Trapping mechanism of O2 in water ice as first measured by Rosetta spacecraft

1. MNRAS 469, S818–S823 (2018) doi:10.1093/mnras/stx3359
Advance Access publication 2018 January 8
Trapping mechanism of O2 in water ice as first measured by Rosetta
spacecraft
Diana Laufer,‹
Akiva Bar-Nun† and Adi Ninio Greenberg
Department of Geophysics, Tel-Aviv University, Ramat Aviv, P.O. Box 39040, 6997801, Tel Aviv, Israel
Accepted 2017 December 14. Received 2017 December 9; in original form 2017 April 14
ABSTRACT
One of the most surprising measurements of the Rosetta Orbiter Spectrometer for Ion and
Neutral Analysis (ROSINA) instrument on the comet 67P/Churyumov–Gerasimenko was the
detection of O2 along with N2 and noble gases which were measured for the first time in
comets, along with the major constituents, water, CO, and CO2. The O2 high abundance
of 1–10 per cent relative to H2O was calculated, with an average value of 3.80 ± 0.85 per
cent (Bieler et al. 2015). The strong correlation of molecular oxygen abundance with water,
suggests the formation of O2 by radiolysis or photolysis of water molecules and the trapping
of O2 in the amorphous ice, together with the other gases while the comet was formed. In this
study, we present new experimental results for trapping O2 with N2 and Ar in amorphous ice
in order to understand the direct measurements of the Rosetta spacecraft. The discovery of O2
in comets challenges our understanding of the composition of the volatiles in the outskirts of
the young solar system and their delivery to the Earth’s atmosphere.
Key words: comets: general – comets: individual: 67P/Churyumov–Gerasimenko – methods:
laboratory – ISM: molecules.
1 INTRODUCTION
Comets are considered pristine objects at the edge of the solar
system, the link between interstellar ices and the early solar system.
Their elemental and isotopic composition provides clues about the
origin of volatiles and water on Earth (Laufer et al. 1999; Morbidelli
et al. 2000; Owen & Bar-Nun 2001; Altwegg et al. 2015; Marty et al.
2016).
The Rosetta observations reveal the most detailed information
on the composition and activity of the nucleus of the comet
67P/Churyumov–Gerasimenko (67P/C–G). The mission ended on
30 September, after 2 yr of continuous measurements around the
comet 67P/C–G, from large heliocentric distances, at about 4 au,
to its perihelion passage (1.24 au) and back, up to ∼3.8 au (Taylor
et al. 2017). Our experimental results of gas trapping in amorphous
ice can explain the observed activity of the comet 67P/C–G from
gas trapped and frozen in water ice along with dust grains and the
increased activity around and after perihelion of the comet. From
the direct observations, the most abundant gases in the coma were
H2O and CO2 which together with CO constitute 95 per cent of
the volatiles. The significant abundance of O2 (Bieler et al. 2015)
and the gases measured for the first time N2 and Ar (Rubin et al.
2015a, Balsiger et al. 2015), all supervolatiles, indicate that 67P/C–
G never experienced high temperatures. The surprising detection
E-mail: dianal@post.tau.ac.il
† Deceased, 2017.
of molecular oxygen in the coma of comet 67P/C–G by ROSINA–
DFMS (Rosetta Orbiter Sensor for Ion and Neutral Analysis, Dou-
ble focusing mass spectrometer), with an O2/H2O abundance ratio
in the range 1–10 per cent, with an average ratio of 3.80 ± 0.85
per cent (Bieler et al. 2015) was confirmed also by the previous
(1986) Giotto Neutral Mass Spectrometer (NMS measurements)
in situ measurements of comet 1P/Halley with similar amounts of
molecular oxygen of 3.7 ± 1.7 per cent (Rubin et al. 2015b).
The large amount of O2 strongly correlated with H2O was one
of the most surprising results of the Rosetta mission (Bieler et al.
2015).
One of the possible explanations for its constant high relative
abundance is that the O2 was already formed by radiolysis (frag-
mentation) of water ice (Bar-Nun et al. 1985; Teolis et al. 2005) in
the protosolar nebula (PSN) by UV photons, electrons, and ions. The
possible chemical reaction is simple, 2H2O → 2H2 + O2 through
OH and O radicals. The resulting H2 is desorbed and cannot be
preserved in the ice (Laufer et al. 1987) however the O2, as a bigger
and heavier molecule, can diffuse between the ice layers (Teolis
et al. 2005).
O2 was observed depleted in molecular clouds as ρ Oph A and
Orion (Melnick Kaufman 2015). Taquet et al. (2016) found a
good correlation between the species measured by ROSINA and
observations in dark cloud ρ Oph A. Mousis et al. (2016) calculated
the energy constraints and concluded that O2 could be trapped in
the ice grains in the stage of the molecular cloud.
Experimental studies by Zheng, Jewitt Kaiser (2006),
showed a very low yield of H2O2, and no O3 (Yabushita,
C
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2. Trapping mechanism of O2 in water ice S819
Table 1. Gas abundances calculations from experimental results.
Comet 67P/C–G
coma composition a
Gas
mixture
Ratio
gas/H2O in
ice sample Depletion
Binding
energy
Edes (K)
Calculated
PSN
Ori A PPD
class II
H2O 1 1 1 5050 1 1e
CO 0.16 – – – 1420 0.6 c 0.3e
CO2 0.41 – – – 2530 0.41 d 0.03e
N2 8.80 × 10−4 0.97 0.23 0.23 1155 3.8 × 10−3 0.038e
O2 0.038 0.24 0.228 0.95 1660 0.04 0.01–0.06f
Ar 1.20 × 10−5 0.19 0.038 0.2 866 5.34 × 10−5 0.02e,g
N2/O2 0.023 4 1.01 0.21 – 0.09 –
Ar/N2 (9.1 ± 0.3) × 10−3 b 0.19 0.16 0.84 – 1.44 × 10−2 –
aBieler et al. 2015; Le Roy, et al. 2015; Marty et al. 2016.
bBalsiger et al. 2015.
cCollings et al. 2015; He, Acharyya Vidali 2016; Smith, May Kay 2016.
dBisschop et al. 2006; Mousis et al. 2008; Smith, May Kay 2016.
eFuruya Aikawa 2014.
fGoldsmith et al. 2011.
gEstimated from Simpson et al. 1998 and Furuya Aikawa 2014.
Hama Kawasaki 2013), thus explaining the low abundance of
H2O2/O2 = (0.6 ± 0.07) × 10−3
(Bieler et al. 2015) and lack of O3
in the direct observation of ROSINA (Bieler et al. 2015).
Previous experimental studies on physical properties of O2, N2,
and CO2 have been performed to estimate their abundance in in-
terstellar clouds (Burke Brown 2010; He, Acharyya Vidali
2016) and calculating binding energies of atoms and molecules to
different substrates.
O2 has a higher binding energy, 1660 K (He, Acharyya Vidali
2016), as compared to N2, 1155 K (Bisschop et al. 2006; Mousis
et al. 2008; Smith, May Kay 2016), and CO, with a binding
energy of 1420 K (Bar-Nun, Notesco Owen 2007; Collings et al.
2015; He, Acharyya Vidali 2016; Smith, May Kay 2016), CO2
and H2O having much higher values, 2530 and 5050 K, (Burke
Brown 2010; Drozdovskaya et al. 2016). Ar with a lower binding
energy of 866 (Smith, May Kay 2016), as shown in Table 1. From
these values is expected that O2 could be trapped better than other
supervolatiles such as CO, N2, and Ar.
The measured abundance in comet 67P/C–C of supervolatiles
such as N2 and Ar, as compared to previous experimental results
from our laboratory, show a low temperature formation of ∼30 K
with an abundance similar to the experimental value with a de-
pletion in the N2/CO ratio of ∼25 as compared to solar abundance
(Notesco Bar-Nun 2005; Bar-Nun, Notesco Owen 2007; Rubin
et al. 2015a).
The correlation between trapping of CO2, N2, and Ar in amor-
phous ice was also studied by us (Ninio-Greenberg, Laufer
Bar-Nun 2017) showing that CO2 enhances the trapping of ad-
ditional gases.
The aim of this experimental study is to determine the effective-
ness of trapping and release the already formed O2 in the molecular
cloud stage, as compared to N2 and Ar, within the amorphous wa-
ter ice, thus explaining the constant high value of its abundance in
comet 67P/C–G.
2 EXPERIMENTAL METHODS
The experimental setup (Fig 1) was described previously (Bar-Nun
et al. 1987; Laufer et al. 1987). Gas mixtures of O2, N2, Ar and water
vapour were deposited through a diffuser on a 17 cm2
cold surface
at temperatures of 40–60 K, to form a 100 μm thick gas-laden
amorphous ice, in a cryogenically pumped vacuum chamber (CTI-
Cryotorr-72) to a pressure of 10−6
torr, during the sample formation.
At these temperatures, the formed water ice is amorphous and highly
porous, and gases are trapped in the lattice (Bar-Nun et al. 2013).
The fluxes of the gases and water vapour evolving from the ice,
when the sample was heated, were monitored by a Riber QX−
100
quadrupole mass filter, recalibrated for each experiment.
The formed ice sample was then heated at a rate of 1 K min−1
up to 200 K, when the ice sample evaporated. At 90 K the heating
process was stopped for ∼15 min to remove residual gas.
Upon heating, changes occur in the ice, and all the gases evolve
together from it in distinct peaks, related to ice phase transfor-
mations and sublimation, as was observed in our previous studies
(Bar-Nun et al. 1987; Bar-Nun et al. 2013). Not all the ice trans-
formed to crystalline form (Jenniskens Blake 1996) and the gas
remained in the ice until evaporation.
The ratios of the released gases versus water were calculated
in the main temperature ranges, 110–130 annealing process, 140–
160 K, the transformation of the amorphous ice to cubic ice and
160–190 K, the transformation to hexagonal ice and sublimation of
the ice sample. Since the ice sample formation is at low pressures,
no clathrate hydrate could be formed.
3 RESULTS
In this study, we compare the trapping efficiency of O2, N2, and Ar in
amorphous ice. In these sets of experiments, gas-laden amorphous
ice was formed from mixtures of N2, O2, Ar and water vapour at
40–60 K. At these deposition temperatures and pressures, the gases
are trapped in the ice and not frozen in the amorphous ice.
3.1 Trapping of O2 in amorphous ice
Gas-laden amorphous ice was formed from mixtures of gas and
water vapour containing filtered air (N2 and O2) and Ar with a ratio
of H2O: N2: O2: Ar of 1:1:0.3:0.3 to form 100 μm thick layers on a
17 cm2
cold surface at temperatures of 40–60 K, in the cryogenically
pumped vacuum chamber (CTI-Cryotorr-72) at a pressure of 10−6
torr. After the deposition was finished, the samples were then heated
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3. S820 D. Laufer, A. Bar-Nun and A. Ninio Greenberg
Figure 1. The vacuum chamber producing thin gas-laden amorphous ice samples (100–200 μm), at low temperatures (30–60 K) and low pressures (10−8–10−6
torr), as a ‘comet’ simulation (a). The sample is formed on the 3.5 × 5 cm deposition plate (b).
at a 1 K min−1
rate, measuring the desorption of the gas species by
the quadrupole mass filter.
The abundance ratios in the released volatiles from the ice are
very different from the initial relative abundances in the gas phase.
During the heating process, the trapped gases were released to-
gether from ice during the annealing process (100–130 K), the
transformation to cubic ice (140 K), the further transformation to
hexagonal ice (160 K) and together with water sublimation. The
trapping efficiencies in the ice are O2 N2 Ar (Fig 2).
The lower Ar flux can be explained by its low gas pressure in the
gas mixture on the trapping efficiencies. This effect was observed
in our previous studies (Laufer et al. 1987).
The trapping efficiencies of gases in the ice are O2 N2 Ar ex-
plaining the high abundance of molecular oxygen in the ROSINA’s
measurements in the coma of comet 67P/C–G, and the previous
measurements from comet 1P/Halley (Rubin et al. 2015b).
The high abundance of O2 in the coma may explain its low
detection in molecular clouds. From Hershel observation of O2
in Orion (Goldsmith et al. 2011) a kinetic temperature between
65–120 K was calculated. As compared to our experiments, in this
temperature range, only ∼10–20 per cent of the trapped O2 in
ice is released to the gas phase, thus explaining its low observed
abundance (Fig. 2).
In Fig. 3, N2 flux was divided by 4 and Ar flux was multiplied
by 3, marked as ‘norm.’ index, for better comparing the trapping
efficiency, the gas ratios were normalized to O2 in the gas–water
vapour mixture thus, we could compare their trapping efficiencies.
The ratios were calculated from the ratios of the integrated fluxes
in each temperature range gas release. Lower ratios of O2/N2 as
compared to O2/Ar can also be explained by the pressure effect
(Laufer et al. 1987) since the flow pressure of N2 is four times
higher and the Ar is at three times lower pressure.
Figure 2. Gas release from ice as a function of the temperature. The trapping
efficiencies in amorphous ice are O2 N2 Ar.
Figure 3. Released gas fluxes in the main temperature ranges: 110–135 K
ice annealing, 140–160 K cubic transformation, 160–190 K hexagonal trans-
formation and ice evaporation. In the legend ‘norm.’ is for the normalized
value to the O2 in the gas mixture.
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4. Trapping mechanism of O2 in water ice S821
The ratios of N2/Ar are almost constant as observed in the mea-
surements from 67P/C–G (Balsiger et al. 2015) and also in our
experiments of trapping N2 and Ar with CO2 in amorphous water
ice (Ninio-Greenberg, Laufer Bar-Nun 2017).
Different gas–water correlation measured in comet 67P/C–G be-
tween pre- and post-equinox (Fougere et al. 2016; Hoang et al. 2017)
can be also explained by gas ratios as compared to H2O release in
the different temperature ranges.
The different measured gas ratios in the coma may indicate the
temperature on the surface or from the interior of the nucleus.
The experimental results are similar to the experimental results
of Collings et al. (2004), but the gas distribution in the tempera-
ture ranges is different as a result of formation temperature. In our
experiments, the gas is trapped in the ice (not frozen) and is re-
leased mainly during the ice transformations and water evaporation
whereas in Collings et al. (2004, 2015) experiments, the samples
are formed at 10 K thus most of the gases are released between 20
and 60 K. Our experimental results together with the direct observed
activity on comet 67P/C–G support a higher temperature formation,
below ∼30 K (Rubin et al. 2015a).
Additional measurements later on, followed the illumination of
the comet increasing the water flux in anticorrelation with CO2 the
in correlation with other gases such as O2 and N2 (Mall et al. 2016;
Gasc et al. 2017; Hoang et al. 2017).
From the water production rates, the comet is calculated to have
lost 0.12 ± 0.06 per cent of its mass during the perihelion passage
(Marshall et al. 2017).
In Table 1, gas abundances in the experiments are compared with
the direct measurements of the composition from the ROSINA data
when the comet was still away from the sun (Bieler et al. 2015;
Le Roy, et al. 2015; Marty et al. 2016). The biding energies Edes
(K) between the gases and amorphous water ice are also shown.
The ‘depletion’ was calculated by dividing the ‘gas ratio in the
ice’ by the ‘ratio in the gas mixture’. The composition of the PSN
was derived, by dividing the direct measurements of 67P/C–G by
the experimental depletion factor, for the gases N2, Ar, and O2.
The PSN calculations were compared with the direct measurements
and calculations of the Class II Protoplanetary Disks (PPD) Ori
A (Simpson et al. 1998; Goldsmith et al. 2011; Furuya Aikawa
2014).
For the PSN, the calculated value for N2, 3.8 × 10−3
is lower than
5.8 × 10−2
at 10 K (Drozdovskaya et al. 2016), strongly depending
on the initial gas mixture, pressure, and higher ice temperature
formation at 45–50 K.
CO relative abundance in PSN was calculated from the N2
depletion value (Rubin et al. 2015a) from N2/CO ratio of
(5.70 ± 0.66) × 10−3
for the Rosetta measurements as predicted
by Bar-Nun, Notesco Owen (2007) and for CO2 as calculated
from our experiments of CO2 trapping in amorphous ice (Ninio-
Greenberg, Laufer Bar-Nun 2017).
The abundance of CO2 in 67P/C–G vary strongly, as observed by
the Rosetta instruments, since CO2 is frozen at temperatures up to
80 K, or upcoming from the inner layers (Mall et al. 2016). Thus,
CO2 abundance in the PSN may vary as related to the temperature.
For the measurements in Ori A PPD Class II, shown in Ta-
ble 1, relatively high Ar and low CO2 ratios are temperature de-
pendent. Using our experimental data (Ninio-Greenberg, Laufer
Bar-Nun 2017), a temperature range of ∼60–80 K is determined,
in between the estimated temperatures of 65–120 K (Goldsmith
et al. 2011).
The values for O2 observations are not very well understood.
Further observations are needed for better comparison.
Figure 4. Time effect on gas trapping in amorphous ice. No drop in the
flux rate was observed even after 48 h.
Figure 5. Gas-laden ice sample during the heating process. No change was
observed even after 2 d at a constant temperature of 140 K at the cubic
transformation temperature.
These results can explain the different abundances measured in
comets depending gas composition and their physical conditions in
which they formed and their location relative to the snow line.
3.2 Time effect on gas release
In order to understand the effect of ice structure influence on gas
release, in some experiments the sample was kept at constant tem-
peratures of 120, 150, 160 K for 30 min, and up to 20 and 48 h at
140 K as shown in Fig. 4. The trapped gas continues to desorb from
the gas-laden ice layer as the heating process resumes. After 48 h at
a constant temperature of 140 K (Fig. 4), no changes were observed
during imaging the gas-laden ice as shown in Fig. 5.
This activity strongly depending on the temperature can explain
the comet activity from the upper layers of the nucleus, with the
interior in its pristine form, as suggested by Capria et al. (2017).
The trapped gas is released during the phase transition thus ex-
plaining the comets continuous activity also when receding from
the sun (Meech et al. 2009; Guilbert-Lepoutre et al. 2014).
Subsequently resuming the heating process, the gas continued
to be released during the transformations and ice evaporation, thus
explaining gas release from comets after many close approaches to
the sun.
These results may explain the observed activity of main belt
comets (MBC), with a lower water flux rate as compared to comet
67P/C–G (Snodgrass, et al. 2017).
3.3 Recondensation of ices
The OSIRIS camera and VIRTIS spectro-imager on board the
Rosetta spacecraft observed brittle terrain and smooth regions on
both lobes of the comet (El-Maarry et al. 2015) and active circu-
lar flat-floored pits (Oklay et al. 2016). A diurnal cycle of water
ice deposits of H2O and CO2 ices was measured (De Sanctis et al.
MNRAS 469, S818–S823 (2018)
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5. S822 D. Laufer, A. Bar-Nun and A. Ninio Greenberg
Figure 6. Gas-laden amorphous ice formed on a crystalline ice layer at
50 K. The gas is released only from the upper ice layer. N2 and O2 being
released at the same ratios.
2015; Pommerol et al. 2015; Filacchione et al. 2016). Both H2O and
CO2 can refreeze on the surface of the nucleus, along with trapped
volatiles, on the night side where the temperatures may drop to
about 50 K.
The recondensation process was studied in experiments by form-
ing a gas-laden amorphous ice on a pre-deposited hexagonal ice
layer. The first H2O layer was formed at 50 K, heated to 160 K
to form a hexagonal ice and cooled back to 50 K. An additional
layer of gas-laden amorphous ice containing N2 and O2 was then
deposited (Fig. 6). Upon heating, gases were released only from the
upper layer and didn’t penetrate the crystalline ice layer. For better
comparison, N2 flux was normalized to the O2 flux. The N2 and
O2 are trapped in similar ratios, although, using an air mixture, N2
flow was at a higher partial pressure and Ar was significant depleted
(Laufer et al. 1987).
These results can explain Rosetta’s measurements of short-lived
outbursts near sunrise and the observed correlation of H2O and O2.
The source of the continuous jets must be from an internal source
emerging from deeper layers of the nucleus.
Jets activity observed on comet 67P/C–G during sun exposure
can be explained also by our previous studied jets from frozen CO2
covered by water ice (Laufer et al. 2013) and by the thermal model
of comet activity of Prialnik, A’Hearn Meech (2008).
4 CONCLUSIONS
From our experiments, we can conclude that comets formed at the
edge of the solar system by trapping volatile gases in amorphous
water ice according to their physical properties. Despite O2 low
abundance in molecular clouds, the formed O2 by radiolysis can be
trapped and preserved in the ice in high abundances. The O2/H2O
high ratio can be explained by trapped O2 in the PSN stage, as
shown in this study and by Moussis et al. (2016). Additional higher
ratios as measured by Alice (Feldman et al. 2016) may be explained
by secondary reactions in the coma (Dulieu, Minissale Bockelée-
Morvan 2017), taking into account the modulation of the solar wind
in the vicinity of the comet nucleus (Goldstein et al. 2017).
Further in the inner solar system, the comet nucleus undergoes
a differentiated structure due to poor thermal conductivity, where
the deep interior didn’t experience extreme temperatures, holding
the supervolatiles, while the shallow layers are covered with ice de-
pleted compacted dust. Some of the released water and the volatiles
recondensed on the surface of comet 67P/C–G at low temperatures
disappeared when exposed to the Sun.
The experimental results in our study are in agreement with the
direct measurements of the ROSINA and VIRTIS instruments and
can explain the activity observed on the surface of the nucleus of
comet 67P/C–G. The O2 is trapped in the amorphous ice more ef-
ficiently as expected from the binding energy as compared to N2
and Ar. The gas is released from the internal ice layers, depend-
ing strictly on the water temperature, thus providing continuously
observed ratios together with the water sublimation.
The coma composition as measured by Rosetta is a glimpse in the
comet activity during its passages around the sun. The fluctuations
in the composition indicate diurnal and seasonal variations showing
the link between the coma and the nucleus temperature, but still
may not reflect the interior of the nucleus.
From the ROSINA measurements on comet 67P/C–G and this
study we can conclude that cometary material agglomerated the
PSN stage with similar gas abundance and the location relative to
the snow line.
Ongoing analyses of data linking between the outburst events and
the measurements in the coma are required to explain the surprising
large O2/H2O ratio.
Further laboratory experiments are needed to understand the
abundances of the noble gases in the PSN as compared to the values
from Rosetta mission.
ACKNOWLEDGEMENTS
The authors acknowledge support from the Israel Ministry of Sci-
ence and Technology and Space through the Israel Space Agency
grant 3–11480. We thank Dina Prialnik and Gila Notesco for their
assistance and the fruitful discussions on the manuscript. The au-
thors thank the ROSINA team, especially Kathrin Altwegg, for the
productive discussions. We thank Israel Silver and Dor Zvulun for
computer assistance. We thank the reviewers, for the careful reading
and insightful comments and suggestions.
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