Icarus 156, 136–142 (2002)
doi:10.1006/icar.2001.6763, available online at http://www.idealibrary.com on
Energy Distributions for Desorption of Sodium and Potassium
from Ice: The Na/K Ratio at Europa
R. E. Johnson
Engineering Physics, University of Virginia, Charlottesville, Virginia 22903
E-mail: [email protected]
F. Leblanc
Engineering Physics, University of Virginia, Charlottesville, Virginia 22903; and Service d’Aéronomie du CNRS, 91371 Vierrières-Le-Buisson, France
and
B. V. Yakshinskiy and T. E. Madey
Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers University, Piscataway, New Jersey 08854
Received June 7, 2001; revised September 13, 2001
The sputtering and decomposition of the surface of Europa by
fast ions and electrons lead to the production of an atomosphere
containing sodium and potassium atoms. Here time-of-flight energy
distributions are measured for Na and K sputtered from a vapordeposited ice by 200-eV electrons. These data are then used in a
Monte Carlo simulation for alkalis in Europa’s atmosphere. Na/K
ratios versus distance from Europa are calculated and compared
to the recent observations in the range 6 to 18 Europan radii from
the surface. Normalizing to the observations, the Na/K ratio for
the loss rates is ∼27 and the ratio for the average surface source
rates is ∼20. These ratios are very different from the Na/K ratio
at Io and are larger than the Na/K ratio suggested for Europa’s
putative subsurface ocean, consistent with fractionation on freezing
and upwelling of ocean material. c 2002 Elsevier Science (USA)
Key Words: Europa; surface; satellite; atmosphere; composition.
1. INTRODUCTION
Sodium and potassium atoms escaping from a number of Solar
System objects are seen via their resonance lines near the peak
of the solar spectrum. Early observations of Na near Io (Brown
and Chaffee 1974) were the first indication that a moon of Jupiter
had an escaping atmosphere (Matson et al. 1974). Subsequently,
Na was seen at Mercury (Potter and Morgan 1985), the Moon
(Potter and Morgan 1988), and more recently, Jupiter’s moon
Europa (Brown and Hill 1996). Whereas photodesorption appears to be a primary desorption process at the Moon and
Mercury (Yakshinskiy and Madey 1999, Mendillo et al. 1999),
Europa’s surface is exposed to an intense flux of energetic ions
and electrons trapped in Jupiter’s magnetic field (Cooper et al.
2001, Paranicas et al. 2001). This causes electronically induced
decomposition and sputtering of the surface (Johnson 1990,
Johnson and Quickenden 1997, Madey et al. 1998). Therefore,
the ultimate source of sodium and potassium for Europa’s atmosphere has been suggested to be the radiation-induced decomposition of a surface brine (Johnson 2001). This is thought to be
material from Europa’s putative underground ocean which may
have reached the surface at various times in the past. If this is the
case, then the sputter-produced atmosphere should contain other
species from this ocean which might be detected by remote sensing (Johnson et al. 1998), an exciting prospect. Alternatively, the
observed Na and K atoms in Europa’s atmosphere could be material from Io implanted into Europa’s surface and subsequently
sputtered (Brown and Hill 1996).
The observation of alkalis at Europa is consistent with models
suggesting that Europa has frozen, hydrated salts on its surface
(Fanale et al. 1974, Kargel 1991, McCord et al. 1999, 2001,
Zolotov and Shock 2001). Charged particle irradiation can preferentially deplete such materials of sodium (Wiens et al. 1997,
Chrisey et al. 1988, Madey et al. 1998), leaving behind, for
instance, the frozen hydrated sulfuric acid (Johnson 2001) suggested by Galileo spacecraft data (Carlson et al. 1999). Most of
the ejected sodium atoms return to the surface but their large
excursion distances lead to redistribution across the surface of
Europa. This populates the icy regions of Europa with alkalis as
would implanation from the jovian plasma or micrometeorites.
Therefore, measurements of the desorption of alkalis from an
ice surface are needed.
Yakshinskiy and Madey (2001) measured the electronically
stimulated desorption efficiency of potassium from an ice surface as well as the mean ejection energy. In this paper we present
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c 2002 Elsevier Science (USA)
All rights reserved.
SODIUM/POTASSIUM RATIO AT EUROPA
the first laboratory measurements of the energy distributions for
electronically stimulated desorption (sputtering) of sodium and
potassium from ice. These data are used in a recently developed
Monte Carlo simulation for a sputtered-produced atmosphere at
Europa (Leblanc et al. 2001). The dependence of the sodium and
the potassium densities versus distance from Europa’s surface is
calculated and used to obtain the Na/K ratio in Europa’s atmosphere. These are compared to the ratios obtained from Brown’s
(2001) observations. In this manner the relative surface source
rates for Na and K are determined and the Na/K ratios in the optical layer and at depth are estimated. It is found that the ratio of
the loss rates is somewhat larger than the solar abundance ratio
(e.g., Allen 1991) and is larger than the proposed concentration
ratios suggested for Europa’s ocean, consistent with fractionation during upwelling (Kargel 1991, Zolotov and Shock 2001).
It is also very different from the Na/K ratio at Io, in agreement
with Brown’s (2001) conclusions.
2. LABORATORY SIMULATION
The experimental measurements were performed at Rutgers
University in an ultrahigh vacuum environment. We dose the
surface of an amorphous ice (formed by vapor deposition onto a
metal substrate held at 100 K) with Na and K atoms from SAES
Getters sources (Yakshinskiy and Madey 2001). Upon adsorption of the alkalis, electron transfer to the ice forms solvated
Na+ and K+ in the surface and subsurface layers (Barnett and
Landman 1993, Fuke et al. 1999) as indicated by X-ray photoelectron spectroscopy (Yakshinskiy and Madey 2001). In
Europa’s icy equatorial regions sublimation occurs and elsewhere the bombarding plasma produces H2 O vapor by sputtering the ice-rich regions. As essentially all of the sublimed H2 O
and ∼80% of the sputtered H2 O return to the surface (Johnson
1990), Europa’s surface layer is formed by vapor deposition as
in the laboratory experiment. In addition, most of the Na and K
atoms that are desorbed or sputtered from Europa return to the
surface, and a large fraction of those adsorb in ice-rich regions
(Johnson 2000). Therefore, the procedure for formation of the
laboratory samples roughly reproduces the atmospheric Na or
K absorbed on ice grains in Europa’s regolith. The results here
may also be representative of loss of alkalis from the hydrated
minerals on Europa’s surface, but the dominant source of atmospheric alkalis is sputtering (desorption) from ice-rich regions
(Johnson 2000).
At temperatures on the order of 100 K, the solvated alkalis
have a relatively low diffusion rate so the adsorbed alkalis would
typically be surficial. However, diffusion of the adsorbed or intrinsic alkalis is enhanced under charged-particle irradiation.
This irradiation-enhanced diffusion is important in maintaining
a surface population of alkalis (McGrath et al. 1986) but it also
makes the laboratory measurement of the absolute desorption
cross section difficult.
Photodesorption is the likely dominant steady-state source of
the lunar sodium atmosphere (Yakshinskiy and Madey 1999),
137
based on the measured photodesorption cross section and energy
distributions. In contrast, electronic sputtering by the fast ions
and electrons is believed to be the dominant source of desorbed
alkalis at Europa (Johnson 2000, Johnson et al. 1998). That is, the
jovian charged particle energy flux to Europa’s surface is dominated by energetic (>10 keV) protons and electrons (Cooper
et al. 2001, Paranicas et al. 2001). These incident particle primarily produce secondary electrons from a few eV to ∼1 keV
(average energy ∼50 eV). Since desorption of alkalis from ice
exhibits a threshold less than ∼4 eV, with the yield increasing
with increasing electron energy (Yakshinskiy and Madey 2001),
the seconday electrons act to desorb alkalis. The desorption
mechanism has been described by Yakshinskiy and Madey
(1999, 2001). It involves electron transfer to adsorbed ionic Na+
to make neutral Na in a repulsive state, causing the neutral to
desorb. Therefore, we bombard ice surfaces dosed with either Na
or K atoms using 200-eV electrons, representative of the upper
limit of secondary electrons produced by the incident energetic
ions and electrons. We note that except near threshold, the ejecta
energy destributions are very insensitive to the incident electron
energy.
Using the surface ionization detector described elsewhere
(Yakshinskiy and Madey 2000) and time-of-flight analysis of the
ejected alkalis, the energy distributions of the desorbed sodium
and potassium are obtained. Since incident electrons are also
capable of removing H2 O (Johnson 1990, Sieger et al. 1998),
the total amount of material removed in obtaining a spectrum is
kept to less than a monolayer.
For the energy distributions derived from measured time-offlight distributions shown in Fig. 1, experimental constraints
FIG. 1. Measured energy distributions of Na (stars) and K (triangles) desorbed by 200-eV electrons; data displayed normalized to 1 at their peak values.
Lines are fits using C EU x /(E + U )2+x with U = 0.052 eV and x = 0.7 for
Na and U = 0.02 eV and x = 0.25 for K with extended tails (E −2 ) fitted to the
last few data points. C is a normalization constant. Inset: Energy distribution of
Na (stars) and energy spectrum (dashed line) extracted independently from the
observations of Europa sodium clouds (Leblanc et al. 2001).
138
JOHNSON ET AL.
cause uncertainities at very low energies (<0.02 eV) and very
high energies (>1 eV). The spectra are shown normalized at
the peak values and are seen to have a dependence roughly that
seen in other electronic sputtering processes. Each exhibits a
peak at low energies with a slowly decreasing but nonthermal
tail which decays roughly as E −2 (Johnson and Liu 1996). The
Na distribution is broader than the K distribution. The peak occurs at ∼0.04 eV for Na and at a slightly smaller energy for
K. Within the uncertainties (∼ ± 0.01 eV at the peaks), these
are similar to that found in the sputtering of H2 O by keV heavy
ions (∼0.03 eV; Reimann et al. 1984). The electronic sputtering of ice by fast light ions also exhibits a large contribution at
low ejecta energies (Johnson 1998). That these spectra all have
significant low-energy contributions may be due to the fact that
the surface is porous and composed of a frozen volatile. The
maxima in the energy spectra measured here are lower than that
observed for sputtering of Na from Na2 SO4 by energetic heavy
ions (∼0.14 eV) (Wiens et al. 1997). Moreover, all of these spectra exhibit nonthermal tails at the largest ejecta energies, which
for incident heavy ions have a momentum transfer component
that extends to higher energies than measured here.
In analyzing his observations, Brown (2001) assumed the energy spectra for ejection of Na and K were roughly the same.
Here it is seen that this is not an unreasonable assumption
although there are noticeable differences, as seen in Fig. 1.
Yakshinskiy and Madey (1999, 2000, and in preparation) also
measured the energy distributions for desorption of Na and K
from silicon dioxide by electrons). The mean energy of K ejected
from a silicate is also slightly lower than that for Na, but different energetics are found for desorption of Na and K from other
oxide surfaces (Madey et al. 1998). Assuming a linear dependence at low E and a form C EU x /E + U )2+x , where x and U
are fitting parameters and C is a normalization constant, the data
were fit. This form for the energy spectra of Fig. 1 was used in
the atmospheric model described below.
3. ATMOSPHERIC MODEL
Leblanc et al. (2001) (hereafter, LJB) constructed a Monte
Carlo model of the sputtered sodium atmosphere at Europa.
This was normalized to an extensive set of observations. In this
model, atoms are launched from the surface according to a spatial, energy, and angular distribution determined by sputtering
of Europa’s surface. The ejected atoms are tracked through the
ambient O2 atmosphere (∼1015 O2 /cm2 ; Hall et al. 1995) until
they return to the surface, reach the boundary of our simulation
volume, or are ionized by electrons and swept away by the jovian
magnetosphere’s motional electric field. The simulation is carried out in Europa’s rest frame accounting for the gravitational
forces of Jupiter and Europa. In the simulation the centrifugal, Coriolis, and radiation pressure forces, as well as the local
ionization rate, are all accounted for as described in LJB. Because of Jupiter’s significant gravity, a “banana-shaped” cloud
of Na is seen. Figure 2 is from LJB with the surface source rate
FIG. 2. Simulated Na cloud at Europa showing position of Europa relative to Jupiter at which modeling was done, near western elongation (∼290◦ ). Jupiter
is represented by the circle on the axis x = 0. Lines are contours of equal density, and axes are indicated: positive x toward the sun; y-axis is the E/W direction in
Fig. 3 with positive y away from Jupiter and N/S perpendicular to the plane shown. Scale on right is density (Na/cm3 ) in powers of 10 (from Leblanc et al. 2001).
SODIUM/POTASSIUM RATIO AT EUROPA
normalized to the sodium observations. The spatial distribution
of the surface source in LJB was assumed to vary as the cosine
of the angle from the apex to the hemisphere trailing Europa’s
motion. This is the hemisphere preferentially irradiated by the
energetic ions (Pospieszalska and Johnson 1989, Cooper et al.
2001) and electrons (Paranicas et al. 2001). The LJB simulation described the observations well within ∼20 RE but was less
successful at much larger distances due to temporal variability,
Europa’s changing position during the measurements, and Io’s
sodium. Here we focus on distances less than 20 RE for which
Brown (2001) observed the Na/K ratios.
In modeling the sodium cloud, LJB suggested an energy spectrum which appeared to reproduce the principal aspects of the
cloud morphology. This spectrum is remarkably close to that
reported in Fig. 1. The inset to Fig. 1 shows the agreement between the energy distribution found in LJB (dashed line) and
the laboratory measurements of the sodium energy distribution
described in Section 2 (stars). Therefore, the measurements presented here support the assumption that the observed sodium
is mostly ejected from icy surface layers (Johnson 2000). In
the simulation described above we use the fits to the measured
energy spectra shown in Fig. 1. For both Na and K atoms we
extend the data in Fig. 1 to higher ejecta energies by attaching
an energetic tail. This roughly accounts for a contribution due
to collision cascade sputtering from more reactive materials, as
discussed in LJB. At the distances of interest here (<∼20 RE ),
the densities are not very sensitive to the tail nor are they very
sensitive to the spatial variation in the ionization lifetime and to
radiation pressure. Therefore, for K we use an average electron
impact ionization rate which is ∼1.2 times (Brown 2001) that
of the local Na ionization rate in LJB.
The simulation region in LJB is divided into cells, and the
times for atoms ejected from the surface to pass through each
cell are recorded and summed. From the residence times the
steady-state density in each cell can be calculated using a surface flux and the number of atoms in the simulation. Using the
local densities the morphology of the sodium and potassium
clouds can be calculated as shown for sodium in Fig. 2. Since
the atoms in each cloud can be seen by their resonance fluorescence, we calculate the line-of-sight column densities. That is,
we integrate the number of atoms per unti volume lying along an
observation direction. Rather than normalize the Na and K simulations separately to the observations, we calculate the ratios
of the line-of-sight column densities versus distance. Therefore,
this calculation has only one free parameter, the ratio of the rate
of desorption of sodium and potasium. In Fig. 3a we give the ratios of the line-of-sight column densities versus distances from
Europa using the fits to the energy distributions in Fig. 1. In
Fig. 3a we use equal surface source rates for Na and K. This
allows the actual surface source rates to be obtained by scaling
to observation. These ratios are calculated for Europa near the
western elongation (∼290◦ ), the geometry shown in Fig. 2. Ratios of the line-of-sight column densities are calculated versus
distance from Europa along axes through Europa’s position:
east/west (E/W) scans toward Jupiter (east) and away from Jupiter
139
FIG. 3. The ratios of the line-of-sight column densities of Na to K versus.
distance from Europa given in Europa radii, RE . Line-of sight densities are
integrations of the density along the x-axis. Europa is near the western elongation
(Fig. 2). (a) Scans of simulation data along east/west axis in Europas orbital plane
with surface source rates Na/K = 1. Circle line: E/W, toward Jupiter (−y).
Square line: E/W away from Jupiter (+y). The bars indicate the root mean
square deviation for three runs with an average of 720,000 particles in each run.
(b) Squares with bars are data of Brown (2001). Line is average of the scans in
(a) with surface source ratio Na/K = 20.
(west) inside Europa’s orbital plane. The differences of the ratios
in Fig. 3a are primarily due to the differences in mass between K
and Na and to the shifted energy spectrum for K compared to Na.
Because the Na/K ratios in Fig. 3a are larger than one for equal
source rates, sodium atoms more easily reach the distances considered. It is also seen that the dependence on distance from
Europa is somewhat different in the two observation directions,
primarily due to the influence of Jupiter. As is the case for neutrals from Io’s atmosphere, and as seen in Fig. 2, the atoms
preferentially “fall” toward Jupiter. Therefore, the Na/K ratio is
somewhat smaller in the jovian direction, as K can escape more
readily and remain in the atmosphere longer than it does on the
antijovian side of Europa.
Shown in Fig. 3b are the ratios of the sodium and potassium
column densities near Europa observed by Brown (2001). These
data are extracted from the line intensities for sodium and potassium. They are averages of observations on either side of Europa
140
JOHNSON ET AL.
TABLE I
Na/K
Europa atoms observeda
Europa escapeb
Europa surface source rateb
Europa oceanc
Solar d
Io atmospherea
Lunar atmospheree
Mercury atmosphere f
Earth oceang
25
27
20
14–19
20
10
6
80–190
45
a
Brown (2001).
Calculated here.
c Zolotov and Shock (2001). They give mass ratio
8–11. These are number density ratios.
d Allen (1991).
e Potter and Morgan (1997).
f Potter and Morgan (1988).
g Lide (1996).
b
roughly along the E/W direction (Brown 2001) and are plotted
versus distance from Europa. At these distances, the Na/K ratios are nearly independent of the distance from Europa. This is
consistent with the average of the E/W scans from the simulations in Fig. 3a, which is also shown in Fig. 3b normalized to
Brown’s (2001) data. The ratios calculated in this region using
the measured velocity distribution are also nearly independent
of distance from Europa.
By normalizing the average of the E/W scans in Fig. 3a to the
observations in Fig. 3b, we obtain a ratio, Na/K ≈ 20 ± 4, for the
average surface source rates. From these simulations we can also
obtain the ratio of the net loss rates of sodium and potassium.
Primarily because of the effect of Jupiter, this is not simply a
ratio of that fraction of the energy spectra in Fig. 1 which is larger
than Europa’s escape energy. We found that ∼30% of the Na and
∼25% of the K ejected from the surface are lost by either leaving
the simulation region or being ionized in the simulation region. If
we ignore the fraction of those ionized alkalis that are picked up
and accelerated back into the surface, the globally averaged loss
ratio obtained from the above numbers (Na/K) is ∼1.3 times the
ratio of the source rate. This is similar to the atmospheric ratios
in Fig. 3a. Using the ratio of the surface source rates estimated
above, the Na/K ratio for the loss rates is ∼27. Since the ionized
fraction reimpacting the surface for K is likely larger than that
for Na, this is a lower limit.
Although the source rate for sodium was converted into a
rough estimate of sodium surface concentration in Johnson
(2000) and LJB, this required estimating the average sputtering yield and the charged particle flux. To convert the ratio of
source rates into a ratio for the surface concentrations requires
knowledge of the relative desorption cross sections. Yakshinskiy
and Madey (2001) estimate roughly similar desorption cross
sections for 200-eV electrons. These are ∼5 × 10−19 cm2 (within
a factor of 2) for the Na and the K. Therefore, to first order, the
ratio of the average surface concentration is the same as the ratio
of the source rates.
In Table I, the Na/K ratios found here are compared to the solar
value and the atmospheric ratios at Io, the Moon, and Mercury.
Possible sources of alkalis at Europa are Na+ and K+ which
are formed at Io and eventually implanted in Europa’s surface.
Since these are both minor ions in the jovian plasma, there are no
accurate measurements of this ratio in the plasma near Europa.
However, we showed earlier that the implantation rate is much
smaller than the loss rate (Johnson 2000, Leblanc et al. 2001).
It is also seen in Table I that, as pointed out by Brown (2001),
the Na/K ratio at Io is very different from that found at Europa,
suggesting that Io is not a primary source of the alkalis observed.
4. DISCUSSION
The chemical composition of the optical surface of Europa
(∼1 mm) is altered by the incident radiation (Carlson et al.
1999, Cooper et al. 2001). In this layer implantation and sputtering of alkalis compete with alkalis brought to the topmost
atomic layers by mixing or by irradiation-enhanced diffusion.
Due to sputter redistribution across Europa’s surface, the source
of alkalis for the atmosphere is likely dominated by electronic
sputtering (desorption) from the icy regions (Johnson 2000).
Such regions are also the source of the dominant atmospheric
component, O2 (Johnson 1990). Comparing the estimated loss
rate to the implantation rate, shows Europa to be a net source of
sodium (Johnson 2000, Leblanc et al. 2001). Brown (2001) also
concluded this by comparing the observed atmospheric ratios
of Na/K at Europa and Io. In this he assumed that the energy
spectra for ejected Na and K were identical and estimated the
sputter contribution to the observed alkali atmosphere.
Here we present measurements of the energy spectra (Fig. 1)
for electronic sputtering of Na and K from ice. The energy spectrum for desorbed sodium atoms is close to that extracted by
Leblanc et al. (2001) from observations (see inset to Fig. 1).
This supports the assumption in our previous work that electronic sputtering from an icy surface is the dominant ejection
process for sodium. It can be seen in Fig. 1 that the ejected
sodium and potassium energy spectra are similar, the potassium
ejecta being on average somewhat less energetic. These newly
measured energy distributions were used in a recent model to
simulate the Na and K atmospheres at Europa. The calculated
Na/K ratios of the line-of-sight column densities from the simulations were then normalized to the ratios of the line-of sight
column densities observed by Brown (2001). This allowed us to
obtain relative surface source rates and relative loss rates.
In Fig. 3a we give the ratios of line-of-sight column densities
for Na and K calculated from the alkali cloud simulations in
which the newly measured energy distributions were used. These
ratios can be scaled either to known surface source rates or to
new observations as they become available. The averages of the
E/W scans in Fig. 3a were normalized here to the observations
in Fig. 3b and are seen to have roughly the same dependence on
SODIUM/POTASSIUM RATIO AT EUROPA
distance from Europa as the observations. On normalizing the
Na/K ratios to the observations of Brown (2001), the ratio of the
average surface source rates is ∼20 ± 4.
Because the desorption efficiencies are similar for Na and K,
the ratio of the concentrations in the optical layer is also ∼20. The
globally averaged Na atomic concentration in the surface layer
is ∼0.01–0.005 (Johnson 2000, Leblanc et al. 2001), implying a
potassium concentration of ∼0.0005–0.0003 in the optical layer.
Since the surface can be depleted in the alkalis, and the incident
fast ions and electrons cause diffusion of alkalis in the irradiated
layer, the ratio of surface concentrations is not the best indicator
of the concentration below the layer penetrated by the radiation.
In steady state, the ratio below the charged-particle penetration
depth is roughly equal to the ratio of the total loss rates (Johnson
and Sittler 1990). Using the fractions of the sputtered sodium and
potassium determined from the simulations, the concentration
ratio, Na/K, below the radiation penetration depth is ∼27.
The Na/K ratios obtained here are compared in Table I to the
cosmic ratio, the ratios for other objects, and the ratio expected
in Europa’s putative ocean. In agreement with Brown (2001),
the Na/K ratio at Europa differs significantly from that at Io.
Therefore, Io is not likely to be the source of the observed alkalis. The fact that Europa has endogenic sources of alkalis lends
support to the hypothesis of a geologically active surface, possibly associated with an underground ocean. This could be the
“salty” Europa proposed by a number of authors (Fanale et al.
1974, Kargel 1991, Zolotov and Shock 2001). Preferential loss
of alkalis from such salts would be consistent with atmospheric
alkalis and with the observation of hydrated sulfuric acid in the
dark chaos regions (Carlson et al. 1999, Johnson 2001).
The Na/K ratio, ∼27, obtained here for a depth under the surface below that for radiation penetration is somewhat larger than
the ratio for the cosmic abundance. This ratio is also larger than
that predicted by models of Europa’s ocean based on formation
of Europa from carbonaceous chondrites (∼14–19; Zolotov and
Shock 2001). Increased fractionation occurs during freezing and
dehydration of the material as it reaches Europa’s surface, enhancing the Na/K ratio (Zolotov and Shock 2001). Therefore, the
relative loss rates determined here and the relative concentrations
in the model ocean are not inconsistent. Since sodium atoms are
lost only ∼1.3 times faster than potassium atoms and the surfaces are relatively young, the globally averaged ratio (∼27)
can be used to constrain models for fractionation on transport
of warm ice to Europa’s frozen surface.
ACKNOWLEDGMENTS
This work was supported by grants to the University of Virginia by the NASA
Planetary Geology and Geophysics Program, the NASA Planetary Atmospheres
Program and the NSF Astronomy Program, and by a grant to Rutgers University
from NASA’s Planetary Atmospheres Program.
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