The origin of the 3.4 micron feature in Wild 2 cometary particles and in ultracarbonaceous interplanetary dust particles

0.71 MB·English

by  Graciela Matrajt

The origin of the 3.4 µm feature in Wild 2 cometary particles and
in ultracarbonaceous interplanetary dust particles
G. Matrajt
Astronomy Department, University of Washington, Seattle, WA 98195
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[email protected]
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G. Flynn
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J Department of Physics, SUNY-Plattsburgh, Plattsburgh, NY 12901

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3 D. Brownlee

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P D. Joswiak
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. and
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- S. Bajt
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r DESY Photon Science Notkestr. 85 22607 Hamburg Germany
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[ ABSTRACT

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We analyzed 2 ultra-carbonaceous interplanetary dust particles and 2
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7 cometary Wild 2 particles with infrared spectroscopy. We characterized the
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carrier of the 3.4 µm band in these samples and compared its profile and the
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1. CH2/CH3 ratios to the 3.4 µm band in the diffuse interstellar medium (DISM),
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in the insoluble organic matter (IOM) from 3 primitive meteorites, in asteroid
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1 24 Themis and in the coma of comet 103P/Hartley 2. We found that the 3.4 µm
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v band in both Wild 2 and IDPs is similar, but different from all the other astro-
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X physical environments that we compared to. The 3.4 µm band in IDPs and Wild
r 2 particles is dominated by CH groups, the peaks are narrower and stronger
a 2
than in the meteorites, asteroid Themis, and the DISM. Also, the presence of
the carbonyl group C=O at ∼1700 cm−1 (5.8 µm) in most of the spectra of our
samples, indicates that these aliphatic chains have O bonded to them, which is
quite different from astronomical spectra of the DISM. Based on all these obser-
vations we conclude that the origin of the carrier of the 3.4 µm band in IDPs
and Wild 2 samples is not interstellar, instead, we suggest that the origin lies in
the outermost parts of the solar nebula.
Subject headings: Wild 2 cometary particles; interplanetary dust particles; 3.4
µm feature
– 2 –
1. Aim of paper
In this work we performed a coordinated study of cometary and interplanetary particles.
We first used an electron microscope to locate the carbonaceous materials in the samples
and determine their morphological aspects. We then performed in situ infrared spectroscopy
directly on the carbonaceous materials to investigate their 3.4 µm band and the presence
of other organic-related peaks (carbonyl, aromatics, etc). Finally we compared the 3.4 µm
feature of our samples to the 3.4 µm feature of other astrophysical environments.
2. Introduction
The NASA Stardust spacecraft returned particles collected from the coma of comet
81P/ Wild 2 (hereafter Wild 2). Hundreds of cometary particles ranging from 1 µm to
100 µm in size were collected by impact into aerogel with an encounter velocity of 6.1
km/s (Brownlee et al. 2006). The examination of these samples has provided many unex-
pected findings, including the presence of refractory minerals (Simon et al. 2008), the pres-
ence of chondrule-like objects (Nakamura et al. 2008) and low abundance of presolar grains
(Stadermann & Floss 2008). Indigenous organic materials are also observed in Wild 2 sam-
ples (Sandford et al. 2006; Matrajt et al. 2008; Cody et al. 2008, 2011; Gallien et al. 2008;
Wirick et al.2009;De Gregorio et al.2010,2011;Clemett et al.2010;Nakamura-Messenger et al.
2011). The abundance of organic matter in the Wild 2 samples was lower than expected
because the most of the submicron material was destroyed upon capture (Brownlee et al.
2006). Coordinated analyses of organic material have shown that the organic material
in the Wild 2 particles is very diverse in its morphology, isotopic and chemical composi-
tion, abundance, spatial distribution and complexity (Matrajt et al. 2008; De Gregorio et al.
2010, 2011; Nakamura-Messenger et al. 2011). Infrared spectroscopy (Sandford et al. 2006;
Keller et al. 2006; Mun˜oz Caro et al. 2008; Bajt et al. 2009) has shown that aliphatics are
present in most tracks and particles. In some cases, the aliphatic molecules areindistinguish-
able from the organics intrinsic to aerogel (Mun˜oz Caro et al. 2008). But in most cases the
organics are very different from the compounds found in aerogel, which is mainly dominated
by CH groups (Bajt et al. 2009).
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Interplanetary dust particles (IDPs) are materials collected in the Earth’s stratosphere
usuallyconsideredtobeamongthemostprimitivesamplesofthesolarsystem(Brownlee et al.
1976; Sandford 1987). Most IDPs are very carbon-rich, having in average 10-12 wt% C con-
tent (Schramm et al. 1989). The carbonaceous materials in these IDPs are made of organic
molecules (Thomas et al. 1993; Flynn et al. 2003), including aromatic and aliphatic com-
pounds (Clemett et al. 1993; Keller et al. 2004). These carbonaceous phases often have H
– 3 –
andNisotopic anomalies (Messenger 2000; Aleon et al. 2003; Keller et al. 2004) proving that
they are indigenous and suggesting that they formed through low-temperature chemical re-
actions (Messenger 2000; Keller et al. 2004; Floss et al. 2006) in a presolar cold molecular
cloud or at the edges of the protoplanetary disk.
In the present study we analyzed with Fourier transform infrared spectroscopy two
IDPs and two Wild 2 samples that have been previously characterized by other analytical
(Matrajt et al. 2008, 2012). These past studies revealed that all of these samples have
carbonaceous materials with 15N and D excesses and it was suggested that this is primitive
organic matter that has changed little or not at all since the formation of the Solar System
(Flynn et al. 2003; Keller et al. 2004; Matrajt et al. 2008, 2012, 2013). However, owing to
their small size, the nature of these phases has been poorly constrained. In this work we
characterized these organic materials with FTIR to determine 1) the nature of the organics;
2) the characteristics of the 3.4 µm band and 3) the origin of these carbonaceous materials
(solar vs interstellar).
3. Samples
In this study we worked with two IDPs, that we nicknamed Chocha and GS and two
fragments from two Stardust tracks that we nicknamed Febo and Ada.
3.1. GS
Particle GS (curatorial name L2055-R-1,2,3,4,5 cluster #7) is a Grigg-Skjellerup timed-
collection IDP. Calculations (Messenger 2002) predicted that 1 to 50% of the total flux of
IDPs>40µmindiameter collected afterEarthpassed throughcomet26P/Grigg-Skjellerup’s
dust stream in April 2003 would originate from this comet. A dedicated collection of this
dust stream was organized by NASA known as Grigg-Skjellerup collection. Our sample is
an ultra-carbonaceous particle made of > 90 % carbon, anhydrous minerals (mainly olivines
and diopside) and Fe-Mg carbonates. Previous studies of the carbonaceous materials of this
particle showed that it is composed of several carbonaceous textures which have N isotopic
anomalies (Matrajt et al. 2012).
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3.2. CHOCHA
ParticleChochaisanIDPfromcollectorflagW7154. Itisananhydrousultra-carbonaceous
particle made of > 95% carbon. It also contains anhydrous minerals, mainly olivine, py-
roxene (diopside) and Fe-Ni sulfides (pyrrhotite and pentlandite). Previous studies of the
cabonaceous materials of this particle showed that it is composed of several carbonaceous
textures which all have N isotopic anomalies (Matrajt et al. 2012).
3.3. FEBO
Particle Febo is fragment # 2 from Stardust track # 57. The particle is made mainly
of pyrrhotite and fine-grained material and also contains small silicates. Previous studies
of the carbonaceous materials, found in the periphery of the pyrrhotite and between the
small fine grains, showed that they have several textures and N and H isotopic anomalies
(Matrajt et al. 2008).
3.4. ADA
Particle Ada is fragment # 2 from Stardust track # 26. The particle is made mainly
of tridymite and fayalite. Previous studies of the carbonaceous materials, found in the
periphery of the particle, showed that they have several textures and N and H isotopic
anomalies (Matrajt et al 2008).
4. Methoods
4.1. Sample preparation
The Wild 2 particles were received from NASA inside aerogel chips, also known as
keystones. The entire aerogel chips and the IDPs were embedded in acrylic, then cut with a
diamond knife to a thickness of less than 50 nm to make it transparent to the electron beam.
Acrylic was then dissolved out from the cut sections with chloroform vapors, following the
methodology developed by Matrajt & Brownlee (2006).
– 5 –
4.2. Transmission Electron Microscopy (TEM)
All microtome slices were studied with a 200 keV Tecnai field-emission electron micro-
scope in transmission mode. We used a CCD Orius camera to study the morphologies and
textures of the carbonaceous materials. We also used a Gatan Imaging Filter (GIF) detector
to acquire carbon maps.
4.3. Fourier Transform Infra Red (FTIR) spectroscopy
Fourier transformed Infrared (FTIR) spectroscopy is a technique often used for the
in situ identification of organic functional groups. The mid infrared spectral region, from
650 to 4000 cm−1, shows unique absorption features characteristic of organic materials. We
used the infrared microscope located on beamline U2B of the National Synchrotron Light
source at Brookhaven National Laboratory to study samples Febo, GS and Chocha. Spectra
were obtained over a range of 4000 to 650 cm−1 and with an energy resolution of 4 cm−1
and a spatial resolution of 3-5 µm, using a Thermo-Nicolet Continuum FTIR bench (KBr
beamsplitter) in transmission mode, and a MCT-A detector. Sample Ada was analyzed
at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, using a
Thermo-Nicolet Magna 760 FTIR bench (KBr beamsplitter) and a SpectraTech Nic-Plan IR
microscope in reflectance mode, and an MCT-A detector. The preliminary IR data of Ada
was published in a conference abstract (Wopenka et al. 2008).
5. Results
Table 1 shows the peak assignments for all the peak positions found in all four samples
and it also shows which assignments were found for each of the samples studied. Table 2
shows the CH /CH ratios. Ratios were calculated using the optical depth of the CH peak
2 3 3
at ∼ 2956 cm−1 and the optical depth of the CH peak at ∼ 2926 cm−1 (Sandford et al.
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1991). We made a baseline correction by fitting the baseline with a straight line across
the range from 3100 cm−1 to 2800 cm−1. Table 3 shows a comparison of peaks from the
3.4 µm region and the C=O peak between our samples and other objects (DISM, comet
103P/Hartley, Murchison IOM, Orgueil IOM, Tagish Lake, asteroid 24 Themis).
The IR peak assignments and interpretations were done based on former IR studies of
IDPs (Keller et al. 2004; Matrajt et al. 2005; Mun˜oz Caro et al. 2006) and the Tagish Lake
meteorite (Matrajt et al. 2004) and are as follows: 3255 cm−1 is the OH stretch in water,
carboxylic acids or alcohols. 2990 cm−1 is a =C-H stretching 2950-2956 cm−1 is the CH
3
– 6 –
asymmetric stretching in aliphatic hydrocarbons. 2918-2920 cm−1 is the CH asymmetric
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stretching in hydrocarbons. 2896 and 2860-2870 cm−1 are the CH symmetric stretchings in
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hydrocarbons. 2845-2855 cm−1 is the CH symmetric stretching in aliphatic hydrocarbons.
2
2160 cm−1 is a C=C stretching vibration in alkenes. 1740 cm−1 is the carbonyl (C=O)
in esters. 1730, 1714-1717 and 1700 cm−1 are C=O stretching in ketone and carboxylic
acids. 1685 cm−1 is the H-O-H stretching in water. 1480 and 1447 cm−1 are CH and CH
3 2
bending vibrations, respectively. 1435 cm−1 is C=C stretching in aromatics. 1386 cm−1 is
the CH symmetric bending. 1270 and 1240 cm−1 are C-O-C vibrations in esters. 1190 and
3
1147 cm−1 are unknown. 1065 cm−1 is a C-OH vibration in secondary cyclic alcohol. 987,
970 and 910 cm−1 are CH=CH bending vibrations. Absorption at 1650-1654 cm−1 is the
C=C stretching in aromatics. 1448 cm−1 is the CH bending in aliphatics or the CO3− in
2
carbonates. 1418 cm−1 is the C=C stretching in aromatics. 1350 cm−1 is the CH bending
3
in aliphatic hydrocarbons. 1220 cm−1 is the CH wagging mode. 1160 cm−1 is CH twisting
2 2
mode. 1216, 1136 and 1106 cm−1 are Si-O stretching in silicates. 1070, 1060 and 952 cm−1
are the Si-O stretching in pyroxenes. 930 cm−1 is the Si-O stretching in silicates and 887
cm−1 is Si-O stretching in olivine.
5.1. IDP GS
5.1.1. TEM
Energy filtered transmission electron microscopy (EFTEM) carbon maps revealed that
most of the microtomed area of the particle is made of carbonaceous material (Figure 1).
The bright field (BF) images of the different carbonaceous areas revealed several types of
morphologies: spongy, globular, smooth, dirtyandvesicular. Thesemorphologies, previously
described in adjacent sections of this same IDP and other IDPs (Matrajt et al. 2012) can
be described as follows (Figure 2): vesicular morphology is characterized by having small
vesicles or voids found in a C-rich smooth material. Usually the voids are smaller than
∼
the section thickness ( 50-70 nm). Globular morphology is characterized by round-shaped
structures that may be hollow or filled. Dirty morphology is characterized by a carbonaceous
material that has mineral grains (typically sulfides) embedded in it. Spongy morphology
is characterized by a lace mesh-like material. Smooth morphology is characterized by a
shapeless and textureless material.
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5.1.2. IR
Figure 3 shows a FTIR spectrum of the entire particle. Peaks are observed at 3255,
2951, 2920, 2896, 2870, 2845 and 1070 cm−1. Also, a broad band from 1545 to 1455 cm−1 is
observed. Fromthepeak assignments we deduced that thisparticle containswater bondedto
its structure, probably to carbonates. It also contains organics that correspond to aliphatic
hydrocarbon chains containing symmetric and assymetric stretchings and silicates. The
broadbandfrom1545and1455cm−1 correspondstocarbonates. Figure4showsthealiphatic
stretching peak area (3000-2800 cm−1) zoomed. The CH /CH band depth ratio found was
2 3
1.0 (Table 2).
5.2. IDP Chocha
5.2.1. TEM
EFTEM carbon maps revealed that > 95% of the microtomed area of the particle is
made of carbonaceous material (Figure 5). The bright field (BF) images of the different
carbonaceous areas revealed several types of morphologies (Figure 6): spongy , vesicular,
smooth, globular, and dirty. These morphologies were previously described in adjacent
sections of this same IDP and other IDPs (Matrajt et al. 2012).
5.2.2. IR
Figure 7 shows a FTIR spectrum of the entire particle. Peaks are observed at 2956,
2920, 2847, 1740, 1654, 1448, 1350, 1220 and 1160 cm−1. There is also a broad band centered
at 3270 cm−1. From the peak assignments (Table 1) we deduced that this particle contains
aliphatic hydrocarbon chains containing symmetric and assymetric stretchings. The organic
material alsocontains carbonyl, probablyintheformofesters, andeither olefinic oraromatic
C=C molecules. The particle also has silicates. The broad band is the OH stretch in water.
The CH /CH ratio found was 4.6 (Table 2).
2 3
– 8 –
5.3. Wild 2 Febo
5.3.1. TEM
EFTEM carbon maps of the microtomed section revealed several small areas that are
carbon-rich (Figure 8). The BF images of the different carbonaceous areas reveal several
types of morphologies (Figure 9): dirty , vesicular and smooth. The dirty and vesicular mor-
phologies were previously described in adjacent sections of this same particle (Matrajt et al.
2008) and are identical to morphologies identified in IDPs (Matrajt et al. 2012).
5.3.2. IR
Figure 10 shows two FTIR spectra from two areas of the particle. The left one was
acquired primarily on top of the sulfide area (black area of the particle in figure 8). The
right spectrum was primarily acquired from the fine-grained area of the particle (arrow in
Figure 8), which is the area where all the C-rich materials were observed. Peaks in the
left spectrum are observed at 2954, 2920, 2855, 1730, 1717, 1700, 1685 and 1650 cm−1.
There is also a broad band between 1070 and 957 cm−1 centered at 1010 cm−1. The peak
assignments indicate that this portion of the particle contains hydrocarbons with aliphatic
chains, carbonyl in ketones and carboxylic acids and water bonded to the structure of the
organic molecules. There is also some evidence of aromatic or olefinic C=C molecular bonds.
The broad band is the Si-O stretch in silicates.
The peaks in the right spectrum are observed at 2950, 2920, 2860, 1730, 1060, 952, 930
and 887 cm−1. The peak assignments indicate that this side of the particle has aliphatic
hydrocarbon chains and carbonyl in ketones and carboxylic acids. There is also evidence of
olivines and pyroxenes. The CH /CH band depth ratio was 1.96 (Table 2).
2 3
5.4. Wild 2 Ada
5.4.1. TEM
EFTEM carbon maps of microtomed sections revealed several small areas that are
carbon-rich (Figure 11). The BF images of the different carbonaceous areas reveal two types
of morphologies: globular and smooth. These morphologies were previously described in ad-
jacent sections of this same particle (Matrajt et al. 2008) and are identical to morphologies
identified in IDPs (Matrajt et al. 2012).
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5.4.2. IR
Figure 12 shows two FTIR spectra of two different microtomed sections. Peaks are
observed at 2954, 2918, 2847, 2160, 1714, 1418, 1216, 1136and 1106cm−1. Peak assignments
(Table 1) indicate that this particle contains chains of aliphatic hydrocarbons, some of which
have C=C groups attached to them. There is also evidence of carbonyl in ketone and
carboxylic acids and either olefinic or aromatic C=C bonds. Olivines are the main silicate
present in this sample. The CH /CH band depth ratio was 4.3 (Table 2).
2 3
5.5. Acrylic
Although the acrylic embedding medium we used for our samples was removed from
sections using chloroform vapors, we measured a piece of acrylic with FTIR under the same
experimental conditions used for our samples to have a reference spectrum and ensure that
the interpretations of the organics in the samples are not biased by the organics found in
acrylic. Figure 13 shows a FTIR spectrum of this acrylic. The peaks observed are at 2990,
2949, 1727, 1480, 1447, 1435, 1386, 1270, 1240, 1190, 1147, 1065, 987, 970 and 910 cm−1.
Peak assignments (Table 1) indicate that acrylic is composed of aliphatic hydrocarbons,
ketones and carboxylic acids, aromatics and esters. There are also secondary cyclic alcohols.
Figure 14 shows a comparison of acrylic with IDP Chocha and Wild 2 particle Ada. The
spectrum of acrylic is very different from the other two spectra. First, the peaks in the 3000
cm−1 region are shifted in the acrylic toward higher values (2995 and 2950 cm−1), while
both in Chocha and Ada these peaks are in similar positions and shifted toward lower values
(2917 and 2848 cm−1). Acrylic has a C=C-H stretching that is absent in Ada and Chocha.
Acrylic is dominated by CH while Ada and Chocha aredominated by CH groups (Table 1).
3 2
Second, the relative heights of these peaks are very different in the acrylic spectrum. Third,
Ada, Chocha and acrylic have a peak around 1700 cm−1, but in the acrylic this peak is very
narrow and strong comparing to the one found in our samples and its position is slightly
shifted to lower values comparing to our samples. Additionally, sample GS lacks a peak at
this position (Figure 3), clearly indicating that the presence of this peak in our samples is
not related to the acrylic embedding medium. Forth, all the peaks below 1500 cm−1 in the
acrylic spectrum are narrower and stronger (more intense) than in Chocha (and they are
absent in Ada). Fifth, some peaks in the acrylic spectrum (in the 1000 cm−1 region) are
absent in our samples. In general, peaks in the acrylic spectrum are better defined (more
net) and narrower and stronger than in the samples we studied. It is clear that the organic
material measured in our samples is unambiguously indigenous to the particles and not a
contamination from the acrylic embedding medium. The CH /CH band depth ratio could
2 3
– 10 –
not be calculated because acrylic does not have a CH peak at 2920 cm−1.
2
6. Discussion
The textures and morphologies of the carbonaceous materials we found in our samples
are identical to the morphologies found in carbonaceous materials from carbonaceous chon-
drites (Garvie & Buseck 2004, 2006; Nakamura-Messenger et al. 2006), from interplanetary
dust particles (Matrajt et al. 2012) and from other Wild 2 particles (Matrajt et al. 2008;
De Gregorio et al. 2010; Matrajt et al. 2013). These carbonaceous materials are organic re-
fractory molecules, given that they survive atmospheric entry or hypervelocity impact into
aerogel (Matrajt et al. 2012, 2013). Both IDPs and Wild 2 samples suffered from heating
while being decelerated either in the stratosphere or aerogel. The effects of heating on the
organics in these type of samples are poorly known. However, past studies have shown that
pyrolyzed terrestrial kerogens tend to increase their CH /CH ratios as well as their degree
3 2
of aromatization (Ehrenfreund et al. 1991). For example, under the effects of heating the
Orgueil meteorite decreased its CH /CH ratio. It was suggested that such a decrease means
3 2
that the -CH groups are engaged in thermally labile structures (i.e. bounded to N and/or
3
O). This would cause a faster decomposition of CH groups relative to the CH groups and
3 2
would change the CH /CH ratios (Ehrenfreund et al. 1991). It could also be due simply to
3 2
the general loss of H from these materials during pyrolysis (Jones 2012a). This is, however,
not the case for the samples analyzed in our study. Both IDPs and Wild 2 particles contain
abundant O and N bonded to their organic materials (Flynn et al. 2003; Matrajt et al. 2005;
Sandford et al. 2006; Matrajt et al. 2008; De Gregorio et al. 2010; Matrajt et al. 2012, 2013)
indicating that the CH groups did not decompose by outgassing of thermally labile struc-
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tures during deceleration. CH groups are simply less abundant in IDPs and Wild 2 samples
3
relative to other astrophysical environments. Therefore, we believe that the CH /CH ratios
2 3
discussed in the following sections reflect the primary composition of the organic molecules
from the parent bodies of our samples.
In the following paragraphs we will discuss the characteristics of the 3.4 µm feature of
this organic refractory material and we will compare this feature to the one observed in the
interstellar medium, meteoritic material fromcarbonaceous chondrites (CCs), comet Hartley
2 and asteroid Themis. Because infrared spectroscopy is primarily a qualitative analytical
technique, these comparisons will remain purely qualitative.
Themeteoriticmaterialthathasbeenpreviouslyinvestigated withinfraredspectroscopy
consists of two different components. First, a general carbonaceous component of the me-
teorite, which consists of all the carbonaceous materials present in the sample. Second, a