Ammonia abundances in comets

Adv. Space Res. Vol. 9, No. 3. pp. (3)169—(3)176, 1989 Printed in Great Britain. All rights reserved.

0273—1177/89 WOO + .50

Copyright © 1989 COSPAR

AMMONIA ABUNDANCES IN COMETS S. Wyckoff, S. Tegler and L. Engel Department of Physics, Arizona State University, Tempe, Arizona, 85287, U.S.A.

ABSTRACT Spectra of comets Halley, Hartley-Good, Thiele and Borrelly were obtained with the 4-rn telescopes at Cerro Tololo and Kitt Peak Observatories and with the Multiple Mirror Telescope. The emission band strengths of the NH 2 bands were measured to determine NH2 column densities in all four comets. Production rates which were determined using two independent models (Haser and vectorial) agree within the observational errors and indicate that a simple two-step decay model approximates well the distribution of NH2 in the comae of these four comets. If the observed NH3 derives from a single parent with a point-source origin, the most likely parent is NH3 which for the photolytic process, NH3 + he’ -.~ NH3 + H, has a branching ratio -..95%. This branching ratio leads to ammonia-to-water abundance ratios ranging from 0.01% to 0.4% in the four comets, indicative of significant di~Ferences. Three independent determinations of the ratio in comet Halley give Q(NH3)/Q(H20) = 0.002 ±0.001. No significant difference in the ammonia abundance was found before and after perihelion in comet Halley, which indicates that the ice component of the nucleus is chemically homogeneous within the observational uncertainties. The ammonia abundance ratio determined for comet Halley 14 March 1986 is an order of magnitude smaller than that inferred from the spacecraft ion mass spectrometer data. INTRODUCTION Formation of planetary systems has only recently become an observational science with the discovery of the first bona-fide proto-planetary disk gravitationally bound to the T Tau star, HL Tau /1/. Evidence now indicates that virtually all star formation occurs in the cores of dense giant molecular clouds /2/, while formation of a planetary system is considered a natural process in the collapse of a rotating molecular cloud. Hence understanding the history of the solar system should provide valuable information about the general problem of planetary system formation. The ice component of comet nuclei contains the unchanged nucleogenic record of the primitive solar nebula and therefore clues to its formation. Comets are the best chemical diagnostics of conditions in the outer regions of the early solar nebula. They are unique solar system objects which have presumably undergone little chemical processing since they condensed in the solar nebula 4.5 billion years ago. Current theories of star formation and evolution indicate that icy and rocky planetesimals from which comets, planets and other minor solar system bodies formed, first condensed from the solar nebula within iO~~years after the protosun and solar nebula formed from collapse and fragmentation of a giant molecular cloud /3/. Direct knowledge of molecular abundances in the early solar nebula would provide valuable insights into the physical and chemical conditions which prevailed in the outer solar system (r ~4 AU) at the time of formation of the outer planets and satellites. Recent models reconstruct these conditions in the solar nebula /4,5,6/. A key parameter in the models is the time scale for dynamical mixing of the solar nebula material during its brief 106_I year lifetime. The extent of processing by kinetic chemical reactions in the solar nebula is another highly uncertain, but vital fact required to develop realistic models of the (3)169

(3)170

S. Wyckoff ec at.

early solar nebula. The net chemical reactions N

2 + 3ff2

—~

2NH~

CO+2H2—+CH4 could have been driven to efficient rates by solid Fe catalysts presumed present in the solar nebula /4,5,6/. In chemical equilibrium the above reactions would be driven to the right. For non-equilibrium chemistry, the above reactions would have proceeded incompletely. The third alternative is that the chemical composition of the primitive solar nebula remained unaltered after collapse of the giant molecular cloud, and the solar system objects retained their original interstellar medium compositions. Thus the abundance ratios N2

NH3 and

CH4 are key diagnostics to the extent of chemical processing in the solar nebula subsequent to collapse of the giant molecular cloud /6/. In ground-based comet spectra emission bands of NH2 are 2A~ —~ usually X2B prominent features observed near 5700 A, 6000 A, 6300 A and 7230 A arising from a A 1 transition which produces vibronic bands at the above wavelengths. Photolysis of NH3 by solar radiation results in the photodissociation process NH3 + hv -+ NH2 + H with a 95% branching ratio /7/. Hence if NH3 is the dominant parent species, then the NH2 observed in the visible region of the spectra of comets essentially represents the ammonia abundance. Ammonia is one of several parent molecules expected in the ice component of a comet nucleus, the others being N2,CO, CO~CIh and 1120. Analysis of in situ and ground-based data of comet Halley confirmed that H2 0 is the dominant ice, comprising 80% of the volatile component of the nucleus with CO contributing 10-20% of the icy component /8/. Thus C03, CH4, N2 and NH3 are expected to be trace parent constituents in the comet nucleus, but remain key chemical diagnostics of conditions in the solar nebula. Here we report measurements of ground-based NH2 spectra in four comets. We conclude that the NH3/H20 abundance ratio found for comet Halley is —~5-10times less than determined from the GIOTTO ion mass spectrometer (IMS) data /7, 9/. OBSERVATIONS Spectra were obtained on 14 March 1986 and 15 March 1986 of comet P/Halley in the 6800 - 7600 A region (1 A FWHM resolution) using a photon-counting CCD detector at the Cassegrain focus of the 4-rn telescope at Cerro Tololo Observatory. Additional details of the observations and reductions are discussed elsewhere /10/. In this paper the NH3 analysis for 14 March is discussed. Analysis of the 15 March data leads to similar results /10/. The NH2 band near 7350 A arising from the transition (0,5,0) (0,0,0) and the underlying continuum is presented in Figure 1. The integrated NH3 (0,5,0) .—~ (0,0,0) fluxes were obtained from the spectrum after the dust continuum background had been subtracted. Spectra of comet P/Borrelly were obtained in the 4750 - 7000 A spectral region (5 A FWHM resolution) using the 4.5-rn Multiple Mirror Telescope on 20 November 1987. A photon-counting Reticon detector was used to obtain spectra off-set from the comet nucleus by 6.5 arcsec. Further details of these observations are given in /11/. The spectrum of comet Borrelly showing the NH2 (0,10,0) —~ (0,0,0) band and the underlying solar continuum is displayed in Figure 2. In Figure 3 we present the region of the spectrum showing the NH2 (0,9,0) —~ (0,0,0) bands and the [Ofl lines arising from the inetastable ‘D state. Spectra of comets Halley, Hartley-Good and Thiele were obtained 16 November 1985 with the 4-rn telescope at Kitt Peak Observatory using an intensified CCD detector with the Cassegrain spectrograph. Observational details are given in /12/.

Ammonia Abundances in Comets

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Spectrum of P/Halley obtained 7800 km from nucleus taken with CTIO 4-rn telescope on 14.4 March 1986 (UT). —

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P/BORRELLY 1007—NOVEUBER—20.3 UT WTRnlcoN ;(5v—o)

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Fig. 2.

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WAVELENGTH (A)

Spectrum of P/Borrelly obtained 2500 km from nucleus taken with F. L. Whipple 4.5-rn Multiple Mirror Telescope on 20.3 November 1987 (UT).

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Spectrum of P/Borrelly obtained 2500 km from nucleus taken with F. L. Whipple 4.5-rn Multiple Mirror Telescope on 20.3 November 1987 (UT).

(3)172

S. Wyckoff et a!.

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Spectrum of P/Halley obtained 8500 km from nucleus taken with KPNO scope on 16.2 November 1985 (UT). ~0IU

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Spectrum of C/Hartley-Good obtained 10,000 km from nucleus taken with KPNO 4-rn telescope on 16.1 November 1985 (UT).

(3)173

Ammonia Abundances in Comets

NH

2 COLUMN DENSITIES

The integrated fluxes, F, of the NH2 emission bands corrected to outside the Earth’s atmosphere of all comets are given in Table 1. The projected distances, p, from the nucleus and the NH2 band measured are indicated in the table. Also included in Table 1 are details of NH2 measurements in comet Halley made on 13 March 1986 which have been reported elsewhere /13/. TABLE 1- Observed Fluxes and Column Densities NH2Band Comet Halley

Borrelly Hartley-Good Thiele

f

A 7350 5700

p (km) 7800 8500

F 2) (erg s~ cm (1.6±0.3)xiO_13 (2.4 ±0.4) x

(10,0)”

5700

5000

(4.0 ±0.9) x 10~~

2.0 x iO’~

(10,0)

5700 5700 5700

2500 10,000 7700

(6.6±1.3)x iO~ (1.3±0.3)x 10_14 (8.9±1.7)x i0’~

4.6 x 1010 1.8 x 1010 3.3 x 10’°

(v~, vi’)

(A)

(5,0)~

(io,o)t

(io,o)f (io,o)f

16 November 1985, aperture = 1” x 5.8”. 14 March 1986 aperture = 1” x 3.5”. 20 November 1987 aperture = 1” x 3”.

13 March 1986 /13/, aperture The column density, N, is given by

=

(molecNcm2) 2.3 x 10~ 1.8 x lO~~

-

1” x 2.7”.

4,rF N=— where ~ is the solid angle subtended by the observing aperture at the comet, and g is the fluorescence efficiency which for a molecular band is =

~

irF 0A~,51,

f.JI~IIW,I~II

where irF0 is the fluorescing solar flux, ~ is the wavelength of the band origin, oscillator strength, and ~ is the branching ratio for downward transitions, ~t.jIpIs

=

ITIVII

is the band

AVI5~I EAVI,.,, Vt,

where AVIV,, is the Einstein transition rate from the upper to the lower electronic levels in the molecule. The fluorescence efficiencies have been calculated and discussed in /11/. NH2 PRODUCTION RATES A model is needed to convert the column densities into production rates. We have shown /13/ that the Haser /14/ and vectorial models /15,16/ give essentially the same results for NH2. We interpret our previous results to mean that the Haser model of a simple point-source is a very good representation for the parent species of NH2. Since NH3 photodissociates directly into NH2 95% of the time, the production rate of NH2 is then essentially the production rate of NH3 in a comet presuming that ammonia is the dominant parent of the observed NH2. The production rates computed with the Haser /14/ model are given in Table 2. For completeness we have also included in Table 2 an NH2 production rate for 13 March 1986 of comet Halley /13/ which has been corrected with revised fluorescence efficiencies /11/.

(3)174

S. Wyckoff ez at. TABLE 2

-

Production

Cornet

Rates

Q(NH

r

2)

~

(AU)

(rnolec s~)

Halley (pre-perihelion)

1.71

2.4 x 1026

0.0006

Halley (post-perihelion)

0.90

2.2 x i0~

0.004

Halley (post-perihelion)

0.88

1.6 x 1027

0.002

Borrelly Hartley-Good Thiele

1.40 0.85 1.41

2.4 x 10” 3.2 x 10” 3.8 x 1023

0.0007 0.0001 0.0002

The water production rate for comet Halley on 14 March 1986 was well-determined since this is the date that GIOTTO transited the coma. The GIOTTO ion mass spectrometer measurements give Q(H2O) = 6 x 1029 molec s /8/. The IUE observations /17/ agree very well with the in situ water production rates on that date. The water production rates for comets Borrelly, Thiele and Hartley-Good were measured from the [01] line flux at 6300 A using the method of Spinrad /18/. The method assumes that the process H2O+hi’—~H3+O(’D) occurs with a branching ratio of 3% in the cornet. Since solar Ly a accounts for —‘half the photodissociation of H2O /19/, the accuracy of the water production rate determined from the emission line atdepends 6300 A 1D) level therefore ultimately resulting from to the the ground theismeta-stable O( assumed that the value corresponding on knowing thetransitions Ly a flux at time thefrom comet observed. We to solar minimum /18/ applied to the observations of the comets.

Corrections to the [01] fluxes of

comets Thiele and Hartley-Good were made due to overlying contributions from line blends in the NH 2 (0,8,0) band. The corrections were < 30% in both cases. The water production rate determined from

[01] in comet Borrelly agrees within 25% of the interpolated H20 production rate measured with the IUE 28 days prior to and 7 days after our observations with the IUE /20/. The water production rates for comets Hartley-Good and Thiele were also determined from the [01[ line fluxes assuming the solar minimum solar fluxes determined the water photodissociation rates. If we assume that NH3 is the sole parent species of NH2, then the ammonia production rates can be calculated from the photodissociation branching ratio of NH3 into NH2, namely, 0.95. The ratios of the ammonia-to-water production rates given in Table 2 then represent the relative abundances of these parent volatiles in the four comets. We estimate that the production rate ratios have accuracies of —~± 50%. If NH3 is not the only parent of the observed NH2, then the ammonia-to-water abundance ratios in Table 2 represent upper limits. DISCUSSION

Our ground-based spectroscopic analysis of the ammonia to water abundance ratios in the four comets indicates a range of a factor of 40, Q(NH3)/Q(H20) -‘0.01 - 0.4%. Three independent deternunations of the ammonia abundance have now been determined from ground-based spectra for comet Halley. Tire

average ammonia abundance in comet Halley is found to be =

0.002 ± 0.001

where the error represents the observational uncertainty pius estimated errors in the production rates and the g-factors in the three measurements.

The pre- and post-perihelion ammonia abundances in

Ammonia Abundances in Comets

(3)175

cornet Halley agree within the drors quoted. The sublimating surface of the nucleus had eroded several meters between the two measurements for comet Halley given in Tables 1 and 2. Therefore we interpret the constancy of the ammonia abundance in Halley to mean that no chemical inhomogeneities in the volatile component of the nucleus were detected within the observational uncertainties. The consistency between the [OIl, GIOTTO IMS and IUE water production rates for comets Borrelly and Halley indicates that the [OIl water production rates for comets Hartley-Good and Thiele are probably accurate to ±50%. Thus the range of greater than an order of magnitude in the ammonia abundances derived (Table 2) for the four comets is significant. The variation in ammonia abundance among the four comets may represent inhomogeneities in the primitive solar nebula at the time the comets condensed 4.5 billion years ago. Alternatively the variations in NH 3 abundances among comets could indicate different condensation temperatures /21/. The ammonia-to-water abundance ratio for comet Halley derived from the GIOTTO IMS spacecraft data is Q(NH3)/Q(H20) -‘1 - 2% /7,9/. Thus the spacecraft and ground-based determinations of the ammonia abundance in comet Halley lead to results which may be discrepant by an order of magnitude. The greatest uncertainties in the ground-based spectral analysis are the calculation of the g-factors, and the production rates. The estimated error in the g-factors is -‘20% /11/. The Haser and vectorial models give virtually the same NH2 production rates, and we estimate conservatively that the model contributes ~30% uncertainty to the results. The water production rate for comet Halley is probably determined to better than 50% by the GIOTTO and TUE data. The agreement between our determination of the [01] H20 production rate in cornet Borrelly and that determined from TUE data using the vectorial model /20/ indicates agreement to -‘25%. Therefore the major source of the discrepancy in the ammonia abundances determined for comet Halley is probably the interpretation of the GIOTTO IMS data. Both Allen /7/ and Wegmann /21/ found Q(NH3)/Q(H2O) -‘1-2% in comet Halley from independent analyses of the GIOTTO IMS data. Since NH~is a minor contributor to the mass 17 channel of the IMS, where 011+ is the major contributor, indirect methods must be used to determine the ammonia abundance. The most abundant ion within the contact surface of comet Halley was detected in the mass 19 amu channel and attributed to 113 0+, thus indicating the significance of ion-neutral reactions near the nucleus. Charge exchange reactions of ammonia with 1130+ produce NH~at a significant rate in the comet. Therefore the ammonia abundance is inferred from the ratio of counts in the channels at 19 and 18 amu. In fact Allen /7/ fits models to the variation in = as a function of distance from the nucleus and found that models with Q(NH3)/Q(H20) -‘1-2% gave the best fit. However, this analysis has been criticized by Marconi and Mendis /22/ who suggest that by including an additional ionization source (such as a larger uv solar flux), the ammonia abundance determined from the IMS data can be substantially reduced, which would bring the GIOTTO results in closer accord with the ground-based spectroscopy. REFERENCES 1.

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