ICARUS 54, 388-405 (1983)

Studies of Proton-irradiated Cometary-Type Ice Mixtures

MARLA H. MOORE,*-' BERTRAM DONNt RAJ KHANNA,*AND MICHAEL F. A'HEARN*

University of Maryland, College Park, Maryland 20740, and tNASAlGoddard Space Flight Center,Green belt, Maryland 20771

Received October 21, 1982; revised January II, 1983

Radiation synthesis has been proposed as a mechanism for changing the nature of the outer fewmeters of ice in a comet stored 4.6 billion years in the Oort cloud and may explain some of thedifferences observed between new and more evolved comets. Cometary-type ice mixtures werestudied in a laboratory experiment designed to approximately simulate the expected temperature.pressure, and radiation environment of the interstellar Oort cloud region. The 2.5- to 15-Arminfrared absorption features of thin ice films were analyzed near 20'K before and after I MeVproton irradiation. Various ice mixtures included the molecules HO, NH3, CH 4, N2, C3H8, CO. andCO2. All experiments confirm the synthesis of new molecular species in solid phase mixtures at20'K. The synthesized molecules, identified by their infrared signatures, are C2H6, CO 2, CO. N2(O,NO, and CH, (weak). Synthesized molecules, identified by gas chromatographic (GC) analysis ofthe volatile fraction of the warmed irradiated ice mixture, are C2H, or C2 H,, and C3H8. When CH 4 ispresent in the irradiated ice mixture, long-chained volatile hydrocarbons and CO2 are synthesizedalong with high-molecular-weight carbon compounds present in the room temperature residue.Irradiated mixtures containing CO and H20 synthesize CO2 and those with CO2 and H20 synthesizeCO. Due to radiation synthesis, - I% of the ice was converted into a nonvolatile residue containingcomplicated carbon compounds not present in blank samples. These results suggest that irrespec-tive of the composition of newly accreted comets, initial molecular abundances can be altered andnew species created as a result of radiation synthesis. Irradiated mixtures exhibited thermolumi-nescence and pressure enhancements during warming; these phenomena suggest irradiation syn-thesis of reactive species. Outbursts in new comets resulting from similar radiation induced exo-thermic activity would be expected to occur beginning at distances of the order of 100 AU.

1. INTRODUCTION

Cometary Theory and Observations

It is a widely accepted postulate thatcomets were formed simultaneously withthe solar system (Delsemme, 1977; Donnand Rahe, 1982) and have been stored for4.6 billion years in the Oort cloud region.Cometary nuclei in this interstellar environ-ment experience an ionizing particle fluxdue to cosmic rays and an ultraviolet pho-ton flux. Ultraviolet photons penetrate ma-terial to - 10- I cm and affect only the come-tary surface. Cosmic rays however haveconsiderable penetrating power as shown inTable I for protons in water ice.

' NAS/NRC Research Associate at NASA/GSFC.

The exposure of comets to the environ-ment of interstellar ionizing radiation mayresult in significant chemical and physicalchanges in the outermost irradiated ice(Shul'man, 1972; Donn, 1976; Whipple,1977). The subsequent reprocessing maysignificantly alter the original properties ofthe ice and account for some of the differ-ences observed between new and moreevolved comets. The purpose of this re-search was to experimentally study the ef-fects of l-MeV proton radiation on ice mix-tures expected to occur in comets. Thel-MeV protons simulate the chemically ef-fective low energy range of cosmic rays andare also experimentally convenient.

Observations support the hypothesis thatwater is the dominant volatile in most com-ets (e.g., Jenkins and Wingert, 1972; Keller

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0019-1035/83 $3.00Copynrght © 1983 by Academic Press, Inc.All nghts of reproduction in any form reserved.

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES

TABLE I

RANGE OF PROTONS IN WATER ICE'

Proton energy (MeV) Range (cm)

0.5 9.1 x 10-4b|.0 2.1 x 10-3b

5.0 3.5 x 10-2b10.0 1.1 x 10-1I

100.0 7.6'

,The range of protons in water ice isequal to the range of protons in liquid watersince we have adopted the same mass den-sity (p) for both phases. The integrated lossof energy of the penetrating proton, whichdetermines the proton range, is proportionalto the mean density of electrons (N,) in thetarget (e.g., Anderson and Zeigler, 1977).Since N, -p, the range of a proton can becalculated if different densities occur due tophase changes or ice compactions.

b Northcliffe and Shilling (1970).c Barkas and Berger (1964).

and Thomas, 1975; Jackson et al., 1976;Weaver et al., 1981). In some comets, car-bon dioxide or carbon monoxide may alsobe a major volatile (Delsemme, 1977; Feld-man et al., 1974; A'Hearn et al., 1977;Delsemme and Combi, 1979), whereas am-monia and methane are examples of mole-cules suggested as minor constituents in thecometary ice.

In most respects, the observed featuresof new and more evolved comets are verysimilar. There are, however, some come-tary observations which support the ideathat new comets have a more volatile icysurface layer than evolved comets. Onetype of observational data is based on themeasured rate of variation of cometarybrightness for comets of different ages.Whipple (1978) showed that a typical newcomet brightens rapidly and becomes visi-ble at great solar distances and then moreslowly near perihelion (n - 2.4, where theobserved cometary brightness is - r n, thecomet-sun distance is r, and the value of nis derived for each comet by curve fitting).The behavior of new comets at great solardistances supports one of the conclusions

of Oort and Schmidt (1950) that these com-ets contain more volatile gases. After peri-helion, new comets show statistically thesame luminosity decrease as older comets(h - 3.4). Whipple (1978) suggested thatthis could arise from the buildup of meteor-itic material. A buildup of a chemical insu-lating layer may also occur. These observa-tions imply that the supply of more volatileice has been exhausted before the first peri-helion is reached. Marsden and Sekanina(1973), based on the statistics of cometaryorbits, concluded that observed cometswith q > 3 AU (where q = perihelion dis-tance) are mostly new comets. Since thereis no a priori reason for that statistical dis-tribution, a possible explanation is that af-ter their first apparition comets are muchfainter than they were initially and are notas readily detectable at these distances.

The possible existence of super-volatileices on the surface of new comets is sup-ported also by the observations of tails innew distant comets. Observations analyzedby Sekanina (1973) showed that each of twonew comets exhibited a particle tail whichhe estimated had been released at distancesas large as 15 AU. His calculations showedthat a substance more volatile than watersnow, and probably comparable to or evenmore volatile than methane, was requiredto supply the necessary momentum to liftthe grains into the tail. Cometary activity atvery large heliocentric distances is also en-visioned by Sekanina (1982) to explain thedust observed around the relatively inac-tive new comet, Bowell (1980b).

What is definite is that "new" cometsshow enhanced activity at large heliocen-tric distances. The cause of this behavior,as just discussed, is generally interpreted tobe super-volatile ices in the outer shell ofthe nucleus. Although the differencesamong individual comets are large, Whip-ple (1977) adopted, as a working model, alayered cometary nucleus. For a newcomet, this structure includes an outerlayer formed of very volatile material ex-tending to depths of a few hundred grams

389

MOORE ET AL.

per square centimeter. Whipple speculatedthat this frosting could result from cosmicray irradiation of the ice during the 4.6-bil-lion year storage time in the Oort cloud.Radiation-synthesized radicals would beconcentrated in this outermost layer. Theseradicals would react exothermically withmoderate solar heating at solar distances r> 3 AU resulting in the vaporization of thefrosting. An outflow of solid particleswould be produced. If this frosting is re-moved during the first perihelion passage,the brightness of the comet could decreaseby several magnitudes during the next ap-parition. This model could explain the large"disappearance rate" of newer cometswith perihelion distance q > 3 AU.

An apparently opposite speculation byDonn (1976) was that intense radiation syn-thesis would tend to polymerize the simple,volatile ices resulting in a darker, less vola-tile outer zone with a lower albedo com-pared to the inner protected ices. He sug-gested that this less volatile outer zone wasin contradiction with the greater activity ofnew comets. However, this may not be thecase. This layer may not be completelyconverted to inert polymer resulting in amixed-polymer reactive ice shell close tothe surface. This heavily irradiated outerlayer will be darker and absorb a largerfraction of sunlight than the original icy ma-terial. The additional heating would makethis shell significantly more active than theoriginal material that is exposed in almost-new comets. Thus, Whipple's or Donn'sprediction leads to more active new com-ets.

Cosmic Ray Environment

The interstellar particle radiation envi-ronment is modeled as a cosmic ray fluxwhich, according to cosmochemical evi-dence, has been roughly constant through-out the age of the solar system (Meyer etal., 1974). Although the flux and spectrumof interstellar cosmic rays cannot be di-rectly measured, it is estimated that 93% ofthe particle flux is due to protons, 6% he-

lium nuclei, and 1% heavier weight nuclei(Meyer et al., 1974). The interstellar elec-tron cosmic ray flux is 1/10 to 1/100 the pro-ton flux for energies below 100 MeV (Gold-stein et al., 1970). An estimate of the totalgalactic cosmic ray intensity is 8 x 102 MeVcm- 2 sec- 'ster- '; the mean energy of a cos-mic ray proton is 2 x 103 MeV (Shul'man,1972). A cometary surface exposed to thisradiation for 4.5 billion years would un-dergo 3.5 x 10'7 impacts/cm 2 .

The energy density deposited in a come-tary icy nucleus over 4.5 billion years hasbeen estimated by Donn (1976), and inde-pendently by Whipple (1977). For energiesgreater than 100 MeV, the extrapolated in-terstellar cosmic ray proton differential en-ergy spectrum is assumed to be a power lawof the form dNIdE - E-e, where y - 2.5(Goldstein et al., 1970). Donn (1976) usedthis energy spectrum and assumed that thedistribution was valid to energies as low as10 MeV. These data were combined withthe stopping power of water ice to estimatethe accumulated energy deposited in an icycometary nucleus as a function of depth.The calculated results indicate that suffi-cient energy is accumulated in the top lay-ers of a cometary nucleus for significant ra-diation synthesis to occur. The top 10 cm ofthe cometary ice effectively stops protonswith energies below 120 MeV. Protons withgreater energy penetrate to deeper ice lay-ers. Because the number of cosmic ray pro-tons is decreasing as -2-5 the concentrationof reacted molecules diminishes in a corre-sponding manner. Table II compares thepercentage of reacted molecules predictedby Donn (1976) and Whipple (1977).

Three previous experiments have beencarried out on the effects of particle radia-tion on ice mixtures at low temperature: (I)Berger (1961) irradiated a mixture of water,ammonia, and methane at 770 K with 12MeV protons to a total incident fluence of1.24 x 1014 protons cm 2 (calculated fromBerger assuming the incident flux given as0.5 ,AA/sec was to read 0.5 MxA/cm 2 ). Heestimated that the number of proton im-

390

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES

TABLE 11

ESTIMATED ENERGY ACCUMULATED IN COMETARY Nuci.El IN 4.5BILLION YEARS

Absorbed fluencein top 20 cm

ice layer

Donn (1976)3.49 x 1019 MeV cm-2

Whipple (1977)2.4 x 10 19 MeV cm-2

Approximate % of moleculesreacted, in 20-cm layer

located at:

Depth of Dept]Surface I m 2 r

50

34

16

30

n ofn

10

17

Note. Oort cloud dose - 41 eV/molecule. An *'Oort cloud dose" isdefined as the eV/molecule estimated to be deposited in the top 20-cm layerof a cometary ice stored for 4.5 billion years in the Oort cloud region. Usingthe average absorbed fluence predicted by Donn (1976) and Whipple (1977),2.9 x 1019 MeV/cm2 , results in an Oort cloud dose = 41 eV/molecule (theaverage molecular weight adopted was 17 g/mol).

a The amount of absorbed energy required to create a new product istypically 100 eV/molecule (e.g., Davis and Libby, 1964).

pacts cm-2 during the experiment ap-proached the order of magnitude of thosesustained by a comet the age of the solarsystem (actually, 10'7 proton impacts cm-2is calculated based on data discussed byShul'man, 1972 or Donn, 1976). Analysis ofthe volatile and nonvolatile fractions afterirradiation showed the presence of acetone,the suggestion of purine or pyrimidine com-pounds, and urea. (2) Oro (1963) irradiateda mixture of water, ammonia, and methaneat 770K with 5-MeV electrons and mea-sured a 4.6% conversion of labeled ['4C]me-thane from a volatile to a nonvolatile form.Analysis of the nonvolatile residue sug-gested the presence of purines and pyrimi-dines. (3) Pirronello et al. (1982) irradiatedan ice mixture of equal parts of isotopicallylabeled water and carbon dioxide at -90Kwith 1.5-MeV helium ions (1013_1014 inci-dent ions over a 4-cm2 sample) and ob-served the synthesis of formaldehyde,H2CO. The production rate of H2CO was-3.7 molecules per incident ion for ice-1000 monolayers thick. The formalde-hyde was detected using a mass spectrome-ter which analyzed the volatiles released by

sublimation from the irradiated ice duringwarming. Experiments investigating the ef-fects of radiation in pure ices have beencarried out by various investigators. Theseinvestigations include y-irradiated CH4,H20, and NH3, and e-irradiated C2H2 andD20 (Davis and Libby, 1964; Matheson andSmaller, 1955; Floyd et al., 1973; Glasel,1962, respectively). All of these experi-ments suggest that interesting low-tempera-ture chemistry can occur as the result ofparticle radiation synthesis.

An extensive program at Leiden has con-tributed significant results on the effects ofultraviolet photolysis on low-temperatureice mixtures (e.g., Greenberg, 1982; Haganet al., 1979). These mixtures (e.g., CO +H20 + NH3 + C0 2) are simulations of icemantles on interstellar grains. Their resultsshow: (1) the synthesis of new products at10°K, (2) luminescence and pressure en-hancements during warming, and (3) aroom-temperature residue. The comparisonbetween ultraviolet and particle irradiatedices is a complex task and one that is notaddressed in this paper. In general, it is ex-pected that at the molecular level, all kinds

391

MOORE ET AL.

of ionizing radiation have qualitatively thesame effect (e.g., Dertinger and Jung,1980).

Recent work has shown that charged par-ticles effectively erode frozen volatiles(Brown et al., 1980a,b). The number ofmolecules sputtered in a 7-10'K ice per in-cident 1.5-MeV He ion is: CH4, 120, CO2,120, NH3, 140, SO2, 32, H20, 8 (Johnson etal., 1982). These studies are used to predictthe effects of competition between collec-tion and loss of volatiles on icy bodies andthe nature of the surface composition. Im-plications for comets are also discussed byJohnson et al. (1982).

II. EXPERIMENTS

In previous experiments the effects ofparticle radiation on low-temperature iceswere deduced by analyzing the volatileswhich sublimed during warming or by ana-lyzing the residue after warming to roomtemperature. The present research on pro-ton-irradiated cometary-type ice mixturesmeasured the infrared absorption spectrumof the ice at 20'K, before, during, and afterirradiation. The ice characteristics duringwarming and the residue after warming toroom temperature were also investigated.Mixtures include some combination of thefollowing molecules: H20, NH3, CH4, N2,C3H8, CO, and CO2. These combinationsapproximately simulate the abundant ele-mental composition of comets. The extentof chemical changes is not expected to besensitive to the molecular composition; thespecific product will depend on composi-tion. More experimental details are givenelsewhere (Moore, 1981). The sputtering ofice films, the expected difference betweenthin films and bulk ice samples, and theemission of secondary electrons from thealuminum sample substrate are discussedby Moore. In general, these effects aresmall and therefore assumed to have negli-gible influence on these experiments.

The source of protons used in this experi-ment was a High Voltage Corporation 2.5-MeV Van de Graaff accelerator at the

NASA/GSFC. The proton energy was IMeV, and the beam current was of order10-7 A (1.25 x l0 l protons cm-2 sec 'overthe 5-cm 2 sample area). This flux incidenton a 1-Mm-thick ice film for 4 hr results inan absorbed fluence equivalent to that pre-dicted for the top 20-cm layer of a cometexposed for about 1.5 billion years in theOort cloud. The absorbed fluence (MeVcm- 2) in an ice film was calculated from:incident flux x irradiation time x (stoppingpower x ice density)2 x ice thickness. Toprevent contamination of the film from theaccelerator vacuum chamber, a 3.1 x 10-5-cm-thick nickel foil (0.64-cm diameter) wasplaced in the beam tube and was used toisolate the Van de Graaff vacuum from theice sample vacuum area. Approximately3% of each proton's energy was lost as theresult of passage through the nickel foil.

Figure I is a schematic of the experimen-tal arrangement and shows the low-temper-ature closed-cycle cryostat attached to anion-pumped vacuum system and the Van deGraaff accelerator. Figure 2 is a cut-awaydrawing of the sample area of the cryostat.Ice films were formed by condensing a gasmixture on the aluminum substrate whoseminimum temperature was near 20°K, sev-eral degrees above the lowest temperatureof the cryostat's cold end due to thermalresistance. The substrate could be main-tained at any temperature between 20 and300°K with a temperature stability of+ 1.0°K. One surface of the aluminum sub-strate was polished to a high reflectivity.The opposite side was anodized with sap-phire. This anodizing provided a low ther-mal resistivity along with a high electricalresistivity (required for the measurement ofthe proton beam current). After deposition,the ice sample was irradiated with l-MeVprotons.

2 The electronic stopping power for l-MeV protonsis 308 MeV cm2 g-' (Northcliffe and Shilling, 1970).The density of water ice was assumed to be I g cm 1.A I -Am-thick ice film absorbs approximately 3% of theenergy of each l-MeV proton. Our calculations can berescaled for a different ice density.

392

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES

TacMT -Fr I - PROTON BEAM

GAS SUBSTRATE FOILHANDLINGFACILITY

IONPUMP

CLOSED VAN DE GRAAFFCYCLE

CRYOSTAT ACCELERATOR BEAM TUBE

FIG. I. Arrangement of spectrophotometer, low-temperature cryostat, and Van de Graaff generator.Ice sample is shown facing in the direction of the proton beam. If rotated 180°, the infrared spectrum ofthe ice sample can be measured.

The infrared spectrum of the ice could bemeasured before, during, and after irradia-tion by rotating the ice sample. A Perkin-Elmer 621 double-beam spectrophotometerwas used to measure the absorption spec-trum of the deposited ice between 2.5 and15 ,am. The sample beam from the spec-

trometer was directed onto the depositedice film. This beam passed through the ice,reflected from the aluminum mirror sub-strate, and then passed a second timethrough the ice before it was recombinedwith the reference beam of the spectrome-ter. By using the measured infrared spec-

FIG. 2. The sample area of the cryostat. Ice samples are formed on the cooled aluminum mirrorsubstrate by condensation of premixed gases.

393

MOORE ET AL.

trum of the ice, the equivalent film thick-ness of each molecular constituent in theice was calculated using the standard Lam-bert-Bouguer radiation law and known ab-sorption coefficients. The total thickness ofa deposited ice mixture was assumed to bethe sum of the thicknesses of each molecu-lar component.

The analysis of the spectrum of irradiatedice mixtures from 2.5 to 15 ,Am involved theidentification of new absorption features inthe infrared spectrum of the original ice. Inan original mixture containing H20 + NH3+ CH4 (see Fig. 3, Before), the v, and V3

bands of ammonia and the v, band of watercombine to form a large absorption bandcentered near 3300 cm -'. The v) fundamen-tal of methane forms a sharp line centeredat 3110 cm-'. Near 1600 cm-', the V4 funda-mental of ammonia and the v2 fundamentalof water combine to form a small band. At1300 cm -' is the V4 fundamental of methane,at 1090 cm-' the i 2 fundamental of ammo-nia, and at -800 cm-' the libration band ofwater. When CO, was substituted as asource of carbon, its V3 fundamental wasobserved at 2342 cm-', and v 2 at 654 cm-and 660 cm-' (unresolved); CO as a sourceof carbon gives a sharp absorption funda-mental at 2124 cm- '; and when substitutingN2 for NH3 as a source of nitrogen there areno absorptions in the 2.5- to 15-Am region.Gaseous carbon dioxide and water vapor,present in the spectrophotometer, were dif-ficult to completely remove through purg-ing. The narrow absorptions typical ofthese gases are superimposed on the spec-tra of all ice mixtures and are especiallynoticeable around 2.5-4, 4.2, and 5-7.5Am. All spectra have been drawn from theoriginal data but no attempt was made toexactly reproduce the line positions ofthese gas absorptions. These atmosphericlines have been completely removed fromthe 100% lines (Figs. 3-5) for convenience.Identification of synthesized species wasbased on numerous reference spectra of20'K deposits of H2O + NH3 + CH4 icescontaining a fourth molecule (less than 10%

by volume). The selection of the fourthmolecule was based on its expected synthe-sis.

Several blank samples were studied un-der experimental conditions identical tothose using ice mixtures. These blank sam-ples monitored the possible contribution ofcontaminants. A typical blank spectrumshowed that the gases H20 and CO2 werecryopumped in small amounts onto the low-temperature substrate. These moleculesoriginated from the unbaked walls of thevacuum system. No other molecular con-taminants were observed.

When irradiated ice samples werewarmed, the volatile fraction could be col-lected and analyzed using (e.g.) gas chro-matographic techniques. The detection ofthermoluminescence from other irradiatedsamples during warming was made using aI P21 cooled photomultiplier. Concurrentpressure enhancements in the vacuum sys-tem were recorded by monitoring the ionpump current which is directly proportionalto the pressure.

A summary of the experiments includedin this paper is given in Table III. Experi-ments are divided into six sets determinedby the composition of the initial ice mix-ture. Comments are included to help directthe reader to the corresponding figure ortable in the discussion.

111. RESULTS

Low Temperature

Experiment set I was a study of irradi-ated H20 + NH3 + CH4 ice mixtures at20'K using I-MeV protons. Ice mixtureswithin this and within other "sets" showedgenerally similar results after irradiation.Figure 3 shows the infrared spectrum of anH20 + NH3 + CH 4 ice (set la) before andafter irradiation. A list of the new absorp-tion lines observed after irradiation is givenin Table IV. In addition to the synthesis ofethane (C2H6), CO2, and CO, the 2040-cm-Iline is suggestive of an N-N-N bondedmolecule. The synthesis of CO2 was con-

394

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES 395

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396

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MOORE ET AL.

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PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES 397

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MOORE ET AL.

TABLE III

EXPERIMEIONTS

Thick- Irradi-ness ation(tim) time

(hrI

Absorbed Incident fluxfluence (E,,- = I MeVI

(MeV cm -- (MeV cm "sec 1)

la H.O + NH 1 + CH,

cb

defghi

Ila H20 + NH, + "CH.bcdefg

III H2O + N2 + CH,IV HO + N, + Co,

Va H2O + N, + CO

b H20 + NH, - COcd

Vla H.() + NH, 4 CH,'b

(2 :5:4)

(2: 7:4)(1 :10 7)(I:3: 2)(I :4: 2)(I :6:3)(I :3 :3tI 3 :4)-IS :9: 71

nc5

nc(20 13 . 16)

ncnc

(1:1:3)(1 :1 :4)/1: I: 1)'

2.7 6.3 I x t0" 2.0 x 10' (max)

2.62.22.3I .53.22.93.0)

15- 2.5-5.024

--5.0-9.0

1.7I.42.8

0.62.02.82.08.16.02.3

--2.5-1.22.8

-2.6I 1.42.1

-3.9--3.5

1.5

6 x 10'22 x 10tl4 x 10"2 x 10"2 x 10''I X 1I19 x III"

-8 x 101,-7 x lO"

3 x lo"

I x 10"'4 x 10"'I x 1(1''I x lol9 x 101'

3 5 x 101115.I x 101115s5 x 101115.3 x lo"'8.8 x 10"',7.4 x lo11"1.2 x lo0l

2 x loll (max)--2 x 101l (max)-2 x lo"11 (max)-2 x 1)1o" (max)-2 x 1011 (max)-- 2 x 10)11 (max)-.2 x 101l (max)* 2 x 10l (max)2 x 10l (max)

Spectrum: Fig. 3.Table IV

Residue: Fig. 7

Luminescence: lFig. 6

(1: I 1) ' 0.1 -2.1 S5 x 10" 2 x lOll (max) Spectrum: Fig.Table V

15. I :l1 d 0.7 --2.9 5 x 101 -2 x 10l (max) Spectrum: Fig.Table VI

(2: >I : 2'(2 3 1)(5: 10: 1)(9 :5:4)(7 4 :3)'

0.20.40.7

ncnc

--2.0tlo1.8

<4.0<4.0

I Mass ratio approximated by calculating equivalent thickne!coefficients (unless otherwise noted).

nc. Not calculated.Ratio measured in gas phase mixture prior to deposition.Ratio very uncertain.Proton energy = 1.5 MeV, ice temperature = 77"K.

f Incident fluence.

firmed in experiment set II using [' 3C]meth-ane which resulted in the formation of['3C]carbon dioxide at a rate proportional tothe incident flux. A decrease in the CH4lines is noticeable in Fig. 3, after irradia-tion.

Some coloration of the irradiated ice wasobserved in this and other experiments al-though not all irradiated films were visuallychecked for coloration due to the awkwardlocation of the viewing window. Whenviewed, however, a central circular yellow-ish area was noticeable where the irradiat-ing beam was most intense; fainter colora-tion extended over the entire sample. Nocoloration appeared in a small crescent-shaped area at the edge of the ice film

I X 11o,7 x 10122 x 10l2 x 104'2 x lO"'

-2 x 1(11 (max)-2 x 101l (max)

1.3 x 1l)11

4,

5-

Residue: Fig. 7Residue: Fig. 7

ss of each molecular component using appropriate absorption

where the thermal radiation shield of thecryostat shadowed the ice from the protonbeam.

The volatile fraction of a similarly irradi-ated ice (set Ih) was collected in a liquidnitrogen-trapped tube as the ice waswarmed above 77°K. Gas chromatographicanalysis (designed to be most sensitive tothe detection of organic compounds C,-C10) was used to identify CH4, C2H6 orC2H4, and C3H8 in the ratio 31: 22: 1.

The decrease in strength of the V4 12CH4and 13CH4 absorption line as a function offluence was measured quantitatively (set laand set hIg, respectively) after an averageabsorbed fluence of 1.5 x 1014 MeV/cm2

which is '-19 eV/molecule or half the Oort

398

Expen-ment

set

Icemixture

Approxi-matemassratio,

Comments

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES

TABLE IV

WAVENUMBERS OF NEW ABSORPTION FEATURESAFTER IRRADIATING H20 + NH3 + CH4 ICE

cm- Identification Unidentifiedfeatures(cm ')

2968 C2H, 29552930 C2 H6 22602875 C2 H6 21702342 CO2 21002282 1CO2 14702140 Co 13802040 N-N-N species, 1340 (broad band)

1460-1453 C2H61368 C2H6 (?)818 CH 6650 CO,, (654

and 660 cmlines wereunresolved)

Similar to the N-N-N stretch of the v2 fundamen-tal of solid NH4N3 at 90'K, Dows et al. (1955).

cloud dose (see Table 11). This is estimatedto be equivalent to the energy absorbed inthe top 20-cm layer of cometary ice in - 1.5billion years or to the energy absorbed in a20-cm layer at a depth of about I m in acomet irradiated for 4.5 billion years. A66% decrease in the line strength of CH4occurred. The average CH4 content beforeirradiation was 46% by mass.

Substitution of N2 for NH3 in the ice mix-ture was studied in experiment set III.Qualitatively similar results were obtainedeven though the difference between thebond energies of these molecules is 5.4 eV/molecule. N20 was identified only in exper-iment set III.

In the following two experiments (sets IVand Va), CO2 or CO was used instead ofCH4 as the source of carbon. In these mix-tures, N2 was substituted for NH3 as asource of nitrogen to circumvent the reac-tion between NH3 and CO2 which results in(NH4)NH2CO2. Figure 4 shows the infraredspectrum of an H20 + N2 + CO2 ice (setIV) before and after irradiation. A list ofnew absorption lines observed after irradia-

tion is given in Table V. The intensity of theunidentified lines at 2240, 1260, and 1243cm-' was observed to decrease when theice was warmed from 20 to 26°K suggestingthat very reactive radical species may bepresent. Further work is necessary beforeidentification of these reactive species canbe made. A decrease in the intensity of theCO2 line and subsequent increase in the COline as a function of absorbed fluence wasobserved. The intensity of the v3 CO2 (2143cm-') line decreased -18% after the icemixture had absorbed 5 x 1012 MeV/cm 2

which is --14 eV/molecule or approxi-mately one-third the Oort cloud dose. TheCO2 content before irradiation was 55% bymass.

In set Va, an ice mixture of H20 + N2 +CO was irradiated until the absorbedfluence was 4.6 x 1013 MeV/cm 2. The infra-red spectrum of this ice, the thickness ofwhich was -I Am, is shown in Fig. 5. A listof new absorption lines observed after irra-diation is given in Table VI. In addition tothe synthesis of C0 2, a small signature ten-tatively attributed to CH4 is observed. Adecrease in the intensity of the CO absorp-tion line and subsequent increase in the CO2absorption line as a function of absorbedfluence was measured (set Vb). The inten-sity of the CO absorption line decreased-46% after the ice mixture had absorbed

TABLE V

WAVENUMBERS OF NEW ABSORPTION FEATURESAFTER IRRADIATING H2 0 + N2 + CO2 ICE

cm-, Identification Unidentifiedfeatures(cm ')

2143 2 CO 2240f2100 '3CO 21801870 NO 20501303 CH4 1260h660 CO2 (654 and

660 cm-'lines were 1243,unresolved) 1038

Possibly associated with radical species.

399

MOORE ET AL.

TABLE VI

WAVENUMBERS OF NEW ABSORPTION FEATURESAFTER IRRADIATING H 2 0 + N, + CO ICE

cm- Identification Unidentifiedfeatures(cm-')

23432282

1300 small650

12CO2

CH4'

CO2 (654 and660 cm-' lineswere unresolved)

224522402180

1460 (broad band)

1015

I Tentative.

-5 x 103 MeV/cm2 which is -19 eV/mole-cule or approximately one-half the Oortcloud dose. The average CO content beforeirradiation was 22% by mass.

In experiment sets IV and V, some smallincrease in the CO2 absorption line oc-curred during irradiation due to CO2 cry-opumped from the unbaked walls of thevacuum system. Because of this CO2, itwas not possible to derive an accurate reac-tion rate for the synthesis of CO from C0 2,or CO2 from CO. However, the fact that thenew species are formed is not in question.In general, the rate constants for the pro-duction in the ice mixture of CO from C0 2,and CO2 from CO seem to be of the sameorder of magnitude based on over simplifiedfirst-order reactions which do not representthe complex radiation chemistry in thecometary ice mixture. However, if the rateconstants (k) are the same order of magni-tude [i.e., k(CO -- CO2) k(C0 2 - CO)],then, when either CO or CO2 is initiallypresent in a primitive comet, nearly equalfractions of both molecules will result dueto radiation synthesis in the outer few me-ters of the comet.

Warmup of Irradiated Ices

During slow warming of an irradiated,thicker H20 + NH3 + 13CH 4 ice (set llc),luminescence was visually detected and

photometrically measured. Concurrentpressure enhancements were also re-corded. An example of the luminescenceand simultaneous pressure bursts observedduring warming from 22 to 320K is given inFig. 6. Upon recooling to 20'K from 260K(see A, Fig. 6), luminescent activity wasextinguished and did not reoccur until theice was again warmed to 260K. While in-creasing the temperature from 30 to 90'K,the thermoluminescent activity was rela-tively low but increased again between 90and 150'K. Within the higher temperatureregion broader, smoother curves of emis-sion vs time were recorded. A more de-tailed discussion of the observation of ther-moluminescence and the simultaneouspressure bursts is planned for a later paper.

The observation of luminescence in theirradiated ice mixture below 40'K suggeststhat reactive species formed during irradia-tion recombine exothermically to produceelectronically excited species. These irre-versible reactions result in the observed lu-minescence and pressure enhancements.Although there are several specific mecha-nisms which could explain the radiation-in-duced thermoluminescence and concurrentpressure enhancements, there is at presentinsufficient data on which to choose themechanisms most important in this experi-ment. In the case of burst No. 3, Fig. 6, itwas estimated that approximately 0.5% ofthe initial mass of the ice sample was re-leased between 26 and 280K. Blank samplesof unirradiated ices showed some apparentluminescence during warming from 20'Kwhich was not accompanied by pressurebursts. The intensity of the emissions wasabout one order of magnitude lower thanthat observed for irradiated ice. Theseemissions were attributed to light scatteredinto the photodetector from the gas ioniza-tion region of the ion pump.

After warming, all irradiated ice mixturesleft a nonvolatile residue on the sample sub-strate at room temperature. This opaqueyellowish film often appeared to have a cir-cular concentration of material where the

400

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES

135 -;Cco

Q,244I-24

23UJI

- TEMPERATURE 2 20

TIME ( AR UNITS) 20

FIG. 6. Luminescence and pressure enhancements in an irradiated H20 + NH3 + CH, ice mixtureduring incremental warming from T = 19 to 28.50K. In most cases pressure enhancements occursimultaneously with luminescent emissions (e.g., luminescent emissions 1-3). Luminescent emissionsand pressure enhancements marked with an asterisk occur while the temperature is constant at -281K.These emissions may result from a series of exothermic reactions in the ice.

proton beam was most intense. Since a yel-lowish coloration was observed in the irra-diated ice at 20'K, it is possible that someof the residue material is formed at 20'K.The infrared spectra of several residues (setVIa,b; set Id) are shown in Fig. 7. Domi-nant absorption features occur near 3.4 and9.9 gm (see Moore and Donn, 1982, for dis-cussion of the infrared spectrum). Severalresidues were weighed on a microbalancein order to estimate the quantity of ice con-verted into residue as the result of irradia-tion. Using this technique, it was estimatedthat 0.9 + 0.2% of the ice mass was con-verted into a nonvolatile residue after theabsorption of an energy density equivalentto 1.7 X 107' MeV/cm 3. Preliminary gaschromatographic analysis of the residuesrevealed literally hundreds of organic com-pounds which were not present in similarlytreated blank samples. Amino acid analysisof the residue was inconclusive due to diffi-culties in separating the water-soluble (con-taining amino acids) fraction. Luminescent

emissions and yellowish residues were alsoobserved in photolyzed interstellar-typeices by Greenberg (1982).

IV. DISCUSSION

Based on our laboratory simulations, wepredict that cosmic radiation can signifi-cantly reprocess the upper icy layers ofnew comets over 4.6 billion years. We havelisted the various infrared signatures ofmolecules synthesized at 20'K during pro-ton irradiation (Tables IV-VI). Particularlynoticeable, in ice mixtures containing meth-ane, is the decrease in the strength of the V4

CH4 line by 66% for an absorbed energydensity believed similar to that at a depth ofI m in a new comet. An extrapolation of ourdata to include even higher absorbed en-ergy densities, believed typical of the outer40-cm ice layer of new comets, suggeststhat more than 79% of the original methanewould have reacted to form a new product.The inner region of a comet will contain alarger fraction of primitive methane since

401

MOORE ET AL.

WAVELENGTH MICRONS31 35 41I . , , I

UNEXTRACTED/ SET 3, b

9 10 11 12I ' .1 3 K

EXTRACTEDSETP a,

X5

X1090

80 EXTRACTD /

2915

I r ' i I I I I -,3000 2500WAVENUMBER (cm- 1 )

,tl 1,O -

1010

I

I 10 01 100 1 000 900 800

FIG. 7. Infrared spectra of residues from irradiated ice mixtures.

this volume accumulates less cosmic rayenergy.

Proton-irradiated ice mixtures containingH20 and CO, or H2O and CO2 resulted inthe synthesis of CO2

3 or CO (respectively)at rates which appear to be approximatelysimilar. This result suggests that CO, con-centrated in the outer layers of new comets,may be a significant component of the su-per-volatile ice required to eject materialfrom the heads of new comets at distancesof 15 AU or even larger (distance predictedby Sekanina's calculations, 1973). This vol-atile layer would be removed during thefirst perihelion passage resulting in a lessactive comet during subsequent appari-tions. As mentioned earlier this mechanismis postulated as an explanation for the large"disappearance" rate of newer comets.

Since a prominent emission from come-tary comae is due to the CN radical, it isinteresting to note that no signature of asynthesized CN-bonded molecule was de-

' Irradiation of an H20 + NH1 + '3CH4 ice also pro-duced I3CO,.

tected in these experiments. The detectionin the ice mixture of 1% HCN, for example,would not be possible in our experimentsby virtue of the location of HCN's strong-est infrared absorption in a region obscuredby water, coupled with its weaker absorp-tion coefficient. The detection of moleculescontaining CN polymers in the room-tem-perature residue requires techniques moreelaborate than those used in this experi-ment. It is expected, however, that an ap-preciable concentration of the CN speciesin the starting mixture would yield an arrayof interesting biogenic compounds. Furtherwork on the detailed analysis of residues isplanned.

Our studies support Whipple's (1977)postulate that the outer layer of a newcomet would be a volatile icy frosting con-taining reactive species formed as the resultof cosmic ray irradiation. Although the na-ture of the reactive species was not deter-mined using infrared techniques (i.e., radi-cal species were not positively identified),irradiated ice mixtures were observed toexhibit luminescence and concurrent pres-

90

80-

z

WLU

.0

IM60k

50k

I -.-.

402

!

70L S

60-

80 -

PROTON-IRRADIATED COMETARY-TYPE ICE MIXTURES

sure enhancements during warming.4 Theluminescence and concurrent release ofmaterial we observed in the laboratorywould occur when a new cometary nucleusis warmed to 320K and this could occur at aheliocentric distance of more than 100 AU(A, = 0.65 and AIR = 0). Conceivably, emis-sions could occur at a variety of distancesless than 100 AU as cooler subsurface-irra-diated ices are warmed through the appro-priate temperature interval. This type ofexothermic activity, if sufficiently ener-getic, could result in outbursts of materialfrom new comets at distances beyond thosesuggested by Sekanina (1973). Above 50'K,the exothermicity of the reactions involvedin the thermoluminescence has not beenmeasured quantitatively. Our data suggeststhat some enhancement in pressure occurs.Although the luminescent emissions couldnot be directly observed in comets, radia-tion-induced activities could contribute tothe increased brightness and activity in newcomets at heliocentric distances between5.3 and 3 AU if sufficient material is re-leased as the result of corresponding pres-sure enhancements. Another possible con-tribution to the increased activity of newcomets at this distance is an exothermicevent associated with the phase changes ofwater (e.g., Patashnick et al., 1974; Smolu-chowski, 1981). The magnitude of the radia-tion induced contribution would decreasewith successive returns of the comet toperihelion as deeper, less irradiated iceswere exposed.

The results of our experiments also sup-port Donn's (1976) postulate that cosmicray radiation would tend to polymerize thesimple volatile molecules resulting in a lessvolatile material. In our laboratory simula-tions, approximately 1% of the volatile icemass was converted into a complicatednonvolatile residue. The energy density ab-

' The experimental results are for irradiation ofcompact, condensed gases in the form of ice. Theouter layers of a new comet are more likely to be alow-density snow (Donn, 1963). There is little experi-mental work on such material.

sorbed in our experimental ices is the sameas the estimated absorbed energy density ina new cometary nucleus at a depth of ap-proximately I to 2 m. An even larger ab-sorbed energy density occurs near the com-etary surface. Using the calculated massconversion rate of -1% we find that acomet with a 10-km radius could accumu-late in its outer 2 m, 2.3 x 10° kg (over 10million tons) of residue in 4.6 billion years.

During vaporization of ices from the sur-face of a comet, some fraction of a similarnonvolatile residue is expected to remainon the underlying material. This organicresidue would result in the formation, tosome degree, of an inert, chemical-insulat-ing coating on the underlying ice. Cometaryobservations indicate a decrease in activityin comets from the upper mantle to the core(Whipple, 1977). This could be explained asa systematic change in the composition orvolatility of the material, or as an increasein cohesiveness of the material. On theother hand, during vaporization of icesfrom the cometary surface, some of the res-idue is probably carried into the coma. Thismaterial could be a source of complex or-ganic compounds in the solar system.

V. SUMMARY

Laboratory Observations

1. New molecular species were synthe-sized in the solid phase in cometary-typeice mixtures as a result of proton irradiationat 20'K. The synthesized molecules identi-fied were: C2H6, C0 2, CO, N20, NO, andCH4 (weak).

a. Proton irradiation of ice mixturescontaining CH4 and H20 resulted in thesynthesis of CO2. When CH4 was 46% ofthe initial ice mixture, more than two-thirdsof the methane was depleted due to its con-version into other molecules. The energydensity absorbed during these experimentswas comparable to the estimated accumu-lated energy density at a depth of -I meterin a new cometary nucleus.

403

MOORE ET AL.

b. Proton irradiation of ice mixturescontaining CO and H20 resulted in the syn-thesis of CO2; those containing CO2 andH20 resulted in the synthesis of CO. Therate constants for these solid phase reac-tions seem to be of the same order of mag-nitude and under these conditions newcomets would have nearly equal fractionsof CO and CO2 in their outer layer if eitheris initially present.

2. During warming, irradiated ice mix-tures exhibited thermoluminescent activityand concurrent pressure enhancements.This activity was most intense between 20and 320K, and 100 to 150'K. These temper-ature ranges roughly correspond to cometsat solar distances r > 100 AU, and r = 2-5AU, respectively.

3. Coloration of the irradiated ice was ob-served at 20'K. Coloration implies a changein the visual albedo of the cometary surfaceice as a result of proton irradiation.

4. In irradiated ice mixtures approxi-mately 1% of the volatile ice mass was con-verted into a nonvolatile residue. The non-volatile residue contained hundreds of newcarbon-containing compounds not presentin blank samples.

Cometary Implications

1. It is suggested by our laboratory obser-vations that outbursts of new comets begin-ning at distances up to perhaps 100 AU aredue to the release of radiation-synthesizedmaterial as the ice is warmed to 320K. Ob-servations of new comets at very large dis-tances may detect these activities, or theresult of these activities (i.e., the observa-tion of remnant particle tails).

2. Based on our laboratory experiments,new comets are expected to contain thelargest fraction of nonprimitive (synthe-sized) molecules within their outer few me-ters. New comets are also predicted to havea significant depletion of methane in theirouter layers compared to their inner vol-ume of ice, or compared to more evolvedcomets. During vaporization and ionizationof the outer ice layer of a new comet, a

different ion and molecular concentrationwill result in the coma than for the similarpassage of a more evolved comet. Gaschromatographic-mass spectrometric mea-surements from a rendezvous satellitethrough the coma of comets of differentages could measure these concentrations.Such measurements could also be made bysufficiently sensitive infrared spectroscopyin the 2.5- to 15-Am region.

3. More viable studies would includespectroscopic observations of the nucleusat large heliocentric distances, pre- andpost perihelion for comets of different ages.Equipment to do this should be available inthe not too distant future.

ACKNOWLEDGMENTS

This research was funded by NASA Grant NSG-5172-Sup 7 and was part of the Ph.D. thesis submittedto the Astronomy Program at the University of Mary-land by one of us (M. H. Moore). The authors ac-knowledge Cliff Walters, Laboratory of ChemicalEvolution at the University of Maryland, who did thegas chromatographic analysis of the residue; E.Gardner, Department of Chemistry at the Universityof Maryland, who did the gas chromatographic analy-sis of the volatiles; and members of the Radiation Sim-ulation Facility. NASA/GSFC, who operated the Vande Graaff accelerator.

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