Near infrared spectra of the surface of Enceladus returned by Cassini show the presence of an absorption feature at 3.53 μm, ascribed by Brown et al. (2006) to “short chain organics,” and by Newman et al. (2007) to hydrogen peroxide. We assign this feature tentatively to methanol. Variations in the peak position of the feature suggest that methanol in the “tiger stripes” region may be segregated from the water ice, and not homogeneously distributed in the ice matrix. The photolytic destruction of methanol implies that methane or methanol itself must be continually deposited on the surface. On Enceladus, methanol may be generated photochemically from a mixed methane/water ice, or deposited from the plume itself. The variation in the concentration of methanol over the surface could be used to distinguish between these two processes

Goguen, Jay D.

Hodyss, Robert

Johnson, Paul V.

Kanik, Isik

Parkinson, Christopher D.

Stern, Julie V.

Yung, Yuk L.

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95284/1/grl26203.pd

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Geophysical Research Letters

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Methanol on Enceladus

Caltech Authors - Main

Methanol on Enceladus
Robert Hodyss,1 Christopher D. Parkinson,2 Paul V. Johnson,1 Julie V. Stern,1
Jay D. Goguen,1 Yuk L. Yung,3 and Isik Kanik1
Received 26 May 2009; revised 5 August 2009; accepted 11 August 2009; published 15 September 2009.
[1] Near infrared spectra of the surface of Enceladus
returned by Cassini show the presence of an absorption
feature at 3.53 mm, ascribed by Brown et al. (2006) to
‘‘short chain organics,’’ and by Newman et al. (2007)
to hydrogen peroxide. We assign this feature tentatively to
methanol. Variations in the peak position of the feature
suggest that methanol in the ‘‘tiger stripes’’ region may be
segregated from the water ice, and not homogeneously
distributed in the ice matrix. The photolytic destruction of
methanol implies that methane or methanol itself must be
continually deposited on the surface. On Enceladus,
methanol may be generated photochemically from a
mixed methane/water ice, or deposited from the plume
itself. The variation in the concentration of methanol over
the surface could be used to distinguish between these two
processes. Citation: Hodyss, R., C. D. Parkinson, P. V.
Johnson, J. V. Stern, J. D. Goguen, Y. L. Yung, and I. Kanik
(2009), Methanol on Enceladus, Geophys. Res. Lett., 36, L17103,
doi:10.1029/2009GL039336.
[2] Recent near infrared spectra of the surface of Ence-
ladus obtained by Cassini show the presence of an absorp-
tion feature in the C-H stretch region at 3.53 mm [Brown
et al., 2006]. Brown et al. [2006] ascribe this feature to
‘‘short chain organics.’’ Newman et al. [2007] attribute this
feature to hydrogen peroxide, although the observed width
of the feature is significantly narrower than is seen in spectra
of hydrogen peroxide. A careful analysis of hydrogen
peroxide peak widths and positions under a variety of
conditions has led Loeffler and Baragiola [2009] to con-
clude that hydrogen peroxide is not the carrier of the 3.53 mm
band. Here, we present evidence that methanol is the origin
of the 3.53 mm band. Methanol may be delivered to the
surface by deposition of methanol present in the plume, or it
may be formed photochemically from mixed ices of water
and methane. The 3.53 mm feature varies in both peak
position and intensity over the surface of Enceladus, and
these variations might be used to determine the source of
the methanol.
[3] Figure 1 compares reflectance spectra acquired by
Cassini VIMS of Enceladus with transmission spectra of
ices obtained in our laboratory. The spectrum of a 1%
mixture of methanol in crystalline water ice at 140 K is
an excellent match to the feature at 3.53 mm in the global
average spectrum of Enceladus, both in peak position and
line width. On Enceladus, this band varies in position from
3.53 to 3.56 mm, with FWHM between 0.02 to 0.03 mm
[Newman et al., 2007]. The FWHM of our methanol band is
0.02 mm, in contrast to the FWHM of H2O2 in water ice,
which varies from 0.03 to 0.15 mm [Newman et al., 2007],
suggesting the feature is more likely due to methanol. We
saw little variation in the shape or position of this band with
temperature and phase of the water ice matrix. In the spectra
of the ‘‘tiger stripes’’ region on Enceladus, the peak position
of the band has shifted to 3.55 mm. As shown in Figure 1,
this shift is partly replicated by a thin film of pure,
amorphous methanol ice. This may indicate that the meth-
anol in this region is segregated from the water ice into
domains of pure methanol, or that a different matrix is
present in the warmer regions, perturbing the position of the
band. In the spectrum of the region between the tiger stripes,
the 3.55 mm feature is broadened, possibly indicating the
presence of both segregated methanol and methanol isolated
in the water ice matrix. Ion irradiation may also be capable
of altering the width and intensity of this band [Brunetto et
al., 2005; Palumbo et al., 1999].
[4] Methanol exhibits two other bands in the near infra-
red, at 2.27 and 2.34 mm. Sandford and Allamandola [1993]
measured the integrated absorbances of these features in a
water-rich matrix. The 2.27 mm band has a band strength of
4  1019 cm  molecule1, and the 2.34 mm band has a
band strength of 6  1020 cm  molecule1. These bands
are significantly weaker than the 3.53 mm band (7.5 
1018 cm  molecule1; the ‘‘weak IS mix’’ from Hudgins et
al. [1993]). Although the 2.27 mm band might be detectable
in the highest SNR ground based spectra [Emery et al.,
2005], another feature they ascribe to NH3 covers the 2.235
to 2.275 mmwavelength range. It is plausible that the 2.27 mm
methanol feature contributes part of this observed absorption.
The weaker 2.34 mm feature is not expected to be detectable
and was not detected in the Emery spectra.
[5] On Enceladus, methanol may be present within the
warmer interior, from either a primordial source or from in
situ formation [Matson et al., 2007], and could then be
released in the plume. In either case, methanol would
condense on the surface, leading to the observed feature.
This hypothesis is strengthened by recent data from the
Cassini INMS indicating trace quantities of small organic
compounds in the plume [Waite, 2008].
[6] Another possible source of methanol on Enceladus is
photochemical generation from water and methane. Both
water and methane are known components of the Enceladus
plume [Waite et al., 2006], and could thus be deposited on
the surface. Photolysis of methane/water ices has been shown
to produce methanol, through the reaction of hydroxyl and
methyl radicals formed through photolysis and hydrogen
atom abstraction [Hodyss et al., 2009]. The warmest areas
GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L17103, doi:10.1029/2009GL039336, 2009
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1Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
2Department of Atmospheric, Oceanic, and Space Sciences, University
of Michigan, Ann Arbor, Michigan, USA.
3Division of Geological and Planetary Sciences, California Institute of
Technology, Pasadena, California, USA.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL039336$05.00
L17103 1 of 3
of the tiger stripes have a temperature inferred to be at least
145K [Spencer et al., 2006], but other regions of Enceladus
should be at temperatures controlled by insolation, ranging
from 33 to 75 K [Spencer et al., 2006]. These lower
temperatures should allow for condensation of some frac-
tion of the plume methane onto the surface. Experiments
have shown detectable methane in co-condensed methane/
water mixtures up to 74 K [Bernstein et al., 2006]. Subse-
quent photochemistry could then lead to the formation of
methanol, and the 3.53 mm band.
[7] Methane is not expected to be spectrally detected in
the surface ice at the 1.6 ± 0.4% concentration measured by
INMS in the plumes. Emery et al. [2005] use high SNR
spectra with better spectral resolution than VIMS to put an
upper limit of 2–7 wt% methane in Enceladus surface ice
based on their spectra at 2.32 and 2.38 mm where interfer-
ence from water ice absorption features is minimal. At
3.32 mm, the methane absorption on Enceladus is not
detected because it is masked by the strong absorption from
the shoulder of the 3 mm water ice band. Although methane
also has an absorption feature near 3.53 mm, it is an order of
magnitude weaker than the methanol feature (A(CH3OH)
7.5  1018 cm mol1 [Hudgins et al., 1993]; A(CH4)
 6  1019 cm mol1 [Grundy et al., 2002]). Thus the lack
of detection of methane features anywhere in the spectrum is
consistent with the fidelity of the observed spectra and the
INMS methane concentration in the plume. Conversion to
methanol allows the 3.53 mm absorption to become detect-
able due to the increased intrinsic strength (A value) of the
methanol band.
[8] Since hydroxyl radicals are continually produced by
photolysis of the water ice, all methanol should eventually
be oxidized to carbon dioxide [Hodyss et al., 2009], if not
shielded by continually deposited materials from the plume.
The reverse reaction, hydrogenation of CO2 to formalde-
hyde, methanol, or methane does not occur in irradiated ices
[Strazzulla et al., 2005]. Carbon dioxide is therefore a sink,
and has in fact been detected on Enceladus [Brown et al.,
2006]. Methanol must be continually deposited onto the
surface by the plume, or be continually produced on the
surface through photochemical oxidation of deposited meth-
ane for the 3.53 mm band to be extant. Parkinson et al.
[2007] estimate the flux of water accreting on the surface is
at least 105 cm/year. Is this a sufficient rate to shield the
methanol from complete oxidation? From the photodestruc-
tion cross-section given by Cottin et al. [2003] (2.7  1019
cm2) and the flux of far ultraviolet radiation at Enceladus
(1.1  1012 photons cm2 s1 at 4.4–12.4 eV [Madey et al.,
2002, Table 1], we can calculate a half life for methanol of
27 days. Water ice is strongly absorbing in the ultraviolet at
wavelengths shorter than approximately 200 nm where
photons have sufficient energy to photolyze water. For
instance, below 155 nm, 90% of the light is absorbed within
the top 0.1 mm of a water ice surface [Cottin et al., 2003], a
thickness which is deposited in 37 days on Enceladus at the
flux given by Parkinson et al. [2007]. From these consid-
erations it seems clear that the deposited water ice should
provide sufficient shielding to protect methanol from com-
plete oxidative destruction.
[9] It is important to note that photolysis of the surface
ices is not the only form of radiation processing that will
occur on Enceladus. Chemistry may also be induced by
charged particle irradiation, and sputtering by both charged
particles and ultraviolet photons will play a role in altering
the surface. In general, the chemical effects of charged
particles will be similar to that of ultraviolet photons for
the water ice-organic mixtures we describe here. Charged
particles and ultraviolet photons will both produce hydroxyl
radicals that will lead to oxidative chemistry and eventual
conversion of organics to CO2. However, charged particle
irradiation will differ from photolysis in its distribution
across the surface, and in its penetration depth into the
ice. Enceladus is irradiated by magnetospheric protons at a
energy flux of approximately 1  1012 eV cm2 s1,
corresponding to 1 107 ions cm2 s1 at 100 keV [Loeffler
et al., 2006]. The energy flux of far ultraviolet radiation given
above is approximately 9  1012 eV cm2 s1(calculated as
the average photon energy times the flux), and so ultraviolet
photolysis should be the more important process.
[10] We have described two possible sources of methanol
on the surface of Enceladus, viz., 1) deposition by the plume
from the interior, and 2) generation photochemically on the
surface from water and methane. These two scenarios will
lead to different distributions of methanol on the surface,
shown schematically in Figure 2. The composition of the
condensing gasses on the surface is related to the temper-
ature of the surface, which is warmer closer to the vent. In
scenario 1, methanol will be deposited over all ice surfaces
it comes into contact with, as even the highest inferred
temperature on Enceladus’ surface (145 K) is still low
Figure 1. Laboratory transmission and Cassini VIMS
reflectance spectra. The vertical dashed line is at 3.530 mm.
Dashed line: pure methanol at 100 K; solid line: 1%
methanol in crystalline water at 140 K; crosses: Enceladus
global average (methanol band position 3.530 mm); squares:
Enceladus around tiger stripes (methanol band position
3.547 mm); stars: Enceladus tiger stripes (methanol band
position 3.547 mm). The 1% methanol in water sample was
deposited at 20 K at a rate of 2.5 mm/hour, and then warmed
to 140 K at approximately 1 K/min. For a detailed
description of the location on Enceladus of the tiger stripes
spectra, see Brown et al. [2006, Figure 1A].
L17103 HODYSS ET AL.: METHANOL ON ENCELADUS L17103
2 of 3
enough for methanol to condense. Methanol concentrations
should be highest immediately adjacent to the plume vents
and decrease with increasing distance from the vent, as
shown in Figure 2b. Scenario 2 is more complex: if the
methanol is generated photochemically from methane then
the amount of methane deposited becomes important. The
coldest areas of Enceladus will trap more methane than
warmer regions; the high temperatures of the tiger stripes
themselves (145 K) will prevent methane from sticking.
However, the amount of methane available for trapping in the
ice will be greater closer to the vent. The balance between
these two constraints will lead to a bimodal distribution, with
the greatest concentration of surface methanol some distance
from the plume vent (Figure 2c). Of course, some combina-
tion of these two processes may also occur, resulting in a
hybrid distribution. Further study of the photochemical
formation and degradation rates of methanol in water ice
would enable the construction of a model of the processes
described here. In combination with careful mapping of the
strength and position of the methanol feature across Encela-
dus’ surface, the model would constrain the origin of the
methanol, and yield insights into the role of plume deposition
and chemistry on Enceladus’ surface.
[11] Acknowledgments. This work was performed at the Jet Propul-
sion Laboratory (JPL), California Institute of Technology, under a contract
with the National Aeronautics and Space Administration (NASA). YLY
was supported by NASA grant NNG06GF33G to the California Institute of
Technology. Financial support through JPL’s Research and Technology
Development program is gratefully acknowledged. We thank Bonnie
Buratti and James Bauer (JPL) for providing the Cassini VIMS data, and
Arthur L. Lane (JPL) for useful discussions.
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J. D. Goguen, R. Hodyss, P. V. Johnson, I. Kanik, and J. V. Stern, Jet
Propulsion Laboratory, California Institute of Technology, Pasadena, CA
91109, USA. (robert.p.hodyss@jpl.nasa.gov)
C. D. Parkinson, Department of Atmospheric, Oceanic, and Space
Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
Y. L. Yung, Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA.
Figure 2. (a) A plume vent on Enceladus. The composition of the gasses condensed on the surface is related to the
temperature of the surface, which is warmer closer to the vent. (b) Surface methanol concentration as a function of distance
from the vent if methanol is deposited directly from the plume. (c) Surface methanol concentration as a function of distance
from the vent if methane is deposited from the plume and photochemically converted to methanol.
L17103 HODYSS ET AL.: METHANOL ON ENCELADUS L17103
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