Publications of the Astronomical Society of the Pacific 88:342-348, June 1976

PHYSICAL PROCESSES IN COMET KOHOUTEK

C. R. O'DELL NASA Marshall Space Flight Center Received 1976 March 3

Observational results are brought together to establish the mass loss of Comet Kohoutek (1973 XII) in H2O, solid particles, and other molecules. The production rate of molecules is seen to have flattened prior to peaking near perihelion passage, and then fell below the rate for the incoming branch of the orbit. The total mass loss was about 1 X 1014 gm. All abundances are consistent with a clathrate dominant form of water. Comparison of the sizes of the nucleus as calculated from photometry and the rate of water loss requires that either an unrealistically high albedo be assumed or that only a portion of the nucleus was covered by water, in contrast with previous expectations for a new and probably first-passage comet. More- over, if the preperihelion estimates of the nuclear brightness are rejected as being contaminated, the resulting values indicate an ice-covered nucleus of 2.1 km radius. Key words: abundances — comets

Introduction nucleus. This conclusion was quantified best by Comet Kohoutek — 1973/ (1973 XII) provided an Delsemme (1971), who refined and defended his con- unusual opportunity to study a moderately bright clusions (Delsemme 1973) against criticism (Wallis comet by modern means. As the reduced data ap- 1972). This assumption is further strengthened by the pear, we can develop a more complete picture of what spectroscopic detection of the I^O^ ion in the coma happened in at least this one comet. Although it is a and tail (Wehinger et al. 1974). Therefore, we are sample of one, it is perhaps more justified to gen- probably justified in adopting measurements of La, eralize about comets from the one well-studied object OH, and Ha in Comet Kohoutek as measures of the than to claim peculiarities from the way it was ex- H2O release rate. pected to be on the basis of earlier concepts. In this Four important sets of data are available; La pho- paper we shall collate the various observations to tometry from Skylab, La rocket photometry, OH ob- determine the total mass-loss properties, evaluate the servations from a research aircraft, and Ha observa- model for a water-ice-dominated surface for new tions at high spectral resolution. The most extensive comets, and to test the compatibility of measurements data are the Skylab and rocket measurements reported with the proposed clathrate form for water ice. by Meier et al. (1975), which supersedes their earlier papers (Page 1975; Carruthers et al. 1974). Observa- I. Mass Loss tions were made with a wide-field electronographic The combination of the results of the many measure- camera, calibrated, reduced to fluxes, and then hy- ments of Comet Kohoutek lead to a rather complete drogen-production rates (Ç), atoms sec-1 ster-1) were description of the total mass loss from this comet. calculated. An independent measurement of the La This is a very important fact since the fragmentary image was made after perihelion passage by rocket quantitative results on previous comets allowed older observation. Feldman et al. (1974) performed La arguments (e.g., enormous amounts of mass loss re- spectrophotometry from a rocket. Blamont and Festou main undetected) to be continued. In this section (1974) took advantage of the high-contrast, low- we will show that the mass loss from the dominant extinction environment of the Ames Research Center molecule H2O has been quantified, that the relative Galileo II research aircraft to determine the OH pro- abundances of other molecules are accurately bounded, duction rate. and that the particle mass-loss rate is a substantial Huppler et al. (1975) made extensive observations fraction of the total mass loss. of the Ha emission-line intensity over the coma. Their derived La production rates are uncertain because . H2O Mass Loss Α of the uncertainties in the excitation rate by Lß, which The original discovery of extensive Lyman alpha excites the Ha. Huppler et al. point out that they did (La) halos and intense OH emission in Comets Ben- not have a L/3 profile that applies to the entire solar nett (1970II) and Tago-Sato-Kosaka (1969IX) im- disk. This limitation is probably eliminated by the mediately led to the qualitative conclusion that water recent Skylab spectra of Lß of the quiet region of the ice was their source and the dominate ice form on the sun (Nicolas et al. 1975). However, the calibration for

342

the solar L/3 line center has been questioned by Levas- of visual-light molecules such as C2 and CN and is the seur, Meier, and Tinsley (1975), who seek to explain driving force for removing particles, the source of the the intensity of atmospheric terrestrial Ha. If one scattered light cotinuum, from the nucleus. We have uses the more recent line profile and calibrations, the used the summary of visual observations prepared by Huppler et al. values of the production rate should be Deutschman (1974) who gathered the data from many increased by a factor of 6.5. This would put their sources, correcting them only for aperture effects. We values much greater than the more accurately cali- have summarized his data in Figure 1 at and beyond brated La data. An adjustment is needed and we have 20 days from perihelion. Since a wide spread of in- raised all of their values by only a factor of 2.82 to dividual estimates occurs for any given time interval, bring them into agreement with the La values derived we have determined a normalized dispersion depend- by other observers in the overlapping time intervals. ing on the square root of the number of observations. All of these data are summarized in Figure 1. Such errors are of course very large at those smallest If we set a goal of establishing a total mass loss of heliocentric distances where only a few observations H2O, then we need to know the rate of loss at helio- were made. In fact, those observations made nearly at centric distances beyond which these determinations perihelion are probably very inaccurate also since were made. To do this, we have assumed that the total they were made through a variable density filter. The visual luminosity is a fair measure of the H2O produc- photometric estimates of the brightness were then tion rate. This assumption is probably valid since normalized to the hydrogen observations over the H2O evaporation probably controls the rate of release region of overlap and used to define the mass-loss

Δ Τ (DAYS) Fig. 1 — Production rates for H20-related species are shown as a function of time before ( + ) and after ( — ) perihelion. Filled circles indicate results of L« observations; open circles, Ha observations; OH, the OH molecule production rate; and Ρ the results of visual photometry normalized to the La and Ha results. The curve gives the most reasonable fit to the composite data.

rate at greater heliocentric distances. molecules (Delsemme and Miller 1970). We see that These data can be used to estimate the total mass all of the rates are compatible with the clathrate limit. loss of H2O. The La data are over a sufficiently large This means that probably no more than 50% by mass of spatial volume that they are a measure of both hy- other gases is being lost. drogen atoms in each original water molecule, since the secondary product OH will also have been photo- C. Particle Mass Loss dissociated (Delsemme and Rud 1973). Using the The scattered light continuum can be used to deter- curve shown in Figure 1, we integrate to find a total mine the number of particles in the coma. Again, the H2O mass loss between ±60 days (1.56 AU) around data of A'Hearn and Cowan (1975) can be used since perihelion as 6.4 X 1013 gm. Although mass loss oc- they gave continuum intensities at 4520 Â. O'Dell curred beyond this interval, the fractional addition is (1973) has shown that the ratio of this continuum to negligibly small. the nearby Δυ = +1 band sequence of C2 can give It is important to note that the loss rate was asym- an estimate of the number ratio. More specifically, metric, being about a factor of two lower after peri- we showed that the number ratio would be N(C2)/ helion. If one accepts as definitive the quantative La N(P) = 2.2 X 104W, where W (in angstroms) is the measurements made near perihelion, there was a peak equivalent width of the band with respect to the con- at perihelion. It is apparent that the rate of loss tinuum and an assumed particle radius of 0.1 μ was reached an early plateau maximum, before perihelion, taken. This then allows one to calculate the mass-loss stayed at about that same level for about 20 days, rate for particles as peaked, and then began to lose mass more slowly on the way out from the sun. Such fadings of comets and ^(P) -fí%n P(P) / ^ \ ¢(02) m such asymmetries are certainly not unknown (Whipple 5W(H20) W \ 0.1 μ, / Ç(H20) ^ 1962; Sekanina 1970), which gives further credence to where 5$(P) and 9^(^0) are the mass-loss rates for the present interpretation of the production-rate pro- particles and water, respectively, p(P) is the particle file. Reasons for this behavior are discussed later in density, and s is the particle radius. The assumption this paper. has been made that C2 and the particles have equal lifetimes in the coma, an assumption supported by the B. Other Molecule Mass Loss absence of systematic variations of W in the several Due to the extensive optical and radio measure- diaphragm sizes used (A'Hearn and Cowan 1975). ments, we now have a more complete picture of the With the exception of a single, early preperihelion molecular mass-loss rate than was previously possible. point, which gives an even smaller value of W, the The most useful optical observations are probably data indicate log W = 2.70 ± 0.24 preperihelion and those of A'Hearn and Cowan (1975) who used them to log W = 3.09 ± 0.21 postperihelion. Using equation derive accurate C2 and CN production rates. Two (1), an assumed particle radius of 0.3/^, and ¢)(^0)/ radio detections of molecules have been reported, = 50 from Figure 2 we obtain äft(P)/äft(H20) = CH3CN by Ulich and Conklin (1975) and HCN by 0.8 preperihelion and 0.3 postperihelion. The particle Heubner, Synder, and Buhl (1974, 1976). Both detec- radius assumed is compatible with the mean size cal- tions seem indisputable, but the calculated rates of culated by O'Dell (1974) for Comet 1970 II from sev- production are more uncertain than for the optical eral methods of determination. Although the accuracy data. Upper limits were also set for NH3, CH3OH, CN, of these ratios is unknown, they clearly indicate two CO, and N2O (Mango et al. 1974) from radio measure- things; the mass being lost by particles is a significant ments, although none of these upper limits reached as fraction of the water mass loss, and relatively more low as the observed H2O production rate. particles escaped before perihelion. It is important to The observed molecule production rates are sum- note that if these same Ç)(H20)/Ç)(C2) ratios apply for marized in Figure 2, where we also show the averaged Comet 1970 II then its much stronger continuum in- hydrogen production rates. It is obvious that there is dicates that particle mass loss dominated there (Stokes a substantial gap between the hydrogen rate and that 1972). of nonwater molecules. We have presented the CH3CN results for only the Heubner et al. (1976) discussion of II. Radius of the Nucleus the production rate. Their detailed discussion of the If the albedo is known, the radius of the nucleus of molecular population distribution argues that the pro- Comet Kohoutek can be estimated by two methods. duction rate is some two orders of magnitude less than The two methods can be used together to determine in the original announcements (Ulich and Conklin both the radius and the albedo under certain assump- 1975). The clathrate model for water ice sets an upper tions. The first method depends upon the photometric limit of about one trapped molecule per seven water brightness of the nucleus while the second depends

—40 -30 -20 -10 0 10 20 30 40 Δ Τ (DAYS)

Fig. 2 — Production rates for individual observations of nonwater molecules are shown for various times, labeled according to the molecular designation. Averaged hydrogen production rates (H) are shown for comparison. upon the water-loss rate. The methods used below values is discussed below. were first developed by Delsemme and Rud (1973). Under the assumption that the entire nucleus is At large heliocentric distances the reduction of the covered by ice, Delsemme and Rud (1973) showed brightness of the coma allows a determination of the that equating the absorbed energy and the energy brightness of the light reflected from the nucleus. This carried away by evaporation of H2O can lead to a measurement, together with an assumed phase func- determination of (1 —A)S from knowledge of (XE^O) tion, allows the determination of AS, where A is the and the heliocentric distance. As pointed out in sec- albedo and S is the apparent surface area of the comet tion IA, Ç)(H20) was less after perihelion and we must 2 {πΚΝ where RN is the nuclear radius). Roemer calculate (1 —A)S for the incoming and outgoing (1973a,fc; 1974) observed Comet Kohoutek at preperi- branches of the orbit. Adopting Ç)(H20) = 2.2 X 1029 helion distances of 4.4, 4.2, and 2.1 AU and postperi- mol sec-1 at —20d(r=0.18) and Ç)(H20) = 1.2 X helion at 2.5 AU. The preperihelion data using a Lam- 1029 mol sec--1 at H-20d gives (1 —A)S = 6.06 and 3.2 2 2 bert phase law showed the 1/r dependence expected km , respectively. The resultant values of A and RN for a contamination-free observation of the nucleus. are given in Table I, where we also give the values The average reduced luminosity (i.e., normalized to calculated by Delsemme and Rud for several other 1 AU heliocentric and geocentric distance) was 10^7, comets. The derived values for Comet Encke are while the single postperihelion value was 13^7. These formal, as Delsemme and Rud properly point out that observations lead to the values of AS = 151 and 9.5 this periodic comet is probably not now covered by km2, respectively. The wide discrepancy of these ice; moreover, the observational data are much less

TABLE I Albedos and Radii of Comets

Comet (1 —A)S AS A RN (km2) (km2) (km) Kohoutek 1973 XII (preperihelion data) 6.06 151 0.97 7.0 Kohoutek 1973 XII (postperihelion data) 3.2 9.5 0.73 2.0 Kohoutek 1973 XII (combined data) 6.06 9.5 0.60 2.2 Tago-Sato-Kosaka 1969IX* 5.9 9.55 0.63 2.2 Bennett 1970II* 15.7 29.3 0.66 3.8 Encke 1971II* 0.09 1.95 0.96t O.SOf

*Data taken from Delsemme and Rud (1973) f Delsemme and Rud did not calculate A and RN for Comet Encke

extensive for this comet. cannot use this argument to test Comet Kohoutek as Sekanina (1974) has argued that the nuclear magni- the variation in phase angle was too small to distin- tudes of Roemer have been used indiscriminatingly in guish between the Lambert and the Lunar phase laws. determining AS for many comets due to mistaking the The difficulties of applying Delsemme and Rud's inner coma or particulate cloud for the nucleus; how- method here are very important. In the usual division ever, this criticism would not seem to apply to Comet of comets into new and old types. Comet Kohoutek is Kohoutek because of the consistency of the preperi- extremely new and, if anywhere, the Delsemme and helion data indicating a well-defined reduced lumi- Rud method should be applicable. Two alternatives nosity. The same is true for the data used by Del- for Kohoutek can be advanced to explain these very semme in deriving values for Comets Tago-Sato- high formal albedos. Kosaka and Bennett. The sharp division of pre- and 1. The surface of Comet Kohoutek was covered by postperihelion data argues that a very significant debris, with only part being ice exposed to direct solar change occurred during perihelion passage or that the heating. preperihelion data were contaminated. 2. During the inner orbit, the surface was domi- If one accepts the idea of preperihelion contaminated by an ice with a latent heat of evaporation sig- nation, then we should calculate using the postperi- nificantly higher than the 11,500 cal mole-1 assumed helion value of AS together with both values of for H20. (1~A)S. This gives an average radius of 2.1 km and The former interpretation is not acceptable, since an albedo of about 0.67. the change in (1 — A)S and AS indicates the ice-covered If one accepts the preperihelion magnitude mea- assumption fits after perihelion, which is contrary to sures of the nucleus it is difficult to understand the what is expected due to heating. The latter interpre- very high resulting calculated preperihelion albedos. tation seems unlikely as water has one of the highest The particles imbedded in the ice have the low albedo latent heats of evaporation of the compounds readily of about 0.1-0.2 (Ney 1974) after release, which would formed from the observed elements. Therefore, it ap- keep the albedo of the surface down. Moreover, it is pears that the preperihelion nuclear magnitude data difficult to find materials of this high albedo (0.97) on must be rejected. For further discussion we shall take the earth. If the assumptions made in calculating Rn to be 2.1 km. (1 — A)S do not apply, i.e.,not complete coverage by III. Discussion ice or that water ice is not the controlling species, then In the above sections we have established several the formal value of (1 —A)S is of little use beyond set- important features in the behavior of Comet Kohoutek. ting an approximate lower limit to S. The same type of 1. The comet displayed a marked asymmetry in the problem was encountered by Delsemme and Rud (1973) rate of mass loss in H2O before and after perihelion. for Comet Encke, where the values of (1 —A)S and AS 2. The rate of mass loss flattened before perihelion. were very different and they did not derive values of A 3. The nonwater molecular abundance is com- and Rn. In the case of Encke, they noted that the patible with these molecules coming from clathrate values of the reduced magnitude of the nucleus were ices in the nucleus. consistent only if a lunar phase law applies and since 4. The derived albedo and nuclear radius are com- this was inconsistent with the assumption of a full ice parable to other well-studied comets. cover, they concluded that Comet Encke's surface was We will use these results to develop a picture of nearly ice free and the method could not be used. We what happened to Comet Kohoutek.

It is helpful to compare the total mass loss to that of postperihelion nucleus was an irregular surface only the entire nucleus. In section IA, we showed the H2O partially ice covered, the remainder being a matrix of loss to be about 6.4 X 1013 gm and in sections IB and dust. Such an object would be of lower intrinsic IC that the additional loss from other molecules and luminosity and progressively poorer in dust emission dust was about 50% more. Adopting the radius of 2.1 at its next passage, growing more inert and dust-matrix km, we find that the total mass loss of 1 X 1014 gm dominated. Relatively few passages would be re- represents an average layer of only l.7lpNm thickness or quired to reduce it to the dust-matrix-dominated form _3 a mass fraction of 2 X 10 pN, where pN is the average that is deduced for Comet Encke (Sekanina 1969), or density of the entire nucleus. This means that we are the irregular shapes deduced for comets undergoing dealing with modifications of the surface of the comet great outbursts (Sekanina 1972). and not a change in the overall nucleus. I would like to thank W. F. Huebner, R. R. Meier, The marked variation in the dust/gas ratio before and M. F. A'Heam for transmitting data on Comet and after perihelion means that the surface area losing Kohoutek prior to publication, and to Dr. Meier and gas changed considerably. Using the theory originally Professor A. H. Delsemme for their comments on the advanced by Whipple (1951) and refined by Finson calibration of the solar L/3 flux and the derivation of and Probstein (1968), Gary and O'Dell (1974) found photometric radii, respectively. that the velocity of escaping particles is reduced by the gravity of the nucleus. Using their equation (3) REFERENCES for the zero-velocity case, we find for the maximum A'Heam, M. F., and Cowan, J. J. 1975, Ap. J. 80, 852. size particle being lost d(max) = 27 yg3K(H20)/ Blamont, J. E., and Festou, M. 1974, Icarus 23, 538. 2 3 Carruthers, G. R., Opal, C. B., Page, T. L., Meier, R. R., and {4n pdPNGRu ), where Vg is the thermal velocity of Prinz, D. K. 1974, Icarus 23, 526. H2O, pd is the particle density, and G the gravitational Delsemme, A. H. 1971, Science 172, 1126. constant. Inserting values gives ¿(max) — 1m at Δ Τ 1973, Ap./. (Letters) 14, L163. = — 20 days. This value is much larger than the Delsemme, A. H., and Miller, D. C. 1970, Planetary and particles accounting for the scattered light, therefore, Space Set. 18, 717. all of the particles loosened are removed by gas drag. Delsemme, A. H., and Rud, D. A. 1973, Astr. and Ap. 28, 1. Deutschman, W. A. 1974, Center for Astrophysics Preprint No. This means that the variation in the gas/dust ratio in 135. dicates a physical change in the areas of the gas loss. Feldman, P. D., Takacs, P. Z., Fastie, W. G., and Donn, B. This could be either that the icy regions emitting gas 1974, Science 185, 705. after perihelion simply have less dust or that some of Finson, M. L·., and Probstein, R. F. 1968, Ap. ]. 154, 327. the gas is coming from more solid, matrix regions that Gary, G. Α., and OOell, C. R. 1974, Icarus 23, 519. Huebner, W. F., Snyder, L. E., and Buhl, D. 1974, Icarus 23, have lost their ice-dominated surfaces. 580. The picture of the history of the nucleus of Comet 1976 (private communication). Kohoutek that fits the principal observations follows. Huppier, D., Reynolds, R. J., Roesler, R. L., Scherb, F., and The nucleus has a radius of about 2 km, the surface of Trauger,]. 1975, Ap./. 202, 276. which was initially covered by a mixture of water ice, Levasseur, A.-C., Meier, R. R., and Tinsley, B. A. 1975 (private communication). possibly in the form of clathrates, and particles. As Mango, S. Α., Johnston, K. Ji, Chui, M. F., Cheung, A. C., and the comet approached perihelion, significant portions Matsakis, D. 1974, Icarus 23, 590. of the icy areas of the surface were depleted, leading Meier, R. R., Opal, C. B., Keller, H. U., Page, T. L., and Car- to a flattening off of the total H2O production rate in ruthers, G. R. 1976, Astr. and Ap. (in press). spite of the still increasing solar heating. Near peri- Ney, Ε. P. 1974, Icarus 23, 551. Nicolas, K., Bartoe, J. D., Kjeldseth-Moe. O., and Tousey, R. helion, the surge of solar heating allowed subsurface 1976, J. Geophys. Res. (in press). ices to be removed. After perihelion, the remaining O'Dell, C. R. 1973, Icarus 19, 137. icy spots on the surface served as the principal sources 1974, ibid. 21, 96. of water with the warmer bare regions emitting less Page, T. L. 1975, Comet Kohoutek, NASA SP-355; (Washing- gas per unit area, giving a lower total water release ton: U.S. Printing Office), p. 37. Roemer, Ε. 1973a, Mercury 2, 19 (No. 4). after perihelion when measured at the same helio- 1973b, ibid 2, II (No. 2). centric distance. The total mass removed from the 1974, Z.A.I/. CircM/ar No. 2671. nucleus accounts for a thin layer on the average; but, Sekanina, Z. 1969, Ap. J. 74, 1223. probably much of the mass was removed in restricted, 1970, I.A.U. Circular No. 2276. icy areas of greater depth. 1972, Asteroids, Comets ù Meteoric Matter, IAU Colloq. No. 22, Nice, France. The resultant changes in Comet Kohoutek due to its 1974, I.A.U. Colloquium No. 25, NASA SP-393 (Washing- inner solar system passage were quite large. The ton: U.S. Printing Office). initial ice-covered surface was modified by removal of Stokes, G. M. 1972, Ap.J. 177, 829. about 0.2% of the total mass from selected regions. The Ulich, B. L., and Conklin, E. K. 1975, Comet Kohoutek,

NASA SP-355 (Washington: U.S. Printing Office), p. 119. Lew, H. 1974, Ap.J. (Letters) 190, L43. Wallis, M. 1972, Science 178,78. Whipple, F. L. 1951, Αρ./. 113, 464. cu Wehinger, Ρ. Α., Wyckoff, S., Herbig, G., Herzberg, G., and 1962, ibid. 67, 1.