The condensation and vaporization behavior of H2O: CO ices and implications for interstellar grains and cometary activity

ICARUS 7 6 , 2 0 1 - - 2 2 4

(1988)

The Condensation and Vaporization Behavior of H20"CO Ices and Implications for Interstellar Grains and Cometary Activity S C O T T A. S A N D F O R D AND L O U I S J. A L L A M A N D O L A NASA~Ames Research Center, Mail Stop 245-6, Moffett Field, California 94035

Received December 3, 1987; revised April 5, 1988 A number of parameters associated with the physical behavior of CO, HzO, and 1-120 : CO ices have been determined using infrared spectroscopy. The C O - C O surface binding energy in pure CO ices is found to be (AHs/k) = 960 ± 10 K, in good agreement with a previously determined value of 955 K. In HzO-rich ices containing minor amounts of CO, the HzO-ltzO surface binding energy is found to he (AHs/k) = 4815 ± 15 K and 5070 -.+ 50 K for unannealed and annealed ices, respectively. The determined CO-HzO volume binding energies are (AHv/k) = 5550 -+ 350 K and 5800 --- 350 K for unannealed and annealed ices, respectively. CO is observed to condense in substantial quantities into H20-rich ices at temperatures twice as high as those required for condensation in pure C O . Under interstellar conditions, CO can condense onto HzO-rich ice grains at temperatures up to 50 K. When HzO : CO ices condensed at 10 K are warmed they lose substantial amounts of CO between 30 and 65 K, 125 and 150 K, and 150 and 175 K. These ranges are associated with processes involving molecular rearrangement and sublimation of the HzO matrix and diffusion of CO through the H20 matrix. The presence of CO in 1-120 ice modestly increases the "effective volatility" of the HzO. The implications of these data for cometary models and our understanding of cometary formation are considered. The amounts of CO and COz observed in Comet Halley are consistent with ices produced in the interstellar phase. When the nucleus is warmed upon approaching the Sun, volatiles such as CO can filter through porous, amorphous HzO ice of the comet and escape, resulting in activity further from the Sun than expected for pure H20 ices. Some of the mobile volatiles can collect in pockets and be released suddenly, producing outbursts and jets. The dominant behavior, however, will be mediated by H20 as long as the concentrations of more volatile species is less than 30%. ©1988AcademicPress.Inc.

I. INTRODUCTION

the widespread use of CO as a probe of interstellar conditions in the gas phase, Since the discovery of CO in the inter- however, its equally important role in the stellar medium by Wilson et al. in 1971, chemical and physical properties of intergas-phase CO has been found to be both stellar and c o m e t a r y ices has not been fully abundant and ubiquitous in interstellar appreciated. space. Because it is photostable, it can surWhile there have been many studies of vive in a wide variety of astrophysical envi- CO in solids at low temperatures, few are ronments (see, for example, van Dishoek pertinent to CO in astrophysical ices. The and Black 1987, and references therein) and infrared spectra of CO in pure films (Ewing it must have been one o f the major molecu- and Pimentel 1961, L e g a y and L e g a y - S o m lar c o m p o n e n t s present in the protostellar maire 1982, L e g a y - S o m m a i r e and L e g a y nebula. The important role CO plays in in- 1982), and CO in inert matrices (Maki 1961, terstellar chemistry has b e c o m e widely rec- Dubost 1976, Dubost and Abouaf-Marguin ognized during the last decade (see, for ex- 1972, Jiang et al. 1975, Pacansky and Enample, Tarafdar and V a r d y a 1987). Despite gland 1986) have been investigated in de201 0019-1035/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

202

SANDFORD AND ALLAMANDOLA

tail. However, CO in astrophysical ices models must rely, we have measured the is intimately mixed with other molecules condensation-vaporization properties of which can have a strong effect on its physi- CO in pure and H20-rich ices. Binding encal and chemical behavior (Sandford et al. ergies, diffusion coefficients, and related 1988). While the condensation, vaporiza- parameters have been determined and their tion, and diffusion behaviors of pure solid astrophysical implications are discussed. CO and H20 are individually well known The infrared spectral properties of these (e.g., Shinoda 1969, Hobbs 1974, and refer- ices, including band positions, full-widthences therein), detailed studies of the va- at-half-maxima (FWHM), band profiles, porization and trapping properties of mixed and integrated absorbances (band molecular ices have only recently become strengths) are reported in detail in another available (Bar-Nun et al. 1985, 1987, Laufer paper (Sandford et al. 1988). et al. 1987, Schmitt and Klinger 1987). I1. EXPERIMENTALPROCEDURES While many molecules are present in interstellar ices, including CO, HCO, NH3, Only a brief summary of the experimenH2CO, and CH3OH, the major component tal procedures used is given here since a is H20 (Allamandola 1984, Tielens and A1- detailed description of the equipment and lamandola 1987). Comparison of laboratory techniques are given elsewhere (Allamandata (Sandford et al. 1988) with astronomi- dola et al. 1988). cal observations (Lacy et al. 1984, Whittet All the gas mixtures were prepared in a et al. 1985, Geballe and Wade 1985, Geballe greaseless, glass vacuum system calibrated 1986) shows that, when solid CO is present, to allow for careful control of the concenits concentration with respect to H20 lies trations of species mixed in each sample between I and 20%. Similarly, CO is bulb. H20 (initially 99.5% pure) was first thought to be a component of cometary thoroughly degassed at room temperature ices, with H20 still being the dominant and then further purified by three freezecomponent. For example, it has been sug- thaw cycles under vacuum. CO (purity gested that CO is responsible for the activ- greater than 99.5%) was taken directly from ity of comets at large solar distances where a lecture bottle and used without further the volatility of H20 may be insufficient to purification. explain the observed activity (Delsemme After allowing ample time for complete 1982). Although the concentration of CO mixing, samples were slowly condensed with respect to H20 is typically inferred to onto a CsI window maintained at 10 K and be between 10 and 20% in most comets suspended in a vacuum chamber. Deposi(Woods et al. 1986, Festou et al. 1986), tions were made at pressures of about 5 × concentrations as high as 50% have been 10-8 mbar. Infrared spectra show that unreported in several cases (see Lust 1981, der these conditions a mixed molecular and references therein). The similarity be- amorphous ice is formed, not a crystalline tween cometary and interstellar ices may or clathrate structure. The flow of gas onto not be accidental since it has been sug- the CsI window was regulated by a migested that comets are formed either by the croflowmeter and samples were typically accretion of preexisting interstellar grains deposited at a rate of 1.6 × 10-5 mole/hr (5 which had icy mantles or through the accre- /xm/hr). Sample deposition usually took tion of grains that had formed by the con- about 5 min. The window temperature was densation of protosolar gases at large solar controlled by a closed-cycle helium refrigdistances (Donn and Rahe 1982, Greenberg erator. Using a resistive heater, the temper1982). ature of the CsI window could be mainIn order to expand the data base on tained at any point between 10 and 280 K. which astronomical observations and The temperature was measured with an ab-

ASTROPHYSICAL H20 : CO ICES solute accuracy of _+2 K and a relative accuracy of +0.2 K with an Fe-Au/chromel thermocouple placed in a small hole inside the window mounting. The resistive heater could be controlled in such a manner as to allow for sample warm-up or cool-down at predetermined rates. All the warm-ups described in this paper were done at a rate of 2 K/rain. The CsI window is rotatable. Besides facing the deposition port, the window can be turned toward an ultraviolet lamp which simulates the interstellar radiation field. The window can also be rotated into the beam axis of a Fourier transform infrared spectrometer. Infrared transmission spectra were measured from 4000 to 400 cm -1 (2.5 to 25 ~m). The resolution of the spectrometer was determined to be 1 cm -j (the observed width of an unresolved line). Spectral positions are accurate to _+0.2 cm -~ because of oversampling. Spectra taken during warm-up experiments were taken while the temperature was held constant unless stated otherwise. The number of CO and H20 molecules within the ice subtending the infrared beam were determined from the infrared spectra using NH20 --

f r.dv AH20

f rvdv

Nco - - , Aco

(1)

where f r~dv is the integrated area (in absorbance) of a spectral band due to the molecule of interest and A is the absorption intensity of that band in cm/molecule. There are often large infrared intensity differences between vibrational transitions of a molecule in the gas phase and those same vibrations when the molecule is in the solid phase. Similarly, differences can also be produced when the molecule is placed in solid matrices with different compositions. Recently, Schmitt et al. 0988) have shown that Aco for CO trapped in H20 ices is also temperature-dependent. The value of Aco decreases uniformly and reversibly by about 40% as the temperature is raised from

203

I0 to 100 K. Consequently, the column density of an individual component in a mixed ice can only be accurately determined using the A values measured from an ice having the same composition and temperature. The bands used here, and their absorbance values (A values), are listed in Table I. III. RESULTS AND DISCUSSION

In the following two sections the binding energy of CO in pure CO ices and H20 in HEO:CO ices are determined. These are followed by four sections in which the sticking efficiency of CO to H20 ices, the CO loss from H z O : C O ices during warmup, the CO binding energy in H20 ices, and how CO alters the effective volatility of H20 ice are examined.

A. The Surface and Volume Binding Energies of CO in Pure CO Ice In one series of experiments, pure CO was deposited at 10 K and warmed steadily at the rate of 2 K/rain while the strength of the CO fundamental at 2138.6 cm -j was simultaneously monitored. No CO loss was observed up to a temperature of 24 K. Measurable CO loss started at 24 K. CO sublimed rapidly above 30 K and was entirely gone by 33 K. In a second series of experiments CO was deposited while the substrate was held fixed at temperatures between 20 and 40 K. All depositions were made for 1 min. It was found that CO did not freeze onto the cold finger at temperatures above 33 K. Small amounts of CO did stick at 32 K, but the entire deposit sublimed away in less than 15 rain. Deposition at a temperature of 31 K yielded a similar result although it took approximately 20 min for the ice to sublime away. CO was retained for successively longer periods at temperatures of 30 and 28 K. CO loss at 30 K was monitored after time intervals of 3, 7, 10, 14, 18, 22, 26, and 33 rain. CO loss was monitored at 28 K after time intervals of 10, 21, 30, 40, 55, 70, 80, 95, and 174 min. The losses of CO as a

204

SANDFORD

AND ALLAMANDOLA TABLE I

INFRARED BANDS AND THEIR ABSORBANCE VALUES

Molecule

Mode

Band position (cm i (/~m))

Band absorbance (era/molecule)

H20 (pure) (10 K) H20 (pure) (10 K)

O - H stretch

3280 (3.05)

2.0 x 10 ~6"

O - H stretch

3289 (3.05)

2.1 x 1 0 t6"

H20 (pure)

O H bend Libration C = O stretch C = O stretch C ~ O stretch CO bend

1660 760 2140 2140 2340 660

8.4 2.8 1.1 1.7 2.1 4.1

HzO (pure) CO (pure) CO (in H20) CO, (in H20) COz (in H20) " From h From ' From a From crease by 1988).

(6.02) (13.16) (4.67) (4.67) (4.27) (15.15)

x z x x x x

10 10 10 10 10 10

tSb ~7~ 17c 17J ~6J 17J

H a g e n et a/. (1981). d ' H e n d e c o u r t and Allamandola (1986). Jiang et al. (1975). Sandford et al. (1988) (applicable at 10 K). Aco values dea factor of about 4% per 10 K upon warm-up (Schmitt et al.

function of time for the 28 and 30 K pure CO ices are shown in Fig. 1. In general, the observed deposition-sublimation behavior is consistent with the models of Langmuir (1916) and Frenkel

(1924) in which it is assumed that the sticking efficiency is equal to 1.0 and that reevaporation takes place at a rate given by R~ = vo e x p ( - A H ~ / k T ) ,

0.9

o7i! 0s tJ

",,

i 131K

...... \ 0K

"-

0.4 0

10

20

30

40

50

60

70

80 90 100 110 120 130 140 150 160 170 180 TIME, rain

FIG. 1. CO loss as a function of time for pure CO at 28 and 30 K. The solid lines and points represent the data and the d a s h e d lines represent the theoretical losses expected using the derived C O - C O surface binding energy o f ( A H J k ) = 960 K.

(2)

ASTROPHYSICAL H20 : CO ICES where u0 is the lattice vibrational frequency of the CO molecule within the CO matrix, AHs is the binding energy of the CO to a surface site on the matrix, k is the Boltzman constant, and T is the ice temperature in Kelvin. The loss rate, Rt, of CO within the infrared beam of the spectrometer is then

Rt = (ab/am)Uo exp(-AHs/kT),

(3)

where ab is the area of the substrate probed by the infrared beam and am is the area of a CO molecular site. The observed CO loss rate is also given by Rt =

ab [(~'A//')i -- ('/'my)f] tf

Aco

,

(4)

where tf is the time over which the loss rate is measured, (rAu)~ is the integrated band strength at time ti = 0, (¢Au)f is the integrated band strength at time tf, and Aco is the integrated absorbance of solid CO in cm/molecule. Equating (3) and (4) and solving for the surface binding energy, (AHJk), yields AH~

k

Tln[vot~-~co{(¢Av)i-(¢Av)f}]" (5)

The size of the CO lattice site in CO ice is 3.4 × 3.4 x 4.6 A (Barrett and Meyer 1965). This implies a mean site area of about 16.0 ,~2, the value we use for am. The lattice vibrations of solid CO fall at 50.5 and 86.0 cm -1 (Ron and Schnepp 1967), corresponding to an average v0 value of 2.0 x 1012/sec. Aco for the CO fundamental in pure CO ice is 1.1 x 10 -T7 cm/molecule (Jiang et al. 1975). These parameter values and the observed loss rates of CO shown in Fig. 1 were used in Eq. (5) to calculate the surface binding energy of CO in pure CO ices. The average of the results for six measurements at 28 K and six measurements at 30 K are listed in Table II. The common formalism of expressing binding strengths in terms of (AHs/k), i.e., in Kelvin, is used throughout this paper. The 28 and 30 K experiments

205

yield a mean binding energy of (AHs/k) = 960 -+ 10 K for CO on the surface of solid CO ice. This is in very good agreement with the value of 955 K derived by Shinoda (1969) for the c~-form of crystalline CO ice at 0 K. The CO loss curve predicted using the 960 K surface binding energy is plotted as dashed lines in Fig. 1 for comparison with the experimental data. Solid CO packs in a face-centered cubic structure (Barrett and Meyers 1965). In this form each CO molecule is surrounded by 12 other molecules, 6 in a plane containing the central CO and 3 each in the planes above and below. A CO molecule in a surface site has only 9/12 the number of nearest neighbors it has in the bulk material. If we make the simple assumption that the binding energies scale as the number of nearest neighbors, an equivalent CO volume binding energy of approximately (AHv/k) = 1280 K is deduced.

B. The Surface and Volume Binding Energies of H20 in H20 : CO Ices The surface and bulk binding energies of H20 molecules in H20 : CO (initially 20 : 1) ices were determined in a manner similar to that described in the previous section. The gas samples were deposited onto the 10 K substrate and an infrared spectrum measured (Fig. 2). Each sample was then warmed up at a rate of 2 K/rain to the temperature at which the loss of H20 and CO was to be monitored. The temperature was maintained at that value for the duration of the experiment. H20 loss was determined as a function of time by measuring the strength of the broad O - H stretching band near 3290 cm -1. No measurable loss of H20 was observed from ices held at 50 K for 6 hr, 100 K for 5 hr, or 125 K for 21 hr. Monitoring H20 loss above 125 K requires special care since H 2 0 undergoes a phase transition from amorphous to cubic ice above 130 K. Unfortunately, there is little agreement in the literature on the time scales required for the completion of this transformation. Pub-

206

SANDFORD AND ALLAMANDOLA TABLE II EXPERIMENTALLY DETERMINED CO AND H20 BINDING ENERGIES a System examined

Ice sample

CO on CO

Pure CO

H20 on H 2 0

U n a n n e a l e d H20 A n n e a l e d H20

CO in H20

U n a n n e a l e d H20 a U n a n n e a l e d HzO ~ Annealed H20 a

A n n e a l e d H20 ~

Temperature (K)

28 30 150 152 145 152 150 152 150 152 140 145 152 140 145 152

Surface binding energy t'

Volume binding energy c

(AH]k)

(AHv/k)

955 + 968 ± 4825 + 4806 + 5027 +5090 ± (3160 (3150 (3470 (3470 (2930 (3460 (3350 (3220 (3760 (3670

4 K 2 K 5 K 5 K 87 K 36 K K) K) K) K) K) K) K) K) K) K)

(1270 K) (1290 K) (7720 K) (7690 K) (8040 K) (8140 K) 5049 ± 48 5043 +_ 28 5551 ± 48 5552 +_ 28 4682 +- 33 5539 ± 225 5359 +_ 151 5150 ± 33 6024 ± 226 5868 ± 151

K K K K Ky K K Kr K K

a All m e a s u r e m e n t s were taken at a pressure of 5 x 10 `8 mbar. Initial samples deposited at 10 K with H20 : CO = 20 : 1. After w a r m - u p to 150 K the ratio lies between 100 : I and 200 : 1. Stated errors represent the standard deviation of all the calculated values at each temperature. b N u m b e r s in p a r e n t h e s e s represent binding energies calculated from the o b s e r v e d volu m e binding energies using nearest-neighbor scaling. c N u m b e r s in p a r e n t h e s e s represent binding energies calculated from the observed surface binding energies using nearest-neighbor scaling. J Calculations were made using a n f v a l u e of 3/16 and Do = 330 cm2/sec (see Section IIIE). e Calculations were made using an f v a l u e of 1 and Do = 330 cm2/sec (see Section IIIE). c Diffusion probably assisted by the a m o r p h o u s HzO ice to cubic H20 ice transition,

lished values include a few seconds at 150 K (McMillan and Los 1965), 18 rain at 135 K (Hardin and H a r v e y 1973), 45 min at 145 K (Hagen et al. 1981), and 45 rain at 155 K (Rice et al. 1978). Recently, Bar-Nun et al. (1987) studied an ice in which the transition took place in 10 min as the ice was warmed from 136 to 160 K. The determination o f the binding energies of H20 in both unannealed and annealed ices required two sets of experiments. The binding energies of unannealed ice were determined by warming the ice directly from 10 K to the desired sublimation temperature at a rate of 2 K/min with no additional time provided for annealing. For the sample warmed directly to 150 K, spec-

tra were taken after 60, 120, 360, and 480 rain, while the spectra were taken after 20, 40, 60, 80, and 100 min for the sample warmed directly to 152 K. The spectra were used to determine the sublimation rates at each temperature. Because of the time required to warm the ice to the measurement temperature and the time required to monitor the H20 loss, some annealing was unavoidable. In order to examine this effect, another experiment was carried out in which a sample was warmed from 10 to 145 K and then allowed to anneal for 3.5 hr before being warmed to 150 K. The sublimation rate at 150 K was then measured after 30, 60, 90, 120, and 150 rain. The observed loss rate for this experiment was

ASTROPHYSICAL H20 : CO ICES

207

WAVELENGTH,um 2.5

3

4

5

8

10

i

I

i

i

I

2170

2150

2130

25

2110

WAVENUMBERS, cm -1

.

3600

.

.

2800

,

.

2000

1200

400

WAVENUMBERS, cm -1

FIG. 2. The infrared transmission spectrum of a typical H20 : CO = 20 : 1 amorphous ice deposited at 10 K. The inset is an expansion of the region containing the C~---O fundamental. This band falls at 2137 cm -I when CO is incorporated in H20 matrices. Note the side band that occurs at 2152 cm -~ (see Sandford et al. 1988, for a discussion of this band).

nearly identical to that of the original 150 K experiment, indicating that little additional annealing occurred during the 3.5 hr the ice

was held at 145 K. The H20 losses for these "unannealed" ices as a function of time are shown in Fig. 3.

1.0 0.9 0.8 Z

0.7 ,,,

150K (3.5 hr ANNEALING AT 145K)

0.6 ;¢~ 0.5

/ ~

Z 0.4

-" ~

(DIRECT WARM UP)

2

150K(DIRECTWARMUP)

\,,

~ 0.3 u. 0.2 0.1

I0

2

I

I

I

I

40

60

80

100

I

I

120 140 TIME, min

I

I

I

I

160

180

200

220

I

I

240 260

FIG. 3, H~O loss as a function of time from unannealed H20 : CO ices held at constant temperature (150 and 152 K). H20 : CO (initially 20 : 1) samples were deposited at 10 K. Two were warmed directly to 150 and 152 K. The third was warmed to 145 K, allowed to anneal for 3.5 hr, and then warmed to 150 K. The solid lines and points represent the data and the dashed lines represent the theoretical losses expected using the derived H 2 0 - H 2 0 surface binding energy of (AHs/k) = 4815 K for unannealed ice.

208

SANDFORD

AND ALLAMANDOLA

1.05

1.00 ¢3 zm

_z 0.95

;~0.90 Z 0

~.~'

o.85I

0.80

152K

0.75 0

I 20

I 40

I 60

l 80

I 100

I 120

I I I 140 160 180 T I M E , rain

I 200

I 220

t 240

I 260

I I 280 300

FIG. 4. H20 lOSS as a function of time from annealed H20 : CO ices held at constant temperature (145 and 152 K). A n H20 : CO (initially 20 : 1) sample was deposited at 10 K. The sample was first w a r m e d to 125 K and held there for 21 hr. It was then w a r m e d to 140 K. After 5 hr at 140 K, the ice was w a r m e d to 145 K and held there for the 145 K m e a s u r e m e n t s . After 5 hr the ice was further w a r m e d to 152 K and additional m e a s u r e m e n t s were made. T h e solid lines and points represent the data and the dashed lines r e p r e s e n t the theoretical losses expected using the derived H 2 0 - H 2 0 surface binding energy of (AHs) = 5070 K for annealed ice.

Similarly, loss rates were determined for ice mixtures at 145 and 152 K that had been given ample time to anneal. The sample was deposited at 10 K and then warmed to 125 K. This sample showed no H20 loss after 21 hr and was then warmed to 140 K. No H20 loss was observed at this temperature after 5 hr and the ice was subsequently warmed to 145 K. Minor, but measurable, H20 loss occurred at this temperature. The loss was monitored after 30, 60, 120, 210, and 300 min. After a total of 5 hr at 145 K, the ice was warmed to 152 K and additional H 2 0 loss was noted after 20, 40, 60, 90, 120, and 150 rain. The H~O loss from these annealed ices is plotted as a function of time in Fig. 4. As expected, the loss rates at comparable temperatures are substantially smaller (about 5 times) for the annealed ices than for the unannealed ices. Noting that the H:O loss rate from the 150 K ice, which had previously spent 3.5 hr at 145 K, was simi-

lar to that of the unannealed 150 K ice suggests that, in this case, the ice was still largely unannealed. In contrast, the sample annealed for 21 hr at 125 K, 5 hr at 140 K, and 5 hr at 145 K before ultimately being warmed to 152 K loses H20 at a rate more characteristic of annealed ice. Thus we do our part to add to the confusion concerning the transition time scale from amorphous to cubic ice by suggesting that it lies between 3.5 and 5.0 hr at 145 K. While not directly comparable since the ice described here contains minor amounts of CO, our transformation time is in apparent disagreement with the findings of Hardin and Harvey (1973) and to a lesser extent with those of Hagen et al. (1981), but may be consistent with the observations of McMillan and Los (1965), Rice et al. (1978), and Bar-Nun et al. (1987). The effect of the CO in our ices is not expected to be large since most of the CO is lost during the warm-up from 10 to 150 K and the H20 : CO value in the 150 K

ASTROPHYSICAL HzO : CO ICES ice lies between 100 : 1 and 200 : 1 (see Section IIID). It seems safe to conclude that the transition from amorphous to cubic ice is very sensitive to sample preparation method, temperature, and thermal history. In any event, all the reported transition times are rapid compared to the time scales associated with the warm-up of ices in cometary nuclei, and the annealed values will be the most appropriate for the majority of cometary applications (see Section IVC). The surface binding energies of unannealed and annealed H20 ices were determined using the formalism presented in the previous section. The integrated absorbance of the 3290 cm -1 H20 band in 150 K ice is Auto = 2.1 x 10-16 cm/molecule (Hagen et al. 1981). The lattice modes of H20 ice fall at about 65 cm -1 (Bertie et al. 1969), implying that 1'0 = 2.0 x 10Wsec. Assuming a density of 1 g/cm 3, the area am of a molecular site in H20 ice is about 10.2 A 2. These values of v0, am, and Ar~o, and the data shown in Figs. 3 and 4, were used to calculate the H 2 0 - H 2 0 surface binding energies listed in Table II. These data indicate an average surface binding energy of (AH~/k) = 4815 -+ 15 K for the unannealed ice and (AHJk) = 5070 --- 50 K for the annealed ice. The predicted H20 loss curves using these binding energy values are plotted as dashed lines in Figs. 3 and 4. The value of (AHJk) = 4815 K for the unannealed ice should be considered an upper limit for amorphous ice since some molecular rearrangement occurs during warm-up to the measurement temperature and during the measurement interval. Similarly, the value of (AH0 = 5070 K for the annealed ice should be considered a lower limit for cubic ice since the transition from amorphous ice to cubic ice may not have been completed during these experiments. The exact density of the ice sample at 150 K is uncertain and may be slightly less than the value of 1 g/cm 3 we used. If it is as low as 0.9 g/cm 3 (the density of cubic ice), the values of (AHJk) are lowered by about 30

209

K. The experimental uncertainties remain the same. H20 molecules in cubic ice occupy sites with tetrahedral symmetry with each molecule having four nearest neighbors (Hobbs 1974). Presumably H~O molecules in amorphous ice have a similar number of nearest neighbors. Molecules on the surface will have two or three nearest neighbors depending on the surface site, so the surface binding energy should be about ~(~ 1 + ~) = 0.625 that of binding energies in the bulk. This yields corresponding average volume binding energies of approximately (AHv/k) = 7705 and 8110 K for the unannealed and annealed ices, respectively. It should be kept in mind that the binding energies derived above are strictly applicable only for H20 ice that contains 0.5 to 1% CO at 150 K. The H 2 0 - H 2 0 binding energies derived here, however, are probably not very different from those of pure H20 ices. Above 125 K the CO in the ice is in substitutional sites (see Section IIID) and there is about a 1 in 100 chance that any one of the four neighbors of an H20 molecule is CO. If we make the assumption that the binding energy of each molecule in the ice is directly related to the number and type of nearest neighbors, and we use the H 2 0 - C O binding energy derived in Section IIIE, the derived H 2 0 - H 2 0 surface binding energies for H20 containing 1 part in 100 CO should be less than 2% lower than that expected for pure H20 ice, an effect smaller than the errors associated with these measurements. Note also that the HzO-H20 volume binding energies derived above fall in the 71308350 K range of published H20 self-diffusion energies (Hobbs 1974).

C. The CO Sticking Efficiency in H20 Ice To examine the CO sticking efficiency appropriate for H20-rich ices, an H20 : CO (20 : 1) mixture was deposited on a CsI window held at different temperatures. After deposition the H20/CO ratio was determined using the 3290 cm -~ H20 band and the 2137 cm -1 CO band (Fig. 2). Figure 5

210

SANDFORD AND ALLAMANDOLA 1.2 1.1 , 1.0 O 0.9 -r

0.8

~

0.7

>- 0,6 U Z

N o.5 ¢J u,,

0.4 z 0.3 v'

0.2

0 0.1 x

10

20 30 40 50 TEMPERATURE OF OEPOSITION, K

60

70

FIG. 5. The sticking efficiency of CO in H20-rich ices as a function of temperature. The H20 : CO ratio of the gas was 20: 1. The efficiencies are normalized to a value of 1.0 at 10 K and all values are corrected for the temperature dependence of Aco.

shows a plot of the CO sticking efficiency as a function of substrate temperature. In the plot it is assumed that both H20 and CO stick to the Csl window with 100% efficiency at 10 K. Under our experimental conditions, measurable amounts of CO can be condensed out of the gas phase at temperatures as high as 65 K in the presence of H20. This is more than twice the temperature at which pure CO condenses in our experiments. The decreased volatility of CO on H20 is due to the additional forces induced by the interaction of the CO with the polar H20 matrix. These attractive forces are not present in the pure CO system. Of course, the exact upper limit to CO sticking will depend on variables such as the deposition rate and thermal conductivity of the ice sample, as well as the pressure in the cell. The sticking efficiency, or retention time appropriate for the incorporation of CO into interstellar ices, and the implications for comets are described in Section IVBi. The higher CO condensation tempera-

tures observed in these experiments are not due to trapping resulting from large numbers of H20 molecules piling onto the CO molecules before they can " r e b o u n d " from the H20 substrate. At the deposition rate used, each surface site received about 5 new molecules per second. CO molecules on the surface undergo approximately 4 x I0 H lattice vibrations during the time it takes a monolayer to grow and would easily escape before the next molecular ice layer is deposited if the effective surface binding energy was not substantially larger than that for pure CO.

D. CO Loss from H20 Matrices during Warming Figure 6 shows the changes in the 2137 cm -j CO fundamental band profile as an HzO : CO (20 : 1) ice is warmed from 10 to 150 K. CO condensed into H20-rich amorphous ice samples at 10 K occupies one of two sites in the ice (Sandford et al. 1988). The major site, which contains approximately 80% of the CO, is thought to be sub-

ASTROPHYSICAL H20 : CO ICES stitutional, i.e., one in which the CO molecule is frozen into a position that would otherwise have contained an H20 molecule. The remaining 20% of the CO occupies a second site which is believed to be interstitial. CO in the major site absorbs at 2136.7 cm -j, while CO in interstitial sites produces a 2152 cm J shoulder (see the in'

'

'

'

~

fl0K

'

r

Ir I

I

I

~d~j

'OOK

I

i

2180

~//l/v/1

i

1 1 ~

i

i

2160 2140 2120 WAVENUMBERS, cm-1

1

2100

FIG. 6. The spectral change of the CO-stretching band in an H20 : CO (initially 20 : 1) ice as a function of temperature. Note that the 2152 cm ~side band disappears by 100 K and that the band begins to develop narrower subpeaks at 2134, 2136, and 2144 cm ~above 125 K (see Section Ill and Sandford et al. 1988). The vertical bars represent I% transmission. The crosshatched areas at the bottom indicate the typical position and FWHM of the interstellar CO and X : C N bands.

211

set in Fig. 2 and the top five spectra in Fig. 6). The positions of these bands relative to the position of the fundamental band of gas phase CO (2143 cm -j) indicates that the interstitial site is repulsive while the substitutional site is attractive. CO leaves the ice during warm-up from 10 to 125 K. Below 125 K, CO loss, where it occurs, ceases immediately when the warm-up is stopped and the temperature is held fixed. CO loss resumes when the warm-up is continued. This implies that, below 125 K, the loss of CO is dominated by H20 lattice annealing processes and CO release from traps, rather than simple diffusion of CO through the H20 lattice. Figure 6 illustrates that CO in interstitial (2152 cm -1) sites is lost more rapidly than CO in substitutional (2137 cm -J) sites. This is consistent with the first site being repulsive while the second is attractive as discussed by Sandford et al. (1988). Unfortunately, since CO is lost from both sites simultaneously, it is not possible to say how much of the interstitial CO is lost directly from the ice and how much is converted into substitutional CO with subsequent loss from the ice. By 80 K all the CO is in substitutional sites. The appearance of subpeaks at 2134, 2136, and 2144 cm -~ between 100 and 150 K implies that new sites form in this temperature range. These new sites are presumably related to the transformation of the amorphous ice to cubic ice and may be indicative of the formation of a CO clathrate. Without diffraction data we cannot ascertain the relative importance of these two crystalline structures. H o w e v e r , if a clathrate structure is present, it has formed at a temperature, pressure, and CO concentration f a r below those typically used to produce laboratory CO clathrates (cf. Davidson et al. 1987, Schmitt and Klinger 1987). Figure 7 shows how the CO absorption strength (main peak plus 2152 cm ~ shoulder) decreases with temperature. Curves are shown for H20 : CO (20 : I) gases deposited at 20, 30, 40, and 50 K. The dotted line indicates the slope of the decrease in ab-

212

SANDFORD AND ALLAMANDOLA

sorption expected for the temperature de- simultaneous loss of the entire H20 matrix pendence of Aco alone (Schmitt et al. 1988). due to sublimation. Slopes steeper than the dotted line indicate These observations are in good qualitaCO loss. This figure shows that appreciable tive agreement with previous findings for CO loss occurs in three temperature do- CO in H20 ices (Bar-Nun et al. 1985) and mains: (i) 30-65 K, (ii) 125-150 K, and (iii) for Ar in H20 ices (Bar-Nun et al. 1987, 150-175 K. Laufer et al. 1987). In Laufer et al. (1987) it We interpret the CO loss observed in do- is suggested that amorphous H20 ices can main (i) to be the result of H20 lattice rear- exist in two metastable forms, one stable at rangements associated with the conversion T < 85 K, the other stable between 85 and of the interstitial CO to substitutional CO. 136 K. It is interesting to note that our doThis is supported by the spectral sequence main (i) corresponds closely to their first in Fig. 6 where the higher loss of interstitial temperature range. Since the high-temperaCO (2152 cm -~ shoulder) relative to substi- ture end of our domain (i) is associated with tutional CO (2137 cm -I main peak) in the the final loss of CO in interstitial sites, this 30-65 K region indicates that the loss is suggests that the low-temperature form of accompanied by molecular rearrangement. amorphous ice is rich in interstitial sites. We ascribe CO loss in the 125-150 K range (domain ii) to the transformation of amor- E. The Surface and Volume Binding Energies o f CO in 1t20 Ice phous ice into cubic ice. As the H20 molecules rearrange during the phase transition, The experiments discussed in Section the mobility of species within the lattice is IIIB can also be used to determine the bindincreased. The final loss of CO between 150 ing energies of CO in H20-rich ices. CO and 175 K (domain iii) coincides with the continuously leaves our samples at temper-

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40

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70

80 '

9 o ' 100

110 . . 120 . .130

140 150 '

TEMPERATURE, K

Fl6. 7. The decrease of CO absorption for H20 : CO ices as they are warmed from different initial deposition temperatures. The four samples were deposited at 20, 30, 40, and 50 K. No H20 loss was observed below 145 K. The dotted line s h o w s the decrease in absorption expected from the temperature d e p e n d e n c e of Aco (the line has been shifted upward for clarity). Slopes steeper than the dotted line indicate CO loss. Particularly large losses occur between 30 and 65 K and 125 and 150 K. The remaining CO is lost between 150 and 175 K during the sublimation of the entire H20 matrix.

ASTROPHYSICAL H20 : CO ICES atures above 140 K. At temperatures over 145 K both CO and H20 leave the sample, with the fractional CO loss always higher than the fractional H20 loss. In this regime, the CO loss proceeds via two separate processes. First, CO molecules diffuse through the H20 matrix. Some of these reach the surface and escape. Second, CO loss is enhanced near the surface when overlying H20 molecules sublime and expose the CO, which can then escape. Thus, some CO is lost because it wanders to the surface (a process controlled by diffusion, which depends on the C O - H 2 0 volume binding energy) and some CO is lost because the surface comes to it (a process controlled by the H20 loss rate, which depends on the HEOH20 surface binding energy). The rate of CO loss by diffusion can be used to calculate the volume binding energy of CO in H20 ice. First, the observed CO loss rate associated with diffusion can be found from --(TA~,)f]COab~ [(TAp)f ]

,

(6) where the variables have their previous definitions. The term in the first set of brackets represents the total CO loss rate, while the term in the second set of parenthesis corrects for the fraction of CO loss controlled by H20 sublimation. If CO can diffuse a distance df in time t~, then the CO loss rate can also be written as g t - f fl(Pcodfab )

tf

'

(7)

where pco is the number density of CO in the ice, (pfodfab) is the number of CO molecules within the diffusion distance df of the ice surface, f is the fraction of this CO that reaches the surface, and/3 is the fraction of the CO that reaches the surface which escapes before it can diffuse back into the ice volume. The f a c t o r f i s a function of sample geometry and varies from 3/16 when df < d,

213

where d is the ice sample thickness, to 1 when de -> d. The average distance a CO molecule diffuses in time tf is given by de = (v~D~ftf), where Df is the diffusion coefficient of CO in H20 at temperature Tf. Substituting this expression for df into Eq. (7), equating (6) and (7), and solving for Df yields 1 ( [ ( r A v ) i - (TA/2)f]CO Df - f2/32t f t A~op-'~o X L(,rA1,)iiHzo j .

(8)

The C O - H 2 0 volume binding energy can be calculated from these values of Df using the standard formula Df = Do exp(-A H v/ kT), and solving for (AHv/k):

AHv

--~ -

T In (Df/Do),

(9)

where Do is the diffusion constant for the C O - H 2 0 system. Equations (8) and (9) were used to calculate (AHv/k) for the 140, 145, and 152 K experiments for the annealed ice and the 150 and 152 K experiments for the unannealed ice presented in Section IIIB. We used a value ofAco = 1.7 x 10-17 cm/molecule, appropriate for CO in H20-rich ices at 10 K (Sandford et al. 1988). Calculations were made assuming that/3 = 0.375, i.e., that the fraction of the CO that escapes once it has reached the ice surface scales as [1 - (AHs/AHv)] in H20 ice. A value of 330 cm2/sec, determined for the self-diffusion of Hf180 in H20 ice (Delibaltas et al. 1966), was used for Do. This is not strictly appropriate for CO in H20, but the following analysis demonstrates that this assumption does not introduce a large error in the derived values of (AHv/k). The range of possible (AHv/k) values was calculated using Eqs. (8) and (9) by choosing f values lying at the extremes of 3/16 and 1. These are listed in Table II for both f values. Stated errors in Table II represent only the standard deviation of the values calculated at each temperature. The actual volume bind-

214

SANDFORD AND ALLAMANDOLA

ing energy should lie within the range listed. We estimated the best (AHv/k) value by comparing the calculated diffusion scale d~ for each f value with the ice layer thickness d and adjusting the f value iteratively until a consistent solution was obtained. These data imply a C O - H 2 0 volume binding energy of (AHv/k) = 5550 K for the 150 and 152 K unannealed ices and (AHv/k) = 5800 K for the 145 and 152 K annealed ices. While the binding energy of CO in the unannealed ice is expected to be several hundred degrees lower than that of the annealed ice (as was the case for the HzOH20 binding energies), the uncertainties in our calculations preclude ascribing too much significance to the observed difference. Four different uncertainties are associated with the H 2 0 - C O volume binding energies summarized in Table III. First are the statistical uncertainties which correspond to standard deviations of 30 to 230 K (Table II). Second, there is uncertainty in the calculated energies due to the range o f f values possible. The maximum uncertainty produced by this effect is about 500 K ( f = 3/16 v e r s u s f = 1). Since our iterative process constrains the actual f values better than this, this uncertainty probably lies below 300 K. Third, the Do value used may be inaccurate. Our assumed value of 330 cm2/ sec (for H2J80 in H20 ice) could potentially

differ from that of CO in H20 ice by up to a factor of 10 (but probably less). Using the extreme values of D0 = 30 and 3000 cm2/sec produces energies which differ by about 350 K. Finally, there is a small uncertainty associated with the Aco value. We used Aco = 1.7 × 10 17 cm/molecule which is appropriate for CO in H20 at 10 K. Schmitt, Greenberg, and Grim have shown that this value has a relative temperature dependence and could possibly be as much as a factor of 2 lower at 150 K. However, reducing Aco in Eq. (6) by half lowers our calculated (AHv/k) values by less than 100 K. Taken together, we conservatively estimate that the calculated values of 5550 and 5800 K for the unannealed and annealed ices are accurate within -+350 K. Again, assuming the surface and volume binding energies scale as the number of nearest neighbors in the matrix and assuming that all the CO is in substitutional sites, we can use the derived volume binding energies to estimate surface binding energies. The above values for the volume binding energy of CO in H20 ices correspond to surface binding energies of (AHflk) = 3470 and 3630 K for unannealed and annealed ices, respectively. Finally, note that the C O - H 2 0 binding energies calculated from the 140 K annealed ice experiment are substantially

TABLE Ill AVERAGED CO AND H2O BINDING ENERGIES System examined

CO on CO H20 on H20 CO in H20

Ice sample

Pure CO Unannealed H20 Annealed H20 Unannealed H20 Annealed H20

Surface binding energy ~'

Volume binding energy b

(AHJk)

(AHv/k)

960 _+ 10 K 4815 +_ 15 K 5070 _+ 50 K (3470 K) (3630 K)

(1280 K) (7705 K) (8110 K) 5550 _+ 350 K 5800 _+ 350 K

Numbers in parentheses represent binding energies calculated from the observed volume binding energies using nearest-neighbor scaling. h Numbers in parentheses represent binding energies calculated from the observed surface binding energies using nearest-neighbor scaling.

ASTROPHYSICAL H20:CO ICES

215

of the 3290 cm -1 band area of the H20: CO ices ratioed to the area of the same band in the spectra of the pure H20 ice at the same temperature. Each ratio was normalized to a value of 1.0 at the deposition temperature. It is apparent from Fig. 8 that the presence of I part in 20 of CO in the H20 matrix at 10 K produces a total excess H20 loss, prior to sublimation at 150 K, of about 5%. The CO-enhanced H20 loss occurs predominantly between 40 and 80 K. Note that this encompasses the temperature region in which CO loss is rapid and interstitial CO is converted into substititional CO (see Figs. 6 and 7). This suggests that the extra H20 is lost when the metastable ice, rich in interstitial sites, rearranges to the form containing only substitutional sites. The loss rate above 80 K is essentially identical to that of pure H20 ice, supporting the idea that the transition is involved. A similar H20 loss enhancement is not seen between 130 and 150 K when the H20 transforms from amorphous to cubic ice. This may be due to temperature-dependent differences in CO concentration. The effect of CO would be expected to decrease as the temperature rises since the CO/H20 ratio decreases with warm-up in these thin samples. By 130 K there may not be enough CO in the ice

lower than those of the higher temperature experiments (Table II). This suggests that the 140 K ice sample was still transforming from amorphous to cubic ice during the measurements. The energy liberated during this exothermic phase transformation corresponds to (AH/k) = 218 K per molecule (Ghormley 1968). This energy enhances the mobility of the CO molecules and results in a lower inferred C O - H 2 0 binding energy at this temperature. The difference between the 140 K and 145-152 K binding energies can be understood if the phase transformation went approximately 20% of the way to completion during the 5 hr of the 140 K measurement series. A summary of our best estimates for the CO-CO, HzO-H20, and C O - H 2 0 binding energies in ices is given in Table III.

F. The Enhancement of H20 Volatility Due to Entrapped CO First, pure H20 was deposited at 10 K and the strength of the 3290 cm -j band monitored at 20, 30, 40, 50, 65, 80, 100, 125, and 150 K. The ice was warmed at 2 K/min and held at each temperature for half an hour during the measurement. These data were used in conjunction with the warm-up spectra of the H20 : CO (20 : 1) mixtures discussed in Section IIID. Figure 8 is a plot

1.05 ,a,

1.00

__~

~ - ~ . ~

S %

,~

.1u.i 0 , 9 5 > I,~c 0 . 9 0

0.8!

............

..~

DEPOSIT TEMP, K -----20 ..... 30 - 40 ...... 50 .

.

,

.

10

20

30

40

,

.

.

506007

,

,

80

90

.

,

,

,

,

,

.

100110120130140150160

W A R M UP T E M P E R A T U R E ,

K

FIG. 8. Effect of entrapped CO on the sublimation rate of H20 ice. Ices were condensed from an H20 : CO = 20 : 1 gas mixture at 20, 30, 40, and 50 K and subsequently warmed. The H20 present in each sample is normalized to 1.0 at the deposition temperature and ratioed to the H20 loss observed from pure H20 ice.

216

SANDFORD AND ALLAMANDOLA

to substantially affect H20 loss during the amorphous ice to cubic ice phase transition. Note, however, that the excess H20 loss between 40 and 80 K is somewhat insensitive to concentration. This is demonstrated by the 50 K deposition shown in Fig. 7. Due to the lower sticking efficiency of CO at 50 K, this sample had a CO concentration 3 times lower at this temperature than the other ices, and yet it also produced a 5% excess H20 loss. This implies that enhancement of H20 loss in these ices primarily occurs during the low-temperature lattice transformation, but only when CO is present. The 40-80 K range of excess H20 loss in these experiments is higher than the 30-45 K range reported by Bar-Nun et al. (1987) for an H20 : Ar ice. This difference may be due to variations in experimental conditions such as the effects of Ar vs CO or differences in trapped species concentration (it is not clear what the Ar/H20 ratio was in the experiments reported by BarNun et al. (1987)). IV. ASTROPHYSICALIMPLICATIONS Much of the theoretical work on astrophysical ices proceeds with the assumption that the physical properties of pure components provide a reasonable approximation for their behavior in mixed ices. For many properties, however, this is incorrect. The interactions between different species in the ice can be quite strong and can affect processes like accretion, surface and bulk diffusion, and evaporation to such an extent that it becomes impossible to speak of the properties of an individual component. Instead, the collective properties of the material must be considered. A . Interstellar Ices

The position, width, and profile of the 2137 cm 1 solid state CO feature in the spectra of infrared sources in molecular clouds demonstrate that, in most cases, CO is intimately mixed in an ice matrix dominated by polar molecules, of which H20 is

probably the major constituent (Sandford et al. 1988). In a few cases the position and width of the band show that CO is frozen in nonpolar ices in which CO2 may be the major component. Models of interstellar ice accretion which consider only the thermodynamic properties of pure materials are, therefore, inappropriate and any conclusions derived from such models regarding grain temperature constraints are questionable. For example, many calculations of condensation in molecular clouds are based on the assumption that the individual ice components behave independently (e.g., Nakagawa 1980, Leger 1983, Yamamoto et al. 1983). These models are strictly only applicable to the relatively rare interstellar cases where pure ices may be present. Such calculations predict CO will accrete on grains in dense clouds only at temperatures below 20 K, while our data demonstrate that it can condense at much higher temperatures if H20 is present. This is because the C O - H 2 0 surface binding energy is about three times larger than the CO-CO surface binding energy (Table III). The CO residence times on the surface of solid CO versus solid H20 are listed as a function of temperature in Table IV. This shows that even for grain temperatures as high as 50 K, CO can accrete onto H20-rich grains in interstellar clouds. B. The F o r m a t i o n H i s t o r y o f C o m e t s i. The s o u r c e o f c o m e t a r y CO. CO has been detected in the comae of several comets (Cosmovici et al. 1982, Feldman 1986, Festou et al. 1986, Woods et al. 1986). Inferred gas-phase abundances of CO fall between 10 and 20% that of H20. Although it is unclear how much of the observed CO is the result of photodissociation of other parent molecules, it is generally assumed that CO is present in cometary ices. The CO may be trapped in amorphous ices or in clathrate hydrates (Delsemme and Miller 1971). The result that CO can be condensed and trapped in HzO-rich ices at much higher temperatures than deduced for pure CO

ASTROPHYSICAL H20: CO ICES

217

tion behavior is not controlled by H20 or some other more abundant component. COMPARISON OF THE C O RESIDENCE TIME ON C O ii. The s o u r c e o f c o m e t a r y C 0 2 . CO2 was AND H 2 0 ICES AS A FUNCTION OF ICE detected in Comet Halley by a variety of TEMPERATURE a techniques (Combes et al. 1986, KranTemperature CO on CO C O on H 2 0 kowsky et al. I986, Feldman et al. 1986). (K) residence time residence time The abundance of CO2 relative to H20 was (years) (years) inferred from these data to fall between 1 10 7.9 × 1021 1.6 × 10143 and 3.5%, although it has been suggested 20 1.1 _+ 10 ~ 3.5 x 1055 that the CO2 abundance may be higher dur30 1.2 x 10 6 2.7 × 1025 ing outbursts. There are several possible 40 3.2 x 10 9 7.6 x 1017 sources of cometary CO2. First, molecular 50 -2.2 x 10 l0 CO2 could have been produced during an 60 -2.1 x l0 t interstellar or protostellar phase prior to ice 70 -5.4 x 10 ~ 80 -1.1 × 10 -l formation and subsequently incorporated 90 -8.9 x 10 -4 into a comet or precometary ice grains by 100 -1.9 x 10 -5 condensation. Second, CO2 could have been formed within precometary ice grains R e s i d e n c e t i m e w a s c a l c u l a t e d u s i n g the i n v e r s e o f or in the coma via in situ photochemical E q . (2), i.e., tev = ug t exp(AHJkT). V a l u e s f o r (AHflk) w e r e t a k e n f r o m T a b l e I I I a n d u0 = 2 x 10~2/sec. reactions. Unfortunately the abundance of interstellar COE cannot be measured from Earth because of telluric CO2, and estimates of the modifies some preconceptions that are precometary CO2/H20 ratio must rely on commonly associated with the formation of theoretical models. Published models of the comets. It is often assumed that the pres- gas-phase chemistry in interstellar clouds ence of CO in comets implies that they which exclude grain surface chemistry were not heated above 25 K during forma- yield steady-state values of CO2/CO that lie tion and have not been exposed to tempera- between 1.5 x 1 0 - 2 and 5 × 10 5, dependtures much in excess of this value since ing on the assumed cloud conditions 0gleformation (see Spinrad 1987 for a recent sias 1977, Prasad and Huntress 1980). In review and discussion). For example, models which include both grain surface reYamamoto et al. (1983) and Yamamoto actions and gas-phase chemistry, CO2 can (1985) have made calculations based on the dominate CO if the controversial reaction assumptions that condensation occurred in CO + O is possible on grain surfaces at low a manner that fractionated the CO from temperatures (Tielens and Hagen 1982, H20 and that molecular species sublime d'Hendecourt et al. 1985, Grim and d'Henand condense independently. In particular, decourt 1986). they assume that CO condenses into comeIn addition to the direct condensation of tary ices only at temperatures below 25 K. gas-phase CO2, it is possible that CO2 was In contrast, CO can be condensed in signifi- formed within the ice by radiation. Irradiacant amounts in the presence of H20 ices at tion of the ice could have occurred either temperatures as high as 50 K and, once fro- on the surface of the comet or in the interzen into the ice, CO can be retained at even stellar/precometary phase. Cosmic ray efhigher temperatures (e.g., Figs. 5 and 7). fects will dominate photon-induced pro-. Clearly, the physical properties of pure CO cesses once the comet has formed since ices can only be used to help establish the they can penetrate about I0 m into the ice, conditions of the precometary formation re- while ultraviolet radiation can only penegion if CO is a major component of the trate the first 1000 .~t. Although cosmic rays comet under consideration and its sublima- readily form CO2 within ices (Moore et al. TABLE IV

218

SANDFORD AND ALLAMANDOLA

1983, and references therein), they are unable to produce CO2 throughout the entire comet. CO2 was observed from Comet Halley during its 1986 apparition. This comet, with a period of about 76 years, has made many orbits around the Sun and lost many tens of meters from its original surface. As the time required to substantially alter the top few meters of a comet by cosmic rays is on the order of 109 years (cf. Moore et al. 1983, Johnson et al. 1986), there is insufficient time between perihelion passages to explain the CO2 abundances seen in Comet Halley in terms of cosmic ray production. We are forced to conclude that cosmic rays are not the major contributor to the production of cometary CO2. Despite its insignificant role in the alteration of the interiors of comets, ultraviolet radiation is important during the precometary phase. Any CO2 produced within the interstellar ices would be incorporated throughout the comet during accretion. The photolysis of any ice mixture containing a source of carbon and HzO invariably results in the production of CO2 (cf. Hagen et al. 1979, d'Hendecourt et al. 1986, Sandford et al. 1988). The quantum efficiency for the conversion of CO into CO2 via in situ ultraviolet photolysis in simple binary H20 : CO (20 : 1) ices is on the order of a few percent (Sandford et al. 1988). From other experiments it is known that if other molecules are present they tend to screen out the ultraviolet photons and the efficiency usually drops (Allamandola et al. 1988). Adopting a quantum efficiency of 1%, we find that a precometary ice C 0 2 / H 2 0 value of 3% is consistent with an ultraviolet fluence in the photolytically important region below 2000 A of about 1019 photons/ cm 2 for an optically thin ice. This is a relatively modest exposure. The ultraviolet flux in the diffuse interstellar medium is approximately 108 photons/cmZ/sec. The flux within dense molecular clouds is less than this and depends on the optical depth of the cloud. At least a tenth of the photons will penetrate to optical depths of 3, a thou-

sandth to optical depths of 10, and only a millionth to an optical depth of 20 (Hagen et al. 1979). Thus, the ultraviolet fluence required to produce the observed CO2 could be accumulated in the interstellar cloud phase in as little as 104 to 5 × 106 years provided the optical depth of the cloud is less than 10. For clouds with larger optical depths, internal sources of radiation produce an ultraviolet flux between 10-5 and 10 -4 that of the diffuse interstellar field (Norman and Silk 1980, Prasad and Tarafdar 1983). Consequently, sufficient photoprocessing can occur in dense clouds on time scales of 10 6 t o 10 8 years, a period comparable to the lifetime of most molecular clouds (Elmegreen 1985). Ultraviolet exposure could also have occurred during an active protosolar or T-Tauri phase of the Sun. Thus, substantial amounts of CO2 could have been produced in the ice phase via ultraviolet photolysis prior to cometary formation. Finally, the possibility that CO2 is produced by solar irradiation of ice grains ejected into the coma must also be considered. The solar ultraviolet flux at 1 AU is about 1013 photons/cm2/sec. With this flux, an optically thin ice grain must be exposed for about 20 days to produce the required amount of CO2. This is much longer than the evaporative time scale for an optically thin ice grain at 1 AU. Therefore, the observed CO2 is not produced in the ice particles after cometary ejection. We conclude that, if CO2 exists in cometary ices, it probably has an interstellar/ precometary origin. While the CO2 may include contributions from molecules accreted directly from the gas phase, much of the CO2 must have been produced within precometary ice grains by photolysis. C. C o m e t a r y Activity i. A c t i v i t y in n o r m a l c o m e t s . H20 controls the surface temperature and vaporization behavior of the majority of cometary nuclei (Delsemme 1982, 1983, Spinrad

ASTROPHYSICAL HzO:CO ICES 1987, and references therein). For a particularly nice description of the vaporization processes, see Schmitt and Klinger (1987). HzO mediates the behavior of ices, even when more volatile components are present, provided that it is a major constituent. Most cometary models account for the transition of amorphous H20 ice to cubic H20 ice as the comet warms during its approach to the Sun. This transition is important to consider since (i) it must occur to some extent within all comets containing amorphous ice, (ii) it can supply additional energy to the ice, and (iii) it can alter the comet's thermal conductance and diffusion properties (Klinger 1981, Smoluchowski 1981a,b, Herman and Weissman 1987). In most models the transformation is driven by surface warming due to close passages to the Sun. In these cases, a layer of cubic ice surrounds an amorphous ice core. The cubic ice layer propagates downward as the temperature of the nucleus increases and as overlying cubic ice is sublimed away (cf. Prialnik and Bar-Nun 1987). Multiple apparitions may drive the cubic ice layer to the center of the comet (Herman and Weissman 1987). Others have noted that if the 26A1 content in comets was similar to that inferred from the primitive meteorite AIlende, then the energy released during its decay may have altered the entire comet to cubic ice during or shortly after formation of the nucleus (Prialnik et al. 1987). Thus, the most appropriate binding energies for problems involving sublimation from the cometary surface are those listed in Table IlI for the annealed samples. The presence of CO in the ice produces some new effects not seen in pure H20 ices. Much of the CO leaves the ice at temperatures lower than the sublimation point of H20, particularly between 30 and 65 K. If cometary nuclei are porous, CO released during cometary warm-up through this temperature range is likely to escape the comet entirely. The result would be minor cometary activity at larger heliocentric distances than those associated with the sublimation

219

of H20, provided sufficient quantities of gas are released (Delsemme 1982, 1983). Calculations suggest that, after multiple revolutions around the Sun, many cometary cores will reach sufficiently high temperatures to degas a significant fraction of their CO in this manner (Herman and Weissman 1987). Any gases ejected by this slow, steady process would, of course, be CO-rich. If cometary nuclei are not porous, the CO released at temperatures below 150 K may not be able to immediately escape the comet. Instead, gaseous CO might accumulate in pockets and cracks within the ice. This possibility is discussed in the following sections. However, it is also clear that theories that assume H20 controls the vaporization behavior of comets are valid for HzO-rich cometary ices, even if they contain small amounts of other, more volatile, components. For example, once above 80 K, the H20 lost during warm-up to 150 K is not greatly affected by the initial presence of CO at concentrations of 1 part in 20 (see Fig. 8). One reason that CO does not affect the behavior of the ice during the amorphous ice to cubic ice phase transition in our experiments is that its abundance is very low due to previous CO loss. CO is also lost at these temperatures by diffusion. Thus, provided the sample is thin enough, the properties of a l0 K H20 : CO ice that is slowly warmed to 150 K will be largely those of a pure H20 ice. In a large sample, the CO reservoir within the bulk material will constantly replenish the CO near the surface and in this case one might expect some H20 loss enhancement during the transition from amorphous ice to cubic ice. ii. Activity in volatile-rich c o m e t s . Despite the predominance of H20 in most comets, components more volatile than H20 appear to control sublimation rates in a few comets (Feldman 1978, A'Hearn and Cowan 1980, Houpis and Mendis 1981). Examples include comets Morehouse 1908 III, Humason 1962 VIII, Kohoutek 1973 XII, and West 1976 VI. Models that assume H20

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controls the vaporization behavior fail to fit the observations of these comets. The observed behavior is similar to that of comets controlled by H20, except that it is manifested at larger heliocentric distances (Delsemme 1982, 1983). The observations can, however, be fit assuming the sublimation is controlled by more volatile constituents such as CO and CO2. Here we only address the question of how large the CO/H20 or CO2/H20 ratio must be before the H20 no longer regulates the behavior of the ice. In H20-rich ices, H20 controls the ice behavior because it forms relatively strong H-bonding networks through which the other molecular components must migrate. Thus, one would expect that CO and CO2 should begin to strongly influence the behavior of the ice when their concentrations are sufficiently large that the H20 molecules cannot form a complete network. The largest amount of contaminants a traditional clathrate can contain before disruption is about 15% (see Davidson et al. 1987, and references therein). However, these structures are much more organized than amorphous ices produced at low temperatures and pressures. Infrared studies have shown that the three-dimensional H-bonded network of amorphous H20 ice begins to break down once the concentration of non-H-bonding impurities exceeds 30% (Hagen et al. 1983). Beyond this point the H-bonding is incomplete and we expect the behavior of the ice to become progressively more influenced by the impurities (provided, of course, that they are more volatile than H20). This implies that in comets like Kohoutek 1973 XII and West 1976 VI, whose activities appeared to be controlled by species more volatile than H20, the concentration of CO and/or CO2 must be in excess of 30% in some regions. iii. Cometary outbursts. Cometary outbursts are relatively common (Delsemme 1982, Wyckoff 1982, Beyer 1962, and his many previous papers in Astron. Nachr.). Outbursts are traditionally thought to be

due to "explosions" produced by rapid transitions from amorphous ice to cubic ice in localized regions (Smoluchowski 1981a) and/or gas release from pockets of material rich in molecules more volatile that H20 (Whipple 1980, Feldman et al. 1986). CO and CO2 are the most commonly considered molecules for the latter case. These pockets may be due to heterogeneities trapped within the nucleus during accretion (a possibility somewhat at odds with observations that suggest that cometary nuclei are largely homogeneous (Donn and Rahe 1982)) or to localized enrichment in volatiles by diffusional processes. Much of the CO trapped in H20 ices is released during warm-up by a combination of molecular reorganization during H20 annealing and CO diffusion. If cometary nuclei are porous, this CO will escape and produce some nuclear activity at large solar distances. If cometary surfaces are not porous, however, the released gaseous CO may accumulate into pockets within the ice or beneath the amorphous ice-cubic ice interface. The sudden release of gas from these pockets, triggered by sublimation of the surrounding nonporous ice or by macroscopic displacements within the nucleus (comet quakes), might then produce outburst activity. Here, the CO-HzO binding energies for unannealed ices are the most appropriate for modeling processes that occur prior to the amorphous H20 ice to cubic H20 ice transition, and those for annealed ices are most appropriate for processes that occur after the transition (Table III). V. CONCLUSIONS

Properties associated with the vaporization behavior of CO, H20, and H 2 0 : C O ices have been determined using infrared spectroscopic techniques. The CO-CO surface binding energy in pure CO ice was found to be (AHs/k) -- 960 _+ 10 K, in good agreement with the previously published value of 955 K. The H20-H20 surface binding energies on H20 ices containing minor amounts of CO are (2~Hs/k) = 4815 + 15

ASTROPHYSICAL H20: CO ICES

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K and 5070 + 50 K for unannealed and an- H20 when they are sufficiently abundant nealed ices, respectively. Finally, the CO- (>30%) to disrupt the H-bonding network H20 volume binding energies on H20 ices of the H20 ice. Third, in H20-rich ices containing CO were (AHv/k) = 5550 + 350 above 65 K, H20 controls the sublimation K and 5800 + 350 K for unannealed and regardless of the volatility of other compoannealed ices, respectively. nents. Less abundant species which have Under the experimental conditions used, higher volatilities than H20 (such as CO) substantial amounts of CO condense out of can be slowly released from the ice at temthe gas phase onto H20 ices at tempera- peratures substantially lower than the sublitures as high as 65 K. This is in sharp con- mation temperature of H20. This intermetrast with the 30 K temperature required for diate release is the result of processes its condensation onto pure CO ices. Once involving reorganization of H20 molecules trapped in an H20 lattice, some CO remains in the lattice and volatile diffusion through in the solid state up to relatively high tem- the lattice. This gradual release can contribperatures (150 K). H20: CO ices condensed ute to the formation of a coma at large at 10 K gradually lose CO as the ice is heliocentric distances or may result in the warmed. Major CO losses are observed be- accumulation of gas pockets within the nutween 30 and 65 K, 125 and 150 K, and 150 cleus. Gas release from such pockets could and 175 K. The 30-65 K losses occur dur- then be triggered by sublimation or physical ing a period of H20 rearrangement in which destruction of the pocket walls and result in CO originally trapped in metastable, inter- jetting or outburst activity. stitial sites within the amorphous ice either Finally, ultraviolet photolysis of ices escapes or is trapped in substitutional sites. containing HzO and simple carbon-containBetween 125 and 150 K amorphous ice un- ing molecules produces CO2. The amount dergoes a phase transition to cubic ice and of CO2 inferred to be present in Comet Haladditional CO leaves the matrix. Above 125 ley can be explained in terms of photoK CO loss by diffusion also becomes impor- production in the ice grain phase prior to tant. Finally, between 150 and 175 K the cometary formation. Adequate ultraviolet last CO is lost when the H20 matrix itself radiation could have been supplied either sublimes. The presence of CO in an H20 ice during a precometary/interstellar phase or lattice increases the effective volatility of possibly during the protosolar phase. the H20. When CO is initially present in concentrations of 1 part in 20 about 5% ACKNOWLEDGMENTS more H20 is lost during warm-up between The authors thank A. H. Delsemme and A. Bar-nun 20 and 80 K than when CO is not present. for helpful reviews which resulted in an improved verThese results are applicable to the study sion of this paper. We also thank Schmitt, Greenberg, of comets. First, the presence of CO in a and Grim for showing us their results on the temperature dependence of Aco prior to publication. comet does not necessarily imply that the comet formed at temperatures below 25 K, REFERENCES as is often stated. The observation that volA'HEARN, M. F., AND J. J. COWAN 1980. Vaporizaatile molecules like CO can be condensed tion in comets: The icy grain halo of comet West. and maintained in the solid state at much Moon Planets 22, 41-52. higher temperatures in the presence of H20 ALLAMANDOLA, L. J. 1984. Absorption and emission than for the pure volatile weakens the lowcharacteristics of interstellar dust. In Galactic and Extragalactic IR Spectroscopy (M. Kessler and P. temperature constraints associated with the Phillips, Eds.), pp, 5-35. Reidel, Dordrecht. conditions under which comets formed. ALLAMANDOLA, L. J., S. A. SANDFORD, AND G. J. Second, molecular components more volaVALERO 1988. Photochemical and thermal evolution tile than H20 can only exert a major influof interstellar/precometary ice analogs. Icarus 76, ence on the sublimation behavior of the 225-252.

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