Effect of the presence of carbon dioxide on the
Triton’s icy surface
M. Á. Satorre, J. Cantó, M. Domingo, R. Luna, C. Millán and C. Santonja1
Departamento de Fı́sica Aplicada, Escola Politècnica Superior d’Alcoi, Universitat
Politècnica de València 03801 Alcoi, Alacant, SPAIN msatorre@fis.upv.es
Summary. Triton has an icy surface composed by different molecules (N2 , CH4 ,
CO, CO2 , H2 O,...). One of the proposed spatial distribution of them contemplates
the possibility that carbon dioxide appears segregated from the rest due to differential sublimation. This work researches about the variation on the temperature
of sublimation of methane and nitrogen codeposited with carbon dioxide as minor
components. Our investigation shows a large variation on the characteristic temperature of sublimation of both volatiles. Our results let suggest different trapping
mechanisms for both molecules explaining their retention up to the sublimation temperature of carbon dioxide. The perspective opened by these experiments referring
to the composition of the carbon dioxide terrain has also been discussed.
1 Introduction
Triton’s surface is mainly formed by frozen nitrogen. But other minor components as methane, carbon monoxide, carbon dioxide and water are also present
[1]. Carbon dioxide and water have a higher temperature of sublimation than
the others. This difference suggests the presence of two different terrains, one
composed by so called volatiles and the second one formed by water and carbon dioxide. Nowadays it is still in discussion whether carbon dioxide exists
in a separate terrain from water or whether it is mixed with it [2, 3]. If carbon
dioxide forms a separate terrain it would be interesting to study its interaction
with other volatile molecules present at the surface. Up to now, this kind of
interaction has not been studied as it has been made for water [4].
2 Experiments
2.1 Experimental set up
A detailed description of our system can be consulted in [5]. Figure 1 shows
a scheme for the experiments we have used for this work. Basically it is a
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high vacuum chamber with a base vacuum around 10−7 mbar. A compressor
is used to cool down the sample holder to 10 K. A resistor and a temperature
controller let vary up to 300 K this temperature with an error minor than 0.5
K. On the cold finger is connected a Q-sense Quartz Crystal Microbalance
(QCMB) with a frequency of oscillation about 5 MHz, its precision is around
1 Hz. The QCMB has a gold platted surface, optically plane, and it serves to
reflect a laser beam for interferometry.
The vacuum chamber has five windows used to attach different techniques.
For the experiments presented here, two opposite suprasil windows are used for
an He-Ne laser beam and another one, at the bottom of the vacuum chamber,
is used as a port for the Mass Spectrometer (MS). It is a quadrupole AccuQuad
Residual Gas Analyzer 100 D, capable to analyze gases whose atomic weights
ranges from 1 to 100 amu, with a resolution close to 0.5 amu.
The measured and registered magnitude is the partial pressure of a gas.
Selecting up to 10 different molecules the MS takes values of pressure every
few (4 or 5) seconds. It makes possible to study the temporal evolution of
the pressure therefore to monitor the sublimation of molecules as a function
of the temperature, in real time. The so called “pre-chamber” is a vacuum
Fig. 1. Scheme of the set up used in TPD experiments
chamber (base pressure 10−3 mbar) connected to the gas bottles. Proportions
in mixtures are controlled from the partial pressure of each gas measured with
a capacitance gauge.
Effect of the presence of carbon dioxide on the Triton’s icy surface
3
2.2 Experimental procedure
When the base vacuum is obtained in the chamber (base pressure 1×10−7
mbar) the sample holder is cooled down to the temperature of deposition
(usually about 15 K).
When the desired temperature is established, the mixture prepared in
the pre-chamber passes through a needle valve. The velocity of deposition is
regulated with the aperture of this valve and the diferential pressure between
the chamber and the pre-chamber.
All gases or mixtures in the pre-chamber are arranged from chemically pure
components (5.0 grade gas Carburos Metalicos for nitrogen and PRAXAIR
for methane and carbon dioxide, 4.5 and 5.0 grade respectively).
The rate of deposition is monitored with the QCMB which frequency is
measured precisely with a frequencymeter and the interference patterns of two
polarized laser beams.
Gas composition during deposition is confirmed with the mass spectrometer. In all the experiments presented here 95% is CO2 and 5% is N2 or CH4 .
After one minute of a background deposition (no preferential directions of
deposition) an approximately 0.1 micron film is formed. Once the sample is
deposited, the system rises its temperature at a certain rate (0.5 − 1 K min−1 )
while the partial pressure of each gas is registered. This kind of experiments
are known as “Temperature Programmed Desorption” (TPD) (see for example
[6, 7]).
As the goal of this research is to determine the effect that carbon dioxide
has in the characteristic sublimation temperature of methane and nitrogen, it
should be taken into account two main error sources. The first one is that N2
appear as contaminant in vacuum systems. It has been avoided with a series
of “no deposition” experiments that consent us to quantify the contaminants
present in the chamber and their rate of deposition. The second one is the
way in which the MS detect molecules, it ionizes and breaks molecules. If
CO2 sublimes it will be detected at the MS as a molecule of mass 44 but also
possible fractions as CO or O will be measured. Therefore a percentage of
CO2 is detected as pieces of 28 and 16 amu. In our case it is an undesired
effect because them could be confused with N2 and CH4 , respectively. This
misleading result has been taken into account and suppressed.
3 Results
Results presented here are part of a set of experiments of frozen ice of binary
mixtures of carbon dioxide (95%) and methane or nitrogen (5%), deposited
at 15 K. After deposition the ice is warmed up at 0.5 K/min, the molecules
sublimated from the ice are analyzed in real time as exposed in 2.2.
The characteristic temperature of sublimation of each molecule was determined for our experimental set up (see table 1), previously to the experiments with mixtures. The procedure to establish the temperature has been
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taken from [6] because the conditions in their experimental set up were similar to ours. This procedure obtains the sublimation temperature as the onset
of sublimation (extrapolation of the partial pressure to a baseline pressure).
The error in these measurements has been estimated taking into account the
method used. Other methods fixe these temperatures from the peaks of maximum desorption. It gives little shifts respect to our results with no significance
for the conclusions extracted here.
Table 1. Characteristic sublimation temperatures of the used molecules in our high
vacuum chamber.
Sublimation
Gas Temperature
(± 3 K)
CO2
CH4
N2
90
42
25
Figures 1 and 2 presents the curves obtained for TPD experiments of nitrogen (Fig. 2) and methane (Fig. 3) co-deposited with carbon dioxide. Figures
plot temperature of sublimation, in kelvin, against pressure, in arbitrary units.
In both figures (Fig. 2 and Fig. 3), dotted lines represent carbon dioxide and
solid lines nitrogen and methane, respectively.
The first important result is the variation of the initial temperature of
sublimation. Both, methane and nitrogen, starts the sublimation around 47
K. These implies a variation of the characteristic sublimation temperature of
nitrogen greater than 20 K and for methane it varies only 5 K. It is necessary
to highlight that carbon dioxide is able to delay the initial sublimation point
of both molecules.
As a second result, in TPD experiments of pure molecules, after this first
release the pressure drop abruptly, but in mixtures with carbon dioxide they
present a continuous decrease that do not falls to the base pressure. It implies
a mechanism of continuous variation of CO2 structure.
Third, last molecules release the ice during carbon dioxide sublimation,
starting at 87 K.
Between the first release and the final one, different bumps appear with
slightly variations for nitrogen and methane. May be the most clear point is
about 73 K, with a higher slope for nitrogen than for methane.
The possible mechanism that explains the desorption of volatiles retained
in CO2 is still in discussion. All explanations in this sense point out a porous
material as a retainer, but differ about the physical state of CO2 [8]. This
discussion is over the goal of that contribution that try to study the possibility
that the carbon dioxide terrain includes a few % of volatile molecules in it.
Effect of the presence of carbon dioxide on the Triton’s icy surface
5
Fig. 2. Nitrogen desorption from carbon dioxide ice
Fig. 3. Methane desorption from carbon dioxide ice
Respect to the physical mechanisms that explain the peaks of release of
trapped molecules about 47 K, 73 K and 87 K should correspond respectively
to: gas adsorbed on the CO2 surface, gas desorbed when carbon dioxide ice
reaches its maximum density [9], and finally, molecules trapped in the final
structure that carbon dioxide gets after warming up.
4 Astrophysical implications
Triton presents an atmosphere mainly composed by gases and vapors of
molecules in equilibrium with the icy surface. It is proposed, based on obser-
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vations and theoretical models, that it consists in two different terrains, the
first one composed by an intimate mixture of volatiles (nitrogen, methane,
and carbon monoxide), and the second one by water and carbon dioxide. It
seems that those late molecules form an intimate mixture but it is not excluded the possibility that water and carbon dioxide appear in a separated
terrain of pure ices. The capability of water ice in retaining molecules has been
widely studied, and it has been demonstrated that volatiles could be retained
by water up to the sublimation temperature of water in the laboratory at
around 160 K. This capability it is not studied in the case of carbon dioxide.
In this article we have shown results that demonstrate that if methane
or nitrogen are initially present in the matrix of carbon dioxide they are
retained at higher temperatures than that relevant for Triton. In this system
the temperature varies in a few degrees around 35 K. It makes impossible a
complete segregation of the volatile molecules from both carbon dioxide (this
article) and water (see for example [4]). If finally it is confirmed that exits a
separate terrain of carbon dioxide it would imply that in a certain moment
of its history Triton has suffered and increase of its temperature above the
sublimation temperature of the carbon dioxide with all the implications that
it would represent. If this possibility is no plausible it would imply that carbon
dioxide could not exist as a pure ice but it would be plenty of impurities.
Acknowledgments
We wish to thank the Ministerio de Educación y Ciencia (Co-financed by
FEDER funds) AYA2004-05382.
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