Bland, P.A.

Daly, L.

Fougerouse, D.

Hallis, L.J.

Jourdan, F.

Lee, M.R.

Reddy, S.M.

Rickard, W.D.A.

Saxey, D.W.

Timms, N.E.

English

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ATOMIC SCALE DEPTH PROFILE OF SPACE WEATHERING IN AN ITOKAWA OLIVINE GRAIN.  
L. Daly1 M. R. Lee1, L. J. Hallis1, P. A. Bland2, S. M. Reddy2,3, D. W. Saxey3, W. D. A. Rickard3, D. Fougerouse2,3, 
N. E. Timms2, F. Jourdan2. 1School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 
8QQ, UK. (luke.daly@glasgow.ac.uk). 2Department of Applied Geology, Curtin University, GPO Box U1987, 
Perth, WA 6102, Australia. 3Geoscience Atom Probe Facility, Advanced Resource Characterisation Facility, John de 
Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. 
 
 
Introduction:  Space weathering is an important 
process in modifying the physical and chemical proper-
ties of the regoliths of airless worlds [1]. This process is 
mostly due to bombardment from ions within the solar 
wind (SW), with minor contributions from solar flares 
and micrometeorite impacts [1]. Transmission electron 
microscopy (TEM) studies of Itokawa grains indicate 
that space weathering typically produces 20-40 nm thick 
amorphous and vesiculated rims, together with occa-
sional Fe nanoparticles [2, 3]. Space weathering also 
causes the implantation of elements, in particular the no-
ble gasses He and Ne [4], as well as volatile elements 
such as H [5]. SW implanted H is particularly important 
as it may react with silicates to produce OH and H2O, 
which have been observed in interplanetary dust parti-
cles (IDPs) [5]. In turn, these reactions may have signif-
icant implications for the origin of water in the inner so-
lar system. Therefore, it is important to quantify the dis-
tribution and speciation of SW-implanted H, and here 
we present the first atom probe microscopy (APM) 
study of Itokawa particles, revealing the attenuation 
depth and profile of SW irradiation.  
Methods: Itokawa particle RA-QD02-0279, a 164 
µm diameter grain consisting of olivine and plagioclase, 
was mounted onto a 100 µm diameter glass rod with ar-
aldite. The grain was sputter coated with 200 nm of Cr, 
to dissipate charge, and protect the immediate surface of 
the grain from any electron or ion beam damage (Fig. 
1). Two APM specimens were extracted normal to the 
surface of the olivine portion of the grain (Fig. 1), and 
sharpened to a needle-shape with a 10-15 nm Cr cap us-
ing focused ion beam (FIB) milling using a TESCAN 
LYRA3 FIB-Scanning electron microscope (SEM), at 
Curtin University. These specimens were analysed on 
the Cameca LEAP 4000X HR Geoscience atom probe 
at Curtin University.  
Results: 51.5 and 55.9 million atoms were collected 
from each specimen. In both APM data sets evaporation 
commenced within the protective Cr coating ensuring 
the outermost portion of the grain was analysed, where 
any space weathering features may be present. No feld-
spar was observed. The composition of olivine was Fa30. 
Helium and Ne were absent from both datasets. How-
ever, small reductions and spikes in density were de-
tected in the outermost 20-30 nm of the tip in both spec-
imens (Fig. 2). The high-density regions in each dataset 
are enriched in Mg relative to the rest of the specimen. 
Furthermore, a gradual enrichment in OH and H2O ions 
was detected extending from the outer surface of the 
grain to a depth of ~50 nm (Fig. 3). 
 
Figure 1: Itokawa particle RA-QD02-0293. Atom 
probe specimens were extracted from the red box. 
Discussion: Vesiculated textures and Fe nanoparti-
cles have previously been described in space-weathered 
rims of Itokawa [6]; such features are not detected here. 
Density variations detected here may either represent a 
new atomic-scale feature of space weathering, or imma-
ture examples of the features described in [6]. The ab-
sence typical space weathering features previously de-
scribed in Itokawa grains imply that this rim is immature 
and has not been exposed to SW as long as other Ito-
kawa grains (e.g. [6]). 
Hydrogen ions are a main component of SW [5], and 
so implantation of H+ ions from SW is interpreted to be 
the cause of anomalously high levels of OH and H2O 
ions detected in the outer 50 nm of the particle. The pen-
etration depth of SW for this particle was approximately 
50 nm, with a linear attenuation profile (Fig. 3). This 
finding is broadly consistent with Monte Carlo simula-
tions of space weathering implanted H+ ions [3]. Meas-
ured abundances of He and Ne for other Itokawa grains 
with space-weathered rims are above the detection lim-
its of APM [4, 7]. The absence of He and Ne within 
these rims therefore indicates that our Itokawa grain was 
1495.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
not exposed for as long as the grains studied by Nagao 
et al., [4], in agreement with our interpretation that this 
is an immature space weathered rim. 
 
Figure 2: Atom probe specimen of a space weathered 
rim of Itokawa. The blue and teal dots represent in-
dividual atoms of Cr and Mg, respectively. The pro-
tective Cr cap is clearly visible where the blue atoms 
are concentrated at the top of the needle. Pink 
isosurfaces depict Mg-rich, high-density regions 
close to the surface of the olivine grain. 
Despite minimal exposure to the SW we still ob-
serve a rimwards enrichment in OH and H2O, up to 1.2 
at.% (Fig. 3). This finding indicates that even brief SW 
exposure results in a substantial implantation of H, 
which bonds with O to produce OH and H2O [5]. Here, 
an enrichment of ~1 at. % water across a 50 nm depth in 
a 100 µm diameter grain would result in a 0.1 at% en-
richment in water for the whole grain. For smaller 
grains, this effect would be magnified substantially.  
Our work has important implications for under-
standing the origin of water in the inner solar system. It 
has been suggested that solar, comets or carbonaceous 
chondrites could be the source of Earth’s water [8, 9]. 
However, the D/H ratio of Earth’s water is too heavy 
compared to that of the Sun, while it is too light to be 
derived from the majority of known cometary material 
[8]. Carbonaceous chondrites could be a source, as their 
D/H ratios are indistinguishable from terrestrial ratios 
[9]. However, a several isotope systems, particularly 
Ru, are inconsistent with such a suggestion [10]. 
 
Figure 3 Atom probe specimen of a space weathered 
rim of Itokawa. The blue dots represent OH ions, 
which are enriched up to 1.6 at. % in the outer 50 nm 
of the particle (left hand side). 
In addition, as shown here, small grains exposed to 
the SW can be infused with large amounts of water. 
Therefore, small grains may represent an extra reservoir 
of water that could contribute somewhat to the terres-
trial water abundance and D/H ratios we observe via the 
accretion of small SW irradiated particles. 
Conclusion: Space weathering of Itokawa grains 
has been evaluated by atom probe for the first time. 
Density variations within the rim, and a lack of detecta-
ble noble gasses (He and Ne), suggest that this grain was 
only briefly exposed to the SW. However, we still ob-
serve a rimward enrichment in OH and H2O extending 
to 50 nm depth. The implantation of volatiles such as O 
and H, as well as their combination to produce OH and 
water molecules, indicates that SW implantation may be 
a further mechanism to seed planetesimals with water. 
References:  [1] Chapman C.R., (2004) Ann. Rev. of 
Earth & Planet. Sci., 32, 539-567. [2] Noguchi T., et al., 
(2011), Science, 333, 6046, 1121-1125. [3] Noguchi T. 
et al., (2014) MAPS, 49, 2, 188-214. [4] Nagao K., et al., 
(2011) Science, 333, 6046, 1128-1131. [5] Bradley J.P., 
et al., (2014) PNAS, 111, 5, 1732-1735. [6] Thompson 
M. S. et al., (2014) Earth, Planets & Space, 66, 89. [7] 
Edmondson P.D. (2011) Scripta Mat., 65, 731-734 [8] 
Robert F., (2001) Science, 293, 5532, 1056-1058. [9] 
Robert F., (2006), Meteorites & the Early Solar System 
II, Eds: Laruetta D.S., & McSween Jr. H.J. [10] Fischer-
Gödde M., & Kleine T., (2017) Nature, 541, 525-527. 
Acknowledgements: JAXA and the Hyabusa sam-
ple curation team for the loan of this specimen. 
1495.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


Atomic Scale Depth Profile of Space Weathering in an Itokawa Olivine Grain

Enlighten

ATOMIC SCALE DEPTH PROFILE OF SPACE WEATHERING IN AN ITOKAWA OLIVINE GRAIN.  L. Daly1 M. R. Lee1, L. J. Hallis1, P. A. Bland2, S. M. Reddy2,3, D. W. Saxey3, W. D. A. Rickard3, D. Fougerouse2,3, N. E. Timms2, F. Jourdan2. 1School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK. (luke.daly@glasgow.ac.uk). 2Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6102, Australia. 3Geoscience Atom Probe Facility, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia.   Introduction:  Space weathering is an important process in modifying the physical and chemical proper-ties of the regoliths of airless worlds [1]. This process is mostly due to bombardment from ions within the solar wind (SW), with minor contributions from solar flares and micrometeorite impacts [1]. Transmission electron microscopy (TEM) studies of Itokawa grains indicate that space weathering typically produces 20-40 nm thick amorphous and vesiculated rims, together with occa-sional Fe nanoparticles [2, 3]. Space weathering also causes the implantation of elements, in particular the no-ble gasses He and Ne [4], as well as volatile elements such as H [5]. SW implanted H is particularly important as it may react with silicates to produce OH and H2O, which have been observed in interplanetary dust parti-cles (IDPs) [5]. In turn, these reactions may have signif-icant implications for the origin of water in the inner so-lar system. Therefore, it is important to quantify the dis-tribution and speciation of SW-implanted H, and here we present the first atom probe microscopy (APM) study of Itokawa particles, revealing the attenuation depth and profile of SW irradiation.  Methods: Itokawa particle RA-QD02-0279, a 164 µm diameter grain consisting of olivine and plagioclase, was mounted onto a 100 µm diameter glass rod with ar-aldite. The grain was sputter coated with 200 nm of Cr, to dissipate charge, and protect the immediate surface of the grain from any electron or ion beam damage (Fig. 1). Two APM specimens were extracted normal to the surface of the olivine portion of the grain (Fig. 1), and sharpened to a needle-shape with a 10-15 nm Cr cap us-ing focused ion beam (FIB) milling using a TESCAN LYRA3 FIB-Scanning electron microscope (SEM), at Curtin University. These specimens were analysed on the Cameca LEAP 4000X HR Geoscience atom probe at Curtin University.  Results: 51.5 and 55.9 million atoms were collected from each specimen. In both APM data sets evaporation commenced within the protective Cr coating ensuring the outermost portion of the grain was analysed, where any space weathering features may be present. No feld-spar was observed. The composition of olivine was Fa30. Helium and Ne were absent from both datasets. How-ever, small reductions and spikes in density were de-tected in the outermost 20-30 nm of the tip in both spec-imens (Fig. 2). The high-density regions in each dataset are enriched in Mg relative to the rest of the specimen. Furthermore, a gradual enrichment in OH and H2O ions was detected extending from the outer surface of the grain to a depth of ~50 nm (Fig. 3).  Figure 1: Itokawa particle RA-QD02-0293. Atom probe specimens were extracted from the red box. Discussion: Vesiculated textures and Fe nanoparti-cles have previously been described in space-weathered rims of Itokawa [6]; such features are not detected here. Density variations detected here may either represent a new atomic-scale feature of space weathering, or imma-ture examples of the features described in [6]. The ab-sence typical space weathering features previously de-scribed in Itokawa grains imply that this rim is immature and has not been exposed to SW as long as other Ito-kawa grains (e.g. [6]). Hydrogen ions are a main component of SW [5], and so implantation of H+ ions from SW is interpreted to be the cause of anomalously high levels of OH and H2O ions detected in the outer 50 nm of the particle. The pen-etration depth of SW for this particle was approximately 50 nm, with a linear attenuation profile (Fig. 3). This finding is broadly consistent with Monte Carlo simula-tions of space weathering implanted H+ ions [3]. Meas-ured abundances of He and Ne for other Itokawa grains with space-weathered rims are above the detection lim-its of APM [4, 7]. The absence of He and Ne within these rims therefore indicates that our Itokawa grain was 1495.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)not exposed for as long as the grains studied by Nagao et al., [4], in agreement with our interpretation that this is an immature space weathered rim.  Figure 2: Atom probe specimen of a space weathered rim of Itokawa. The blue and teal dots represent in-dividual atoms of Cr and Mg, respectively. The pro-tective Cr cap is clearly visible where the blue atoms are concentrated at the top of the needle. Pink isosurfaces depict Mg-rich, high-density regions close to the surface of the olivine grain. Despite minimal exposure to the SW we still ob-serve a rimwards enrichment in OH and H2O, up to 1.2 at.% (Fig. 3). This finding indicates that even brief SW exposure results in a substantial implantation of H, which bonds with O to produce OH and H2O [5]. Here, an enrichment of ~1 at. % water across a 50 nm depth in a 100 µm diameter grain would result in a 0.1 at% en-richment in water for the whole grain. For smaller grains, this effect would be magnified substantially.  Our work has important implications for under-standing the origin of water in the inner solar system. It has been suggested that solar, comets or carbonaceous chondrites could be the source of Earth’s water [8, 9]. However, the D/H ratio of Earth’s water is too heavy compared to that of the Sun, while it is too light to be derived from the majority of known cometary material [8]. Carbonaceous chondrites could be a source, as their D/H ratios are indistinguishable from terrestrial ratios [9]. However, a several isotope systems, particularly Ru, are inconsistent with such a suggestion [10].  Figure 3 Atom probe specimen of a space weathered rim of Itokawa. The blue dots represent OH ions, which are enriched up to 1.6 at. % in the outer 50 nm of the particle (left hand side). In addition, as shown here, small grains exposed to the SW can be infused with large amounts of water. Therefore, small grains may represent an extra reservoir of water that could contribute somewhat to the terres-trial water abundance and D/H ratios we observe via the accretion of small SW irradiated particles. Conclusion: Space weathering of Itokawa grains has been evaluated by atom probe for the first time. Density variations within the rim, and a lack of detecta-ble noble gasses (He and Ne), suggest that this grain was only briefly exposed to the SW. However, we still ob-serve a rimward enrichment in OH and H2O extending to 50 nm depth. The implantation of volatiles such as O and H, as well as their combination to produce OH and water molecules, indicates that SW implantation may be a further mechanism to seed planetesimals with water. References:  [1] Chapman C.R., (2004) Ann. Rev. of Earth & Planet. Sci., 32, 539-567. [2] Noguchi T., et al., (2011), Science, 333, 6046, 1121-1125. [3] Noguchi T. et al., (2014) MAPS, 49, 2, 188-214. [4] Nagao K., et al., (2011) Science, 333, 6046, 1128-1131. [5] Bradley J.P., et al., (2014) PNAS, 111, 5, 1732-1735. [6] Thompson M. S. et al., (2014) Earth, Planets & Space, 66, 89. [7] Edmondson P.D. (2011) Scripta Mat., 65, 731-734 [8] Robert F., (2001) Science, 293, 5532, 1056-1058. [9] Robert F., (2006), Meteorites & the Early Solar System II, Eds: Laruetta D.S., & McSween Jr. H.J. [10] Fischer-Gödde M., & Kleine T., (2017) Nature, 541, 525-527. Acknowledgements: JAXA and the Hyabusa sam-ple curation team for the loan of this specimen. 1495.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)

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Atomic Scale Depth Profile of Space Weathering in an Itokawa Olivine Grain

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