Meridional transport of HCN from SL9 impacts on Jupiter

Transcription

1 P.1 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 1 3 Icarus ( ) Meridional transport of HCN from SL9 impacts on Jupiter Caitlin A. Griffith, a,,1 Bruno Bézard, b Thomas Greathouse, c Emmanuel Lellouch, b John Lacy, c Douglas Kelly, d and Matthew J. Richter e 14 Department of Planetary Science University of Arizona, Tucson, AZ 85721, USA 70 LESIA, Observatoire de Paris, Meudon, France University of Texas, Austin, TX, USA 16 d Steward Observatory, Tucson, AZ, USA e Physics Department, U.C. Davis, Davis, CA 95616, USA Received 10 September 2003; revised 30 January Abstract In July 1994, the Shoemaker Levy 9 (SL9) impacts introduced hydrogen cyanide (HCN) to Jupiter at a well confined latitude band around 44, over a range of specific longitudes corresponding to each of the 21 fragments (Bézard et al. 1997, Icarus 125, ). This newcomer to Jupiter s stratosphere traces jovian dynamics. HCN rapidly mixed with longitude, so that observations recorded later than several months after impact witnessed primarily the meridional transport of HCN north and south of the impact latitude band. We report spatially resolved spectroscopy of HCN emission 10 months and 6 years following the impacts. We detect a total mass of HCN in Jupiter s stratosphere of 1.5 ± g in 1995 and 1.7 ± g in 2000, comparable to that observed several days following the impacts (Bézard et al. 1997, Icarus 125, ). In 1995, 10 months after impact, HCN spread to 30 and 65 latitude (half column masses), consistent with a horizontal eddy diffusion coefficient of K yy = cm 2 s 1. Six years following impact HCN is observed in the northern hemisphere, while still being concentrated at 44 south latitude. Our meridional distribution of HCN suggests that mixing occurred rapidly north of the equator, with K yy = cm 2 s 1, consistent with the findings of Moreno et al. (2003, Planet. Space Sci. 51, ) and Lellouch et al. (2002, Icarus 159, ). These inferred eddy diffusion coefficients for Jupiter s stratosphere at mbar generally exceed those that characterize transport on Earth. The low abundance of HCN detected at high polar latitudes suggests that, like on Earth, polar regions are dynamically isolated from lower latitudes Elsevier Inc. All rights reserved. Keywords: Jupiter; Atmosphere; Dynamics; IR spectroscopy 1. Introduction The dynamics of the jovian atmosphere is best understood in the troposphere at 0.5 bar where NH 3 clouds trace zonal circulation and the effects of convective upwelling. A weak meridional flow, coupled with vertical transport, arises from frictional drag on the strong zonal flow. Cloud motions at 0.68 bar reveal jet velocities ranging from 70 m s 1 westerly to 150 m s 1 easterly depending on the latitude (Limaye, 1986; García-Melendo and Sánchez-Lavega, 2001). The measurement of high zonal winds ( m s 1 )at 24 bars by the Galileo Probe, albeit of a singular and par- ticularly dry atmospheric region, suggests that the energy responsible for Jupiter s observed weather originates from the interior (Atkinson et al., 1996). Less is known about the dynamics in Jupiter s stratosphere. Zonal averages of Jupiter s temperature at the 150 and 270 mbar levels indicate a decay in the jet velocities with a vertical scale of 2 3 scale heights (Gierasch et al., 1986). Jupiter s circulation at higher altitudes was estimated with models that include seasonally varied insolation, radiative heating and cooling by gases, and frictional damping to drive meridional circulation (Conrath et al., 1990). West et al. (1992) added the effects of aerosol heating. Moreno and Sedano (1997) also included the heating due to aerosols, using an updated spatial opacity distribution for the haze determined from HST images of Jupiter * 110 Corresponding author. 55 address: griffith@lpl.arizona.edu (C.A. Griffith). The aerosol distribution of West et al. (1992) indicates a Visitor at the Observatoire de Paris at Meudon in June 2002 and poleward drift for air below the 10 mbar pressure level, /$ see front matter 2004 Elsevier Inc. All rights reserved. doi: /j.icarus

2 P.2(1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 2 2 C.A. Griffith et al. / Icarus ( ) an equatorward drift for air above this level, and, at northern latitudes, a poleward drift above 1 mbar. Moreno and Sedano s aerosol distribution indicates a different circulation above the 50 mbar pressure level, where air drifts from equator to pole with the exception of the south polar region between 2 and 20 mbar. Both models (West et al., 1992; MorenoandSedano,1997) calculate the residual meridional circulation remaining after the eddy heat flux divergence is subtracted from the adiabatic cooling. The meridional circulation then results mainly from diabatic heating. On Earth, such residual circulation approximates the mean motion of air parcels and thus the transport of tracers. The SL9 impact illuminates the local dynamics of the cyclonic and anticyclonic storms (Simon and Beebe, 1996), and, for the first time, the global circulation in Jupiter s stratosphere. The comet added dust to Jupiter s mbar level and the gases H 2 O, CO 2, CO, CS, and HCN, to Jupiter s upper stratosphere, above mbar (West et al., 1995; Marten et al., 1995; Lellouch et al., 1995, 1997; Moreno, 1998; Bézard et al., 1997; Griffith et al., 1997; Moreno et al., 2003). These pioneer tracers of jovian stratospheric circulation mixed rapidly in longitude so that the impact sites were no longer discernible several months after impact. Subsequent observations of the dust and gases thus revealed the meridional mixing of Jupiter s stratosphere. Cometary dust, observed one month to 3.3 years following the impacts (West et al., 1995; Friedson et al., 1999), drifted towards the equator faster than predicted by West et al. (1992) and Moreno and Sedano (1997). Friedson et al. (1999) concluded, as a result, that the residual meridional circulation does not represent the transport of tracers in Jupiter s atmosphere. They hypothesize that wave transience and nonlinearity dominate diabatic heating in driving vertical motions and thus meridional transport. Friedson et al. estimate the eddy diffusion in the mbar region from Eliassen Palm flux, which quantifies the forcing due to eddies on the zonal winds. The eddy diffusion values derived, 34 Table observations # Date (UT) Longitude Latitude coverage a coverage May 16, 11: , K yy = cm 2 s 1, agree well with the observed 41 12: , spread of dust from SL9 (Friedson et al., 1999). 16 May 18, 09: , :59 47 In Jupiter s upper stratosphere (above 1 mbar), impactproduced gases provide the only tracers of horizontal trans , , : port. Morenoet al. (2003) and Lellouchet al. (2002) find that At 44 latitude, System III. The following impact sites are covered 99 by our data: E ( H 2 O, CO 2, CO, CS, and HCN diffused from the southern ), L (352 ), R + Q2 (47 ), Q1 (64 ), B (71 ), N (73 ), 100 H (101 ). to northern hemisphere in 3 years with an eddy diffusion coefficient 10 times larger than the dust diffusion at lower Table 2 47 altitudes observations 48 This paper investigates the dynamics of Jupiter s stratosphere with observations of the meridional migration of Wavenumber 104 # Date Time Longitude Noise (UT) cm 1 50 HCN. Under the usual quiescent conditions prior to impact, HCN was undetectable in the stratosphere with an up /2/00 10: per limit on its mixing ratio of /2/00 10: above 55 mbar /30/00 13: (Bézard et al., 1995). Following the SL9 impacts, HCN 4 11/30/00 13: /2/00 11: appeared in Jupiter s stratosphere above 0.1 mbar (Marten et al., 1995) in disks as large as 14,000 kilometers in diameter, centered at the impact sites (Bézard et al., 1997; Hammel et al., 1995). The large sizes of the disks indicate that HCN formed from the chemistry of the hot impactproduced plume as it fell on Jupiter s atmosphere (Griffith et al., 1997). Millimeter observations during the impacts (Marten et al., 1995), and three years later (Moreno et al., 2003), indicate that HCN had diffused downwards by less than a scale height and that its total mass remained constant within 30%, the uncertainty of the measurements. Vertical diffusion models (Lellouch et al., 2002) indicate that the material deposited at 0.1 mbar by impact chemistry should diffuse down to 0.5 mbar after 5 years. Because of its relatively slow vertical diffusion and its stability against photolysis (Moses et al., 1995), HCN can be regarded as a chemically inert tracer of horizontal dynamics in Jupiter s stratosphere. Hydrogen cyanide exhibits strong emission features at infrared wavelengths, allowing us to image, at relatively high spatial resolution ( ), the progression of HCN towards the north and south of the impact latitude. We present spectral images recorded 10 months and 6.5 years following the impacts. The 1995 observations reveal rapid zonal transport of HCN, and slow meridional transport north and south of the impact sites, such that HCN remained south of the equator. HCN had spread to the northern hemisphere by 1997 (Moreno et al., 2003), and to high northern latitudes by Yet, despite HCN s fast northward migration, HCN was still concentrated at the impact latitude in We first present our spectral images from 1995 and 2000, followed by the radiative transfer analysis of each spectrum therein. To quantify the spread of HCN with latitude, we use a simple model parametrized with a constant horizontal 55 a 1σ noise level determined from continuum points near HCN emission lines (erg s 1 cm 2 sr 1 cm). 112

3 P.3 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 3 Meridional transport of HCN from SL9 impacts on Jupiter 3 17 Fig. 1. Examples of 1995 data and Gaussian models (solid lines) of , , and cm 1, respectively (Table 2, 73 the emission features. The strong R(5) C 18 2 H 2 line of the ν 5 band at cm 1 Fig. 3). We also observed lines from the ν helps establish the position and width of the nearby HCN 4 band of CH 4 line. The weak line at cm 1 is the R(6) line of 13 C 12 CH 2.The2σ and the ν 5 band of C 2 H 2 to constrain the thermal profile of 20 noise level is ergs s 1 cm 2 sr 1 cm. Spectra are Doppler shifted Jupiter sstratosphere (Table 2, observation5). Centralscans roughly 0.04 cm eddy diffusion coefficient, K yy. The paper closes with a discussion of meridional transport in Jupiter s atmosphere, and comparisons of jovian and terrestrial transport characteristics. 2. Observations spectra On May , we recorded spectral images of Jupiter at the NASA Infrared Telescope Facility (IRTF) at Mauna Kea, Hawaii, using the U. Texas mid-infrared echelle spectrometer Irshell, a 11 spatial by 64 spectral array. Observations focused on the R10 rotational transition of the ν 2 HCN fundamental at cm 1 (Fig. 1). We opened the slit to 1.4 arcsec, corresponding to a spectral resolution of 16,000 and a pixel size of arcsec. However, the detector plane was slightly out of focus and the actual spatial resolution was 2.5 arcsec, providing a spectral resolution of This corresponds to a 10 widthinlatitude at the impact site latitude ( 44 ) on Jupiter. Images are created by positioning the slit north of the impact site, oriented along the jovian east to west direction, and scanning the slit south in 1 arcsec steps. Observation times and positions are indicated in Table 1 and Fig. 2. The initial position of the slit is defined by centering the array on a bright star through the 6th pixel, the array s center. Each scan contains the jovian limb, which was used later to verify our position. A bright region of emission at 15 latitude, contained in the South Equatorial Belt, provides a second constraint for position. We removed atmospheric absorption and flat fielded following the methods of Lacy et al. (1989). Intensity calibration is corrected following Griffith et al. (1997). We multiplied the radiance by 1.15 to correct for the illumination of the secondarymirror (Bézardet al., 1995). Flat fielding provides the major source of uncertainty of a few percent, rather than Poisson noise (S/N > 200) at the HCN line spectralregion (Fig. 1) spectra To record the spread of HCN in Jupiter s stratosphere, we obtained spectra of Jupiter in November and December of 2000 at the IRTF using TEXES, a high resolution cross-dispersed echelle spectrograph (R = 75,000) designed at the University of Texas (Lacy et al., 2002). The spectral region ( cm 1 ) recorded exhibits the R10, R11, and R12 rotational transitions of the ν 2 fundamental of HCN at of Jupiter were created by orienting the length of the slit along the central meridian, positioning the slit initially at the north pole. Each observation was formed from a set of slit positions, which stepped the down the meridian, through the sub-earth point, and on to the southern pole. Throughout the text, we designate each spectrum in terms of the spectral position, the slit position and observation numbers. These correspond respectively to the spectral position within the slit, the position of the slit on Jupiter s disk, and the observation whose suit of slit positions scan Jupiter s north to south pole. A step of 4.3 arcsec allowed for significant overlap in each consecutive slit position, even though only the inner 9.6 arcsec of the 12.5 long slit are considered. The outer pixels of TEXES typically display reduced signal, a result of differential illumination of the slit, and are thus discarded. The inner 32 pixels, having an aperture of are binned in groups of 4 spectra along the slit axis to form pixel apertures, , more similar to the atmospheric seeing. Figure 4 exhibits the averages of the continuum emissions of the first four observations of Table 2, indicating how we confirm the position of each slit placement and the location of Jupiter s limb, needed to derive the jovian airmass of each spectrum. 3. Spectral calculation We interpret the 1995 and 2000 spectra with a radiative transfer calculation that includes line by line techniques. Analyses incorporate the continuum opacity provided by H 2 and He at mole fractions of and 0.10, respectively. Calculations of collision-induced opacity from H 2 H 2 and H 2 He follow the formalism of Birnbaum and Cohen (1976) and Cohen et al. (1982), and include the laboratory measurements of Dore et al. (1983) and Bachet (1988). Helium and hydrogen-broadened coefficients for HCN, C 2 H 2,andCH 4

4 P.4 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 4 4 C.A. Griffith et al. / Icarus ( ) 32 Fig. 2. Spectral images 40, 41, 16, 18, 19 (a to e respectively) from Horizontal and vertical scales indicate the column and the row numbers in the array Panels show, from top to bottom, the continuum intensity averaged between and cm 1, the HCN column abundance, and a sketch of the impacts 89 sites, and the angular distances (arcsec) from the center of the jovian disk as seen from the Earth. are provided by Bézard et al. (1997). The line parameters for HCN and C 2 H 2 come from the GEISA data bank (Husson et al., 1991). The temperature dependence for the Lorentz half widths is T Our analysis of the 1995 observations assumes that HCN lies above 0.1 mbar, in agreement with the pressure level of HCN measured in July 1994 by Marten et al. (1995). We further assume that HCN resides between mbar, with a constant mixing ratio. If HCN is not capped at mbar, the column abundance is 25% lower that the values presented here. This altitude sensitivity occurs because the higher temperatures reached above the 1 µbar level enhance the emission. The large eddy diffusion time of HCN indicates little vertical transport from the time of Marten et al. s observations to ours, ten months later. We assume that in 2000 HCN resides above 0.5 mbar, consistent with the time evolution of the vertical profile of SL9 gases derived by Lellouch et al. (2002). We assume a nominal thermal profile (Fig. 5) formedby smoothing the ASI profile recorded by Galileo (Yelle et al., 2001; Seiff et al., 1998). This profile is specific to one spot in Jupiter s atmosphere, and thus does not account for the variability in Jupiter s tropopause and stratospheric temperatures (Orton et al., 1994). We thus adjusted temperatures in the lower stratosphere and upper troposphere to match the continuum. This technique does not affect the interpretation of the HCN lines, which are formed at much higher altitudes than is the continuum. At line formation altitudes, the uncertainty in the thermal profile contributes to the uncertainty in the HCN abundance. We find that for example a change in temperature of 5 K above 0.3 mbar implies a 25% change in the column abundance. In order to quantify these variations, we analyze the flux of the ν 4 band of methane, a gas which, unlike HCN and C 2 H 2, is uniformly mixed across Jupiter s disk. Variations in CH 4 emissions (Fig. 6)arecausedbythermal rather than compositional differences. We find that these emissions are consistent with stratospheric temperatures 4 7 K cooler in the southern hemisphere compared to the northern hemisphere. Note however that methane and acetylene emissions originate from the 4 25 and mbar

5 P.5 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 5 Meridional transport of HCN from SL9 impacts on Jupiter 5 36 Fig. 2. Continued. 92 pressure levels respectively, lower than the level at which HCN emission occurs (Fig. 7). We also compare acetylene emissions at similar north and south latitudes, where the abundances are roughly similar (Nixon et al., 2001). The latitudinal dependence of these lines changes with spectral feature (thus altitude) and with time. Given these variations, we find temperature differences similar to those derived using methane (Fig. 8). With constraints on the horizontal temperature variations, we analyze HCN features to derive the HCN column abundance at each latitude. Because the HCN lines are optically thin and the temperature lapse rate is small at altitudes where HCN resides, the derived column abundance of HCN is insensitive to the vertical profile of HCN, and depends solely on the integrated areas of the lines. Thus the line areas provide a direct measurement of the HCN column abundances. With over a thousand spectra, we fit the HCN feature with a Gaussian superimposed on a quadratic continuum (Figs. 9 and 10). The three free parameters required to fit the Gaussian are the peak position, line width and height. Since all the lines are unresolved, we use the strong nearby C 2 H 2 features establish the positions and instrument profile widths of the weaker HCN lines (Figs. 1 and 3). An algorithm to minimize the value of χ 2 between the fit and observations is used to determine the height, and thus the integrated area, of the HCN line. We normalize these fits to the column abundances determined from full radiative transfer calculations of spectra from a range of airmasses and HCN intensities to obtain the column abundance for each pixel. The spectra, scans and observations considered in the full radiative transfer analysis are [4 5,12, 1], [1 7, 7, 4], [2 3, 12, 4], and [3 5, 4, 4] (Table 2). Noisy array pixels are not considered in the fitting procedure, and HCN line heights below the 2σ noise level are deemed non-detections. Only jovian airmasses less than 3.6 are used

6 P.6 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 6 6 C.A. Griffith et al. / Icarus ( ) 36 Fig. 2. Continued Results 4.1. HCN in 1995 In May of 1995, 10 months following impact, HCN is detected at all longitudes observed. Yet, its spread with latitude away from the 44 impact band appears heterogeneous even considering the error in the measurements that result from noise (Figs. 2 and 11). For example, the latitudinal spread of HCN recorded at longitude (observation 40) exceeds that recorded at longitude (observation 18), although similar total masses were recorded at these longitudes (Fig. 11). The longitude distribution was also heterogeneous, with column abundances varying by a factor of 2. These observed heterogeneities exceed our estimated errors in the retrieved HCN mass of 25% assuming ±5 Kuncertaintiesin thetemperatureprofile. The regions of high mass do not correlate with impact longitudes, in agreement with observations of CS recorded in the same month (Moreno et al., 2003). The integrated mass column abundance suggests a total HCN stratospheric abundance of 1.5± g, in agreement with the total mass of 1.1± g estimated from several impact sites, several days after the impact (Bézard et al., 1997) HCN in 2000 The total stratospheric HCN abundance remains constant from 1994 to 2000 within measurement errors, with a value of 1.7 ± g indicated by the 2000 observations. Yet, the horizontal distribution of HCN evolves during this time. In the intervening years between 1995 and 1997, HCN migrated across the equator (Moreno et al., 2003). By 2000 we find detectable levels as far north as we could reasonably measure. That is at 60 north (Figs. 12 and 13). Here, the HCN mixing ratio exceeds the upper limit established for Jupiter s quiescent atmosphere (Bézard et al., 1995) byan order of magnitude. Yet, HCN remains concentrated at the impact latitude, of roughly 44. The latitudinal profiles of HCN derived from various lines and at different longitudes agree remarkably well, with the exception of the 20 peak associated with the cm 1 line. This artifact may arise from incorrect assumptions in the latitude temperature field, which were derived for levels below that where HCN resides. The relatively flat profile of the HCN distribution from 10 south to 40 north suggests that once the HCN crosses the equator it is fairly quickly mixed northward. 5. Discussion In order to quantify the meridional mixing of HCN, we adopt the two-dimensional eddy mixing model used by Lellouch et al. (2002) to interpret the origins of jovian H 2 O and CO 2. Ignoring vertical diffusion, we assume that HCN spreads latitudinally from an initial uniform band of material, 10 thick, centered at 44 latitude. (A band thickness of 15%, as used by Lellouch et al. (2002), changes our derived K yy values by 30%.) The initial column abundance was set to and cm 2 for K yy = cm 2 s 1 and K yy = cm 2 s 1, respectively, to achieve the best average fit to the 1995 observations between 60 and 30 latitude (Fig. 14). Our 1995 observations support the underlying premise that the initial zonal mixing of the impact debris significantly exceeds the meridional mixing. Although the zonal distribution was heterogeneous in 1995, the HCN locations did not correspond to the impact latitudes, indicating that the somewhat turbulent zonal flow erased the memory of the impacts initial positions. If we assume that the meridional transport acts uniformly with latitude under a single eddy diffusion coefficient, K yy,then the HCN column abundance, n(θ), a function of latitude, θ, evolves with time according to: t = 1 r 2 cos θ K (cos θ n yy. θ n θ )

7 P.7 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 7 Meridional transport of HCN from SL9 impacts on Jupiter 7 Fig. 3. Examples of jovian spectra taken at the two grating positions observed in Top figure: Spectrum 1 from slit position 10 of observation 1, recorded at 37 latitude and jovian airmass of Bottom figure: Spectrum 4 from slit position 12 of observation 4, recorded at 29 latitude and jovian airmass of Spectra are Doppler shifted approximately 0.02 cm 1. Fig. 4. The median of Jupiter s continuum emission at cm 1 43 Fig. 5. The nominal thermal profile (solid line, lower x-axis), relative to 99 (on 12/2/00) and cm 1 (on 11/30/00). Each slit position contributes 8 spectra (the average of four adjacent spectra), centered a distance Tropospheric temperatures, which vary across Jupiter s disk, were adjusted which thermal departures were derived from methane and acetylene lines. 45 of 1.2 apart. The first pixels (not shown) lie off of Jupiter s disk. Overlapping in the interpretation of each spectrum to match the spectral continuum. The fluxes from adjacent slit positions confirm a scan offset of 4.3.The short dashed and long dashed lines correspond to the C 2 H 2 and CH 4 mixing limb of the planet is clearly defined, and Jupiter s variable temperature in ratios respectively (top x-axis), which were assumed to establish horizontal the upper troposphere is evident. thermal gradients. 104 For a particular value of K yy,wesolveforn(θ) at the time of our observations, that is after a particular time lapse from impact, by using the Crank Nicholson scheme. We find that HCN profile in 1995 indicates latitudinal mixing consistent with an eddy mixing coefficient of K yy = cm 2 s 1 (Figs. 14 and 15). Such a slow mixing cannot account for the presence of HCN in the north- ern hemisphere in 2000 (Fig. 14). Instead a faster eddy mixing of roughly K yy = cm 2 s 1 (Figs. 14 and 15) is indicated. This rapid transport, however, does not reproduce the peaked high abundance of HCN at the impact latitude in 2000 (Fig. 14). Both the 1995 and 2000 data suggest that Jupiter s meridional mixing near the impact latitude is significantly slower than that near the equa-

8 P.8 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 8 8 C.A. Griffith et al. / Icarus ( ) 18 Fig. 6. Radiative transfer models (dashed lines) of pairs of CH 4 spectra Fig. 9. Gaussian models of 13 C 12 CH 2 spectra observed using TEXES in observed at the same jovian airmass (solid lines). Differing stratospheric The spectra recorded at the 10th slit position are shown, with the 1st temperatures cause different emission strengths. Temperatures of Jupiter s (southern most) spectrum on the bottom and subsequent spectra, in ascending order, above. Spectra are displaced by 0.3, 0.2, 0.1, 0.05, 0.2, 0.3, northern stratosphere (top spectra) exceed those in the south (bottom spectra) at 4 25 mbar. 0.4, and 0.5 erg s 1 cm 2 sr 1 cm for better clarity Fig. 11. Gaussian fits to the latitudinal spread of HCN in From top to bottom: Observations 16, 18, 19, 40, and 41. The peak concentrations 96 Fig. 7. Contribution functions of C 41 2 H 2,CH 4, and HCN features, and are located at 41.6, 42.3, 46.6, 47.6, 46.0.TheFWHMare 97 the continuum. These calculations assume the thermal profile of Fig. 5, respectively 19.7,11.8,22.0,28.0,23.1.They axis is the mass per a constant stratospheric mixing ratio for CH 4 of , a mixing ratio of unit area averaged over all pixels in each 4 latitude bin. The mean FWHM for HCN above 0.3 mbar, and the C 2 H 2 mixing ratio depicted is 21.5, eliminating smearing due to the finite spatial resolution of the in Fig. 5. instrument. 100 tor and, potentially, northwards (Fig. 14). The combined 1995 and 2000 observations can be fit with a single K yy profile that captures the slow tropical mixing, the faster middle latitude mixing and the depressed polar transport (Fig. 15). In addition, we find that the HCN abundance is depleted poleward of 50 relative to that reproduced by either K yy (Figs. 14 and 15). This interpretation depends somewhat on whether the HCN peak (Fig. 14) is due to increased HCN or increased temperatures. However, in support for a polar depletion of HCN, we also find that its abundance is depleted south of 50 in 2000, relative to the HCN migration reproduced by either K yy (Figs. 14 and 15). The diffusion rate that governs the introduction of HCN to the northern hemisphere agrees with prior measurements of impact-created CO, HCN and CO 2 in the northern hemisphere in 1997 (Moreno et al., 2003; Lellouch et al., 2002). Moreno et al. (2003) notes that in 1997 both CO and HCN were present in the northern hemisphere, implying an eddy diffusion coefficient of K yy = cm 2 s 1. A similar value of K yy = cm 2 s 1 represents the migration of CO 2 to the northern hemisphere (Lellouch et al., 2002)

9 P.9 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 9 Meridional transport of HCN from SL9 impacts on Jupiter 9 21 Fig. 8. Areas of C 2 H 2 (black dashed lines) and CH 4 features (brown long dashed line). Observations 1 through 4 are represented by red squares, green diamonds, blue up-triangles, and violet down-triangles respectively. The black dashed lines designate the 8-point running average values for 3 different C 2 H lines. The areas of the cm 1 CH 4 feature are shown with magenta Os, along with the running average. Both constituents indicate higher temperatures 79 in the northern hemisphere. Only the cm 1 C 24 2 H 2 line was observed on 11/30/ Fig. 10. Examples of Gaussian models of HCN spectra. Spectra of observation 1 of 2000, recorded at the 10th slit position, are displayed following the protocol of Fig. 9. Latitudes range from 37 (bottom spectrum) to 13 (top spectrum). 103 Since these observations do not spatially resolve the latitudinal variations beyond the aperture of the instrument (10 for HCN and CO, and 27 for CO 2 ), they are not inconsistent with our finding that the transport rate varies with jovian latitude. The slower mixing rate that we derive near the impact latitude, K yy = cm 2 s 1 matches, perhaps coincidentally, that derived from high spatial resolution observations of the impact dust (Friedson et al., 1999) at much deeper levels. However in 1997, the aerosols are not observed to cross the equator as did the impact gases (CO, CO 2,CS, and HCN). The dust migration likely differs from the gas transport because the dust resides at higher pressures ( mbar), a result of sedimentation. Meridional transport

10 P.10 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p C.A. Griffith et al. / Icarus ( ) 22 Fig. 12. Examples of spectra of Jupiter s HCN feature at high and middle northern latitudes, culled from observations 1 (top) and 4 (bottom). Top figure: shown are spectra observed at 31,35,38,52,63, and 69 north latitudes (in green, red, blue, purple, cyan, and orange). Bottom figure: HCN feature observed 79 at 20,21,22, and 49 north latitudes (in green, red, blue, and purple). 46 Fig. 14. Average HCN column abundances from the four 2000 observations (blue diamonds) and the 1995 observations (green squares). Moreno s observations appear as red squares. These data are compared to diffusion models of HCN in Jupiter s stratosphere. Top: The dotted lines, derived with a constant diffusion coefficient (K yy = cm 2 s 1 ) across Jupiter s disk, represent the latitude distribution of HCN in 1995 (green), 1997 (red), and (blue). Bottom: Calculations of latitude distributions for K yy = cm 2 s at equatorial latitudes thus appears to be more sluggish in the lower stratosphere and troposphere than at mbar. Current dynamical models of Jupiter s lower stratosphere (e.g., by Moreno and Sedano, 1997; West et al., 1992; Friedson et al., 1999) do not illuminate the atmospheric dynamics above 10 mbar, because here the temperature, composition, and thus the radiative heating and cooling of Jupiter s atmosphere are not well constrained. In Earth s stratosphere at similar pressures to those addressed here (0.5 1 mbar), the tracers N 2 O and the potential vorticity indicate a horizontal transport consistent with eddy diffusion coefficients ranging from 10 9 to cm 2 s 1

11 P.11 (1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p. 11 Meridional transport of HCN from SL9 impacts on Jupiter Fig. 13. HCN column abundances derived from the four observations of 2000, excluding that from 750 cm 1 feature, whose interpretation suffers from the feature s weak signal and its position on the wing of a strong C 2 H 2 line. Values are averaged over 10 latitude points. One standard deviation corresponds to an 78 uncertainty in column abundance of cm 2. Also shown are the 1996 and 1997 observations from Moreno et al. (2003). Gravity wave dissipation is thought to be the main mechanical forcing of the zonal winds which drives the meridional circulation on Earth. On Jupiter, the role of gravity waves in meridional transport is unknown. Yet, recent observations, including Galileo radio measurements of the jovian thermal profile, indicate their presence in Jupiter s stratosphere (Seiff et al., 1998; Flasar and Gierasch, 1986; Young et al., 1997; Magalhaes et al., 1990; Deming et al., 1989; Orton et al., 1994). In addition, quasi-biennial oscillations have been shown to correlate with terrestrial horizontal mixing. The role of the jovian cousin, quasi-quadrennial oscillations (Leovy et al., 1991; Friedson, 1999), on Jupiter s horizontal transport remains obscure. On Earth, polar vortices form in the winter, and isolate the polar air from lower latitudes. In the spring planetary waves are associated with the breakup of the polar vortex. The depletion of HCN at Jupiter s poles suggests the isola- Fig. 15. Average HCN column abundances from the 1995 and 2000 observations (squares and diamonds) can be fit with models that assume slow polar vortex is also indicated by images of Jupiter at 17 µm, tion of polar air and the presence of polar vortices. A jovian 43 mixing south of 20 latitude (K 99 yy = cm 2 s 1 ), and faster 44 mixing northward (K yy = cm 2 s 1 ). Transport north of 60 which detect a cap of cold air at Jupiter s north pole (Orton and south of 60 was also depressed to K yy = cm 2 s 1 et al., 2002). However, if such a vortex exists, its manifestation would differ from Earth s since, for example, Jupiter s and K yy = cm 2 s 1, respectively. The column abundance was set to cm 2. Calculations of the 1995 and 2000 data are shown with the tilt is small (3.1 ) and seasons are mild dashed and dotted lines. 104 (Newman et al., 1988; Jackman et al., 1988), equal to or smaller than the values that we derive for Jupiter s atmosphere. Horizontal mixing however varies dramatically on Earth: at low and middle latitudes the summer stratosphere is quiescent, in contrast to the vigorous meridional mixing in the winter months. Acknowledgments Research by C. Griffith was supported by the NASA grant NAG to the University of Arizona. Support for B. Bézard comes from the Action Spécifique Grands Télescopes Etrangers and from the Programme National de Planétologie of the Institut National des Sciences de l Univers (INSU). Observations with TEXES were sup-

12 P.12(1-12) ELSGMLTM(YICAR):m5 2004/03/17 Prn:24/03/2004; 16:24 yicar7334 by:ea p C.A. Griffith et al. / Icarus ( ) ported by NSF Grant AST T. Greathouse and M. 1 A., Despois, D., Strobel, D.F., Sievers, A., Chemical and thermal 57 2 Richter were supported by USRA We thank response of Jupiter s atmosphere following the impact of Comet 58 3 Shoemaker Levy 9. Nature 373, the IRTF for their support, technical help and expertise. We 59 Lellouch, E., Bézard, B., Moreno, R., Bockelée-Morvan, D., Colom, P., 4 also thank Dan Jaffe for helping with the observations. 60 Crovisier, J., Festou, M., Gautier, D., Marten, A., Paubert, G., Carbon monoxide in Jupiter after the impact of comet Shoemaker 61 6 Levy 9. Planet. Space Sci. 45, References Lellouch, E., Bézard, B., Moses, J.I., Davis, G.R., Drossart, P., Feuchtgruber, 63 8 H., Bergin, E.A., Moreno, R., Encrenaz, T., The origin of 64 9 Atkinson, D.H., Pollack, J.B., Seiff, A., Galileo Doppler measurements of the deep zonal winds at Jupiter. 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