ABSTRACT. The Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft made infrared observations of Jupiter's atmosphere during the flyby of 2000 ...
The Astrophysical Journal, 602:1063–1074, 2004 February 20 Copyright is not claimed for this article. Printed in U.S.A.
THE NITROGEN ISOTOPIC RATIO IN JUPITER’S ATMOSPHERE FROM OBSERVATIONS BY THE COMPOSITE INFRARED SPECTROMETER ON THE CASSINI SPACECRAFT M. M. Abbas,1 A. LeClair,2 T. Owen,3 B. J. Conrath,4 F. M. Flasar,5 V. G. Kunde,6 C. A. Nixon,6 R. K. Achterberg,7 G. Bjoraker,5 D. J. Jennings,5 G. Orton,8 and P. N. Romani5 Received 2003 June 25; accepted 2003 October 7
ABSTRACT The Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft made infrared observations of Jupiter’s atmosphere during the flyby of 2000 December to 2001 January. The unique database in the 600–1400 cm1 region with 0.53 and 2.8 cm1 spectral resolutions obtained from the observations permits retrieval of global maps of the thermal structure and composition of Jupiter’s atmosphere, including the distributions of 14NH3 and 15NH . Analysis of Jupiter’s ammonia distributions from three isolated 15NH spectral lines in eight latitudes is 3 3 presented for evaluation of the nitrogen isotopic ratio. The nitrogen isotopic ratio 14N/15N (or 15N/14N) in Jupiter’s atmosphere in this analysis is calculated to be 448 62 [or ð2:23 0:31Þ 103 ]. This value of the ratio determined from CIRS data is found to be in very close agreement with the value previously obtained from the measurements by the Galileo Probe Mass Spectrometer. Some possible mechanisms to account for the variation of Jupiter’s observed isotopic ratio relative to those of various astrophysical environments are discussed. Subject headings: ISM: abundances — ISM: evolution — planets and satellites: individual (Jupiter) — solar system: formation
1. INTRODUCTION
lifetimes that become Type II supernovae (Chin et al. 1999; Jose´ & Hernanz 1998; Woosley & Weaver 1995). It is, however, recognized that the current models do not produce sufficient amounts of 15N to account for the Galactic abundance, and it is not yet clear whether 14N or 15N is the more abundant primary isotope produced. Owen et al. (2001) have summarized the available measurements of the nitrogen isotopes in diverse astrophysical environments that indicate a wide range of values of 14N/15N (or 15N/14N), varying from 100 (or 102) in the Large Magellanic Cloud (LMC), 500–1000 [or ð1 2Þ 103 ] near the Galactic center, 300–400 [or ð3:3 2:5Þ 103 ] in the local ISM, 273 (or 3:66 103 ) in Earth’s atmosphere, and 435 (or 2:3 103 ) in Jupiter’s atmosphere. The observed variations in the nitrogen isotopic ratio reflect the production rates of the two isotopes by nucleosynthesis in various regions of the Galaxy and isotopic fractionation and dynamical processes in various astrophysical environments and therefore have the potential to provide valuable information about the evolutionary processes in the regions involved. The variation is expected to be a consequence of temporal as well as spatial effects. The 14N/15N ratio in the ISM increases with time with the production of 14N from nuclear processing in stars. As pointed out by Owen et al. (2001), this implies that the 14N/15N ratio is expected to be higher in the local ISM now, compared with the value 4.6 Gyr ago at the time of formation of the solar system. Also, with the increasing degree of nuclear processing toward the Galactic center with higher levels of 14N production, the 14N/15N ratio is expected to decrease with distance from the Galactic center to the local ISM and the solar system, as observed. The measurements of nitrogen isotopic ratios in the solar system also show a great deal of variation with recently measured values of 200 (or 5 103 ) in the solar wind (Kallenbach et al. 1998), 333 (or 3 103 ) in the lunar surface (e.g., Hashizume et al. 2001), and 293 (or 3:41 103 ) for Mars (Mathew & Marti 2001). Measurements for Jupiter obtained from the Galileo
14N/15N
in Measurements of the nitrogen isotopic ratio various astrophysical environments have been of great scientific interest in recent years because of their relevance to several issues dealing with (1) the formation of nitrogen isotopes by nucleosynthesis in supernovae and in the CNO cycle and (2) the evolution of these isotopes in the Galaxy as manifested in the local interstellar medium (ISM), the solar system, and the planets. The measurements of the nitrogen isotopic ratios, however, have led to a number of conflicting results that have been referred to as the ‘‘nitrogen puzzle’’ (e.g., Audouze 1977; Kerridge 1980, 1982; Geiss & Boschsler 1982; Dahmen, Wilson, & Matteucci 1995; Chin et al. 1999; Owen et al. 2001). Nitrogen is believed to be formed in the interior of stars in the CNO cycle by the p and -capture reactions (e.g., Clayton 1968; Audouze 1977; Harwit 1998). Observational evidence appears to indicate that the production of 14N involves both primary and secondary components in long-lived intermediate and low-mass high-metallicity stars (e.g., Chin et al. 1999). 15N is believed to be synthesized in massive stars with short 1 NASA Marshall Space Flight Center, Huntsville, AL 35812; mian.m.abbas@nasa.gov. 2 University of Alabama in Huntsville, Huntsville, AL 35899; andre.leclair@msfc.nasa.gov. 3 University of Hawaii Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822; owen@ifa.hawaii.edu. 4 Department of Astronomy, Cornell University, Ithaca, NY 14853; barney.j.conrath@gsfc.nasa.gov. 5 NASA Goddard Space Flight Center, Greenbelt, MD 20771; mike@ leprss.gsfc.nasa.gov, gordon.l.bjoraker@gsfc.nasa.gov, donald.e.jennings .1@gsfc.nasa.gov, paul.romani@gsfc.nasa.gov. 6 University of Maryland, College Park, MD 20742; virgil.g.kunde@ gsfc.nasa.gov, conor.a.nixon.1@gsfc.nasa.gov. 7 Science Systems and Applications, Inc., 10210 Greenbelt Road, Suite 600, Lanham, MD 20706; richard.achterberg@gsfc.nasa.gov. 8 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109; glenn.s.orton@jpl.nasa.gov.
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Probe Mass Spectrometer measurements indicate a value of 14N/15N of 435 (or 2:3 103 ). This depletion of 15N is consistent with the nitrogen isotopic ratio of 526þ585 169 (or 3 14NH and 15NH profiles ob 10 ) obtained from 1:9þ0:9 3 3 1:0 tained from ISO-SWS observations of Jupiter (Fouchet et al. 2000). However, the uncertainty in the latter determination invites reevaluation of Jupiter’s ammonia. In this paper we present the nitrogen isotopic ratio in Jupiter’s atmosphere, obtained from Composite Infrared Spectrometer (CIRS) infrared observations during the flyby of the Cassini spacecraft of 2000 December to 2001 January, which yielded a large number of spectra in the 600–1400 cm1 region. Analysis of the uniquely observed spectra provides global maps of the thermal structure and composition of Jupiter’s atmosphere, including profiles of the two isotopes of ammonia. The nitrogen isotopic ratio can be obtained from the retrieved ammonia abundances. 2. OBSERVATIONS The Cassini spacecraft carries a high spectral resolution infrared imaging spectrometer capable of making limb-scanning and nadir-viewing measurements of thermal emissions of the atmospheres of Saturn and Titan, the rings, and the icy satellites. The spacecraft flew by Jupiter from 2000 December to 2001 January, yielding an opportunity for extensive observations of Jupiter’s atmosphere in the nadir-viewing mode during the period of its closest approach, at a spacecraft-toJupiter distance in the ð137 250ÞRJ range. The measurements were made with high- and low-apodized spectral resolutions of 0.53 and 2.8 cm1, respectively, and spatial resolutions on the disk of 0.27 mrad, corresponding to 6.0–3.5 of great circle arc at the subspacecraft point. The low-resolution set consists of about 20,000 scans in each of the two focal planes FP3 and FP4, and the high spectral resolution database consists of about 11,000 scans in FP3 and about 13,000 scans in FP4. A brief summary of the instrument and the observational modes is given here for convenience. A more detailed description of the instrument design and operational characteristics has been given in a previous publication (Kunde et al. 1996).
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The CIRS instrument, telescope, and FP1 are passively cooled and maintained at a temperature of 170 K, while the FP3 and FP4 are maintained at 80 K with a single passive cooler radiating to space. An example of the averaged spectrum of Jupiter observed in FP3 during December 29 to January 15 over the N5 to N15 latitude range with 2.8 cm1 resolution in the 600–1100 cm1 region is shown in Figure 1, exhibiting the collision-induced S(1) line of H2 near 600 cm1 and vibrational-rotational lines of C2H2, C2H6, and NH3. Figure 2 shows the averaged spectrum for the same period and the latitude range observed in FP4 in the 1050–1400 cm1 spectral region with 2.8 cm1 resolution, exhibiting vibrational-rotational lines in the CH4 band near 1300 cm1. Spectra similar to those shown in the two figures have been employed for the retrieval of Jupiter’s thermal structure and composition discussed in the following sections. 3. THERMAL STRUCTURE AND COMPOSITION Global maps of the thermal structure and composition of Jupiter have been retrieved from CIRS’s observed Jupiter spectra by using analytical inversion techniques that have been developed for the interpretation of planetary spectra. Several preliminary results based on analyses of CIRS’s spectra for Jupiter’s global thermal structure, dynamics, and constituent distributions involving NH3, PH3, C2H2, and C2H6 in particular have already been presented (e.g., Achterberg et al. 2004; Flasar et al. 2003; Nixon et al. 2003). However, for self-consistency and to avoid any systematic biases, the thermal structure and gas distributions employed in
2.1. Composite Infrared Spectrometer The CIRS consists of a pair of far- and mid-infrared thermal emission Fourier transform spectrometers sharing a 50 cm beryllium Cassegrain telescope. The spectrometers are designed to measure thermal emissions in the 10–1400 cm1 spectral region from the atmospheres of Titan and of Saturn, its rings, and icy satellites during the 4 yr orbital tour of the Saturnian system. The far-infrared spectrometer is a MartinPuplett–type polarizing interferometer with a focal plane (labeled FP1) consisting of two thermopile detectors with a 3.9 mrad field of view covering the 10–600 cm1 region. The mid-IR interferometer is a conventional design with two focal planes (labeled FP3 and FP4) covering the 600–1400 cm1 region in two bandpasses: FP3 with a 1 10 photoconductive HgCdTe detector array covering 600–1100 cm1, and FP4 with a 1 10 photovoltaic HgCdTe array covering 1100– 1400 cm1. Each detector pixel in both arrays has a field of view of 0.276 mrad, providing a capability for making atmospheric limb measurements. The CIRS spectral resolution may be varied from 0.5 to 20 cm1 (apodized). The higher resolution of 0.53 cm1 is an order of magnitude better than the 4.3 cm1 resolution of the IRIS instrument on Voyager.
Fig. 1.—Example of the averaged spectrum of Jupiter observed in FP3 from December 29 to January 15 over the latitude range N5 –N15 with 2.8 cm1 resolution in the 600–1100 cm1 region, exhibiting the collisioninduced Sð1Þ line of H2 near 600 cm1 and vibrational-rotational lines of C2H2, C2H6, and NH3.
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Fig. 2.—Averaged spectrum for the data in Fig. 1, observed in FP4 in the 1050–1400 cm1 spectral region with 2.8 cm1 resolution, exhibiting vibrational-rotational lines in the CH4 band near 1300 cm1.
calculations of the nitrogen isotopic ratios discussed in this paper were retrieved independently, using the alternative analytical techniques discussed in the following section. 3.1. Analytical Techniques The data analyzed in this paper are based on the apodized low-resolution (2.8 cm1) spectra obtained from 2000 December 30 to 2001 January 15 and the high-resolution (0.53 cm1) spectra obtained from 2000 November 30 to December 13. The low-resolution spectra obtained from both FP3 and FP4 have been used for retrieval of P=T profiles and for ammonia (14NH3) and phosphine (PH3) distributions, whereas the high-resolution (0.53 cm1) spectra are the basis for retrieval of the C2H6 and 15NH3 distributions. The observed database that encompasses latitudes of N75 to S75 over a wide range of longitudes provides a unique opportunity for retrieving global maps of Jupiter’s thermal structure and gas distributions. The emission angles involved in the observations vary with latitude from 0 –30 at low to mid-latitudes and 60 –70 at high latitudes. For the analysis discussed in this paper, a zonally averaged subdivision of the database was constructed for 10 latitude bins over the range extending from N65 to S65, with the variation of emission angles in each latitude bin limited to 5 except at low latitudes, where the emission angle varies over the 0 –30 range. The binning process limits the uncertainty of air mass in the radiative transfer calculations to 3%–4%. The zonally averaged spectrum for each latitude bin comprises about 1000–1500 scans in both FP3 and FP4 and about 500–700 scans for the high-resolution FP3 spectra. The radiative transfer calculations employed in the analysis in this paper are based on precalculated correlated-k (c-k)
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distributions (Goody & Yung 1989; Lacis & Oinas 1991). This rapid radiative transfer calculation technique is based on the premise that over a given frequency interval the absorption coefficient k distributions at different pressure levels throughout the atmosphere are simply correlated in frequency space. This technique permits precalculation of the k distributions for a given atmosphere and application, leading to very rapid radiative transfer calculations suitable for iterative procedures. The c-k distributions were generated by line-byline calculations for the high- and low-resolution spectra with Hamming function apodization. The parameters involved in the calculation of the arrays for c-k distributions are chosen to provide sufficiently rapid calculations with accuracies generally 1%–2% compared with full line-by-line calculations. For the current applications, the cumulative distribution function g versus k arrays for each gas were precalculated for 50 Gaussian quadrature points for a grid of 24 pressures and 21 temperatures representing Jupiter’s atmosphere. The basic Jupiter atmospheric model, and the initial-guess model for retrieval of atmospheric parameters, is that constructed from the Galileo probe data (Seiff et al. 1998; Yelle, Griffith, & Young 2001), along with atmospheric chemistry model calculations (Romani 1996). The atmospheric model employed here in the calculations includes H2, He, CH4, CH3D, 14NH3, 15NH3, C2H2, C2H6, C2H4, C4H2, CH3, and PH3. Some additional complex hydrocarbons have also been identified but have not been included. The P=T and the vertical volume mixing ratio profiles of some of the gases included in the Jupiter atmospheric model are shown in Figures 3 and 4. The hydrogen and helium volume mixing ratios have been assumed to be 0.863 and 0.134, respectively. No cloud effects have been included in the analysis discussed in this paper, which is limited to retrieval of useful information above 400 mbar levels only. The required positive and negative shifts in continuum levels in the 600–1000 cm1 region were found to be inconsistent with inclusion of cloud opacities in the radiative transfer model for tropospheric levels. The molecular spectral data employed in the radiative transfer calculations are from the recent GEISA (JacquinetHusson et al. 1999) and HITRAN (Rothman et al. 1998) compilations. The collision-induced absorptions for H2, He, and CH4 pairs have been incorporated by using the model provided by Borysow et al. (1985), Borysow & Frommhold (1991), and Birnbaum, Borysow, & Orton (1996). The methods for retrieval of atmospheric parameters such as the vertical P=T and gas volume mixing ratio profiles from observed spectra are based on inverse solutions of the radiative transfer equation. The basic principle behind various inversion techniques that have been developed utilizes the information contained in the spectra relating to the vertical variability or distribution of the atmospheric parameters indicated by the contribution functions that characterize the atmospheric region from where the observed radiance may be originating (e.g., Deepak et al. 1985; Goody & Yung 1989). The inversion technique employed in the analysis presented here is based on a nonlinear least-squares fitting technique for observed and calculated radiances that has been effectively used for retrieval of atmospheric parameters from infrared observations of Earth’s atmosphere (Abbas et al. 1985, 1987, 1991; Guo 1987). This technique is based on a physical approach employing selected frequencies with sharply peaked contribution functions providing unique information about atmospheric parameters, with radiances originating from various levels of the atmosphere. Ideally, it is desirable to have a
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Tmax ðiÞ, and qmax ðiÞ is obtained with a nonlinear relaxation equation. The corrected full atmospheric profile (Pj , Tj , qj ; j ¼ 1, N layers) for all layers is obtained from the retrieved set by (1) log interpolation between the layers corresponding to the peak values and (2) a scaled merging of the retrieved parameters at the layers below the bottom peak of the contribution function set with the model profile. For example, if the difference between the retrieved and the model volume mixing ratios at the bottom peak level is indicated by a scale factor SF, this procedure provides the mixing ratios at layers below the lowest peak level by qret ð jÞ ¼ qmod ð jÞ½1 þ xðSF 1Þ, where x represents the normalized contribution function at the level j. (3) At heights above the top peak level of the contribution function, the retrieved atmospheric parameters are extrapolated with the same scale factor as at the top peak level of the contribution function. (4) A similar procedure is applied for retrieval of the temperature profiles. The iterative procedure is continued until the rms residual error becomes less than a predefined noise level of the observations. For appropriately chosen frequencies providing contribution functions with a single maximum, convergence to a solution usually takes place rapidly. The solution, however, is rejected if the convergence does not take place, and the frequencies that lead to very broad or multipeaked contribution functions during the course of the iterative process are rejected. 3.2. Retrieval of Thermal Structure The composite P=T profile for each latitude bin was obtained from the averaged 2.8 cm1 resolution data by combining information from about 18 CH4 emission lines in the 1300 cm1 region for stratospheric temperatures, and Fig. 3.—Model P=T profile of Jupiter’s atmosphere
set of frequencies sensitive to the opacities of the retrievable parameters, with contribution functions approaching delta-like functions in the pressure range of interest. However, with the spectral resolution and the nadir-viewing mode employed in the CIRS/Jupiter observations, the contribution functions are generally broad with the peaks spanning a narrow range of levels, thus limiting the pressure range with unique information. The inversion techniques, however, may still be used and composite atmospheric profiles constructed by utilizing a priori information, interpolation techniques, and photochemical model considerations. The uniqueness of the retrieved atmospheric parameters is then compromised by the accuracy of the assumed information. The inversion procedure adopted is as follows. The atmosphere is divided into 200 layers of equal thickness extending from a 5 bar pressure level at the bottom to about 0.001 mbar at the top. A model atmosphere (Figs. 3 and 4) consisting of P=T and gas composition volume mixing ratio profiles (Pj , Tj , qj ; j ¼ 1, N layers) is assumed. A set of isolated frequencies i ¼ 1, K with a minimum overlap of interfering gases is selected to provide contribution functions peaking at specific levels in the atmosphere at layers j corresponding to the pressure, temperature, and volume mixing ratio q identified as Pmax ðiÞ, Tmax ðiÞ, and qmax ðiÞ, respectively, where the subscript ‘‘max’’ refers to values at the peak levels of the contribution functions. An iterative process is started to minimize the difference between the observed and the calculated radiances in a least-squares sense, and a corrected set of atmospheric parameters Pmax ðiÞ,
Fig. 4.—Model vertical gas mixing ratio profiles of the gases included in Jupiter’s atmospheric model.
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the observed spectra following the analytical techniques outlined in x 3. The retrieval of 14NH3 and 15NH3 were carried out in spectral regions where the latter two gases have minor contributions to opacity. For self-consistency, however, the retrieved distributions of the two gases were used in the radiative transfer model for nitrogen isotopic ratio calculations. 3.3.1. Ethane
Ethane (C2H6) with the 9 band in the 800–870 cm1 region has a small contribution to opacity in the spectral region of ammonia lines, and the effect of its partial overlap was minimized by appropriate choice of the retrieval frequencies. The ethane distributions employed in the radiative transfer calculations used in the analysis presented here were obtained from the high-resolution 0.53 cm1 spectra utilizing the analytical technique outlined in x 3. The details of an analysis for retrieval of ethane distributions with an alternative analytical technique are discussed in another publication (Nixon et al. 2003). 3.3.2. Distribution of
14
NH3 and PH3
The ammonia distributions were retrieved from the observed spectra in FP3 in the 860–960 cm1 spectral region with 2.8 cm1 resolution, employing about 38 selected frequencies. The normalized contribution functions, defined here as ð@ =@ ln uÞð@B =@ ln pÞ (where is the transmittance, u is the atmospheric column density of the active gas, B is the
Fig. 5.—Contribution functions ð@B =@T Þð@ =@ ln pÞ for the nadir view CH4 band data peak in the 2–20 mbar pressure range and for the H2 span in the 200–600 mbar range.
about 20 frequencies in the collision-induced H2 Sð1Þ line in the 600 cm1 region for tropospheric temperatures. Figure 5 shows the contribution functions ð@B =@T Þð@ =@ ln pÞ for nadir viewing in the CH4 band peaking at 2–20 mbar pressure levels with frequencies from 1283 to 1347 cm1 and in H2 at 200–400 mbar levels with frequencies from 600 to 675 cm1 and at 400–600 mbar from 747 to 772 cm1. The latter spectral region with contribution functions peaking at levels below 400 mbar, however, has some small contribution from C2H6, as well as from cloud opacity, but does not play any significant role. The P=T profiles in the intermediate region and at levels above 2 mbar and below 400 mbar are merged with the model profiles by scaling with the contribution functions as discussed in x 3.1. The retrieved P=T profile for some selected latitudes (0, N20, and S20 ) is shown in Figure 6 along with the model profile. The thermal structure retrieved in this analysis is similar to and in fair agreement with that obtained by Achterberg et al. (2004). 3.3. Retrieval of Composition The retrieval of only those gases with significant contributions to opacity in the spectral regions relevant to retrieval of 14NH3 and 15NH3 are considered in this paper. The opacities of gases with minor contributions are included with the assumed model profiles. The collision-induced absorptions of H2, He, and CH4 pairs are included with precalculated absorption coefficient arrays and with the assumed volume mixing ratios of 0.863, 0.134, and the model profile shown in Figure 4, respectively. The vertical and latitudinal distributions of 14NH3, 15NH3, PH3, and C2H6 were retrieved from
Fig. 6.—Retrieved P=T profile for some selected latitudes (0, N20, and S20 ), along with the model profile.
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Fig. 9.—Comparison of the averaged observed and synthetic spectra for +5 to +15 latitudes in FP3 with 2.8 cm1 resolution, employed for retrieval of tropospheric thermal structure. Fig. 7.—Contribution functions ð@ =@ ln uÞð@B =@ ln pÞ for some selected frequencies employed for retrieval of 14NH3.
Planck function, and p is the pressure) for some selected frequencies employed for retrieval of 14NH3 are shown in Figure 7. The peaks of the contribution functions lie in a narrow range of 300–600 mbar pressure levels over a rapid falloff of the ammonia concentration at lower pressures. The retrieval algorithm is thus employed only to evaluate a scale factor of the ammonia profile relative to the model profile of Figure 4. The scale factors representing the distribution of ammonia retrieved from the averaged observed spectra based on about 1500 scans in each latitude bin are shown in Figure 8. The scale factors indicate ammonia concentrations below
Fig. 8.—Scale factors representing the distribution of ammonia retrieved from the averaged observed spectra based on about 1500 scans in each latitude bin.
Fig. 10.—Comparison of the averaged observed and synthetic spectra for +15 to +25 latitudes in FP3 with 2.8 cm1 resolution, employed for retrieval of C2H6 distributions.
Fig. 11.—Comparison of the averaged observed and synthetic spectra for 25 to +25 latitudes in FP3 with 2.8 cm1 resolution , employed for retrieval of
14NH
3
distributions
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Fig. 12.—Comparison of the averaged observed and synthetic spectra for +5 to +15 latitudes in FP4 with 2.8 cm1 resolution, employed for retrieval of stratospheric thermal structure.
the model values over most of the globe except at high latitudes and in the equatorial region. The PH3 mixing ratio profiles were retrieved from the averaged 2.8 cm1 spectra obtained from observations made during December 29 to January 15, by utilizing isolated PH3 spectral lines in the 1038–1122 cm1 region. Since the vertical profile of PH3 (Fig. 4) has a rapid falloff with height in the troposphere, a retrieval procedure similar to that for ammonia was employed by scaling the model profile by least-squares fitting of about 18 frequencies. The retrieval of PH 3 abundances for the 13 latitude bins indicates enhancements relative to the model profiles by factors of about 2–4. The uncertainty and variation of PH3 abundances from the model profile, however, has an insignificant effect on retrieval of NH3 isotope profiles. Comparison plots of the observed and calculated spectra with the retrieved P=T and gas profiles are shown in Figures 9–12 for some selected latitudes over the 600–1350 cm1 range, representing the spectral regions for retrieval of the thermal structure and retrieval of C2H6, PH3, and NH3 distributions, indicating good fits with rms errors of less than 4%–6%. 3.3.3. Distribution of
15
NH3 and the Nitrogen Isotopic Ratio
15NH
The distribution of 3 was retrieved from the averaged 0.53 cm1 resolution data obtained from 2000 November 29 to December 13, comprising about 6000 scans in the latitude ranges N5 –N45 and S5 –S45 in eight bins of 10 each, with about 700 scans in each latitude bin. The initial-guess value of the isotopic ratio of 15NH3 to that of 14NH3 was assumed to be the terrestrial value of 3:66 103 as employed in the HITRAN database (Rothman et al. 1998). The three relatively isolated 15NH3 lines at 863, 883, and 903 cm1 are clearly
identifiable in the observed spectra for all latitudes (Figs. 11 and 13). Since the 15NH3 lines are much weaker than those of 14NH , the continuum radiance levels at the wings of these lines 3 must match the continuum levels calculated with the retrieved P=T and gas profiles, in order to retrieve the contribution from 15NH . A small positive or negative radiance shift, generally 3 0%–5%, was applied to the observed spectra in order to match the continuum radiances. The retrieved 15NH3 abundances from a total of 24 observed spectral features (eight discrete observations of the three isolated lines) were compared with the corresponding 14NH3 values to evaluate the 15N/14N ratios in Jupiter’s atmosphere and are shown in Tables 1A and 1B. A comparison of some selected plots of the observed and calculated 15NH3 features is shown in Figure 13, along with the spectra calculated with an assumed terrestrial value of the 15NH /14NH ratio of 3:66 103 , indicating a clear depletion 3 3 of 15NH3 in the observed value on Jupiter. The estimated error bars shown on the averaged observed spectra represent noise equivalent spectral radiance (NESR) ð1 3Þ 109 (W cm2 sr1/cm1). The feature in the observed spectra between 860–861 cm1 does not appear to correspond to any of the gases in the model atmosphere and remains unidentified. The retrieved data based on eight latitudes and three 15NH spectral features provide calculation of 24 values of 3 14NH /15NH ratios that yield the nitrogen isotopic ratios 3 3 15N/14N in Jupiter’s troposphere that are shown in Tables 1A and 1B, along with the inverse ratios 14N/15N and the relative values (14N/15N)/(14N/N15), with Earth’s ratio taken as ð14N/15NÞ ¼ 273 (or 3:66 103 ). The nitrogen isotopic ratio derived from the analysis presented here leads to the mean value 14N/15N ¼ 448 61 [or ð2:23 0:31Þ 103 ]. Compared with Earth’s nitrogen isotopic ratio, the three representations of Jupiter’s nitrogen isotopic ratio become ð14N/15NÞ/ð14N/15NÞ ¼ 1:64 0:23 or equivalently ð15N/14NÞ/ð15N/14NÞ ¼ 0:61 0:08. The uncertainty in Jupiter’s nitrogen isotopic ratio relative to that of Earth indicated above represents the quadrature sum of the statistically determined random errors arising from heavy ammonia retrieval errors, represented by the standard deviation of the mean retrieved from 24 heavy ammonia spectral features, of 5%; normal ammonia random retrieval errors of 12%; and random errors arising from uncertainty in temperatures of 1 K, estimated as 4%. The uncertainty shown is the total 1 random uncertainty of the quadrature sum of the three: 14% of the mean value. The retrieval of the nitrogen isotopic ratio presented here is based on retrieval of the thermal structure and 14NH3 and PH3 distributions from the 2.8 cm1 data set obtained from observations during 2000 December 30 to 2001 January 15 and retrieval of C2H6 and 15NH3 distributions from the 0.53 cm1 data set obtained during 2000 November 30 to December 13. This mixing of data sets in two different resolutions and time periods, with the objective of minimizing the retrieval uncertainties, could be a source of error if the spectra are not calibrated appropriately and if the isotopic ratio calculations are made with disregard to this difference. However, the following considerations are expected to minimize any possible discrepancies arising from the mixing of the data sets: (1) No significant change is expected in the retrieved zonal mean quantities employed in the ratio calculations over similar latitudinal bins in the time difference between the two data sets. A single global parameter representing the nitrogen isotopic ratio is the desired quantity. (2) The calibration of two data sets was evaluated by normalization of radiances
Fig. 13.—Comparison of some selected plots of the observed and calculated 15NH3 features, along with the calculated spectra with assumed terrestrial value of the 15NH3/14NH3 ratio of 3:66 103 , indicating a clear depletion in the observed value.
Fig. 13.—Continued
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JUPITER’S NITROGEN ISOTOPIC RATIO TABLE 1A Ratios Retrieved
15NH /14NH 3 3
Scale Factor
Retrieved
15NH /14NH 3 3
Latitude Range (deg)
863 cm1
883 cm1
903 cm1
863 cm1
883 cm1
903 cm1
+35 to +45 ................................. +25 to +35 ................................. +15 to +25 ................................. +5 to +15 ................................... 15 to 5.................................. 25 to 15................................ 35 to 25................................ 45 to 35................................ Mean ...................................... Standard Deviation ................
0.66 0.32 0.56 0.79 0.47 0.64 0.77 0.73 0.62 0.16
0.47 0.47 0.33 0.58 0.47 0.51 0.47 0.57 0.48 0.08
0.83 0.91 0.68 0.55 0.64 0.81 0.64 0.83 0.74 0.13
2.42E3 1.17E3 2.05E3 2.89E3 1.72E3 2.34E3 2.82E3 2.67E3 2.26E3 5.90E4
1.72E3 1.72E3 1.21E3 2.12E3 1.72E3 1.87E3 1.72E3 2.09E3 1.77E3 2.83E4
3.04E3 3.33E3 2.49E3 2.01E3 2.34E3 2.97E3 2.34E3 3.04E3 2.70E3 4.58E4
wherever possible. (3) The zonal mean P=T and 14NH3 profiles were treated as the basic data needed for calculation of the spectra, with 15NH3 features treated as perturbations in the background spectra. The continuum levels of the observed spectra at the base of the 15NH3 features were normalized to the calculated spectra with all of the retrieved profiles, with positive/negative radiance shifts of the observed spectrum. The area under the curve of the heavy ammonia features thus essentially provides their abundance. However, despite the above considerations, in order to verify the validity of our assumptions we carried out an alternative set of retrievals utilizing only the high-resolution 0.53 cm1 spectra obtained during 2000 November 30 to December 13. The results for the P=T and the gas retrievals were found to be similar to those obtained from the 2.8 cm1 data set, with higher residuals and uncertainties. The mean value of the nitrogen isotopic ratio for Jupiter’s atmosphere relative to that of Earth obtained from the high-resolution data set only, following an analysis similar to that for the mixed data set discussed earlier, was found to be 0:60 0:21 [or ð2:20 0:77Þ 103 ], with the uncertainty representing the standard deviation of the mean, larger than the previously determined value with a smaller uncertainty. The mean value calculated in this analysis is within the uncertainty of the value presented earlier, thus validating our assumptions. 4. DISCUSSION The agreement between the CIRS result reported here and the in situ determination by the Galileo probe is reassuring. Owen et al. (2001) have suggested that the Jovian value of 15N/14N ¼ ð2:3 0:3Þ 103 represents the average value in the solar nebula and thus must also be the solar value of this important isotope ratio. They proposed that the difference between this ratio and the value of 15N/14N ¼ 0:366 103
measured in the Earth’s nitrogen stems from the difference in the original molecular carriers of nitrogen to Jupiter and to Earth. For Jupiter, N2 must have been the predominant carrier, while Earth apparently received its nitrogen originally in the form of N compounds such as NH3. This conclusion follows from the history of nitrogen in the Galaxy. As discussed earlier, the 14N isotope is believed to be produced in low to intermediate mass long-lived stars, whereas 15N is ejected by massive stars with short lifetimes that become supernovae. This difference in sources leads to a gradual increase in 14N in the Galaxy, since massive star formation was more prevalent in early times. It also explains the observed gradient in 14N/15N from a high value of 1000 near the Galactic center to 300–400 near the position of the Sun. Once in the ISM, enrichment of 15N/14N in N compounds can occur by means of ion-molecule reactions, leading to a maximum 30% increase in 15N/14N in HCN or NH3 relative to N2 (Terzieva & Herbst 2000). The gradual increase in 14N in the Galaxy with time implies that the isotopic ratio 14N/15N in the ISM is higher today than it was in the interstellar cloud that collapsed to form the solar system. As noted by Owen et al. (2001), this is consistent with the value of 14N/15N measured in HCN from comet Hale-Bopp being lower than current values in interstellar HCN (Jewitt et al. 1997). The cometary value is representative of the conditions in the solar nebula at the time of formation of the solar system, 4.6 Gyr ago. The value of 14N/15N in Jupiter’s atmosphere is higher than the cometary value because Jupiter’s nitrogen was supplied as N2, not HCN. This general agreement is encouraging, but the scenario requires further development and testing. The role of radiation pressure on dust in astrophysical environments and laboratory measurements on individual dust grains has been discussed by Abbas et al. (2003), and the possible role of presolar grains in
TABLE 1B Summary Parameter Mean of all....................................... Standard deviation ........................... Standard deviation of mean............. Temperature (1 K) uncertainty ........ 14NH uncertainty ............................ 3 Total uncertainty ..............................
Retrieved
15NH /14NH 3 3
0.61 0.16 0.033 0.025 0.073 0.08
Scale Factor
Retrieved
15NH /14NH 3 3
2.23E03 5.84E04 1.21E04 9.14E04 2.67E04 3.08E04
Retrieved
14NH /15NH 3 3
448 117 24 ... ... 62
1074
ABBAS ET AL.
contributing anomalous nitrogen to Jupiter will be explored elsewhere. More measurements of nitrogen isotopes in comets can be carried out from Earth and by spacecraft, specifically Rosetta. A direct determination of 15N/14N in the solar wind will be accomplished by the Genesis spacecraft. If the ammonia lines in the spectrum of Saturn are sufficiently strong, CIRS will be able to determine 15N/14N on that planet as well. If the same value for this ratio is found on Saturn as on Jupiter, it will be a signal that nitrogen was also brought to Saturn in the form of N2. This in turn will again require low-temperature,
solar-composition planetesimals as the delivery agent (Owen & Encrenaz 2003).
Useful suggestions and comments on this manuscript by Narayana Bhat, John M. Davis, Joseph A. Nuth III, and S. T. Wu are gratefully acknowledged. Thanks are also due to Mitzi Adams and Allyn Tennant for useful suggestions on the data analysis presented here.
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