Spencer, J.R. and W. M. Calvin 2002. Condensed O2 on Europa

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Spencer, J.R. and W. M. Calvin 2002. Condensed O2 on Europa
Condensed O2 on Europa and Callisto
John R. Spencer1
Wendy M. Calvin2
Lowell Observatory
1400 W. Mars Hill Rd.
Flagstaff, AZ 86001
(928) 774-3358
Fax: (928) 774-6296
[email protected]
Dept. Geological Sciences, MS172
University of Nevada, Reno
Reno, NV 89557
Corresponding author: John Spencer
Submitted to the Astronomical Journal, April 30 2002.
High signal-to-noise spectra of Europa and Callisto show a 0.3% deep 5770 Å absorption
band, due to condensed O2, at the same wavelength as a stronger band previously
identified on Ganymede. Excellent longitudinal coverage for Europa shows that unlike
Ganymede, where the band is much stronger on the trailing side, Europa shows no
significant longitudinal variation in the O2 band strength.
In recent years, spectroscopy of the surfaces of the icy Galilean satellites has revealed the
presence of several species other than water ice. O2 (Spencer et al. 1995, Calvin et al.
1996) and O3 (Noll et al. 1996) have been identified on the trailing side of Ganymede, as
well as SO2 on the trailing side of Europa (Lane et al. 1981, Noll et al. 1995), and the
leading side of Callisto (Noll et al. 1997). Galileo NIMS has seen CO2, and possible SH, C-N, and C-H features on Callisto and Ganymede (McCord et al. 1998), hydrated salts
or sulfuric acid on Europa (McCord et al. 1999, Carlson et al. 1999a), and H 2O2 on
Europa (Carlson et al. 1999b).
The presence of condensed O2 on Ganymede is inferred from a pair of weak (<2%
deep) absorption bands at 5770 and 6275 Å, which require the absorption of a photon by
two adjacent O2 molecules and so are produced only in high-density condensed O2. The
high vapor pressure of condensed O2 at Ganymede surface temperatures suggests that the
O2 is trapped in bubbles or crystal defects in a water ice matrix (Calvin et al. 1996,
Johnson and Jesser 1997, Johnson 1999): the traps may themselves be produced by
charged-particle irradiation. HST observations show that Ganymede’s O2 is concentrated
at low latitudes (Calvin and Spencer 1997): the warmer temperatures at low latitudes may
allow the coagulation and growth of radiation-triggered bubbles in the ice, providing sites
for concentration of the O2 (Johnson and Jesser 1997). Laboratory measurements of
H2O/O2 ice mixtures have reproduced the O2 absorption bands seen on Ganymede (Vidal
et al 1998, Baragiola and Bahr 1998), but in experiments so far the O2 is gradually lost at
temperatures above 70 K, leading to the more radical suggestion that the O2 is exposed
on the surface in small frost patches with daytime temperatures below 70 K, perhaps due
to extremely high albedo, or even in an atmospheric haze (Baragiola and Bahr 1998;
Baragiola et al. 1999, Bahr et al. 2001).
While it might be thought that Ganymede’s intrinsic magnetic field (Kivelson et al. 1996)
would protect the surface from bombardment by the low energy particles (ion speed <
corotation speed) that could produce the observed strong trailing side concentration of O2
on Ganymede, sufficiently low-energy particles (< 20 keV for protons) may be able to
penetrate the field on the trailing side due to E × B drift associated with the corotational
electric field of Jupiter's magnetosphere (Cooper et al. 2001). Such particles might also
be expected to produce a similar abundance of O2 on Europa’s trailing side, however, but
earlier studies (Spencer et al. 1995) did not show an O2 band on Europa.
We obtained new CCD spectra of the Galilean satellites with the Ohio State
University CCD spectrograph at the Lowell Observatory Perkins 72” telescope in June
and November 1997, using similar observational and reduction techniques to those used
previously (Spencer et al. 1995), with the difference that in the new observations we used
an off-axis autoguider to track a nearby guide star, providing more consistent centering of
the satellite in the spectrograph slit. Typical total integration time per satellite per night
was about 300 seconds. Observations are logged in Table 1. Various gratings and slit
widths were used, giving variable spectral resolution.
We concentrated on the 5770 Å rather than the 6275 Å band, because the latter is
weaker and broader and also overlaps a sharp telluric 6280 Å CO2 band, and is thus
harder to study from the Earth’s surface. To remove Fraunhofer lines as precisely as
possible, we ratioed the icy satellite spectra to Io spectra taken the same night, rather than
to a solar-type standard star. We chose Io because its ice-free surface composition made
it less likely to have surface O2 than the icy satellites. Ratioing to Io introduced a strong
curvature in the spectra due to Io’s very different continuum shape: this was removed by
dividing by a cubic polynomial fit to the spectrum, excluding the 5875 – 5910 Å region
where sodium emission from Io was prominent in the ratios.
We did not correct for wavelength-dependent atmospheric extinction, but the
airmass difference between the Io and icy satellite spectra was generally less than 0.04
(Table 1). Ratios of the same satellite at different airmasses show negligible telluric
absorptions in the 5700 – 5900 Å region, and placed an upper limit of 0.05% on the
strength of any atmospheric features due to atmospheric extinction resulting from a 0.04
airmass difference. We calibrated wavelengths by comparing Frauhofer line positions to
a solar spectrum (A’Hearn et al. 1983) before ratioing: estimated wavelength uncertainty
is 1 Å.
The ratio spectra generally contain artifacts larger than the noise at some wavelengths,
due to imperfectly canceled Fraunhofer lines, or telluric features. However, the improved
observations and analysis reveal a previously unseen weak 5770 Å O2 band in the
Europa/Io and Callisto/Io ratios (Figure 1). Though the individual spectra are noisy, all
spectra show a consistent drop in reflectance between 5805 and 5770 Å, as expected from
O2 absorption. The shape of the absorption band on Europa is more apparent when all
Europa/Io ratio spectra are averaged (Figure 2). The band seems to have identical shape
to that on Ganymede, and is seen with similar strength on the leading and trailing sides.
The single Callisto spectrum, also shown on Fig. 2, has unexplained features that may be
artifacts centered at 5550 Å. The Callisto O2 band is not much stronger than these
unexplained features, but its perfect wavelength match to the Europa and Ganymede
features provides good evidence that it is a real feature.
The depth of such broad shallow bands, ratioed to Io’s complexly curved continuum, is
difficult to measure precisely. In Figure 2 the 5630 – 5810 Å region of the O 2 absorption
is excluded from the cubic fit so that the fit does not decrease the band depth, at the risk
of introducing artifacts due to treating the region of interest differently from its
surroundings. Band depth is 0.30% for the average Europa spectrum, 0.34% for the
leading side, and 0.24% for the trailing side: the leading/trailing difference is probably
not significant. Depth is 0.33% in the single Callisto spectrum, which covers Callisto’s
trailing hemisphere. For comparison, the maximum depth of the O2 band on Ganymede’s
trailing side (Spencer et al. 1995, and Figure 2) is 1.8%, though as this depth was
obtained from a Ganymede / Callisto ratio spectrum, the true band depth on Ganymede
may be slightly higher if the Callisto spectrum also contained O2. The absorption
minimum is at 5771 ± 1 Å, consistent with the 5773 ± 1 Å reported previously for
Ganymede (Spencer et al. 1995).
The O2 band was not seen in previous reductions of Europa spectra (e.g, Figure 7 of
Spencer et al. 1995), probably because all spectra were ratioed to Callisto rather than Io.
Europa and Callisto (at the one longitude observed so far) apparently have very similar
O2 absorption band strength, so the band disappears in Europa/Callisto ratios. The
similar O2 bands on Europa and Callisto, despite their very different mean surface
compositions, might lead to suspicion that the band is an artifact, perhaps due to a feature
in Io’s spectrum. However, we consider this unlikely. If the feature were on Io, it would
have to be a reflectance excess with the same shape and wavelength dependence as the
indubitably real Ganymede O2 absorption, but of opposite sign, and this seems highly
unlikely. No instrumental artifact should appear only in Europa and Callisto spectra, but
not concurrent Io spectra (Io does have a smaller mean distance from Jupiter, and thus
more potential for artifacts due to Jupiter light contamination, but on several dates
Europa was at a similar or smaller distance to Jupiter than was Io). We thus consider the
only plausible explanation of the data, however surprising, to be that a weak O2 band is
present at similar strength, 0.3%, at all longitudes on Europa and at least one longitude on
O2 abundance is difficult to constrain from these observations, as the intrinsic strength of
the 5770 Å band is a very strong function of the density of the O2, which is unknown. On
Ganymede we estimated a maximum absorption SDWKOHQJWKRIPLIWKHEDQGZDVDV
VWURQJDVWKDWRIVROLG.22 as reported by Landau et al. (1962), (Spencer et al. 1995). In
Fig. 2 we use new measurements of the strength of the 5770 Å band in 22 by Calvin et
al. (2002) to match the observed band depths on Callisto, Ganymede, and Europa. The
were appropriate for the Galilean satellite O2SDWKOHQJWKVRIURXJKO\PRQ&DOOLVWR
PRQ(XURSDDQGPon Ganymede’s trailing side would be implied, though it can
EHVHHQIURP)LJWKDWSKDVH22 does not match the band shape precisely. Translation
from path length to absolute abundance then requires knowledge of the typical visiblewavelength photon path length in the H2O matrix (if the O2 is dispersed in H2O ice), and
is thus even more uncertain.
The presence of condensed high-density O2 on Europa and Callisto constrains hypotheses
for its formation on all the icy satellites, though we leave detailed exploration of these
constraints to future papers. The presence of O2 at all longitudes on Europa in similar
amounts, in contrast to Ganymede, suggests that it is not generated by low-energy plasma
bombardment, which on Europa, due to the lack of a deflecting magnetic field, strongly
favors the trailing hemisphere. The idea that O2 might be formed at all longitudes on
Ganymede, but destroyed or buried by micrometeorite bombardment on the leading side
(Calvin and Spencer 1997), is also challenging to reconcile with the lack of an obvious
leading/trailing asymmetry on Europa, which will have an even greater leading/trailing
bombardment asymmetry due to its greater orbital speed. The reduced abundance of O2
on Europa and Callisto compared to Ganymede also requires explanation: possible
explanations for Europa might include scavenging of the oxygen by sulfur, which is
probably more abundant on Europa’s surface than on Ganymede’s (Carlson et al. 1999a),
or surface erosion by charged particle bombardment. For Callisto, the lower surface ice
abundance than on Ganymede is likely to be part of the explanation for the lower O2
The observations confirm the highly oxidizing nature of Europa’s surface inferred from
the earlier detection of H2O2 and possible H2SO4. If these oxidants can be transported
from the surface to the interior, they could conceivably provide an energy source for
possible Europan organisms (Chyba 2000).
This work was supported by NASA grant NAG5-4262. Thanks are due to Ray
Bertram and Mark Wagner for assistance with the OSU spectrograph, and to John Cooper
for insights into plasma/satellite interactions.
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Airmass Airmass Airmass
l Resn.,
Notes: Spectral resolution is defined as the full-width-half-maximum of an unresolved
spectral line. CML = central meridian longitude.
Orbital Longitude
0.3% band depth
Wavelength, Angstroms
Figure 1 Spectra of Europa (single, unlabeled lines) and Callisto (double, labeled, line),
divided by contemporaneous Io spectra to remove Fraunhofer lines, arranged according
to the central longitude of Europa or Callisto at the time of the observation. The band
depth scale is also shown. Spectra are divided by a cubic fit to correct for the large
difference in continuum shape between Io and Ganymede (Spencer et al. 1995): the fit is
shown by the horizontal lines. Sodium emission from Io, which appears as a negative
feature at 5893 Å, has been cropped out of the spectra. A weak 5770 Å absorption band
due to O2 is seen in all spectra as a drop in the relative reflectance between 5805 and
5771 Å (vertical lines).
Reflectance / Continuum
C. Trailing
E. Trailing
E. Leading
E. Average
G. Trailing
Solid O2
Wavelength, Angstroms
Figure 2 Averages of all Europa/Io ratios from Figure 1, and separate averages of
Europa’s leading and trailing hemispheres (labeled “E.”), showing the weak O2 band.
Our single Callisto/Io ratio, from Fig. 1 (labeled “C.”) and a Ganymede trailing
hemisphere spectrum from 1994/04/05, ratioed to Callisto (labeled “G.”), from Spencer
Europa and Ganymede respectively. Unlike Fig. 1, the spectra are normalized to a cubic
fit which excludes the 5630 – 5810 Å region of the O2 band, to allow more accurate
measurement of the band depth. Finally we show laboratory spectra of all three phases of
solid O2, with arbitrarily vertical scaling, from Landau et al. (1962). All spectra are
offset vertically for clarity. Vertical lines show the wavelengths of minimum reflectance
(5771 Å) and return to continuum (5805 Å) of the Ganymede O2 feature, for comparison
with Europa and Callisto.
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