Aerogel tracks made by impacts of glycine: Implications for formation of bulbous tracks in aerogel and the Stardust mission

Authors


Corresponding author. E-mail: m.j.burchell@kent.ac.uk

Abstract

Abstract– Impacts of small particles of soda-lime glass and glycine onto low density aerogel are reported. The aerogel had a quality similar to the flight aerogels carried by the NASA Stardust mission that collected cometary dust during a flyby of comet 81P/Wild 2 in 2004. The types of track formed in the aerogel by the impacts of the soda-lime glass and glycine are shown to be different, both qualitatively and quantitatively. For example, the soda-lime glass tracks have a carrot-like appearance and are relatively long and slender (width to length ratio <0.11), whereas the glycine tracks consist of bulbous cavities (width to length ratio >0.26). In consequence, the glycine particles would be underestimated in diameter by a factor of 1.7–3.2, if the glycine tracks were analyzed using the soda-lime glass calibration and density. This implies that a single calibration for impacting particle size based on track properties, as previously used by Stardust to obtain cometary dust particle size, is inappropriate.

Introduction

The use of aerogel as a collector of cosmic dust in space is well established. Aerogel is a high-porosity, low-density material, and impacts by particles at speeds in excess of a few km s−1 can result in tracks in the aerogel which contain substantial fractions of the original impactor. A review of aerogels and their use as dust collectors in space is given in Burchell et al. (2006). The NASA Stardust mission to comet 81P/Wild 2 collected dust samples in SiO2 aerogel and in impact craters on aluminum foil, during a fly by of the comet at 6.1 km s−1. The Stardust mission was launched in 1999, had its cometary encounter in 2004, and returned samples to Earth in 2006. A description of the collector design is given in Tsou et al. (2003). An overview of the preliminary results of the initial analysis of the returned samples is given in Brownlee et al. (2006); with later reviews in Sandford (2008) and Burchell and Kearsley (2009).

The aerogel used to collect cometary dust in the Stardust mission was divided into blocks, with surface areas 40 × 20 mm, and 30 mm depth. The aerogel had a density of 12 ± 2 mg mL−1 at the front face, smoothly varying to 50 ± 5 mg mL−1 at the rear of the blocks. Note that the front face density given here is larger than usually quoted for the mission, but reflects the values of the aerogel flown. The aluminum foil used as a collector by Stardust was grade Al1100 with a thickness of 102 μm. Impacts at 6.1 km s−1 were estimated to result in peak shock pressures of approximately 800 MPa when impacting the aerogel (Trigo-Rodríguez et al. 2008, assuming a mean aerogel density of 20 mg mL−1 as suggested by Burchell et al. 2009a), compared with 60–90 GPa when impacting the aluminum foil (Kearsley et al. 2009). Studies of the returned dust collector showed many tracks in the aerogel (Hörz et al. 2006; Burchell et al. 2008) and many craters in the foils (for a detailed discussion of large craters see Kearsley et al. 2008a; and for small craters, see Price et al. 2010). An analysis of the observed tracks suggests that, on average, the aerogel behaved as if it had a mean density seen by the captured particles of 20 mg mL−1 (Burchell et al. 2009a).

The initial analysis of the Stardust aerogel revealed the presence of three types of tracks, designated A, B, and C by Hörz et al. (2006) and Burchell et al. (2008). Type A tracks (2/3 of the total numbers and sometimes referred to as “carrot” or “champagne flute”) have a relatively long, narrow appearance with values of the ratio between the maximum width and total length of <0.11 (Burchell et al. 2008). Type B tracks (1/3 of the total and sometimes referred to as “turnip”) are characterized by a wide, bulbous cavity with narrow stylus-type tracks emerging from the end of the cavity and a maximum width/total length ratio in the range 0.11–0.35 (Burchell et al. 2008), although the ratio of cavity width/cavity length ranges from 0.16 to 0.66 (Trigo-Rodríguez et al. 2008). Type C tracks (only 2% of the total observed and not assigned a familiar name) have a broad bulbous cavity, but no styli, and the maximum width/total length ratio is >0.35 (Burchell et al. 2008) or lies in the range 0.35–0.46 (Trigo-Rodríguez et al. 2008). A broad range of impact crater shapes have also been reported in the Stardust aluminum foils (Kearsley et al. 2008a, 2009; Price et al. 2010).

This variety of track morphologies from identical impact conditions in terms of impact speed, direction (normal incidence), and target, implies a range of structures and compositions for the impacting dust particles. In Kearsley et al. (2009), the variety of aerogel tracks was considered alongside the various crater morphologies in the foil––the basic assumption being that the corresponding foil craters arose from impacts of similar particles compared with those captured in the aerogel. A scheme was developed in Kearsley et al. (2009), which describes the observed variations in shape as arising from the nature of the impactors. This has now been documented for a wide range of impactor types (Kearsley et al. 2012). Well consolidated, robust grains of single minerals or polymineralic “rocks” produce type A tracks and simple bowl-shaped craters. If the impacting grain has a weak internal structure (either a good crystallographic cleavage or noninterlocking subgrains) this may be reflected in partial break up in a broad cavity near the aerogel surface, and formation of multiple styli (type B tracks). Foil crater morphology is less sensitive to internal grain weakness, and the particles that create type B tracks in aerogel may again produce bowl-shaped craters on foil, albeit of shallower depth if the impactor has a low bulk density. Only when there is a major difference in internal subgrain sizes is a more complicated crater morphology produced with multiple subpits. Depending on how much fine-grained and weak bound material is present within the impactor, and the degree of fragmentation, development of a bowl-shaped cavity may dominate the aerogel track. Indeed, if no significant subgrains exist, no styli may occur, resulting in just a bulbous cavity, i.e., a type C track. It has been suggested that disintegration of an impactor on entry into the aerogel may result from mechanical weakness of the particle, or from volatile driven expansion arising from heating of volatiles inside the particle (Hörz et al. 1998; Trigo-Rodríguez et al. 2008).

Such schemes to explain the nature of the impactor, based on the morphology of the aerogel tracks or craters, rely heavily on insights from laboratory impact experiments using analog projectiles. In the case of the craters in the aluminum foil, an extensive data set of impact experiments were carried out using abundant flight-spare foil as targets (Kearsley et al. 2006, 2007, 2008a, 2008b, 2009). However, for aerogel, while in general, a wide range of experiments have been carried out on a variety of aerogels (see for example Hörz et al. 1998, 2009; Burchell et al. 1999a, 2001, 2009a; Kitazawa et al. 1999), there has been (relatively) less systematic documentation of the track properties created by diverse projectile types. Indeed, in terms of detailed particle size versus track dimension calibrations for Stardust made using flight-spare aerogel, only three shots were carried out, all using soda-lime glass beads (Burchell et al. 2008). The suite of shots was limited by paucity of flight-spare aerogel for impact testing, and the need to use simple particles of known size range (monodisperse) and shape (spherical), with an appropriate density (approximately 2.4 g cm−3) to carefully constrain the calibration. Soda-lime glass beads were selected as suitable projectiles, but these detailed calibration shots only produced type A tracks. Thus, all attempts to obtain the size of the impacting dust grain from the observed Stardust aerogel tracks have been based on a limited data set that strictly relates to only one track type (albeit the most frequent, accounting for some 2/3 of all Stardust tracks, see Burchell et al. 2008).

In the work reported here, we extend the understanding of aerogel track production. We focused on bulbous, type C tracks produced in high-speed impacts in the laboratory performed using a two-stage light-gas gun. As there is no flight-spare Stardust aerogel available, we used a new batch of aerogel produced by the same group who made the original Stardust aerogel and with the same protocol. To produce tracks with the characteristic type C appearance, we used glycine projectiles. Glycine (C2H5NO2) is an amino acid, and is commonly found in proteins. It was chosen as it has a low thermal decomposition temperature (initiating at 233 °C with complete decomposition at 290 °C). If expansion of track cavities is primarily driven by thermal decomposition of impactors, then glycine is likely to show the effect well. To cross-calibrate velocity and target conditions with the original Stardust calibration, we also looked at impacts of soda-lime glass beads into an aerogel block from the same batch as used here for the glycine shots.

Method

The aerogel used was manufactured in the same facility as the original Stardust aerogel, and its manufacture is described in Jones (2007). It has a density gradient from the front to rear face, with an initial low density of 12 mg mL−1 near the front and 50 mg mL−1 at a depth of 30 mm, similar to the flight aerogel. Thus, although manufactured at a later date, the aerogel is similar to the Stardust flight aerogel.

The impacts were obtained using a two-stage light-gas gun (described in Burchell et al. 1999b). This fires a sabot that is discarded in flight and whose contents proceed to the target. Here, the contents were either glycine powder or soda-lime glass beads. Approximately 1 mg of the relevant projectile material was placed in the sabot for a shot. When the gun was fired, this material was accelerated along the gun barrel until the sabot was discarded in-flight, leaving the released powder to produce multiple impacts on the aerogel target suspended in the gun’s target chamber, which had been evacuated to typically 0.5 mbar. The impact speed was selected beforehand to be approximately 6.1 km s−1, but as this can vary slightly from shot to shot, it is measured in-flight to within a few percent (see Table 1 for actual values).

Table 1.   Impact speed and mean projectile size in each shot.
Projectile typeMean diameter (μm)Impact speed (km s−1)Impact kinetic energy (mJ)
Soda-lime glass63.8 ± 0.86.266.96
Glycine75 ± 236.305.10
Glycine140 ± 406.2132.2
Glycine241 ± 516.37177

Two types of projectiles were used, soda-lime glass and glycine. The soda-lime glass beads were spherical and near monodispersive (diameter of 63.8 ± 0.8 μm). They were supplied by Whitehouse Scientific, and were from the same batch used in the original calibration described in Burchell et al. (2008). The glycine was supplied commercially, and was found to be broadly polydisperse in size and irregular in shape. Sieving was used to put the glycine into separate size ranges. A set of carefully measured sieves (pass ranges of 54–66, 93–130, and 181–210 μm) were used to separate the glycine into three nominal size ranges. However, given that the grains of glycine were not spherical (see Fig. 1), their size in each sieve fraction had to be measured again by microscopy, and a mean diameter was found for each sample (see Table 1 for diameters and relevant speeds for each individual shot).

Figure 1.

 Glycine grains after sieving in the ranges a) 54–66 μm, b) 93–130 μm, and c) 181–210 μm.

After impact, the aerogel blocks were removed from the gun and examined using a Leica stereo microscope equipped with a camera. The scale on the images was regularly checked against standard stage micrometers.

Results

Four shots were performed (one with glass, three with glycine projectiles), speeds are given in Table 1 and average track dimensions in Table 2. Five tracks were imaged in the glass shot, and six in each of the glycine shots. The size of the entrance hole diameters was obtained by taking several (usually 4 or 5) individual diameter measurements on each hole and averaging to get a mean value per hole. These average values were then averaged in turn over the five or six impacts in each shot to get a mean size per shot. The track volume was obtained using the model of Burchell et al. (2008), who used a pair of back-to-back frustra to approximate the track shapes (see fig. 3 in Burchell et al. 2008 for an example). As a test of the volume calculation method, one glycine track was approximated as 10 circular slices, and the overall volume was found. The results for the track volume of the two methods for that track agreed to within 3%. Thus, a finer-grained approximation to obtain track volume was not used, and the results given here are from the two frustra method.

Table 2.   Results of track measurements from each aerogel sample.
Projectile type and size (μm)Total track length (μm)Entrance hole diameter (μm)Maximum track width (μm)Depth along track at which max. width occurs (μm)Captured particle diameter (μm)Track volume (mm3)
  1. aOnly one track in this sample had an observable terminal grain.

Glass: 63.822,608 ± 1641272 ± 52956 ± 1231737 ± 68468 ± 65.6 ± 1.3
Glycine: 75757 ± 190168 ± 59420 ± 139208 ± 8014a0.049 ± 0.029
Glycine: 1401646 ± 368273 ± 48730 ± 177546 ± 13040 ± 80.31 ± 0.18
Glycine: 2413429 ± 979784 ± 1571536 ± 2091310 ± 54729 ± 32.9 ± 1.6

The soda-line glass shot is discussed first. The tracks were the classic type A carrot shape. The entrance holes were not clean circles in appearance, having smooth curved regions in some places, but jagged edges elsewhere in the same entrance hole, and were slightly depressed below the original surface plane of the aerogel; an example is shown in Fig. 2, and is very similar in appearance to those in the original calibration work (see fig. 6 in Burchell et al. 2008). On several occasions, we have also fired these projectiles at thin foils and obtained circular holes. This indicates that the projectiles are still spherical when they reach the target in our gun. Thus, given that a smooth, spherical projectile hit the aerogel in Fig. 2, and did not produce a smooth fully circular hole, it can readily be seen that we cannot infer the projectile’s cross-sectional shape from the entrance hole. Indeed only part of the rim of the entrance hole in Fig. 2 (the right half) has what looks like a sharp, regular-curved appearance with radial splits that may have released stress as the surface bent inward. However, the lower left quadrant has a more textured appearance and irregular shape. Thus, the entrance hole did not totally form as a circular hole, punched cleanly in the aerogel by passage of the projectile, and which then flexed inwards causing radial tears in the rim. Some other mechanism also occurred in the lower left quadrant. We plot the track volume and entrance hole diameter versus kinetic energy in Fig. 3, shown along with the data from the original calibration work. The new results for track volume and entrance hole diameter vary only slightly from the previous work, and are within 1σ of the previous values. To aid in this comparison in Fig. 3a, we show the trend line from the fit to the original data presented as eqn. 11 in Burchell et al. (2008). However, in Burchell et al. (2008) the data were not shown in the form in Fig. 3b here, and so no previous fit exists. Therefore, we fit the original data from Burchell et al. (2008) and obtain the trend line shown in Fig. 3b, which has the form: entrance hole diameter (μm) = 1504 × (kinetic energy in Joules)0.28±0.04. The ratio of maximum track width to total track length was found to have an average value of 0.043 ± 0.007, well within the <0.11 bound defined by Burchell et al. (2008) for the Stardust type A tracks. Thus, under similar impacts, there is no significant change indicated in the response of the new aerogel compared with the original samples.

Figure 2.

 Entrance hole in aerogel made by passage of a 63.8 μm diameter soda-lime glass bead at 6.26 km s−1. Although impacted by a projectile with a spherical shape, a purely circular hole did not result. The hole shape is thus not a good indicator of the projectile shape (see main text for further discussion).

Figure 3.

 Comparison of results of new shot with soda-lime glass beads and original calibration data from Burchell et al. (2008). a) Track volume versus kinetic energy. b) Entrance hole diameter versus kinetic energy.

Examples of the entrance holes and track shape (seen from the side) for glycine impacting the aerogel are shown in Fig. 4. In each impact, there is a broad cavity. There were no long styli emerging from beneath these cavities, although in several cases, there was a short squat protrusion at the end of the cavity as if some of the impactor had survived, but with insufficient energy to penetrate much further than the cavity (examples are shown in Fig. 4). These protrusions are at most about 1/9 of the total track length, and are neither long enough nor relatively narrow enough to be considered styli as in the case of type B tracks. It was difficult to find significantly sized terminal grains at the bottoms of the cavities, as can be seen from the results in Table 2. This is compatible with the lack of large, discrete styli in the tracks. The ratio of maximum width/total track length in the tracks is shown in Fig. 5. For all the glycine tracks combined, the ratio of maximum width/total track length has a mean value of 0.49 ± 0.14 and ranges from 0.26 to 0.74. This is clearly distinct from the type A tracks, and is very similar to the values found for type B Stardust tracks (which range from 0.2 to 0.7 although in this case the ratio used is restricted to the cavity width/cavity length ratio) and Stardust type C tracks (which range from 0.35 to 0.46); Trigo-Rodríguez et al. (2008). Glycine tracks therefore closely resemble those Stardust tracks that contain a broad cavity.

Figure 4.

 Examples of entrance holes (upper row) and track shape (as seen from the side) in aerogel made by impacts of glycine particles. For track shapes, the impacts were from the left. The mean impactor size is given beneath each column.

Figure 5.

 The shape of each glycine track in aerogel is given by the maximum track width divided by the total track length, which is shown here versus total track length. This is very similar to the range of values for type B tracks observed in Stardust aerogel, which range from 0.2 to 0.7 and type C tracks, which range from 0.35 to 0.55 (see Fig. 4b and Trigo-Rodríguez et al. 2008).

We also investigated the track volume to kinetic energy and entrance hole diameter (EHD) to original particle diameter (OPD) relationships for the glycine impacts and compared these against those relations previously published for soda-lime glass impacts in Burchell et al. (2008). This comparison is shown in Fig. 6. There is a clear difference between the results for glycine and soda-lime glass, which may be quantified by obtaining a numerical fit to the glycine data sets and comparing to the published results for soda-lime glass. To aid in this comparison, we show the trend lines from Burchell et al. (2008) for the soda-lime glass data. For the new glycine data, we show trend lines obtained as follows: We obtained a fit to volume V (in mm3) as a function of impact kinetic energy E (Joules) given by:

image(1)
Figure 6.

 Comparison of results of the glycine shots with soda-lime glass beads (both new and original from Burchell et al. 2008). a) Track volume versus kinetic energy. b) Entrance hole diameter versus original particle diameter.

where the regression coefficient of the fit was 0.850. This gives an aerogel excavation energy (i.e., the inverse of the slope) of 81 mJ per mm3. Similarly, for the entrance hole diameter (EHD) as a function of original particle diameter (OPD), we find:

image(2)

where the dimensions of EHD and OPD are measured in μm, and the regression coefficient of the fit was 0.905. In both cases, we can ignore the intercept on these linear fits as they are compatible with zero. We can then compare to the equivalent values for soda-lime glass projectiles, where the slope parameter in the equivalent fit to Equation 1 was found to be 600 ± 170 and for Equation 2 was 5.0 ± 1.4, in both cases, clearly distinct from glycine impacts.

Discussion

Our data may be used to consider models of track formation. Iida et al. (2010) offer a tripartite scheme for track shape similar to the A, B, C scheme used above. Like Kearsley et al. (2009), Iida et al. associated each track type with, respectively, strong well-consolidated grains, mixtures of fine-grained and less-fragile grained materials, and fragile aggregates of fine-grained materials. They go further, however, in noting that the maximum lateral growth of tracks is observed to occur early along a track, and propose that this width depends on the shock wave caused near the impact point, i.e., it depends on the impact speed, size of projectile, and aerogel properties only and no other particle properties. Herein, we can test this model by looking at tracks from soda-lime glass and glycine grains of similar size. From Fig. 7, it appears that the maximum track width does not depend solely on original particle diameter, with a clear difference between the results for soda-lime glass and glycine (this is also true for entrance hole diameter shown in Fig. 6b, which also depends on particle type and not just original particle size). Thus, this aspect of the model of Iida et al. 2010 does not appear correct. Even for equal-sized particles, the maximum track width occurs at different depths along a track. Further, the maximum track width does not simply scale with impact energy. This supports the hypothesis that the cavities in type B and C tracks develop differently to the initial part of type A tracks.

Figure 7.

 The maximum track width is shown plotted against the original size of the impactor for both soda-lime glass and glycine impacting Stardust flight-spare aerogel or Stardust equivalent aerogel at approximately 6.1 km s−1. It can be seen that under similar impact conditions, the maximum track width depends on some other property or properties of the impactor than just its original diameter.

The shape of the glycine tracks in the aerogel is clearly both qualitatively and quantitatively different to those made by the soda-lime glass beads. In previous work (Burchell et al. 2008), grains of the serpentine mineral lizardite were fired into Stardust grade aerogel. The resultant track shape was indeed comparable to type B to C, i.e., a broad single cavity (see Fig. 8), with few, if any styli. Lizardite typically contains 14 wt% H2O, and thus the formation of a bulbous cavity in aerogel might be taken as evidence for volatile driven expansion as a driving mechanism. Certainly, the hypervelocity capture of hydrous phyllosilicates is known to involve high temperature surface processing (Okudaira et al. 2004; Noguchi et al. 2007). Rapid dehydration must therefore involve expulsion of vapor. However, scanning electron microscopy of the source sample for the lizardite grains used in Burchell et al. (2008) shows that it is composed of natural porous aggregates of small crystals, and might be expected to be mechanically weak at the μm scale. It could therefore also be cited as an example of how a cavity can grow from the disruption of a mechanically weak impactor, which fragments into finite subgrains that are able to disperse sideways. In comparison with track images in Fig. 4, the lizardite track appears to have more jagged peaks along its wall than the glycine track walls that are relatively smoother. This may reflect different mechanisms driving the growth of the cavity, i.e., discrete solid impactor fragments propelled radially in the lizardite shot as opposed to broader vapor-pressure driven expansion in the glycine shot. The roles of particle internal structure and volatile content in development of subtle aerogel track features are explored further in the artificial aggregate projectiles described by Kearsley et al. (2012), and may be accentuated by changes in impactor behavior on encountering different density aerogel targets.

Figure 8.

 Side view of track made in Stardust grade aerogel by impact (from the top) of a grain of lizardite. (Reproduced from fig. 5b, Burchell et al. 2008.)

It does not automatically follow that, in general, organic particles impacting aerogel will all behave as the glycine did here. In an earlier paper using organic projectiles, large grains of poly (methyl methacrylate) (PMMA) and poly (ethyl methacrylate) (PEMA) were fired into aerogel (density 60 kg m−3) at speeds of approximately 5 km s−1 (Burchell et al. 2004). Both PMMA and PEMA are thermally stable up to about 300 °C. The PMMA grains were 200–300 μm in diameter, and those of PEMA were between 100 and 150 μm; the grains in both samples were spherical in shape. After impact, the paper reported on a captured PMMA grain of 187 μm diameter, and a PEMA grain of 130 μm diameter. The lack of a well-defined initial grain size meant that mass loss during capture could not be quantified, but in both cases, a substantial quantity of material was retained at the end of the tracks, in strong contrast to the glycine studied here. Interestingly, terminal grains of both PMMA and PEMA gave recognizable Raman spectra in situ in the aerogel after capture (Burchell et al. 2004). Attempts to obtain Raman spectra from the particles observed in situ in the glycine aerogel tracks here failed to show the distinctive spectra of glycine (which was observed in raw, unshot grains), but did show D and G carbon peaks. This indicates some surface processing of the captured grains. The shape of the PMMA and PEMA tracks is also interesting. Although not elaborated in Burchell et al. (2004), they are not both classic carrot-shaped, type A tracks. The PMMA, at first glance does indeed appear to be a classic type A (fig. 2a in Burchell et al. 2004), but has a width/length ratio of about 0.11, right on the limit used in Burchell et al. (2008). The shape of the PEMA track (fig. 4a in Burchell et al. 2004), has no broad initial cavity with stylus beneath (i.e., is not a type B track), but rather appears more like a broad, short carrot-shaped, more akin to some of the glycine tracks seen here (e.g., the 140 μm example in Fig. 4), and has a width/length ratio of about 0.3, at the lower limit of the range of values for glycine tracks here and well above the type A limit. Further description of aerogel tracks from impact of organic materials can be found in Kearsley et al. (2012).

In a separate study, firing 20 μm mono-dispersive polystyrene microparticles into aerogel of density 25–35 kg m−3, it was found that classic carrot-shaped type A tracks resulted (Burchell et al. 2009b), even in impacts at 6.1 km s−1. However, although not noted in that paper, the tracks became relatively broader at the higher speeds. The captured polystyrene grains at the end of the tracks in these high-speed impacts were heavily eroded, having lost 86% of their mass on average, but had done so as type A tracks. Interestingly, the grains showed no significant mass loss in lower speed impacts until the impact speed exceeded 2 km s−1. In that paper, it was also reported that under thermogravimetric analysis, the polystyrene grains did not lose mass until heated above 400 °C. A model calculation in Burchell et al. (2009b) compared the incident kinetic energy (in terms of kJ mol−1) with the chemical energy needed to break all the chemical bonds in the styrene, and found it insufficient to do so below impact speeds of approximately 10 km s−1. However, if selective bond breaking occurred, mass loss could also occur. A significant source of prolonged, intense heating lies in the molten aerogel that builds up around a particle as it penetrates into the target aerogel block. Examples of molten aerogel coatings on captured soda-lime glass beads are shown in fig. 14 in Burchell et al. (2009a), where there is also evidence of mechanical damage (cracking and fracturing of the projectiles, even in cases which produced type A tracks). Figure 16 in Burchell et al. (2009a) shows evidence for melting and thermal ablation of a projectile, and similar evidence of temperatures sufficient to melt alumina, is given in fig. 15 of Hörz et al. (2009). In addition, it may be that once heated, the polystyrene decomposes differently to materials such as glycine and lizardite, and there may be stronger release of gas in the latter case aiding the cavity formation (unlike the polymer where there is no oxygen reservoir, for example).

Hence, the response of grains to impacts may be a complicated mixture of their own properties, the impact speed (and energy), and the properties (mostly density) of the aerogel. Together these can result in (1) fragmentation of mechanically weak particles and those with low thermal decomposition temperatures creating broad cavities (and type B or C tracks depending on whether there were any large, robust subgrains to then form styli after the cavity), or (2) type A tracks, forming from more robust (mechanically and thermally), well-consolidated particles, with terminal grains similar in size or smaller than the original grain; in the latter case after losing mass by heating causing chemical bond breaking or mechanical ablation of softened material.

Implications for Stardust

If we accept that cavities and long relatively slender styli in aerogel tracks are developing from different mechanisms, this raises questions about the calibration of the Stardust cometary dust particle sizes. This was performed for the initial reports of Hörz et al. (2006) and Burchell et al. (2008) using entrance hole diameters for large particles and track volume for other particles. Both these parameters were obtained from impacts of soda-lime glass projectiles. The results presented here, however, suggest that using the original soda-lime calibration, the entrance hole diameter method underestimates the original (glycine) particle diameter by a factor of 1.7. Similarly, the use of track volume and a constant aerogel excavation energy of 1.7 mJ per mm3 soda-lime glass compared with 81 mJ per mm3 found here for glycine (this value is the inverse of the slope in Fig. 6a) underestimates the original energy by a factor of 48. This would have underestimated the original (glycine) mass by the same factor and, at a fixed impact speed, the original diameter by a factor of 3.6, assuming we had known the correct density, or 3.2, if we used the incorrect soda-lime density to convert mass to diameter. Thus, by use of the wrong calibration, both methods suggest an underestimate of original particle size by a factor of around 1.7–3.2 in particle diameter.

Although traces of glycine with δ13C values indicative of an extraterrestrial origin have been reported in Stardust aerogel exposed to the cometary environment (Glavin et al. 2008; Elsila et al. 2009), it is not thought that single dust particles with significant glycine content were encountered. Indeed, distinct grains of glycine were not detected in Stardust; rather, glycine was found as trace amounts trapped in aerogel away from tracks, i.e., which had probably spread through the aerogel as a vapor after impact, or on the surfaces of foils around the aerogel. However, the use of glycine in this study was not because we imagine large single grains impacting Stardust aerogel near the comet. Rather, it was as a proxy for materials with a low thermal decomposition temperature. It then allows us to consider what might happen to the derived cometary dust particle sizes and masses, if they had similar properties.

The tracks found in aerogel reported in Hörz et al. (2006) and Burchell et al. (2008) were 65% type A, 33% type B, and 2% type C. If all type C tracks formed the same way as described here, the correction to the previously published size distribution would be straightforward, but of negligible impact, as it applies to such a small fraction of the data. The more serious issue is that of the cavities in the observed type B tracks. If these cavities are formed in the same way as cavities in type C tracks, then their contribution to the track volume would have been underweighted when calculating particle size from the track volume. We must also consider that while glycine has a low thermal decomposition temperature, and its behavior may give an indication of an important process that may be occurring, the true degree of underweighting need not be the factor of 1.7–3.2 found here; it will depend on what type of material drove the cavity expansion. This is particularly significant, as the type B tracks are not uniformly distributed in the particle size distribution, but disproportionally make up the larger tracks (indeed all the largest tracks are type B). It is therefore the particles at the coarser end of the cometary dust cumulative size distribution that may have been underestimated in size, and correction for this effect might result in a slightly less steep cumulative size distribution for Wild 2 dust.

Conclusions

The use of aerogel to collect dust particles in space is a valuable tool in cosmic dust studies. However, attempting to infer the original, preimpact size distribution of these dust grains is difficult. For the Stardust mission, which flew past comet 81P/Wild 2 at a known speed, this task was easier, as the impact speed and angle were well constrained. However, the nature of the dust itself dictates the nature of the observed aerogel tracks that are split by convention into three types. That there is a difference between the mechanism which produces the long slender carrot tracks and the broad cavities is demonstrated here.

For Stardust, the existing calibration of Burchell et al. (2008) is appropriate for the 65% of impacts that produced type A carrot-shaped tracks. However, a “one-size fits all” calibration is not sufficient when trying to determine original particle size from study of more complex aerogel tracks. Our data here should not, however, be taken as a definitive calibration of aerogel track cavity formation. That may well depend on the nature of the volatile material involved. Furthermore, it does not provide a calibration for cavities produced by fragmentation of mechanically weak dust grains. Therefore, we do not attempt a detailed re-evaluation the 81P/Wild 2 cumulative size distribution here. Instead, this awaits further calibration work using a combination of experiments with dust grains that are mechanically weak, dust grains with multiple subgrains embedded in a range of volatile materials and, if possible, dust grains embedded in ice. The results then need to be combined with an assessment of what caused each observed track cavity to try to match the correct calibration to each track.

Acknowledgments–– M. J. B. and M. C. P. acknowledge a grant from STFC (UK) that funds their work. The authors thank the referees for useful comments on the manuscript.

Editorial Handling–– Dr. Bradley De Gregorio

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