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SUPPLEMENTARY INFORMATION
associated with the proposed letter to Nature:
« Evidence for the presence of complex organic matter in Titan’s aerosols by in situ analysis »
By G. Israël et al.
Reference article: 2005-05-06080
Version: 05/10/2005
CONTENTS OF FIGURES
 Figure 1.
Laboratory chromatogram of pyrolysis products of a tholin
(laboratory analogue of Titan's aerosol).
 Figure 2.
Schematic of the ACP-GCMS transfer interface
 Figure 3.
Sampling and transfer phases during the descent
 Figure 4.
Nominal oven pressurisations during the descent
 Figure 5.
Evolution of the ratio (m/z=28)/(m/z=27) during ACP 2-3
transfer
 Figure 6.
Probable structure of Titan’s aerosols analysed with the ACP
experiment.
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Evidence for the presence of complex organic matter in Titan’s aerosols by in
situ analysis
By G. Israël et al.
Supplementary information
1. The Huygens ACP instrument
Our knowledge of the chemical composition of Titan’s aerosols and of the chemical
pathways leading to their production was based on assumptions done from: i) the state of the
art of theoretical chemistry and physics applied to Titan’s atmosphere by the way of
numerical modeling1; ii) the results obtained from laboratory experiments which simulate
Titan’s atmospheric chemistry2,3. Regarding this second approach, we have undertaken a
systematic pyrolysis-GCMS analysis of tholins from various origins representative of
chemical pathways that are likely to be relevant for Titan. Results from this study are shown
in supplementary Fig. 1, and they clearly demonstrate that NH3 and HCN are two of the major
pyrolysis products of the studied tholins studied.
The Cassini-Huygens mission provided the first opportunity to collect direct in situ
information about these aerosols and the Aerosol Collector Pyrolyser (ACP) was selected to
achieve this task. The ACP experiment, coupled with the GCMS experiment of the Huygens
probe, provided the first direct chemical analysis of these atmospheric particles. The primary
objective of the experiment was indeed to determine the chemical makeup of the aerosols in
the Titan's lower stratosphere. Moreover, in the lower atmosphere where the probe’s
instruments operated, photochemical aerosols settling down from the upper atmospheric
layers may act as condensation nuclei and yield cloud particles. As a consequence, the other
main objective of ACP was to search for the relative abundances of organics condensed on the
particles down to the middle troposphere.
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A complete description of the ACP experiment has already been published4. The main
components of the instrument are: 1) a sampling system with a stainless steel filter and its
mechanism for its translation; 2) a pump unit to drain off the atmosphere of Titan; 3) an oven
and its gate valve; 4) a subsystem for the transfer of ACP gas products to GCMS. The
sampling system requires an inlet tube extending from the Probe’s fore dome and an exhaust
tube which allows the gas to be vented externally. The gas flow, on reaching the level of the
oven, follows a path perpendicular to the oven/gate valve assembly.
The sampling of the aerosols was performed in two regions of the atmosphere: first
from 130 down to 35 km, and then, from 25 down to 20 km altitude. In its sampling position,
the filter front face extended a few millimetres beyond the probe fore dome. The translation
mechanism allowed the filter to be either outside the Huygens probe to collect aerosols, or
inside the probe. In this last position, the filter is in the ACP oven where evaporation and
pyrolysis of the sampled particles can be carried out. All the components, mechanisms and
seals were studied to avoid any organic contamination. The capture of atmospheric aerosols
was achieved: i) by impaction processes on the front part of the filter, in the upper
stratosphere (down to 80 km, first segment of the first sampling); ii), by filtration, using the
pump unit that forces the gas flow through the filter in the lower stratosphere and troposphere.
During the two samplings, the collecting target's temperature stayed very close to that of
Titan's atmosphere. Therefore, it can be assumed that the more volatile components of the
collected aerosols and cloud particles remained trapped.
After each sampling, ACP then converted the collected matter by evaporation and
pyrolysis in the oven into gases which are transferred to the (GCMS) (see supplementary Fig.
2). The oven is a pyrolysis furnace with a small dead volume. Resistance heaters could heat
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the filter, and hence the collected aerosols, up to 600°C. After each sampling the matter
collected was submitted to three different treatment steps in order to discriminate the origin of
the gases released from the sample: i) during the first step, the sample once in the oven
remains slightly above the very cold temperature at which it was collected, ii) the sample is
then heated to 250°C to vaporize its volatile part; and iii) the sample is finally heated to
600°C to thermally decompose (pyrolyse) its solid “refractory” part into gaseous species.
After each of these steps, in order to efficiently transfer the gases evolved from the
oven, mixed with the corresponding sampled atmospheric gases, to the GCMS, the gases were
flushed by a gas at high pressure (see supplementary Fig. 2). To avoid possible interferences
with Titan's atmospheric dinitrogen, labelled dinitrogen
15
N15N (or
15
N2) was used as the
flushing gas. To ensure a transfer of the gas with minimal dilution, each injection into the
GCMS was done by pressurizing the oven to 2.5 bars with
15
N2 and then rapidly
depressurising it down to 2.1 bars (“piston effect”). To be sure that no information on the
oven content has been lost, each transfer phase is composed by a series of six injections of
0.875 s each. Each injection is followed by an MS analysis period of 4.750 s, within which
three successive mass scans are performed (see Fig. 4 in reference4).
The two ACP samples were analysed for each transfer by GCMS for a given fraction of its
operational time. A systematic analysis of all the transferred gases by direct MS was achieved
using the ion source IS2, entirely devoted to ACP. A complete GCMS analysis was used only
once (at To+73 min, i.e. for the analysis of the species evolved from the pyrolysis at 600°C of
the first sample) because it was consuming too much time (10 min) to be repeated several
times during the probe descent. ACP was being turned off at To +110min (11 km nominal
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altitude). Data products from the GCMS that are referencing to ACP were provided for
analysis to the ACP Science Team.
2. Technical behaviour during the descent
During the Huygens probe descent, the electric valves and all the mechanisms were
operating in complete conformity with the specifications. The actuation of the pump unit was
accomplished properly, first from the lower stratosphere down to the middle troposphere
(between 80 and 35 km), and then in the middle troposphere (between 25 and 20 km altitude)
(see supplementary Fig. 3). Readings from a number of pressure and temperatures sensors
were taken during the probe descent. In particular, the temperature sensors placed on the
internal wall of the oven and on its heating element, confirmed that the temperatures of the
aerosols sample reached the specified step values of 250°C and 600°C for analysis. It has also
shown that during the first step analysis the so called “ambient” temperature was very low,
not above – 70°C.
We have noticed that a part of the ACP operation was not nominal. As shown on
supplementary Fig. 4, the recording of the temporal evolution of the pressure inside the oven
proved that the pressurisation and depressurisation effect during the gas transfers from ACP to
GCMS did not operate adequately after the “ambient” temperature step and the 250°C heating
step. The reason for such a behaviour is that, during part of the transfer period, before the
actuation of the heater element at 250°C, the temperature at the base of the oven was beyond
the range at which a fluorosilicon gasket ensures the gas tightness of the oven. This
explanation is advanced because at the time of ACP gas transfer to the GCMS, a number of
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temperature sensors which checked the thermal conditions of ACP reached values 20 to 30°C
lower than expected, and not in accordance with the thermal modelling of the instrument.
However, supplementary Fig. 4 clearly shows that because of the heaters effect (at 250°C) the
injections done after this one were nominal. This is confirmed when we look at the evolution
of m/z=16 in the spectra obtained after the pyrolysis at 600°C, (see Fig. 3a of the Letter). The
regular decrease of the intensity of the peak after each injection is in conformity with the
curves obtained during the design of the ”piston effect” (see Fig. 17, in reference4).
3. Chemical information on Titan’s aerosols composition deduced from the ACP
measurements
ACP data show that Titan atmospheric aerosols are composed of an organic refractory
part, particle nucleus made of carbon, hydrogen and nitrogen atoms (supplementary Fig. 6).
The pyrolysis at 600°C of this solid nucleus produces hydrogen cyanide (HCN) and ammonia
(NH3). These molecules are thus fingerprints of the chemical structure of the aerosols solid
core, proving the inclusion of nitrogen in the aerosols. They are also indicators of the potential
presence in the core molecular structure of nitrile groups (-CN), amino groups (-NH2, -NHand -N<) and /or imino groups (-C=N-). This nucleus can induce the condensation of volatile
atmospheric species present in Titan's stratosphere (supplementary Fig. 6), which probably
cover its surface (the present paper does not deal with this coating which will be evaporated at
low temperatures).
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References to supplementary information
1. Yung, Y.L., Allen, M. & Pinto, J.P. Photochemistry of the atmosphere of Titan :
comparison between model and observations. The Astrophysical Journal Supplement Series
55, 465-506 (1984).
2. Cabane, M. & Chassefière, E. Laboratory simulations of Titan's atmosphere: organic gases
and aerosols. Planetary and Space Science 43, 47-65 (1995).
3. Khare, B.N. et al. Optical constants of organic tholins produced in a simulated Titanian
atmosphere : from X-ray to microwave frequencies. Icarus 60, 127-137 (1984).
4. Israel, G., Niemann, H., Raulin, F., Riedler, W, Atreya, S., Bauer, S., Cabane, M.,
Chassefière, E., Hauchecorne, A., Owen, T., Sable, C., Samuelson, R., Torre, J.P., VidalMadjar, C., Brun, J.F., Coscia, D., Ly, R., Tintignac, M., Steller, M., Gelas, C., Condé, E. and
Millian, P. The Aerosol Collector Pyrolyser (ACP) experiment for Huygens. ESA Special
Publications 1177, 59-84 (1997).
Figure captions – supplementary information
Supplementary Figure 1. - Laboratory chromatogram of pyrolysis products of a tholin
(laboratory analogue of Titan's aerosol).
The tholin sample was produced under controlled conditions (experimental setup previously
described20). After the tholin production and collection, a tube was filled with 0.47 mg of
sample, inserted into the pyrolyser system (Curie point instrument), and flushed with He. The
sample was pyrolysed at 750°C, and the resulting vapour was injected into a laboratory gas
chromatograph mass spectrometer (GC-MS). The different gaseous products were separated
by the GC, and analysed with the MS at the GC column outlet, enabling the identification of
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the different products by their mass spectrum. The Poraplot Q column of the GC is 25 m long,
0.32 mm wide, with a 2.5 m particle trap. The GC temperature program is as follows: hold at
60°C for 2 min, then increase to 240°C at a rate of 10°C/min, hold at 240°C for 30 min, then
raise to 250°C at a rate of 10°C/min, and hold for 9 min for a total GC run is 60 min and a
total MS analysis of 50 min. The last 10 min are used to clean the column at 250°C. The
Helium flow used was 1.5 ml/min. The label “lab” indicates that the product origin is the
laboratory atmosphere introduced through the injection loop.
Supplementary Figure 2. - Schematic of the ACP-GCMS transfer interface
In the configuration shown, the filter is in its inner position after the aerosols sample has
been collected (the atmospheric gas flow follows a path perpendicular to the oven assembly
not shown in the figure). The scheme shows the way the gas products, obtained while the
filter is in the oven, are transferred after heating. Three valves (V1, V2, and VT) are mounted
on the oven body: V1 supplies a labelled gas (15N2) - stored in a gas tank (GT) - to carry the
gas samples through V2, from the oven to the GCMS. In order to transfer the gas samples
with minimal dilution from the effluent gas (15N2), each injection into the GCMS is done by
pressurizing the oven to 2.5 bars with N2, and then rapidly depressurizing it down to 2.1 bars.
Each transfer of the sample is completed after 6 injections of 0.875s.
In order to obtain a background analysis, the opening for 5 s of the venting valve VT and of
the valve VAA allows the oven gas content to be drained off into Titan’s atmosphere via the
exhaust tube (see Section 1) and to evacuate any residual traces of pyrolysis products in the
transfer line. Two transfers, composed of a series of 2 injections of 0.875 s are done to obtain
the «background spectra». Titan’s atmospheric gases contribute mainly to the volume’s
content during this background operation and it can be assumed that if after the 6 injections
(«signal spectra») which occur just before the venting, a contribution due to the heating or
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pyrolysis products still remains, it is fairly diluted. This allows a fair comparison between the
two spectra (see Fig. 1 and 2 in the «Letter to Nature»).
The GCMS gas sampling system has three subsystems: the direct atmospheric sampling, and
the ACP sample line. The ACP line, that is presented in Fig. 2, interfaces either with the gas
chromatograph (GC) columns (coupled to ion sources -IS 3, IS4, IS5 – of the MS, used as a
detector), and the mass spectrometer using the dedicated ion source IS2. The source IS1
connected to the direct GCMS atmospheric sample (not shown on the schema) has the same
characteristics and performances as the ion source IS2.
Supplementary Figure 3. - Sampling and transfer phases during the descent.
Supplementary Figure 4. - Oven pressurisations during the descent.
Supplementary Figure 5. - Evolution of the ratio R=(m/z=28)/(m/z=27) during ACP 2-3
transfer.
A signature at m/z=27 was observed on the mass spectra measured during the transfer of the
gases evolved from the first and second aerosol samples pyrolysed at 600°C. Several
molecules may be at the origin of this signature, but it was shown (see the main text in the
Letter to Nature) that HCN may be the main contributor at m/z=27. Another possibility for
this signature is a crosstalk of m/z=28 on m/z=27 (a part of the ions with m/z=28 are detected
at m/z=27) as has been observed by the GCMS experiment. If that was the case, the ratio
R=(m/z=28)/(m/z=27) should remain unchanged whatever the intensity of the m/z=28 signal
is. However, we observe on this figure that this ratio R varies by a factor of about 3 during the
successive mass scans corresponding to the successive flushing of the ACP oven. Then one
may conclude that the temporal variation of R corresponds to the arrival at the MS level of
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molecules with m/z=27 that are not linked to the N2 molecules at m/z=28. Error bars on R are
calculated from counting statistics on m/z=27 and m/z=28.
Supplementary Figure 6. - Probable structure of Titan’s aerosols analysed with the ACP
experiment.
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Supplementary Figure 1. - Laboratory chromatogram of pyrolysis products of a tholin
(laboratory analogue of Titan's aerosol).
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To Exhaust tube
OVEN
to
FILTER
Sampling lines
GC-MS
GC Columns
+
Ion sources IS3-5
atmosphere
MS
Ion source IS2
Supplementary Figure 2. – Schematic of the coupling between ACP and GCMS. For more
details see: http://www.aerov.jussieu.fr/experience/ACP/
ACP - SAMPLING & TRANSFER PHASES
160
150
Descent altitude vs probe time (from DTWG-1)
First Sampling :
0:06:43 (129.88 km) - 1:00:00 (34.69 km)
140
130
120
Second Sampling :
1:17:16 (25.06 km) - 1:28:30 (19.95 km)
110
100
Z (km)
90
80
Pump Activated
0:23:30 (80.41 km)
70
60
Transfer Ambient : 1:39
50
40
Transfer 250 °C : 1:42
Transfer 650 °C : 1:13
30
Transfer 650 °C : 1:47
Transfer 250 °C : 1:08
20
Transfer Ambient : 1:05
10
0
0:00:00
0:14:24
0:28:48
0:43:12
0:57:36
1:12:00
1:26:24
1:40:48
1:55:12
Probe time (post T0)
Supplementary Figure 3. - Sampling and transfer phases during the descent
2:09:36
2:24:00
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Supplementary Figure 4. - Oven pressurisations during the descent
2000
1800
1600
1400
1200
1000
800
600
400
200
10
:5
7:
21
10
:5
7:
23
10
:5
7:
25
10
:5
7:
27
10
:5
7:
29
10
:5
7:
31
10
:5
7:
33
10
:5
7:
34
10
:5
7:
36
10
:5
7:
38
10
:5
7:
40
10
:5
7:
42
10
:5
7:
44
10
:5
7:
46
0
UTC Time
Supplementary Figure 5. - Evolution of the ratio R=(m/z=28)/ (m/z=27) during ACP2-3
transfer.
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Refractory
organics
Small fraction of
Condensates
Supplementary Figure 6. - Probable structure of Titan’s aerosols analysed with the ACP
experiment.