Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon
  from IRTF/SpeX observations
Distributions of H2O and CO2 ices on Ariel, Umbriel,
Titania, and Oberon from IRTF/SpeX observations
W.M. Grundy1,2, L.A. Young1,3, J.R. Spencer3, R.E. Johnson4,
E.F. Young1,3, and M.W. Buie2
1. Visiting/remote observer at the Infrared Telescope Facility, which is operated by the University
of Hawaii under contract from NASA.
2. Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff AZ 86001.
3. Southwest Research Institute, 1050 Walnut St., Boulder CO 80302.
4. Univ. of Virginia Dept. Engineering Physics, 116 Engineer’s Way, Charlottesville VA 22904.
Published in 2006: Icarus 184, 543-555.
Abstract
We present 0.8 to 2.4 µm spectral observations of uranian satellites, obtained at IRTF/SpeX on 17
nights during 2001-2005.  The spectra reveal for the first time the presence of CO2 ice on the
surfaces  of  Umbriel  and Titania,  by means of  3 narrow absorption bands near  2 µm.  Several
additional, weaker CO2 ice absorptions have also been detected. No CO2 absorption is seen in
Oberon spectra, and the strengths of the CO2 ice bands decline with planetocentric distance from
Ariel through Titania.  We use the CO2 absorptions to map the longitudinal distribution of CO2 ice
on Ariel, Umbriel, and Titania, showing that it is most abundant on their trailing hemispheres.  We
also  examine  H2O  ice  absorptions  in  the  spectra,  finding  deeper  H2O  bands  on  the  leading
hemispheres  of  Ariel,  Umbriel,  and  Titania,  but  the  opposite  pattern  on  Oberon.   Potential
mechanisms to produce the observed longitudinal and planetocentric distributions of the two ices
are considered.
KEYWORDS: Ices; satellites of Uranus; surfaces of satellites; infrared observations; spectroscopy.
1.  Introduction
The discovery of CO2 ice on the surface of the uranian satellite Ariel (Grundy et al. 2003) raised a number
of questions regarding its origin and ultimate fate. Is it a relic of Ariel’s primordial volatile inventory?  Was it
delivered by more recent impactors?  Or is it being produced in situ in chemical reactions enabled by energetic
radiation?  How long does exposed CO2 ice survive on the satellite’s surface, and what processes act on it?
How stable is it against sublimation and sputtering?  Does it participate in radiolytic chemistry, such as envi-
sioned by Delitsky and Lane (1997, 1998), Hudson and Moore (2001), and Johnson (2001)? 
Questions such as these could potentially be addressed by means of a survey of CO2 ice as a function of
longitude and planetocentric distance among the major uranian satellites, the basic physical properties of which
are listed in Table 1. The results of such a survey of the four largest satellites are presented in this paper.
Table 1.  Major satellites of Uranus.
Satellite
Distance from
Uranus
(105 km)
Orbital Period
(days)
Radiusa
Massb
(1023 g)
Surface gravity
 (cm s-2)
 Visual
full disk
albedoc
Miranda 1.30   1.41 236    0.66  7.9 0.307
Ariel 1.91   2.52 579 13.5  26.9 0.350
Umbriel 2.66   4.14 585 11.7  22.9 0.189
Titania 4.36   8.71 789 35.3  37.8 0.232
Oberon 5.81 13.46 761 30.1  34.7 0.205
aRadii are from Thomas (1988).
bMasses are from Jacobson et al. (1992).
cFull disk albedos at 0.55 µm and 1˚ phase are from Karkoschka (1997) Table II.
2.  Observations and Reduction
We obtained infrared spectroscopy of uranian satellites on seventeen nights during 2001-2005 at NASA’s
Infrared Telescope Facility (IRTF) on Mauna Kea, as tabulated in Table 2. The data were collected using the
short cross-dispersed mode of the SpeX spectrograph (Rayner et al. 1998, 2003), which covers the 0.8 to
2.4 µm wavelength range in five spectral orders, recorded simultaneously on a 1024 × 1024 InSb array. Urani-
an satellite observations were done at an average airmass of 1.26. Maximum airmasses were 1.61, 1.77, 1.57,
and 1.46 for Ariel, Umbriel, Titania, and Oberon, respectively. When observing at higher airmasses, we used
an image rotator to keep SpeX’s slit oriented near the parallactic angle on the sky-plane, to minimize
wavelength-dependent slit losses from differential atmospheric refraction. During 2001-2002 we used the
0.5 arcsec slit, and during 2003-2005 we used the 0.3 arcsec slit, resulting in measured spectral resolutions
(λ/∆λ for unresolved OH airglow emission lines) between 1300 and 1400 and between 1600 and 1700, respect-
ively.
All spectra were collected as pairs, with the telescope being offset along the slit between two locations, re-
ferred to as A and B beams. Pairs of A and B spectral frames were subtracted prior to extraction using the
Horne (1986) optimal extraction algorithm as implemented by M.W. Buie et al. at Lowell Observatory (e.g.,
Buie and Grundy 2000).
Table 2.  Circumstances of observations
UT date of observation
mid-time
Sky conditions and
H band image size
Sub-observer
longitude ( )˚
Sub-observer
latitude ( )˚
Phase
angle ( )˚
Total integration
(min)
Ariel
2001/07/05.59 Cirrus, 1.1” 233.8 -23.1 1.91   50
2001/07/08.61 Clear, 1.1” 304.8 -23.2 1.79   48
2002/07/16.55 Partly cloudy, 0.7” 294.8 -19.3 1.65 140
2002/07/17.56 Partly cloudy, 0.8”   79.8 -19.4 1.60 108
2003/08/05.50 Clear, 0.6” 200.0 -15.9 0.95   84
2003/08/09.51 Clear, 0.6”   53.6 -16.0 0.75 156
2003/09/07.40 Clear, 0.6” 219.8 -17.2 0.70   90
2003/10/04.24 Clear, 0.6”   93.5 -18.1 1.88 108
2003/10/08.33 Clear, 0.9” 316.6 -18.2 2.03 132
2004/07/15.50 Clear, 0.6” 159.9 -11.1 1.99 112
Umbriel
2001/07/07.59 Cirrus, 1.0” 219.8 -23.0 1.83   52
2004/07/05.59 Thin cirrus, 0.8” 216.2 -10.9 2.32   80
2004/07/16.60 Thin cirrus, 0.8”   92.1 -11.1 1.95   74
2004/07/27.48 Clear, 0.6” 317.6 -11.4 1.51 184
2005/09/18.39 Clear, 0.6” 159.9 -  9.4 0.85 196
Titania
2001/07/06.56 Cirrus, 0.9” 237.0 -23.0 1.87   56
2001/07/07.55 Cirrus, 1.0” 277.8 -23.0 1.83   36
2003/10/08.24 Clear, 0.9”   98.0 -18.1 2.03   64
2004/07/15.60 Clear, 0.6” 213.9 -11.1 1.98   64
2005/10/13.38 Clear, 0.6” 299.6 -10.2 1.93 106
Oberon
2001/07/06.60 Cirrus, 0.9” 164.0 -23.0 1.87   32
2001/07/08.55 Clear, 1.1” 216.2 -23.1 1.79   48
2005/10/13.27 Clear, 0.6” 110.6 -10.2 1.93 124
Uranian satellite observations were interspersed between observations of nearby solar analog reference
stars HD 210377 and HD 214572. We also observed more distant, but better-known solar analogs 16 Cyg B,
BS 5968, BS 6060, HD 219018, and SA 112-1333. Those observations showed that HD 214572 is a reasonable
solar analog in this spectral range, and that HD 210377 was also usable as a solar analog after correcting for its
effective temperature, which is a few hundred K cooler than the sun. From the solar analog observations ob-
tained each night we computed telluric extinction and corrected the star and satellite observations to a common
airmass. We then divided the airmass-corrected satellite spectra by the airmass-corrected solar analog spectra.
This operation eliminated most instrumental, stellar, and telluric spectral features, and produced spectra pro-
portional to the satellite disk-integrated albedos. Cancellation of telluric features was sometimes imperfect near
1.4 and 1.9 µm, where strong and narrow telluric H2O vapor absorptions make sky transparency especially
variable in time. Additional spurious features originate in scattered light from Uranus, such as at 0.94 µm.
These could be either positive or negative, depending
whether Uranus was closer to the object or sky beam
position, and only affected Umbriel and Ariel spectra
for longitudes near 0 or 180˚. Examples of final, nor-
malized albedo spectra are shown in Fig. 1.
Wavelength calibration was derived from telluric
airglow emission lines, primarily those of OH, ex-
tracted from the satellite frames. The dispersion
ranges from 0.00023 to 0.00054 µm per pixel from
the shortest to longest wavelengths. The wavelength
uncertainty is no more than a pixel in regions having
abundant sky emission lines, which include the
wavelengths where the satellites exhibit absorption
features analyzed in this paper.
Our spectra could not be photometrically calib-
rated, since variable slit losses from different tracking
time scales combine with variable focus and seeing to
undermine photometric fidelity of comparisons
between the faint satellites and the much brighter sol-
ar analog stars. However, other sources of photo-
metry exist for the uranian satellites in our wavelength range (e.g., Karkoschka 1997) and these could be used
to scale the spectra to an albedo scale, enabling the use of quantitative radiative transfer models.
3.  Analysis
The spectra of all four satellites in Fig. 1 look broadly similar to one another. A gentle reddish spectral
slope extends from 0.8 to about 1.4 µm, followed by a water ice absorption complex between 1.45 and 1.7 µm,
and two more H2O ice absorptions between 1.9 and 2.2 µm and from 2.25 µm to the limit of our spectral cov-
erage at 2.4 µm. The H2O ice bands are weakest in the spectrum of Umbriel, the satellite with the lowest al-
bedo of the four (Karkoschka 1997). Lower albedos and weaker H2O bands typically go together, since the
same dark materials (probably carbonaceous, in the case of uranian satellites) which suppress albedos are also
effective at masking H2O ice absorptions. Although this dark material has a profound effect on the satellites’
albedos, H2O ice could still be the most abundant surface material on all four satellites, considering that
thermal segregation of H2O ice, as seen on the jovian satellites, is improbable at the low surface temperatures
of the uranian satellites (Spencer 1987). Impact gardening probably leads to intimate mixing between dark and
Figure 1: Example spectra of uranian satellites, normalized at
J band and offset upward with zero levels indicated by tick marks
on the ordinate axis. Gaps around 1.4 and 1.85 µm coincide with
high atmospheric opacity. The Ariel spectrum is an average of all
spectra with subsolar longitudes between 210˚ and 330˚. The
Umbriel spectrum is from 2005/09/18 UT and the Titania and
Oberon spectra are both from 2005/10/13 UT.
icy components, enabling even small quantities of dark material to significantly suppress surface albedos
(Clark 1981).
3.1.  Water ice
The shapes of the H2O ice absorption bands and their depths relative to one another, especially between
1.50 and 1.57 µm and around 1.65 µm, confirm that most of the H2O ice on all four satellites is crystalline at
the ∼mm depths sampled by these wavelengths (e.g., Grundy et al. 1999; Hansen and McCord 2004). Amorph-
ous ice lacks 1.57 and 1.65 µm absorption bands and exhibits somewhat different shapes for the 1.5 and 2 µm
bands (Grundy and Schmitt 1998; Schmitt et al. 1998). The H2O features in the satellite spectra are consistent
with simple Hapke models (e.g., Hapke 1993) having no amorphous ice at all, although we can include up to
10 to 20% amorphous ice before the discrepancies become conspicuous.
This predominance of crystalline ice is not surprising, since it is the more thermodynamically stable, lower
energy phase. Crystalline H2O has also been reported on other outer solar system surfaces at comparable and
greater heliocentric distances, both icy satellites and trans-neptunian objects. In fact, everywhere that the phase
of H2O ice on these objects’ surfaces has been identified, it is crystalline (e.g., Buie and Grundy 2000; Bauer et
al. 2002; Grundy and Young 2004; Jewitt and Luu 2004).
Under certain circumstances, energetic charged particles or UV photons can disrupt the crystal structure of
ice (e.g., Kouchi and Kuroda 1990; Johnson 2000; Moore and Hudson 1992; Johnson and Quickenden 1997),
but evidently this effect is so inefficient that ice on the uranian satellites is not amorphous at mm depths.
Nature does tend to find its way to thermodynamically favored states such as crystallinity, in the case of H2O
ice. Thermal recrystallization goes as e
, where EA is an activation energy, k is the Boltzmann constant,
and T is the temperature. Laboratory studies of crystallization rates of pure H2O ice at much warmer temperat-
ures (Delitsky and Lane 1998; Jenniskens et al. 1998), if extrapolated over many orders of magnitude, suggest
that thermally activated crystallization could be extremely slow at uranian satellite surface temperatures.
However, even if the low thermal crystallization rates implied by that extrapolation are correct, the presence of
impurities in the ice, or of non-thermal energy inputs from less energetic charged particles, photons, or even
micrometeorites may help overcome thermal limits (e.g., Brown et al. 1978). Sputtering might also remove
H2O more efficiently than it is amorphized. Consequently, we do not subscribe to the idea proposed by Bauer
et al. 2002 and by Jewitt and Luu (2004) that the existence of crystalline ice on the surfaces of these bodies im-
plies recent heating episodes.
The H2O ice absorption bands of jovian and saturnian satellites tend to be deeper on their leading hemi-
spheres (e.g., Clark et al. 1984; Grundy et al. 1999; Cruikshank et al. 2005). Similar behavior is exhibited by
the uranian satellites, except for Oberon, as shown in Fig. 2. Integrated areas of the 1.5 µm water ice absorp-
tion complex were computed by normalizing each spectrum to a line fitted to continuum wavelengths on either
side of the absorption band (1.29 to 1.32 and 1.72 to 1.75 µm, in this example), then integrating one minus the
normalized spectrum over the band interval (1.42 to 1.72 µm). Ariel, with the most complete longitudinal
sampling, shows a sinusoidal pattern of variation of its integrated H2O ice band, with maximum H2O absorp-
tion coinciding with the leading hemisphere (90˚ longitude in the left-handed IAU system). Fewer longitudes
were observed for the other satellites, but Umbriel and Titania both show more H2O absorption on their leading
hemispheres. The trend for Oberon appears to be reversed, but confirming observations are needed.
Magnetospheric charged particle bombardment can drive sputtering as well as radiolytic destruction of H2O
ice, and could be responsible for the observed leading-trailing asymmetries. The magnetic field of Uranus ro-
tates rigidly, at the same frequency as Uranus spins on its axis, with a period of 17.9 hours. The satellites orbit
more slowly (see Table 1), so the magnetosphere overtakes them from behind and its charged particles prefer-
entially strike their trailing hemispheres (Cheng et al. 1986; Lanzerotti et al. 1987). Magnetospheric charged
particles spiral along the magnetic field lines at gyroradii inversely proportional to the strength of the magnetic
field. The dipole component of the field diminishes as the inverse cube of the distance, so gyroradii grow as the
cube of the distance from Uranus. For protons, the dominant ions in the uranian magnetosphere (Bridge et al.
1986), the energies at which gyroradii match the radii of the satellites range from ∼100 keV for Miranda down
to <1 keV for Titania, spanning the ‘dominant’ ∼10 keV energy range for magnetospheric protons (Johnson
2000). That these protons have gyroradii larger than the radii of the satellites in the outer parts of the system
suggests that outer satellites should receive more isotropicially distributed proton bombardment. This tendency
is moderated by the fact that the net plasma flow past the satellites, arising from the greater speed of the urani-
an magnetic field relative to the Keplerian motion of the satellites, is faster further from Uranus. This net
plasma flow increases from 6 up to 39 km sec-1 from Miranda to Titania. However, these speeds correspond to
proton energies only 0.2 to 8 eV, far below 10 keV energies. Although 10 keV protons circle field lines in or-
bits larger than the radii of satellites from Umbriel on out, the orbits are executed relatively quickly compared
with the rate at which the magnetic field sweeps past the satellites. Orbital periods for protons are inversely
Figure 2: Variation of integrated area of the 1.5 µm H2O ice band complex as a function of sub-solar/sub-viewer longitude,
showing large cyclical variations with more H2O ice absorption on the leading hemispheres, at least for satellites with better
longitude coverage, where sine fits to the data are plotted as dashed curves. Data are duplicated outside the 0˚ to 360˚ interval to
better show the periodic trends.
proportional to magnetic field strength, being 0.3, 1, 3, and 13 seconds, from Miranda through Titania. Com-
bining these gyroperiods with net plasma flow rates, each successive proton orbit advances by 2, 14, 60, and
500 km, relative to satellites Miranda through Titania. Only at Titania and beyond does a 10 keV proton ad-
vance far enough between successive loops to begin to have much chance of looping around and hitting the
satellite’s leading hemisphere. Computing the actual three dimensional spatial distribution of impacts on a
satellite’s surface as a function of proton energy is beyond the scope of this paper. It depends on the substantial
tilt and offset of the dipole field of Uranus as well as the distribution of particle pitch angles, which could vary
seasonally. However, the simple arguments presented here suggest that particles in the 10 keV range will
mostly hit trailing hemispheres of satellites, and can only influence the leading hemispheres of the more distant
ones. Additionally, magnetospheric plasma densities decline rapidly with planetocentric distance, more rapidly
than is compensated by the increase in net plasma flow relative to satellite orbital motion (or at least they did
during the Voyager 2 encounter in 1986, near the time of southern summer solstice, e.g., Bridge et al. 1986;
Cheng et al. 1991). Overall magnetospheric plasma sputtering rates should thus be lower on the more distant
satellites. For both of these reasons, we would expect magnetospheric sputtering to produce diminishing lead-
ing-trailing contrasts with planetocentric distance. This expectation is consistent with results from sinusoidal
fits to the observed longitudinal variation in H2O bands, which give fractional variations 93 ± 4%, 69 ± 47%,
and 34 ± 13% for Ariel, Umbriel, and Titania, respectively.
Another mechanism capable of producing leading-trailing asymmetries in the satellites’ H2O ice bands is
magnetospheric delivery of carbonaceous dust particles. Dark particles from the uranian rings which become
electrically charged will experience a Lorentz force from the magnetic field lines sweeping past them. This
force will tend to accelerate them such that they spiral slowly outward, eventually culminating in low speed
collisions onto the trailing hemispheres of the satellites, primarily the inner ones, so this mechanism could also
contribute to the observed decline of leading-trailing spectral contrasts with planetocentric distance. The addi-
tion of these dark particles to the satellites’ trailing hemispheres would suppress their albedos and their ice ab-
sorption bands, by reducing the fraction of photons able to escape from the surface at all wavelengths, as op-
posed to just the wavelengths at which ice absorbs. A possible way to establish limits on the supply of carbon-
aceous grains via this mechanism could involve comparison of visible wavelength albedos between leading
and trailing hemispheres. Voyager images do not reveal dramatic global leading-trailing albedo patterns, but a
detailed comparison, controlling for geological age by selecting specific regions, would be needed to properly
address this question. Such a study is beyond the scope of this paper.
Yet another mechanism which could produce leading-trailing asymmetries diminishing with planetocentric
distance is impact cratering. In general, leading hemispheres suffer more collisions with debris from outside of
the Uranus system (Zahnle et al. 2001, 2003), and gravitational focusing by Uranus causes the inner satellites
to be cratered more frequently and with higher average impact velocities. Large impacts could dredge up
cleaner ice from below the surfaces of the satellites, resulting in deeper H2O ice bands on leading hemispheres.
Dust-sized impactors could sputter away ice and deliver exogenous, non-icy materials, creating the opposite
spectral effect. Relatively little is known about the size distribution, composition, and net effect of smaller im-
pactors on icy surfaces in the Uranus system, but the dust counter on NASA’s New Horizons1  spacecraft
should provide useful information about the dust population in the ecliptic plane at large heliocentric distances.
Oberon is the most distant major satellite. It spends part of its time outside of the uranian magnetosphere in
the solar wind environment (Cheng et al. 1991). This difference between Oberon and the other uranian satel-
1. NASA’s first New Frontiers mission, bound for Pluto and the Kuiper belt (Stern and Cheng 2002, Stern and
Spencer 2003). 
lites offers a possible explanation for its H2O ice bands following a different longitudinal pattern. Closer in,
magnetospheric plasma may be the dominant remover of H2O ice, chiefly from trailing hemispheres, while fur-
ther out, micrometeoroid impacts may increase in relative importance, preferentially sputtering ice from lead-
ing hemispheres.
3.2.  Carbon dioxide ice
Three narrow CO2 ice absorption bands are seen in
some of the spectra around 2 µm. They are most appar-
ent in Ariel spectra, where they were previously noted
Grundy et al. (2003). Here we report that these bands
also appear, although less prominently, in spectra of
Umbriel and Titania. Figure 3 shows the CO2 ice ab-
sorptions, and a general trend of more CO2 ice absorp-
tion on the satellites closer to Uranus (toward the bot-
tom of the figure): no CO2 absorption is evident in
Oberon’s spectrum, the strongest CO2 absorptions are
barely visible in Titania spectra, and the absorptions
become progressively stronger in spectra of Umbriel
and Ariel. Two additional CO2 ice absorptions at 1.578
and 1.610 µm also appear to be present in spectra of
Ariel and possibly Umbriel and Titania. These absorp-
tions are attributable to 21 + 22 + 3 and
1 + 42 + 3 transitions in CO2 ice, respectively
(Quirico and Schmitt 1997; Gerakines et al. 2005). The
21 + 22 + 3 band looks like it is superimposed on
the longer wavelength shoulder of a broader dip. That
feature appears to be a stellar absorption that is slightly
deeper in the sun than in our solar analogues, resulting
in incomplete cancellation. Additional, very weak,
unidentified CO2 ice absorptions from the laboratory
spectra of Hansen (2005) are also visible in the regions
enlarged in Fig. 3. In general, SpeX does not quite
spectrally resolve the cores of these CO2 ice absorp-
tions.
Our spectra show no apparent absorption features at
2.134 µm, where the ‘forbidden’ 23 overtone of CO2
is reported by Bernstein et al. (2005) for CO2 molecules
mixed into H2O ice (this behavior was also noted by B.
Schmitt, personal communication 2002). The absence
of this band couples with the wavelengths and the nar-
row profiles of the other CO2 bands to strongly favor
pure CO2 ice, in the sense of CO2 molecules being associated with their own kind, as opposed to being isolated
in H2O ice. That the CO2 molecules are not isolated in H2O ice is perhaps not surprising. There is some uncer-
Figure 3: Enlarged views of the 1.6 and 2 µm regions of the
same satellite spectra as in Fig. 1, showing narrow CO2 ice
absorption features superimposed on the broad H2O ice bands.
Wavelengths of additional weak, unidentified CO2 ice
absorptions in the laboratory data of Hansen (1997, 2005) are
indicated by vertical line segments. An arbitrarily scaled Hapke
model spectrum (e.g., Hapke 1993) based on those laboratory
data is included at the bottom of each panel to give an idea of
the sorts of band shapes expected for pure CO2 ice, albeit at a
warmer temperature (150 K). An arrow marks the location of
the ‘forbidden’ 23 transition (Bernstein et al. 2005). 
tainty about how long such a thermodynamically unstable situation would last. Laboratory evidence suggests
that, at least at higher temperatures, H2O:CO2 ices mixed at the molecular level can reorganize themselves over
time into segregated zones of pure phases, and thus lose the 23 absorption feature (Bernstein et al. 2005). It is
unclear how rapidly such a reorganization could progress at the uranian satellites’ surface temperatures, but the
apparent ability of H2O to maintain its crystal structure against amorphization hints that such a phenomenon
might not be out of the question.
The spatial distribution of CO2 ice can place valuable constraints on its origin and evolution. Fig. 4 explores
the longitudinal and planetocentric distribution of CO2 ice on the four satellites by plotting the sums of the in-
tegrated areas of the three strongest CO2 ice absorptions near 2 µm from each spectrum versus sub-observer
longitude. The integrated areas for the three absorptions were computed using 1.957 to 1.962 µm and 1.969 to
1.974 µm for the continuum and 1.962 to 1.969 µm for the band interval of the 21 + 3 CO2 band. For the
1 + 22 + 3 band we used 2.002 to 2.008 µm and 2.015 to 2.020 µm for the continuum and 2.008 to
2.015 µm for the band interval. For the 42 + 3 band we used 2.062 to 2.068 µm and 2.072 to 2.078 µm for
the continuum and 2.068 to 2.072 µm for the band interval.
These measurements show more CO2 ice absorption on the trailing hemispheres of Ariel, Umbriel, and Ti-
tania, as well as declining CO2 absorption with distance from Uranus. This distribution contrasts somewhat
Figure 4: Variation of total integrated area of the three strongest CO2 ice bands near 2 µm. Sine fits are plotted as dashed curves.
Both the integrated areas and their longitudinal variation decline with planetocentric distance. No CO2 absorption was detected in
the Oberon spectra.
with reports from Galileo/NIMS observations of Europa, Ganymede, and Callisto (McCord et al. 1997, 1998;
Hibbitts et al. 2000, 2003). Those observations of the 4.25 µm 3 fundamental band showed CO2 to be present
on both leading and trailing hemispheres, although more abundant at the center of the trailing hemisphere of
Callisto. Also, the 3 band is shifted in wavelength from where it occurs in pure CO2 ice, suggesting that in the
jovian system CO2 is trapped or bound up in some other material. The absorption was also observed to be geo-
graphically associated with darker materials (Hibbitts et al. 2000, 2003). Pure CO2 ice in the jovian system
would not be thermally stable, adding weight to the interpretation that it is complexed with some other, less
volatile material. The planetocentric trend was the opposite of what we see in the uranian system: the 4.25 µm
band depth increases with distance from Jupiter (McCord et al. 1998). The 3 band of carbon dioxide has also
recently been detected in saturnian satellite spectra by Cassini/VIMS (e.g., Buratti et al. 2005; Clark et al.
2005). As in the jovian system, the band appears to be shifted from the wavelength of the pure CO2 ice trans-
ition, and may be associated with dark material there, as well.
It is important to realize that the 4.25 µm 3 fundamental absorption detected by NIMS and VIMS is in-
trinsically much stronger than the overtone and combination bands we are observing near 2 µm, by a factor of
a thousand in absorption coefficients (e.g., Hansen 1997, 2005). Accordingly, our detection of much the weak-
er bands requires considerably more condensed CO2 than is indicated by the jovian and saturnian satellite ob-
servations. Simple radiative transfer models (e.g., Hapke 1993; Grundy et al. 2003) can be used to get an idea
of the quantities of CO2 ice required by our observations. A CO2 ice glaze uniformly covering Ariel’s entire
trailing hemisphere would have to be at least 5 µm thick to match the observed absorption bands. It would need
to be thicker if the CO2 ice were more geographically restricted. If the CO2 ice were as segregated as possible
from other materials, in a checkerboard-like pattern, at least 5% of the trailing hemisphere of Ariel would have
to be composed of CO2 ice.
In the following sub-sections, we consider possible sources and sinks of CO2 on the satellites’ surfaces, es-
timating rates where possible, and considering what longitudinal or planetocentric trends various processes
might lead to in light of the observed CO2 ice distribution. We begin with processes which can mobilize or
eliminate CO2 ice.
3.2.1.  Processes which move or destroy CO2
If the CO2 ice on the satellite surfaces is not trapped in a less volatile material such as H2O ice or a carbon-
aceous residue (as is suggested by the non-detection of the 23 band and by the absence of wavelength shifts
relative to pure CO2 ice measured in the laboratory), we can use its known vapor pressure curve (Brown and
Ziegler 1980) and sticking coefficient (Weida et al. 1996) to consider its stability against sublimation over the
course of a seasonal cycle in the uranian system. We used a thermophysical model (e.g., Spencer and Moore
1992) to compute diurnal temperature histories as functions of latitude and season on the satellites, assuming
plausible values of thermal inertia , bolometric bond albedo AB, and emissivity . From the computed temper-
ature histories and the CO2 ice vapor pressure curve, we computed sublimation histories for each latitude band.
Integrating these histories over time, we arrived at seasonally averaged sublimation rates as a function of latit-
ude. Inverting these rates gives the time to sublimate one gram of CO2 ice per square centimeter. Examples are
shown in Fig. 5.
The CO2 sublimation rate is proportional to its vapor pressure, which is an extremely steep function of tem-
perature at uranian satellite surface temperatures, so local seasonal sublimation rates tend to be dominated by
the seasonal maximum temperatures achieved. Polar regions, which are bathed in decades of continuous sun-
light each summer because of the extreme obliquity (98˚), experience the highest peak temperatures and most
rapid seasonal sublimation rates. This latit-
ude dependence of seasonal sublimation
rates will tend to drive accumulation of
CO2 at equatorial latitudes. But even at the
satellites’ equators, appreciable quantities
of CO2 ice could sublimate on time scales
much shorter than the age of the Solar Sys-
tem. Of the four satellites, Ariel spins fast-
est on its axis, with a 2.5 day period. Its
faster rotation reduces the day-night tem-
perature contrast, so for any set of thermo-
physical parameters, Ariel’s equatorial di-
urnal peak temperatures remain lower than
on Umbriel, Titania, and Oberon, with
their longer periods of 4.1, 8.7, and
13.5 days, respectively. As a result, CO2
ice is most stable against sublimation at
low latitudes on the faster rotating, inner
satellites.
The extremely strong temperature de-
pendence makes sublimation rates highly
sensitive to assumed model parameters. For instance, increasing AB from 0.5 to 0.6 reduces absorbed sunlight
by 20%, leading to a few K drop in maximum temperatures. This small temperature drop drives an order of
magnitude increase in the time scale to sublimate 1 g cm-2 of CO2 ice, as shown in Fig. 5. Although we as-
sumed plausible values for albedos, thermal inertias, emissivities, etc., in our computations of temperature his-
tories, the extreme temperature sensitivity makes the sublimation numbers highly uncertain. Our models pro-
duce peak temperatures in the 70 to 80 K range. Peak temperatures from Voyager 2 thermal emission measure-
ments were as high as 85 K (Hanel et al. 1986). If CO2 ice temperatures get that high, sublimation will proceed
much more rapidly than we have estimated in Fig. 5. The thermal emission observed by Voyager is dominated
by the darkest regions which absorb the most sunlight as well as topographic lows that absorb thermal radi-
ation from surrounding areas. Any CO2 ice in these warmest regions would be mobilized very rapidly.
The net result of thermal mobilization will be that CO2 ice migrates away from polar latitudes toward equat-
orial latitudes on relatively short time scales, of the order of hundreds to millions of years. In the equatorial re-
gions, the ice will tend to accumulate in local cold traps: regions of higher albedo, higher thermal inertia, high-
er emissivity, and/or higher elevation. There its texture will slowly evolve toward larger particles with rounded
shapes, or even a compacted crust overlaying more porous ice below (e.g., Clark et al. 1983; Spencer 1987;
Eluszkiewicz et al. 1998; Grundy and Stansberry 2000; Titus et al. 2001).
The above sublimation estimates do not account for the fate of CO2 molecules after they are mobilized. Be-
ing unlikely to collide with other gas molecules, they will generally follow ballistic trajectories. Some will es-
cape, while others will fall back to the surface and stick elsewhere. We estimate escaping fractions following
Chamberlain and Hunten (1987, Eq. 7.1.5). As with sublimation rates, the fraction escaping is extremely sens-
itive to temperature, and escape rates from the warmer poles greatly exceed escape rates from the cooler equat-
orial belts. For the thermal parameters described above, we find maximum polar escape fractions ranging from
Figure 5: Estimates of time in Earth years to sublimate one gram of CO2 per
cm2 as a function of latitude for satellites as labeled by their initials. North and
South latitudes mirror each other, so half of each curve is omitted to reduce
clutter. On the left are models with  = 104 erg cm-2 s-1/2 K-1. Models with
 = 2 × 104  erg cm-2 s-1/2 K-1 are on the right. Solid curves are for AB = 0.5,
dotted curves for AB = 0.6, and dashed curves for AB = 0.7. Emissivity  = 0.9
was assumed for all models. 
1 × 10-9 (for Titania with AB = 0.7) up to 7 × 10-4 (for Umbriel with AB = 0.5). For equatorial latitudes, peak
temperatures are lower and so maximum escape fractions are lower as well, from 1 × 10-10 to 3 × 10-4 for the
same two cases, with  = 10,000 erg cm-2 s-1/2 K-1. For any particular set of thermal parameters, Umbriel has the
highest escape fraction, followed by Ariel, because of their lower masses and gravitational accelerations. Since
these two satellites show the most CO2 ice, the escape of sublimated CO2 to space does not seem to govern the
planetocentric distribution of CO2. Even for the highest equatorial escape rates of any of our models, thermal
escape of CO2 would be extremely slow. For Umbriel with  = 10,000 erg cm-2 s-1/2 K-1 and AB = 0.5, the time
to sublimate 1 g cm-2 at the equator is about 105 years, of which 3 × 10-4 g escape to space, so the time to lose
1 g cm-2 to space is about 3 × 108 years. For other thermal model assumptions and for other satellites, the time
scales are even longer, much longer than the age of the solar system in most cases.
Charged particle sputtering can mobilize CO2, and will act at a much higher rate for CO2 ice than it does for
less volatile H2O ice, assuming the two ices are segregated from one another (e.g., Johnson et al. 1983). Using
equation 3.27 from Johnson (2000), with values from Table 3.4 in that book, we estimate that for typical urani-
an satellite surface temperatures and magnetospheric proton energies, the sputtering rate for CO2 ice should be
some 50 to 100 times higher than for H2O ice. Cheng et al. (1991) estimate seasonal average sputtering rates
for H2O molecules from uranian satellite surfaces in their Table IV. We multiply their H2O sputtering rates by
50 to 100 to get an idea of possible magnetospheric proton CO2 sputtering rates for the uranian satellites, find-
ing values in the 0.6 to 1.4 × 10-8 g cm-2 yr-1 range for Ariel and Umbriel, dropping to 2 to 4 × 10-10 g cm-2 yr-1
for Titania and as low as 0.5 to 1 × 10-10 g cm-2 yr-1 for Oberon. As before, these values can be inverted to get
an idea of how long it would take for one gram of CO2 ice to be sputtered from a square centimeter. For Ariel
and Umbriel, these time scales are in the 100 Myr range, increasing to more than a billion years for Titania and
Oberon.
Sputtered molecules are ejected with more kinetic energy than thermally mobilized molecules generally
have, so they can more readily escape a satellite’s gravitational well. The energy spectrum of sputtered CO2
molecules is uncertain, but crude estimates can be made by assuming it is similar to the energy distribution of
sputtered H2O molecules, which goes approximately as U'(E + U')-2, where U' is a measure of binding energy
and E is the kinetic energy of ejection (e.g., Johnson 1998). The appropriate value of U' for CO2 ice is un-
known. Assuming an H2O-like U' = 0.05 eV (Haring et al. 1984) the fraction of molecules sputtered from the
surfaces of the satellites at greater than escape velocity would be 0.85, 0.40, 0.44, 0.26, and 0.29 from Miranda
through Oberon. If U' is as low as the 0.009 eV value reported by Haring et al. (1984) for CO ice, the escape
fractions would be 0.51, 0.11, 0.13, 0.06, and 0.07. These values are generally smaller than the Cheng et al.
(1991) estimated escape fractions for H2O molecules, since CO2 molecules are more massive (44 AMU as op-
posed to 18 AMU for H2O). Combining the escape fractions with sputtering rates as estimated in the previous
paragraph, we find time scales for one gram of CO2 ice to be sputtered and lost to space to be in the few hun-
dred million year range for Ariel and Umbriel, but longer than the age of the solar system for Titania and
Oberon.
Sputtering will gradually remove CO2 ice from the low latitude cold traps where thermal mobilization
drives it, at least on the trailing hemispheres of the inner satellites, where magnetospheric sputtering rates are
highest and the molecules are less tightly bound, gravitationally, because the satellites are less massive (see
Table 1). The fact that CO2 ice is observed to be most abundant in exactly these regions where it should be re-
moved most rapidly suggests sputtering by magnetospheric protons is not controlling the distribution of CO2
ice on the satellites’ surfaces, and that whatever is controlling that distribution acts on shorter time scales.
Sputtering by solar UV photons could also mobilize CO2 on satellite surfaces (e.g., Johnson 1990). UV
sputtering should affect all longitudes and all uranian satellites equivalently, so although it may be important
for increasing mobility of CO2 molecules, it cannot explain the observed spatial distribution of CO2 ice. Anoth-
er source of energy which can lead to sputtering is micrometeorite impacts, which, if they come from outside
of the Uranus system, preferentially strike the satellites’ leading hemispheres (Zahnle et al. 2003). The abund-
ance of dust-sized impactors at 20 AU is not well known, but could be sufficient to play a role in mobilizing
CO2 ice from the leading hemispheres (e.g., Eviatar and Richardson 1986).
Finally, CO2 molecules can be destroyed by energetic UV or charged particle radiation. Gerakines and
Moore (2001) estimated G value of 1.1 and 8.1 CO2 molecules destroyed per 100 eV incident as protons or UV
photons, respectively, via the reaction path energy + CO2  CO + O. A UV photon with wavelength shorter
than 2275 Å can dissociate a CO2 molecule (Gerakines et al. 1996; Delitsky and Lane 1998). From SORCE
spectra (e.g., Rottman et al. 2004), we estimate about 0.07% of the solar flux to be at or below that wavelength.
Accounting for the geometry of the Uranus system, we obtain seasonal average UV fluxes ranging from 0.5
through 0.8 erg cm-2 s-1 for equatorial through polar latitudes on the uranian satellites. These fluxes correspond
to destruction of the order of 1018 CO2 molecules, or ∼10 g cm-2 yr-1, if all of the UV photons are absorbed
within the CO2 ice. This is a very high destruction rate, compared with other processes we have considered.
Magnetospheric charged particles can also dissociate CO2 molecules. Using the (Gerakines and Moore
2001) G value of 1.1 molecule per 100 eV, along with dose rates from Cheng et al. (1991, Figure 23), we es-
timate radiolytic destruction rates in the 10-10 to 10-7 g cm-2 yr-1 range for Oberon through Miranda. These rates
are much lower than the photolytic destruction rates estimated in the previous paragraph, but are comparable to
the magnetospheric sputtering rates estimated previously.
Of course, not all CO and O resulting from photolysis and radiolysis of CO2 will be lost. Some will recom-
bine to make new CO2 molecules while others will react with water or other locally available materials to form
new molecules, some of which could be quite complex (e.g., Benit et al. 1988; Delitsky and Lane 1997, 1998).
Indeed, it seems likely that a carbon cycle much like the one envisioned by Johnson et al. (2005) for the jovian
system also operates on the surfaces of the uranian satellites, with carbon atoms repeatedly recycled between
CO2 and other molecules such as carbonic acid (H2CO3) and formaldehyde (H2CO).
The relatively high destruction and loss rates estimated in this section, especially from UV photolysis as
well as from sputtering and radiolysis, suggest that there must be a recent, or more likely an ongoing source for
the observed CO2 ice. Accordingly, we next turn our attention to possible sources of CO2.
3.2.2.  Possible sources of CO2
Out-gassing from satellite interiors is a possible source for CO2. Ariel, with its relatively deep CO2 absorp-
tion bands, also shows clear evidence of past geologic activity (Plescia 1987), but Umbriel, with the next
strongest CO2 bands, shows no evidence for recent activity, at least not on the hemisphere observed by Voy-
ager (Plescia 1989). Titania, with still less observed CO2, may have some relatively youthful provinces, but its
surface is mostly ancient (Zahnle et al. 2003). Although Voyager was unable to view more than about half of
each satellite’s surface, the presence of CO2 ice does not seem to correlate particularly well with the existence
of less-cratered, younger regions or with evidence of recent geological activity. Either could be an indicator of
ongoing or recent release of volatiles from the satellites’ interiors. An out-gassing source for the observed CO2
ice thus does not look especially promising.
Impactors could deliver CO2 to the uranian satellites, or could release it from their sub-surfaces. If exogenic
delivery or impact exhumation were dominated by frequent, smaller impactors, they would tend to distribute
CO2 through the Uranus system according to a consistent pattern, and the balance between delivery and loss
rates would govern the observed planetocentric and longitudinal distributions. Infrequent, larger impactors
could produce a more random distribution pattern, in which a local high abundance of CO2 might simply point
to a single recent impact event. Zahnle et al. (2003) estimated cometary cratering rates on the uranian satellites.
Owing to gravitational focusing, cratering rates per unit area increase with proximity to Uranus, being
something like a factor of 4 or 5 higher on Ariel than on Oberon, and impactors strike with average speeds a
factor of almost 2 higher on Ariel than on Oberon (Zahnle et al. 2003). Impactors from outside the Uranus sys-
tem would preferentially strike the satellites’ leading hemispheres, so the observation that CO2 ice is predomin-
antly on the trailing hemispheres would require mobilization and redistribution, followed by preferential re-
moval from the leading hemispheres, possibly by dust impact driven sputtering.
CO2 molecules could form through radiolytic action on the satellites’ surfaces, with oxygen coming from
the ubiquitous H2O ice, and carbon arriving via implantation of magnetospheric ions (Strazzulla et al. 2003). A
magnetospheric source of carbon ions would neatly explain the observed distribution of CO2 ice. However, the
Voyager 2 spacecraft did not detect carbon ions in the magnetosphere during its Uranus flyby in 1986 (Bridge
et al. 1986; Krimigis et al. 1986), making a magnetospheric source of carbon ions seems improbable. However,
magnetospheric delivery of larger carbonaceous particles from the uranian rings remains a possibility, as men-
tioned previously.
There is also no shortage of dark and presumably carbonaceous material already on the satellites’ surfaces,
as evidenced by their low visible albedos. Recent investigations of radiolysis at interfaces between carbon-
aceous materials and H2O ice (Mennella et al. 2004; Gomis and Strazzulla 2005) show that CO2 can be pro-
duced from purely local materials, even when the carbon and oxygen are not initially present in a single phase.
The low albedos of the satellites’ surfaces, along with temperatures too cold for thermal segregation of H2O ice
as well as abundant craters indicative of impact gardening, suggest relatively intimate mixing between H2O ice
and dark particles. In one experiment by Gomis and Strazzulla (2005), H2O ice deposited on a carbonaceous
substrate was irradiated at 80 K with 200 keV Ar+ ions. CO2 was efficiently produced at the interface. It would
be interesting to see if proton, electron, or even UV irradiation produces similar results. Any energy source
capable of breaking bonds in H2O ice or in the dark carbonaceous materials, and mobilizing radicals containing
O or C should be able to contribute to this process.
Whether or not radiolysis can produce CO2 rapidly enough to replace CO2 lost to UV photolysis is an im-
portant question to consider. Prior to saturation in the Gomis and Strazzulla (2005) experiment, each 200 keV
Ar ion produced about 30 CO2 molecules, or 7 keV per CO2 molecule produced. This is orders of magnitude
more energy than the ∼12 eV per CO2 molecule destroyed according to the photolytic G value from Gerakines
and Moore (2001). From these numbers, radiolytic production rates would be far below photolytic destruction
rates. For production not to be overwhelmed by photolytic destruction, much more efficient production path-
ways are necessary. It is possible that lower energy protons or electrons are more effective at producing CO2
from the available precursors, and it is also possible that the expected granular texture of the ice and carbon-
aceous material enhances production of CO2, because of its large surface area. Additionally, photolysis may
not be a purely destructive process. We are not aware of laboratory studies of photolytic production of CO2
from H2O ice and carbonaceous precursors at uranian satellite surface temperatures, but it probably can occur
(e.g., Bernstein et al. 1995; Wu et al. 2002). Photolytic production would not, however, be able to explain the
observed longitudinal distribution of CO2 ice. Another possibility is that more of the satellites’ surfaces are in-
volved in production than in destruction. Such a configuration is plausible, since CO2 will tend to accumulate
in localized cold traps, owing to its high thermal mobility, unlike the global distribution of the non-volatile pre-
cursors: H2O ice and carbonaceous solids. As noted previously, the spectra are consistent with CO2 being con-
fined to a relatively small fraction of the surface area, as little as 5%.
3.2.3.  Discussion
The planetocentric and longitudinal distributions of CO2 and H2O ices in the uranian system offer valuable
clues regarding their origin and fate. Having considered candidate sources and sinks on the satellite surfaces,
we now review the trends expected from these mechanisms and compare them with the observational data,
which shows CO2 ice to be most abundant on the inner satellites, and especially on their trailing hemispheres,
and H2O ice to be more abundant on leading hemispheres from Ariel through Titania, but on the trailing hemi-
sphere of Oberon.
Sublimation mobilizes CO2 relatively rapidly at seasonal maximum uranian satellite surface temperatures,
which are higher at polar latitudes, so CO2 accumulates at low latitudes. Relatively little is lost to space. At low
latitudes, sublimation rates are much lower, but still rapid enough to drive CO2 ice to accumulate in local cold
traps, and to evolve in texture. Sublimation does not depend on longitude, and has no real effect on the much
less volatile H2O ice. Voyager images of Umbriel show a bright equatorial feature (‘Wunda’) near the center of
the trailing hemisphere. It is tempting to speculate that CO2 ice is concentrated there, however, similar low-lat-
itude, bright features are not evident on the trailing hemispheres of Ariel or Titania.
Impactors preferentially strike leading hemispheres and inner satellites. Impacts could dredge up fresh H2O
ice from the subsurfaces, primarily of the leading hemispheres of the inner satellites, consistent with the obser-
vations of deeper H2O ice bands there. An impact source of CO2 could be consistent with the observed planeto-
centric trend, but not with the observed longitudinal trends. An impact loss mechanism could explain the ob-
served CO2 longitudinal trends, but not the planetocentric trend. Impacts apparently cannot explain the ob-
served CO2 ice distribution, at least not without the help of additional mechanisms.
Magnetospheric charged particle sputtering preferentially mobilizes CO2 and H2O ices from the trailing
hemispheres of the inner satellites, exactly those regions where CO2 ice is observed. Sputtering rates are estim-
ated to be well below sublimation rates for CO2, but are probably the dominant sink for H2O ice. A much great-
er fraction of sputtered CO2 molecules should escape to space, compared with sublimated CO2 molecules. Cor-
relations with indicators of recent geological activity in Voyager images are not evident, arguing against an
out-gassing source of CO2 from the satellite interiors.
Radiolytic production of CO2 driven by magnetospheric irradiation should act most rapidly on the trailing
hemispheres of the inner satellites, because magnetospheric densities are higher closer to Uranus. The longit-
udinal and planetocentric pattern of radiolytic production is consistent with the observed distribution of CO2,
and appears to be the best candidate source for CO2 on the satellites’ surfaces. The fact that our estimated rates
of radiolytic production are considerably below the estimated photolytic destruction rate is troubling. This dis-
crepancy may point to problems in our estimated production rates or could imply that much more surface area
is involved in production than is actually covered with CO2 ice, in order to maintain an equilibrium. The idea
that CO2 ice may only cover a small fraction of the surface area of the satellites’ trailing himespheres is at least
consistent with the spectral evidence.
Finally, it seems probable that CO2 ice on the uranian satellites’ surfaces participates in a radiolytic and
photolytic carbon cycle (e.g., Johnson et al. 2005). If so, carbonic acid is also likely to be present, and might be
detectable by means of its characteristic infrared absorption bands. With hydrogen being more readily lost to
space than heavier carbon and oxygen, chemistry at the satellites’ surfaces probably tends to be oxidizing and
acidic. Such a surface environment may help explain why NH3, an alkali, has not been convincingly detected,
despite its presence being suggested by geomorphologic evidence (Croft and Soderblom 1991; Kargel et al.
1991). A tentative report of NH3 in a spectrum of Miranda (Bauer et al. 2002) has yet to be confirmed. If NH3
were confirmed to be present on the surface of a uranian satellite, the chemical implications would be quite re-
markable.
4.  Conclusion
Seventeen nights of IRTF/SpeX observations of the 4 largest uranian satellites reveal the existence of CO2
ice on Umbriel and Titania for the first time, as well as confirming its presence on Ariel. The observations also
provide information about the longitudinal and planetocentric distributions of CO2 and H2O ices. These distri-
butions likely result from equilibria between source and sink mechanisms, and can tell us something about the
relative importance of the processes involved. Magnetospheric sputtering removal of H2O ice from trailing
hemispheres of Ariel, Umbriel, and Titania is consistent with the observation of deeper H2O ice absorption
bands on the leading hemispheres of those satellites, with diminishing leading-trailing asymmetries with plan-
etocentric distance. For Oberon, magnetospheric sputtering is greatly reduced, and the leading-trailing asym-
metry appears to be reversed. The deeper H2O ice bands on Oberon’s trailing hemisphere may point to an ef-
fect of micrometeoroid sputtering that only becomes important where magnetospheric effects become negli-
gible. Radiolytic production of CO2, driven by magnetospheric charged particle bombardment, appears most
consistent with the observation of deeper CO2 ice absorption bands on the trailing hemispheres of Ariel, Um-
briel, and Titania, as well as declining CO2 absorption with planetocentric distance and its absence from the
surface of Oberon.
Thermally-driven processes such as grain growth, volatile transport, sintering, and solar gardening can be
expected to influence the texture and distribution of CO2 ice on the surfaces of the uranian satellites. The sea-
sonally-averaged distribution of sunshine should drive CO2 ice away from polar latitudes. Low latitude regions
which radiatively cool fastest will cold-trap CO2 ice, and if enough ice accumulates to raise the albedos of
these regions, the process will be self-reinforcing. Uppermost surfaces of CO2 ice particles can radiatively cool
faster than subsurface depths where solar energy is absorbed, so sublimation will tend to preferentially remove
CO2 from below the uppermost surface. Loss to space driven primarily by sputtering by UV photons and mag-
netospheric charged particles limits the residency time of CO2 ice in these cold traps. Additional limits on CO2
accumulation come from UV photolysis, which is expected to break CO2 bonds at a relatively high rate, lead-
ing to a chemical cycle in which carbon, oxygen, and hydrogen are repeatedly combined into new species and
broken apart again, forming a complex mixture which should include CO2 as well as species like carbonic acid
and formaldehyde. More detailed comparisons between leading hemispheres (which see relatively little mag-
netospheric radiolysis) and trailing hemispheres should be able to shed more light on the relative importance of
radiolysis and photolysis.
If radiolysis by magnetospheric particles is indeed the source of CO2 ice on the uranian satellites, produc-
tion rates on Miranda should be even higher than on Ariel. However, Miranda’s lower gravity will allow a
much larger fraction of sublimated and sputtered CO2 molecules to escape. Spectral observations of Miranda
could provide useful constraints on the balance between sublimation and radiolytic production.
ACKNOWLEDGMENTS: We are grateful to W. Golisch, D. Griep, P. Sears, S.J. Bus, J.T. Rayner, and K. Crane
for assistance with the IRTF and with SpeX, R.S. Bussmann for contributing to the reduction pipeline,
M.R. Showalter for the Rings Node’s on-line ephemeris services, and NASA for its support of the IRTF. We
especially thank K.A. Tryka for sharing with us an unpublished manuscript on NH3 and CO2 stability in the ur-
anian system, which contained many useful ideas for modeling the fate of CO2 ice on the satellites’ surfaces.
Scientific discussions with B. Schmitt and M.P. Bernstein also provided useful ideas. We thank the free and
open source software communities for empowering us with many of the tools used to complete this project,
notably Linux, the GNU tools, LATEX, FVWM, Tcl/Tk, TkRat, and MySQL. We acknowledge the significant
cultural role and reverence for the summit of Mauna Kea within the indigenous Hawaiian community and are
honored to have had the opportunity to observe there. Finally, we are grateful for funding from NSF grants
AST-0407214 and AST-0085614 and from NASA grants NAG5-4210, NAG5-10497, NAG5-12516, and
NNG04G172G, without which this work could not have been done.
References
Bauer, J.M., T.L. Roush, T.R. Geballe, K.J. Meech, T.C. Owen, W.D. Vacca, J.T. Rayner, and K.T.C. Jim 2002. The
near infrared spectrum of Miranda: Evidence of crystalline water ice. Icarus 158, 178-190.
Benit, J., J.P. Bibring, and F. Rocard 1988. Chemical irradiation effects in ices. Nucl. Instr. and Methods in Phys.
Res. B32, 349-353.
Bernstein, M.P., S.A. Sandford, L.J. Allamandola, S. Chang, and M.A. Scharberg 1995. Organic compounds pro-
duced by photolysis of realistic interstellar and cometary ice analogs containing methanol. Astrophys. J. 454,
327-344.
Bernstein, M.P., D.P. Cruikshank, and S.A. Sandford 2005. Near-infrared laboratory spectra of solid H2O/CO2 and
CH3OH/CO2 ice mixtures. Icarus 179, 527-534.
Bridge, H.S., J.W. Belcher, B. Coppi, A.J. Lazarus, R.L. McNutt Jr., S. Olbert, J.S. Richardson, M.R. Sands, R.S.
Selesnick, J.D. Sullivan, R.E. Hartle, K.W. Ogilvie, E.C. Sittler Jr., F. Bagenal, R.S. Wolff, V.M. Vasyliunas,
G.L. Siscoe, C.K. Goertz, and A. Eviatar 1986. Plasma observations near Uranus: Initial results from Voyager 2.
Science 233, 89-93.
Brown, W.L., L.J. Lanzerotti, J.M. Poate, and W.M. Augustyniak 1978. “Sputtering” of ice by MeV light ions.
Phys. Rev. Lett. 40, 1027-1030.
Brown, G.N. Jr., and W.T. Ziegler 1980. Vapor pressure and heats of vaporization and sublimation of liquids and
solids of interest in cryogenics below 1–atm pressure. Adv. Cryogenic Eng. 25, 662-670.
Buie, M.W., and W.M. Grundy 2000. The distribution and physical state of H2O on Charon. Icarus 148, 324-339.
Buratti, B.J., D.P. Cruikshank, R.H. Brown, R.N. Clark, J.M. Bauer, R. Jaumann, T.B. McCord, D.P. Simonelli,
C.A. Hibbitts, G.B. Hansen, T.C. Owen, K.H. Baines, G. Bellucci, J.P. Bibring, F. Capaccioni, P. Cerroni, A.
Coradini, P. Drossart, V. Formisano, Y. Langevin, D.L. Matson, V. Mennella, R.M. Nelson, P.D. Nicholson, B.
Sicardy, C. Sotin, T.L. Roush, K. Soderlund, and A. Muradyan 2005. Cassini Visual and Infrared Mapping Spec-
trometer observations of Iapetus: Detection of CO2. Astrophys. J. 622, L149-L152.
Chamberlain, J.W., and D.M. Hunten 1987. Theory of Planetary Atmospheres: An Introduction to Their Physics and
Chemistry. Academic Press, San Diego.
Cheng, A.F., P.K. Haff, R.E. Johnson, and L.J. Lanzerotti 1986. Interactions of magnetospheres with icy satellite
surfaces. In Satellites, Eds. J.A. Burns and M.S. Matthews, Univ. Arizona Press, Tucson, 403-436.
Cheng, A.F., S.M. Krimigis, and L.J. Lanzerotti 1991. Energetic particles at Uranus. In Uranus, Eds. J.T. Bergstralh,
E.D. Miner, and M.S. Matthews, University of Arizona Press, Tucson, 831-893.
Clark, R.N. 1981. The spectral reflectance of water-mineral mixtures at low temperatures. J. Geophys. Res. 86,
3074-3086.
Clark, R.N., F.P. Fanale, and A.P. Zent 1983. Frost grain size metamorphism: Implications for remote sensing of
planetary surfaces. Icarus 56, 233-245.
Clark, R.N., R.H. Brown, P.D. Owensby, and A. Steele 1984. Saturn’s satellites: Near-infrared spectrophotometry
(0.65-2.5 microns) of the leading and trailing sides and compositional implications. Icarus 58, 265-281.
Clark, R.N., R.H. Brown, R. Jaumann, D.P. Cruikshank, R.M. Nelson, B.J. Buratti, T.B. McCord, J. Lunine, K.H.
Baines, G. Bellucci, J.P. Bibring, F. Capaccioni, P. Cerroni, A. Coradini, V. Formisano, Y. Langevin, D.L. Mat-
son, V. Mennella, P.D. Nicholson, B. Sicardy, C. Sotin, T.M. Hoefen, J.M. Curchin, G. Hansen, K. Hibbitts, and
K.D. Matz 2005. Compositional maps of Saturn’s moon Phoebe from imaging spectroscopy. Nature 435, 66-69.
Croft, S.K., and L.A. Soderblom 1991. Geology of the uranian satellites. In Uranus, Eds. J.T. Bergstralh, E.D.
Miner, and M.S. Matthews, University of Arizona Press, Tucson, 561-628.
Cruikshank, D.P., T.C. Owen, C. Dalle Ore, T.R. Geballe, T.L. Roush, C. de Bergh, S.A. Sandford, F. Poulet, G.K.
Benedix, and J.P. Emery 2005. A spectroscopic study of the surfaces of Saturn’s large satellites: H2O ice, tholins,
and minor constituents. Icarus, 175, 268-283
Delitsky, M.L., and A.L. Lane 1997. Chemical schemes for surface modification of icy satellites: A road map. J.
Geophys. Res. 102, 16385-16390.
Delitsky, M.L., and A.L. Lane 1998. Ice chemistry on the Galilean satellites. J. Geophys. Res. 103, 31391-31403.
Eluszkiewicz, J., J. Leliwa-Kopystynski, and K.J. Kossacki 1998. Metamorphism of solar system ices. In Solar Sys-
tem Ices, Eds. B. Schmitt, C. de Bergh, and M. Festou, Kluwer Academic Publishers, Boston, 119-138.
Eviatar, A., and J.D. Richardson 1986. Predicted satellite plasma tori in the magnetosphere of Uranus. Astrophys. J.
300, L99-L102.
Gerakines, P.A., and M.H. Moore 2001. Carbon suboxide in astrophysical ice analogs. Icarus 154, 372-380.
Gerakines, P.A., W.A. Schutte, and P. Ehrenfreund 1996. Ultraviolet processing of interstellar ice analogs I.: Pure
ices. Astron. & Astrophys. 312, 289-305.
Gerakines, P.A., J.J. Bray, A. Davis, and C.R. Richey 2005. The strengths of near-infrared absorption features relev-
ant to interstellar and planetary ices. Astrophys. J. 620, 1140-1150.
Gomis, O., and G. Strazzulla 2005. CO2 production by ion irradiation of H2O ice on top of carbonaceous materials
and its relevance to the Galilean satellites. Icarus 177, 570-576.
Grundy, W.M., and B. Schmitt 1998. The temperature-dependent near-infrared absorption spectrum of hexagonal
H2O ice. J. Geophys. Res. 103, 25809-25822.
Grundy, W.M., and J.A. Stansberry 2000. Solar gardening and the seasonal evolution of nitrogen ice on Triton and
Pluto. Icarus 148, 340-346.
Grundy, W.M., and L.A. Young 2004. Near infrared spectral monitoring of Triton with IRTF/SpeX I: Establishing a
baseline. Icarus 172, 455-465.
Grundy W.M., M.W. Buie, J.A. Stansberry, J.R. Spencer, and B. Schmitt 1999. Near-infrared spectra of icy outer
solar system surfaces: Remote determination of H2O ice temperatures. Icarus 142, 536-549.
Grundy W.M., L.A. Young, and E.F. Young 2003. Discovery of CO2 ice and leading-trailing spectral asymmetry on
the uranian satellite Ariel. Icarus 162, 223-230.
Hanel, R., B. Conrath, F.M. Flasar, V. Kunde, W. Maguire, J. Pearl, J. Pirraglia, R. Samuelson, D. Cruikshank, D.
Gautier, P. Gierasch, L. Horn, and P. Schulte 1986. Infrared observations of the uranian system. Science 233, 70-
Hansen, G.B. 1997. The infrared absorption spectrum of carbon dioxide ice from 1.8 to 333 microns. J. Geophys.
Res. 102, 21569-21587.
Hansen, G.B. 2005. Ultraviolet to near-infrared absorption spectrum of carbon dioxide ice from 0.174 to 1.8 µm. J.
Geophys. Res. 110, E11003.1-18.
Hansen, G.B., and T.B. McCord 2004. Amorphous and crystalline ice on the Galilean satellites: A balance between
thermal and radiolytic processes. J. Geophys. Res. 109, E01012.1-19.
Hapke, B. 1993. Theory of reflectance and emittance spectroscopy. Cambridge University Press, New York.
Haring, R.A., R. Pedrys, D.J. Oostra, A. Haring, and A.E. DeVries 1984. Reactive sputtering of simple condensed
gases by keV ions III: Kinetic energy distributions. Nucl. Instr. Meth. in Phys. Rev. B5, 483-488.
Hibbitts, C.A., T.B. McCord, and G.B. Hansen 2000. Distributions of CO2 and SO2 on the surface of Callisto. J.
Geophys. Res. 105, 22541-22557.
Hibbitts, C.A., R.T. Pappalardo, G.B. Hansen, and T.B. McCord 2003. Carbon dioxide on Ganymede. J. Geophys.
Res. 108, 1956.1-22.
Horne, K. 1986. An optimal extraction algorithm for CCD spectroscopy. Pub. Astron. Soc. Pacific 98, 609-617.
Hudson, R.L., and M.H. Moore 2001. Radiation chemical alterations in solar system ices: An overview. J. Geophys.
Res. 106, 33275-33284.
Jacobson, R.A., J.K. Campbell, A.H. Taylor, and S.P. Synnott 1992. The masses of Uranus and its major satellites
from Voyager tracking data and Earth-based uranian satellite data. Astron. J. 103, 2068-2078.
Jenniskens, P., DF. Blake, and A. Kouchi 1998. Amorphous water ice: A solar system material. In Solar System Ices,
Eds. B. Schmitt, C. de Bergh, and M. Festou, Kluwer Academic Publishers, Boston, 139-155.
Jewitt, D.C., and J. Luu 2004. Crystalline water ice on the Kuiper belt object (50000) Quaoar. Nature 432, 731-733.
Johnson, R.E. 1990. Energetic charged-particle interactions with atmospheres and surfaces. Springer-Verlag, New
York.
Johnson, R.E. 1998. Sputtering and desorption from icy surfaces. In Solar System Ices, Eds. B. Schmitt, C. de Bergh,
and M. Festou, Kluwer Academic Publishers, Boston, 303-334.
Johnson, R.E. 2001. Surface chemistry in the jovian magnetosphere radiation environment, In Chemical Dynamics
in Extreme Environments, Ed. R. Dessler, Adv. Ser. In Phys. Chem. World Scientific, Singapore, 11, Ch. 8, 390-
Johnson, R.E., and T.I. Quickenden 1997. Photolysis and radiolysis of water ice on outer solar system bodies. J.
Geophys. Res. 102, 10985-10996.
Johnson, R.E., L.J. Lanzerotti, W.L. Brown, W.M. Augustyniak, and C. Mussil 1983. Charged particle erosion of
frozen volatiles in ice grains and comets. Astron. & Astrophys. 123, 343-346.
Johnson, R.E., R.W. Carlson, J.F. Cooper, C. Paranicas, M.H. Moore, and M.C. Wong 2005. Radiation effects on the
surfaces of the Galilean satellites. In Jupiter: The Planet, Satellites, and Magnetosphere, Eds. F. Bagenal, T.E.
Dowling, and W.B. McKinnon, Cambridge university Press, New York, 485-512.
Kargel, J.S., S.K. Croft, J.I. Lunine, and J.S. Lewis 1991. Rheological properties of ammonia-water liquids and crys-
tal-liquid slurries: Planetological applications. Icarus 89, 93-112.
Karkoschka, E. 1997. Rings and satellites of Uranus: Colorful and not so dark. Icarus 125, 348-363.
Kouchi, A., and T. Kuroda 1990. Amorphization of cubic ice by ultraviolet irradiation. Nature 344, 134-135.
Krimigis, S.M., T.P. Armstrong, W.I. Axford, A.F. Cheng, G. Gloeckler, D.C. Hamilton, E.P. Keath, L.J. Lanzerotti,
and B.H. Mauk 1986. The magnetosphere of Uranus: Hot plasma and radiation environment. Science 233, 97-
Lanzerotti, L.J., W.B. Brown, C.G. Maclennan, A.F. Cheng, S.M. Krimigis, and R.E. Johnson 1987. Effects of
charged particles on the surfaces of the satellites of Uranus. J. Geophys. Res. 92, 14949-14957.
McCord, T.B., R.W. Carlson, W.D. Smythe, G.B. Hansen, R.N. Clark, C.A. Hibbitts, F.P. Fanale, J.C. Granahan, M.
Segura, D.L. Matson, T.V. Johnson, and P.D. Martin 1997. Organics and other molecules in the surfaces of Cal-
listo and Ganymede. Science 278, 271-275.
McCord, T.B., G.B. Hansen, R.N. Clark, P.D. Martin, C.A. Hibbitts, F.P. Fanale, J.C. Granahan, M. Segura, D.L.
Matson, T.V. Johnson, R.W. Carlson, W.D. Smythe, G.E. Danielson, and the NIMS team 1998. Non-water-ice
constituents in the surface material of the icy Galilean satellites from the Galileo near-infrared mapping spectro-
meter investigation. J. Geophys. Res. 103, 8603-8626.
Mennella, V., M.E. Palumbo, and G.A. Baratta 2004. Formation of CO and CO2 molecules by ion irradiation of wa-
ter ice-covered hydrogenated carbon grains. Astrophys. J. 615, 1073-1080.
Moore, M.H., and R.L. Hudson 1992. Far-infrared spectral studies of phase changes in water ice induced by proton
irradiation. Astrophys. J. 401, 353-360.
Quirico, E., and B. Schmitt 1997. Near-infrared spectroscopy of simple hydrocarbons and carbon oxides diluted in
solid N2 and as pure ices: Implications for Triton and Pluto. Icarus 127, 354-378.
Plescia, J.B. 1987. Geological terrains and crater frequencies on Ariel. Nature 327, 201-204.
Plescia, J.B. 1989. Cratering history of the uranian satellites: Umbriel, Titania, and Oberon. J. Geophys. Res. 92,
14918-14932.
Rayner, J.T., D.W. Toomey, P.M. Onaka, A.J. Denault, W.E. Stahlberger, D.Y. Watanabe, and S.I. Wang 1998.
SpeX: A medium-resolution IR spectrograph for IRTF. Proc. SPIE 3354, 468-479.
Rayner, J.T., D.W. Toomey, P.M. Onaka, A.J. Denault, W.E. Stahlberger, W.D. Vacca, M.C. Cushing, and S. Wang
2003. SpeX: A medium-resolution 0.8–5.5 micron spectrograph and imager for the NASA Infrared Telescope
Facility. Publ. Astron. Soc. Pacific 115, 362-382.
Rottman, G.J., L. Floyd, and R. Viereck 2004. Measurement of the solar ultraviolet irradiance, in Solar Variability
and Its Effect on the Earth’s Atmosphere and Climate System, Eds. J. Pap, P. Fox, C. Frohlich, H.S. Hudson, J.
Kuhn, J. McCormack, G. North, W. Sprigg, S.T. Wu, AGU Monograph Series, Washington DC, pp. 111-125.
Schmitt, B., E. Quirico, F. Trotta, and W.M. Grundy 1998. Optical properties of ices from UV to infrared. In Solar
System Ices, Eds. B. Schmitt, C. de Bergh, and M. Festou, Kluwer Academic Publishers, Boston, 199-240.
Spencer, J.R. 1987. Thermal segregation of water ice on the Galilean satellites. Icarus 69, 297-313.
Spencer, J.R., and J.M. Moore 1992. The influence of thermal inertia on temperatures and frost stability on Triton.
Icarus 99, 261-272.
Stern, S.A. and A. Cheng 2002. NASA plans Pluto-Kuiper belt mission. EOS 83, 101, 108-109.
Stern, A., and J. Spencer 2003. New Horizons: The first reconnaissance mission to bodies in the Kuiper belt. Earth,
Moon, & Planets 92, 477-482.
Strazzulla, G., G. Leto, O. Gomis, and M.A. Satorre 2003. Implantation of carbon and nitrogen ions in water ice.
Icarus 164, 163-169.
Thomas, P.C. 1988. Radii, shapes, and topography of the satellites of Uranus from limb coordinates. Icarus 73, 427-
Titus, T.N., H.H. Kieffer, K.F. Mullins, and P.R. Christensen 2001. TES premapping data: Slab ice and snow flurries
in the martian north polar night. J. Geophys. Res. 106, 23181-23196.
Weida, M.J., J.M. Sperhac, and D.J. Nesbitt 1996. Sublimation dynamics of CO2 thin films: A high resolution diode
laser study of quantum state resolved sticking coefficients. J. Chem. Phys. 105, 749-766.
Wu, C.Y.R., D.L. Judge, B.M. Cheng, W.H. Shih, T.S. Yih, and W.H. Ip 2002. Extreme ultraviolet photon-induced
chemical reactions in the C2H2-H2O mixed ices at 10 K. Icarus 156, 456-473.
Zahnle, K., P. Schenk, S. Sobieszczyk, L. Dones, and H.F. Levison 2001. Differential cratering of synchronously ro-
tating satellites by ecliptic comets. Icarus 153, 111-129.
Zahnle, K., P. Schenk, H. Levison, and L. Dones 2003. Cratering rates in the outer solar system. Icarus 163, 263-
