Isotopic fractionation of nitrogen in ammonia in the troposphere of Jupiter

L117

The Astrophysical Journal, 657: L117–L120, 2007 March 10 䉷 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.

ISOTOPIC FRACTIONATION OF NITROGEN IN AMMONIA IN THE TROPOSPHERE OF JUPITER Mao-Chang Liang,1,2 Bing-Ming Cheng,3 Hsiao-Chi Lu,3 Hong-Kai Chen,3 M. S. Alam,3 Yuan-Pern Lee,4

and Yuk L. Yung2

Received 2006 November 11; accepted 2007 January 29; published 2007 February 21

ABSTRACT

Laboratory measurements of the photoabsorption cross section of NH3at wavelengths between 140 and 220 nm 15

are presented for the first time. Incorporating the measured photoabsorption cross sections of NH3and NH3into a

15 14

one-dimensional photochemical diffusive model, we find that at 400 mbar, the photolytic efficiency of NH3is about 15

38% greater than that of NH3. In addition, it is known that ammonia can condense in the region between 200 and 14

700 mbar, and the condensation tends to deplete the abundance ratio of NH3and NH3. By matching the observed

15 14

ratio of NH3 and NH3 at 400 mbar, the combined effect of photolysis and microphysics produces the ratio of

15 14

in the deep atmosphere, in excellent agreement with the Galileo spacecraft measurements. The

⫺3

(2.42Ⳳ 0.34) # 10

usefulness of the isotopic composition of ammonia as a tracer of chemical and dynamical processes in the troposphere of Jupiter is discussed.

Subject headings: atmospheric effects — planetary systems — planets and satellites: individual (Jupiter) —

radiative transfer

1.INTRODUCTION

Ammonia can condense to form ammonia ice at 720 mbar for the solar N/H ratio, 840 mbar for 3 times solar N/H, or 1000 mbar for 4 times solar N/H (Atreya & Wong 2005; Atreya et al. 2005). The N/H ratio in the deep atmosphere of Jupiter is constrained to be 3–4 times solar abundance (Folkner et al. 1998; Atreya et al. 2003; Wong et al. 2004), setting the con-densation level to be at ∼900 mbar. Condensation processes usually preferentially select heavier isotopologues. Photolysis can either enhance (e.g., see Cheng et al. 2006; Liang et al. 2004) or deplete (e.g., NH3, this work; OC

34

S, Leung et al.

15

2002) the abundance of heavy isotopologues. So the isotopic composition of molecules provides a tool for understanding chemical and dynamical processes in the atmospheres of plan-ets. For example, the highly depleted D/H ratio in HDO and H2O in the upper atmosphere of Mars (Krasnopolsky et al.

1998) can be satisfactorily explained by condensation/evapo-ration (Bertaux & Montmessin 2001) and photolytic processes (Cheng et al. 1999).

The15N/ N isotopic ratio in the atmosphere of Jupiter has14

been determined by various groups (Encrenaz et al. 1978; To-kunaga et al. 1980; Fouchet et al. 2000, 2004; Owen et al. 2001; Abbas et al. 2004). The observed15N/ N ratios in am-14

monia at∼400 mbar from the Infrared Space Observatory Short Wavelength Spectrometer (Fouchet et al. 2000) and Cassini spacecraft Composite Infrared Spectrometer (Abbas et al. 2004; Fouchet et al. 2004) are, respectively, 1.9⫹0.9_⫺1.0# 10⫺3 and . In the deeper atmosphere somewhere

⫺3

(2.23Ⳳ 0.31) # 10

between 0.9 and 2.9 bar, the in situ Galileo Probe Mass Spec-trometer (GPMS) returned a value of (2.3Ⳳ 0.3) # 10⫺3 (Owen et al. 2001), similar to that at higher altitudes. These values are a factor of ∼2 less than the terrestrial value and the cometary (Hale-Bopp) value of

⫺3

3.68 # 10

1_{Research Center for Environmental Changes, Academia Sinica, Taipei 115,} Taiwan; mcl@rcec.sinica.edu.tw.

2_{Division of Geological and Planetary Sciences, California Institute of} Tech-nology, Pasadena, CA 91125; yly@gps.caltech.edu.

3_{National Synchrotron Radiation Research Center, Hsinchu Science Park,} Hsinchu 30076, Taiwan; bmcheng@nsrrc.org.tw.

4

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan; yplee@mail.nctu.edu.tw.

(Jewitt et al. 1997). Recently from the solar wind

⫹0.5 ⫺3

3.1_⫺0.4# 10

record of nitrogen archived on the Moon, Hashizume et al. (2000) found that15N/ N is depleted by at least 24% (or the14

ratio ⱗ2.8 # 10⫺3) relative to the terrestrial value. From the

Solar and Heliospheric Observatory, Kallenbach et al. (1998)

directly measured the 15N/ N ratio in the solar wind to be14

. The ratio is found to be in

⫹1.7 ⫺3 ⫺3

5.0_⫺1.0# 10 (2.2Ⳳ 0.5) # 10

the local interstellar medium (Dahmen et al. 1995) and ∼10 in the Large Magellanic Cloud (Chin et al. 1999). It is⫺2

important to note that the ratio could be very different between molecules (e.g., Hutsemekers et al. 2005). The observed de-pleted abundance of NH3in the troposphere of Jupiter (relative

15

to the terrestrial value) leads Owen and colleagues (e.g., Owen et al. 2001; Owen & Encrenaz 2003) to propose that the tro-pospheric N/ N can be used to indicate the solar nebula value.15 14

In this Letter, we investigate the ammonia isotope ratio by considering the photolytic processes of ammonia, based on the laboratory measured photoabsorption cross sections of NH3and

its isotopologues, in the troposphere of Jupiter.

2.MEASUREMENTS OF PHOTOABSORPTION CROSS SECTIONS FOR NH3AND NH3

15 14

The photoabsorption cross sections (Fig. 1) for NH3 and 15

NH3in the wavelengths between∼140 and 230 nm are mea-14

sured at a spectral resolution of 0.02 nm, using vacuum ultra-violet light produced in the National Synchrotron Radiation Research Center in Taiwan.5

The spectral range is associated with two transitions:A R X(∼165–200 nm) andB R X(∼140– 170 nm). To our knowledge, the absorption cross sections of NH3are new. Considering all possible systematic errors, ex-15

perimental uncertainties of cross sections are estimated to be within 10% of reported values. See Cheng et al. (2006) for experimental details.

The absorption maxima in the transition of NH3are 15

A R X

redshifted from those of NH3 by∼0.02 nm for the vibrational 14

band

v

p0; the shifts increase gradually to 0.24 nm for

v

p . The widths for the corresponding vibrational bands of NH3

15

13

and NH3are similar. The maximal absorption cross sections of 14

5

Digitized cross sections at 0.02 nm spectral resolution are available at http://ams-bmc.nsrrc.org.tw.

L118 LIANG ET AL. Vol. 657

Fig. 1.—Top: Absorption cross sections (in units of megabarn, 1 Mbarn p

cm2_{) in the spectral region 140–220 nm for NH}

3and NH3. For best

⫺18 15 14

10

visualization, the cross section of NH3is offset by⫺5 Mbarn. Bottom: Frac-14

tionation factor (e) of NH3, defined by , wherej and are

15 ₁₀₀₀₍

j/j0⫺ 1) j0

the cross sections of NH3and NH3, respectively. See Liang et al. (2004)

15 14

for details ofe factor.

Fig. 2.—Vertical profile of NH3(solid line) used in the model. Crosses are measurements by Edgington et al. (1999). Dashed and dotted lines represent an upper and lower bound of measured NH3mixing ratios, respectively. The profiles used in this study are consistent with that obtained by Fouchet et al. (2000).

this transition of NH3are greater than those of NH3by 15%–

15 14

23% for the first three vibrational bands and by 5%–9% for the other bands. Similarly, the absorption maxima of bands in the transition of NH3are redshifted from those of NH3by

15 14

B R X

0.08–0.24 nm; the widths for the vibrational bands of NH3are 15

slightly smaller than the corresponding bands of NH3. However, 14

the behavior of the maxima for absorption cross sections of NH3 15

and NH3 in the transition is different from that in the 14

B R X

transition; those of NH3increase by 6%–40% for bands 15

A R X

relative to those of NH3 and 3%–29% for bands

 14

v

p0–2

but decrease by 3%–6% for bands .

 

v

p6–11

v

p3–5

The oscillator strength (Herzberg 1950) f is defined by , wherej is the cross section in megabarns

⫺6

1.13 # 10

∫

j dn (p10 cm2_{) and}

n is the wavenumber (cm⫺1). The value of

⫺18

f integrated over 165–220 nm for the transition of NH3 15

A R X

is 0.00858, only 7.3% greater than that (0.0800) of NH3 in 14

theA R X transition. Although absorption maxima and band-widths of the transition for all four deuterated NH3

14

A R X

isotopologues varied substantially with the number of D (2_H)

atoms in each isotopologue, their f-values are almost identical (Cheng et al. 2006). For the transition of NH3, the

15

B R X

value of f integrated over 144–165 nm is 0.0104, 16% smaller than the value of 0.0124 for NH3. We also observed similarly

14

large variations of f-values forB R X transitions among four deuterated isotopologues of NH3in the previous work (Cheng

et al. 2006). For instance, the f-values of NH2D and ND3

14 14

are smaller than that of NH3 by 23%–27%. Thus, the 14

transition is affected by vibrational excitation,

presum-B R X

ably due to vibronic coupling (Lin 1976; Liao et al. 1999). The shift of band origin between isotopologues reflects the difference in zero-point energy of the excited and ground states for these species. The absorption maximum (216.76 nm) of in the transition of NH3is redshifted from that

 15

v

p0 A R X

of NH3by only 0.02 nm, or⫺4 cm . In contrast, the shifts

14 ⫺1

between four deuterated isotopologues of NH3 are 0.80, 0.82,

and 0.85 nm to the blue for each increase in the number of D atoms, or 171, 176, and 185 cm . Notably, other vibrational⫺1 bands of NH3in A state are all redshifted from those of NH3,

15 14

whereas those of the three deuterated isotopologues of NH3are

all blueshifted. According to calculations (McCarthy et al. 1987; Cheng et al. 2006), the A state of NH3dissociates into NH2(

2_B 1)

⫹ H with a small barrier. Because the solar flux decreases rapidly as the wavelength decreases in this spectral region, we expect that the isotopic photo-induced fractionation has opposite effects for D- and N-isotopologues. The fractionation factor of NH3

15 15

is presented in the lower panel of Figure 1. The figure shows that enhanced photolysis of NH3occurs at wavelengths greater

15

than∼210 nm.

3.MODEL DESCRIPTION AND SIMULATION RESULTS By analogy with mechanisms that fractionate the isotopic com-position of water in the atmosphere of Mars (Bertaux & Mont-messin 2001), we consider photolysis and condensation pro-cesses of ammonia in the troposphere of Jupiter. We first calculate the isotopic fractionation of ammonia due to photolysis, and dimensional models are sufficient for such purpose. The one-dimensional Caltech/JPL KINETICS model is used in our study. A detailed description of the model has been given elsewhere (e.g., Gladstone et al. 1996; Moses et al. 2005). Due to the complexity of chemical and (micro)physical processes involving ammonia, we fix the vertical profiles of ammonia based on the observations (e.g., Edgington et al. 1999) and defer self-consis-tent modeling (dynamics, microphysics, and photochemistry coupled calculation) to a later paper. The vertical profile of am-monia is shown by the solid line in Figure 2, which is the same as the one used in a companion paper (Cheng et al. 2006); this is set to be the reference profile.

Because of absorption by CH4and C2H6in the upper atmosphere

of Jupiter, UV photons at wavelengths shorter than∼160 nm are absent in the troposphere (Gladstone et al. 1996; Moses et al. 2005). The measured photoabsorption cross sections for ammonia isotopologues (Fig. 1) between 140 and 230 nm are suitable for our investigation in the troposphere. The photoabsorption of am-monia mostly takes place in the region between ∼200 and 700 mbar (see Fig. 3, dashed line), covering the region of interest at∼400 mbar.

The photolytic efficiencies of NH3and NH3are shown in

15 14

Figure 3. Over the region of interest, the photolysis of NH3 15

prevails, resulting in a depletion of the abundance of NH3 15

([ NH3]). The column-integrated photolysis rate of ammonia 15

is∼1012

molecules cm⫺2 ⫺1s and is insensitive to the prescribed ammonia vertical profiles. We define ( NH3)/J( NH3),

15 14

No. 2, 2007 JOVIAN15N/ N14

L119

Fig. 3.—Vertical profile ofb, the ratio (solid line) of the J (photolysis rate

coefficient) values of NH3and NH3in the atmosphere of Jupiter, calculated

14 15

using the reference ammonia profile (Fig. 2, solid line). The photoabsorption rate of NH3is overplotted by a dashed line using the cross section presented in Fig. 1. The maximum rate (1.0) corresponds to_{9.1 # 10}5molecules cm⫺3 ⫺1s .

Fig. 4.—Vertical profile of [ NH3]/[ NH3], obtained by matching the Cassini

15 14

measurement (Abbas et al. 2004) at 400 mbar, represented by the diamond with the reported one-j statistical error bar. Calculations coupled with ammonia

pho-tolysis and microphysics processes are represented by the solid line. For com-parison, results that only consider photolysis and vertical mixing processes are shown by the dotted line. The arrow indicates the Galileo GPMS measurements (Owen et al. 2001) in the deep atmosphere. The dashed line denotes a sensitivity study to the changes of ammonia condensation rates based on the vertical eddy mixing coefficients used by Gladstone et al. (1996). See text for details. where J is the photolysis rate coefficient, andg p ([ NH3]/

15

[ NH3])/([ NH3]0/[ NH3]0), where [ NH3]0 and [ NH3]0 are

14 15 14 15 14

the abundances of NH3 and NH3 in the deep atmosphere,

15 14

respectively. The g in photochemical equilibrium equals 1/b. At 400 mbar,b p 1.38. This impliesg p 72.5%. Below the ∼550 mbar altitude level, our model showsg p 66%. In con-trast, NH2D shows an enrichment of the abundance above

14

450 mbar level and depletion below (Cheng et al. 2006). The g value for NH2D has a maximum of 143.9% at 300 mbar

14

and levels off at a value about 70% below∼550 mbar level. Note that the above calculations are based on photochemical equilibrium arguments. See, for example, Yung et al. (1997) and Liang et al. (2007) for an in-depth discussion on the iso-topic composition of species in photochemical equilibrium and the role of dynamics in modifying photochemical effects.

Varying the concentration profiles of NH3 can modify the

results presented above. For this, we change the NH3 profiles

shown in Figure 2. With lower NH3 abundance (dotted line),

( ) at 400 mbar. For higher NH3abundance

g p 76.5% b p 1.31

(dashed line),g p 66.6% b p 1.50( ). All these sensitivity stud-ies give . From the Cassini measured [ NH3]/

15

g p 72%Ⳳ 5%

[ NH3] p , [ NH3]0/[ NH3]0 p

14 _(2.23Ⳳ 0.31) # 10⫺3 15 14

is inferred. This value is greater than the

⫺3

(3.10Ⳳ 0.48) # 10

GPMS value of(2.3Ⳳ 0.3) # 10⫺3below 0.9 bar level, where the photolysis-induced fractionation is negligible (see Fig. 3), although the difference is not significant. However, the FWHM of the contribution function of the observations (e.g., see Fouchet et al. 2004) is as wide as the region where the photolytic process of ammonia is important; the total column photolysis rate of ammonia would be a better indicator to represent the photo-chemical effect of ammonia. The column integrated b p and ; the values are insensitive to the selection 1.323 g p 75.6%

of ammonia vertical profiles. The implied [ NH3]0/[ NH3]0 is

15 14

, close to the value calculated at 400 mbar.

⫺3

(2.95Ⳳ 0.41) # 10

So 400 mbar level is selected for the following discussion. The microphysical processes of ammonia and vertical eddy mixing can also affect the isotopic composition of ammonia. Following the same method as that by Fouchet et al. (2000) and Bertaux & Montmessin (2001), we can expressg as [X(NH3)/

X(NH )] , where X(NH3) and X(NH ) are the volume mixing

a⫺1

3,0 3,0

ratios at higher altitudes and in the deep atmosphere, respectively. The fractionation coefficient a is determined experimentally. Since the ammonia condensation rate has not been calculated/ measured, we estimate the rate based on the ammonia mixing ratio profile prescribed by the solid line in Figure 2 and the eddy mixing coefficients from Moses et al. (2005) based on Edgington et al. (1999) observations. Assuming X(NH3, 0) p 2 # 10⫺4 (e.g., Abbas et al. 2004), we find that ammonia can condense in the region between∼300 and 800 mbar. The lower and upper limits are set by photolysis and and saturation relation of NH3,

respectively. Over the∼300 and 800 mbar region, the ammonia microphysics process is more important than the photolysis, re-sulting in the dilution of the depletion of N caused by photolysis15

(see Fig. 4). Applying the a measured for the liquid-vapor transition of NH3 (Thode 1940; Jancso & van Hook 1974),

15

we obtain at 400 mbar and infer [ NH3]0/ 15

g p 92.3%

, in excellent agreement with

14 ⫺3

[ NH ] p(2.423 0 Ⳳ 0.34) # 10

the GPMS measurements. Note that the a used in the above calculation is extrapolated from the values measured at higher temperature (∼195–240 K vs. 130 K for the region of interest in Jupiter).

The above estimation of the isotopic fractionation of ammonia due to ammonia condensation is calculated by assuming the “open” cloud system, where the condensed phase is in isotopic equilibrium with the vapor phase and the condensed particles leave the system immediately after their formation. There is another model that results in smaller isotopic fractionation: the “closed” cloud model, in which the condensed and vapor phases stay in the same air parcel. See, for example, Fouchet & Lellouch (2000) for a detailed description of the two models. The latter model gives [ NH3]0/

15

.

14 ⫺3

[ NH ] p(2.273 0 Ⳳ 0.32) # 10

To estimate the sensitivity of the results to the changes of vertical eddy mixing coefficients, a case based on the Gladstone et al. (1996) eddy profile is shown by the dashed line in Figure 4; the tropospheric eddy mixing coefficient in this case is 103_cm2_{s ,}⫺1

a factor of 10 less than that (∼104

cm2

s ) used by Moses et al.⫺1 (2005). In this case, the inferred [ NH3]0/

15 _{[ NH ] p 3.02 #}14 3 0

, suggesting that vertical transport plays a crucial role in the

⫺3

10

L120 LIANG ET AL. Vol. 657 In contrast, the abundance of NH2D is less affected (!10%) by

the selection of vertical eddy mixing coefficients. The main reason for this is that the microphysical processes dominate the isotopic fractionation in NH2D.

To test the above estimation of the ammonia condensation rate, we perform a microphysics calculation using the Community Aerosol and Radiation Model for Atmospheres (CARMA; Toon et al. 1988). We find that at∼400 mbar, the ammonia ice production rate ranges between!1 # 10⫺15 and g cm s (relative humidity ∼100%–120%),

ob-⫺12 ⫺3 ⫺1

9 # 10

tained over a variety of initial conditions (i.e., mean radius of condensation nuclei of 0.01–0.5mm, number of condensation nuclei of 0.1–100 cm , and initial relative humidity of NH⫺3 3

of 100%–300%), which could be much greater than the nominal production rate (∼10⫺16g cm⫺3 ⫺1s ) used in Figure 4, suggesting that the microphysical processes of ammonia could be more efficient than we expect from vertical eddy mixing processes to homogenize the isotopic composition of ammonia between 400 mbar level and lower altitudes.

4.DISCUSSION AND SUMMARY

Elemental and isotopic measurements provide a wealth of information for the formation and evolution of the solar system. The currently favored formation scenario for giant planets sug-gests that heavier elements are enriched compared with the solar or protosolar abundances. The degree of enrichments (roughly by a factor of3Ⳳ 1, compared with solar abundances, for Jupiter) represents the fraction of core mass to the sur-rounding gaseous envelope (Mizuno 1980; Pollack et al. 1996), as well as the conditions such as temperature for trapping these elements in the icy planetesimals for the formation of giant planets (e.g., Owen & Encrenaz 2003). Molecular nitrogen is the biggest N reservoir in solar nebulae. However, it is so volatile that it can hardly be incorporated into planets during their formation. Ammonia is likely to be the major N carrier in giant planets. It has been shown that ion-molecule reactions

(Terzieva & Herbst 2000) in interstellar clouds could enrich N/ N in HCN relative to N2. Yet the observed N/ N for

15 14 15 14

Jupiter smaller than that for the comet Hale-Bopp leads Owen et al. (2001) to suggest that the measured15N/ N in ammonia14

represents the protosolar value. In contrast, our calculation sug-gests that the inferred N/ N in the lower atmosphere of Jupiter15 14

could be modified by the photolytic processes of ammonia above∼700 mbar level.

We demonstrate that vertical eddy mixing coefficients at and below 400 mbar altitude levelk103_cm2 _s⫺1_{can greatly dilute}

the ammonia photolytic effect. No evident latitudinal variation of N/ N in ammonia was found (Abbas et al. 2004), suggesting15 14

that the ammonia abundances at 400 mbar at latitudes between Ⳳ40⬚ are controlled primarily by the microphysics of ammonia. The formation of ice particles followed by dynamical processes such as convection (Gierasch et al. 2000; Ingersoll et al. 2000) provides an alternative explanation to the observed NH3/ NH3

15 14

at 400 mbar and in the deep atmosphere; the evaporation of ammonia ice supplies the loss of ammonia due to photolysis. Further modeling coupled with photochemistry, dynamics, and microphysics could provide valuable information for the dynam-ical properties in the troposphere of Jupiter. The isotopic com-position measurements of NH2D and NH3at various altitudes

15

and latitudes will be needed to constrain model parameters (e.g., atmospheric dynamics). Laboratory measurements of the pho-toabsorption cross sections of ammonia isotopologues and frac-tionation coefficients (a) due to condensation at lower temper-ature (∼150 K) are required to refine our calculation.

We thank Geoff Blake and John Eiler for helpful discussion, and Andy Ackerman, Xin Guo, Run-Lie Shia, and Giovanna Tinetti for assisting the CARMA simulation, and Chris Par-kinson for useful comments. Special thanks are due the referee Emmanuel Lellouch for his insightful comments. This work was supported by NASA grant NNG06GF33G to the California Institute of Technology. B.-M. C. was supported by the National Science Council of Taiwan (grant NSC95-2113-M-213-006). REFERENCES

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