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Short Communication
The capabilities of ROSINA/DFMS to measure argon isotopes at comet
67P/Churyumov–Gerasimenko
M. Hässig a,n, K. Altwegg b,c, J.J. Berthelier d, U. Calmonte b, J. De Keyser f, B. Fiethe g, S.
A. Fuselier a,h, T.I. Gombosi e, L. Le Roy c, T. Owen i, M. Rubin b
a
Southwest Research Institute, Space Science and Engineering, 6220 Culebra Rd., San Antonio, TX 78238, USA
Space Research and Planetary Sciences, University of Bern, Switzerland
c
Center for Space and Habitability (CSH), University of Bern, Switzerland
d
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Univ. Pierre et Marie Curie, Univ. Versailles Saint-Quentin & Centre National de la
Recherche Scientifique, Paris, France
e
Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI, USA
f
Space Physics Division, BIRA-IASB, Brussels, Belgium
g
Institute of Computer and Network Engineering, TU Braunschweig, Braunschweig, Germany
h
University of Texas at San Antonio, San Antonio, TX, USA
i
Institute for Astronomy, University of Hawaii, Honolulu, HI, USA
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 3 September 2014
Received in revised form
6 November 2014
Accepted 11 November 2014
Little is known about the noble gas abundances in comets. These highly volatile atoms are possible
tracers of the history of cometary matter including the thermal evolution. They can help quantify the
contribution of cometary impacts to terrestrial oceans and help elucidate on the formation history of
comets and their role in the formation and evolution of planetary atmospheres. This paper focuses on
argon and the capabilities to measure this noble gas with in situ mass spectrometry at comet 67P/
Churyumov–Gerasimenko, the target of the European Space Agency's spacecraft Rosetta.
Argon may have been detected by remote sensing in a single Oort cloud comet but to date nothing is
known about the isotopic abundances of argon in comets. Furthermore, no detection of argon in a
Jupiter-family comet has been reported. Comet 67P/Churyumov–Gerasimenko belongs to the group of
Jupiter-family comets and originates most likely in the Kuiper belt.
Onboard Rosetta is ROSINA/DFMS (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double
Focusing Mass Spectrometer). DFMS has unprecedented mass resolution and high sensitivity and is
designed to measure isotopic ratios including argon (Balsiger et al., 2007). Argon measurements using
the DFMS lab model (identical to the flight model) demonstrate this capability. At very least, this mass
spectrometer has the resolution and sensitivity to reduce the upper limit on the argon outgassing rate
relative to water by more than three orders of magnitude (for 38Ar). Most likely, ROSINA/DFMS will
provide the first detection of argon in a Jupiter-family comet together with the first determination of the
36
Ar/38Ar ratio at a comet.
& 2014 Elsevier Ltd. All rights reserved.
Keywords:
Argon isotopes
Comet
ROSINA/DFMS
Rosetta
1. Introduction
Noble gases are possible tracers for the history of cometary
matter and might help to solve the riddle about the role played by
comets in the formation and evolution of planetary atmospheres
(Owen and Bar-Nun, 2001). Noble gases are highly volatile and
chemically inert. Therefore they can be used to trace the thermal
evolution of cometary matter. Comparison of the noble gas
abundances in planetary atmospheres and comets can be used to
n
Corresponding author. Tel.: þ 1 210 522 2449.
E-mail address: [email protected] (M. Hässig).
further increase our knowledge about cometary bombardments
during the evolution of the planet. Little is known about the noble
gas content of cometary matter. This content is mainly estimated
from laboratory experiments measuring the trapping efficiency of
noble gases in growing amorphous ices (Owen et al., 1992).
The three major isotopes of argon are 36Ar, 38Ar, and 40Ar. Of
these, 36Ar and 38Ar are primordial; 40Ar is produced by decay of 40K.
There exist several measurements of 38Ar/36Ar in the solar system
(see Table 1). The low Martian value is attributed to atmospheric
escape, which has also produced a 5–6 times enrichment in D/H on
this planet compared to the D/H in Earth's ocean water.
In 1997, sounding rocket observations provided a possible detection of argon for C/1995 O1 (Hale-Bopp). An upper limit for the argon
http://dx.doi.org/10.1016/j.pss.2014.11.015
0032-0633/& 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Hässig, M., et al., The capabilities of ROSINA/DFMS to measure argon isotopes at comet 67P/Churyumov–
Gerasimenko. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.11.015i
M. Hässig et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Table 1
Isotopic measurements of
36
Ar/38Ar in the solar system.
Ar/38Ar
Earth atmosphere
Bulk solar wind
Meteorites
Mars atmosphere
36
Reference
5.3197 0.008
5.4707 0.003
4.417 0.06 (HL) – 5.87 7 0.70 (N)
4.2 7 0.1
Lee et al. (2006)
Heber et al. (2012)
Ott (2002)
Atreya et al. (2013)
outgassing rate of (1.170.3) 1029 s 1 (Stern et al., 2000) and the
well-established outgassing rate for H2O of 1.1 1031 s 1 (Biver et al.,
1997; Colom et al., 1997) results in an upper limit for the ratio of the
outgassing rates of water to argon of o100737.5 (at perihelion). The
sounding rocket measurements were not capable of resolving the
isotopic abundances of argon in Hale-Bopp. The deduced Ar/O ratio
was 1.870.96 times the solar value (Stern et al., 2000). Therefore
Hale-Bopp may be enriched in argon compared to the solar ratio by up
to about a factor of 2. Laboratory studies of the capture of noble gases
in ice show that a solar Ar/O ratio requires an equilibrium temperature
of the nucleus that is below 30 K (Owen and Bar-Nun, 1995). Even
colder temperatures would be required to enhance this ratio. To
explain the concentration of noble gas in the terrestrial and Martian
atmospheres by cometary bombardment, a depletion (factor 10–100)
of the solar Ar/O ratio has been suggested (Owen and Bar-Nun, 2001)
although some impacts of nearly solar Ar/O ratios are necessary to
explain the noble gas concentration in Venus' atmosphere.
The Far Ultraviolet Spectroscopic Explorer (FUSE) conducted a
detailed search for argon in three different Oort cloud comets. This
search did not detect any argon emission. However, it was possible
to decrease the upper limit for the outgassing rate of argon relative
to water for Oort cloud comets. The upper limits on the Ar/O ratio
were (4.2 710) 10 3 for C/T1 and (2.4 7 1.2) 10 4 for C/A2
(Weaver et al., 2002). The argon concentrations are depleted by at
least a factor of 10 for C/A2 relative to the solar ratio (Grevesse
and Sauval, 1998). This ratio depends strongly on the origin of the
signal for oxygen. Weaver et al. (2002) deduced this ratio by
estimating the Ar/H2O ratio and then making corrections for
possible contributions to oxygen in the comet. The lowest H2O/
Ar ratio of 3850 7 1923 was found for C/A2. Assuming a CO/H2O
ratio of 0.65, this results in the aforementioned lowest Ar/O ratio
for the comet (Weaver et al., 2002). The deduced nucleus temperature from laboratory studies and the assumed CO/H2O ratio
suggests a formation temperature that is higher than 61 K for C/A2
and somewhat lower than 61 K for C/T1. The Ar/O ratio is depleted
for those comets compared to the solar ratio but is at the upper
limit of Owen and Bar-Nun (1995) obtained from laboratory
studies. It is important to note that these ratios are upper limits
for argon and that no argon emission was detected in the search.
These measurements and upper limits are for an unknown
combination of argon isotopes, including 40Ar. This isotope of
argon is a decay product of 40K. 40K only exists in rocks and not in
ice. Depending on the amount of rock on the comet surface, the
40
Ar contribution to the total argon signal is an open question.
Finally, the sample of comets in which argon was detected is very
small and contains only comets that probably originate in the Oort
cloud. No detection of argon has been made to date for any Jupiterfamily comet and nothing is known about the isotopic abundances
of argon in any comet.
Comet 67P/Churyumov–Gerasimenko (67P) is a Jupiter-family
comet and originates most probably in the Kuiper Belt region. This
comet is also the target of the European Space Agency's Rosetta
mission (Schulz et al., 2007). Rosetta will escort the comet for an
extended period of time from almost 4 AU through perihelion.
During its inward (and partly outward) journey, Rosetta will
determine the physical, chemical, and morphological characteristics of the nucleus and the surrounding coma.
ROSINA onboard Rosetta consists of two different types of mass
spectrometers and the COmet Pressure Sensor (COPS) (Balsiger
et al., 2007). The two mass spectrometers are the Reflectron-type
Time of Flight (RTOF) and the Double Focusing Mass Spectrometer
(DFMS). They are well suited to study the cometary coma, with
both high dynamic range and mass resolution to determine
isotopic ratios of very low abundant species (Balsiger et al., 2007).
DFMS has high mass resolution (m/Δm of 3000 at the 1% level at
mass/charge 28 u/e) and a dynamic range of 105 per mass spectrum
and 1010 for a total mass scan. It can resolve mass peaks at mass/
charge 18, 36 and 38 u/e and therefore measure argon isotopes and
their abundance relative to H2O. The Ar/O ratio will be challenging to
deduce since the possible contributing sources to the oxygen signal
have to be determined. However, no detection of argon in neither a
Jupiter-family comet has yet been reported nor the detection of 38Ar
in any comet. Therefore the observations of ROSINA at comet 67P are
important to shed more light on the role of Kuiper belt objects in the
formation and evolution of planetary atmospheres and DFMS could
possibly make the first detection of 38Ar in any comet.
2. DFMS
DFMS is a mass spectrometer with a combination of a 901
electrostatic analyzer as an energy analyzer and a 601 magnet as
momentum analyzer (Balsiger et al., 2007). The mass spectrometer
is designed to measure the elemental, molecular and isotopic
composition of the neutral gas and the cold thermal part of the
ion distribution close to the comet. Neutrals are first ionized by
bombardment with electrons in the ion source. As with any neutral
mass spectrometer, this violent ionization can cause molecules to
fragment, leading not only to a signal for the parent molecule but
also signals of the fragments. To determine the total signal of a
molecule this fragmentation has to be taken into account.
3. Measurements with the lab model of DFMS on Earth
Measurements with the lab model of DFMS on Earth (this model is
nearly identical to the one in space) reproduced the terrestrial isotopic
ratios for 38Ar/36Ar, 36Ar/40Ar, and 38Ar/40Ar (see Table 2). The
measurements were part of a detailed calibration campaign with the
lab model. To measure terrestrial argon, the calibration chamber was
filled with a gas of mainly argon and some background gas. Measurements were dominated by argon and therefore statistical uncertainties
are small.
4. Argon measurements in the early phase of the mission
The conditions are quite a bit different for measurements with the
flight model of ROSINA/DFMS on board of Rosetta, where argon will
not be the dominant gas. There are several different signals expected
for mass per charge measurement 36 and 38 u/e, where a possible
argon detection would take place. The species that interfere with the
signals for argon can be divided into two groups of origin: the
spacecraft and the comet.
Table 2
Isotopic measurements of terrestrial argon with the flight spare model of DFMS on
Earth. All isotopic ratios are within uncertainties compatible with terrestrial ratios.
The uncertainties are due to calibration of 5% and the ion statistics of 3–6%.
Isotopic ratio Measurement
36
38
38
40
Ar/ Ar
Ar/40Ar
Ar/36Ar
Literature
3
(3.219 70.177) 10
(6.08 7 0.43) 10 4
0.1908 7 0.0141
Statistical uncertainty (%)
3
3.379 10
6.35 10 4
0.1878
3
5
6
Please cite this article as: Hässig, M., et al., The capabilities of ROSINA/DFMS to measure argon isotopes at comet 67P/Churyumov–
Gerasimenko. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.11.015i
M. Hässig et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
The contamination from the spacecraft due to spacecraft outgassing (Schläppi et al., 2010) does not contain any argon. Rather, it
contains HCl and C3 both at mass/charge 36 u/e (see Fig. 1). At
mass/charge 38 u/e, there are H37Cl, C2N and C3H2 in increasing
mass (see Fig. 1). As a noble gas, argon is lighter than all of these
molecules and is therefore located to the left (lower mass direction) of
these molecules and separated at the resolution of the instrument.
This is true as long as the intensity of the hydrochloride molecule is
similar or lower than that for argon. However, the signal due to
spacecraft outgassing for these molecules has remained nearly constant for a long period of time after the initial rapid outgassing of the
spacecraft (Schläppi et al., 2010). Therefore the signal for those
molecules is well known and can – if necessary – be corrected for.
The detection capabilities for ROSINA/DFMS for an integration time of
20 s is 1 cm 3 (Balsiger et al., 2007). The signal for HCl is at the limit
of detection capabilities in May 2014. Therefore, no signal is detected
for the less abundant H37Cl at mass/charge 38 u/e. Nonetheless; the
location of the peak is marked in Fig. 1. The same is true for C2N (see
also Fig. 2). By further increasing the integration time, the detection
capabilities decreases, but the signal due to spacecraft outgassing will
increase with the integration time. Therefore the detection capabilities for 38Ar depend also on the integration time and not only on the
signal due to spacecraft outgassing.
The signal expected from the comet at mass/charge 38 u/e is
mainly C2N a fragment of HC2N and possibly C3H2. Both molecules
are well separated from 38Ar at the resolution of the instrument.
No detection of HCl in comets has been made and therefore no
signal is expected for the less abundant isotope of molecule at
mass/charge 38 u/e from the comet.
At mass/charge 36 u/e, signal from H234S (an isotope of H2S)
and C3 are expected from the comet. The detection capabilities for
36
Ar depend on the signal for H234S, since the peak of 36Ar is
Fig. 1. Left: A simulated spectra with a signal of 36Ar relative to H234S of 1/200. The
peak shape of the narrow Gaussian was fitted for the C3 and then adapted for the
two other peaks. A fitting of the peaks could reproduce the ratio between 36Ar and
H234S. The signals HCl and C3 are due to spacecraft outgassing and are expected to
be in the similar range or lower for the mission duration. The red and cyan lines are
the simulated signal for H234S and the blue line is the simulated signal for 36Ar. The
squares indicate the measurement points and the sum of the simulated 36Ar and
H234S. The shoulder on the left side can be identified and a separation of 36Ar and
H234S by fitting is achieved. right; A simulated spectra with a signal of 38Ar relative
to 36Ar, assuming a terrestrial value. The signal of C3H2 is detected in the
background of Rosetta due to spacecraft outgassing. In earlier measurements of
the spacecraft background signals for H37Cl and C2N are detected but are lower
than the detection capabilities in this spectra. Therefore only the location of the
peak is marked but no peak is actually visible.
3
detected at the limit of sensitivity as a shoulder on the H234S peak.
To estimate the detection capabilities of 36Ar relative to H234S, the
peak shape was deduced from a high peak close in mass to 36,
since the peak shape changes very little from one mass to another.
At the peak centered at H234S, the same peak shape was used to
add a signal at 36Ar. The concentration was increased until a
detectable change in the peak shape in form of a shoulder was
visible. This concentration represents the detection capabilities of
36
Ar relative to H234S of 0.5% (see Fig. 1).
4.1. Estimation of upper detection capabilities for
38
Ar relative to H2O
Nothing is known about the isotopic ratio of argon for comets
and any detection of 38Ar will be a first. Since there are no other
mass peaks near 38Ar, the upper limit for the detection of 38Ar
within 20 s of integration time is simply the detection capabilities
of the instrument. Assuming a water density of 1 109 molecules
per cm3 ( 1 10 7 mbar), the ratio between the detection
capabilities for 38Ar and the water density covers more than seven
orders of magnitude (see Fig. 2). If the solar ratio of 36Ar/38Ar of 5.5
(Vogel et al., 2011) is assumed, then this limit would lower the
current upper limit for detection of argon at comets by almost four
orders of magnitude. Furthermore, 36Ar would dominate the
detection limit for argon given for C/A2 in Weaver et al., 2002.
However, the detection capabilities can be further decreased with
longer integration time, making the detection of 38Ar and therefore argon almost guaranteed.
4.2. Estimation of upper detection capabilities for
36
Ar relative to H2O
To estimate the upper detection capabilities for 36Ar relative to
H2O, an estimate for the signal expected for H234S must be determined
first. The highest signal of H2S relative to H2O was detected for C/1995
O1 (Hale-Bopp) (Bockelée-Morvan et al., 2004). The production rate of
H2S relative to water for C/1995 O1 (Hale-Bopp) was measured and is
1.5% (Bockelée-Morvan et al., 2000). The highest isotopic ratio for
34 32
S/ S was again detected for C/1995 O1 (Hale-Bopp) on H2S
(Bockelée-Morvan et al., 2004). The isotopic ratio for 34S/32S measured
Fig. 2. Detection capabilities of ROSINA/DFMS for the isotopes of argon relative to
the outgassing rate for water. There exist one argon measurement for Hale-Bopp
and a lowest upper limit for argon relative to water for LINEAR which is lower than
that for Hale-Bopp, but no detection of argon was achieved. None of these
measurements resolve the isotopic abundances of argon and only the total is
measured. ROSINA/DFMS will be able to resolve the isotopic ratios of argon, while
the detection capabilities for 36Ar depend on H234S abundance relative to water.
The 38Ar detection is limited only by the instrument's detection capabilities and
almost four orders of magnitude lower than the current upper limit for Ar/H2O
from observations of C/2001 A2 LINEAR.
Please cite this article as: Hässig, M., et al., The capabilities of ROSINA/DFMS to measure argon isotopes at comet 67P/Churyumov–
Gerasimenko. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.11.015i
M. Hässig et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
for H232S/H234S was 16.573.5 (Crovisier et al., 2004a) or 6.0671.06%.
From these numbers, the production rate of H234S relative to H2O is
0.091% or 9.1 10 4. The upper limit production rate of argon relative
to H2O is minimal for C/A2 and is 0.02570.009%. Assuming the
detected argon is dominated by 36Ar this provides an upper limit for
36
Ar relative to H2O of 2.5 10 4. The accuracy of the ratio between
36
Ar/H2O could be improved, reducing the upper limit by a factor of
50 with ROSINA/DFMS if the ratio of outgassing rates of H234S/H2O
is similar to that of Hale Bopp. Given the fact that those outgassing
rates are the highest measured for comets to date (Bockelée-Morvan,
2011) this ratio results in an upper limit of the outgassing rate of
H234S/H2O. The estimate of the lower limit for H234S/H2O was
determined by combining the lowest abundance of H2S/H2O (0.12%
in Bockelée-Morvan, 2011) and the lowest measured isotopic abundance of 34S/32S (3.7% in Bockelée-Morvan (2011) by Jewitt et al.
(1997)). The outgassing ratio for H234S/H2O would be 0.0045% or
4.5 10 5, decreasing the detection capability for the outgassing ratio
36
Ar/H2O to 2 10 7 and three orders of magnitude compared to the
upper limit given for C/A2.
Long-term observations of comet Hale-Bopp (Biver et al., 1998)
show that the outgassing rates of H2S relative to OH beyond 3 AU on
the inbound leg are similar. Therefore the ratio of H2S relative to H2O
deduced close to perihelion approach may be similar once the
comet's activity increases during its approach to the Sun. The outgassing rates for C/A2 for argon were deduced shortly after perihelion
approach. Argon is a noble gas and highly volatile, therefore it is very
likely that this atom will be among some of the first material that
comes off the comet. As the comet gets closer to the Sun, the ratio of
outgassing rates of 36Ar/H2O could be higher than the ratio at or
shortly after perihelion. Therefore the detection capabilities for 36Ar
could be even lower in the early phase, as the comet slowly becomes
active. However, the detection capabilities for 38Ar/H2O are much
lower, and strict constraints can be made on the isotopic abundances
in comets.
5. Summary and outlook
Noble gases are chemically inert and highly volatile and could
be used to trace the thermal history of cometary matter. They
might also help to solve the riddle about the role played by comets
in the formation and evolution of planetary atmospheres (Owen
and Bar-Nun, 2001). To improve our knowledge of the cometary
bombardment, a comparison of noble gas abundances in planetary
atmospheres and comets could be of great help. Argon may have
been detected in one Oort cloud comet (Hale-Bopp), but this
detection could not be confirmed so far for other comets. In other
Oort cloud comets, a lower limit on the detection compared to
water was lower than this specific observation at comet HaleBopp. So far, there has been no in situ or remote sensing detection
of argon in comets that belong to other families (like Jupiterfamily). Furthermore, no measurement of any comet has been
capable of resolving isotopic abundances of this noble gas. Nothing
is known about the diversity of noble gas concentrations within
different locations of origin for comets.
67P is a Jupiter-family comet and the target of the Rosetta
mission. It is the first comet of this family where argon may be
detected. ROSINA/DFMS, with its high sensitivity and ability to
resolve isotopic ratios, is capable of measuring argon isotopic
ratios with unprecedented accuracy for a space-born mass spectrometer. These measurements have been proven using the flight
spare model on Earth. The capability to detect 36Ar at comet 67P
depends on the signal ratio of H234S to 36Ar. However, even with
the highest estimation for H234S /H2O, the upper limit for 36Ar/H2O
would be reduced by at least a factor of 50 and up to three
orders of magnitude below the current upper limit. The detection
capabilities for 38Ar and a possible measurement for the isotopic
ratio of argon depend on the detection capabilities of the instrument and therefore depend also on the integration time. For a
water density of 109 molecules per cm3 and the sensitivity limit for
argon, the argon detection limit can be improved by almost four
orders of magnitude compared to the ratio of Ar/H2O in C/A2.
Given the current knowledge about argon abundances in comets,
ROSINA/DFMS capabilities are well suited to make a first in situ
measurement in a comet.
Acknowledgments
We thank the following institutions and agencies that supported
this work. Research at Southwest Research Institute is supported by
JPL subcontract under NASA prime Contract NNX148F71G. Work at
the University of Bern was funded by the State of Bern, the Swiss
National Science Foundation, and the European Space Agency's
PRODEX Program. Work at BIRA-IASB was supported by the Belgian
Science Policy Office via PRODEX/ROSINA PEA 90020. Research
at University of Michigan was supported under JPL subcontract
1266313 under NASA prime Contract NMO710889. The work on
ROSINA in France was supported by through Grants CNES/ROSETTA/
98-70 to 04–70.
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Please cite this article as: Hässig, M., et al., The capabilities of ROSINA/DFMS to measure argon isotopes at comet 67P/Churyumov–
Gerasimenko. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.11.015i