, K, Ti, Zn X
1298-C11 11 - clean X
1298-C129x13 - Na, Mg, Fe X
1298-C13 10 - Na, Mg, Fe X
1298-C14 10 - Na, Mg, Fe, Cu X
LDEF Tray G-10 (Earth end)
1172-C5 9 - Na, Mg,Ti,Fe X
1172-C6 9 - clean (trace Na, K) X
1172-C7 11 - Na, K X
1172-C8 10 - clean (trace Na, K) X
1172-C10 9 - clean (trace Na, K) X
1172-C11 10 - Na, K, Ti, Cr X
LDEF Tray H-11 (Space end)
1255-C1 20 32 Ti, Zn (trace Na, Mg) X
1255-C223x27 - Na, Mg, K, Ca, Fe X
1255-C4 11 - Ti, Zn X