The Dark Spot in the atmosphere of Uranus in 2006: Discovery, description, and dynamical simulations

Icarus 201 (2009) 257–271

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The Dark Spot in the atmosphere of Uranus in 2006: Discovery, description, and dynamical simulations ✩ H.B. Hammel a,∗ , L.A. Sromovsky b , P.M. Fry b , K. Rages c , M. Showalter c , I. de Pater d , M.A. van Dam e , R.P. LeBeau f , X. Deng f a

Space Science Institute, Boulder, CO 80303, USA Space Science and Engineering Center, University of Wisconsin, Madison, WI 53706, USA c SETI Institute, Mountain View, CA 94043, USA d University of California, Berkeley, CA 94072, USA e W.M. Keck Observatory, Kamuela, HI 96743, USA f Mechanical Engineering, University of Kentucky, Lexington, KY 40506, USA b

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 18 March 2008 Revised 29 July 2008 Accepted 12 August 2008 Available online 13 February 2009 Keywords: Uranus, atmosphere

We report the first definitive detection of a discrete dark atmospheric feature on Uranus in 2006 using visible and near-infrared images from the Hubble Space Telescope and the Keck II 10-m telescope. Like Neptune’s Great Dark Spots, this Uranus Dark Spot had bright companion features that exhibited considerable variability in brightness and location relative to the Dark Spot. We detected the feature or its bright companions on 16 June (Hubble), 30 July and 1 August (Keck), 23–24 August (Hubble), and 15 October (Keck). The dark feature—detected at latitude ∼28 ± 1◦ N with an average physical extent of roughly 2◦ (1300 km) in latitude and 5◦ (2700 km) in longitude—moved with a nearly constant zonal velocity of 43.1 ± 0.1 m s−1 , which is roughly 20 m s−1 greater than the average observed speed of bright features at this latitude. The dark feature’s contrast and extent varied as a function of wavelength, with largest negative contrast occurring at a surprisingly long wavelength when compared with Neptune’s dark features: the Uranus feature was detected out to 1.6 μm with a contrast of −0.07, but it was undetectable at 0.467 μm; the Neptune GDS seen by Voyager exhibited its most prominent contrast of −0.12 at 0.48 μm, and was undetectable longward of 0.7 μm. Computational fluid dynamic simulations of the dark feature on Uranus suggest that structure in the zonal wind profile may be a critical factor in the emergence of large sustained vortices. © 2009 Elsevier Inc. All rights reserved.

1. Introduction A striking difference between the atmospheres of the ice giants Neptune and Uranus has been the existence of multiple Great Dark Spots in the former (Smith et al., 1989; Hammel et al., 1995; Sromovsky et al., 2002), and the marked absence of such features in the latter (Smith et al., 1986). This distinguishing difference had been attributed to the lack of dynamical activity in general on Uranus, suggestive of inhibited vertical convection perhaps via a ✩ Based on observations with both the NASA/ESA Hubble Space Telescope and the W.M. Keck Observatory. Hubble is administered by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated under NASA Contract NAS5-26555. The Keck Observatory, which was made possible by the generous financial support of the W.M. Keck Foundation, is operated as a scientific partnership by the California Institute of Technology, the University of California, and NASA. Corresponding author. E-mail address: [email protected] (H.B. Hammel).

*

0019-1035/$ – see front matter doi:10.1016/j.icarus.2008.08.019

©

2009 Elsevier Inc. All rights reserved.

stably stratified atmosphere or due to the lack of an internal heat source (e.g., Allison et al., 1991, and references therein). Recent observations of Uranus have shown evidence of an increase in detectable activity as the planet’s northern hemisphere has experienced increased insolation (Hammel et al., 2005b; Sromovsky and Fry, 2005). The last Uranus equinox was in 1965, prior to the development of astronomical imaging devices capable of documenting discrete features on the 3.8-arcsec disk of Uranus. Even the southern hemisphere of Uranus has now begun to show short-term dynamic activity (Hammel et al., 2005a). (We use the IAU convention for defining the pole position, where the southern hemisphere has been pointing toward the Sun and Earth for the past few decades.) All of the confirmed discrete atmospheric features on Uranus to date have been bright structures. Prior unconfirmed reports of discrete dark features on Uranus included: sketches made in the early 1900s (Alexander, 1965); a single mention in Smith et al. (1986) based on low-contrast ultraviolet imaging during the 1986

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H.B. Hammel et al. / Icarus 201 (2009) 257–271

Voyager encounter (Voyager images yielded no visible-wavelength dark spots); and a single sighting in near-infrared observations taken in 1993 at a ground-based observatory (Wild et al., 1993) that was never published since it could not be confirmed. From 1994 through early 2006, Hubble Space Telescope images showed no independently confirmed discrete dark features despite regular coverage. Karkoschka (1998) mentions a dark feature in a single 1997 Hubble image at 0.34 μm at latitude 31◦ S, detectable only when updated low-noise flatfields were employed in processing (E. Karkoschka, personal communication), and never seen in any subsequent Hubble images. No dark features were seen in any near-infrared adaptive optics images with exquisite quality taken prior to 2006 at the Keck II 10-m telescope (Hammel et al., 2005b; Sromovsky and Fry, 2005). Here, we report the first confirmed detection of a Uranus Dark Spot (UDS) seen with both Hubble and Keck at multiple epochs (Fig. 1). 2. Observations 2.1. Hubble images We discovered the dark feature in images of Uranus in late August 2006 (Table 1) using the High Resolution Camera (HRC) on Hubble’s Advanced Camera for Surveys (ACS). The UDS was detected in the northern hemisphere of Uranus in multiple images (Fig. 2), along with a bright companion (BC) detectable in red and methane-band images. The altitude levels to which different filters are sensitive is discussed thoroughly in Sromovsky et al. (2007; see their Fig. 3). The mean resolution in these images was of order 0.07 or ∼1000 km on Uranus (Table 2 shows pixel scales and effective resolutions with this and other camera systems). We later located pre-discovery images of the UDS and its bright companion in June 2006, also obtained with Hubble’s ACS/HRC, though the filter selection differed (Table 1). We were unable to detect the features in Wide Field Planetary Camera 2 images taken with Hubble throughout that time period, due mostly to the features’ low contrast as well as the slightly larger pixel size (0.046 versus 0.027 ). At several different times during the past decade, Hubble observations of Uranus have generated complete longitudinal coverage of these latitudes, at the same wavelengths (e.g., Karkoschka, 1998, 2001). Other than the aforementioned Karkoschka (1998) dark feature, we know of no Hubble images with discrete dark features prior to June 2006, including images with the same camera and filters used here. 2.2. Keck images We also obtained images of Uranus with the 10-m Keck II telescope on Mauna Kea during two different observational windows in 2006 (July/August and October), using the near-infrared camera NIRC2 coupled to the adaptive optics system (Table 1). The dark feature was detected in a few Keck adaptive optics (AO) images, along with multiple detections of associated, extensive bright structure surrounding the UDS, reminiscent of the bright companion structure seen with Neptune’s Great Dark Spots, discussed further below. The mean resolution in the Keck narrow-angle images was of order 0.04 or ∼600 km on Uranus (Table 2). No discrete dark feature was seen in earlier Keck images of Uranus taken over the past decade, usually with the same camera and filter system (Hammel et al., 2001, 2005a, 2005b; Sromovsky and Fry, 2005). 2.3. Image reduction For the Hubble observations, we reduced the images, navigated the planetary disks, and measured feature locations using procedures described in Sromovsky and Fry (2005). The Keck data

were analyzed following procedures discussed in de Pater et al. (2006). 3. Dark feature characteristics 3.1. Latitude and spatial extent of dark feature Fig. 3 and Table 3 summarize our measurements of the UDS’s location and zonal velocity. Based on all detections, the average latitude of the Dark Spot is 27.7 ± 0.8◦ N. At its maximum in a Hubble F658N image on 23 August, the storm extended about 2.4◦ in latitude and 5.7◦ in longitude (Figs. 1 and 2), corresponding to roughly 1600 by 3000 km. Note that its apparent size varied with wavelength and time (see Fig. 3), and was often difficult to assess due to the feature’s low contrast. Over the period of the 2006 observations, from June through October, the dark feature’s latitude appeared to be constant to within our ability to measure it, with no significant latitudinal drift. Similarly, Neptune’s northern GDS features did not exhibit latitudinal drift over a timescale of several years (Sromovsky et al., 2002); in contrast, Neptune’s southern Great Dark Spot (GDS-89) drifted ∼15◦ over one year (Hammel et al., 1995; Sromovsky et al., 2002). Further observations of the UDS in subsequent years (if they can be conclusively identified as this same feature) may constrain its latitudinal stability. The Uranus dark feature falls squarely within a range of latitudes that Hammel et al. (2001) defined as an “active” region: the regions from 23◦ –32◦ in both hemispheres are known for their proclivity of features. We can only speculate whether this region is simply unstable and hence prone to production of multiple features. Similar latitudes on Saturn showed “storm alleys” in Cassini observations (e.g., Dyudina et al., 2007); likewise, a current planetary-scale disturbance on Jupiter may be linked to the presence of storms at mid latitudes (e.g., Sánchez-Lavega et al., 2008). Another interesting possibility is that the underlying atmospheric disturbance that created the UDS has been in existence for several years, and the many bright northern features near this latitude are detectable tracers of the motion of the (hitherto unseen) dark underlying disturbance, which may not necessarily be stable in latitude and/or longitude. 3.2. Zonal velocity of dark feature The longitudes of the Uranus dark feature (Fig. 3) are measured relative to the assumed 17.24-h internal rotation period of Uranus (Seidelmann et al., 2002) and are, by convention, between 0◦ and 360◦ . However, as the feature drifts in longitude, it may circumnavigate the planet an unknown integral number of times between observations with wide temporal separations (i.e., its longitude would be greater than 360◦ ). Fig. 3 shows measured longitudes for the UDS as well as the bright companion (that feature is discussed in more detail below); for observations after June, Fig. 3 also shows the measured longitudes plus multiples of 360◦ . For both features, the best fit to a constant drift velocity model was obtained if the feature circumnavigated the planet twice between June and August (i.e., lonfit = lonmeasured + 720◦ ), and thrice between June and October (lonfit = lonmeasured + 1080◦ ). We computed fits for the data weighted by the uncertainties in their measured longitudes, as well as fits that were unweighted (Table 3, Fig. 3). The errors in the unweighted fit are estimated from the standard deviation of the points around the fit line instead of by a priori estimated measurement errors. Similar fits were conducted for the bright companion. For the UDS, the principle source of measurement errors is the difficulty of locating the feature borders for this low-contrast

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Fig. 1. Great Dark Spot of Uranus and its bright companion clouds. Upper panel: this color-composite image was created from images taken with Hubble’s ACS High-Resolution Camera (HRC) on 23 August 2006 at wavelengths of 0.550 μm (assigned to blue), 0.658 μm (assigned to green) and 0.775 μm (assigned to red). The blue and green images were deconvolved to enhance contrast using simple Fourier deconvolution with a point-spread function (PSF) obtained from Tiny Tim software (Krist, 1995). To restore appropriate spatial resolution, these images were then reconvolved with the core of the PSF (i.e., the PSF modified by setting it to zero outside the first diffraction minimum, in this case a radius of 2 pixels for the HRC). The blue image was further processed to correct for limb darkening using a Minnaert function with an exponent of 0.7, partway between no limb darkening (0.5) and Lambertian limb darkening (1.0). The red image was not deconvolved based on past aesthetic experience: strong limb brightening at this wavelength leads to an unnatural bright-red planetary edge. The inset image shows a magnified view of the dark spot with enhanced contrast. The north pole of Uranus is to the right near the 3 o’clock position. The bright band in the southern hemisphere is at 45◦ S. Lower panel: this false-color image was taken with the Keck’s NIRC2 camera on 30 July 2006 at a wavelength of 1.6 microns. The inset shows a magnified view of the complex of bright features associated with the dark feature seen at shorter wavelengths. The orientation is the same as in the upper panel.

260 H.B. Hammel et al. / Icarus 201 (2009) 257–271 Fig. 2. Uranus Dark Spot and bright companion in Hubble and Keck images from 2006. Each map’s central latitude is 30◦ N (planetocentric), with the map’s central longitude being the expected location of the UDS. Each column is a different night except for the last four columns, which were all obtained on 15 October at Keck. The images in the second and third October columns were reordered from Table 1 to facilitate night-to-night comparisons. The UDS is most evident in Hubble images on 16 June and 23 August (columns 1 and 4); it is on the limb in the 24 Aug Hubble data (column 5) and thus difficult to detect and display. In the first two columns of 15 October Keck data (columns 6 and 7), the dark spot is nestled within the surrounding bright material (e.g., the top three images in column 6: frames n103, n104, and n105). Each panel subtends 30◦ in latitude and 60◦ in longitude. The labels in each panel are abridged Hubble or Keck filename (upper left), filter (lower left), and the central west longitude (lower right). The Hubble images generally sense to greater depth than the Keck images; hence they show the UDS whereas the Keck data show surrounding bright structure at higher altitude (see Fig. 3 of Sromovsky et al., 2007, for a quantitative estimate of altitude versus filter bandpass).

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Table 1 Observation log for Uranus images in 2006. Filename

Date = DOYa

UT time

Detectorb

Filterc

Exp.

Dark feature

(s)d

Con.e

Latitude

Longitude

Bright companion Con.e

Latitude

Longitude

– – – 110.2 ± 0.5 109.1 ± 0.4

– – – 0.04 0.05

– – – 32.1 ± 0.1 32.6 ± 0.3

– – – 108.2 ± 0.6 108.1 ± 0.8

j9dh35h0 j9dh35h1 j9dh35h2 j9dh35h5 j9dh35h9

16 16 16 16 16

Jun = 167 Jun = 167 Jun = 167 Jun = 167 Jun = 167

16:09:44 16:11:18 16:13:00 16:24:52 16:39:56

Hubble ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC

n0002 n0003 n0004 n0005 n0006 n0007

30 30 30 30 30 30

Jul = 211 Jul = 211 Jul = 211 Jul = 211 Jul = 211 Jul = 211

10:51:37 10:53:28 10:55:25 10:58:08 11:01:06 11:04:08

Keck NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA

H H H K K K

4 × 15. 4 × 15. 4 × 15. 5 × 24. 5 × 24. 5 × 24.

– – – – – –

– – – – – –

– – – – – –

0.22 0.23 0.21 0.49 0.39 0.62

27.2 ± 0.4 26.9 ± 0.9 27.3 ± 0.8 28.2 ± 0.3 28.7 ± 0.3 28.7 ± 0.7

157.2 ± 0.5 157.0 ± 0.6 156.9 ± 0.4 161.1 ± 0.4 160.3 ± 0.5 160.4 ± 0.5

n0064 n0065 n0066 n0070 n0071 n0072

1 1 1 1 1 1

Aug = 213 Aug = 213 Aug = 213 Aug = 213 Aug = 213 Aug = 213

11:54:53 11:56:30 11:58:08 12:13:05 12:15:42 12:18:23

Keck NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA

H H H K K K

4 × 15. 4 × 15. 4 × 15. 4 × 30. 4 × 30. 4 × 30.

– – – – – –

– – – – – –

– – – – – –

0.11 0.10 0.10 0.83 0.91 1.09

26.9 ± 0.7 26.9 ± 0.4 26.9 ± 0.5 31.1 ± 0.4 30.5 ± 0.3 30.7 ± 0.1

175.1 ± 1.8 175.5 ± 1.5 175.3 ± 1.3 179.9 ± 0.9 179.8 ± 1.1 179.8 ± 0.4

j9q351ly j9q351lz j9q351m0 j9q351m1 j9q351m2

23 23 23 23 23

Aug = 235 Aug = 235 Aug = 235 Aug = 235 Aug = 235

09:57:24 10:06:14 10:15:49 10:18:00 10:25:12

Hubble ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC

F250W F250W F550M F658N F775W

450. 450. 46.f 354.f 46.f

– –

– –

−0.02 −0.02 −0.03

– – 27.7 ± 0.5 27.8 ± 0.2 27.5 ± 0.5

– – – – –

– – – – –

– – – – –

j9q351m3 j9q351m4

23 Aug = 235 23 Aug = 235

10:27:25 10:34:35

Hubble ACS/HRC ACS/HRC

F892N F892N

350. 350.

– –

– –

– –

0.09 0.09

29.8 ± 0.5 29.5 ± 0.2

26.4 ± 0.1 25.7 ± 0.4

j9q356rk j9q356rl j9q356rm j9q356rn j9q356ro j9q356rp j9q356rq j9q356rr j9q356rs j9q356rt

24 24 24 24 24 24 24 24 24 24

Aug = 236 Aug = 236 Aug = 236 Aug = 236 Aug = 236 Aug = 236 Aug = 236 Aug = 236 Aug = 236 Aug = 236

17:55:55 17:58:42 18:01:00 18:03:11 18:10:35 18:13:41 18:17:11 18:24:01 18:30:51 18:38:20

Hubble ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC ACS/HRC

F775W F435W F550M F658N F850LP F850LP F892N F892N F892N F775W

64.f 45.f 46.f 354.f 102.f 102.f 330. 330. 330. 46.f

−0.01

27.5 ± 0.1 – – 27.4 ± 0.1 28.4 ± 0.1 28.4 ± 0.1 – – – 27.8 ± 0.2

39.5 ± 0.6 – – 36.7 ± 0.8 42.4 ± 0.3 40.4 ± 0.6 – – – 38.4 ± 0.4

0.01 – – – 0.03 – 0.02 0.07 0.07 –

28.3 ± 0.3 – – – 28.2 ± 0.7 – 29.4 ± 0.1 29.6 ± 0.1 28.5 ± 0.1 –

44.9 ± 0.7 – – – 38.0 ± 0.3 – 37.5 ± 0.5 37.2 ± 0.8 39.3 ± 0.4 –

n0103 n0104 n0105 n0106 n0107 n0108

15 15 15 15 15 15

Oct = 288 Oct = 288 Oct = 288 Oct = 288 Oct = 288 Oct = 288

05:30:40 05:32:28 05:34:18 05:36:53 05:39:54 05:42:51

Keck NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA

H H H K K K

4 × 15. 4 × 15. 4 × 15. 5 × 24. 5 × 24. 5 × 24.

– – –

27.6 ± 0.2 27.1 ± 0.3 27.6 ± 0.3 – – –

151.7 ± 0.9 152.3 ± 0.7 150.3 ± 0.9 – – –

0.15 0.16 0.17 0.81 0.64 1.10

27.6 ± 1.0 28.5 ± 0.9 27.7 ± 0.5 31.0 ± 0.1 30.5 ± 0.4 31.0 ± 0.3

149.1 ± 3.4 144.1 ± 2.7 143.5 ± 2.7 150.2 ± 1.0 149.5 ± 1.4 150.4 ± 1.1

n0134 n0135 n0136

15 Oct = 288 15 Oct = 288 15 Oct = 288

07:01:40 07:05:36 07:08:10

Keck NIRC2/NA NIRC2/NA NIRC2/NA

H H H

4 × 15. 4 × 15. 4 × 15.

−0.05 −0.04 −0.03

27.8 ± 0.3 26.9 ± 0.7 28.0 ± 0.2

154.4 ± 0.4 153.3 ± 0.5 154.5 ± 0.4

0.15 0.15 0.14

27.8 ± 0.6 27.5 ± 0.4 27.1 ± 0.7

151.3 ± 1.5 151.4 ± 1.5 150.7 ± 1.7

n0137 n0138 n0139 n0140 n0141

15 15 15 15 15

Oct = 288 Oct = 288 Oct = 288 Oct = 288 Oct = 288

07:10:47 07:12:44 07:15:01 07:17:57 07:21:51

Keck NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA NIRC2/NA

Jg Jg Jg Kg Kg

4 × 15. 4 × 15. 4 × 15. 5 × 24. 5 × 24.

– –

– – 26.1 ± 0.8 – –

– – 152.9 ± 0.5 – –

0.07 0.05 0.06 0.74 0.97

26.4 ± 0.8 27.6 ± 1.4 25.8 ± 0.3 30.1 ± 0.6 29.7 ± 1.3

150.7 ± 1.5 153.6 ± 1.4 151.3 ± 1.1 150.8 ± 0.5 150.8 ± 0.6

n0142 n0143 n0144 n0145 n0152 n0154 n0156

15 15 15 15 15 15 15

Oct = 288 Oct = 288 Oct = 288 Oct = 288 Oct = 288 Oct = 288 Oct = 288

07:26:27 07:27:08 07:28:36 07:29:19 07:51:35 08:00:32 08:07:48

Keck NIRC2/WA NIRC2/WA NIRC2/WA NIRC2/WA NIRC2/WA NIRC2/WA NIRC2/WA

Jh Jh Hh Hh Pbeta Hcont Jcont

4 × 1. 4 × 1. 4 × 1. 4 × 1. 10 × 10. 10 × 10. 12 × 10.

– – – – 29.1 ± 0.2 27.9 ± 0.5 –

– – – – 154.2 ± 0.5 152.0 ± 0.6 –

0.04 0.03 0.13 0.13 0.02 0.02 0.33

29.7 ± 0.3 148.7 ± 0.7 30.5 ± 0.6 148.6 ± 0.8 29.9 ± 0.9 148.7 ± 1.0 29.4 ± 0.7 148.7 ± 2.2 30.2 ± 0.7 149.6 ± 0.7 29.2 ± 0.5 148.9 ± 0.6 30.1 ± 0.1 154.8 ± 0.5 (continued on next page)

F475W F606W F330W F814W F814W

15. 10. 210. 70. 70.

– – –

−0.02 −0.02

– – – 27.9 ± 0.1 28.5 ± 0.3

– –

−0.02 −0.02 −0.03 – – –

−0.02 −0.05 −0.05 −0.07

−0.02 – –

– – – –

−0.01 −0.02 –

25.3 ± 0.1 26.1 ± 0.1 26.0 ± 0.2

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Table 1 (continued) Filename

n0160 n0161

Date = DOYa

15 Oct = 288 15 Oct = 288

UT time

09:19:58 09:23:51

Detectorb

Keck NIRC2/NA NIRC2/NA

Filterc

H H

Exp.

Dark feature

(s)d

Con.e

Latitude

Longitude

Con.e

Bright companion Latitude

Longitude

4 × 15. 4 × 15.

−0.07 −0.04

26.9 ± 0.5 28.0 ± 0.2

154.1 ± 0.4 154.1 ± 0.2

0.05 0.04

29.0 ± 0.7 28.8 ± 0.4

150.6 ± 0.3 149.4 ± 0.3

DOY = Day of year, used in Fig. 3. Keck’s NIRC2 camera has both narrow-angle (NA) and wide-angle (WA) settings. c Bandpasses for the Hubble and Keck filters are illustrated in Fig. 6; filter details are given in the references provided in that figure’s caption. d Keck exposure times are shown as number of coadds times the exposure of each coadd. e The contrast values are lower limits, since we have not actually resolved any of these features. Fig. 2 shows images with measured contrasts, although the order differs from this table (see notes g and h below). a

b

f g h

For these Hubble images, the gain was 4.235; otherwise the Hubble gain was 2.216. In column 2 of Fig. 2, the two K images are shown above the three J images to facilitate comparison with other nights. In column 3 of Fig. 2, the two H images are shown above the two J images to facilitate comparison with other nights.

Table 2 Spatial resolution of images in 2006. Camera

Pixel size ( )

Each pixel at Uranusa

Diffraction limita

Effective resolution (km)

Hubble ACS/HRC Keck NIRC2/NA Keck NIRC2/WA

0.027 0.010 0.040

380 km 140 km 560 km

0.052 at 0.5 μm, 0.094 at 0.9 μm 0.030 at 1.2 μm, 0.040 at 1.6 μm, 0.055 at 2.2 μm 0.030 at 1.2 μm, 0.040 at 1.6 μm, 0.055 at 2.2 μm

740, 1330 600b , 560, 770 1120c , 1120c , 1120c

a We assume diffraction-limited images, which is frequently the case with the Keck AO-corrected images. Occasionally, poor seeing degrades the effective resolution. During our observations, 1 at Uranus subtends about 14,000 km. b For the Keck narrow-angle J-band, the telescope rarely reaches the diffraction limit; our experience suggests that 0.043 is a more typical value. c For the Keck wide-angle mode, effective resolution is determined by Nyquist sampling, i.e., two pixels (0.080 ).

barely-resolved feature. For the BC, the principle source of error was the innate complexity of the structure, making feature positions problematic at best. The various fit parameters in Table 3 provide a quantitative estimate of the uncertainties inherent in these measurements, and the dispersion in the actual longitude measurements can be seen directly in Fig. 3 (panels c, d, g, and h). The unweighted fits (Table 3) allow us to account for unanticipated spatial deviations during our estimation of uncertainties in the fitted parameters. For example, such deviations may include feature drift as a function of time, and the disagreement between the computed UDS and BC drift rates may thus arise because uneven temporal sampling leads to different weighting of non-uniform motion: i.e., no UDS data were obtained with Keck in July/August; and many measurements were made of the BC in Keck October data. We find reasonable agreement for both the UDS and BC when using an unweighted fit that excludes the October Keck data (Table 3): 43.12 ± 0.09 m s−1 for the UDS and 42.86 ± 0.13 m s−1 for BC. The simplest interpretation for the discrepancy of the October data is that the feature did not always drift at a constant rate. Such behavior is perhaps not surprising, given that other Uranus features appear to exhibit variable drift rates: e.g., a very bright feature at 30◦ N (Sromovsky et al., 2007) and a bright feature near 34◦ S (Sromovsky and Fry, 2005). Regardless of how it is computed, the dark feature’s zonal velocity is higher than all other velocities measured to date near that latitude (Fig. 4). As discussed below in the modeling section, deviations from a smooth zonal wind profile are likely to create such vortex features, and such deviations could account for the significant dispersion of feature velocities seen at these latitudes. Another possible explanation exists for the UDS’s deviation from the overall trend exhibited by bright features: the underlying “disturbance” may be at an atmospheric level that differs from that of the average bright feature. Vertical wind shear can be estimated from horizontal temperature gradients under the assumptions of hydrostatic and geostrophic balance (Conrath et al., 1991). Thermal observations of Uranus suggest a general decay of wind speed with height, consistent with the dark-spot-producing disturbance being deeper than the surrounding bright features. Fig. 13 of Conrath et

al. (1991) suggests very small shears in the 28◦ N region, however, making it difficult to create a quantitative case for this or to estimate how many scale heights of displacement might be required. It may be possible to derive limited relative altitude estimates from the features’ wavelength dependence and center-to-limb reflectivity changes; this is work in progress (discussed further below). Finally, we note that if a vortex—such as that which presumably formed the dark feature—controls the motions of bright features, and if the brightest of the bright features preferentially form on the poleward side of the vortex, then the actual wind profile might be that of the vortex (i.e., the dark feature), while the commonly observed profile (e.g., that determined from the brightest features) is in fact somewhat offset in latitude from its true position. 3.3. Temporal variability of the dark feature With only four distinct observation windows (Hubble in June and August, and Keck in July/August and October), we cannot yet deduce much about the dark feature’s long-term temporal variability. A feature with similar structure does appear to be present in 2007 Keck II images at a planetographic latitude of 28.6◦ N; it is currently under investigation. There is definite evidence for shortterm variation in the contrast of the UDS, discussed below. We do know that no dark feature has been detected in earlier Hubble ACS images and Keck images (see references in Sections 2.1 and 2.2). If such features are short-lived (lasting just weeks or months), then we must concede the possibility that such a feature may have come and gone unnoticed, since our temporal coverage with Hubble and Keck is far from complete. However, other possible explanations—viewing geometry changes, wavelength choice, varying spatial resolution or S/N (Karkoschka, 2001)—cannot account for the lack of past detections of such a feature. If this feature becomes physically more extended, or increases in contrast relative to the surrounding atmosphere, it could be detectable in ground-based images taken with smaller-aperture telescopes. The past reports of dark features suggest this phenomenon may have occurred in the past, but this is the first time such a feature on Uranus has been tracked and verified by two independent facilities. Likewise, Neptune data are too limited to permit any detailed com-

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Fig. 3. Uranus Dark Spot and bright companion latitudes and longitudes. In panels (a) and (e), a circle marks the central latitude of each measurement; latitudinal extremes are marked with plus signs; and asterisks mark the latitude of the longitudinal extremes. The straight line indicates the overall mean latitude of all measurements, and the surrounding gray regions mark the 1-σ uncertainty. In panels (b) and (f), the central longitude of each feature measurement is marked with a circle, its longitude extremes are marked with plus signs, and asterisks mark the longitude of the latitudinal extremes. For the later measurements, we replot the longitudes adding integral multiples of 360◦ . The lines are linear fits to either all the points [solid in both (b) and (f)], or all points except those in October = DOY 288 [dotted in (b) and dashed in (f)]. From the slopes, we compute the zonal velocities at that latitude relative to the 17.24-h rotational period of Uranus (Table 3). In panels (c) and (g), we plot longitude residuals for the fits to all the points [solid lines in panels (b) and (f)]. In the two bottom panels (d) and (h), we plot longitude residuals for fits that exclude the October data for both the Dark Spot (dotted line), and the bright companion (dashed line). Together, the two Hubble measurements and the first Keck set exhibit a stable drift rate that corresponds to a zonal velocity of 43.0 ± 0.2 m s−1 (Table 3). The scatter for the first three sets of measurements reflects the difficulty of measuring these features’ positions, but the offset seen in the fourth set (DOY 288) likely indicates intrinsic feature motion (see text). Table 3 Measurements of Uranus Dark Spot and bright companion structure. Feature

# of points

Latitude (◦ N)

Latitude extent (◦ )

Longitude extent (◦ )

Longitudinal drift rate (◦ /day)

Zonal velocity (m s−1 )

Dark Spot DS all (weighted) DS all (unweighted) DS excluding October data (weighted) DS excluding October data (unweighted)

21 21 10 10

27.7 ± 0.8 27.7 ± 0.8 27.9 ± 0.5 27.9 ± 0.5

∼2.2 ∼2.2 ∼2.4 ∼2.4

∼4.6 ∼4.6 ∼5.7 ∼5.7

9.266 ± 0.002 9.281 ± 0.016 9.398 ± 0.002 9.402 ± 0.017

42.56 ± 0.01 42.63 ± 0.08 43.10 ± 0.02 43.12 ± 0.09

Bright companion BC all (weighted) BC all (unweighted) BC excluding October data (weighted) BC excluding October data (unweighted)

44 44 21 21

28.9 ± 1.4 28.9 ± 1.4 28.9 ± 0.9 28.9 ± 0.9

∼3.5 ∼3.5 ∼3.0 ∼3.0

∼5.7 ∼5.7 ∼5.2 ∼5.2

9.227 ± 0.002 9.257 ± 0.012 9.444 ± 0.002 9.436 ± 0.011

41.92 ± 0.01 42.06 ± 0.06 42.90 ± 0.02 42.86 ± 0.13

parisons with this feature, since the Neptune features were never tracked by two independent facilities (ground-based spatial resolution now surpasses that available during the Voyager encounters by a full order of magnitude: point-spread functions of 0.04 now versus 0.4 then). 3.4. Wavelength dependence, altitude, and shape of the dark feature We measured feature contrast (≡ DNfeature /DNbackground − 1) for the dark and bright features. To determine DNbackground , we ex-

tracted scans along the row and column centered on the feature. We typically sampled from 15 to 50 pixels on either side of the feature (when a feature was very close to the limb, this was shortened to 15–35 pixels). We then fit second-order polynomials to the extracted regions, and used those fits to estimate the background value (see examples in Fig. 5). The row and column contrasts were computed separately and then averaged to obtain a final value. The image-to-image contrast variation was usually larger than the row-vs.-column variation. Some of the variation is due to intrin-

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Fig. 4. Uranus Dark Spot zonal velocity in context. The Uranus Dark Spot and its companion are shown as filled circles, overplotted with velocities measured for past bright features within the same latitudinal range. Three models are shown: the symmetric fit from Allison et al. (1991; dot–dash line); the asymmetric fit of Karkoschka (1998; dashed line); and the local linear fit to the Sromovsky and Fry (2005) binned and long-term drift measurements (solid line, from Sromovsky et al., 2007). No published fits to date provide an adequate representation of the aggregate data set, suggesting complexity for the zonal winds at this latitude.

sic change in contrast due to feature variability; for Keck data, the variation was sometimes due to seeing fluctuations. This Uranus Dark Spot is not detected shortward of 0.5 μm, whereas Neptune’s Great Dark Spots had maximum contrast near 0.5 μm (Fig. 6). Similarly, Uranus bright companions are not detected shortward of 0.7 μm, unlike those of Neptune. Nevertheless, the morphological similarity of the companion features associated with the Uranus and Neptune dark spots (discussed further below) suggests a similar underlying structure and/or formation mechanism. With Keck, the dark feature was still detectable in the wideangle (WA) mode despite its lower spatial resolution, which we attribute to its wavelength dependence. The Keck WA filters— “Paschen Beta” and “H continuum” (images in second to last column of Fig. 2)—have narrower bandpasses than the Keck narrowangle (NA) filters near the same wavelengths (Fig. 6). This may serve to isolate a specific altitude level at which the feature has higher contrast: those filters probe relatively deep levels in the atmosphere (deeper than 3 bars in a clear atmosphere). In contrast, the “Jcont” filter probes to about the 1.5-bar level; the broadband K filter probes higher atmospheric levels; and the J and H filters are so broad that they cover many depths in the atmosphere. A discrete dark feature mentioned by Karkoschka (1998) was detected at 0.34 μm, and was located in the southern hemisphere at latitude 31◦ S. That feature had no dark counterpart longward of 0.34 μm, yet if that feature had similar characteristics to the UDS we report here, it should have been readily detected at the longer wavelengths. Thus its wavelength dependence is more reminiscent of Neptune’s GDS-89 than the UDS. It is not obvious why that dark

feature on Uranus would differ from the 2006 UDS, though it is likely related to feature altitude. From the wavelengths at which the UDS was detected (Fig. 5, Table 1), we surmise that the feature lies between 1 and 4 bars, and more likely closer to the deeper end of that range. The bright companions are probably in the 200–600 mbar range, based on features of roughly similar spectral characteristics at similar altitudes, e.g., a bright feature in 2004 that was near 220 mbars (Sromovsky and Fry, 2005) and a complex of very bright features seen in 2005 that ranged from 300 to 600 mbars (Sromovsky et al., 2007). Detailed radiative transfer calculations of the Uranus dark feature and the planet’s zonal banding will better constrain relative altitudes. This modeling work—not only of images in Hubble and Keck filters where the feature was seen, but also images through other filters where the feature was known to be present but below the detection threshold—is in progress, but is beyond the scope of this paper. With our limited imaging capability, we cannot determine whether this Uranus Dark Spot is a dark-colored cloud or a dark hole in a bright cloud deck. The Great Dark Spot seen by Voyager on Neptune appeared to be the latter based on cloud morphology, though a rigorous analysis was not conducted (Smith et al., 1989). The Neptune Great Dark Spots seen by Hubble (Hammel et al., 1995; Sromovsky et al., 2002) suffered from worse spatial limitations than this Uranus Dark Spot: Neptune subtends only 2.3 compared with 3.4 for Uranus. Other wavelength-dependent dark structure is seen on Uranus, specifically contrast reversals in the southern polar collar (at least in 1997) and in the equatorial region: bright at wavelengths longward of 0.5 μm; dark in the UV. Such phenomena are likely related to general absorption of aerosols at UV wavelengths; i.e., bright bands reverse contrast because they absorb at UV wavelengths that would be otherwise dominated by Rayleigh scattering. To within our ability to measure it, the dark feature appeared to retain its elongated aspect ratio throughout the observing period. However, the feature’s low contrast makes such measurements very challenging, and we cannot rule out the type of shape changes that typified Neptune’s GDS-89 (Hammel et al., 1995; Sromovsky et al., 2002). We have no repeated filters between the June and August Hubble datasets, and limited detections in the Keck dataset, so we cannot assess if the feature’s shape and contrast have actually changed, or if the changes are simply due to wavelength. 4. Bright companion structures 4.1. Characteristics of bright companion structures In 2006, the dark feature appeared to be accompanied by companion features, dubbed in the aggregate as “BC” (Table 1). The bright companion discussed here may—or may not—be related to a Bright Complex (also dubbed BC) seen at 30◦ N in 2004–2005 by Sromovsky et al. (2007); our assessment of the bright companion reported here is based solely on data obtained in 2006. The 2004– 2005 Bright Complex was extremely bright, whereas the 2006 UDS companion structure’s brightness was just barely above that of the surrounding atmosphere in many Hubble images, making an accurate position difficult to measure. Such low contrast marks a considerable difference from Neptune’s bright companions, which dominated the whole-disk reflectivity at some wavelengths (e.g., 0.89 μm). Furthermore, Sromovsky et al. (2007) noted no dark feature in the vicinity of the Bright Complex in those Hubble images, although it might have been present at too low a contrast to be detectable.

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265

Fig. 5. Feature contrast measurements. We illustrate our contrast computations for the dark feature and its bright companion in Hubble image j9dh35h9. In the left column, the location of the dark feature is indicated in the image with a single white pixel, and the “crosshairs” mark the rows and columns that we extract (in the image, south is up). In the plots below the image, we show the extracted data numbers (DNs) as a solid line; we mark those used for the background fit (squares, 15–50 pixels from the feature location) along with the second-order polynomial fit (dashed line). Asterisks indicate the specific values of the feature and background from which the contrast was computed (contrast ≡ DNfeature /DNbackground ) − 1. The polynomial does a good job excluding those values from the dark spot that might “contaminate the background” for the bright spot (and vice versa). The right column shows the same information for the bright feature.

Our Keck images showed companion features as well, but presented different problems. First, there was an extensive bright structure with many subcomponents, all of which covered significant latitudinal and longitudinal expanse. This made determination of “single” position very difficult. Furthermore, the Keck images showed that different components varied significantly in brightness over quite short periods of time, further confusing location measurements. For example, the two top images in Fig. 2 were both obtained at H (1.6 μm) just 49 h apart, yet

the distribution of brightness among the subcomponents differs markedly. We acknowledge that our estimates of a “bright companion” position are merely rough aggregate measurements, as discussed above and as can be seen in Figs. 1 and 2. Fig. 3 further illustrates the dispersion in the measured latitudes for the feature. The value of BC measurements in this paper lies in connecting “bright companion” data with “dark spot” data over the long term. The BC longitudes align well with those of the UDS, giving us confidence

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clouds surrounding Neptune’s Great Dark Spot as imaged by Voyager in 1989, a similar formation process may be at work on Uranus. 5. Numerical simulations of the Dark Spot of Uranus The 2006 vortex appeared to be relatively stable, lasting from at least June through October. Our simulations therefore examined the relative viability of vortices as functions of zonal wind profile and vortex strength in the vicinity of the observed latitude of ∼28◦ N. The objective was to determine the conditions that are most opportune for a vortex-like feature to persist on time scales similar to that of the observed feature. Specifically, the simulations examined several zonal wind profiles proposed for Uranus and assessed the relative viability and behavior of a vortex within those profiles at various latitudes in the region of the observed UDS. The simulations used the Explicit Planetary Isentropic Coordinate General Circulation Model (EPIC GCM version 4.03; Dowling et al., 2006), which had already been used to conduct simulations of vortices on Neptune (LeBeau and Dowling, 1998), along with a few early simulations of vortices on Uranus (Deng and LeBeau, 2007). Those early UDS simulations suggested the possibility of month-long vortices comparable to the observed feature. However, subsequent analysis of the observational data presented here allowed for an improved representation of the UDS within the numerical model that more accurately reflects likely conditions on Uranus. 5.1. The EPIC model

Fig. 6. Wavelength-dependent contrast of ice giant Dark Spots and bright companions. We plot contrast as a function of wavelength for the features on Uranus, both the dark feature (filled circles in lower panel) and its bright companion structure (filled circles in upper panel). Note the order of magnitude difference in the contrast axes. When more than one companion feature was visible, the contrast shown here is the brightest component. Since none of these features is resolved, these contrast values are likely lower/upper limits (i.e., bright spots may be brighter; the dark spot may be darker). For comparison, we show Neptune GDS and BC contrasts determined from Voyager data (open squares connected by solid lines; Smith et al., 1989) and from Hubble images (open triangles, dark spots only; Sromovsky et al., 2002). The Uranus Dark Spot is not detected shortward of 0.5 μm, whereas Neptune’s Great Dark Spots had maximum contrast near 0.5 μm. Similarly, Uranus bright companions are not detected shortward of 0.7 μm, unlike those of Neptune. We also indicate the filter bandpasses for Voyager filters in the upper panel (UV, Blu, Ora, Vio, MeU, and MeJ; http://pds-rings.seti.org/voyager/iss/); in the lower panel we show the filters for Hubble (shortward of 1 μm; http://www3. cadc-ccda.hia-iha.nrc-cnrc.gc.ca/hst/hgroups) and Keck (longward of 1 μm; http:// www2.keck.hawaii.edu/realpublic/inst/nirc2/filters.html).

that our measured aggregate BC zonal velocity is accurate. The zonal velocity itself was already discussed above (see Section 3.2). 4.2. Comparison with other bright companions To within our ability to measure their positions, the bright companion material was spatially associated with the dark feature, suggesting a physical relationship as was clearly seen for Neptune’s bright companion features (Smith et al., 1989; Hammel et al., 1995; Sromovsky et al., 2002). Stratman et al. (2001) examined a threedimensional model of Neptune bright companion clouds, both for Voyager’s southern-hemisphere GDS-89 (Smith et al., 1989) and for Hubble’s northern-hemisphere GDS-94 (Hammel et al., 1995). Their results support the hypothesis that the bright companions are methane clouds that form at or just below the tropopause and that they are caused by lifting in a manner analogous to the formation of orographic clouds. Since the detailed structure of the Uranus bright companions is so highly reminiscent of the bright

Model construction was based on a hybrid “pressure–potential temperature” coordinate that vertically defines a series of atmospheric layers. Within each of these layers, the Navier–Stokes equations were solved on an oblate spheroid coordinate system of longitude and latitude. Initialization inputs included the zonal wind profile, the vertical temperature-pressure structure, and the vertical shear in the wind profile. The computations were conducted on a whole-globe domain encompassing the full pole-to-pole latitudinal range and 360◦ of longitude. This domain was divided into a grid with a uniform spacing of 0.7◦ in latitude and longitude, corresponding to a 512 × 256 grid for the full globe. This grid has been successfully applied to dark spot simulations on Neptune (Deng and LeBeau, 2007); however, for the smaller UDS, this grid was relatively coarse. The coarse grid allowed for more rapid simulation, allowing us to consider a larger variety of vortex strengths, vortex locations, and wind profiles than would be possible with a finer grid. Still, the presented results should be considered preliminary given the limited resolution of the model, with more detailed analysis of the vortex motions reserved for future finer-resolution studies. Numerical integration and stability were treated using standard options in the EPIC model (Dowling et al., 1998). Specifically, instability due to small-scale gravity waves was managed with a global sixth-order hyperviscosity and a Fourier filter applied poleward of 45◦ latitude, and the standard time step was 60 s using the thirdorder Adams–Bashforth method. 5.2. Model structure The vertical pressure–temperature profile (Fig. 7) was the standard observed profile (Lindal et al., 1987) in the stratosphere and the upper troposphere (Fig. 7a). To extend the profile into the deeper troposphere, we follow procedures established by earlier EPIC investigators (LeBeau and Dowling, 1998; Stratman et al., 2001; Deng and LeBeau, 2007; Dowling et al., 2007). We assumed

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267

Fig. 7. Uranus vorticity-model input parameters. (a) Vertical temperature–pressure profiles are shown for Uranus. The N 2 = 0.09 × 10−4 s−2 profile corresponds to that used in the current simulations: the Lindal et al. (1987) profile above 630 mbars, and the constant N 2 profile below 630 mbars. (b) The values for the square of the Brunt–Väisälä (buoyancy) frequency N—computed with EPIC for the two profiles in panel (a)—are overlaid by the locations of the layer boundaries (dashed lines) in the simulation grid. The triangles represent the location of the layer midpoints for the constant N 2 profile.

a constant Brunt–Väisälä frequency, N, and generated the profile by solving: T N − T0 T T N − T T0

=

p p0

R gas /c p

,

TN ≡

g2 N 2c p

,

(1)

where T and p are the computed temperature–pressure profile, g is gravitational acceleration, R gas is the ideal gas constant, c p is the specific heat at constant pressure, and T 0 and p 0 are a reference point on the Lindal et al. profile. Our reference point on the Lindal profile is 630 mbars; below this we used a constant profile with N = 0.003 s−1 (Fig. 7b), where N is derived from the Lindal profile. The atmosphere was divided into ten layers in altitude covering pressures from about 2 mbars to 7.5 bars (Fig. 7b). The current simulations assumed a uniform zonal wind profile with depth except within the top two layers of the model, above the proposed vortex region. Under these conditions in the EPIC model, layer thickness is roughly uniform across the globe, meaning variations in potential vorticity corresponded directly to those in absolute vorticity. As with previous EPIC vortex studies (LeBeau and Dowling, 1998; Stratman et al., 2001; Deng and LeBeau, 2007; Dowling et al., 2007), our simulations did not attempt to generate the initial vortex. Rather, we placed the vortex into the background defined by the zonal wind profile and then allowed it to evolve. The initial dimensions of the ellipsoidal vortex were set at 7.8◦ in longitude and 3.9◦ in latitude, slightly larger than the dimensions of the August observations (the model vortex initially became more compact with time and we have little knowledge of where the observed vortex is in its overall evolution). The model vortex’s central layer was placed at about 1000 mbars, the upper end of the proposed range for the observed UDS. The initial vertical extent of the vortex covered a pressure range of approximately 700 to 1400 mbar. The ellipsoidal vortex was subsequently defined by a closed potential vorticity contour, which is approximately a material boundary in the EPIC code.

The vortex was placed in the flow field by changing the local winds to reflect the superposition of an ellipsoidal vortex onto the background wind profile. The flow-field modification for the vortex was consistent with a Gaussian distribution with a second-order Rossby correction on the velocity as described by Dowling et al. (2007). The stream function corresponding to a quasi-geostrophically balanced vortex was computed, and from this the necessary changes in the velocity field were determined. The resulting initial potential-vorticity distribution had a Gaussian profile and its strength was defined by the maximum vortex velocity relative to the background zonal wind. We considered maximum relative velocities between 30 and 130 m s−1 . The resulting relative vortex velocity fields were largely independent of the underlying zonal wind structure. The initial vortex then evolved within the numerical simulation. Similar to the previous EPIC vortex studies, the vortex experienced a rapid adjustment phase within the first few simulated days, which in this case resulted in the vortex becoming more compact and more cylindrical in terms of its velocity distribution (only anticyclonic vortices were retained, since initially cyclonic features became immediately unstable). Our analysis focused on the subsequent, more gradual evolution of the vortex after this adjustment phase. 5.3. Assumed zonal wind profile In our numerical investigation, the other key parametric variable (aside from initial vortex conditions) was the shape of the zonal wind profile. Several different curves have been fit to the data available for the uranian zonal wind profile as shown in Fig. 8a, which compares these profiles against cloud-tracking data (Hammel et al., 2005b; Sromovsky and Fry, 2005). A symmetric model best fit the data from the Voyager 2 encounter (Smith et al., 1986; Allison et al., 1991), and also provided a good fit to Keck data taken in 2003 (Hammel et al., 2005b). Asymmetric fits have

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Fig. 8. Uranus zonal wind profiles and initial constructed profiles. (a) We show zonal wind profiles, including fitted and constructed profiles, compared with the cloud-tracking data (open symbols) from Hammel et al. (2005b) and Sromovsky and Fry (2005). (b) This panel shows the correlated gradients in absolute vorticity based on the profiles in (a).

also been suggested (Karkoschka, 1998; Sromovsky and Fry, 2005). From the perspective of vortex dynamics, of more interest than these zonal velocity profiles are their concomitant absolute vorticity profiles (Fig. 8b), where absolute vorticity, ω + f , is computed by

ω+ f =−

1 ∂u r R ∂λ

+ 2Ω sin λ.

(2)

Here, u is the zonal wind velocity, r and R are the longitudinal and meridional radii of curvature, and λ is latitude. The Coriolis parameter is f = 2Ω sin λ, where Ω is the planetary rotation rate, and ω is the local (relative) vorticity. The three fit profiles are similar in terms of their absolute vorticity magnitudes. However, the mid-latitude variations in the Sromovsky and Fry (2005) profile results in a region of nearconstant absolute vorticity about latitude 32◦ N. Previous simulations of Neptune’s GDS-89 suggested that a region approaching uniform potential vorticity was advantageous to stable vortices on Neptune (LeBeau and Dowling, 1998; Deng and LeBeau, 2007), hence this correlation may not be coincidental. We thus constructed an additional profile to extend this “fixed vorticity” a few more degrees southward, to fully encompass the region of the uranian dark spot. For this set of constructed profiles, we modified the profile of Sromovsky and Fry (2005) between 23 and 33◦ N using the following formula (cf. LeBeau and Dowling, 1998):

−

1 ∂(ur )

+ f = (ω + f )0 + Q y β ∗ R 0 (λ − λ0 ), ∂λ   1 ∂ . (ω + f ) β∗ ≡ R ∂λ λ=λ0

rR

(3)

Here, λ0 is defined as 28◦ N and the constant R 0 is the meridional radius at λ0 . The value (ω + f )0 and the parameter β ∗ were evaluated at 28◦ N from the Sromovsky and Fry profile, with ω representing the local vorticity. The parameter Q y scales β ∗ , with

a value of zero corresponding to a region of constant absolute vorticity; our profile corresponded to Q y = 0. Between 14 and 23◦ N, as well as from 33 to 40◦ N, the constructed profile was gradually merged back into the original profile. Fig. 8 shows this constructed profile and its absolute vorticity profile (both labeled Q y = 0). This profile was a reasonably close match to the observed zonal wind data; however, it notably did not capture the three data points near 22◦ N. This constructed profile allowed us to further investigate the conditions most likely to produce long-lived vortices on Uranus. 5.4. Results of model simulations We summarize the simulation results in Table 4. The essential output was the lifespan of the vortex, given in terms of days (i.e., 24 h). We assessed the vortex lifespan by tracking the evolving potential vorticity contour that defines the vortex. Once the vortex had stretched to a longitudinal dimension of more than 20◦ or had lost the coherence of a single compact vortex, it was considered to have been effectively sheared out by the zonal wind. Table 4 also indicates the center latitude of both the initial vortex and the vortex when it reached the end of its lifespan. The standard evolution of these vortices was to drift equatorward during the simulation, with the most rapid drift occurring in the first several days of the simulation. This drift is one reason why multiple initial vortex latitudes are considered in these simulations. The meridional drift is consistent with standard beta drift models as applied to terrestrial cyclones (Wang and Holland, 1996), where an anticyclonic vortex will generate equatorward motions. However, the current grid is too coarse to resolve a clear beta-gyre structure in these simulations. 5.4.1. Effect of zonal wind profile Fig. 9 illustrates the effect of the zonal wind profile on vortex shearing. Here, initial vortices of the same dimensions and same

The Dark Spot on Uranus in 2006

Table 4 Uranus vortex simulations on the coarse grid. Profilea

u max (m s−1 )b

λI

λF

(◦ )d

Lifespan (days)

Smith Smith Karkoschka Karkoschka Sromovsky Sromovsky Sromovsky Sromovsky Sromovsky Sromovsky Sromovsky Sromovsky Sromovsky Qy =0 Qy =0 Qy =0

70 70 70 70 30 50 70 90 110 130 70 70 70 30 50 70

28 30.8 28 31 28 28 28 28 28 28 29.6 29.8 30 28 28 28

24.8 28.1 24.6 28.0 27.0 26.3 25.4 24.6 24.4 24.3 27.6 27.9 28.4 28.5 28.3 28.0

23 16 25 19 14 20 23 27 27 28 26 27 26 23 29 31

(◦ )c

Comment

Merged with shear layer Merged with shear layer Merged with shear layer

a The fit profiles were from Smith et al. (1989), Karkoschka (1998), and Sromovsky and Fry (2005); the Q y profiles were constructed as described in the text. b c d

Initial maximum velocity of vortex. Initial latitude of vortex center. Final latitude of vortex center.

269

strength (u max = 70 m s−1 ) were placed in five different zonal wind fields on the coarse grid. The evolution of the vortex contour in the central layer (layer 6) for each profile is shown in a 47◦ by 30◦ frame centered on the vortex. The vortices placed in the Smith profile sheared most rapidly, steadily losing coherence as a vortex-like feature (Table 4). Vortices placed in the Karkoschka and Sromovsky profiles sheared more slowly, although the rate was sensitive to the initial vortex latitude. Unlike the fit profiles, the constructed profile maintained a compact spot with little noticeable shearing through most of the run, never reaching 20◦ in length. Instead, these spots disappeared due to interaction with the shear region equatorward located near the bend in the absolute vorticity profile at 22◦ N, eventually merging into the shear. As shown in Table 4, the rate of vortex drift appears to be somewhat correlated to the gradient in absolute vorticity. The Smith and Karkoschka profiles, with a stronger gradient in the vicinity of 28◦ N, drifted equatorward faster than the Sromovsky profile, while the Q y = 0 profile exhibited minimal drift over the lifespan of the vortex. Shifting the initial vortex location northward in an attempt to have the vortex drift to 28◦ N rather than starting at that latitude yielded a few-day increase in the lifespan of the vortex in the Sromovsky profile. Similar efforts with the Smith and Karkoschka profiles reduced the lifespan by several days.

Fig. 9. Coarse-grid sensitivity of vortex lifetime to zonal wind profiles. For these simulations, a 70-m s−1 vortex was initially placed in five different zonal wind profiles using a whole-globe domain. The number indicates the initial latitude of the vortex, all of which are between 28 and 31◦ N. The dimensions of each frame were 47◦ by 30◦ . The vortices placed in the profiles of Karkoschka (1998) and Smith et al. (1989) were clearly shearing by 20 days. The Sromovsky and Fry (2005) vortices remained more compact, while the Q y = 0 vortex evidenced the least amount of shearing.

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Fig. 10. Sensitivity of vortex lifetime to maximum vortex wind velocity. We examined the evolution of different strength vortices initially placed at 28◦ N in the profile of Sromovsky and Fry (2005). This plot presents the evolution of the vortex major axis measured in degrees of longitude; the vortex lifespan is defined as the time to reach a 20◦ length. There were considerable gains in lifespan as the vortex velocity was increased from 30 to 90 m s−1 , but thereafter the increases in lifespan were minimal.

5.4.2. Effect of vortex strength In an attempt to extend the vortex lifespan, we considered several different maximum vortex velocities in the Sromovsky and Fry profile (Fig. 10). While increasing the vortex strength from 30 to 90 m s−1 did increase the lifespan steadily, the effect appeared to reach an asymptote beyond 90 m s−1 . This suggested that just increasing the vortex strength was not a means to significantly increase vortex lifespan. Similar results occurred with the Q y = 0 profile as seen in Table 4. The effect of vortex strength on drift rate was not studied in this analysis, being more appropriate for a higher resolution set of simulations. 5.4.3. Next steps for vortex modeling Interestingly, none of the modeled vortices survived as a compact feature for more than a month, whereas the real feature was clearly detected for longer than that. To resolve this issue, future studies may include variations in the atmospheric vertical pressure–temperature profile and the vertical wind shear, as well as alterations in the vortex structure. We also plan to explore deeper vortices (we worked at the upper limit of the proposed range of vortex depth), which may exhibit different dynamical behavior, particularly if there are vertical wind shears at those altitudes. The most probable means to extend vortex lifespan is to better resolve the vortex motions with a finer grid, to yield more accurate simulations. The results of the coarse grid simulations shown here will greatly reduce computational costs by guiding our approach. For example, the coarse grid simulations suggested that a near-constant absolute vorticity gradient was more likely to minimize meridional drift, which would be more consistent with the observations. With the reduced gradient profiles we were also able to achieve slightly longer lifespans, the longest being the Q y = 0 profile at 31 days. Stronger vortices can also extend the lifespan, but as mentioned above this effect appears to reach an asymptote. Such conclusions will provide useful directions for future finer-

resolution numerical investigations of the dark spots on the ice giants. 6. Discussion We report the first definitive detection of a discrete dark feature on Uranus. Given the lack of knowledge of vortex creation on Uranus, this detection may be purely a chance sighting of a shortlived feature. Yet no similar feature has been seen in many years of Hubble and Keck imaging. The appearance now, in conjunction with other atmospheric changes evidenced in the equinoctial season, demonstrates the dynamic and evolving seasonal nature of the planet’s atmosphere. Our preliminary simulation data suggest that a longer-lived vortex such as was seen in 2006 may require a region of relatively uniform and low potential-vorticity gradient in the vicinity of the vortex. Such a region is not directly consistent with simple smooth sinusoidal zonal wind profiles, but instead suggests local detailed variations in the profile’s structure near the vortex latitude. Cloud-tracking data near the latitude of the vortex support this interpretation. It is yet to be determined whether such zonal wind structure might be the result of seasonal changes, creating a window of opportunity for a vortex to appear, or instead is a permanent feature that has been lurking in the darkness of the long uranian winter. If Uranus is becoming more “Neptune-like” as its sub-solar latitude crosses the planet’s equator, then such features may become common in the next decade. This can only be assessed by continued long-term observations of Uranus with facilities that provide very high spatial resolution. Acknowledgments Uranus Hubble observations were supported through STScI GO Grants 10170, 10534, 10805, 11118, 11190, and 11156. The

The Dark Spot on Uranus in 2006

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