Submillimeter In-Situ Gas Phase Investigation of the Tholin Hypothesis
Brian J. Drouin and Robert Hodyss,
California Institute of Technology, Jet Propulsion Laboratory, Pasadena CA, 91109
Introduction
The atmosphere of Titan has fascinated astronomers, physicists, chemists and biologists
more and more as new observations and simulations increase understanding of this frozen
pre-biotic world. Observations and laboratory studies have largely focussed on the
presence of haze that obscures visual and infrared imaging. These aerosol laden clouds
of methane are believed to be continuously supplied with organic seed material dropping
from the radiation processed upper atmosphere.
Sediments consistent with
experimentally derived tholin condensates have been imaged on the surface using
infrared imaging (in methane windows) [Brown et al. 2006] and radar [Elachi et al.
2006]. These carbon-nitrogen compounds are known to produce amino-acids and other
complex organic species in an abiotic aqueous environment [Khare et al. 1984]. It is
even likely that such processing has occurred on the surface of Titan during short periods
following meteoric impacts or cryovolcanism [Artemieva and Lunine 2003]. Searching
out and analyzing such locations would achieve major goals in the search to understand
the origins of life and the nature of the early earth.
The Huygens probe [Tomasko et al. 2005, Niemann et al. 2005, Fulchignoni et al. 2005]
successfully operated in Titan’s atmosphere and survived on the surface for several hours
– thus paving the way for future, more ambitious, surface sampling. But how do we
really know what to look for in such a mission? Certainly laboratory efforts have
contributed to the successes of the Cassini-Huygens mission to the Saturn/Titan system
[reference review]. However, the laboratory must continue to provide the guidance
necessary to push the analysis of present and forthcoming mission results and to plan the
next missions. Perhaps the most pressing questions that remain to be pinned down are
still the most obvious ones, such as: Which atmospheric phenomena contribute to the
creation of the organic haze layers? Which radiative mechanism dominates the
atmospheric chemistry (or is it a concert of radiation mechanisms)? Does the atmosphere
selectively fractionate/condense chemicals on time-scales shorter than diffusion and
deposition? Are the solid compounds produced in laboratory simulations causally linked
to gaseous processes that can be quantified in the Titanian atmosphere? Mitigating these
unknowns can confidently determine which instruments will be successful in future
missions that search for pre-biotics in this specific environment? Certainly there are
many ways to reduce uncertainty in the tholin hypothesis and we present one in which the
rotational spectra of atmospheric species can be exploited both in the laboratory and with
new telescopes.
Many experiments have been done in efforts to elucidate the understanding of Titan’s
activated atmosphere and the nature of surface sediments. These experiments each
reproduce the atmospheric chemistry of Titan through mixture of nitrogen, methane and
sometimes ammonia or water, which is then activated by electrical or coronal discharge,
electron bombardment or photolysis.
(reasons to use a flow system from Clarke 2000)
The laboratory investigation of the atmospheric photochemistry of planets and satellites is mainly carried out in
static systems. These studies are often poor models of chemical processes in atmospheres because: (1) much
higher mixing ratios of minor constituents must be used to accurately determine the amount of reactant
consumed and to obtain sufficient products for analysis, (2) secondary photolysis of the initial photoproducts
often occurs, (3) wall reactions occur, and (4) most of the starting material is converted to products to obtain
enough for spectroscopic analysis.
(More from Szopa 2006)
However, the existing plasmas simulations introduce experimental biases compared with the conditions of
aerosols production in Titan’s atmosphere: chemistry is induced by electrons instead of photons; the solid
analogues are produced and deposited on solid surfaces; direct analysis of the particles inside the reactive
chamber is not easy. In order to avoid some of these experimental problems, we have developed another
method of production of Titan’s aerosols analogues.
Prior to 1991 experiments were primarily qualitative in nature and served to identify
species that could be confirmed in Voyager’s IRIS spectra [Hanel et al. (1981)].
Thompson et al 1991 was the first such experiment to emphasize the need for quantitative
analysis of the simulated atmosphere.
Thompson 1991 showed that quantitative analysis was quite important for classifying the
different experiments utilizing a metric for plasma activation based on the percentage of
ion-pairs of the parent gas. The plasma is said to have a “low-dose” if it is less than 5%
ionized (1.4 eV/molecule), a condition that ensures most collisions in the gas occur with
neutral species and the plasma can be considered thermally cold. Using this dosage
criteria and an activation dependent metric Thompson was able to put the measured
product yields at two different pressures in context with the work of Toupance et al.
[1974]. The results of these two quantitative experiments indicate that the lower pressure
discharge produces less saturated hydrocarbons and nitriles and higher power favors
nitriles over hydrocarbons. The experiments did not effectively measure amines that may
have been present in abundances less than or equal to the similar molecular weight
hydrocarbons. Thompson 1991 directly applies their experiments to a Titan atmosphere
model that incorporates 1 MeV electrons bombarding the atmosphere down to the 0.1
mbar pressure level. This pressure is below the experimental range, but the extrapolative
model was successful in producing most observed species to with factors of 5 of the
Voyager IRIS spectra. This alone shows that the electrical discharge methods employed
are analogous (in mechanism) to the dominant processes in Titan’s upper stratosphere.
They specifically point out that photo-induced chemistry only becomes a significant
factor in the middle stratosphere. The authors specifically address the need for
quantitative measurements of the simulated Titan atmosphere at these lower pressures
where magnetospheric activity is strongest.
Thompson et al 1994 follow up this work with mass flux measurements that indicate the
magnetospheric chemistry can be a major contributor to the stratospheric haze as well as
surface deposition.
A series of laboratory measurements of activated mixtures of N2/CH4 has been performed
at LISA that has focussed on the observables in the mid- to far-infrared wavelengths
(Vanssay 1994,) . Initially these measurements were similar to Thompson, with a plasma
discharge upstream of the gas analysis the results of these experiments are summarized in
Coll 1999a. This series of papers [Vanssay 1994 ib id] has established a list of species
that now have been searched for with the moderately high resolution CIRS onboard
Cassini. Much of the early success of CIRS can be credited to the concerted efforts to
explain the Voyager IRIS data with laboratory simulants and this commensurate plan put
into motion before CIRS arrived at the Saturnian system.
Describe INMS results here – in situ detection of ammonia, acrylonitrile (C2H3CN),
propionitrile (C2H5CN) and formaldimine (CH2NH).
Go into importance of in situ detection for discovery of intermediates, chemical pathways
and full systematic observation. Then describe prior measurements of activated systems.
Optical and electrical measurements of an activated Titan simulant mixture were
performed by Nascimento et al 1998. The emission spectra of a number of radical and
diatomic species were measured and quantified.
Gazeau et al. (2000) developed a photochemical reactor/cell that could probe the reaction
chamber directly with IR.
Subsequent experiments (e.g. Tran et al 2003) have rallied towards a primarily
photochemical mechanism for the formation of organics, nitriles and haze. (more about
Tran).
A correct hypothesis should incorporate contributions from both ionizing radiative
sources, with appropriate considerations for altitude.
In general laboratory efforts can contribute to planetary science through fundamental
research or with simulated environments. Fundamental spectroscopic and kinetic
measurements allow the complex soup of discordant observations to be understood in a
reductionist fashion. Conversely, it is very difficult to obtain a ‘big picture’ from the
patchwork modeling that inevitably results from the small (but growing) observational
datasets of planetary atmospheres. Certainly our strong global understanding of the earth
atmosphere has grown out of regular, timely measurements coupled with continuing
research into fundamental properties of the species present. For remote systems the
fundamental research must be augmented by the in-situ style measurement of simulated
environments because the number of unknowns is constrainable and the chemistry is
immune to model problems.
Paubert et al 1987 pioneered the search for rotational spectra of Titan’s atmosphere and
reported detection of hydrogen cyanide using the IRAM telescope. (Measurement of CO
should be here). Using improved receivers Bézard achieved higher sensitivity with the
same telescope and added cyanoacetylene (Bézard 1992) and acetonitrile (Bézard 1993)
to the detected list. (Someone must have gotten cyanoacetylene here). Motivated by
experimental simulations (de Vanssay et al. 1995 and Coll et al. 1995), Marten et al
utilized even more sensitive receivers at IRAM in an effort to identify cyanodiacetylene,
but sufficed only to improve measurements of the previously discovered cyanides. With
the strict dipole emission source of rotational spectra, non-detections can readily be
turned into upper limits in the disk-averaged models and therefore provide strong
feedback to the experimental efforts, which initially suggested a particular abundance of
a species.
Overall rotational emission detection is a very promising method for discovery of high
polarity, low abundance species in remote environments. Receiver technology and
frequency coverage has steadily improved simultaneously with array techniques that
provide spatial resolution. Furthermore, the spectrometers are inherently high resolution,
with bandwidth and channel width determined electronically, this allows the astronomer
to measure line profiles that can be disseminated into narrow, upper stratospheric, and
wider, lower stratospheric, components. This technique allows concentration as a
function of altitude to be extracted for the disk-averaged globe. Clearly a systematic
effort utilizing experimental methods, atmospheric modeling and rotational emission
detection could lead to a stronger understanding of the stratospheric sources for the Titan
haze and sediment.
Millimeter and submillimeter laboratory spectroscopy has long been useful for qualitative
identification of polar gas-phase species and is naturally connected to the field of
millimeter and submillimeter astronomy. The technique is inherently high resolution and
typically results in a high quality frequency measurement of the molecular spectra that
ultimately leads to gas-phase structure and dipole moment measurements. The frequency
and dipole measurements directly transfer to astronomical needs for quantifying gases
detected via rotational bands, however, quantitative (concentration) analyses are rarely
reported in the laboratory (cite N2O5?) due to the difficulties with source power
characterization. Recent improvements in the stability of local oscillators for Herschel’s
HIFI instrument, as well as a long history of R&D for communications has culminated in
laboratory sources for molecular spectroscopy that are reliable [Drouin et al 2005] and
now quantifiable. Furthermore, the rotational spectra of transient species are routinely
studied using techniques quite similar to those reported for Titan simulants. However, no
study has reported a quantitative analysis of an activated planetary atmosphere simulant
to date. The present work serves as a proof of principal for quantitative analyses of
complex mixtures that will no doubt be improved upon many times as the technology and
cleverness of the researchers improves.
Experimental
The JPL submillimeter spectrometer[Drouin et al. 2005] was fitted with two mass
flow controllers, two hollow electrodes, polyethylene windows and a liquid nitrogen trap,
see Figure 1. The operating temperature of 200 K was maintained through passive
cooling with methanol chilled by liquid nitrogen and circulated through an envelope
surrounding the discharge region. A 3000 Volt, 0.240 Ampere discharge was maintained
within 110 mTorr (0.00014 atmospheres) nitrogen and methane. This pressure was
actively maintained using the mass-flow controllers following the gas regulators and the
discharged gas was continuously pumped through the system using a vacuum pump.
Condensable discharge products either deposited upon the cell walls or were collected in
a liquid nitrogen trap in line to the vacuum pump. Plastic (polyethylene) windows were
fitted directly onto the cold, grounded hollow cathode and onto an insulated extension
near the hot hollow anode.
Submillimeter radiation was produced through direct multiplication from a
voltage-tuned crystal oscillator (Yttrium Iron Garnet – YIG) in the microwave
(centimeter) wavelengths. This spectral source is highly monochromatic, polarized and
directional. However, the radiation is not collimated as in LASER sources, and therefore
careful quasi-optical alignment is required for efficient coupling of the source and
detector system. For frequencies between 300 and 700 GHz (wavelength 1.0 - 0.43 mm)
quasi-optical mirrors of the ‘front-fed Dragonian’ design [Chang and Prata 2004] have
been built that convert the expanding beam from a submillimeter diagonal horn to a one
inch
(-30 dB radius) parallel beam. These mirrors are implemented at the source
multipliers and with Schottky detectors to enable efficient coupling of the submillimeter
beam through the cell. For frequency measurements above 700 GHz (wavelength < 0.43
mm) quasi-optical lenses made of polyethylene were utilized for source/ detector
alignments. The inherent polarization of the multiplier source is used to gain a factor of
two in pathlength (effective length of the absorption cell) by first passing the radiation
through a polarization grid aligned with the beam polarization, then, utilizing a roof-top
reflector at 45o on the opposite end of the cell, the beam is passed through the cell a
second time with the polarization rotated by 90o. Upon reaching the polarization grid the
rotated beam is then reflected into the detector aperture. Use of the Dragonian optics
enables ~10% of the source power to be coupled into a Schottky (room temperature)
detector, making this cost-effective, albeit inherently less sensitive, detector competitive
with liquid helium cooled detectors. Furthermore, the improvement in source optics
removes the need for any quasi-optical lenses that contribute to absorption loss in the
system. A computer program controls the source frequency and monitors the
demodulated detector signal while the spectrum is scanned for characterization of the
plasma. Further details of the spectrometer system, particularly the multiplier
characteristics, are available in reference [Drouin et al. 2005].
Survey scans from 570-648 GHz and 770-930 GHzm using frequency
modulation, were done to search for any unexpected compounds and to characterize
species that were not well cataloged. An example of one of the survey spectra is shown
in Figure 2. Quantitative analyses were performed utilizing direct absorption spectra
taken using amplitude modulation. The spectrum shown in Figure 4 gives a hydrogen
isocyanide concentration of 3.2 ppm.
Low sensitivity scans, utilizing the room temperature Schottky diode detector in
the 570-625 GHz range, quickly identified chemicals such as hydrogen cyanide (HCN),
carbon monoxide (CO), water (H2O), ammonia (NH3), methyl cyanide (CH3CN),
methylenimine (CH2NH) and cyanoacetylene (HC3N). Quantitative analyses of direct
absorption spectra shows that HCN, CH3CN and HC3N are present in the cell at
concentrations about 200x the mixing ratio at the equivalent pressure in Titan’s
atmosphere (water, methylenimine and ammonia have only been characterized in Titan’s
ionosphere). Water was not added to the system, but is detected at conditions consistent
with its source being the cold cell walls.
Table 1. Quantitative analyses of absorption spectra.
Species
Concentration comment
CN
4.09 ppm
N = 7/2, J = 5
CN
5.10 ppm
N = 7/2, J = 4 Harm. Cont.
CO
8.93 ppt
HCN
161 ppm
From 13C
HCN
160 ppm
From 15N
HNC
3.15 ppm
J=5
HNC
3.25 ppm
J=6
H2O
449 ppm
on walls too
NH2
~2.5 ppt
NH3
333 ppm
saturated
NH3
420 ppm
From 15N
H2CN
9.29 ppm
HC2N
~75 ppm
No dipole measurement
CNCN
< 160 ppb
no detection
CH2NH
50.6 ppm
Catalog off 1-50 MHz
HC3N
~80 ppb
CH3CN
23.0 ppm
CH3NC
0.73 ppm
Catalog off 5 – 27 MHz
The detection system was then improved by utilizing the laboratory’s most
sensitive detector, a liquid helium-cooled, Silicon bolometer. Survey scans in the same
spectral window then revealed a number of radical and unstable chemical species present
at < 20 ppm. Compounds thus far identified in the survey spectrum include: cyanide
radical (CN), hydrogen isocyanide (HNC), amidogen (NH2), methyleneamidogen
(H2CN), cyanomethylene (HC2N) and methyl isocyanide (CH3NC). Many of these
species have been quantified using direct absorption spectroscopy and known quantum
mechanical parameters from the JPL and CDMS databases. The quantitative results are
summarized in Table 1. During assignment and quantification of the spectra several
database/literature spectroscopic deficiencies were identified including: 1) Only
extrapolative predictions for CH3NC and CH2NH; 2) no predictions for hydrazine (N2H4),
or methylamine (CH3NH2); 3) and no dipole moment measurement for cyanomethylene.
The survey spectrum shown here, as well as spectra recorded from 770 - 930 GHz are
sufficient to update the database for CH3NC and CH2NH, however, further analysis to
identify signatures due to N2H4 and/or CH3NH2 is beyond the scope of this spontaneous
research project.
An electrical measurement of the ballast resistance allows the power dissipated in
the plasma/electrode system to be separated from the total power provided by the DC
power supply. With the Rb = 7.76 k ballast resistance 38-80% of the power goes into
the N2/CH4 plasma as well as the heating of the electrodes and coolant. A method
described by Thompson et al., for comparison of their results to the work of Toupance,
allows an upper estimate of the plasma ‘dosage’ based on electrical measurements of the
plasma circuit. This formula is given in Eqn 1 and allows a dosage upper limit of 180
eV/molecule to be determined for the measurements described thus far.
Eqn 1) dosage = V I (1 - (I Rb/V))t/N
In Eqn 1 the voltage (V) is that applied by the DC power supply and current (I) is the
measured current of the total circuit including plasma/electrode resistance as well as the
ballast resistance, t is the residence time of the molecules in the flow cell and N is the
total number of molecules in the cell volume enclosed by the electrodes. At 100 mTorr
and 200 K the 2 Liter cell contains approximately N = 1019 molecules and residence times
are close to one second as deduced from the flowmeter settings.
A range of voltage settings (1050-3000 Volts) permits exploration of this
simulated Titan atmosphere under a range of dosages down to ~5 eV/molecule. A
systematic determination of product yield for HCN and NH3 was undertaken at both 300
K and 200 K in an effort to discern effects from an ‘overdose’ that occurs when the
dosage ( > ~30 eV/molecule) is large enough to multiply ionize the nitrogen molecules.
These experiments verify a marked change in product yield trends within the weak-dose
regime and above.
Figure 4 shows the concentration dependence upon discharge
voltage and Figure 5 shows the deduced product yield as a function of dosage.
Modeling
Comparison
Discussion
Conclusion
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Figure 1. Apparatus
DAQ
PC
lock in amp
DC power supply
N2
coolant out
modulation
detector
T
0o C
rooftop
reflector
5 cm
P
RF synthesizer
T
100 cm
multiplier chain
polarizing grid
cathode
coolant in
anode
vacuum pump
CH4
Figure 2. Survey scan of 0.5 mm region.
Figure 3. Direct absorption spectrum of HNC J = 5  4.
543890.00
543895.00
543900.00
543905.00
Frequency (MHz)
450
100
400
90
350
80
concentration (ppm)
concentration (ppm)
Figure 4. ([HCN] - ), ([H13CN]/0.011 - ), and ([NH3] -▲) vs. plasma voltage at
a) 300 K and b) 200 K.
300
250
200
150
70
60
50
40
30
100
20
50
10
0
1000
1500
2000
Discharge Voltage
2500
3000
0
1000
1500
2000
Discharge Voltage
2500
3000
Figure 5. HCN and NH3 product yields vs. energy dosage
Product Yield (molecule/heV)
6.E-04
300 K (HCN)
300 K (H13CN)
300 K (NH3)
200 K (HCN)
200 K (NH3)
5.E-04
4.E-04
3.E-04
2.E-04
1.E-04
0.E+00
0
30
60
90 120 150 180 210 240 270
Energy (eV/molecule)
Figure 6. Profiles compared with experimental data. CH4, CO, C2H2, HCN, CO2, C2N2,
HC3N profiles from Teanby et al. Icarus 181 (2006) 243-255, CH3CN profile from
Marten et al. Icarus 158, (2002) 532-544.
CH4
0.01
CO
C2H2
HCN
0.1
CO2
exp
exp/200
C2N2
pressure(mb)
HC3N exp
1
HCN exp
CO exp
H2O exp
HC3N exp
10
CN exp
HNC exp
H2CN exp
100
CH2NH exp
CH3CN exp
CH3NC exp
1000
1.E-13
NH3 exp
1.E-10
1.E-07
vmr
1.E-04
1.E-01
CH3CN
CH4 exp