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Ion irradiation experiments
Mv. Space Res. Vol. 13, No. 10, pp. (10)189—(10)198, 1993 Printed in Great Britain. All rights resreved.
0273—1177~3$24.00 Copyright @ 1993 COSPAR
ION IRRADIATION EXPERIMENTS G. Strazzulla, G. Leto and M. E. Palumbo lstituto diAstronomia, UniversiM di Catanja, Città Universitaria, 1-95125 Catania, Italy
ABSTRACT We report new results from recent experimental studies on physico-chemical effects induced in frozen gases and mixtures (CO, Ca 2, CH4, H20 and SO2) simulating ice targets in space (frosts on external planets and satellites, and comets). In particular we have studied, by UV-Vis-IR spectroscopy, the formation of new molecules, the alteration of band profiles and the spectral behavior of the original compounds. The ratio CO/CO2 has been measured, as a function ofion Iluence, in several frozen gases (or mixtures). These results are discussed in the light of their relevance in some planetary environments. In particular, we suggest that ion irradiation, which is especially intense at lo, can modify the spectral properties of frozen SO2 thus contributing to explain the spectral behavior of this Jovian moon. The production, by cosmic ion irradiation, of CO and CO2 at the surfaces and/or in the atmospheres of Titan, Triton, Pluto and Charon is discussed. Some predictions of the expected CO/CO2 ratio at the surfaces and/or in the atmospheres of those objects are provided. INTRODUCTION The presence of intense fluxes of charged particles impinging on the solid surfaces of planets, satellites, and rings in the Outer Solar System produces a number of effects the knowledge of which appears to be essential for understanding the evolution of these objects. This type of research is based on laboratory simulations of relevant targets (e.g. molecular solids) bombarded with charged particles under physical conditions more or less similar to the astrophysical ones. Since the measurements in 1978 /1/ of very large sputtering yields for water ice bombarded by keV-MeV ions, molecular solids have been intensively studied in several laboratories. In particular, the physico-chemical effects induced by fast ions colliding with solids of astrophysical interest (frozen gases, carbonaceous and organic materials, silicates, etc.) have been investigated (e.g. /2,3,4/). The most studied effects are erosion (sputtering) and chemical changes in the target material. Erosion, as a consequence of the ion-target interaction, causes the ejection of atoms or molecules (both already present and newly formed) from the target. Chemical changes include the formation of new molecular species residing on the target and, in particular, the formation of a refractory residue. Another effect, which could be of primary relevance, but has been, with a few exceptions, /5/ neglected, is the fate of the incoming ion, e.g. its chemical bonding with target atoms. Here we report new results on physico-chemical effects induced on frozen gases and mixtures (CO, CO2, CH4, H20 and SO2) simulating ice targets in space (frosts on external planets and satellites, and comets). In particular we have studied, by UV-Vis-IR spectroscopy, the formation of new molecules, the alteration of band profiles and the spectral behavior of the original compounds. In the Solar System frozen surfaces are continuously bombarded by energetic ions from solar wind and flares, planetary magnetospheres and galactic cosmic rays. Many detailed applications of the above type of experiments have been discussed in recent years. These include modifications undergone by interstellar and interplanetary grains /2,6,7,8,9/, comets /2,10,11,12/, asteroids /10/ and satellites in the outer Solar System /13,14,15,16,17/. The laboratory results that have been applied in the references above concern mainly the sputtering and production of organic colored materials. Here we discuss our, and others’, new experimental results concerning the production of new species, in the light of their relevance in some planetary environments. UV-VIS-IR SPE~TROSCOPY To irradiate frozen gases and obtain “in situ” UV-Vis-IR spectra, 1 =2.27-25 a scattering ~m) or tochamber an UV-Vis-NIR was faced(190-3200 to an FTIR nm) Perkin-Elmer (mod. lambda 1710) spectrophotometer 19) dispersive spectrometer (4400-400 through cm KBr windows. The vacuum was better than i0~ (10)189
(10)190
G. Strazzulla et al. TABLE 1 Spectroscopic Properties of some Molecules in the Solid State
molecule
vibrational mode
i’ 1) (cm
A (pm)
3300 2205 1655 810
3.03 4.53 6.04 12.34
2139 2136 3010 1300 2340 656 35714 29412 21739 2459 1340 1151 525
4.68 4.67 3.32 7.69 4.27 15.24 0.28 0.34 0.46 4.07 7.46 8.69 19.05
A (10—’~cm/mol) literature” this work
a (10~cm’) res = 2 cm’
H 20
CO Gil4 CO2
0
—
H stretch
0—H bend libration C E 0 stretch in pure CO ice in H20 ice C — H stretch C — H deformation C = 0 stretch C=Obend
502 combination (Mi + 113) asymmetric stretch (113) symmetric stretch (i,~) (P2)
“/18/;
20
21
0.8 3
1.1 2.8
1 1.7 0.6 0.6
1.1 > 1.5 0.6 0.5
76b
1.94
3.01 1.3 0.97 8.88
11b
005b 32b 04b
b/19/
mbar. Frosts were accreted onto a silicon crystal (111) or on a quartz substratum put in contact with a cold finger (10-300 K) by admitting gas (mixtures) into the chamber through a needle valve. All of the spectra shown in the following sections are ratioed to a background including the substratum. The substratum forms an angle of 45 degrees with the light beam and the ion beam so that, during irradiation, spectra can be obtained without tilting the sample. 2 spot on the target (greater During than thecondensation, spot of the light icesbeam) can beand bombarded currents lower by 3 keV thanions. few micro-ampere/cm2. The beam produces Since a 2x2 cm the penetration depth in frozen organic targets is only about 0.05 ~m for 3 keV He ions and such thin targets would show very small JR features, the samples have to be irradiated during deposition. Each experiment was stopped only when good spectra were obtained. For calibration purposes, many spectra of pure gases (mixtures) have been taken at different rates of deposition, i.e. at different gas pressures (10~-10—~ mbar). The sample thickness was determined from previously obtained calibration data. These latter had been obtained by depositing a quantity of ice whose thickness was checked by counting the interference fringes of a reflected laser light (514 nm). Thus, when a target was simultaneously irradiated at a known ion current, we knew both the number of molecules deposited per time unit and the number of ions impinging in the time unit: i.e. we were able to evaluate the irradiation dose of that experiment (in eV/16 amu). Moreover, taking “in situ” spectra, we could stop the experiment only when the irradiated sample was such that it showed a spectrum with well defined features. [Note that here the doses are given in eV per small molecule (16 amu), because this is a convenient way to characterize chemical changes /2/ and to compare them with experiments on other targets and, with some caution, to other projectiles]. For the main absorption bands of all the studied ices, we have evaluated the absorption coefficient a (cm~) and the integrated absorbance A (cm/mol). Our results are listed in Table 1, where they are compared with those reported in refs. /18,19/. The agreement between the two sets of data is generally good. EXPERIMENTAL RESULTS As is already well-known (e.g. /20/), ion irradiation of simple ices produces new species that either remain in the low temperature target or are directly released (chemical sputtering) in the gas phase. If the irradiated samples are warmed up they release old and new species in the gas phase, each species being released at different temperatures depending on the species itself, on the matrix in which it is embedded, and on the irradiation dose. The results presented here refer in particular to the production, at low temperature, of CO 2 from H20:CO mixtures; of CO from H20:C02 mixtures; and of both species from H20:CH4 mixtures. The CO/CO2 ratio frozen on the target has been measured, both as produced at. low T (10K) and as a function of T (10-200K). We have also evaluated that ratio as it would appear in the gas phase, e.g. in the atmosphere of a celestial body that can bind gravitationally all of the species released from its surface. In view of its relevance for lo we have also studied the variation in the absorption properties of SO2 ice in the UV-Vis-NIR region, in response to ion irradiation.
Ion Irradiation Expedments
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H
20:CO Mixtures
Pure CO ice at 10 K shows an absorption feature at 2140 cm_i, due to the fundamental vibrational mode CEO, with FWHM 4 cm’ measured on an optical depth scale. Spectra of pure CO ice have been compared with those of CO irradiated with 3 keV He ions and it has been shown that the FWHM of the band increases /21/. This is a general result: ion irradiation causes the broadening of the bands. This is then an alternative to the broadening due, for instance, to shape effects (roughness, oblateness, etc...). 1. Figure The formation, 1 shows aafter comparison irradiation, between of a irradiated new band and at 2340 not irradiated cm1 is evident, pure CO which in the is easily spectral attributed range 2100—2400 to CO cm 2. The CO/CO2 ratio can then be evaluated by using the measured integrated absorbances (Table 1) and is reported in Figure 2.
par. — —
0.8
.~
—
—
CO
-,-
CO 12 eV/18.mu
-
-
0.6
0.4
-
ii
0.2-
o_______ 2400
2300 2200 w~v.nurraber (cm’)
2100
Fig. 1. JR spectra of pure CO ice and CO irradiated with 12 eV/l6amu, in the spectral range 2400-2100 cm~’. 10
—
—~
*
__—o—
----_±---
8
—— -~
8
“
0.1-,
-
/
ACO H~0:C0 ~- H~0~C0. 0c0~ * i4~O:CM4
./
I II
0.01’ 0
-
20
..,‘,,,
do..
40 (eV/l6amu)
60
Fig. 2. CO/CO2 ratio vs ion irradiation dose for different ices. Dashed lines, connecting points due to similar samples, have been drawn to guide the eye.
The spectral properties of CO strongly depend on the host molecules when it is mixed in with other species. Many ice mixtures have been already studied /22,23,24/. The CO band 1. profile This latter in H20 has icegenerally is very peculiar: lower intensity it shows than a the ion irradiation intensity of the 2152 cm_icm~ band decreases as already noted for UV irradiation mainformer. featureUpon at ~2136 cm~ and the a secondary feature at 2152 /22/. In Figure 3 a spectrum of an unirradiated mixture (H 20:CO=lO:1) is compared with that obtained after irradiation 1) (8is eV/l6amu). due to CO The formation, after irradiation, of new bands is evident, the most prominent of which (at 2340 cm 2. The CO/CO2 ratio has been measured and is reported in Figure 2. If we compare this ratio with that coming out from irradiation of pure CO, we note its strong decrease due to CO oxidation driven by
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G. StrazzuiJa etal.
the ion-induced alteration of water, whose main effect is the formation of OH radicals along with oxygen and hydrogen.
}i.0:CO— 10,1 —
—
—
—
}iO,CO
+
8
.v/i8.mu
1—
0.8-
.~
0.8-
0.4-
0.:
~
2400
,~
~.
2300 wavenurnber
2200 (crn’)
2100
Fig. 3. JR spectrum of a H
2O:CO=10:1 mixture compared with that of an irradiated mixture (8 eV/l6amu).
H2O:CO2 Mixtures When pure CO2 is irradiated with 3 keV He ions, new bands appear. Among these the most intense peak occurs at 2141 cm’ and is due to CO. The CO/CO2 ratio has been measured after different doses and is reported in Figure 2. Similarly, when H2O:CO2 mixtures are irradiated, new bands appear. Again the most intense of these new bands (at 2142 cm’) is due to CO. The CO/CO2 ratio has been measured after different doses and is reported in Figure 2. Jf we compare this ratio with that obtained from irradiation of pure CO2 we find that they are indistinguishable. This fact testifies that the status of oxidation depends on the C/O ratio in the original target. In fact it does not vary too much: from O/C=2 (pure C02) to O/C=3 (mixture 1120:C02=1:1). In the case of irradiation of H2O:CO mixtures, this ratio varies by an order of magnitude from O/C=1 (pure CO) to O/C=11 (mixture H2O:CO=10:1). H20:CH4 Mixtures In Figure 4 a spectrum of an unirradiated mixture (H20:CH4=1:1) is compared with that obtained after irradiation (34 eV/l6amu). Once 1) are again duethe to formation, CO and CO after irradiation, of new bands is evident, the most intense of which (at 2138 and 2342 cm 2 respectively. The CO/CO2 ratio has been measured and is reported in Figure 2. Warm-up Effects The results presented above have been obtained at very low temperatures (10 K) and may be appropriate to “simulated” planetary surfaces during particular stages of their evolution or seasons. Because of the expected temperature variation on the planetary surfaces, it is interesting to study the variation in the spectral behavior and in the chemical composition, driven e.g. by a differential release in the gas phase, of the various species produced by ion irradiation. In particular, we have studied the variation of the CO/CO2 ratio in the frozen irradiated target, as a function of temperature. By subtraction we could also estimate this ratio as it would figure in the gas phase, in the hypothesis that each released molecule remains gravitationally trapped i.e. in the planetary atmosphere. The results are exemplified in Figure 5a and 5b. Figure 5a refers to the CO/CO2 ratio obtained from a mixture H20:C02=1:1 irradiated at 27 eV/l6amu. Figure 5b reports the results obtained by irradiation (34 eV/l6amu) of a mixture H2O:CH4=1:1. Some general conclusions can be drawn: since the CO molecules begin to leave the target at temperatures lower than CO2 molecules, the ratio in the solid phase decreases. The details of this decrease depend on the different environment the species are outgassed from, which, in turn, depends on the original mixture and irradiation history. Looking also at Figure 2, it is, however, clear that when mixtures containing water are considered, the CO/CO2 ratio, in the solid phase, is less than I whatever the irradiation dose and the temperature is. In the gas phase the situation changes: the CO/CO2 ratio is much greater than I for T lower than about 100 K, because only CO is released; it decreases to the initial value of the solid phase, when the temperature increase is such as to let all CO and CO2 evaporate, if no atmospheric loss occurs. Note that in planetary environments the temperature parameter can be substituted, with caution, for the time parameter: in fact, when the temperature is such (say 100 K) that CO2 begins to be released, then the CO/CO2 ratio is expected to vary with time as it does, in the laboratory,
Ion Irradiation Experiments
(10)193
it.0,CH.—1,1 —
—
—
H.0,CH~+ 3? .V/18~mu
—
1-
-
II
0.6-
Il
0.4-
II II
0.8
.~
—
-
II I 0.2
-
i 0
I
II
I I III I \
II II ,
-.-..
~
~
I 2400
~
I
-
-
—----.-—
-
....I,.,,
2300 wavenumber
I
2200
2100
(cm~)
Fig. 4. IR spectrum of a H
20:CH4=1:I mixture compared with that of an irradiated mixture (34 eV/l6amu).
with temperature.
10
‘‘I’’\’i’’’I’’’I’’I’’
1o0,.,~
after 27 eV/l~6amuon N,0:C0,=1:l
after ~4 eV/L6amu on H50:CH4—l:1
\ gas phase
1
10
\
.
.
\ r., S.-’ o
ga. phase
‘S
13--e
“.-
‘5
1-
————
‘S
~EI
0.1-
.
0.1
-
-
solid phase
solid phase
a) 0.01
-
A-.——
III,
II
50
100
150
b) ______________________
200
Temperature (K)
250
50
100
150
200
250
Temperature (K)
Fig. 5. a) Expected CO/CO2 ratio vs temperature in the surface and in the atmospere of a planet, from a H20:C02=1:1 mixture irradiated with 27 eV/l6amu (see text). b) Expected CO/CO2 ratio vs temperature in the surface and in the atmospere of a planet, from a H2O:CH4=1:I mixture irradiated with 34 eV/l6amu (see text). Dashed lines have been drawn to guide the eye.
SO2 Ice UV-Vis-NIR (250-1250 nm) transmittance spectra, normalized to I at 1020 nm, of pure SO2 ice deposited at 10 K and of SO2 irradiated with He (37 eV/I6amu) and D (42 eV/l6amu) ions are shown in Figure 6. The spectrum of pure SO3 ice is characterized by a very strong band at about 280 urn and two weaker bands at about 340 and 460 nm. Moreover, the frost appears transparent in the continuum. At first glance, this spectrum seems very different from the reflectance spectra reported in the literature (e.g. /25/) which exhibit a band at about 340 urn; have a flat, highly-reflecting continuum at greater wavelengths; are completely opaque at wavelengths lower than 310 nm; and do not exhibit any feature at 280 nm. Probably the difference is due to the different thicknesses traversed by the light: at higher pathlengths the very strong absorption centered at 280 nm is completely saturated. Upon irradiation, the SO2 frost layers become progressively darker and darker, and a strong absorption in the continuum appears at progressively higher wavelengths. The absorption bands become progressively less evident, and at
(10)194
(3. Strazzulla Ct at.
II
‘‘I’’’I’’’I’’’I~’’
0.9
o
.~“ .•
C2
0.6
*‘
0.?
-
,
1
As deposited
—
I,,
0.6
3 keY He 37 (.V/lS AMU)
/
/ /
.~
.~‘
-
*1,—I ~
I
/ —
— —
1.5 keV 0~
-
42 (eV/18 MW)
1
0.9
:1 0.4
o
-
0,3 0.0
S.
0.1
a
—
400
I
I
600
600
I
1000
1900
Wavelength (nm) Fig. 6. UV transmission spectra of SO
2 ice deposited at 10 K. Spectra are normalized to 1 at 1020 nm. Solid line refers pure SO2 ice 0.3 jzm thick; short dashed line refers to SO2 deposited under He+ irradiation (37 eV/16 amu); long dashed line refers to SO2 deposited under D~irradiation (1.5 keV, 42 eV/16 amu).
higher doses they appear as shoulders over the declining continuum. This event has to be ascribed to the chemical effects induced by the beam that produce mainly sulfur trioxide along with a sulfate and S8, as evidenced by Moore /26/ from a study of IR spectra of proton irradiated SO2 frost. ION IRRADIATION IN THE SOLAR SYSTEM The production of new molecular species by iois irradiation of simple frozen gases may have a fundamental importance in several ice-objects in the Solar System. In principle, when a given molecular species, say CO, is observed on the surface of a satellite or a planet, different conclusions about its evolutionary history may be drawn. These depend on whether the species is believed to be native or produced “in situ” by some process(es), as may be the case of ion irradiation. In the following we discuss new results, obtained either by us or other groups, in the light of their relevance on objects in the external Solar System. Jupiter’s Satellites Voyager results indicate that sulfur and oxygen are the most abundant ions in the lo torus /27,28,29/. These ions are supposed to be ejected, as neutrals, from a sulfur-rich Ionian surface. Some authors proposed an louisa sulfur volcanism with colors due to the fast quenching of unstable allotropes of sulfur /30,31/. Observations of the 4 pm SO2 band on Jo indicate that a large amount of SO2 exists on its surface. Although ultraviolet observations by Voyager and IUE show little SO2, it has been suggested that SO2 must be intimately mixed with sulfur (or other materials) so that at every wavelength the darker component dominates the spectrum /32/. The experimental results shown above (see Figure 6) suggest a possible influence of ion irradiation, particularly intense at lo, in modifying the spectral properties of frozen SO2 causing strong absorption in the UV that masks the spectral signature at 280 nm. We are investigating the possibility of reconciling UV and IR observations by a process induced by ion irradiation. Several possible mechanisms for the transport of sulfur and oxygen from b’s surface to the torus have been investigated /33,16,26/. Among these, direct sputtering from the surface, caused both by MeV magnetospherical ions (H, He, 0, 5) and by corotating ions, cannot he considered as the only source of neutrals in the bo torus /16,17/. Nevertheless, this mechanism dominates over sublimation on most of b’s surface. Recent IR (2.5-5 pm) observations of Io /34/, confirmed the presence of absorption bands attributable to sulfur dioxide frosts (the most intense being at 4.07 pm), and detected four additional infrared spectral signatures. Salama ~ /34/ attributed the 2.97 and 3.15 ~m band to H20 and the 3.85 and 3.91 pm band to H2S. They,.qualitatively matched the observations with H20 (‘-.‘ 0.1 percent) and H2S (‘-.‘ 3 percent) mixed with sulfur dioxide at 100 K, but the
Ion Irradiation Experiments
(I0)195
formation process(es) of those trace species is (are) not completely clarified. They also suggested a possible exogenic source, namely the insertion of energetic protons in the SO 2 ice. Laboratory experiments to confirm or disprove such a hypothesis are feasible and are planned in our lab. SO2 ice has been proton irradiated (at too low doses to detect species incorporating the projectile) and the main product of irradiation has been found to be sulfur trioxide along with a sulfate and S~/26/. However, those species show strong bands only at wavelengths greater than 5 pm. A new, very important class of absorption features on Jo has been discovered at 2.1253 pm by /35/ and tentatively attributed to CO2 multimers or “clusters” on Jo. Larson ~ /36/ confirmed the observation but challenged its identification, and suggested the further study of sulfur compounds, eventually modified by radiation processes, to search for a more plausible identification. Water ice is supposed to be a major constituent of the surface of Europa, Ganymede and Callisto, although the question about the presence of other species is still unresolved. There have been many considerations of the effects of magnetospherical ion-irradiation of these bodies, making use of the particle fluxes measured by Voyager /37,38,39,40/. It has been also shown that, after ionization, sulfur atoms from Jo can be trapped in the magnetosphere and then implanted mainly in the trailing surface of the satellites. This could explain the finding, by spectroscopic measurements in the UV, suggesting that sulfur atoms form S-0 bonds in Europa’s ice /41/. Recent laboratory studies on the alteration of the UV-Vjs reflectance spectra of water ice by ion irradiation /42/ showed a decrease in the reflectance in the UV, attributed to rearrangement processes that affect the physical structure of ice. Those authors conclude that the ratio in reflectance between the trailing and leading hemispheres of Europa and Ganymede is likely to be due to a different ion irradiation of the two hemispheres rather than to the implantation of reactive species such as sulfur. Saturn’s Satellites Arguments similar to those applied to the Jovian major moons have been used for Saturn’s satellites. By using particle fluxes measured by Voyager, it has been shown that molecules, eroded from water-rich surfaces, which are likely to escape because of the low mass of these satellites, travel along elliptical orbits around the planet. Detailed calculations show that photodissociation, loss and transport processes of the eroded species can well explain the production of a heavy ion torus in the inner magnetosphere, as observed by the Pioneer and Voyager plasma experiments /43,44,45/. The effects of UV photons, electrons and charged particle interaction with Titan have been discussed many times, with particular emphasis on the production and condensation of organic gases /46/. Due to the thick atmosphere of the satellite (of the order of 1500 mbar) only the most penetrating radiation (magnetospheric electrons and cosmic rays) impinges on the surface. This latter is thought to be an ocean, one to several kilometers deep, consisting mainly of ethane, the dominant end product of methane photolysis /47/. It is thus extremely difficult to establish whether ion irradiation plays an important role in modifying Titan’s surface. In our opinion, ions could have played a relevant role in the early stages of Titan’s life when its atmosphere was formed, probably by a devolatihization of the ices that accreted to form the satellite /48/. The present atmosphere is composed mainly of nitrogen (.65-.98 mole fraction), methane (.02-.1 mole fraction) and, perhaps, argon (0-25 mole fraction). Trace elements 5-1.5xl04 include the mole products fraction) of and CO photolysis 9 mole Their in such a reducing atmosphere requires special hypotheses. In methane’s and, fraction). interestingly, of presence two oxygen-containing compounds: CO (6x10 the framework 2 (1.5x10 of the hypothesis that the original surface would be made of clathrate hydrate (CH 4-7H2O), it is assumed that CO is incorporated in the ice along with other gases such as nitrogen and argon. CO would then be outgassed and, reacting with OH, would produce carbon dioxide. The source of the OH is thought to be particles of ices bombarding the orbiting Titan. On the basis of our results we can suggest, alternatively, that both CO and CO2 are formed, by the harder radiation environments in the early stages of the Solar System, directly on the surface made of methane and water. The CO/CO2 ratio is expected to be less than 1 on the surface (see Figure 5) but, owing to the very different outgassing temperatures, a ratio much higher than I is expected to be measured in the atmosphere. It is well known that lapetus has the most unusual photometric properties in the Solar System. Its leading hemisphere is among the darkest surfaces, while a large fraction of its trailing hemisphere is nearly as bright as snow. A number of models have been proposed to account for this asymmetry. In recent years, two lines of thought have been particularly debated. The first one /49,50/ proposes that the dark areas contain organic chromophores produced by UV irradiation of methane-rich ice and that the albedo pattern results from ballistic redistribution of surface material in response to a gradient in the impact flux. The second /51,52/ proposes that the dark material is a native component of an original icy surface concentrated in a thin devolatilized regolith produced by bombardment of dust from Phoebe. Similar material dominates the surfaces of D-type asteroids. However, both models are not fully accredited. The two models agree on one point: they both reject a previous hypothesis which states that the dust from retrograde Phoebe accumulates on the leading hemisphere of prograde lapetus, eventually cancelling the original icy surface from view. This is based on the fact that the spectral behavior in the 0.3-2.2 pm region is quite different for the two satellites. Phoebe has a flat spectrum (C-type asteroid-like), and the dark side of lapetus a red spectrum (D-type asteroid-like). To overcome this problem it has been suggested that only the outermost layers of Phoebe’s crust are carbonized by cosmic protons. Thus meteoritic impacts on Phoebe lead to the injection of materials which, coming from deeper layers, differ in color from the carbonized ones located on the surface /15/. Typical material arriving on Japetus from Phoebe could then have different spectral properties from those of Phoebe’s surface.
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G.Strazzullaetal.
Uranian Satellites Voyager 2, upon its close approach to Uranus and its moons, discovered radiation belts more intense than those of 2 sec _1) of, almost exclusively, Saturn, comparable to Van Allen belts at Earth and high fluxes (10 6107 ions cm protons at higher (greater than 500 keV) energies /53/. A first explanation for the possible cause of the dark coloration of the solid surfaces, inferred from the observations, was that over 100 years, 1016 protons per square centimeter produce dark organic residues on a surface supposed to have been originally covered by frozen methane /54,14/. A problem however arises: does methane exist on Uranian satellites and rings? As far as we know no observational evidence has been found for that so far. Earth-based observations clearly indicate the presence of water ice (or frost) on Ariel, Umbriel, Titania and Oberon. The presence of a dark, spectrally bland component has been inferred on/in water-ice surfaces /55/. Moreover, the color temperature at the distance of the Uranian satellites is about 80 K /56/, i.e. too high to avoid a rapid sublimation of methane ice. However, all the theoreticians agree on the fact that in the proto-uranian nebula the temperature was so low that methane condensed as a major component on protoplanetesimals /57,58/ and on newly formed satellites /59/. This temperature remained low for some time before the Sun’s transition through a high luminosity (T Tau) phase. After this phase, whose duration is estimated at a million years, the temperature dropped to the present value. During the T Tan phase the enhanced fluxes of energetic ions would have caused the formation of organic materials down to depths of several centimeters. In the post-T Tau phase further proton irradiation would have produced darker and darker material. The above astrophysical scenario has been successfully simulated in some experiments and discussed in the recent literature /17/. The satellite surfaces have very likely been completely resurfaced since the end of the T Tau phase. However, meteorite impacts have exposed a water-rich surface mixed with some organic contaminants left over after the proto-uranian nebula phase. Following laboratory experiments, it is interesting to note that whatever the kind of organics irradiated, they are changed into residues of very low albedo, after proton irradiation doses corresponding to 100-10,000 years at Uranus. This time is much shorter than the roughly estimated resurfacing time. This implies that, whatever the nature of the freshly resurfaced areas on the Uranian satellites, as long as they contain carbon compounds, they can be rapidly darkened by ion bombardment. Thus an interesting question about the nature of the dark component on Saturn’s satellites is still unresolved: is it an organic material darkened by particle irradiation or an inorganic one? A key point, suggested by the experiments, to confirm the former case, would be the observation (or time exclusion) of the presence of spectral signatures due to those materials, or to the products synthesized along with time organic dark refractory materials, e.g. CO and CO 2. The Neptunian Satellites Neptune has eight known moons /60/, two of which, namely Triton and Nereid, were known prior to Voyager 2’s encounter with the planet /61/. There is little information on Nereid, whose origin by capture has however been suggested /62/. Spectrophotometric observations in the 0.3-2.5 urn range give indications of the presence, on Triton’s surface, of nitrogen and methane in a frozen state /63/. Moreover, Voyager observations showed that nitrogen is the main constituent of Triton’s atmosphere and that methane is scarcely represented /64/. In the presence of an atmosphere, the ions that can irradiate the surface amid possibly give rise to new organic products are the most penetrating ones, i.e. the cosmic rays. It has been suggested that a frozen layer of methane, located on the surface, can be altered by ions to produce an organic crust /13/. A contimsuous cycling of nitrogen and methane from the surface to the atmosphere, and viceversa, causes the accumulation of the organic residue in the underlying layers. Such a material could play a role in causing the solid-state greenhouse effect. that has been invoked as one of the mechanisms possible to explain the geyser-like plumes discovered by Voyager /65/. Of enormous relevance is the very recent discovery, by Cruikshank and co-workers, of spectral signatures attributable to frozen CO and CO2 /66,67/. The detection of CO could, at least, be consistent with the idea that the satellite has a comet-like composition, with the obvious implications for its origin. On the basis of our experimental results we can suggest an alternative explanation: since CO2 present on the surface could have come from the interior of the satellite /66/, CO could have been produced by ion irradiation of frozen CO2. If this is the case, we expect the CO/CO2 ratio to be lower than 1 at the surface and higher in the atmosphere. If the surface undergoes a temperature increase ~ 100 K, the ratio, in the atmosphere, would be smaller, The planned search for gaseous CO in the atmosphere /67/ appears to be extremely important. Note that H20 should not be a source of oxygen, since it has not been detected by 3 pm spectroscopy /68/. Pluto-Charon Observations of recent Pluto-Charon occultation amid transit. events provided a more detailed knowledge of the system. From 1.5-2.5 pm spectroscopy before and during a total eclipse of the satellite, it turned out that Pluto has a methane frost-rich surface, in contrast with Charon whose surface is prevalently made of water ice /69/. 3 pm spectroscopy of the whole system confirmed the presence on Pluto of methane, whose band shape seems, however, to be different from that of Triton /68/. These recent data confirm previous observations of absorptions attributed to layers of methane frosts /70,71/. Moreover, Charon seems to have hemispheres of two different colors, the Pluto-facing side being neutral in colour and the opposite hemisphere being reddish, similar to Pluto /72/.
Ion Irradiation Experiments
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It is worth noticing that, if Pluto and Charon have the same origin, then the same process(es) that altered the two surfaces in different ways has (have) to be invoked. On the basis of the suggestion that cosmic-ion irradiation of Pluto’s surface produces dark layers of organic material down to a depth of several centimeters /13/, it has been argued that, in the absence of an atmosphere, methane, which was originally present on Charon’s surface, must have sublimated and escaped /73/, while on Pluto any water frost present, and not observed, must have been covered with methane cycling from the atmosphere and organic residues produced by cosmic-rays and/or UV photolysis /74/. In this scenario cosmic-ion irradiation could also explain time peculiar feature of Charon’s ahbedo as due to coloring material left over by the original methane. The experiments described above demonstrate that, assuming an initial composition for the two objects made of water and methane (1:1), new species form after ion-irradiation. In particular, the ratio CO over CO 2 may be predicted. On Charon’s surface one has to expect a CO/CO2 ratio to be lower than 1. In fact, if CO and CO2 form before and during methane sublimation, then most of CO would follow the fate of methane, diffusing from the layer where it was produced, sublimating and, being unable to form an -atmosphere, being lost in space. The much less volatile C02, produced in larger quantity because of the availability of water, would remain mixed in with the water and with coloring organic residues, and could be detectable. On Pluto’s surface the presence of a thicker organic crust, caused by a continuous processing of methane cycling from the surface to the atmosphere and viceversa, would mask water and, possibly, CO2. The CO/CO2 would be lower than I in the (unseeabhe) deeper layers, but greater than 1 in the atmosphere. The possible detection of CO (and CO2) on these objects, being extremely relevant, does not however necessarily imply a native presence of these components on the planetary surfaces. In fact, ion-irradiation could have produced them by inducing chemical alteration of water-methane mixtures. ACKNOWLEDGEMENTS This research has been supported by the Italian Space Agency (ASI). REFERENCES 1. W.L. Brown, L.J. Lanzerotti, J.M. Poat.e amid W.M. Augustiniak, Ploys. Rev. Lett. 40, 1027 (1978). 2. G. Strazzulla and R.E. Johnson, in: Comets in the Post-Halley Era, eds R. Jr Newburn, M. Neugebauer, 1. Rahe, Kluwer, Dordrecht 1991, p. 243. 3. G. Strazzulla, G.A. Baratta, RE. Johnson and B. Donn, Icarus 91, 101 (1991). 4. G. Strazzulla, G.A. Baratta and A. Magazzio in: Solid State Astrophysics, eds E. Bussoletti, G. Strazzulla, North-Holland, Amsterdam 1991, p. 403. 5. J.P. Bibring and F. Rocard, Adv. Space Res. 4, 103 (1984). 6. L.J. Lanzerotti, W.L. Brown, J.M. Poate and W.M. Augustiniak, Nature 272, 431 (1978). 273. 7. Johnson, in: Ices in theG.Solar edsSheng, J. Khinger et. al., Dordrecht 8. RE. G. Strazzulla, L. Calcagno, FotiSystem, and XL. in: Ices in Reidel, the Solar System, 1985, eds. p. J. Klinger et al., D. Reidel Pubi. Co, Dordrecht 1985, p. 273. 9. G. Strazzulla, L. Calcagno and G. Foti, Mon. Not. Royal Astron. Soc. 204, 59 (1983). 10. G. Andronico, GA. Baratta, F. Spinella and G. Strazzulla, Astrorm. Astrophys. 184, 333 (1987). 11. R.E. Johnson, J.F. Cooper, L.J. Lanzerotti and G. Strazzulla, Astron. Astrophys. 187, 889 (1987). 12. G. Strazzulla, Icarus 67, 63 (1986). 13. G. Strazzulla, L. Calcagno and G. Foti, Astron. Astrophys. 140, 441 (1984). 14. L. Calcagno, G. Foti, L. Torrisi and G. Strazzulla, Icarus 63, 31(1985). 15. G. Strazzulla, Icarus 66, 397 (1986). 16. G. Strazzulla, L. Torrisi, S. Coffa and G. Foti, Icarus 70, 379 (1987). 17. G. Strazzulla, GA. Baratta, G. Leto and M.E. Palumbo, Earth Moon Planets 56, 35 (1992). 18. L.J. Allainandola, S.A. Sandford and G.J. Valero, Icarus 76, 225 (1988). 19. S.A. Sandford, F. Salama, L.J. Allamandola, L.M. Trafton, D.F. Lester and T.F. Ramseyer, Icarus 91, 125 (1991). 20. M.H. Moore, B. Donn, R. Khanna and M.F. A’Hearn, Icarus 54, 388 (1983). 21. M.E. Palumbo and G. Strazzulla, Astron. Astrophys. 259, Ll2 (1992). 22. S.A. Salama, L.J. Allamandola, A.G.G.M. Tielens and G.J. Valero, Astrophys. J. 329, 498 (1988). 23. B. Schmitt, M.J.A. Grim and M.J. Greenberg, ESA Sp-29O, 213 (1988). 24. M.E. Palumbo and G. Strazzulla, Astron. Astrophys., submitted (1992). 25. D.B. Nash, M. Carr, 3. Gradie, D. Hunter and C. Yoder, in: Satellites, eds. J. Burns and M. Matthews, Univ. of Arizona Press, Thcson 1986, p. 629.
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R.H. Brown, Icarus 56, 414 (1983). R.H. Brown, D.P. Cruikshank and D. Morrison, Nature 330, 423 (1982). A.G.W. Cameron, in: The origin of the solar system, ed. S.F. Dermott, Wiley, New York 1978, p.49. 111. A.J.R. Prentice, in: lime origin of the Solar System, ed. S.F. Dermott, Wiley, New York 1978 p. A.J.R. Prentice, Ploys. Lett. 114 A, 211 (1986). B.A. Smith et al., Science 246, 1417 (1989). D.P. Cruikshank, in: Uranus and Neptune, ed. iT. Bergstralh, NASA CP2330, 1984, p.425.
62. Hi. Reitsema, W.B. Hubbard, L.A. Lebofaki and D.J. Tholen, Science 215, 289 (1982). 63. D.P. Cruikshank and J. Apt, Icarus 58, 306 (1984). 64. L.A. Soderblom, et al., Science 250, 410 (1990). 65. R.H. Brown, R.L. Kirk, T.V. Johnson and L.A. Soderblom, Science 250, 431 (1990). 66. W.B. McKinnon W.B., Nature 354, 431 (1991). 67. 68. 69. 70. 71.
D.P. Cruikshank, private communication (1992). J.R. Spencer, M.W. Buie and G.L. Bjoraker, Icarus 88, 491 (1990). MW. Buie, D.P. Cruikshank, L.A. Lebofsky and E.F. Tedesco, Nature 329, 522 (1987). D.P. Cruikshank and P.M. Silvaggio, Icarus 41, 96 (1980). J. Apt, N.P. Carleton and CD. Mackay, Astrophys. J. 270, 342 (1983).
72. Di. Tholen, M.W. Buie, R.P. Binzel and M.L. Frueh, Science 237, 512 (1987). 73. L.M. Trafton, S.A. Stern and G.R. Gladstone, Icarus 74, 108 (1988). 74. S.A. Stern, L.M. Trafton and G.R. Gladstone, Icarus 75, 485 (1988).
0273—1177~3$24.00 Copyright @ 1993 COSPAR
ION IRRADIATION EXPERIMENTS G. Strazzulla, G. Leto and M. E. Palumbo lstituto diAstronomia, UniversiM di Catanja, Città Universitaria, 1-95125 Catania, Italy
ABSTRACT We report new results from recent experimental studies on physico-chemical effects induced in frozen gases and mixtures (CO, Ca 2, CH4, H20 and SO2) simulating ice targets in space (frosts on external planets and satellites, and comets). In particular we have studied, by UV-Vis-IR spectroscopy, the formation of new molecules, the alteration of band profiles and the spectral behavior of the original compounds. The ratio CO/CO2 has been measured, as a function ofion Iluence, in several frozen gases (or mixtures). These results are discussed in the light of their relevance in some planetary environments. In particular, we suggest that ion irradiation, which is especially intense at lo, can modify the spectral properties of frozen SO2 thus contributing to explain the spectral behavior of this Jovian moon. The production, by cosmic ion irradiation, of CO and CO2 at the surfaces and/or in the atmospheres of Titan, Triton, Pluto and Charon is discussed. Some predictions of the expected CO/CO2 ratio at the surfaces and/or in the atmospheres of those objects are provided. INTRODUCTION The presence of intense fluxes of charged particles impinging on the solid surfaces of planets, satellites, and rings in the Outer Solar System produces a number of effects the knowledge of which appears to be essential for understanding the evolution of these objects. This type of research is based on laboratory simulations of relevant targets (e.g. molecular solids) bombarded with charged particles under physical conditions more or less similar to the astrophysical ones. Since the measurements in 1978 /1/ of very large sputtering yields for water ice bombarded by keV-MeV ions, molecular solids have been intensively studied in several laboratories. In particular, the physico-chemical effects induced by fast ions colliding with solids of astrophysical interest (frozen gases, carbonaceous and organic materials, silicates, etc.) have been investigated (e.g. /2,3,4/). The most studied effects are erosion (sputtering) and chemical changes in the target material. Erosion, as a consequence of the ion-target interaction, causes the ejection of atoms or molecules (both already present and newly formed) from the target. Chemical changes include the formation of new molecular species residing on the target and, in particular, the formation of a refractory residue. Another effect, which could be of primary relevance, but has been, with a few exceptions, /5/ neglected, is the fate of the incoming ion, e.g. its chemical bonding with target atoms. Here we report new results on physico-chemical effects induced on frozen gases and mixtures (CO, CO2, CH4, H20 and SO2) simulating ice targets in space (frosts on external planets and satellites, and comets). In particular we have studied, by UV-Vis-IR spectroscopy, the formation of new molecules, the alteration of band profiles and the spectral behavior of the original compounds. In the Solar System frozen surfaces are continuously bombarded by energetic ions from solar wind and flares, planetary magnetospheres and galactic cosmic rays. Many detailed applications of the above type of experiments have been discussed in recent years. These include modifications undergone by interstellar and interplanetary grains /2,6,7,8,9/, comets /2,10,11,12/, asteroids /10/ and satellites in the outer Solar System /13,14,15,16,17/. The laboratory results that have been applied in the references above concern mainly the sputtering and production of organic colored materials. Here we discuss our, and others’, new experimental results concerning the production of new species, in the light of their relevance in some planetary environments. UV-VIS-IR SPE~TROSCOPY To irradiate frozen gases and obtain “in situ” UV-Vis-IR spectra, 1 =2.27-25 a scattering ~m) or tochamber an UV-Vis-NIR was faced(190-3200 to an FTIR nm) Perkin-Elmer (mod. lambda 1710) spectrophotometer 19) dispersive spectrometer (4400-400 through cm KBr windows. The vacuum was better than i0~ (10)189
(10)190
G. Strazzulla et al. TABLE 1 Spectroscopic Properties of some Molecules in the Solid State
molecule
vibrational mode
i’ 1) (cm
A (pm)
3300 2205 1655 810
3.03 4.53 6.04 12.34
2139 2136 3010 1300 2340 656 35714 29412 21739 2459 1340 1151 525
4.68 4.67 3.32 7.69 4.27 15.24 0.28 0.34 0.46 4.07 7.46 8.69 19.05
A (10—’~cm/mol) literature” this work
a (10~cm’) res = 2 cm’
H 20
CO Gil4 CO2
0
—
H stretch
0—H bend libration C E 0 stretch in pure CO ice in H20 ice C — H stretch C — H deformation C = 0 stretch C=Obend
502 combination (Mi + 113) asymmetric stretch (113) symmetric stretch (i,~) (P2)
“/18/;
20
21
0.8 3
1.1 2.8
1 1.7 0.6 0.6
1.1 > 1.5 0.6 0.5
76b
1.94
3.01 1.3 0.97 8.88
11b
005b 32b 04b
b/19/
mbar. Frosts were accreted onto a silicon crystal (111) or on a quartz substratum put in contact with a cold finger (10-300 K) by admitting gas (mixtures) into the chamber through a needle valve. All of the spectra shown in the following sections are ratioed to a background including the substratum. The substratum forms an angle of 45 degrees with the light beam and the ion beam so that, during irradiation, spectra can be obtained without tilting the sample. 2 spot on the target (greater During than thecondensation, spot of the light icesbeam) can beand bombarded currents lower by 3 keV thanions. few micro-ampere/cm2. The beam produces Since a 2x2 cm the penetration depth in frozen organic targets is only about 0.05 ~m for 3 keV He ions and such thin targets would show very small JR features, the samples have to be irradiated during deposition. Each experiment was stopped only when good spectra were obtained. For calibration purposes, many spectra of pure gases (mixtures) have been taken at different rates of deposition, i.e. at different gas pressures (10~-10—~ mbar). The sample thickness was determined from previously obtained calibration data. These latter had been obtained by depositing a quantity of ice whose thickness was checked by counting the interference fringes of a reflected laser light (514 nm). Thus, when a target was simultaneously irradiated at a known ion current, we knew both the number of molecules deposited per time unit and the number of ions impinging in the time unit: i.e. we were able to evaluate the irradiation dose of that experiment (in eV/16 amu). Moreover, taking “in situ” spectra, we could stop the experiment only when the irradiated sample was such that it showed a spectrum with well defined features. [Note that here the doses are given in eV per small molecule (16 amu), because this is a convenient way to characterize chemical changes /2/ and to compare them with experiments on other targets and, with some caution, to other projectiles]. For the main absorption bands of all the studied ices, we have evaluated the absorption coefficient a (cm~) and the integrated absorbance A (cm/mol). Our results are listed in Table 1, where they are compared with those reported in refs. /18,19/. The agreement between the two sets of data is generally good. EXPERIMENTAL RESULTS As is already well-known (e.g. /20/), ion irradiation of simple ices produces new species that either remain in the low temperature target or are directly released (chemical sputtering) in the gas phase. If the irradiated samples are warmed up they release old and new species in the gas phase, each species being released at different temperatures depending on the species itself, on the matrix in which it is embedded, and on the irradiation dose. The results presented here refer in particular to the production, at low temperature, of CO 2 from H20:CO mixtures; of CO from H20:C02 mixtures; and of both species from H20:CH4 mixtures. The CO/CO2 ratio frozen on the target has been measured, both as produced at. low T (10K) and as a function of T (10-200K). We have also evaluated that ratio as it would appear in the gas phase, e.g. in the atmosphere of a celestial body that can bind gravitationally all of the species released from its surface. In view of its relevance for lo we have also studied the variation in the absorption properties of SO2 ice in the UV-Vis-NIR region, in response to ion irradiation.
Ion Irradiation Expedments
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H
20:CO Mixtures
Pure CO ice at 10 K shows an absorption feature at 2140 cm_i, due to the fundamental vibrational mode CEO, with FWHM 4 cm’ measured on an optical depth scale. Spectra of pure CO ice have been compared with those of CO irradiated with 3 keV He ions and it has been shown that the FWHM of the band increases /21/. This is a general result: ion irradiation causes the broadening of the bands. This is then an alternative to the broadening due, for instance, to shape effects (roughness, oblateness, etc...). 1. Figure The formation, 1 shows aafter comparison irradiation, between of a irradiated new band and at 2340 not irradiated cm1 is evident, pure CO which in the is easily spectral attributed range 2100—2400 to CO cm 2. The CO/CO2 ratio can then be evaluated by using the measured integrated absorbances (Table 1) and is reported in Figure 2.
par. — —
0.8
.~
—
—
CO
-,-
CO 12 eV/18.mu
-
-
0.6
0.4
-
ii
0.2-
o_______ 2400
2300 2200 w~v.nurraber (cm’)
2100
Fig. 1. JR spectra of pure CO ice and CO irradiated with 12 eV/l6amu, in the spectral range 2400-2100 cm~’. 10
—
—~
*
__—o—
----_±---
8
—— -~
8
“
0.1-,
-
/
ACO H~0:C0 ~- H~0~C0. 0c0~ * i4~O:CM4
./
I II
0.01’ 0
-
20
..,‘,,,
do..
40 (eV/l6amu)
60
Fig. 2. CO/CO2 ratio vs ion irradiation dose for different ices. Dashed lines, connecting points due to similar samples, have been drawn to guide the eye.
The spectral properties of CO strongly depend on the host molecules when it is mixed in with other species. Many ice mixtures have been already studied /22,23,24/. The CO band 1. profile This latter in H20 has icegenerally is very peculiar: lower intensity it shows than a the ion irradiation intensity of the 2152 cm_icm~ band decreases as already noted for UV irradiation mainformer. featureUpon at ~2136 cm~ and the a secondary feature at 2152 /22/. In Figure 3 a spectrum of an unirradiated mixture (H 20:CO=lO:1) is compared with that obtained after irradiation 1) (8is eV/l6amu). due to CO The formation, after irradiation, of new bands is evident, the most prominent of which (at 2340 cm 2. The CO/CO2 ratio has been measured and is reported in Figure 2. If we compare this ratio with that coming out from irradiation of pure CO, we note its strong decrease due to CO oxidation driven by
(10)192
G. StrazzuiJa etal.
the ion-induced alteration of water, whose main effect is the formation of OH radicals along with oxygen and hydrogen.
}i.0:CO— 10,1 —
—
—
—
}iO,CO
+
8
.v/i8.mu
1—
0.8-
.~
0.8-
0.4-
0.:
~
2400
,~
~.
2300 wavenurnber
2200 (crn’)
2100
Fig. 3. JR spectrum of a H
2O:CO=10:1 mixture compared with that of an irradiated mixture (8 eV/l6amu).
H2O:CO2 Mixtures When pure CO2 is irradiated with 3 keV He ions, new bands appear. Among these the most intense peak occurs at 2141 cm’ and is due to CO. The CO/CO2 ratio has been measured after different doses and is reported in Figure 2. Similarly, when H2O:CO2 mixtures are irradiated, new bands appear. Again the most intense of these new bands (at 2142 cm’) is due to CO. The CO/CO2 ratio has been measured after different doses and is reported in Figure 2. Jf we compare this ratio with that obtained from irradiation of pure CO2 we find that they are indistinguishable. This fact testifies that the status of oxidation depends on the C/O ratio in the original target. In fact it does not vary too much: from O/C=2 (pure C02) to O/C=3 (mixture 1120:C02=1:1). In the case of irradiation of H2O:CO mixtures, this ratio varies by an order of magnitude from O/C=1 (pure CO) to O/C=11 (mixture H2O:CO=10:1). H20:CH4 Mixtures In Figure 4 a spectrum of an unirradiated mixture (H20:CH4=1:1) is compared with that obtained after irradiation (34 eV/l6amu). Once 1) are again duethe to formation, CO and CO after irradiation, of new bands is evident, the most intense of which (at 2138 and 2342 cm 2 respectively. The CO/CO2 ratio has been measured and is reported in Figure 2. Warm-up Effects The results presented above have been obtained at very low temperatures (10 K) and may be appropriate to “simulated” planetary surfaces during particular stages of their evolution or seasons. Because of the expected temperature variation on the planetary surfaces, it is interesting to study the variation in the spectral behavior and in the chemical composition, driven e.g. by a differential release in the gas phase, of the various species produced by ion irradiation. In particular, we have studied the variation of the CO/CO2 ratio in the frozen irradiated target, as a function of temperature. By subtraction we could also estimate this ratio as it would figure in the gas phase, in the hypothesis that each released molecule remains gravitationally trapped i.e. in the planetary atmosphere. The results are exemplified in Figure 5a and 5b. Figure 5a refers to the CO/CO2 ratio obtained from a mixture H20:C02=1:1 irradiated at 27 eV/l6amu. Figure 5b reports the results obtained by irradiation (34 eV/l6amu) of a mixture H2O:CH4=1:1. Some general conclusions can be drawn: since the CO molecules begin to leave the target at temperatures lower than CO2 molecules, the ratio in the solid phase decreases. The details of this decrease depend on the different environment the species are outgassed from, which, in turn, depends on the original mixture and irradiation history. Looking also at Figure 2, it is, however, clear that when mixtures containing water are considered, the CO/CO2 ratio, in the solid phase, is less than I whatever the irradiation dose and the temperature is. In the gas phase the situation changes: the CO/CO2 ratio is much greater than I for T lower than about 100 K, because only CO is released; it decreases to the initial value of the solid phase, when the temperature increase is such as to let all CO and CO2 evaporate, if no atmospheric loss occurs. Note that in planetary environments the temperature parameter can be substituted, with caution, for the time parameter: in fact, when the temperature is such (say 100 K) that CO2 begins to be released, then the CO/CO2 ratio is expected to vary with time as it does, in the laboratory,
Ion Irradiation Experiments
(10)193
it.0,CH.—1,1 —
—
—
H.0,CH~+ 3? .V/18~mu
—
1-
-
II
0.6-
Il
0.4-
II II
0.8
.~
—
-
II I 0.2
-
i 0
I
II
I I III I \
II II ,
-.-..
~
~
I 2400
~
I
-
-
—----.-—
-
....I,.,,
2300 wavenumber
I
2200
2100
(cm~)
Fig. 4. IR spectrum of a H
20:CH4=1:I mixture compared with that of an irradiated mixture (34 eV/l6amu).
with temperature.
10
‘‘I’’\’i’’’I’’’I’’I’’
1o0,.,~
after 27 eV/l~6amuon N,0:C0,=1:l
after ~4 eV/L6amu on H50:CH4—l:1
\ gas phase
1
10
\
.
.
\ r., S.-’ o
ga. phase
‘S
13--e
“.-
‘5
1-
————
‘S
~EI
0.1-
.
0.1
-
-
solid phase
solid phase
a) 0.01
-
A-.——
III,
II
50
100
150
b) ______________________
200
Temperature (K)
250
50
100
150
200
250
Temperature (K)
Fig. 5. a) Expected CO/CO2 ratio vs temperature in the surface and in the atmospere of a planet, from a H20:C02=1:1 mixture irradiated with 27 eV/l6amu (see text). b) Expected CO/CO2 ratio vs temperature in the surface and in the atmospere of a planet, from a H2O:CH4=1:I mixture irradiated with 34 eV/l6amu (see text). Dashed lines have been drawn to guide the eye.
SO2 Ice UV-Vis-NIR (250-1250 nm) transmittance spectra, normalized to I at 1020 nm, of pure SO2 ice deposited at 10 K and of SO2 irradiated with He (37 eV/I6amu) and D (42 eV/l6amu) ions are shown in Figure 6. The spectrum of pure SO3 ice is characterized by a very strong band at about 280 urn and two weaker bands at about 340 and 460 nm. Moreover, the frost appears transparent in the continuum. At first glance, this spectrum seems very different from the reflectance spectra reported in the literature (e.g. /25/) which exhibit a band at about 340 urn; have a flat, highly-reflecting continuum at greater wavelengths; are completely opaque at wavelengths lower than 310 nm; and do not exhibit any feature at 280 nm. Probably the difference is due to the different thicknesses traversed by the light: at higher pathlengths the very strong absorption centered at 280 nm is completely saturated. Upon irradiation, the SO2 frost layers become progressively darker and darker, and a strong absorption in the continuum appears at progressively higher wavelengths. The absorption bands become progressively less evident, and at
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2 ice deposited at 10 K. Spectra are normalized to 1 at 1020 nm. Solid line refers pure SO2 ice 0.3 jzm thick; short dashed line refers to SO2 deposited under He+ irradiation (37 eV/16 amu); long dashed line refers to SO2 deposited under D~irradiation (1.5 keV, 42 eV/16 amu).
higher doses they appear as shoulders over the declining continuum. This event has to be ascribed to the chemical effects induced by the beam that produce mainly sulfur trioxide along with a sulfate and S8, as evidenced by Moore /26/ from a study of IR spectra of proton irradiated SO2 frost. ION IRRADIATION IN THE SOLAR SYSTEM The production of new molecular species by iois irradiation of simple frozen gases may have a fundamental importance in several ice-objects in the Solar System. In principle, when a given molecular species, say CO, is observed on the surface of a satellite or a planet, different conclusions about its evolutionary history may be drawn. These depend on whether the species is believed to be native or produced “in situ” by some process(es), as may be the case of ion irradiation. In the following we discuss new results, obtained either by us or other groups, in the light of their relevance on objects in the external Solar System. Jupiter’s Satellites Voyager results indicate that sulfur and oxygen are the most abundant ions in the lo torus /27,28,29/. These ions are supposed to be ejected, as neutrals, from a sulfur-rich Ionian surface. Some authors proposed an louisa sulfur volcanism with colors due to the fast quenching of unstable allotropes of sulfur /30,31/. Observations of the 4 pm SO2 band on Jo indicate that a large amount of SO2 exists on its surface. Although ultraviolet observations by Voyager and IUE show little SO2, it has been suggested that SO2 must be intimately mixed with sulfur (or other materials) so that at every wavelength the darker component dominates the spectrum /32/. The experimental results shown above (see Figure 6) suggest a possible influence of ion irradiation, particularly intense at lo, in modifying the spectral properties of frozen SO2 causing strong absorption in the UV that masks the spectral signature at 280 nm. We are investigating the possibility of reconciling UV and IR observations by a process induced by ion irradiation. Several possible mechanisms for the transport of sulfur and oxygen from b’s surface to the torus have been investigated /33,16,26/. Among these, direct sputtering from the surface, caused both by MeV magnetospherical ions (H, He, 0, 5) and by corotating ions, cannot he considered as the only source of neutrals in the bo torus /16,17/. Nevertheless, this mechanism dominates over sublimation on most of b’s surface. Recent IR (2.5-5 pm) observations of Io /34/, confirmed the presence of absorption bands attributable to sulfur dioxide frosts (the most intense being at 4.07 pm), and detected four additional infrared spectral signatures. Salama ~ /34/ attributed the 2.97 and 3.15 ~m band to H20 and the 3.85 and 3.91 pm band to H2S. They,.qualitatively matched the observations with H20 (‘-.‘ 0.1 percent) and H2S (‘-.‘ 3 percent) mixed with sulfur dioxide at 100 K, but the
Ion Irradiation Experiments
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formation process(es) of those trace species is (are) not completely clarified. They also suggested a possible exogenic source, namely the insertion of energetic protons in the SO 2 ice. Laboratory experiments to confirm or disprove such a hypothesis are feasible and are planned in our lab. SO2 ice has been proton irradiated (at too low doses to detect species incorporating the projectile) and the main product of irradiation has been found to be sulfur trioxide along with a sulfate and S~/26/. However, those species show strong bands only at wavelengths greater than 5 pm. A new, very important class of absorption features on Jo has been discovered at 2.1253 pm by /35/ and tentatively attributed to CO2 multimers or “clusters” on Jo. Larson ~ /36/ confirmed the observation but challenged its identification, and suggested the further study of sulfur compounds, eventually modified by radiation processes, to search for a more plausible identification. Water ice is supposed to be a major constituent of the surface of Europa, Ganymede and Callisto, although the question about the presence of other species is still unresolved. There have been many considerations of the effects of magnetospherical ion-irradiation of these bodies, making use of the particle fluxes measured by Voyager /37,38,39,40/. It has been also shown that, after ionization, sulfur atoms from Jo can be trapped in the magnetosphere and then implanted mainly in the trailing surface of the satellites. This could explain the finding, by spectroscopic measurements in the UV, suggesting that sulfur atoms form S-0 bonds in Europa’s ice /41/. Recent laboratory studies on the alteration of the UV-Vjs reflectance spectra of water ice by ion irradiation /42/ showed a decrease in the reflectance in the UV, attributed to rearrangement processes that affect the physical structure of ice. Those authors conclude that the ratio in reflectance between the trailing and leading hemispheres of Europa and Ganymede is likely to be due to a different ion irradiation of the two hemispheres rather than to the implantation of reactive species such as sulfur. Saturn’s Satellites Arguments similar to those applied to the Jovian major moons have been used for Saturn’s satellites. By using particle fluxes measured by Voyager, it has been shown that molecules, eroded from water-rich surfaces, which are likely to escape because of the low mass of these satellites, travel along elliptical orbits around the planet. Detailed calculations show that photodissociation, loss and transport processes of the eroded species can well explain the production of a heavy ion torus in the inner magnetosphere, as observed by the Pioneer and Voyager plasma experiments /43,44,45/. The effects of UV photons, electrons and charged particle interaction with Titan have been discussed many times, with particular emphasis on the production and condensation of organic gases /46/. Due to the thick atmosphere of the satellite (of the order of 1500 mbar) only the most penetrating radiation (magnetospheric electrons and cosmic rays) impinges on the surface. This latter is thought to be an ocean, one to several kilometers deep, consisting mainly of ethane, the dominant end product of methane photolysis /47/. It is thus extremely difficult to establish whether ion irradiation plays an important role in modifying Titan’s surface. In our opinion, ions could have played a relevant role in the early stages of Titan’s life when its atmosphere was formed, probably by a devolatihization of the ices that accreted to form the satellite /48/. The present atmosphere is composed mainly of nitrogen (.65-.98 mole fraction), methane (.02-.1 mole fraction) and, perhaps, argon (0-25 mole fraction). Trace elements 5-1.5xl04 include the mole products fraction) of and CO photolysis 9 mole Their in such a reducing atmosphere requires special hypotheses. In methane’s and, fraction). interestingly, of presence two oxygen-containing compounds: CO (6x10 the framework 2 (1.5x10 of the hypothesis that the original surface would be made of clathrate hydrate (CH 4-7H2O), it is assumed that CO is incorporated in the ice along with other gases such as nitrogen and argon. CO would then be outgassed and, reacting with OH, would produce carbon dioxide. The source of the OH is thought to be particles of ices bombarding the orbiting Titan. On the basis of our results we can suggest, alternatively, that both CO and CO2 are formed, by the harder radiation environments in the early stages of the Solar System, directly on the surface made of methane and water. The CO/CO2 ratio is expected to be less than 1 on the surface (see Figure 5) but, owing to the very different outgassing temperatures, a ratio much higher than I is expected to be measured in the atmosphere. It is well known that lapetus has the most unusual photometric properties in the Solar System. Its leading hemisphere is among the darkest surfaces, while a large fraction of its trailing hemisphere is nearly as bright as snow. A number of models have been proposed to account for this asymmetry. In recent years, two lines of thought have been particularly debated. The first one /49,50/ proposes that the dark areas contain organic chromophores produced by UV irradiation of methane-rich ice and that the albedo pattern results from ballistic redistribution of surface material in response to a gradient in the impact flux. The second /51,52/ proposes that the dark material is a native component of an original icy surface concentrated in a thin devolatilized regolith produced by bombardment of dust from Phoebe. Similar material dominates the surfaces of D-type asteroids. However, both models are not fully accredited. The two models agree on one point: they both reject a previous hypothesis which states that the dust from retrograde Phoebe accumulates on the leading hemisphere of prograde lapetus, eventually cancelling the original icy surface from view. This is based on the fact that the spectral behavior in the 0.3-2.2 pm region is quite different for the two satellites. Phoebe has a flat spectrum (C-type asteroid-like), and the dark side of lapetus a red spectrum (D-type asteroid-like). To overcome this problem it has been suggested that only the outermost layers of Phoebe’s crust are carbonized by cosmic protons. Thus meteoritic impacts on Phoebe lead to the injection of materials which, coming from deeper layers, differ in color from the carbonized ones located on the surface /15/. Typical material arriving on Japetus from Phoebe could then have different spectral properties from those of Phoebe’s surface.
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Uranian Satellites Voyager 2, upon its close approach to Uranus and its moons, discovered radiation belts more intense than those of 2 sec _1) of, almost exclusively, Saturn, comparable to Van Allen belts at Earth and high fluxes (10 6107 ions cm protons at higher (greater than 500 keV) energies /53/. A first explanation for the possible cause of the dark coloration of the solid surfaces, inferred from the observations, was that over 100 years, 1016 protons per square centimeter produce dark organic residues on a surface supposed to have been originally covered by frozen methane /54,14/. A problem however arises: does methane exist on Uranian satellites and rings? As far as we know no observational evidence has been found for that so far. Earth-based observations clearly indicate the presence of water ice (or frost) on Ariel, Umbriel, Titania and Oberon. The presence of a dark, spectrally bland component has been inferred on/in water-ice surfaces /55/. Moreover, the color temperature at the distance of the Uranian satellites is about 80 K /56/, i.e. too high to avoid a rapid sublimation of methane ice. However, all the theoreticians agree on the fact that in the proto-uranian nebula the temperature was so low that methane condensed as a major component on protoplanetesimals /57,58/ and on newly formed satellites /59/. This temperature remained low for some time before the Sun’s transition through a high luminosity (T Tau) phase. After this phase, whose duration is estimated at a million years, the temperature dropped to the present value. During the T Tan phase the enhanced fluxes of energetic ions would have caused the formation of organic materials down to depths of several centimeters. In the post-T Tau phase further proton irradiation would have produced darker and darker material. The above astrophysical scenario has been successfully simulated in some experiments and discussed in the recent literature /17/. The satellite surfaces have very likely been completely resurfaced since the end of the T Tau phase. However, meteorite impacts have exposed a water-rich surface mixed with some organic contaminants left over after the proto-uranian nebula phase. Following laboratory experiments, it is interesting to note that whatever the kind of organics irradiated, they are changed into residues of very low albedo, after proton irradiation doses corresponding to 100-10,000 years at Uranus. This time is much shorter than the roughly estimated resurfacing time. This implies that, whatever the nature of the freshly resurfaced areas on the Uranian satellites, as long as they contain carbon compounds, they can be rapidly darkened by ion bombardment. Thus an interesting question about the nature of the dark component on Saturn’s satellites is still unresolved: is it an organic material darkened by particle irradiation or an inorganic one? A key point, suggested by the experiments, to confirm the former case, would be the observation (or time exclusion) of the presence of spectral signatures due to those materials, or to the products synthesized along with time organic dark refractory materials, e.g. CO and CO 2. The Neptunian Satellites Neptune has eight known moons /60/, two of which, namely Triton and Nereid, were known prior to Voyager 2’s encounter with the planet /61/. There is little information on Nereid, whose origin by capture has however been suggested /62/. Spectrophotometric observations in the 0.3-2.5 urn range give indications of the presence, on Triton’s surface, of nitrogen and methane in a frozen state /63/. Moreover, Voyager observations showed that nitrogen is the main constituent of Triton’s atmosphere and that methane is scarcely represented /64/. In the presence of an atmosphere, the ions that can irradiate the surface amid possibly give rise to new organic products are the most penetrating ones, i.e. the cosmic rays. It has been suggested that a frozen layer of methane, located on the surface, can be altered by ions to produce an organic crust /13/. A contimsuous cycling of nitrogen and methane from the surface to the atmosphere, and viceversa, causes the accumulation of the organic residue in the underlying layers. Such a material could play a role in causing the solid-state greenhouse effect. that has been invoked as one of the mechanisms possible to explain the geyser-like plumes discovered by Voyager /65/. Of enormous relevance is the very recent discovery, by Cruikshank and co-workers, of spectral signatures attributable to frozen CO and CO2 /66,67/. The detection of CO could, at least, be consistent with the idea that the satellite has a comet-like composition, with the obvious implications for its origin. On the basis of our experimental results we can suggest an alternative explanation: since CO2 present on the surface could have come from the interior of the satellite /66/, CO could have been produced by ion irradiation of frozen CO2. If this is the case, we expect the CO/CO2 ratio to be lower than 1 at the surface and higher in the atmosphere. If the surface undergoes a temperature increase ~ 100 K, the ratio, in the atmosphere, would be smaller, The planned search for gaseous CO in the atmosphere /67/ appears to be extremely important. Note that H20 should not be a source of oxygen, since it has not been detected by 3 pm spectroscopy /68/. Pluto-Charon Observations of recent Pluto-Charon occultation amid transit. events provided a more detailed knowledge of the system. From 1.5-2.5 pm spectroscopy before and during a total eclipse of the satellite, it turned out that Pluto has a methane frost-rich surface, in contrast with Charon whose surface is prevalently made of water ice /69/. 3 pm spectroscopy of the whole system confirmed the presence on Pluto of methane, whose band shape seems, however, to be different from that of Triton /68/. These recent data confirm previous observations of absorptions attributed to layers of methane frosts /70,71/. Moreover, Charon seems to have hemispheres of two different colors, the Pluto-facing side being neutral in colour and the opposite hemisphere being reddish, similar to Pluto /72/.
Ion Irradiation Experiments
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It is worth noticing that, if Pluto and Charon have the same origin, then the same process(es) that altered the two surfaces in different ways has (have) to be invoked. On the basis of the suggestion that cosmic-ion irradiation of Pluto’s surface produces dark layers of organic material down to a depth of several centimeters /13/, it has been argued that, in the absence of an atmosphere, methane, which was originally present on Charon’s surface, must have sublimated and escaped /73/, while on Pluto any water frost present, and not observed, must have been covered with methane cycling from the atmosphere and organic residues produced by cosmic-rays and/or UV photolysis /74/. In this scenario cosmic-ion irradiation could also explain time peculiar feature of Charon’s ahbedo as due to coloring material left over by the original methane. The experiments described above demonstrate that, assuming an initial composition for the two objects made of water and methane (1:1), new species form after ion-irradiation. In particular, the ratio CO over CO 2 may be predicted. On Charon’s surface one has to expect a CO/CO2 ratio to be lower than 1. In fact, if CO and CO2 form before and during methane sublimation, then most of CO would follow the fate of methane, diffusing from the layer where it was produced, sublimating and, being unable to form an -atmosphere, being lost in space. The much less volatile C02, produced in larger quantity because of the availability of water, would remain mixed in with the water and with coloring organic residues, and could be detectable. On Pluto’s surface the presence of a thicker organic crust, caused by a continuous processing of methane cycling from the surface to the atmosphere and viceversa, would mask water and, possibly, CO2. The CO/CO2 would be lower than I in the (unseeabhe) deeper layers, but greater than 1 in the atmosphere. The possible detection of CO (and CO2) on these objects, being extremely relevant, does not however necessarily imply a native presence of these components on the planetary surfaces. In fact, ion-irradiation could have produced them by inducing chemical alteration of water-methane mixtures. ACKNOWLEDGEMENTS This research has been supported by the Italian Space Agency (ASI). REFERENCES 1. W.L. Brown, L.J. Lanzerotti, J.M. Poat.e amid W.M. Augustiniak, Ploys. Rev. Lett. 40, 1027 (1978). 2. G. Strazzulla and R.E. Johnson, in: Comets in the Post-Halley Era, eds R. Jr Newburn, M. Neugebauer, 1. Rahe, Kluwer, Dordrecht 1991, p. 243. 3. G. Strazzulla, G.A. Baratta, RE. Johnson and B. Donn, Icarus 91, 101 (1991). 4. G. Strazzulla, G.A. Baratta and A. Magazzio in: Solid State Astrophysics, eds E. Bussoletti, G. Strazzulla, North-Holland, Amsterdam 1991, p. 403. 5. J.P. Bibring and F. Rocard, Adv. Space Res. 4, 103 (1984). 6. L.J. Lanzerotti, W.L. Brown, J.M. Poate and W.M. Augustiniak, Nature 272, 431 (1978). 273. 7. Johnson, in: Ices in theG.Solar edsSheng, J. Khinger et. al., Dordrecht 8. RE. G. Strazzulla, L. Calcagno, FotiSystem, and XL. in: Ices in Reidel, the Solar System, 1985, eds. p. J. Klinger et al., D. Reidel Pubi. Co, Dordrecht 1985, p. 273. 9. G. Strazzulla, L. Calcagno and G. Foti, Mon. Not. Royal Astron. Soc. 204, 59 (1983). 10. G. Andronico, GA. Baratta, F. Spinella and G. Strazzulla, Astrorm. Astrophys. 184, 333 (1987). 11. R.E. Johnson, J.F. Cooper, L.J. Lanzerotti and G. Strazzulla, Astron. Astrophys. 187, 889 (1987). 12. G. Strazzulla, Icarus 67, 63 (1986). 13. G. Strazzulla, L. Calcagno and G. Foti, Astron. Astrophys. 140, 441 (1984). 14. L. Calcagno, G. Foti, L. Torrisi and G. Strazzulla, Icarus 63, 31(1985). 15. G. Strazzulla, Icarus 66, 397 (1986). 16. G. Strazzulla, L. Torrisi, S. Coffa and G. Foti, Icarus 70, 379 (1987). 17. G. Strazzulla, GA. Baratta, G. Leto and M.E. Palumbo, Earth Moon Planets 56, 35 (1992). 18. L.J. Allainandola, S.A. Sandford and G.J. Valero, Icarus 76, 225 (1988). 19. S.A. Sandford, F. Salama, L.J. Allamandola, L.M. Trafton, D.F. Lester and T.F. Ramseyer, Icarus 91, 125 (1991). 20. M.H. Moore, B. Donn, R. Khanna and M.F. A’Hearn, Icarus 54, 388 (1983). 21. M.E. Palumbo and G. Strazzulla, Astron. Astrophys. 259, Ll2 (1992). 22. S.A. Salama, L.J. Allamandola, A.G.G.M. Tielens and G.J. Valero, Astrophys. J. 329, 498 (1988). 23. B. Schmitt, M.J.A. Grim and M.J. Greenberg, ESA Sp-29O, 213 (1988). 24. M.E. Palumbo and G. Strazzulla, Astron. Astrophys., submitted (1992). 25. D.B. Nash, M. Carr, 3. Gradie, D. Hunter and C. Yoder, in: Satellites, eds. J. Burns and M. Matthews, Univ. of Arizona Press, Thcson 1986, p. 629.
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