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Extraterrestrial Ice. A Review J. Kllnger LabOr8tO/r,9 de oleclologle E.P. 68, F38402-St. M8ttin dH6nes Cedex, Fr8nC.9 (Received: December 16, 1982)
The high cosmic abundance of hydrogen and oxygen suggests that important quantities of solid H2O may be found in the solar system and in interstellar space. The particular physical and chemical properties of ice may influence the behavior of several celestial bodies. On the other hand, the identification of the physical state of ice in space may give us information about the thermal history of extraterrestrial matter. Such kinds of studies are feasible now due the recent development in classical earth based astronomy, radioastronomy, space missions, remote sensing techniques, as well as infrared and ultraviolet observations from artificial satellites. The appearance and stability of different ice phases and substances like clathrate hydrates are conditioned in an important manner by temperatures, pressures, and chemical environments that experienced icy substances since condensation. Bombardments by cosmic rays and collisions with interstellar and interplanetary grains are important too. Physical conditions on an icy body can evidently be different if its gravitational field is important or not. For small bodies (=lkm in size) the lifetimes of ice at different heliocentric distances have been calculated by Watson et al. (1962). It turns out that a lifetime comparable to the age of the solar system needs heliocentric distances between 2 and 5 astronomical units (1 AU = 1.5 X 108 km),depending on the albedo of the body. In 1950, Whipple developed his so-called “dirty snowball model” for comets. This model is now widely accepted. As comets have probably condensed at low temperature, cometary ice could be initially amorphous. The phase transition between amorphous and cubic ice probably influences the heat balance of comet nuclei and is perhaps partially responsible for some unexplained phenomena related with cometary activity. The earth is probably the innermost of the planets and satellites of our solar system that bears ice (hexagonal ice and natural gas clathrates). According td Whalley (1981) cubic ice may form in some cases in the high atmosphere and may be responsible for Scheiner’s halo. It has been shown that the conditions in deep craters in the polar regions of the moon may be compatible with the occurrence of ice but recent studies make it doubtfull that ice is really there. There is probably ice and perhaps Cog-clathratehydrate accumulated in the polar regions of Mars. In order to explain several morphological configurations due to fluid erosion, it has been proposed that frozen rivers may exist on Mars or may have existed in the past. At heliocentric distances like that of the giant planets equilibrium temperatures are sufficiently low so that practically all forms of ice are stable or metastable even at low pressure. The bigger icy Satellites of Jupiter and Saturn probably contain high-pressure forms in their interior. Locally high-pressure ice may be created on the surfaces of icy satellitesduring meteorite impacts. These forms of ice may be conserved after relaxation of the impact pressure. A topography of different ice forms could give us good ideas about the evolution of the surfaces. On the other hand, phase transition between different forms of ice may play a role in the geological activity of some icy satellites. A little more than 10 years ago ice has been detected in the interstellar medium. The observed absorption features are compatible with grains of amorphous ice mixed with other molecules.
Introduction According to Cameron’ hydrogen is 2.66 X lo4 times more abundant in the solar system than silicon. If we disregard helium (1.8 X lo3 times more abundant than silicon) that is not very interesting from a chemical point of view, we find that oxygen holds the second place (18.4 times Si) between the most abundant elements followed by carbon (11.7 times Si). So we have good reasons to expect that molecules containing H, 0, and C may exist in the solar system and in interstellar space. If temperatures are sufficiently low, substances like H20, CO, CH4, etc. will occur in solid form. These kinds of frozen volatiles are often called “ices” by astrophysicists. In the present review only ice in the proper sense, which means frozen HzO, will be considered. In some cases clathrate hydrates will be taken into consideration too. Since the first half of our century it has been proposed that ice may exist in space (see, for example, ref 2). Among the earlier work dedicated to extraterrestrial ice only a small number of papers took into account the existence of different forms of ice. Over the past few years (1) Cameron, A. G. W. Center for Astrouhvsics, Cambridge, MA. _ _ Preprint No. 1357, 1980. (2) Van de Hulst,H. C. Rech. Astron. Obs. Utrecht. 1949, 11, 2.
the study of different phases of ice that may exist in space has become a rapidly developing topic in astrophysics. The topic of this paper is to show that a careful study of all forms of ice together with the study of other frozen volatiles may be a good tool for the investigation of the thermal history of the outer solar system and of molecular clouds.
Ice in the Solar System Stability of Ice in the Solar System. When we speak about the stability of ice in the solar system the situation is evidently very different for small icy bodies with a negligible gravitational field or for ice found on the surface, in the atmosphere, or in the interior of a massive planet or satellite. The problem of the stability of an icy sphere of 1 km in size orbiting in the solar system has been considered by Watson et al.3 It turns out that the lifetime of such an ice body will exceed the age of the solar system on a circular orbit at a heliocentric distance of about 3 astronomical units for an albedo of the body of 0.6 (1 AU = 1.5 X lo8 km). When the albedo is zero the same lifetime will be reached at about 4.8 AU. (3) Watson, K.; Murray, B.C.; Brown, H. Icarus 1963, 1, 317.
0 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 27, 1983
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Figure 1. Equilibrium temperature for an ice body as a function of heliocentric distance: (a) for temperatures below this line amorphous ice I a can be condensed; between (a) and (b) cubic ice IC can be condensed, ice l a is metastable; between (b) and (c) IC is metastable, hexagonal ice is obtained by condensation above (b).
We have to state at this point that the lifetimes calculated by Watson et al. considered evaporation as the only mechanism of erosion. More recent studies of Lanzerotti et ala4show that beyond 1.5 AU the erosion of ice due to sputtering by energetic particles becomes more important than evaporation so that the lifetimes calculated by Watson et al. are probably overestimated. If we consider solar radiation as the only heat source it is easy to calculate, for a given albedo and a known heat of evaporation, the equilibrium temperature as a function of heliocentric distance. The result of such a computation is shown in Figure 1. We see that for zero albedo and under present day conditions cubic ice is stable or metastable for heliocentric distances greater than 0.3 AU. In the same situation amorphous ice is stable for heliocentric distances greater than 4.8 AU. In the same way we may ask at what heliocentric distances amorphous or cubic ice can be formed under present day conditions. We see that for zero albedo cubic ice may be condensed at more than 4.8 AU and amorphous ice at more than 8.5 AU. For bodies with high albedo ,values the equilibrium temperature between the asteroid belt and the orbit of Jupiter is so low that even high-pressure forms of ice are stable in vacuum. Ice on Earthlike Planets and on the Moon. The earth seems to be the innermost planet that bears ice. As pressures in natural ice sheets never exceed some hundreds of bars and temperatures never go below 180 K only hexagonal ice can be found. In cold regions natural gas clathrate hydrates are found and are very troublesome for pipeline p e ~ p l e . Cubic ~ ice may very exceptionally occur in the high atmosphere. In fact, since the seventeenth century several observations of a 28O halo around the sun and the moon have been reported. Recently, Whalley6 suggested that this so-called Scheiner’s halo may be due to refraction of light at the angle of minimum deviation between octahedral faces of crystals of cubic ice. If this idea can be confirmed Scheiner’s halo may be the only (4) Lanzerotti, L.J.; Brown, W. L.; Poate, J. M.; Augustyniak, W.M. Nature (London) 1978,272, 431. (5) Miller, S. L. “Physics and Chemistry of Ice”, Whalley, E.; Jones; Gold; Ed.;Royal Society of Canada: 1973, p 42. (6) Whalley, E. Science 1981, 211, 389.
Klinger
evidence in the terrestrial environment for the existence of a form of ice that is not hexagonal. In 1961 Watson et al.7 suggested that permanently shaded places in polar regions of the moon may bear ice. The extension of these “cold traps” is estimated to be about 5X of the total surface of the moon. The idea was that escape mechanisms like ionization, solar wind collision, and gravitation seemed not to be able to remove all the water vapor released by the lunar surface and that a part of this water vapor may have been able to condense in those cold traps. More recent work by Lanzerotti et al.8 shows that sputtering by solar wind particles is an efficient erosion mechanism even in permanently shadowed areas. For this reason it is quite questionable whether there is ice on the moon. Mars is a rather interesting planet for people interested in physics and chemistry of ice. Ice and COz have been thought to form the polar caps on Mars since their discovery. Modern earth-based observational techniques combined with data from the Mariner and Viking missions have greatly improved our knowledge on this subject. Observationsof the south polar cap of Mars obtained by Mariner 7 demonstrated that the seasonal polar caps are essentially composed of solid C02.9 On the other hand, Clark and Mc Cordlo conclude from earth-based spectroscopic data that the residual north polar cap during summer is mainly composed of water ice and hydrated minerals. An indirect confirmation of this statement is the summer temperature of the north polar cap.’l The Viking infrared thermal mapping experiment gave late summer temperatures for the residual polar cap near 205 K. Such a high temperature is not compatible with solid COP or COz clathrate during summer. The preceeding observations evidently do not exclude completely the presence of frozen COz or clathrate hydrates buried under a permanent cover of ice. But the following two conditions must be fulfilled for this: first, the annual mean temperature of the ice cap must be (at least locally) lower than about 153 K for the occurrence of COz clathrate hydrate or lower than 148 K for the occurrence of solid CO,; second, the COz or clathrate layers must be covered by at least 10 m of solid H,O so that seasonal temperature fluctuations cannot reach those layers. If the presence of ice on Mars is not very surprising the presence of free liquid water on the surface of Mars is not compatible with present day temperatures and atmospheric pressures. For this reason it is very surprising that signs of liquid erosion, probably water erosion, have been found on Mars (see, for example, Milton12). Whereas a great number of authors consider that this degradation of Martian landscape occurred during a period with warmer climate and denser atmosphere, Wallace and Sagan13 pointed out that frozen rivers on Mars are compatiblewith present day conditions. A detailed geomorphological study of Valles Marineris is more favorable to erosion during a warmer and wetter period.14 The shape of Maja Vallis (7) Watson, K.; Murray, B. C.; Brown, H.J. Geophys. Res. 1961,156, 3033. (8) Lanzerotti, L. J.; Brown, W. L.; Johnson, R. E. J. Geophys. Res. 1981,86, 3449. (9) G.; Munch, G.; Kieffer, H.H.;Chase, S. C.; Miner, -, -Neiinehauer, .- E.D.Astr&:>. 1971, 76, 716. (10) Clark, R. N.; Mc Cord, T. B. J. Geophys. Res. 1982, 87, 367. - (11) Kieffer, H.H.;Chase, Jr., S. C.; Martin, T. Z.; Miner, E. D.; Yalluconi, F. D. Science 1976,194, 1341. (12) Milton, D.J. J. Geophys. Res. 1973, 78, 3027. (13) Wallace, D.;Sagan, C. Icarua 1979, 39, 385. (14) Bouquet, B.; Bodart-Jourdain, J. In “Nouveaux d6veloppements dans la connaissance du systgme solaire”;INAG/CNRS: Paris, 1982; p 179.
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Extraterrestrial Ice
on the other hand seems due to glacial erosion with perhaps some meltwater flow on the bottom.15 More observational data are necessary in order to determine the real importance of ice and water in the geological history of Mars. Satellites of the Giant Planets. As shown earlier ice is quite stable in the outer solar system. In 1923 Hepburn16 estimated the density of the six inner satellites of Saturn. The low density he found led him to conclude that these bodies might be made essentially of water ice. More recently infrared photometry and infrared reflection spectroscopy have been used to identify ice as well on three of the galilean satellites of Jupiter as on several satellites of Saturn (see ref 17-19). Saturn’s rings too are composed of ice particles or ice covered particles.20 The physical properties of different forms of ice are essential for the history of several bodies of the Jovian and the Saturnian system. Among the Galilean satellites of Jupiter, Io, the innermost one (radius r = 1850 km, mean density p = 3.5 g/cm3) is probably ice free. Europa (r = 1550 km, p = 3.0 g/cm3) is covered with ice. Voyager 2 imaging science results reveal a rather smooth surface with little or no record of intense meteorite bombardment during the early history of the solar system. On the other hand, the surface of Europa reveals a complicated system of bright and dark linear features.21 Ganymede (r = 2635 km, p = 1.9 g/cm3) has a surface that in a general manner has undergone important tectonic activity but conserved at least a small part of heavily cratered terrain. Callisto (r = 2500 km, p N 1.4 g/cm3) has a heavily and rather uniformly cratered surface. Consolmagno and Lewis22modeled the thermal history of the icy galilean satellites. The starting point of these models was homogeneous mixtures of silicates and of ice in a phase corresponding to a temperature of 100 K and the pressure that ice experience at a given depth under the surface. In the case of Europa the model predicts a strong differentiation of the body and the persistence of liquid water until the present time. This liquid water may perhaps explain the resurfacing of this satellite. More recent calculations by Reynolds and CassenB taking into account subsolidus convection show that Europa would be completely frozen in its present state if normal abundance of radioactive elements is the only heat source. Calculations of Cassen et al.24show that it is rather unlikely that tidal dissipation may be able to maintain a liquid water mantle in Europa. So if liquid water was really responsible for the surface modifications of this body these modifications have probably been achieved during early times. In the case of Ganymede the model of Consolmagno and Lewis predicts a strong differentiation as for Europa. The Voyager imaging results suggest that the surface of Ga(15) Bousquet, B.; Rogeon, P. In “Nouveaux DCveloppments dans la connaissance du systime solaire”; INAG/CNRS: Paris, 1982, p 182. (16) Hepburn, P. H. J.Br. Astron. Assoc. 1923, 33, 244. (17) Clark. R. N.: Owensbv. P. D. Icarus 1981.46, 354. (18) Morrison, D:; Cruikshank, D. P.; Pilcher, C: B.; Rieke, G. H. Astron. J. 1976, 207, L 213. (19) Fink, U.; Larson, H. P.; Gautier, T. N., 111; Defers, R. R. Astroph. J. 1976, 207, L 63. (20) Pilcher, C. B.; Chapman, C. R.; Lebofsky, L. A. Science 1970,167. (21) Smith, B. A.; Soderblom, L. A.; Beebe, R.; Boyce, J.; Briggs, G.; Carr, M.; Collins, S. A.; Cook 11, A. F.; Danielson, G. E.; Davies, M. E.; Hunt. G. E.: Ineersoll. A.: Johnson. T. V.: Masurskv, H.: Mc Canlev, J.: Morrison, D.; Oken, T.; Sagan, C.; Shoemaker, E. M:; Strom, R.; Suomi; V. E.; Veverka, J. Science 1979,206, 927. (22) Consolmagno, G. J.; Lewis, J. S. In ‘Jupiter”; Gehrels, T. A., Ed.; University of Arizona Press: Tucson, 1975; p 1035. (23) Reynolds, R. T.; Cassen, P. M. Geophys. Res. Lett. 1979,6,121. (24) Cassen, P.; Peale, S. J.; Reynolds, R. T. Geophys. Res. Lett. 1980, 7, 987.
The Journal of Physical Chemistry, Vol. 87, No. 2 l, 1983 421 l
nymede has undergone less changes than that of Europa. One explanation may be that of Poirier et al.25 They measured the viscosity q of ice VI at room temperature. They found q = 1014 P which is surprisingly low. Parmentier and Headz6showed that if the viscosity of highpressure ices near the melting point is lower than 10’’ P solid-state convection can take place and remove heat. In this way differentiation and melting are prevented. The value for q found by Poirier et al. may in fact indicate that solid convection took place so that Ganymede may be less differentiated than Europa. Callisto obviously did not undergo an important resurfacing which is consistent with the model of Consolmagno and Lewis. With the only exception of Titan (density p = 1.9) all large satellites of Saturn have density values between 1.0 (Tethys) and 1.4 (Dione). For this reason, we suppose that even the interior of those bodies is essentially icy. The radii of those satellites are rather small so that we do not expect a significant differentiation due to radioactive heating (see Lewis2’). It is surprising to see that one of them (Enceladus)has undergone a very complex geological evolution.28 I t has been tried to explain this activity by tidal heating.2e In fact tidal heating successfully explained volcanism on Io. But for Enceladus the situation is quite different: the density and the radius are much smaller for Enceladus than for Io (Enceladus: r = 250 km, p = 1.1 g/cm3; Io: r = 1818 km, p = 3.5 g/cm3). The average volumetric tidal dissipation calculated by Peal et is more than three orders of magnitude too small to maintain a liquid core in Enceladus. Yoder31 proposes that tidal heating of Enceladus may have been stronger in the past or that the tidal heating is episodic rather than continuous. More recently an alternative mechanism has been sugg e ~ t e d :suppose ~~ that amorphous ice has survived the accretion process of the satellite. In this case a rather small tidal or radioactive heating may trigger the phase transition between amorphous and cubic ice. This effect is cumulative as heat is released during the phase transition.% The difference in density may lead to the surface activity. Such a mechanism may work too for phase transition between others ices and during the destruction of clathrate structures. In a quite general manner, Gaffney and Matson proposed an interesting method for further studies of icy satellite^:^^ during crater formation important amounts of high-pressure ices may be formed on the surface. As temperatures in the outer solar system are rather low those high-pressure ices may survive even when the impact pressure has relaxed. A study of the different forms of ice found on the surface of an ice bodymay allow a deduction of the characteristics of the impact and give information on postimpact geological activity. The different forms of ice may be detectable due to their infrared reflection spectra. (25) Poirier, J. P.; Sotin, C.; Peyronneau, J. Nature (London) 1981, 292, 225. (26) Parmentier, E. M.; Head, J. W. J.Geophys. Res. 1979,84,6263. (27) Lewis, J. S. Science 1971, 172, 1127. (28) Smith, B. A.; Soderblom, L.; Batson, R.; Bridges, P.; Inge, J.; Masursky, H.; Schoemaker, E.; Beebe, R.; Boyce, J.; Briggs, G.; Bunker, A.; Collins, S. A.; Hansen, C. J.; Johnson, T. V.; Mitchell, J. L.; Terrile, R. J.; Cook 11, A. F.; Cuzzi, J.; Pollack, J. B.; Danielson, G. E.; Ingersoll, A. P.; Davies, M. E.; Hunt, G. E.; Morrison, D.; Owen, T.; Sagan, C.; Veverka, J.; Strom, R.; Suomi, V. E. Science 1982,215,499. (29) Yoder, C. F. Nature (London) 1979,279, 767. (30) Peale, S. J.; Cassen, P.; Reynolds, R. T. Icarus 1980, 43, 65. (31) Yoder, C. F. E.O.S.1981,42,939. (32) Klinger, J. Nature (London) 1982,299, 41. (33) Ghormley, J. A. J. Chem. Phys. 1968, 48, 503. (34) Gaffney, E. S.; Matson, D. L. Icarus 1980, 44, 511.
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A discussion of the rheology of ices and their relation to the tectonics of icy satellites can be found in a review paper by Comets. Comets are thought to be the most primitive bodies of the solar system and for this reason they are worthy of study in a quite detailed manner. According to Whipple%they are essentially composed of ice mixed with other frozen volatiles and rocky material. There is much indirect evidence that H,O is a major constituent of comet nuclei: molecular fragments like H20+,OH, as well as 0 and H are currently detected in cometary comae. On the other hand, the dependence of production rates on heliocentric distances fits the predictions about the sublimation of water ice. No other volatile substance has the same characteristics. It is believed that about 10’l comets with a total mass much smaller than the mass of the earth exist in the socalled Oort The Oort cloud is as sphere around the sun with a radius of about 150000 AU. The comets in the outer shell say between 20 000 and 150 000 AU are influerye by gravitational perturbations of nearby stars. In this way, some of them are sent as “new comets” to the inner solar system. Close to the sun the volatile substances evaporate what leads to the well-known and sometimes quite spectacular cometary phenomenon. The influence of the planets causes some comets to be captured in periodic orbits, the most famous periodic comet being Halley’s comet. All types of orbits have been osberved from nearly circular ones to hyperbolic ones. A great number of properties of comets are not quite well explained until now, some of them may be related to the physical properties of ice. The light curves for example do not, in a general manner, follow a simple law as a function of heliocentric distance. Some comets show very irregular variations in cometary activity while others under similar conditions do not. The brightness of comets often shows an asymmetry with respect to perihelion. In order to explain the Youtbursts” shown by some comets Patashnick et proposed that the heat freed during the phase transition from amorphous to cubic ice may be the energy source for outbursts. As comets probably formed at low temperature, cometary ice must be supposed to be initially amorphous. More recently it has been proposed34””that the change of the heat conduction coefficient during the phase transition may modify the heat balance of the cometary nucleus. When ice is in the amorphous state it is supposed to be a poor heat conductor. We can estimate that during the phase transition the heat conduction coefficient rises by a factor of ten.39 For this reason the heat conduction to the interior of the nucleus might be quite negligible when the ice is in the amorphous state. On the other hand, a nucleus partially or completely composed of cubic or hexagonal ice may be a quite good heat conductor and the heat conduction to the interior will perhaps not be negligible in the heat balance equation. In this case we may observe a phase lag in maximum activity with respect to perihelion and the activity may be lower with respect to a nucleus of amorphous ice. It is likely that a great number of shortperiod comets contain cristalline ice whereas a comet like Halley (period 75 years) contains amorphous ice in the (35) Poirier, J. P. Nature (London) 1982, 299, 683. (36) Whipple, F. L. Astrophys. J. 1950, 111, 375. (37) Oort, J. Bull. Astron. Inst. Neth. 1950, 11, 91. (38) Patashnick, H.; Rupprecht, G.;Schuerman, D. W. Nature (LORdon) 1974,250, 313. (39) Klinger, J. Science 1980, 209, 271. (40)Smoluchowski, R. Astrophys. J. 1981, 244, L 31. (41) Klinger, J. Icarus 1981,47, 320.
Klinger
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Figure 2. Different interstellar absorption features reported in ref 47.
center of the nucleus.41 It is particularly interesting to compare comet P/Oterma to P-Schwassman-Wachsmann 1. These two bodies are short-period comets orbiting on nearly circular trajectories. Oterma has an eccentricity e = 0.1444 and a semimajor axis a = 3.961 AU. Schwassman-Wachsmann 1has e = 0.1355 and a = 6.388 AU. The difference in activity between these two comets is striking: Oterma has a rather low activity without important outbursts whereas Schwassmann-Wachsmann 1 shows very violent outburst at all heliocentric distances. The author recently proposed the following e x p l a n a t i ~ n :Since ~ ~ for Oterma the “orbital mean temperature”, that is the temperature that the nucleus would have if it would receive at any moment the mean intensity of solar radiation corresponding to its orbit, is higher than the transition temperature between amorphous and cubic ice, we can consider that Oterma contains cubic ice. For SchwassmanWachmann 1 the orbital mean temperature is roughly speaking 20 K lower than the transition temperature. Further the low value of the eccentricity causes the surface temperature to be more sensitive to diurnal than to seasonal variations in solar radiation. The result of this situation is that small portions of the nucleus may be transformed to the cubic state every time the surface is sufficiently heated by solar radiation. This phenomenon may occur in a very erratic manner until all the ice is transformed to the cubic state. We hope to be able to understand comets better when sufficient data from space missions and from earth orbiting infrared and ultraviolet observatories will be available. Ice in Interstellar Space Ice or other substances in a molecular cloud can be identified by their infrared absorption spectra. For this (42) Klinger, J. Icarus, in press.
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Extraterrestrial Ice
Flgure 3. Spectrum of BN compared to a laboratory spectrum of amorphous iceso(solid line). The dasheddotted line is a spectrum for crystalline ice particles of 1 pm dlameter and the dashed line a spectrum of crystalline ice particles of 0.1 pm diameter (after ref 49). The BN spectrum is taken from ref 47. 1000
800
Y
[cm-'
3 6oo
Figure 5. Absorption band of amorphous ice (libration band): (a) amorphous ice condensed at 10 K; (b) the same sample annealed at 130 K; (c) the annealed sample after recooling to 10 K (after ref 53).
c
1 I 3500
I 3000
4
J
Y [ern-' 1
Flgure 4. (a) Absorption spectrum of the OH Stretching band of amorphous ice condensed at 10 K; (b) the same sample annealed at 130 K; (c) the annealed sample recooled to 10 K (after ref 51).
a star is needed that lies behind the cloud. If the emission spectrum of the star is known the identification of ice is possible. For the first time an icelike absorption in the 3-pm region has been reported in NML Cygnus in 1968.43~u Knacke et al.45concluded that less than 10% of the absorption is due to ice. Gillett and Forrest& examined the so-called Becklin-Neugebauer point source (BN). They established that about 7 90 of the observed absorption was ice. In the meantime, a great number of 3-pm absorptions have been observed (see ref 47). All these absorptions are rather similar in shape (Figure 2). What is troublesome is that the shape of the absorption line was very different from laboratory spectra of hexagonal and cubic ice and that the 12-pm libration band could not be found. On the other hand, the 45-pm ice band has been found in the Kleinmann-Low nebula.48 Mukai et al.49tried unsuccessfully to fit the shape of the 3-pm absorption of BN by particles of different size (Figure 3). Leger et a1.60where able to (43) Gillett, F. C.; Stein, W. A.; Low, F. J. Astrophys. J. 1968,153, L 185. (44) Johnson, H. L.; Astrophys. J. 1968, 154, L 125. (45) Knacke, R. F.; Cudabeck, D. D.; Ganstad, J. E. Astrophys. J. 1969,158, 151. (46) Gillett, F. C.; Forrest, W. J. Astrophys. J. 1973,179, 483. (47) Merrill, K. M.; Russel, R. W.; Soifer, B. T. Astrophys. J. 1976, 207, 763. (48) Ericson, E. F.; Knacke, R. F.; Tokusnaga, A. T.; Haas, M. R. Astrophys. J. 1981,245, 148. (49) Mukai, T.; Mukai, S.; Noguchi, K. Astrophys. Space Sci. 1978, 53, 77.
I
lC
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000
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Flgure 6. Absorption band of cubic ice (Libration band): (a) at 140 K; (b) at 10 K (after ref 53).
fit quite well the high-frequency branch of the BN absorption by means of laboratory spectra of amorphous ice condensed at liquid nitrogen temperature. Hagen et al.51 studied the 3-pm absorption of amorphous ice condensed at 10 K. When the same sample has been annealed at 130 K the absorption becomes stronger (Figure 4). This is not without consequences for the quantity of ice present in the interstellar medium. In fact if the interstellar ice is amorphous the abundance may be higher by a factor of 1.550with respect to the estimation given by Gillett and Forrest.& The fact that the long wavelength side of the 3-pm absorption does not fit the spectrum of amorphous ice might be due to an overlap of the OH stretching band and the 3.4-pm band of the C-H stretching vibration of carbon compounds. The 3.4-pm band has effectively been (50) Leger, A,; Klein, J.; de Cheveigne, S.;Guinet, C.; Defourneau, D.; Belin, M. Astron. Astrophys. 1979, 79, 256. (51) Hagen, W.; Tielens, A. G. G. M.; Greenberg, J. M. Chem. Phys. 1981, 56, 367.
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detected in the galactic center source IRS7.52 Leger et al.” as well as Hagen and ti el en^^^ showed that the 12-pm band of amorphous ice is weaker than that of cubic ice and shifted to longer wavelengths (Figures 5 and 6). This might explain that it was not detected in interstellar absorptions. In any way we must be aware that we cannot expect to find pure compounds with only one size of spherical particles in the interstellar medium. We have probably mixtures having perhaps overlapping spectral bands and a wide distribution of particle size. In my sense even the possibility of nonspherical particles should be discussed.
Conclusion The evolution of earth-based instruments, of earth orbiting satellites with infrared and ultraviolet equipment, and of remote sensing techniques combined with space (52) Wickramsinghe, D. T.; Allen, D. A. Nature (London) 1980,287, 518. (53) Hagen, W.; Tielens, A. G. G. M., in Thesis by A. G. G. M. Tielens, University of Leiden, 1982.
missions gives us the possibility to study the solar system and interstellar space in a quite detailed manner. It has been shown that ice is an important substance in space. The study of different forms of ice present on planets and satellites, in comets, in interplanetary dust, and in molecular clouds may give us important information about the evolution of our solar system and of other systems. To achieve this, a great deal of laboratory work is necessary as well on the optical properties, in particular infrared and ultraviolet properties, on mechanical and thermal properties of low temperature forms and high-pressure phases, and on erosion of ices by cosmic particles. The same type of work should be done on clathrate hydrates that are supposed to exist in space. Laboratory studies of ice condensed on silicate particles a t low temperatures may be useful too. Acknowledgment. This work has been sponsored by the French “Institut National d’Astronomie et de GBophysique-CNRS” Grant No. 37-86. Registry No. Water, 7732-18-5.
Ice in Comets A. H. Delsemme Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio 43606 (Received: August 23, 1982; In Final Form: January 14, 1983)
Ices of water and of more volatile gases are believed to be present in the cometary nucleus. Water ice seems to control the vaporization of all short-period comets. More volatile ices (typically COz ice) seem to control the vaporization of some “new”and less-evolved comets. This seems to suggest that cometary decay is essentially a fractionation, the most volatile ices being lost first. An icy grain halo surrounding the nucleus is likely to exist but difficult to detect except in favorable cases. Amorphous ice may initially play an important role, but in the surface layers it is likely to change into cubic ice when the comet comes closer to the Sun than, typically, the orbit of Jupiter. Volatile ices (including water ice) may represent some 60% of the total mass of a comet, the rest being mostly silicate dust with some carbon compounds.
Historical Introduction Comets become spectacular and develop a fuzzy head and one or several tails for a transient time only. At large distances from the sun, the cometary head is a starlike object with no nebular head and no tail a t all. The fuzzy head typically appears a t distances of 3 astronomical units (AU) and the tail a t less than 1.5 AU, although exceptions are known. This suggests that the transient phenomena appear only while a strong interaction prevails between the solar radiation and the solar wind on the one side, and something rather elusive that we call the cometary nucleus, on the other side. The nucleus is too tiny to be resolved in the largest telescopes, but it must be the permanent feature that is the source of all the transient phenomena, even if it is often hidden in the central condensation of light of the cometary head. In 1808, Laplace’ had already written: “the nebulosity that surrounds the comets’ (nucleus) results from the vaporization ...a t its surface, (which) must diminish the excessive heat...coming from the sun.“ However, in 1866, 1872, and 1885, several meteor showers occurred that were soon associated with the orbits (1)J. S. Laplace, “Exposition du Systgme du Monde”, 3rd ed, Paris, 1808, p 130. 0022-3654/83/2087-4214$01.50/0
of well-known comets. For this reason, Laplace’s suggestion was forgotton and a view became very popular for almost a century, namely, that cometary nuclei are loose aggregates of dust and sand particles, which liberate gases when heated by the sun and scatter easily along the cometary orbit. The dust-to-gas mass ratio was implicitly assumed to be of the order of lo3 or lo4;gas was, therefore, assumed to be adsorbed at the surface of the nuclear dust grains, and the temperature of the grains was assumed not to be very much modified by the latent heat of adsorption-desorption. In the 19th century, the radicals C, and CN were identified by molecular spectroscopy in the heads of comets and the ions CO+ and Nz+in their plasma tails. CH was identified in the 1930’s in the spectrum of comet Halley 1910 11, but a breakthrough was reached with comet Cunningham 1941 I by Swings and co-workers who identified several new radicals, OH, NH, and NH,, and ions, CH+, OH+, and C02+. (For details, see Delsemme’s revi ew.2, Bobrovnikoffj deduced that CO,, NH3, and H 2 0 must be the major parent molecules of the observed species (2) A. H. Delsemme, Appl. Opt., 19, 4007 (1980). (3) N. T. Bobrovnikoff, Reo. Mod. Phys., 14, 164 (1942).
0 1983 American Chemical Society
