Lunar and Planetary Science XXXV (2004)
2079.pdf
H2O2 synthesis induced by irradiation of H2O with energetic H+ and Ar+ ions at various temperatures. R. A.
Baragiola,1 M. J. Loeffler,1 U. Raut,1 R. A. Vidal,1 and R. W. Carlson.2 1University of Virginia, Laboratory for
Atomic and Surface Physics, Charlottesville, VA 22904, USA, 2Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, California, 91109.
maintaining this temperature, measured the ratio of
H2O2 to H2O band areas as a function of time (figure
5). From figure 5, one can see that the relative amount
of H2O2 increases with time, which shows that it is not
uniformly dispersed in the water ice. Thus, it indicates
that the H2O2 molecules might be segregated together
in pockets of the water ice.
References:
[1] Carlson, R. W. et al. (1999) Science 274, 38588. [2] Moore, M. H. and Hudson R.L. (2000) Icarus,
145, 282-288. [3] Gomis, O. et al. (2002) Planet. Sp.
Sci. (in press)
Wavenumber (cm-1)
2900
2800
irradiated water film
virgin film
5th polynomial fit
0.2
2700
2600
2500
-1
Wavenumber (cm )
4000
Arbitrary Units
3000
Arbitrary Units
Introduction: The detection of H2O2 on Jupiter’s
icy satellite Europa by the Galileo NIMS instrument
[1] presented a strong evidence for the importance of
radiation effects on icy surfaces. A few experiments
have investigated whether solar flux of protons incident on Europa ice could cause a significant if any
H2O2 production [2,3]. These published results differ
as to whether H2O2 can be formed by ions impacting
water at temperatures near 80 K, which are appropriate
to Europa. This discrepancy may be a result of the use
of different incident ion energies [3], different vacuum
conditions, or different ways of processing the data.
The latter possibility comes about from the difficulty
of identifying the 3.5µm peroxide OH band on the
long wavelength wing of the much stronger water
3.1µm band. The problem is aggravated by using
straight line baselines to represent the water OH band
with a curvature, in the region of the peroxide band,
that increases with temperature. To overcome this
problem, we use polynomial baselines (figure 1) that
provide good fits to the water band and its derivative.
In our experiments we use ice films thicker than
the range of the bombarding ions (100 keV protons,
and 50 and 100 keV Ar+ ions). We found that H2O2 is
produced even at 120 K using 100 keV protons. We
also found that, after irradiation, the H2O2 band qualitatively appears to decrease as we raise the temperature
(e.g. 40 K to 135 K, figure 2). However, when we fit
the baseline we find that the calculated band area (and
resulting H2O2 column density) remains constant until
the film begins to desorb from the gold substrate (figure 3).
Using Ar+ ions we found, as Gomis et al [3], that
heavy ions produce substantially more peroxide than
protons, at the same energy and ice temperature (figure
4).. We attribute this to the well known enhancement
of radiation molecular products in ice when the density
of deposited energy is high. This also explains why
the experiments of Moore et al [2], using weakly ionizing 800 keV protons on films smaller than the range of
the ions, produced much smaller peroxide concentrations. These results can be used to predict that the
population of similarly weakly ionizing high energy
electrons at the icy satellites will be relatively inefficient to synthesize H2O2 in ice.
To study the stability of H2O2 we raised the temperature of an irradiated water ice film to 165 K and
2000
4
2
0
2
3
4
5
Wavelength (microns)
0.1
0.0
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
Wavelength (microns)
Figure 1. An example of the curve fit used when calculating a baseline for the H2O2 region. In this case, a
6x1018 H2O/cm2 water film at 80 K was irradiated with
100 keV protons to a dose of 2.7 x 1015 H+/cm2.
Lunar and Planetary Science XXXV (2004)
2079.pdf
-1
Wavenumber (cm )
2900
2850
2800
2750
2
2950
H2O2 Column Density (10 molecules/cm )
0.20
Optical Depth
0.15
2
1
0
2
3
4
5
Wavelength (microns)
0.10
40 K
80 K
120 K
135 K
0.05
0.00
3.35
3.40
3.45
3.50
+
100 keV H
+
100 keV Ar
+
50 keV Ar
2
17
Optical Depth
3
3.55
3.60
3.65
1
0
3.70
20
40
Wavelength (microns)
60
80
100
120
Irradiation Temperature (K)
Figure 2. The shape of the H2O2 band in a Ar+ irradiated, 3.2x1018 H2O/cm2, film given with temperature as
a parameter..
Figure 4. The H2O2 column density at saturation values
of the irradiation dose for 100 keV H+ ions incident on
6x1018 H2O/cm2 films at various temperatures, 100
keV Ar+ ions incident on 3.2x1018 H2O/cm2 film at 40
and 80 K, and 50 keV Ar+ incident on a 1 µm H2O film
at 80 K.
ratio of H2O 2 to H2O column densities
1.5
17
2
H2O2 Column Density (10 molecules/cm )
0.25
2.0
1.0
0.5
0.0
20
40
60
80
100
120
140
160
180
Temperature (Kelvin)
T = 165 K
0.20
0.15
0.10
0.05
0.00
0
200
400
600
8 00
1000
1200
1400
1600
1800
time (seconds)
Figure 3. H2O2 column density vs temperature. The
hydrogen peroxide was synthesized at 40 K by 100
keV Ar+ ions incident on a 3.2x1018 H2O/cm2 film for
which a saturation column density was reached. The
temperature of the processed film was raised after the
irradiation was stopped.
Figure 5. The abundance of H2O2 relative to H2O as
the irradiated film sublimes at 165 K. Before raising
the temperature to 165 K, the 6x1018 H2O/cm2 film had
been irradiated at 20 K with 8.0 x 1015 keV H+ ions/cm2
at 100 keV.