ICARUS 131, 317–333 (1998) ARTICLE NO. IS975868
High-Resolution 10-micronmeter Spectroscopy of Ammonia and Phosphine Lines on Jupiter
Luisa-Marı´a Lara1 and Bruno Be´zard2 De´partement de Recherche Spatiale, Observatoire de Paris (Section de Meudon), 92195 Meudon Principal Cedex, France E-mail: [email protected]
Caitlin A. Grifﬁth2 Department of Physics and Astronomy, Northern Arizona University, Flagstaff, Arizona 86011-6010
John H. Lacy2 Department of Astronomy, University of Texas, Austin, Texas 78712-1083
Tobias Owen Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822
Received April 2, 1997; revised November 6, 1997 latitudes 30؇–35؇S. This variation is not correlated with the 200-mbar temperature. It can be explained by a decrease of High spectral resolution measurements of NH3 and PH3 lines on Jupiter in the 10.5- to 11.2-m range are presented. Observa- the eddy mixing coefﬁcient near 240 mbar from ȁ4000 to Յ400 ؊1 2 tions, recorded on January 21–23, 1991, cover the 10؇–40؇S cm sec between the two latitude ranges. The PH3 mixing ؊7 latitude range and several longitudes including the Great Red ratio near 580 mbar lies between 1.7 and 2.6 ؋ 10 in the Spot (GRS). Information on the temperature in the upper tropo- observed regions. At all longitudes, PH3 varies smoothly with sphere was retrieved from the continuum radiance at wave- latitude, decreasing by ȁ30% from 10؇ to 35؇S. This variation lengths around 12.8 and 17.8 m. At all observed longitudes, may also reﬂect a decrease in the strength of the eddy mixing the 200-mbar temperature ﬁeld is minimum at latitudes of near 580 mbar or at deeper levels in the atmosphere. 1998 20؇–25؇S near the location of the South Tropical Zone, in Academic Press Key Words: Jupiter, atmosphere; atmospheres, composition; agreement with Voyager infrared retrievals. This minimum atmospheres, structure; infrared observations. temperature is lower over the GRS than at other longitudes. The ammonia mixing ratio at ȁ380 mbar is not signiﬁcantly enhanced over the GRS. The phosphine abundance probed at 1. INTRODUCTION ȁ580 mbar is also not enhanced (within a precision of 10%), suggesting that this molecule is not a precursor of the reddish The encounters of Pioneer 10 and 11 and of the Voyager chromophores. The NH3 abundance at 380 mbar varies highly with latitude and longitude, a possible consequence of the active 1 and 2 spacecrafts revealed a variety of colorful features jovian meteorology. At the resolution of our observations in Jupiter’s atmosphere. Most noticeable is the Great Red Spot (GRS), a long-lived vigorous anticyclonic feature (ȁ8000 km), the NH3 humidity at this altitude ranges between 15 and 100% throughout the available data set. Above the cloud characterized most of the time by a pronounced reddish tops, the NH3 mixing ratio in the 240-mbar region reaches a tint. The potential chromophores proposed to explain the -maximum near 15؇–18؇S and decreases by a factor of ȁ40 at dominant brownish coloration of the planet include phos phorus, nitriles, and sulfur compounds. Prinn and Lewis
(1975) calculated that phosphine (PH3), upwelled from 1 Also at Instituto de Astrofı´sica de Andalucı´a, Apdo. 3004, 18080 the deep atmosphere, can supply enough P to the upper Granada, Spain. 4 2 Visiting astronomer at the Infrared Telescope Facility which is oper- troposphere to cause the red coloration if the eddy diffu- 6 2 Ϫ1 ated by the University of Hawaii under contract with the National Aero- sion coefﬁcient in this region is ȁ 10 cm s . The possibil- nautics and Space Administration. ity that complex nitriles provide Jupiter’s red chromophore
317 0019-1035/98 $25.00 Copyright 1998 by Academic Press All rights of reproduction in any form reserved. 318 LARA ET AL. may be tested with measurements of NH3 or of the simplest TABLE I nitrile HCN, supposed to be intermediates in the produc- Irshell Observations tion of these organic compounds (Woeller and Ponnamper- Resolution uma 1969). Sulfur compounds are unlikely to be the cause Wavenumber (FWHM) Longitude of the coloration, since their precursor, H2S, is scarce in (cmϪ1) (cmϪ1) (System II) the upper troposphere. Observations of the vertical distri- Temperature 560 0.06 0–40Њ, 110–140Њ bution of NH3 ,PH3, and HCN are thus very important, Њ Њ Њ not only as tests of the ﬁrst two hypotheses for Jupiter’s red Clouds 781 0.11 0–40 , 110–140 , 310–360 NH3 lines 892.0 0.05 225Њ, 240Њ, 245–290Њ coloration, but also because constraints on their vertical 915.7 0.10 180Њ, 200Њ distributions could improve our understanding of the hori- 918.6 0.10 110–140Њ zontal variations in the strength of the vertical transport. 921.3 0.10 30–150Њ Grifﬁth et al. (1992) used Voyager infrared spectra to 923.6 0.10 30–50Њ, 110Њ PH3 lines 944.2 0.11 10Њ, 30–50Њ, 190–240Њ compare the tropospheric abundances of NH3 and PH3 in 954.4 0.12 30–50Њ, 120Њ Jupiter’s GRS to the composition of the surrounding South 974.0 0.13 220–250Њ Tropical Zone (STZ). They investigated how the dynamics of the GRS might affect its chemical composition and pos- sibly explain its peculiar color. Surprisingly, they found that, above the 350-mbar region, NH3 was depleted over 2. OBSERVATIONS the GRS relative to the neighboring regions. The results Observations were conducted at the NASA/IRTF on on PH3 were also unexpected, as no signiﬁcant variation of its abundance was detected (within the error margin, the nights of January 21 and 23, 1991 UT. We used the i.e., Ϫ55 and ϩ75%). These observations are at odds with Irshell spectrometer which included a 10 spatial ϫ 64 spec- the usual picture of rising motions in the cold zones causing tral element array (Lacy et al. 1989). The 10-arcsec long an increase in the concentration of gases which are affected slit was oriented along the planetocentric north–south di- Њ by condensation or by photodissociation such as ammonia rection and centered at a latitude of 22 S. Ten latitudes spanning 10Њ to 36ЊS could be simultaneously observed at and phosphine. The depletion of NH might result from 3 a given longitude. The slit was opened to 2 arcsec, yielding some unidentiﬁed chemical processes at work in the GRS. a resolving power of about 104. This represents an improve- In January 1991, B. Be´zard, C. Grifﬁth, and J. Lacy ment by a factor of about 50 over the Voyager infrared conducted an observing program at the Infrared Telescope observations. The pixel spacing along the slit was 1 arcsec Facility (IRTF) devoted to mapping the chemical composi- and the resolution approximately 2 arcsec. We estimate tion of Jupiter at latitudes and longitudes around those of a pointing uncertainty of about 1 arcsec, similar to the the GRS, using 10-Ȑm high-resolution spectroscopy. The pixel spacing. goal was to investigate the chemistry and dynamics of the The full set of observations is summarized in Table I. GRS following the work of Grifﬁth et al. based on Voyager The continuum level in the 560.6 and 781.1 cmϪ1 intervals spectra at a lower spectral resolution. Spectral regions con- was used to derive information on the temperature proﬁle taining HCN lines were observed, after Woeller and Pon- in the upper troposphere. Spectra around 892.4, 916.1, namperuma’s (1969) suggestion that hydrogen cyanide 918.9, 921.7, and 923.2 cmϪ1 include lines of various might be responsible for the brownish coloration of the 14 strengths from NH3 which can be used to retrieve the jovian atmosphere and more speciﬁcally of the GRS. ammonia vertical proﬁle in the upper troposphere (250– Be´zard et al. (1995) did not ﬁnd evidence for hydrogen 600 mbar). Weak phosphine lines, present at 944.2, 954.4, cyanide absorption in excess of 2% of the continuum in and 974.0 cmϪ1 provide information on the mixing ratio any of the regions investigated (between latitudes 10Њ and around 550 mbar. 36ЊS, excluding the GRS). An upper limit on the deep We applied the procedures for ﬂat ﬁelding, correction Ϫ8 tropospheric HCN mixing ratio of 1.0 ϫ 10 was derived. of atmospheric opacity, and intensity calibration described The present study reports the analysis of the spectral in Lacy et al. (1989) and Achterman (1992) with some data for NH3 and PH3 recorded during this same run at adjustments in the data reduction (detailed in Be´zard et al. the IRTF. In Section 2 we present the high–resolution 1995 as well as the data acquisition). Individual spectra measurements. Section 3 describes the radiative transfer were recorded with on–source integration times of 13 s at analysis of these data and the retrieved information on the 560.6 cmϪ1,and9satother wavenumbers. The rms noise Ϫ1 Ϫ2 temperature, and vertical proﬁles of NH3 and PH3 in the equivalent spectral radiance (in units of erg s cm upper troposphere as a function of latitude and longitude. srϪ1/cmϪ1) is about 0.008 at 560.6 cmϪ1, 0.006 at 781.1 cmϪ1, Results are discussed in Section 4 and a conclusion is pre- and 0.004 for wavenumbers at or longer than 883.0 cmϪ1. sented in Section 5. Although the absolute radiance is estimated to be uncer- NH3 AND PH3 ON JUPITER 319 tain by ȁ20%, past experience indicates that the relative this function overlaps with the region where the cloud calibration of the different rows in the detector array is model is set, detailed calculations show that the 560.6-cmϪ1 better than 2%. radiance is virtually insensitive to the NH3 cloud opacity At the time of the observations, the GRS was a faint for reasonable values of the cloud optical depth. This is feature at 27Њ longitude (System II), hardly distinguishable because our cloud model has a base at 650 mbar and a on the guiding monitor. We recorded spectra at ȁ27Њ and particle–to–gas scale height much smaller than the atmo- ȁ110Њ jovian longitudes of the 560.6 and 781.1 cmϪ1 contin- spheric scale height, so that little particulate opacity is left Ϫ1 uum, the 921.3 and 923.6 cm NH3 features, and the 954.4 above the 450–mbar level. Ϫ1 cm PH3 signatures. These observations provided infor- We ﬁrst considered a baseline case, the ‘‘STZ cold’’ mation on temperature and the ammonia and phosphine proﬁle from Grifﬁth et al. (1992) (displayed in their Fig. abundances. As indicated in Table I, at no other longitude 4). We then perturbed this nominal proﬁle to reproduce were we able to simultaneously obtain the extra continuum the observed radiances in the H2–He continuum. We set measurements (temperature information) with the NH3 the maximum variation of temperature (⌬T) at the tropo- and PH3 lines. pause (140 mbar) and smoothly brought it back to zero at 12 and 640 mbar. We assumed a uniform temperature 3. RADIATIVE TRANSFER ANALYSIS proﬁle in the lower troposphere of Jupiter’s disk, following the assumption of Conrath and Gierasch (1986) and the We have compared our observations to synthetic spectra conclusions of Ingersoll and Porco (1978) that strong tem- generated from a line–by–line radiative transfer program perature contrasts cannot be maintained in the convective (described in Be´zard et al. 1995) to derive thermal and region of Jupiter’s atmosphere. Our entire data set of compositional information on Jupiter’s atmosphere. Spec- 560.6–cmϪ1 spectral radiances required proﬁles with ⌬T troscopic data for ammonia and phosphine come from the ranging between Ϫ2 and ϩ4K. GEISA data bank (Husson et al. 1991). H2–broadened Figure 2 shows the temperature retrieved at 200 mbar NH3 line halfwidths were calculated using Brown and Pe- as a function of latitude and for different System II longi- Ϫ0.73 terson’s (1994) formalism. We assumed a T depen- tudes (27Њ [GRS], 5Њ,40Њ, 110Њ, and 140Њ). Each datum dence, an average value for lines in the 2 band (Brown and point bears a formal error bar of Ϯ0.3 K corresponding Peterson 1994). For phosphine, we used the J–dependent toa2%uncertainty in the relative ﬂux. We ﬁnd that tem- halfwidths measured by Levy et al. (1993) near 4–5 Ȑm. perature reaches a minimum at the latitude of the GRS Ϫ0.69 We adopted a T temperature dependence, an average (21ЊS), a maximum at the northern edge of the array close of the H2 and He broadening properties (Levy et al. 1994). to the SEB (12ЊS), and for some longitudes a local maxi- Our models include a cloud layer between 650 and 300 mum around 31ЊS. The latitudinal contrast is clearly more mbar, with a particle scale height (Hp) equal to 0.15 times pronounced at the GRS longitude than at other longitudes. the atmospheric one (Hp ϭ 0.15 ϫ Hg), i.e. equal to that The radiance in the continuum at 781.14 cmϪ1 probes a of the saturated NH3 gas mole fraction. This cloud model deeper pressure range centered around 550 mbar (Fig. 1a). is similar to that used by Conrath and Gierasch (1986) to The latitudinal contrast is less pronounced than around analyze the Voyager infrared spectra in terms of zonal 560 cmϪ1 indicating that the temperature ﬁeld becomes variations of temperature, NH3 gas, and NH3 cloud opacity. more homogeneous with increasing depth. The 781 cmϪ1 The opacity of this cloud layer was adjusted to obtain a radiance is also affected by the ammonia cloud opacity: good ﬁt of the continuum near the NH3 and PH3 lines. an optical depth of 0.5 in our model decreases the radiance Calculated spectra were convolved with a Gaussian pro- by 5% at an airmass of 1.0 and 2% at an airmass of 1.7. ﬁle having a full width at half maximum equal to the resolu- This low sensitivity prohibits us from retrieving the cloud tion of the observations (Table I). opacity from the 781 cmϪ1 continuum. We found that the temperature models constrained by the 560.6 cmϪ1 obser- a. Temperature vations reproduce the 781 cmϪ1 continuum to within about
Before interpreting the NH3 and PH3 absorption lines 5%. A cloud of ȁ0.2 optical depth provides a satisfactory we retrieved information on the tropospheric temperature ﬁt to the observed 781 cmϪ1 continuum radiance at all longi- proﬁle by analyzing the H2–He continuum at 560.6 and tudes. 781.14 cmϪ1. Figure 1a shows the contribution functions (CF) computed at these two frequencies for an airmass b. Ammonia of 1.10. The CF is deﬁned as the transmission weighting function (d exp(Ϫ)/d ln p) multiplied by the Planck func- The 921 cmϪ1 line was recorded uniformly between 25Њ tion (B(T[p])) and convolved with the instrument func- and 145Њ longitude (System II) with a step of 10–15Њ. How- tion. At 560.6 cmϪ1, the CF extends from 450 to 150 mbar ever, we ﬁrst considered observations limited to the GRS (at half maximum), peaking around ȁ300 mbar. Although (ȁ27Њ) and the ȁ110Њ longitudes where the 560 cmϪ1 radi- 320 LARA ET AL.
Ϫ1 Ϫ1 FIG. 1. Contribution functions for the H2–He continuum at 560 and 781.14 cm , for the NH3 lines at 892.16, 893.0, and 921.25 cm , and for Ϫ1 the core of a PH3 line at 944.20 cm . These functions indicate the contribution of each layer to the outgoing emission at an airmass of 1.1. NH3 AND PH3 ON JUPITER 321
The continuum emission around the 921.25 cmϪ1 line originates from the 550 mbar region and is sensitive to the ammonia abundance in this region, in addition to the cloud
opacity. As a boundary condition, we considered a NH3 mixing ratio in the deep troposphere (Ն800 mbar) equal to 2.5 ϫ 10Ϫ4, i.e., twice the solar value. This abundance yielded a good reproduction of the continuum and of the far wings of the ammonia line when a constant cloud opac-
ity of 0.2 was assumed. Lower NH3 abundances required unrealistically high cloud optical depths (Ͼ0.5) in contra- diction with previous analyses (Gierasch et al. 1986, Grifﬁth et al. 1992). The radiance at the line core then established
the NH3 mixing ratio at 330 mbar. It should be noted that, while the cloud opacity affects the emission in the continuum, it has a negligible inﬂuence on the core of the line as illustrated in Fig. 3. The ammonia abundance retrieved above the ȁ380–mbar level is thus essentially unaffected by this parameter. Figure 4 shows a comparison between synthetic and ob- served spectra for a subset of latitudes at both longitudes 27Њ and 110Њ. Since the maximum of the CF is around 380 mbar, our results (Table II) are most reliable at that
pressure level. Figure 5 presents the variations of the NH3 mixing ratio with latitude at the two longitudes. Errors on the retrieved abundances mainly result from uncertainties in the pointing (about 1 pixel, i.e., 1 arcsec) and in the temperature model T(p). To assess the ﬁrst source, we ﬁtted the observed spectra with synthetic calcu- lations incorporating an airmass equal to that of the two adjacent pixels. These new ammonia mixing ratios differ from the nominal ones by ϩ24% and Ϫ18% (at 380 mb) in the least favorable case. Similarly, using a temperature proﬁle departing from the nominal one by Ϯ1 K at the FIG. 2. Latitudinal variations of the 200–mbar temperature (near tropopause yields ammonia mixing ratios which differ at the level of unit optical depth) retrieved from the 560 cmϪ1 radiance data most from the nominal ones by ϩ14% and Ϫ6%. This error at various System II longitudes. analysis indicates that at low airmasses (latitudes toward the equator) the major uncertainty comes from the temper- ance was also observed. Figure 1b displays the contribution ature model, while for higher airmasses (latitudes away function at the line center (921.25 cmϪ1) calculated at an from the equator) the pointing becomes the dominant er- airmass of 1.1 using the NH3 distribution given in Be´zard ror source. et al. (1997). Emission originates from a layer centered The latitudinal variation of the 380 mbar ammonia mix- around ȁ370 mbar and extends (at half maximum of the ing ratio is shown in Fig. 5 for the two longitudes. At the CF) from 420 to 310 mbar. GRS longitude (27Њ in System II), we found that this mixing ϫ Ϫ6 ϫ Ϫ6 The NH3 distribution resembles that used by Grifﬁth ratio varies from 2.4 10 to 4.5 10 in the latitude et al. (1992), that is (i) the NH3 mixing ratio in the lower range 13Њ–39ЊS (Table II). On the GRS itself, the ammonia troposphere (p Ն 800 mb) is constant, (ii) it decreases abundance is about 30% higher than the value at neigh- exponentially with the log of pressure above 800 mbar and boring latitudes, a variation which is only marginally sig- up to 330 mbar, (iii) above 330 mbar, the exponential niﬁcant. At 110Њ longitude, ammonia shows a pronounced decrease is steeper. The weak 921 cmϪ1 line provides infor- minimum in the 13Њ–21ЊS range with a mixing ratio around mation on the NH3 mixing ratio at the 370 mbar level. half that at other latitudes. The spatial variations in the Information above the 330 mbar level is available from NH3 concentration found in these two data sets exceed the 892 cmϪ1 line observations discussed below. We then the uncertainties discussed above and point to real hetero- chose to hold the NH3 mixing ratio constant at a value of geneities over the observed regions. However, the NH3 1 ϫ 10Ϫ9 at 160 mbar. concentration in the GRS itself does not signiﬁcantly differ 322 LARA ET AL.
Ϫ1 FIG. 3. Comparison between an observed spectrum around 921 cm and two synthetic spectra computed with the same qNH3 distribution, but with cloud (between 650 and 300 mb) opacities ranging from c ϭ 0.0 to c ϭ 0.5. Clouds primarily affect the continuum and not the line center, which probes only the very top of the ammonia cloud layer. from the mixing ratio at 110Њ longitude (observed at the depth) probe the 350- to 490-mbar region (Fig. 1b). We same latitude). The retrieved abundances correspond to a modeled these observations with the same NH3 proﬁle as range of NH3 humidity between 40 and 100%, based on above, but allowing two parameters to vary: the ammonia Atreya’s (1986) formula for NH3 saturation. Within uncer- mixing ratios at 330 and 160 mbar. tainties, a saturated proﬁle is found only in the 27Њ–30ЊS Spectra observed at a longitude of 287Њ are compared region at 100Њ longitude. with our best ﬁt synthetic spectra in Fig. 6. The associated
We note that the ammonia proﬁles derived from the NH3 distributions are plotted in Fig. 7. Table III provides Ϫ1 921.25 cm line (described above) also reproduce the 918.6 values of the NH3 mixing ratio at 240 and 380 mbar and and 923.6 cmϪ1 absorption features (Fig. 4) recorded at for the latitudes mapped in this work. We ﬁnd that the the same longitudes (27Њ and 110Њ). 380 mbar NH3 mixing ratio is 3 to 4 times lower at 10ЊS We also analyzed spectra of the stronger ammonia lines than that at more southern latitudes. The 240-mbar abun- located at 891.88 and 892.16 cmϪ1. The core of these lines dance is also lower at 10ЊS than in the 13–21ЊS region. It probes the ȁ270-mbar level (Fig. 1b) and complements appears that our 287Њ longitude scan hits a SEB hot spot information obtained from the 921.25 cmϪ1 line. Unfortu- characterized by a strong depletion in ammonia in the nately, for the longitudes where this doublet was recorded upper troposphere. At latitudes south of 18ЊS, the NH3 (230Њ–290Њ), we did not obtain 560.6 cmϪ1 observations to mixing ratio at 240 mbar decreases continuously from constrain the temperature proﬁle. We therefore relied on about 2 ϫ 10Ϫ8 at 20ЊSto1ϫ10Ϫ9 at 36ЊS, while at 380 the temperature models derived for the 110Њ longitude. mbar remains in the range 3–4 ϫ 10Ϫ6, similar to that Figure 2 shows that, outside the GRS, the temperature derived from the 921 cmϪ1 line at longitudes 27Њ and 110Њ.
ﬁeld is pretty much uniform with longitude. Errors in T(p)ofϮ1 K introduce variations in the NH3 The cores of the 892 cmϪ1 lines provide information on mixing ratio between ϩ30 and Ϫ21% at 240 mbar, while the 240- to 300-mbar region, and the wings (at half line at 380 mbar this variation lies between ϩ13% and Ϫ25%. NH3 AND PH3 ON JUPITER 323
FIG. 4. Comparison between a subset of observed (solid line) and synthetic spectra (dotted line) around 920–925 cmϪ1 at the GRS longitude (27Њ)(a), and around 918–923 cmϪ1 at 110Њ longitude (b). All absorption features are due to ammonia line opacity.
The spatial variations of ammonia derived in this study 954.44/954.64, and 973.97/974.17 cmϪ1 whose cores sample are thus well beyond modeling uncertainties. the 400- to 670-mbar region (Fig. 1). The continuum near
We also recorded the same spectral interval at 248Њ longi- the PH3 absorption features is sensitive to the NH3 vertical tude. The analysis of these spectra yielded results similar proﬁle and cloud opacity. to those derived at 287Њ (Table III). In particular, 10ЊSis We ﬁrst analyzed the spectral range 941.8–944.7 cmϪ1, characterized by a low concentration of ammonia when which includes, besides the PH3 feature, a line from compared with more southern latitudes. The 240-mbar 15 Ϫ1 the NH3 isotope at 943.03 cm . Although a detailed mixing ratio reaches a maximum near 15ЊS, consistent with 15 analysis of the observed NH3 lines was not undertaken 287Њ longitude data. The 380-mbar abundance shows less in this paper, a good reproduction of the nearby variation with a soft maximum around 18–26ЊS. continuum was needed before analyzing the phosphine absorption. In order to ﬁt the continuum, we had to c. Phosphine add some cloud opacity varying with latitude. The optical To study the phosphine distribution in Jupiter’s tropo- depth was chosen to reproduce the average continuum sphere, we investigate three doublets at 944.20/944.40, level between 943.5 and 944.5 cmϪ1. This allowed us to 324 LARA ET AL.
TABLE II is the level of maximum information) are listed in Table Ammonia Mixing Ratio at 380 mbar Derived from the IV. Formal error bars were calculated in the same way as ؊1 921 cm Absorption for ammonia, that is by considering a Ϯ1-arcsec pointing uncertainty and a Ϯ1 K error in the temperature model. 27Њ Longitude (GRS) 110Њ Longitude Because the probed levels are relatively deep, variations Latitude (Њ) Airmass q a Latitude (Њ) Airmass q a of our temperature model induce only very small changes NH3 NH3 in the best ﬁt PH3 proﬁle. Therefore, the most important Ϫ13 1.30 4.5(Ϫ6) Ϫ13 1.13 3.0(Ϫ6) uncertainties come from the 1-arcsec error assumed for Ϫ15 1.33 2.4(Ϫ6) Ϫ15 1.16 2.0(Ϫ6) the pointing. This uncertainty gives rise to a 2–9% uncer- Ϫ18 1.37 2.4(Ϫ6) Ϫ18 1.18 2.2(Ϫ6) Ϫ21 1.42 3.1(Ϫ6) Ϫ21 1.21 3.5(Ϫ6) tainty in the 580-mbar PH3 mixing ratio. Uncertainties Ϫ24 1.48 2.4(Ϫ6) Ϫ24 1.25 4.3(Ϫ6) from pixel-to-pixel calibration add a 2–3% error on the Ϫ27 1.55 2.4(Ϫ6) Ϫ27 1.29 5.2(Ϫ6) phosphine abundance. Ϫ30 1.65 2.4(Ϫ6) Ϫ30 1.35 5.2(Ϫ6) In order to determine whether the abundance of phos- Ϫ33 1.77 3.1(Ϫ6) Ϫ33 1.42 3.9(Ϫ6) phine differs at the GRS compared to other longitudes, Ϫ36 1.95 3.6(Ϫ6) Ϫ36 1.50 3.9(Ϫ6) Ϫ39 2.21 3.6(Ϫ6) Ϫ39 1.60 4.5(Ϫ6) we analyzed spectra recorded at 100Њ and 200Њ.At200Њ longitude, the observed spectra encompass the same spec- a 5.4(Ϫ6) ϭ 5.4 ϫ 10Ϫ6. tral range as do observations at the GRS longitude. Using the same type of distribution as described above, we calcu- lated the synthetic spectra displayed in Fig. 9. The corre- obtain an excellent ﬁt of the phosphine absorption fea- sponding phosphine mixing ratios are listed in Table IV. tures. At 100Њ longitude, phosphine lines were observed at 954.44, We analyzed GRS observations with the temperature 954.64, and 955.23 cmϪ1; synthetic spectra that best agree proﬁles derived from the 560.6 cmϪ1 data taken at the same with these observations (Fig. 10) were obtained with the longitude. Because the PH3 lines are optically thin, a simple 580-mbar PH3 mixing ratios given in Table IV. Figure 11 one–parameter distribution was used to determine its shows the variation of the 580-mbar mixing ratio with lati- abundance near 580 mbar. Below 1250 mbar, the PH3 mix- tude at the three observed longitudes. The error bars range ing ratio was ﬁxed to 7 ϫ 10Ϫ7 (Bjoraker et al. 1986, Grifﬁth from4to10%. We ﬁnd that phosphine at 580 mbar is not et al. 1992); above this level, we assumed a linear variation enhanced over the GRS within error bars. In contrast with of the log of the mixing ratio with the log of the pressure. ammonia, there is no abrupt variation of the mixing ratio Figure 8 shows a comparison between the observations and with latitude, and only a moderate and smooth southward synthetic spectra generated with the best ﬁtting phosphine decrease is found. Also, there is no marked longitudinal distributions. Mixing ratios retrieved at 580 mbar (which variations of the PH3 distribution at this pressure level.
a. Temperature The temperature ﬁeld in the upper troposphere of Jupi- ter was mapped by Gierasch et al. (1986) using Voyager infrared data. They produced zonal averages of the temper- ature at the 150- and 270-mbar levels and did not investi- gate longitudinal variations. With this caveat in mind, our results show the same latitudinal behavior. At 200 mbar, a cold region is found around the STZ between 20Њ and 25ЊS, with temperatures between 107 and 109 K, in good agreement with Gierasch et al.’s (1986) retrievals. Temper- atures increase toward northern latitudes, and reach 110– 112 K at the SEB, also consistent with Voyager IRIS data. Southward of 20Њ–25ЊS, the 200-mbar temperature in- creases and at most longitudes reaches a plateau at about FIG. 5. Ammonia mixing ratio at 380 mbar as a function of latitude. 30ЊS. This behavior was also observed in the temperature Solid line, NH3 abundance at the GRS longitude. Dashed line, NH3 abundance at 110Њ longitude. Error bars, given for the GRS longitude, ﬁeld derived by Gierasch et al. from the Voyager 2 observa- are mainly associated with uncertainties in the thermal proﬁle and in tions. Voyager 1 data indicate instead a steady increase the pointing. in temperature from 25Њ to 50Њ S. Our analysis provides NH3 AND PH3 ON JUPITER 325
FIG. 6. Comparison between observed (solid line) and synthetic spectra (dashed line) between 891 and 893.5 cmϪ1 at 287Њ longitude for a subset of observed latitudes. The absorption feature is due to the aP(2, K) ammonia doublet.
latitudinal scans of the tropospheric temperature at differ- in 1979 and from the Irshell/IRTF observations in 1991 ent longitudes (Fig. 2). The latitudinal dependence of the points to the stability of the jovian circulation pattern. temperature ﬁeld was similar at all longitudes investigated (27Њ, 110Њ,5Њ,40Њ, 140Њ) as indicated in Fig. 2 and described b. Ammonia above. The temperature contrasts were clearly more pro- nounced at the GRS longitude (27Њ) than at other longi- We found that the ammonia abundance around 380 mbar tudes as was the case for Voyager observations (Flasar was about 30% larger over the GRS than at neighboring et al. 1981, Conrath et al. 1986). latitudes (Fig. 5). This enhancement is however only mar- The observed latitudinal pattern of the temperature ﬁeld ginally signiﬁcant with respect to modeling uncertainties. very likely reﬂects the general circulation at work in Jupi- Analyzing various selections of Voyager IRIS spectra, ter’s atmosphere as discussed by Gierasch et al. (1986). Grifﬁth et al. (1992) concluded that the NH3 abundance Upward motion in the STZ and other ‘‘zones’’ is associated below 350 mbar was similar over the STZ and the GRS with adiabatic cooling and colder temperatures, while within Ϫ25 and ϩ50%. The two studies thus agree that downward motion in the SEB and more generally in there is no pronounced enhancement in the NH3 abun- ‘‘belts’’ produces warmer temperatures. The most pro- dance over the GRS in the 350- to 400-mbar region. The nounced temperature minimum at the GRS location simi- mixing ratio at 380 mbar we obtained for the GRS and larly results from the vigorous rising motion characterizing the STZ (2.4–3.1 ϫ 10Ϫ6) is about half that derived by this peculiar anticyclonic feature. The similarity of the tem- Grifﬁth et al. (4.5–5.5 ϫ 10Ϫ6 with an uncertainty of perature maps obtained from Voyager spectra recorded ϩ2.5/Ϫ1.0 ϫ 10Ϫ6). The disagreement is however at the 326 LARA ET AL.
FIG. 7. (a) Ammonia distributions giving the best ﬁts of the 287Њ spectra shown in Fig. 6. At 10ЊS, the observations hit a ‘‘hot spot’’ characterized by a strong depletion of the ammonia abundance compared to the neighboring southward latitudes. (b) Latitudinal variation of the ammonia mixing ratio at 380 and 240 mbar retrieved from the core and the wings of the 892 cmϪ1 absorption feature; thin line, 287Њ longitude; thick line, 248Њ longitude.
limit of the estimated modeling uncertainties and probably abundance ȁ4 times lower than in the STZ. Unfortunately, arises in part from the choice of slightly different anchor we have no information on the GRS at such pressure levels points in the NH3 vertical distribution. that could conﬁrm this ﬁnding. Observations of the Ϫ1 On the other hand, Grifﬁth et al. found that NH3 around 892 cm ammonia feature, which probe the 240 mbar 300 mbar was strongly depleted over the GRS, with an region, were conducted only at longitudes distinct from NH3 AND PH3 ON JUPITER 327
TABLE III Ammonia Mixing Ratio Derived from the 892 cm؊1 Absorption
287Њ Longitude 248Њ Longitude
Latitude (Њ) Airmass 240 mbar 380 mbar Airmass 240 mbar 380 mbar
Ϫ10 1.02 1.2(Ϫ8) 8.8(Ϫ7) 1.16 1.7(Ϫ8) 2.4(Ϫ6) Ϫ13 1.03 2.1(Ϫ8) 1.3(Ϫ6) 1.18 3.4(Ϫ8) 2.6(Ϫ6) Ϫ15 1.05 4.3(Ϫ8) 2.4(Ϫ6) 1.20 4.4(Ϫ8) 3.0(Ϫ6) Ϫ18 1.07 4.8(Ϫ8) 2.8(Ϫ6) 1.23 3.1(Ϫ8) 3.7(Ϫ6) Ϫ20 1.09 2.3(Ϫ8) 3.6(Ϫ6) 1.27 2.3(Ϫ8) 3.7(Ϫ6) Ϫ23 1.12 8.8(Ϫ9) 3.9(Ϫ6) 1.32 8.8(Ϫ9) 3.9(Ϫ6) Ϫ26 1.16 8.5(Ϫ9) 3.8(Ϫ6) 1.37 3.0(Ϫ9) 3.7(Ϫ6) Ϫ29 1.19 9.6(Ϫ10) 3.0(Ϫ6) 1.43 2.1(Ϫ9) 3.4(Ϫ6) Ϫ32 1.24 1.6(Ϫ9) 3.0(Ϫ6) 1.52 1.0(Ϫ9) 3.4(Ϫ6) Ϫ36 1.30 1.0(Ϫ9) 3.4(Ϫ6) 1.62 1.0(Ϫ9) 3.4(Ϫ6)
FIG. 8. Comparison between observed (solid line) and synthetic spectra (dashed line) of the phosphine doublets at 944.20/944.40 cmϪ1 , for the 15 Ϫ1 GRS longitude and a subset of observed latitudes. Besides the PH3 feature, a line from the NH3 isotope at 943.03 cm is also present in the observed interval. 328 LARA ET AL.
TABLE IV Phosphine Mixing Ratio at 580 mbar
27Њ (GRS) 100Њ 200Њ
Latitude Airmass 580 mbar Latitude Airmass 580 mbar Latitude Airmass 580 mbar
Ϫ10 1.14 2.0(Ϫ7) Ϫ10 1.04 2.6(Ϫ7) Ϫ13 1.16 1.9(Ϫ7) Ϫ13 1.06 2.4(Ϫ7) Ϫ16 1.18 1.9(Ϫ7) Ϫ16 1.07 2.3(Ϫ7) Ϫ17 1.09 2.5(Ϫ7) Ϫ18 1.21 1.9(Ϫ7) Ϫ18 1.09 2.3(Ϫ7) Ϫ20 1.11 2.3(Ϫ7) Ϫ21 1.24 1.8(Ϫ7) Ϫ21 1.12 2.3(Ϫ7) Ϫ23 1.14 2.1(Ϫ7) Ϫ24 1.28 1.7(Ϫ7) Ϫ24 1.15 2.1(Ϫ7) Ϫ25 1.18 2.1(Ϫ7) Ϫ27 1.33 1.8(Ϫ7) Ϫ27 1.18 2.1(Ϫ7) Ϫ28 1.22 2.1(Ϫ7) Ϫ30 1.39 1.8(Ϫ7) Ϫ30 1.23 2.0(Ϫ7) Ϫ31 1.27 2.2(Ϫ7) Ϫ33 1.47 1.7(Ϫ7) Ϫ33 1.28 2.0(Ϫ7) Ϫ35 1.33 2.1(Ϫ7) Ϫ36 1.57 1.7(Ϫ7) Ϫ38 1.40 1.9(Ϫ7) Ϫ42 1.49 1.9(Ϫ7) Ϫ46 1.62 1.8(Ϫ7)
FIG. 9. Same as in Fig. 8, but for 200Њ longitude. NH3 AND PH3 ON JUPITER 329
FIG. 10. Comparison between observed (solid line) and synthetic spectra (dashed line) of the phosphine doublet at 954.44/954.64 cmϪ1, for Ϫ1 100Њ longitude and a subset of observed latitudes. The absorption feature centered at 955.23 cm also results from a PH3 line. the GRS because of technical constraints. An analysis of equatorward with respect to the temperature ﬁeld; in addi- 10–Ȑm spectra of the NTrZ, NEB, and GRS similarly tion, the ammonia concentration at 240 mbar decreases concluded that ammonia above ȁ350 mbar decreased with regularly southward of 18ЊS whereas temperature reaches height much faster over the GRS than in other regions a plateau southward of 28ЊS at most longitudes.
(Tokunaga et al. 1980). The unexpected depletion of am- The NH3 distribution around 240 mbar (above the cloud monia in the upper tropospheric levels of the GRS could tops) is well below saturation and probably reﬂects a bal- have a dynamical origin or result from some unknown ance between vertical mixing and photochemical destruc- chemical process (Grifﬁth et al. 1992). tion. Regions of upwelling motion are expected to be asso- Analysis of the 892 cmϪ1 absorption feature allowed us ciated with lower temperatures (due to adiabatic cooling) to retrieve the NH3 mixing ratio in two pressure regions and enhanced ammonia concentrations in the upper tropo- centered at 380 and 240 mbar. At the two longitudes ob- sphere. The lack of any apparent anticorrelation between served, the NH3 proﬁle exhibits a similar behavior (Table temperature and NH3 abundance near 240 mbar suggests III). The 240-mbar mixing ratio varies by more than an that large–scale vertical motions are not driving the NH3 order of magnitude over the latitude range scanned, with distribution at these levels. Small–scale turbulence, com- a maximum around 15Њ–18ЊS. The NH3 mixing ratio scale monly parametrized with the so–called eddy diffusion coef- height we derive varies from 2.4 km in the 13Њ–18ЊS region ﬁcient (K), could be more important for atmospheric mix- down to 1.2 km southward of 30ЊS. This latitudinal varia- ing. Vertical velocities (w), derived from thermal wind tion does not appear to be signiﬁcantly anticorrelated with shear determinations, vary from about Ϫ0.5 to 1.2 ϫ 10Ϫ9 that of the temperature. The maximum appears shifted scale height sϪ1 in the latitude range we observed (Gierasch 330 LARA ET AL.
FIG. 11. Phosphine mixing ratio at 580 mbar as a function of latitude for three different longitudes: 27Њ (GRS), 100Њ, and 200Њ. Error bars, mainly from pointing uncertainties, are indicated for some latitudes among the observed regions.
et al. 1986). With the NH3 scale heights inferred from In fact, considering the uncertainties associated with the the Irshell data, we ﬁnd that eddy mixing prevails over determination of K from Eq. (1), much smaller values of organized atmospheric motion if K is larger than 250–500 K could be allowed for the 30Њ–35Њ S region, in which case cm2 sϪ1. In a one–dimensional framework, K can be de- atmospheric mixing in this region would be dominated by rived from the upward vertical motion of ȁ1 ϫ 10Ϫ9 scale height sϪ1 (Gierasch et al. 1986). At 13Њ–18ЊS, the upward motion 2 K ϭ JHq Ϫ wHq , (1) that would be required to produce the inferred NH3 scale height would be ȁ0.015 cm sϪ1, much larger than the range where J is the net photochemical loss rate, and Hq is the of vertical velocities determined by Gierasch et al. (1986). NH3 mixing ratio scale height (assumed to be much smaller An interpretation of our 240-mbar NH3 data in the than the atmospheric scale height). framework of this simpliﬁed model is that the eddy mixing The photolysis rate for gaseous ammonia at high alti- coefﬁcient is at least 10 times larger at 13Њ–18ЊS than at Ϫ6 Ϫ1 tudes with the Sun at zenith (Jȍ) is about 3 ϫ 10 s 30Њ–35ЊS. We see no obvious explanation for such a behav- (Atreya 1986). The Rayleigh scattering optical depth at ior. It can still be noted that the standard deviation of the
240 mbar is 1.5 at a wavelength of 195 nm, where the NH3 270-mbar temperature ﬁeld as inferred by Gierasch et al. cross–section is maximum. Including the attenuation of shows a pronounced maximum over the retrograde jet at the solar ﬂux due to Rayleigh scattering and averaging 18ЊS. This maximum may reﬂect a large jet instability, over solar angle, we derive a diurnal average of the photol- generating large and/or small scale eddies that could domi- ysis rate equal to 0.045 Jȍ at 15Њ latitude and 0.032 Jȍ at nate the transport of ammonia above the cloud tops. It 30Њ. Further introducing a 0.4 factor to account for recycling should also be noted that the ortho–para hydrogen ratio of NH3 from its photochemical products (Strobel 1973), shows a departure from equilibrium that is maximum at we ﬁnd J values at 240 mbar of 6 ϫ 10Ϫ8 sϪ1 at 15Њ and low latitudes and decreases northward and southward 4 ϫ 10Ϫ8 sϪ1 at 30Њ. Equation (1) then yields K Ȃ 4000 cm2 (Conrath and Gierasch 1984). Smaller scale variations that sϪ1 at 13Њ–18ЊS and Ȃ400 cm2 sϪ1 at 30Њ–35ЊS. The latter correlate with the temperature ﬁeld are superposed on this value is so small that eddy mixing becomes comparable to general pattern (Gierasch et al. 1986). The global equator– vertical motion in determining the NH3 vertical proﬁle. to–pole variation of the disequilibrium ortho–para ratio NH3 AND PH3 ON JUPITER 331 can be interpreted by stronger eddy mixing or a large–scale porally under the inﬂuence of meteorological features as upwelling at low latitudes, possibly resulting from the does water vapor on Earth. larger insolation (Conrath and Gierasch 1984). In both c. Phosphine interpretations, it remains to understand why the signature of such a variation in the diffusion or circulation pattern Phosphine near 580 mbar does not exhibit the large is not visible in the thermal ﬁeld. Nonetheless, it appears variability seen for ammonia. Its mixing ratio smoothly that the large–scale variation of both the ortho–para ratio decreases southward, by 20–30% over the latitude range and the 240-mbar NH3 mixing ratio can be interpreted scanned. In addition, there is no increase of the PH3 mixing with a similar kind of variation in the eddy mixing or within ratio over the GRS within 10%, which suggests that phos- a global low–latitude upwelling/high–latitude downwelling phine is not the precursor for the reddish chromophores. scheme superimposed on the zone–belt pattern. Analyzing Voyager IRIS spectra, Grifﬁth et al. (1992) simi- The value of the eddy mixing coefﬁcient we infer (ȁ4000 larly concluded that phosphine at 600 mbar was not en- cm2 sϪ1 at most) is smaller than that estimated by Strobel hanced over the GRS, with however a lower sensitivity (1973) in his pioneering investigation of the photochemis- (Ϫ55%, ϩ75%). Our mixing ratio at 580 mbar over the GRS (2.3 Ϯ 0.1 ϫ 10Ϫ7) agrees with Grifﬁth et al.’s retrieval try of NH3 in Jupiter. The reason is that our determination within uncertainties. of the NH3 scale height pertains to a region (240 mbar) where the photolysis rate J is lowered due to Rayleigh The PH3 mixing ratio at 580 mbar is about 3 times lower than at pressure levels of a few bars (Bjoraker et al. 1986). scattering from H2 and He. The value of K we derive from At this level, phosphine is therefore already depleted either Eq. (1) is accordingly smaller than when Jȍ is assumed in place of J (Strobel 1973). by direct photolysis or chemical reactions with other com- pounds (Strobel 1983). Upward motion should locally en- The 380-mbar NH3 mixing ratio at 248Њ and 287Њ steadily increases from the SEB (10ЊS) to about 18Њ–20ЊS and re- hance the phosphine concentration. Because this is not mains approximately constant southwards at ȁ3.5 ϫ 10Ϫ6 observed over the GRS, we tend to conclude that upward (Table III). At the longitude of 287Њ, the 10ЊS mixing ratio motion does not dominate the transport of phosphine near is particularly low (3 times less than at 248Њ), suggesting 580 mbar and that eddy mixing may be more important. that this region corresponds to a SEB ‘‘hot’’ spot associated Assuming a PH3 mixing ratio scale height of about 20 km (Kunde et al. 1982) and a vertical velocity of 0.005 cm sϪ1 with a strong downdraft. The NH humidity dips to ȁ13% 3 over the GRS (Conrath et al. 1981), K Ͼ 1 ϫ 104 cm2 sϪ1 using Atreya’s (1986) saturation law. The results at these is enough to ensure that eddy mixing is more efﬁcient two longitudes differ from those obtained from the 921 than upward motion in this regard. This condition is likely cmϪ1 absorption line at longitudes of 27Њ (GRS) and 100Њ. fulﬁlled at 580 mbar as K should increase downward from Our limited data set shows longitudinal variations its 240 mbar value (ȁ400–4000 cm2 sϪ1 from our NH data) amounting to about a factor of 3 for a given latitude. There 3 up to ȁ108 cm2 sϪ1 below the ȁ0.7–bar region where free is no apparent correlation between the latitudinal variation convection should dominate. of ammonia at 380 mbar (Figs. 5, 7) and the temperature The slight and regular decrease of the phosphine abun- ﬁeld near 200 mbar (Fig. 2). The only general behavior is dance southward may also reﬂect a concomitant increase that the NH mixing ratio tends to increase by roughly 3 in the eddy mixing coefﬁcient. Latitudinal variations in Њ Њ Њ 50% from the 10 –18 S range to 33 S and southward. The the solar ﬂux, affecting the photolysis rate, would tend to strongest longitudinal variations occur at the southern edge produce an opposite variation and cannot thus be invoked. of the SEB (10Њ–13ЊS) and of the STZ (25Њ–30ЊS). Voyager The variation of K near 580 mbar required to interpret infrared data indicate a steady increase in the ammonia the PH3 variations is in the same direction as that discussed abundance near the cloud base from the 10Њ–18ЊS region above for the NH3 at 240 mbar, i.e., a southward increase to the 27Њ–35ЊS, with a high longitudinal variability over the latitude range we observed. Alternately, varia- (Gierasch et al. 1986). The correlation with the zonal aver- tions in the deep mixing ratio, possibly associated with a age of the temperature ﬁeld appears also very weak in the change in the convective mixing of the interior, could ex- Voyager data. plain the observed variation. From high–resolution 5-Ȑm The high variability of ammonia at 380 mbar may result observations, Drossart et al. (1990) found a 60% enhance- from meteorological processes. In the whole data set inves- ment in the phosphine abundance below the 1–bar level tigated here, the NH3 humidity at this pressure level varies near 50ЊN as compared to the NEB and the GRS. These in the range 15–100%. This physical parameter is probably observations can be interpreted through an enhanced determined by a balance between condensation losses and strength of the convective mixing at high northern lati- transport of material (horizontal and vertical) averaged tudes. The ‘‘quenching’’ level of this compound would over our ﬁeld of view of about 8000 km. The NH3 humidity then take place at a higher temperature level where its in the condensation region may vary horizontally and tem- abundance is larger (Fegley and Lodders 1994). Our data 332 LARA ET AL. would accordingly require that the convective mixing trans- ACKNOWLEDGMENTS port from the interior gets more sluggish from equator to pole in the southern hemisphere. Observations with Irshell were supported by NSF Grant AST-9020292. We are grateful to the staff and management of the NASA/Infrared Telescope Facility for their support, and to J. Achtermann for his help during the observations. L-M.L. was partially supported by the Spanish 5. CONCLUSIONS Comisio´ n Interministerial de Ciencia y Tecnologı´a under contracts ESP 93–0338 and ESP 94–0719. B.B. acknowledges support from the ‘‘Pro- We have investigated spatial variations of the ammonia gramme National de Plane´tologie’’. and phosphine vertical proﬁle in the 10Њ–40ЊS range and at various longitudes including the Great Red Spot. Infor- REFERENCES mation on the temperature near 200 mbar was retrieved simultaneously from observations at 17.8 Ȑm. We found Achtermann, J. M. 1992. 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