Post-equinox observations of Uranus: Berg’s evolution, vertical structure, and track towards the equator

Icarus 215 (2011) 332–345

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Post-equinox observations of Uranus: Berg’s evolution, vertical structure, and track towards the equator Imke de Pater a,b,c,⇑, L.A. Sromovsky d, Heidi B. Hammel e,f, P.M. Fry d, R.P. LeBeau g, Kathy Rages h, Mark Showalter h, Keith Matthews i a

Astronomy Department, 601 Campbell Hall, University of California, Berkeley, CA 94720, United States Delft Institute of Earth Observation and Space Systems, Delft University of Technology, NL-2629 HS Delft, The Netherlands SRON Netherlands Institute for Space Research, 3584 CA Utrecht, The Netherlands d Space Science and Engineering Center, University of Wisconsin-Madison, Madison, WI 53706, United States e Association of Universities for Research in Astronomy, 1212 New York Avenue NW, Suite 450, Washington, DC 20005, United States f Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, United States g Aerospace and Mechanical Engineering, Saint Louis University, 3450 Lindell Blvd., Saint Louis, MO 63103, United States h SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, United States i Caltech Optical Observatories, California Institute of Technology, MC 301-17, Pasadena, CA 91125, United States b c

a r t i c l e

i n f o

Article history: Received 29 March 2011 Revised 14 June 2011 Accepted 15 June 2011 Available online 28 June 2011 Keywords: Uranus, Atmosphere Infrared observations Adaptive optics

a b s t r a c t We present observations of Uranus taken with the near-infrared camera NIRC2 on the 10-m W.M. Keck II telescope, the Wide Field Planetary Camera 2 (WFPC2) and the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) from July 2007 through November 2009. In this paper we focus on a bright southern feature, referred to as the ‘‘Berg.’’ In Sromovsky et al. (Sromovsky, L.A., Fry, P.M., Hammel, H.B., Ahue, A.W., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M. [2009]. Icarus 203, 265–286), we reported that this feature, which oscillated between latitudes of 32° and 36° for several decades, suddenly started on a northward track in 2005. In this paper we show the complete record of observations of this feature’s track towards the equator, including its demise. After an initially slow linear drift, the feature’s drift rate accelerated at latitudes jhj < 25°. By late 2009 the feature, very faint by then, was spotted at a latitude of 5° before disappearing from view. During its northward track, the feature’s morphology changed dramatically, and several small bright unresolved features were occasionally visible poleward of the main ‘‘streak.’’ These small features were sometimes visible at a wavelength of 2.2 lm, indicative that the clouds reached altitudes of 0.6 bar. The main part of the Berg, which is generally a long sometimes multipart streak, is estimated to be much deeper in the atmosphere, near 3.5 bars in 2004, but rising to 1.8–2.5 bars in 2007 after it began its northward drift. Through comparisons with Neptune’s Great Dark Spot and simulations of the latter, we discuss why the Berg may be tied to a vortex, an anticyclone deeper in the atmosphere that is visible only through orographic companion clouds. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction When Voyager 2 flew by Uranus in 1986, the images returned by the spacecraft showed a rather featureless planet (Smith et al., 1986). Only by considerably enhancing the contrast could one distinguish a few faint cloud features. It took researchers therefore by surprise when, a decade later, Hubble Space Telescope (HST) revealed many cloud features of high contrast (Karkoschka, 1998) in images taken with the near-infrared camera (NICMOS). The brightest features were in the northern hemisphere,1 as far north ⇑ Corresponding author at: Astronomy Department, 601 Campbell Hall, University of California, Berkeley, CA 94720, United States. Fax: +1 510 642 3411. E-mail address: [email protected] (I. de Pater). 1 We adopt the IAU convention for ‘‘north,’’ which means that we were viewing Uranus’s southern hemisphere during the Voyager encounter (1986). 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.06.022

as one could possibly see at that time, a latitude range on the planet that had just come into sunlight after having been in darkness for 40 years. At visible wavelengths, though, these clouds were not much brighter than those seen by Voyager 2, and hence the apparent change from a featureless to an active planet is in part due to a shift in wavelength from the visible to the infrared. Since the late 1990s the planet has been observed regularly with HST, and since 2000 also with the adaptive optics (AO) system on the 10-m Keck telescope (e.g., Hammel et al., 2001, 2005a,b; de Pater et al., 2002; Sromovsky and Fry, 2005; Sromovsky et al., 2007, 2009). Over time, more and more features could be discerned on the planet’s disk, in part because of improvements in detectors/techniques, and in part because Uranus appeared to become much more active while approaching equinox. An example of an image is shown in Fig. 1a. Some of the features are long-lived. For example, the ‘Bright Complex’ near 30° in the northern hemisphere was

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usually present, and occasionally turned very bright (Sromovsky et al., 2007). In this paper, we focus on a feature in the southern hemisphere just north (equatorward) of the polar cloud collar (Fig. 1a). This feature was named the ‘‘Berg’’, because it resembled an iceberg peeled off the polar collar. It oscillated in latitude between 32° and 36° for many years: at least since 2000, probably since before 1994, and possibly even since the 1986 Voyager era (Sromovsky and Fry, 2005). However, in 2005 it started moving monotonically towards the equator (Sromovsky et al., 2009). The morphology of the Berg set it apart from other cloud features. In the Voyager era the most prominent discrete cloud feature was seen at a latitude of 35°, and this feature had two components (Fig. 1b, reproduced from Fig. 12 of Sromovsky and Fry (2005)): a long streaky feature extending over 10–20° in longitude, with a small compact (unresolved) ‘‘dot’’ 2° polewards of the streaky component. Based on its latitude and resemblance in morphology in later images, this feature may be the Berg seen in recent images. The Berg with its 2-component morphology was detected (again) in the early (2000–2002) AO images (Hammel et al., 2001; de Pater et al., 2002), before the AO system was optimized for planetary imaging (in 2003). During subsequent years the feature’s intricate structure and evolution were mapped in detail. Although the feature was usually seen as a distinct entity, occasionally it appeared embedded within a latitudinal band extending over 120° in longitude (e.g., in October 2003, feature A in Fig. 1 of Hammel et al. (2005a)). This occurred when its latitudinal oscillation brought it closest to the bright band centered at 45°S. As mentioned above, Uranus was observed regularly with the Keck AO system since mid-2000 – that is, we typically observed the planet on 3–8 nights per year over a several-month period around opposition. Roughly 10 HST orbits per year were allocated to Uranus observations. The first signs of changes in the Berg’s dynamical behavior were seen in early July 2004, when the small dot brightened considerably for just a few days, during which time it was also visible at 2.2 lm (Fig. 1 in Hammel et al. (2005b); Figs. 2 and 3 in this paper). Although we cannot rule out similar events in prior years, the subsequent sequence of events discussed in this paper (and by Sromovsky et al. (2009)) together with the data from prior years suggests that the Berg’s behavior indeed changed in 2004. The 2004 series of observations suggested vigorous convection, with cloud particles rising up to much higher levels in the atmosphere than usual (above the 1 bar level). Sromovsky et al.

(a)

(2009) speculated that this dynamical activity may be related to the Berg’s subsequent northward migration (toward the equator). Since 2005 the Berg continued to drift northward and its morphology continued to change, as described by Sromovsky et al. (2009). The long streaky component became much more elongated with complex structures. On 8 August 2007 the single dot became 3 dots, one of which had disappeared by August 20 (Figs. 4 and 5 in Sromovsky et al. (2009); Figs. 2 and 3 in this paper). All three dots were also visible at 2.2 lm. Sometimes the elongated streak showed an extra (long) strand towards the west, such as on 27 July 2007 (feature 72714 in Fig. 2 in Sromovsky et al. (2009); Figs. 2 and 3 in this paper). The present paper continues the work of Sromovsky et al. (2009). After a description of the observations in Section 2, we describe and show quantitative measures of: (1) morphological changes in the Berg’s appearance since 2004, (2) its continued track towards the equator, and (3) changes in its vertical structure. In Section 4 we discuss these results. In particular, we address the question whether the Berg is produced by a vortex, and if so, why it may have departed from its latitudinal oscillation.

2. Observations and data reduction Observations of Uranus were obtained in 2007–2009 with the Keck II telescope on Mauna Kea (Hawaii) at near-infrared wavelengths, and intermittently with the Hubble Space Telescope (HST) at CCD wavelengths, using the F791W and F953N filters in 2007 and 2008, and the F845M filter in November of 2009. A summary of the observations is presented in Table 1, with the instrument and filter characteristics in Table 2. At Keck we used the near-infrared camera NIRC2 coupled to the AO system. NIRC2 is a 1024  1024 Aladdin-3 InSb array, which we used in its highest angular resolution mode, i.e., the NARROW camera at 9.94 ± 0.03 mas per pixel (de Pater et al., 2006), which translates roughly to 140 km/pixel (see Table 1). Data were obtained in the broadband J, H, and K0 filters, and on 20 August 2007 also in the narrow band Hcont filter. We note that the NARROW camera over-samples the FWHM (full width at half maximum) of the telescope. This oversampling, in addition to using the AO system, helped to achieve an angular resolution near the diffraction limit of the telescope, which is 46 mas at K0 band and scales linearly

(b)

Fig. 1. (a) Image of Uranus in H band (1.6 lm) taken with NIRC2 coupled to the AO system on the Keck telescope. The image was taken on 4 July 2004. The figure is annotated with features used in the text (the  ring is barely visible). (Adapted from Hammel et al. (2005b).) (b) Morphology of the Berg as seen in 1986 with Voyager 2. The 3 images were taken with the wide-angle camera in the orange filter, and is high-pass filtered to improve contrast. The central latitude in each frame is 35°, and the gridpoint spacing is 10° in longitude. Each subpanel is 10 degrees in latitude. (Adapted from Fig. 12 of Sromovsky and Fry (2005).)

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with wavelength. We obtained typical resolutions in our images between 46 mas (under excellent conditions in K0 band) and 60 mas (a typical resolution in J band). All images were processed using standard near-infrared data reduction techniques (flatfielded, sky-subtracted, with bad pixels replaced by the median of surrounding pixels). The geometric distortion in the images was corrected using the ‘‘dewarp’’ routines provided by Brian Cameron of the California Institute of Technology.2 Photometric calibrations were performed using the stars HD22686 (in 2007), HD161903 (in 2008), and HD201941 (in 2008 and 2009) (Elias et al., 1982). We converted the observed flux densities, FU, to the dimensionless parameter I/F, where I is the reflected intensity and pF the solar flux density at Uranus’s distance (e.g., Hammel et al., 1989):

I r2 F U ¼ ; F X F

ð1Þ

with r Uranus’s heliocentric distance, pF the Sun’s flux density at Earth’s orbit (taken from Colina et al. (1996)), and X is the solid angle (in steradians) subtended by the body or a pixel on the detector. By this definition, I/F = 1 for diffuse scattering from a Lambertian surface, and is equal to the geometric albedo if the object is observed at normal incidence and a solar phase angle of 0°. We compared the median-averaged I/F values in a 100  100 pixel box centered on Uranus (avoiding scattered light from the rings) on all photometric nights, and found no dependence on solar phase angle, a; I/F values between 0.05° < a < 2.06° typically agreed to within 3–5%; at a = 2.62° (14 July 2009) values in J and H bands were 10% higher. To get all Uranus images on a consistent calibration scale, we used the data from 26 July 2007 as a standard, and scaled all images to this date: H-band: (1.05 ± 0.03)  102, J-band: (1.71 ± 0.06)  102, and K0 -band: (1.41 ± 0.08)  104; these values include a 3% absolute calibration uncertainty, and agree well with the numbers derived by Sromovsky et al. (2007). In September 2008 we obtained images of Uranus with the Wide Field Planetary Camera 2 (WFPC2) as part of a SNAP program (GO 11156; PI: K. Rages). We show here data using the F791W and F953N filters, which best reveal faint cloud features. In November 2009 we used HST’s UVIS detector on the newly installed Wide Field Camera 3 (WFC3) (GO 11573; PI: L. Sromovsky). In this paper we only present the data at 845 nm, as we are only interested in the Berg’s equatorward motion and morphology, and these data provided the best combination of angular resolution, contrast, and signal-to-noise to detect cloud features. Calibrated images were delivered by the WFPC2 calibration pipeline calwp2 (Gonzaga et al., 2010) and the WFC3 calibration pipeline calwf3 (Kim Quijano et al., 2009). Cosmic rays were cleaned with the IDL la_cosmic routine (van Dokkum, 2001), and the images were deconvolved using point spread functions (PSFs) generated by the Tiny Tim package (ver. 7.0, Krist, 1995). The images were deconvolved with a PSF 6 arcsec square, then reconvolved with the core of the PSF (out to the first minimum, radius 2.5 pixels for WFPC2, 2.0 pixels for WFC3). The WFC3 deconvolved images were then geometrically corrected using PyDrizzle, a frontend to the drizzle task of the STSDAS dither package (Fruchter and Hook, 2002), and drizzled onto an output image with half the pixel scale of the original (new scale 0.02 arcsec/pixel). The WFPC2 images are not regularly geometrically corrected as the distortion is minimal over the area of the detector that Uranus covers; geometric correction only serves to induce artifacts. Conversion to I/ F (Eq. (1)) was done using image header PHOTFLAM values, and 2 http://www2.keck.hawaii.edu/inst/nirc2/forReDoc/post_observing/dewarp/ nirc2dewarp.pro.

instrument- and filter-weighted solar irradiance averages, using the Colina et al. (1996) solar spectrum. WFC3 F845M images were combined to increase signal-to-noise ratios by remapping all 16 images from an HST orbit to a rectangular latitude/longitude projection (to account for the planet’s rotation), averaging the images, and remapping back onto the viewing geometry for a time near the center of the orbit. Image navigation and deprojection of both HST and Keck images were accomplished via the methods described by Sromovsky et al. (2009). As stated by the latter authors, the rms error in the position of compact cloud features is 0.005600 . Positions are described using planetocentric latitude, i.e., the angle above the equatorial plane as measured from the planet’s center.

3. Results 3.1. Morphology Representative H-band Keck images of the Berg taken from 2004 through 2009 are shown in Figs. 2 and 3. Fig. 2 shows the images on a linear I/F scale (from 0 to 0.015), while Fig. 3 shows the same images after applying a high-pass filter (smoothing the image in Fig. 2 with a 20  20-pixel boxcar function, and subtracting this from the original image) to enhance the contrast of the small features. The Berg is easily recognizable on each frame: from 4 July 2004 through 9 September 2007 it was visible as a bright elongated streak on the southern hemisphere (left side), with one or more bright compact components just south (polewards) of the bright streak. These compact components occasionally showed up in the K0 band (indicated by a small arrow). Although the morphology of the feature started to change in 2004, we always saw just one compact component for the Berg until on 8 August 2007, when there were three separate compact regions. A week later at least one of these features was gone (unfortunately, high humidity on 14 August degraded the quality of the images); on August 20 two features were visible, and only one was left by the first of September. Only one compact region was seen since that time. In December 2007 no bright spot was visible, but in October 2008 a small spot had reappeared. To show the evolution of the central bright spot in a quantitative way, we plot its peak I/F value relative to the background atmosphere as a function of time in Fig. 4. The highest I/F values were seen on 4 July 2004. The dot’s I/F values were quite similar in J and H bands; it was sometimes slightly brighter, and at other times slightly fainter in J band. Although the PSF in J band is known to fluctuate much more over time than in H band, we note that the relative J/H fluctuations in the peak I/F values are relatively small. Clearly, the variations in all J, H, and K0 values are well correlated. The dot brightened steadily throughout July–September 2007, before it disappeared. In September the dot started to fade first in K0 band. Throughout 2007–2009 the elongated streak changed morphology significantly, both in total length and in detailed structure, as shown in Figs. 2 and 3, and quantified in Fig. 5. In the latter figure we show longitudinal profiles through the streak in H band images, integrated over 6° in latitude (the bright dots were outside this latitude range). The background atmospheric profile was fitted with a 6th order polynomial, and subtracted from the scans. All profiles are plotted on the same intensity scale, with the peak value of the streak approximately centered on the x-axis. In the summer of 2004 the streak was very bright; we show both the July 4 and August 12 profiles in the same panel. In subsequent years it was fainter, and the profile was less peaked, or broader in longitude. At the end of July 2007 the feature is very bright again; in early Au-

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335

Fig. 2. Images of Uranus in H band (1.6 lm) taken with NIRC2 coupled to the adaptive optics system on the Keck telescope. The images show the evolution of the Berg from 2004 through 2009. From 4 July 2007 through 9 September 2007 the Berg is visible as a bright elongated streak on the southern (left side) hemisphere, with one or more bright compact components just south (polewards) of the bright streak. These compact components occasionally show up in the K0 -band, in which case the components are indicated by a small arrow.

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Fig. 3. The same images as shown in Fig. 2 after applying a high-pass filter to enhance the contrast of the small features. This was accomplished by subtracting, from each image, a smoothed version of the image obtained using a 20  20-pixel (0.200  0.200 ) boxcar function. Note that one can see the  ring in many frames.

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I. de Pater et al. / Icarus 215 (2011) 332–345 Table 1 Keck and HST observations.

a b c d

Date (UT)

Time range (hr:min)

ra (AU)

Da (AU)

Diam. (00 )

Bb (°)

Bob (°)

ac (°)

Pixel sized (km)

Keck 2007–07–04 2007–07–27 2007–07–30 2007–08–08 2007–08–14 2007–08–20 2007–09–01 2007–09–09 2007–12–12 2008–10–05 2009–07–14 2009–07–26

11:20–11:50 10:30–15:40 11:20–15:30 11:15–15:10 08:05–08:25 10:30–14:50 12:10–12:20 08:10–13:20 04:25–04:35 05:45–09:45 12:10–15:10 11:00–15:40

20.092 20.092 20.092 20.093 20.093 20.093 20.093 20.093 20.095 20.098 20.098 20.098

19.66 19.35 19.32 19.23 19.18 19.14 19.10 19.09 20.15 19.17 19.64 19.47

3.584 3.642 3.649 3.665 3.674 3.682 3.691 3.693 3.497 3.676 3.589 3.621

0.99 0.60 0.52 0.27 0.06 0.15 0.60 0.96 2.83 2.13 8.79 8.61

1.66 1.41 1.38 1.34 1.22 1.15 1.02 0.99 0.07 3.20 6.20 6.33

2.66 2.02 1.91 1.55 1.29 1.01 0.42 0.05 2.79 1.10 2.61 2.30

134.07 131.94 131.68 131.11 130.79 130.50 130.18 130.11 137.41 130.72 133.88 132.70

HST 2008–09–02 2008–09–09 2009–11–11

07:56–07:59 06:14–06:14 13:16–14:01

20.098 20.098 20.097

19.11 19.09 19.53

3.689 3.691 3.608

3.56 3.26 5.14

3.0 3.08 7.47

0.54 0.18 2.35

637.4 636.7 566.7

r and D are the heliocentric and geocentric distance of Uranus, respectively. B and Bo are the apparent sub-Earth and sub-solar planetographic latitudes, similar to the ring inclination angles as seen from Earth and the Sun. a is the solar phase angle. Pixel size refers to the size of a pixel at the center of the disk.

Table 2 Filter characteristics. Instrument

Filter

Central k (lm)

Bandpass (lm)

NIRC2 NIRC2 NIRC2 NIRC2 WFPC2 WFPC2 WFC3

J Hcont H K0 F791W F953N F845M

1.248 1.58 1.633 2.124 0.7969 0.9546 0.8436

0.163 0.023 0.296 0.351 0.1305 0.0053 0.0787

Fig. 4. Peak I/F values for the center bright dot (relative to the background atmosphere) as a function of time. The background atmosphere was fitted with a 6th order polynomial, and subtracted from the scans. We used all images on each day on which the dot was visible, except for 14 August 2007, as the data were of poor quality due to weather and seeing. If the dot was visible in H and J, but not in K0 band, we assigned the feature a zero I/F value in K0 . The 2004–2006 data are from Hammel et al. (2005b), Sromovsky and Fry (2005), and Sromovsky et al., 2009.

gust the structure changed to an asymmetric profile. In and after December 2007 the feature started to slowly fade away. In addition to the changes noted above, several times in 2007 a curved thin ‘tail’ emanated from the Berg, extending outwards to the uranian north-east, trailing with respect to the feature’s motion relative to the planet. This tail appears to come and go; it was visible during 2007 on 27 July, 14 August, and 1 September. In

October 2008 the elongated streak appeared as multiple bright clumps. In July 2009 the region was very faint, and had a doublelobed morphology. The Berg was very close to the equator at that time, at the same latitude as the rings projected on the disk (near the bottom and top of the image on 14 and 26 July, respectively). Fig. 6 shows images taken with HST’s WFPC2 on UT 2 September 2008 using the F791W and F953N filters (top panels) and with WFC3 on UT 11 November 2009 with the F845M filter (bottom panel). The original images are shown on the left; the images on the right side show difference images to enhance the contrast of faint cloud features. In 2008, we simply subtracted a SNAP image at the same wavelength taken 24 h later. In 2009, we subtracted a median of six images taken on 11–12 November 2009. In 2008, the Berg is visible just to the northwest of Ariel, the bright moon visible on the disk. In 2009, the Berg can be seen at 6°S. To get a more quantitative assessment of the changes in morphology and the overall motion of the Berg, we remapped the images to an orthogonal latitude–longitude projection and show an evolutionary sequence in Fig. 7, from 2004 through 2009. All Keck images are taken in H band, and are high-pass filtered; the bottom right image (11 November 2009) is the HST difference image shown in Fig. 6 (bottom panel). For each frame on which a bright dot is visible, the planetocentric latitude and longitude of the bright dot is indicated and the frame is centered on the longitude of the dot. For frames without the bright dot we estimated latitude and longitude of the approximate center of the gap between the two main bright streaks, and the frame is centered on the corresponding longitude. In this case the latitude indicated is the approximate average streak latitude. 3.2. Latitudinal motion A graph of the latitude of the feature as a function of time is shown in Fig. 8. Sromovsky and Fry (2005) showed that the Berg oscillated in latitude between 32°S and 36.5°S, over the years 1986 up to 2004. They obtained a best-fit sinusoidal model with a period of 1000 days and amplitude of 2.1°. As shown before by Sromovsky et al. (2009), the Berg ‘‘escaped’’ its latitudinal confinement in 2005, and moved in the direction of the equator. As shown in Fig. 8, the Berg continued to migrate towards the equator, on the same, almost parabolic, track in latitude–time coordinates as started in 2005. During the first 2 years the Berg moved nearly lin-

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Fig. 5. Longitudinal scans through the streak in images taken between 2004 and 2009, as indicated. Each scan is integrated over 6° in latitude (the latitude of the bright dot was excluded from these scans). The background atmosphere was fitted with a 6th order polynomial, and subtracted from the scans. The intensities are all on the same scale, normalized to 1.0 as shown. The vertically integrated I/F value that corresponds to 1.0 equals 0.03; the average I/F value therefore corresponds to 0.005. The peak values in each scan are approximately centered on the x-axis. The cosine of the emission angle, l, at the location of the Berg is indicated on each panel. The 2004 data are from Hammel et al., 2005b and Sromovsky and Fry, 2005. The 2005 and 2006 data are from Sromovsky et al. (2009).

I. de Pater et al. / Icarus 215 (2011) 332–345

(a)

WFPC2

339

F791W

2008-09-02 07:56 UT

(b)

WFPC2

F953N

2008-09-02 07:59 UT

(c)

WFC3

F845M

2009-11-11 13:46 UT Fig. 6. HST images of Uranus taken with the WFPC2 on UT 2 September 2008 using the F791W filter (a) and F953N filter (b), and with the WFC3 on UT 11 November 2009 using the F845M filter (c). In 2008 two of Uranus’ large moons are visible: Ariel is on the disk, and Titania in the bottom right (south-west) corner of the frame. The Berg feature is visible just to the west and north of (i.e., above) Ariel. These WFPC2 images were taken with the SNAP program (GO 11156); the images on the left are single frames. The images on the right are obtained by subtracting from the images on the left a SNAP image at the same wavelength that was taken 24 h later. The WFC3 image (from GO 11573) is averaged over 16 35-s frames, taken between 13:16 and 14:01 UT, as described in the text. The feature can be seen just south of the equator. On the right is a difference map, where the median from six images taken on 11 and 12 November 2009 was subtracted from the image on the left.

early at a rate of 0.22°/month, which is equivalent to a meridional velocity of 3.7 cm/s. The Berg moved more rapidly in subsequent years. After the summer of 2007 its velocity was 8.9 cm/s (0.53°/month), and between 2008 and 2009 its drift rate increased to 22.7 cm/s (1.35°/month). It may have slowed down to 10.8 cm/s (0.64°/month) once it came to within 8–10° of the equator. The latter change in drift rate, however, could also be caused by an oscillation superimposed on an otherwise smoother motion. Admittedly, one may never be confident that the same feature is seen in each image; however, in December 2007 and October 2008 the feature’s morphology is highly suggestive of it being the Berg. In 2009 the feature is much fainter, but the shape is still similar,

and its location is consistent with the northward track reported by Sromovsky et al. (2009); hence we hypothesize that it is the same feature. This, therefore, is the first time any feature on a giant planet has been tracked from 34° almost to the equator (5°). It is also the first time that the drift rate of a feature has been seen to increase at planetocentric latitudes jhj < 25°, with perhaps a subsequent slow-down at jhj < 8°. 3.3. Vertical structure While the main streak of the Berg feature brightened, dimmed, and changed its longitudinal extent over the last 5 years, it has only been seen in filters that penetrate deeply into Uranus’ atmosphere

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Fig. 7. Evolutionary sequence of the Berg from 2004 through 2009. All images but one are taken at 1.6 lm with NIRC2 on the Keck telescope, and are high-pass filtered; the bottom right image (2009–11–11) is the HST difference image shown in Fig. 3c (at 845 nm). For each frame, the (planetocentric) latitude and longitude of the bright dot is indicated, except where the dot is absent, in which case the central streak location is given.

such as the J (1.2 lm) and H (1.6 lm) near-IR filters used in our Keck 2 observations. The streak has never been seen in the K0 (2.2 lm) band, which cannot penetrate very deeply because of strong absorption by H2 and CH4 in this band. In contrast, since 2004 the small dots south of the main streak have regularly been seen in the K0 band (Figs. 2–4), indicating that they extend to much higher altitudes than the main streak. When these local bright spots are visible at 2.2 lm, they are always very bright in J and H bands, implying that they have significant optical depths. We can use the I/F values of the Berg in the various filters to quantitatively constrain the altitude of its various cloud components. Three filters of particular use are: H, K0 , and Hcont. The penetration depths of these filters into a clear Uranus atmosphere were calculated as follows: the I/F as a function of wavelength was calculated at 10 cm1 resolution, using a 10-term Gaussianweighted exponential sum to represent the absorption within each 10 cm1 wavelength interval (e.g., Sromovsky and Fry, 2007). After computing the I/F for each 10 cm1 interval, we took a further weighted sum of I/F over the passband of each filter, applying spectral weighting appropriate to the filter/detector response function and the solar spectral weighting (see Eq. (11) from Fry and Sromovsky (2007)). The resulting penetration depths for these filters are indicated in Fig. 9a, where we show the I/F of a unit-albedo reflecting layer as a function of pressure in the Uranus atmosphere. We do not use the J-band observations for height discrimination because its penetration profile does not differ sufficiently from that

of the H filter (Sromovsky and Fry, 2007; their Fig. 1). Moreover, the analysis becomes more complicated because the wavelength gap between filters is increased and the seeing quality is significantly different (and worse) for J than for H. The calculations shown in Fig. 9 make use of the latest improvements in methane absorption coefficients by Karkoschka and Tomasko (2010) and reanalysis of the Lindal et al. (1987) radio occultation results by Sromovsky et al. (2011), which resulted in a preferred deep methane mixing ratio of 4%, a thermal structure similar to the Lindal et al. Model F, and a decreasing mixing ratio profile above the methane condensation level similar to that adopted by Karkoschka and Tomasko (2009). Fig. 9a illustrates that the location of high altitude clouds can be constrained by the ratio of reflectivities in K0 and H filters, while the deeper clouds can be constrained by ratios of reflectivities in H and Hcont filters. The quantitative basis for this statement is described in the following. To develop a simplified model of the effect of discrete cloud features on the observed atmospheric reflectivity we build on recent work by Karkoschka and Tomasko (2009) and Sromovsky et al. (2011). These authors show that the main background cloud layer affecting the CCD spectra of Uranus is located in the 1.2–2 bar region at latitudes between about 35°S and 25°N, with a typical optical depth of 1 at CCD wavelengths, and a weak wavelength dependence. This is also consistent with the near-IR results of Irwin et al. (2010) when adjustments are made for differences in the assumed methane mixing ratio profile.

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341

where Idc,k is the radiance of the discrete cloud at wavelength k, Ia=1,k(P) is the radiance of a unit albedo Lambertian reflecting surface at pressure P (shown in reflectivity units in Fig. 9a), and Ibase,k is the radiance of the atmosphere in the absence of the discrete cloud, which we estimate from the radiance of surrounding regions. If the discrete cloud is below the main cloud layer on Uranus, then the effect of the discrete cloud will be mostly additive and the observed radiance can be approximated as

Idc;k ¼ Ibase;k þ f  Ia¼1;k ðPÞ;

Fig. 8. A graph of the latitude of the feature as a function of time. Up to 2004, the Berg was confined to latitudes 32–36.5°S, and its motion could be modeled well with a sinusoidal curve (solid line), as indicated (from Sromovsky and Fry (2005)). The dotted lines indicate expected variations due to inertial oscillations. The latitudes of the feature from 2005 onwards are for the small bright (central) spot, and are about 1°S from the central latitude of the Berg complex, which is what was plotted for prior years. To help guide the eye, a smooth fit is superposed on the data
2 from 2004 onwards; the fit is of the form: y ¼ 31 þ 1:05 x300 þ 365
x300 5:3 0:0021  365 , where x is julian day relative to 11 July 2004, and y is planetocentric latitude. The horizontal dashed line is the latitude at which LeBeau and Dowling (1998) predict a vortex to erode away. (Figure is extended from Fig. 9 in Sromovsky et al. (2009).)

(a)

(b)

Fig. 9. (a) I/F of a unit albedo Lambertian reflector as a function of pressure within a clear uranian atmosphere, viewed with H, Hcont, and K0 filters. Atmospheric attenuation is calculated using the F1 temperature and methane profile of Sromovsky et al. (2011), and the methane absorption coefficient model of Karkoschka and Tomasko (2010). Curves for three different emission angles are shown (l is the cosine of the emission angle). (b) Contrast ratios for two different pairs of filters.

For our model we first consider a discrete cloud above this layer. We model the observed intensity of this cloud as a unit albedo layer of effective fractional coverage f, which blocks the same fraction of background intensity, leading to the relationship

Idc;k ¼ ð1  f Þ  Ibase;k þ f  Ia¼1;k ðPÞ;

ð2Þ

ð3Þ

where here the effective fraction f also includes the effects of attenuation by the aerosols above the discrete cloud perturbation. In both Eqs. (2) and (3) there is an implicit dependence on observer and solar zenith angles, which can be considered the same for our low-phase angle observations. Applying the above equations to two different filters (using filter names in place of wavelength subscripts) we obtain the ratio equations

DK0 Idc;K0  Ibase;K0 Ia¼1;K0 ðPÞ  Ibase;K0 ¼ ; ¼ DH Idc;H  Ibase;H Ia¼1;H ðPÞ  Ibase;H

ð4Þ

for high altitude optically thick cloud elements, and

DH Idc;H  Ibase;H Ia¼1;H ðPÞ ¼ ¼ ; DHcont Idc;Hcont  Ibase;Hcont Ia¼1;Hcont ðPÞ

ð5Þ

for deep cloud contributions. In both cases the left-hand sides are measurable values and the right-hand sides are pressure-dependent theoretical calculations. The pressure at which the two sides match is our estimated discrete cloud pressure. The ratios for DH/DHcont and DK0 /DH as given in the right-hand sides of Eqs. (4) and (5) are shown in Fig. 9b. These equations have thus the virtue of constraining the pressure in a way that does not depend on the effective cloud fraction. Once we determine the pressure of the discrete cloud, its effective fraction can be determined from either Eq. (2) or Eq. (3). However, the interpretation of the fraction must take into account that its value is affected by seeing conditions, as well as fractional coverage (or cloud opacity) and attenuation by overlying aerosols (the gas attenuation effects are already accounted for in the computation of reflecting layer I/F values). The results of our cloud pressure estimates for the small bright spots when visible in the K0 images are given in Fig. 10. Panel (a) provides the DK0 /DH ratio measurements and panel (b) displays the inferred cloud pressures, which reach a lower limit (i..e, highest altitude) of about 550 mb. The effective fractions retrieved are typically of the order of up to a few percent (the highest fraction, 2%, was reached on 4 July 2004), suggesting that the cloud is either patchy or of low optical depth, such as typical cirrus clouds in the Earth’s atmosphere. These results are relatively weakly dependent on the methane mixing ratio chosen, because the main variation is in the K0 band, which is dominated by hydrogen absorption. Thus it is not surprising that these numbers agree also with the values for features visible in K0 band as derived by de Pater et al. (2002) and Hammel et al. (2005a). The Berg’s streak feature, which has never been seen in K0 images is thus always located deeper in the atmosphere than the 1-bar level. Fig. 10c displays the results of our DHcont/DH ratio analysis of the main Berg streak feature for the two dates on which we have Hcont images containing the Berg feature. The first comes from a multi-filter data set described by Sromovsky and Fry (2007) and makes use of images taken with the Keck NIRC2 wide angle camera on 11 August 2004 (images g and i in their Fig. 7). We used ratios measured at three different locations on the main streak, finding pressures of 3.2–3.7 bars and cloud fractions of 0.41–0.475. These are large fractions considering that they include attenuation by the overlying aerosols. Even using

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(a)

(b)

(c) Fig. 10. (a) DK0 /DH ratio for Berg-related bright spots visible in K0 images. (b) Inferred pressures for observations in panel a. (c) Inferred pressures for main streak features based on H–Hcont observations. The parameter f is the effective fractional cloud coverage (Eqs. (2) and (3)).

the smallest fraction, this allows the effective two-way optical depth of the overlying aerosol layers to be no greater than s = 0.9. This is only about one third of the value inferred at CCD wavelengths, and hence requires a wavelength dependence of the order of k1 between 2 and 0.7 lm. The second set of H with Hcont data was taken on 20 August 2007. A detailed view in all three filters (H, Hcont, K0 ) of the Berg feature on this date is shown by the rectilinear projections in Fig. 11. Cloud pressure solutions for features A, B, C, and D marked on the Hcont projection are summarized in Table 3, and plotted in Fig. 10. There is good agreement on the vertical location of the brightest spot using the two different filter pairs to estimate the cloud pressure, which suggests that the cloud might be mainly located near the derived pressure rather than being optically thick and vertically extended. This suggestion is supported by the relatively small effective cloud fractions obtained. The brightest spot (B) certainly has a significant component reaching above the 1.2-bar methane condensation level and thus is likely to be at least partly, if not entirely, composed of methane ice. The cloud pressures for the main streak inferred from the 20 August 2007 observations are significantly smaller (1.8–2.5 bars) as are the cloud fractions (8–13%) than in 2004. This seems to imply that the Berg feature moved to higher altitudes and became optically thinner as it left its oscillating state and began its equatorward drift. We note that in contrast to the DK0 /DH ratios, the DH/DHcont pressure results depend on the methane mixing ratio profile and also strongly on the methane absorption model used in the analysis. For example, when using the Lindal et al. (1987) Model D profile with Irwin et al. (2006) absorption coefficients, the pressures for the main Berg streak features would increase to more than 6 bars. The composition of the deeper features is not well constrained but H2S is one plausible possibility (e.g., de Pater et al., 1991).

Fig. 11. Rectilinear projections of Berg images in Hcont, H and K0 (Kp) filters on 20 August 2007. Features C and D are local brightness maxima in the main streak feature, and A and B denote companion spots.

4. Discussion 4.1. Is the Berg produced by a vortex? As discussed by Sromovsky et al. (2009), the Berg oscillated with a period of 1000 days between planetocentric latitudes of 32°S and 36.5°S for at least 5 years, and perhaps for two decades, before it started to drift northwards in 2005. As the feature is extended both in latitude and longitude, yet remains coherent over time, Sromovsky et al. (2009) suggest that the clouds are generated by a single feature, such as a vortex. A significant brightening in J, H and K0 bands of the small compact cloud on the south side of the Berg was first reported in 2004 (Hammel et al., 2005b), and interpreted as vigorous convection. In Section 3.3 we show that altitudes near 0.6 bar were reached, and the fractional cloud cover for this event was 2%. As the atmospheric temperature at this level is well below the condensation temperature of methane gas, these clouds are likely orographic methane clouds. In 2007 the morphology of the Berg changed significantly, with several compact clouds on the south side, which frequently reached levels up to 0.6–0.7 bar (Figs. 2, 3, 10). Hence the feature appeared to have evolved into a vigorous storm system. The change in morphology as observed over the years may reflect changes in morphology of the underlying vortex. However, where is the vortex? As it is not seen by us, we can only infer its presence based upon the features that we do see, and by making comparisons with Neptune’s dark spots (GDS, DS2 – Smith et al., 1989; DS34 – Hammel et al., 1995; and DS15 – Sromovsky et al., 2001), and the one uranian dark spot, UDS, that was detected in its northern hemisphere (Hammel et al., 2009). A close examination of the images of the best-observed feature, the Great Dark Spot (see Smith et al., 1989; Stratman et al., 2001), reveal that the Bright Companion is constantly changing shape and extent as the GDS evolves. Smaller clouds also appear intermittently, often near the

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I. de Pater et al. / Icarus 215 (2011) 332–345 Table 3 Pressure and cloud fractions inferred for 20 August 2007 Berg features. Feature in Fig. 10

Location

A (dim spot) B (brightest spot) C (main streak) D (main streak)

17°E, 25°E, 28°E, 34°E,

30.4°S 27.0°S 25.0°S 23.8°S

DK0 /DH analysis

DH/DHcont analysis Pressure (bars)

Cloud fraction (f) (%)

0.84 0.59 1.8 2.2

1.7 4.0 8.1 7.2

eastern and western tips of the vortex. The long, narrow tails seen at times on the east side of the Berg do appear to have similarities with these tip clouds or elongations of the main GDS companion cloud. More compact cloud features of the Berg might be either orographic or regions of strong convection, with analogous behavior to the bright clouds seen near the center of Neptune’s DS2. Sometimes, we see three small dots in the Berg system, possibly suggestive of multiple vortices packed together. However, anticyclones at the same latitude, unless interspersed with cyclones at slightly different latitudes (i.e., a Kármán vortex street), would be expected to merge rapidly (Youssef and Marcus, 2003). Since the three dots were not stable, it is possible that underlying multiple smaller vortices did merge at times. Still, this type of vortex behavior has not been observed on the ice giants, which appear to favor larger, stand alone vortices. In addition, the UDS mentioned previously appeared to be one vortex, with companion clouds resembling the Berg in structure. In the end, there are only a limited number of observations from which to infer the structure of a potential underlying vortex from the observed cloud. The GDS observations do indicate that the overall cloud complex about a vortex may extend longitudinally considerably beyond the vortex itself. Further, the simulation results of Stratman et al. (2001) indicate that vortices found deeper in the atmosphere produce more extensive clouds than shallow vortices. It seems consistent with observations to assume the vortex’s longitudinal extent is less than the observed Berg complex. If sufficiently deep, it seems plausible that the size of the Berg’s vortex could be on the order of the observed UDS, which was 5° or 2000 km in longitudinal extent. 4.2. Why did the Berg depart from its latitudinal oscillation? In the present paper we show observations of the Berg that follow it almost to the equator. As discussed in Section 3, the Berg follows an almost parabolic track in latitude–time coordinates, where the drift rate increased from almost 4 cm/s during the first 2 years, increasing by factor of 6 in 2008 when latitudes near 15° were reached; it may have slowed to half this value in 2009, within 10° of the equator. Drifting vortices in the form of cyclones (hurricanes) are wellknown meteorological phenomena in Earth’s atmosphere. These low pressure systems usually form near (but not within a few degrees of) the equator and then move gradually poleward. The cyclonic rotation of the winds roughly corresponds to geostrophic balance in which the outward coriolis force is equal to the inward pressure gradient forces. The coriolis force is a function of the surface normal component of the planetary rotation vector interacting with the local wind velocity. The coriolis parameter, fC = 2X sin h, where X is the angular rotation of the planet and h the latitude, is therefore strongest at the poles and zero at the equator. Before Voyager imaged the Great Dark Spot (GDS) on Neptune over a period of several months, significant meridional drift of a vortex on another planet had not been observed. Unlike cyclones on Earth, the GDS drifted towards the equator, consistent with a geostrophically-balanced high-pressure system in which the wind rotation is anti-cyclonic, causing the coriolis force to act inward

Pressure (bars)

Cloud fraction (f) (%)

0.6

1.2

opposite the outward force of the pressure gradient. The underlying vortex of the Berg, at a latitude where the flow has an anticyclonic vorticity, is likely an anticyclonic system; its equatorward migration is then consistent with geostrophic balance. Vortices approximately obey the conservation of potential vorticity, q, which in a simplified form is:

q¼

x þ fC h

¼ Constant;

ð6Þ

where x is the local relative vorticity, and h is the vertical extent of the vortex. The result is that as a vortex moves towards the equator and the coriolis parameter decreases in magnitude, either the relative vorticity or the height of the vortex must change to compensate. On the ice giants, the strong zonal winds mean that the relative vorticity includes both a zonal wind and vortex component. As such, the potential vorticity equation may be rewritten as:

q¼

xm þ xz þ fC h

¼ Constant;

ð7Þ

where xm and xz are the vortex and zonal wind components of the vorticity. The zonal wind component plus the coriolis parameter form the background conditions to which the vortex must adjust as it changes latitude. While observed by Voyager, the GDS on Neptune drifted in latitude from 27° to 17° over a period of 8 months. The subsequent disappearance of the vortex is commonly postulated as resulting from a breaking down of the geostrophic balance as the coriolis force decreases near the equator. Numerical simulations of the GDS presented in LeBeau and Dowling (1998) suggest that vortices approaching the equator will suffer from strong Rossby wave dispersion and therefore lose coherence. As this dissipation region was approached around 15°, the simulated vortex drift slowed noticeably. Unlike Jupiter and Saturn with their multi-jet zonal winds, Uranus and Neptune’s zonal winds are dominated by a single broad jet throughout the equatorial and mid-latitude regions. These broad jets result in large regions of relatively uniform background potential vorticity gradient which are conducive to vortex drift. As shown in LeBeau and Dowling (1998), the value of this gradient strongly influences the rate of drift by the vortex. Assuming this explanation, the necessary mechanisms for a drifting uranian vortex seem to exist. Assuming this explanation and given the drift rate pattern of the Berg, it appears likely that there exists at least two different background potential vorticity shear regions, one corresponding to the slower drift before 25°, the other the region of more rapid drift between 25° and 10°. The relatively sparse data available for tracking Uranus wind speeds means that there is sizeable degree of uncertainty in zonal wind data fits, and therefore the resulting potential vorticity backgrounds. The final decrease in drift rate near the equator may reflect the dissipation of the underlying vortex. The addition of orographic clouds to ice giant vortex simulations in EPIC (the Explicit Planetary Isentropic-Coordinate atmospheric model; Dowling et al., 1998) started with Stratman et al. (2001), which illustrated that bright companion-like clouds could be generated about a GDS-like vortex. Subsequent simulations by

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Deng and LeBeau (2007) and Deng et al. (2009) have shown that such clouds can be created in simulations of drifting vortices as well. The basic modeling approach is to fix a uniform initial relative humidity (typically between 40% and 80%) and then allow the methane to condense as advected by the flowfields over the course of the simulation. The methane clouds form over and next to the vortex and can last for months in simulations. Although this type of pairing of a drifting vortex and orographic cloud has not been simulated for a southern-hemisphere UDS, the available evidence suggests that a drifting but undetected vortex with an orographic companion cloud is a plausible explanation for the observed motions. A barrier to this explanation is the close proximity of the Berg to the equator in 2009, but perhaps the different coriolis strength and zonal wind pattern may allow closer passage of vortices to the equator on Uranus compared to Neptune. Another explanation is that the cloud survives the decay of the vortex and then due to inertia continues to drift equatorward in a weakened state. However, no known simulations of a companion cloud and a dying vortex have been completed, so the motions of such a cloud during and after the demise of the vortex are at this time no more than a conjecture. Hammel et al. (2009) used the EPIC model to determine the stability of the UDS on Uranus. Depending on the precise zonal wind profile assumed in the calculations, they found typical time scales of order 3–4 weeks before the vortex lost coherence. This time scale is far shorter than the observed lifetime of the Berg’s vortex, at least if it is judged by the persistence of its group of companion clouds, which seems to have been present over a period from 2004 (Sromovsky et al., 2007) through 2007 (Sromovsky et al., 2009), possibly as early as 1998, or even 1986, though the absence of observations between the latter two dates make this earlier date highly speculative (Sromovsky and Fry, 2005). 4.3. Are new Bergs forming? Occasionally, bright features appear at the northern edge of the polar collar at 45°. Sometimes they seem to have separated from the band, such as the feature on 8 August 2007. On 26 July 2009, a feature that had separated from the band looks suspiciously similar to the Berg, with a streak and a bright dot polewards of the streak. Could this feature, perhaps, develop into a new Berg? Only time will tell. 5. Conclusions Sromovsky et al. (2009) reported that the Berg, a prominent feature on Uranus that had oscillated between latitudes of 32° and 36° for several decades, had suddenly started on a northward track in 2005. In this paper we show the complete record of observations of this feature’s track towards the equator, including its demise. This record consists of observations of Uranus taken with NIRC2 at 1–2.5 lm on the 10-m W.M. Keck II telescope, and WFPC2 and WFC3 on HST between July 2007 and November 2009. After an initially slow linear drift (0.22°/month  3.7 cm/s), the feature’s drift rate accelerated at latitudes jhj < 25° (up to 1.35°/ month  23 cm/s), and may have slowed down again at jhj < 8°. These different drift velocities suggest variations in the background potential vorticity gradient at different latitudes. The final decrease in drift rate near the equator may reflect the dissipation of the underlying vortex. Just prior to the Berg’s northward track the small dot brightened considerably (4 July 2004), indicative of vigorous convection, where clouds reached altitudes of 0.6 bar. The cloud filling factor was 2% on this date, about 2–4 times higher than on any other

date that the clouds reached these high altitudes. The main part of the Berg, which is generally a long and sometimes multipart streak, is estimated to be much deeper in the atmosphere than the bright spot, near 3.5 bars in 2004, but rising to 1.8–2.5 bars in 2007 after it began its northward drift. The overall morphology of the Berg changed dramatically, starting in July of 2007; it continued to change until its demise in 2009. The Berg is probably tied to a vortex, an anticyclone deeper in the atmosphere, that is visible only through orographic companion clouds. We reach this conclusion based upon similarities with Neptune’s GDS and Uranus’s UDS, and by reviewing model simulations of orographic clouds created in connection with vortices on ice giants, both stable and drifting vortices. Based upon calculations for Neptune, anticyclones are not expected to survive within 15° of the equator. As we see the Berg near a latitude of 5° (assuming it is the same feature), the survival of at least the companion clouds of a vortex so close to the equator needs to be investigated. Specifically, future modeling studies require computer simulations that include orographic cloud formation models and sufficiently fine resolution to capture the morphology of the Berg and the underlying vortex. Parameters in the model that need to be investigated include, e.g., (1) whether a coriolis strength and zonal wind pattern different from those on Neptune may allow closer passage of vortices to the equator; (2) whether the clouds might survive after the vortex has decayed away, and continue to drift equatorward; (3) what is the sensitivity to changes in the atmospheric vertical pressure–temperature profile, the vertical wind shear, and the background vorticity gradient; (4) what is the dependence of vortex stability and migration on vortex strength, its vertical structure, and its depth in the atmosphere; (5) how does the observed cloud structure relate to the underlying vortex structure, strength, size, and depth. In order to gain a better understanding of the longevity and migration of vortices, on the observational side, Uranus should be monitored at high spatial resolution at visible and near-infrared wavelengths. Hammel et al. (2009) showed that observations at 550–800 nm are ideal to identify dark spots, while H band observations with Keck AO best reveal the bright companion clouds. Through continued monitoring we may, perhaps, witness morphological changes in the long-lived northern Bright Cloud Complex, and perhaps see a drift in latitude in this Complex. Continued monitoring may, some day, show the formation of a new Berg in either the southern or northern hemisphere. Moreover, in addition to obtaining information on vortices, regular monitoring helps to characterize the progression of seasons, with the formation of a north polar cap and enhancement of the northern polar collar. Acknowledgments We thank Marcos van Dam for trading an hour of Keck time in September 2007. The near-infrared data were obtained with the W.M. Keck Observatory, which is operated by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The WFPC2 and the WFC3 845 nm wavelength data were obtained with the NASA/ESA Hubble Space Telescope as part of HST programs GO11156 (SNAP, WFPC2; PI: K. Rages) and GO11573 (WFC3; PI: L. Sromovsky), with support provided by NASA through grants from the Space Telescope Science Institute to the PIs by the Association of Universities for Research in Astronomy, Inc., under NASA Contract NAS 5-26555. This work was further supported in part by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement AST 98-76783. In addition, IdP. acknowledges support from

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NASA Grant NNX07AK70G; HBH from NASA Grants NNX06AD12G and NNX07AO43G; LAS from NASA Grant NNX08A051G; RPL from NASA Grant NNX11AC01G. The authors extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented would have been possible. References Colina, L., Bohlin, R.C., Castelli, F., 1996. The 0.12–2.5 micron absolute flux distribution of the Sun for comparison with solar analog star. Astron. J. 112, 307–315. Deng, X., LeBeau Jr., R.P., 2007. Comparative CFD simulations of the dark spots of Uranus and Neptune. In: 37th AIAA Fluid Dynamics Conference and Exhibit, AIAA 2007-4095, Miami, FL, June 25–28. Deng, X., LeBeau Jr., R.P., Palotai, C., 2009. Numerical investigation of orographic cloud and vortex dynamics on the ice giant planets. In: 1st AIAA Atmospheric and Space Environments Conference, AIAA-2009-3643, San Antonio, TX, June 22–25. de Pater, I., Romani, P.N., Atreya, S.K., 1991. Possible microwave absorption by H2S gas in Uranus and Neptune’s atmospheres. Icarus 91, 220–233. de Pater, I., Gibbard, S.G., Macintosh, B., Roe, H.G., Gavel, D., Max, C.E., 2002. Keck adaptive optics images of Uranus and its rings. Icarus 160, 359–374. de Pater, I., Gibbard, S.G., Hammel, H.B., 2006. Evolution of the dusty rings of Uranus. Icarus 180, 186–200. Dowling, T.E., Fischer, A.S., Gierasch, P.J., Harrington, J., LeBeau, R.P., Santori, C.M., 1998. The Explicit Planetary Isentropic-Coordinate (EPIC) atmospheric model. Icarus 132, 221–238. Elias, J.H., Frogel, J.A., Matthews, K., Neugebauer, G., 1982. Infrared standard stars. Astron. J. 87, 1029–1034. Fruchter, A.S., Hook, R.N., 2002. Drizzle: A Method for the linear reconstruction of undersampled images. The Publications of the Astronomical Society of the Pacific 114, 144–152. Fry, P.M., Sromovsky, L.A., 2007. Keck NIRC2 photometry of Uranus, uranian satellites, and Triton in August 2004. Icarus 192, 117–134. Gonzaga, S., Biretta, J., et al., 2010, in HST WFPC2 Data Handbook, v. 5.0, ed., Baltimore, STScI Hammel, H.B., Baines, K.H., Bergstrahl, J.T., 1989. Vertical aerosol structure of Neptune: Constraints from center-to-limb profiles. Icarus 80, 416–438. Hammel, H.B., Lockwood, G.W., Mills, J.R., Barnet, C.D., 1995. Hubble Space Telescope imaging of Neptune’s cloud structure in 1994. Science 268, 1740– 1742. Hammel, H.B., Rages, K., Lockwood, G.W., Karkoschka, E., de Pater, I., 2001. New measurements of the winds of Uranus. Icarus 153, 229–235. Hammel, H.B., de Pater, I., Gibbard, S.G., Lockwood, G.W., Rages, K., 2005a. Uranus in 2003: Zonal winds, banded structure, and discrete features. Icarus 175, 534– 545. Hammel, H.B., de Pater, I., Gibbard, S.G., Lockwood, G.W., Rages, K., 2005b. New cloud activity on Uranus in 2004: First detection of a southern feature at 2.2 microns. Icarus Note 175, 284–288.

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Hammel, H.B., Sromovsky, L.A., Fry, P.M., Rages, K., Showalter, M., de Pater, I., van Dam, M., LeBeau, R.P., Deng, X., 2009. The dark spot in the atmosphere of Uranus in 2006: Discovery, description, and dynamical simulations. Icarus 201, 257– 271. Irwin, P.G.J., Teanby, N.A., Davis, G.R., 2010. Revised vertical cloud structure of Uranus from UKIRT/UIST observations and changes seen during Uranus’ Northern Spring Equinox from 2006 to 2008: Application of new methane absorption data and comparison with Neptune. Icarus 208, 913–926. Karkoschka, E., 1998. Clouds of high contrast on Uranus. Science 280, 570–572. Karkoschka, E., Tomasko, M., 2009. The haze and methane distributions on Uranus from HST–STIS spectroscopy. Icarus 202, 287–309. Karkoschka, E., Tomasko, M., 2010. Methane absorption coefficients for the jovian planets from laboratory, Huygens, and HST data. Icarus 205, 674–694. Kim Quijano, J. et al., 2009. WFC3 Mini-Data Handbook, Version 1.0 (Baltimore: STScI). Krist, J., 1995. Astronomical data analysis software and systems IV. Astron. Soc. Pacific Conf. Ser. 77, 349–352. LeBeau, R.P., Dowling, T.E., 1998. EPIC simulations of time-dependent, threedimensional vortices with application to Neptune’s great dark spot. Icarus 132, 239–265. Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshelman, V.R., Hinson, D.P., Tyler, G.L., 1987. The atmosphere of Uranus: Results of radio occultation measurements with Voyager 2. J. Geophys. Res. 92, 14987–15002. Smith, B.A. et al., 1986. Voyager 2 in the uranian system: Imaging science results. Science 233, 43–64. Smith, B.A. et al., 1989. Voyager 2 at Neptune: Imaging science results. Science 246, 1422–1449. Sromovsky, L.A., Fry, P.M., 2005. Dynamics of cloud features on Uranus. Icarus 179, 459–484. Sromovsky, L.A., Fry, P.M., 2007. Spatially resolved cloud structure on Uranus: Implications of near-IR adaptive optics imaging. Icarus 192, 527–557. Sromovsky, L.A., Fry, P.M., Baines, K.H., Dowling, T.E., 2001. Coordinated 1996 HST and IRTF imaging of Neptune and Triton. III. Neptune’s atmospheric circulation and cloud structure. Icarus 146, 459–488. Sromovsky, L.A., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., 2007. Dynamics, evolution, and structure of Uranus’ brightest cloud feature. Icarus 192, 558–575. Sromovsky, L.A., Fry, P.M., Hammel, H.B., Ahue, A.W., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M., 2009. Uranus at Equinox: cloud morphology and dynamics. Icarus 203, 265–286. Sromovsky, L.A, Fry, P.M., Kim, J.H., 2011. Methane on Uranus: The case for a compact CH4 cloud layer at low latitudes and a severe CH4 depletion at high latitudes based on re-analysis of Voyager occultation measurements and STIS spectroscopy. Icarus 215, 292–312. Stratman, P.W., Showman, A.P., Dowling, T.E., Sromovsky, L.A., 2001. EPIC simulations of bright companions to Neptune’s great dark spots. Icarus 151, 275–285. van Dokkum, P., 2001. Cosmic-ray rejection by Laplacian edge detection. Publ. Astron. Soc. Pacific 113, 1420–1427. Youssef, A., Marcus, P.S., 2003. The dynamics of jovian white ovals from formation to merger. Icarus 162, 74–93.