DEVELOPMENT OF SAMPLE MOUNTING FOR STARDUST INTERSTELLAR CANDIDATES

Logan Magad-WeissDepartment of Geology and Geophysics

University of Hawai‘i at MānoaHonolulu, HI 96822

ABSTRACT

At least two particles collected in aerogel by the Stardust Interstellar Dust Collector are suspected to be of interstellar origin, with several more interstellar candidates likely to be identified soon. Oxygen isotopic analysis by secondary ion mass spectrometry will provide more insight into the origin of these particles, and possibly definitive proof of their interstellar origin. We developed sample-preparation techniques to extract the interstellar candidates from the aerogel collector and prepared them for isotope analysis. By using grains extracted from meteorites with non-terrestrial oxygen isotopic composition, we will investigate the reliability, accuracy, and precision of our sample preparation and analytical techniques.

INTRODUCTION

NASA's Stardust Mission was launched in 1999 to collect cometary and interstellar dust and return the samples to Earth for study in the lab. Samples from the Jupiter-family comet 81P/Wild 2 have been extensively studied since samples returned to Earth in 2006. One of the most surprising results from the Stardust cometary collection is that some of the rocky component of this comet originated in the inner solar system (Ogliore et al. 2012 ApJ 745 L19). Analyses of potential interstellar dust collected by Stardust are far more challenging due to their scarcity and small size.

The Stardust Interstellar Dust Collector (SIDC) was exposed to the interstellar dust stream for 195 days during two periods between 2000 and 2002. Like the Stardust cometary collector, the SIDC captured high-velocity dust particles in low-density silica aerogel tiles supported by an aluminum frame. The 0.1 m2 collector is composed of 85% aerogel tiles, and 15% of aluminum foils. Hypervelocity particles impacting the aluminum foils create craters and are severely melted, though some residue of the impacting residue can be retained within the crater. Particles impacting the aerogel collector are more gently captured and can retain their original mineralogy as well as their trajectory which can be used to compare with the expected direction of the interstellar dust stream. Preliminary, nondestructive analyses of particles captured by the Stardust Interstellar Dust Collector (SIDC) were recently published (Westphal et al. 2014 Science 345, 786). At least two particles captured in the aerogel were determined to be probable interstellar dust.

The oxygen isotopic composition of the interstellar dust captured by Stardust can help distinguish true interstellar dust, which formed outside the solar system, from interplanetary dust which formed inside the solar system. The 17O/16O and 18O/16O ratios of solar-system material vary by only ~5% percent, whereas presolar grains, which formed around other stars, vary in these ratios by up to a factor of 10. However, some grains that formed outside the solar system have approximately solar oxygen isotopic composition, so normal oxygen isotope ratios do not

40

necessarily disprove an interstellar origin. Conversely, however, highly anomalous oxygen isotope ratios do prove that the particle formed outside the solar system.

Figure 1: Stardust interstellar candidate “Hylabrook”. A) Phase map of Mg-bearing olivine and amorphous phases, from synchrotron STXM data. B) X-ray diffraction pattern of Hylabrook,

showing crystalline phases. Figure is from Westphal et al. 2014 Science 345, 786.

The technique best-suited to precisely measure the oxygen isotopic composition of candidate interstellar grains is secondary ion mass spectrometery (SIMS), or ion probe. The ion probe is destructive: it consumes the sample to make the isotopic measurement. Since there are only a few grains identified so far that are suspected to be of this interstellar origin, it is imperative to develop reliable, robust techniques to extract and prepare the particles for SIMS analysis.

METHODS

Steve Jones (JPL) has fabricated aerogel embedded with spinels from the Allende meteorite. These spinels have oxygen isotopic composition off the terrestrial fractionation line, and so should be easily distinguishable from terrestrial contaminants and the surrounding aerogel by SIMS.

In order to extract the spinel grains in aerogel keystones, a keystoning technique was implemented. This technique was developed by Andrew Westphal’s lab at U. C. Berkeley (Westphal et al. 2004 MAPS 39, 1375), and was adapted to the microscope, stage, and micromanipulators available in HIGP. This was a significant resource for the University of Hawai‘i since only U. C. Berkeley and Johnson Space Center can extract aerogel keystones. Dr. Ryan Ogliore has experience extracting keystones at U. C. Berkeley, and aided in the safe extraction of the keystone.

CONSTRUCTION OF A SURGICAL TABLE

A “surgical table” had to be constructed in order to hold the silica aerogel in place during extraction. The aerogel had to be kept still during keystone extraction. A device to secure it in place was needed because of the aerogel’s extremely low density and tendency to move easily by

41

static charging. The aerogel is very fragile and fractures easily, so a surgical table was necessary to hold the aerogel block in place during extraction without causing damage to the aerogel or the spinels within it.

The surgical table (Figure 2) is a plastic box with plastic wrap that is tightened from the corners by fishing line pulled by guitar tuners. The aerogel block is placed on a Petri dish between the surface of the plastic box and the plastic wrap. The guitar tuners pull the plastic wrap taut, holding the aerogel block in place, without exerting too much force on the aerogel. A hole in the plastic wrap is made for access to the aerogel while the rest of the aerogel is protected from dirt and dust in the lab.

To construct the box, four holes were drilled on each side of a plastic rectangular box,. From there, guitar tuning heads were fitted to the holes, and held in place using bolts, washers and nuts. Four more holes were drilled above the end of the guitar tuning heads so that fishing line could be fed into the box. A rectangle of plastic wrap was then cut out, and aluminum tape was folded over the edges. Plastic rods were also clamped down within the aluminum tape around the saran wrap. Holes were punctured through the aluminum tape and plastic wrap so that fishing line could also be fed through there. The fishing line was then tied around the plastic rod through the hole so that it would not come undone later in the experiment. A Petri dish was then placed on top of the box, and the plastic wrap was laid down on top of it. Once the fishing line was fed through all of the holes above the guitar-tuning heads, it was pushed through the hole within the tuning head itself, wrapped around, and knotted. All of the tuning heads were then turned until each corner of the plastic wrap was taught, and the center of the plastic wrap was flat. This part was critical because the silica aerogel is very light and fragile, and it was necessary to have a flat surface that could hold it down while not crushing it. The final step in the construction of the stage was to burn a hole ~1-4mm wide in the center of the saran wrap. This was accomplished using a soldering iron which successfully created a hole that would enable us to see the aerogel while operating the microscope during extraction. Following the creation of this hole, the aerogel was successfully placed on the Petri dish under the saran wrap, and held down taut.

Figure 2: Surgical table built for safely holding the silica aerogel

42

THE CODE, AND THE CUTS

A Matlab script (originally written by Andrew Westphal) was used in order to send coordinates for cuts to the micromanipulator. The Matlab code writes simple script files of coordinate values that are sent via serial interface to the micromanipulator. Once we were able to communicate with the micromanipulator through the serial port, we modified the Matlab script to function in an Ubuntu-Linux environment. We modified a vise grip that could hold a pulled-glass needle snugly in the micromanipulator. Next, we aligned the needle in row, pitch, and yaw, centered the micromanipulator’s coordinate system above a spinel grain in the aerogel block. Next, we executed three extraction scripts for a 90-degree-bent glass needle (left cut, top cut, bottom cut) and one for a 45-degree-bent glass needle (the undercut).

Figure 3: Top view of the keystone cuts made by the Matlab code

Figure 4: Side view of the keystone cuts made by the Matlab codE

43

Figure 5: Micromanipulator setup showing the 90° needle

THE KEYSTONE

The keystone was removed using a stereoscope to observe the aerogel, and thin bristles on a paint brush to get the aerogel to attach to it. Silica aerogel is very low density, and is moved mostly by electrostatics rather than by gravity. The keystone was transferred onto a welled glass slide, and quickly covered that with another slide. The slides were then clamped together, and using a stereoscope and compound microscope, it was confirmed that the keystone was successfully extracted. We then located the spinel grain of interest inside the keystone, which was now in a different orientation than when it was first extracted. Despite this success, there were difficulties encountered, such as loss of the first attempted keystone. Removal of the keystone was first attempted using a single bristle of a paint brush held on a toothpick with an epoxy. Problems arose when the aerogel was unable to attach to the bristle. The keystone could be moved, however not picked up and placed on a slide for safe keeping. In addition, the single bristle had a tendency to catch on the aerogel and bend, until reaching a point that caused it to fire outward. This flicked the keystone multiple times, making it difficult to find, and eventually lead to it being lost. Correcting for mistakes made in the first extraction attempt lead to a successful extraction in the second attempt.

ACQUIRING A STANDARD AND SECURING THE PARTICLES

In order to accurately measure the oxygen isotope composition of the Allende spinel grain, there needed to be a standard to compare the oxygen isotope composition against. The standard used was a terrestrial grain of Burma spinel curtesy of Dr. Aurelien Thomen at HIGP. Burma spinel has a known oxygen isotope value, and would permit the ion probe to switch between each particle respectively to measure the differences in oxygen isotope composition. To do so, a large piece of Burma spinel was removed using a diamond saw, and was placed in an ion probe mount. The Burma spinel was then epoxied into the ion probe mount using an epoxy recipe at HIGP, and cured in an oven at 70°C for 72 hours. Once the epoxy had cured, the

44

keystone was then placed in the ion probe mount, and epoxied in as well. Using diamond paste, the sample mount could then be polished down until the Allende spinel grain was exposed. The decision to use a larger Burma spinel grain was so that when polishing down to the Allende spinel, the Burma spinel would merely be shaven down so that the two particles are side by side and can be switched between during testing in the ion probe.

Figure 6: Extracted silica aerogel keystone

Figure 7: Ion probe mount displaying Burma spinel grain and silica aerogel keystone

45

CONCLUSION

Removal of this keystone has confirmed that the Matlab code, as well as the stage and micromanipulator are capable of both cutting and removing a keystone safely. We are currently polishing the keystone with diamond paste in order to expose the Allende spinel grain and spinel standard for ion probe analysis using the Cameca ims 1280 in the basement of POST at University of Hawai‘i at Mānoa. The polishing should be completed in the coming week. Comparison of the grain’s oxygen isotope composition against that of a standard terrestrial particle of known composition from the Allende meteorite will determine whether or not this technique of oxygen isotope measurements is accurate and precise. If the particle’s oxygen isotope composition returns a value within the known values of the Allende meteorite, it will indicate that these methods are both precise and accurate, and are closer to use on the suspected interstellar dust candidates.

REFERENCES

Westphal A. J. et al. (2014) Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft. Science 345, 786; DOI: 10.1126/science.1252496

Ogliore R. C., Nagashima K., Huss G. R., Westphal A. J., Gainsforth Z., and Butterworth A. L. (2015) Oxygen isotopic composition of coarse- and fine-grained material from Comet 81P/Wild 2. Geochimica et Cosmochimica Acta 166, 74–91; DOI:10.1016/j.gca.2015.04.028

46