Keck Adaptive Optics Images of Uranus and Its Rings
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Icarus 160, 359–374 (2002)
doi:10.1006/icar.2002.6966
Keck Adaptive Optics Images of Uranus and Its Rings
Imke de Pater
Astronomy Department, 601 Campbell Hall, University of California, Berkeley, Berkeley, California 94720
E-mail: imke@floris.berkeley.edu
S. G. Gibbard and B. A. Macintosh
Lawrence Livermore National Laboratory, Livermore, California 94550
H. G. Roe
Astronomy Department, 601 Campbell Hall, University of California, Berkeley, Berkeley, California 94720
and
D. T. Gavel and C. E. Max
Lawrence Livermore National Laboratory, Livermore, California 94550
Received February 5, 2001; revised April 25, 2002
the light from the planet (Matthews et al. 1982, Terrile and
We present adaptive optic images of Uranus obtained with the Smith 1986). These techniques are still widely used to image
10-m W. M. Keck II telescope in June 2000, at wavelengths be- faint material next to bright planets or stars. The best ground-
tween 1 and 2.4 µm. The angular resolution of the images is ∼0.06– based infrared images of Uranus published to date are those by
0.09 . We identified eight small cloud features on Uranus’s disk, Baines et al. (1998) and Sromovsky et al. (2000). In both sets
four of which were in the northern hemisphere. The latter features of images the ring with its asymmetric brightness distribution
are ∼1000–2000 km in extent and located in the upper troposphere, was easily observed. Due to the relatively low spatial resolution
above the methane cloud, at pressures between 0.5 and 1 bar. Our
(typically at best ∼0.4–0.5 from the ground), light from this
data have been combined with HST data by Hammel et al. (2001,
ring is blended with that of several faint inner rings. Since the
Icarus 153, 229–235); the combination of Keck and HST data al-
lowed derivation of an accurate wind velocity profile. Our images ring is so much brighter than any of the other rings, light from
further show Uranus’s entire ring system: the asymmetric ring, the ring system has usually been completely attributed to the
as well as the three groups of inner rings (outward from Uranus): ring.
the rings 6 + 5 + 4, α + β, and the η + γ + δ rings. We derived the A wealth of information regarding the planet and its ring sys-
equivalent I/F width and ring particle reflectivity for each group of tem was obtained by the Voyager spacecraft (e.g., Smith et al.
rings. Typical particle albedos are ∼0.04–0.05, in good agreement 1986). The planet itself appeared to be rather uninteresting in
with HST data at 0.9 µm. c 2002 Elsevier Science (USA) the sense that practically no cloud features were detected. Yet
the circumpolar bands on Uranus were similar to those seen on
Saturn and Jupiter, although in a completely different sun–planet
1. INTRODUCTION geometry. This provided much information regarding zonal wind
systems on giant planets. Unfortunately, due to an overall lack of
Due to Uranus’s small angular extent and lack of large-scale cloud features it was difficult to determine an accurate wind pro-
brightness variations, images of the planet obtained several file in Uranus’s atmosphere. Moreover, since only the southern
decades ago were of limited interest. Attempts to observe Uranus (visible) hemisphere1 was imaged by Voyager, no information
were renewed after star occultation measurements by Elliot et al. was obtained on its northern counterpart. In addition to atmo-
(1977a,b) indicated the existence of a ring system surrounding spheric observations, Voyager imaged the ring system, discov-
the planet. To image this ring system, however, one had to some- ered a tenth ring, and imaged sheets of dust in between rings at
how avoid contamination from Uranus’s scattered light. This
could be accomplished by imaging the planet at near-infrared 1 We use the IAU convention of the location of the north pole: at the time of
wavelengths in methane absorption bands where Uranus itself the observations the south pole is visible, and the planet’s rotation is retrograde,
is dark, and/or by using coronagraphy to effectively block out i.e., in the westward direction.
359
0019-1035/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.360 DE PATER ET AL.
high phase angles. The individual rings were resolved via star be determined, and hence the overall wavefront reconstructed.
and radio occultation experiments (French et al. 1986, 1991). The deformable mirror then adjusts to compensate for any de-
With the advent of the Hubble Space Telescope (HST), it be- viations of the wavefront from the desired shape. Since the
came possible to image Uranus from near-Earth at high angular wavefront sensor is looking at light already corrected by the
resolution. Karkoschka (1998) discovered small, bright (com- deformable mirror the AO system operates in a closed feedback
pared to the background) cloud features in the northern hemi- loop, updating at a rate of 500–600 Hz.
sphere and lower contrast features in the southern hemisphere. The Shack–Hartmann sensor has no particular requirement
Several years later bright cloud features were also reported from that the light used to measure the wavefront should come from
ground-based infrared images (Sromovsky et al. 2000). On these a point source—in fact, since it operates at visible light wave-
ground-based images only the bright northern features were de- lengths, where the AO system is providing little correction, even
tected; features in the southern hemisphere had too low a con- the image of a point-source star on each subaperture is extended
trast and/or were blended with the south polar haze. Observa- by atmospheric seeing (0.5–1 ). However, the accuracy with
tions of these cloud features helped to constrain the wind profile which the centroid of an object can be measured is linearly pro-
in Uranus’s atmosphere at latitudes where no prior information portional to the signal-to-noise ratio in the image and inversely
existed, refine the wind profile at southern latitudes, and search related to the diameter of the image. Since Uranus is very bright
for time variability in the wind profile at southern latitudes as (m v = 5.7) it can provide a relatively high signal-to-noise wave-
compared to the time of the Voyager encounter. front measurement. However, since it is large (3.65 at the time
HST also provided images of the bright ring, mostly uncon- of our observations) it will provide a wavefront measurement
taminated by other rings (Karkoschka 1997, 2001a). In these with a signal-to-noise measurement roughly four times worse
HST images the fainter inner rings show up in three groups (as than an equivalent brightness star, or (since the measurement is
seen going outward from Uranus): 6 + 5 + 4 rings, α + β rings, photon shot-noise limited) equivalent to a star 16 times dimmer,
and the η + γ + δ rings. The individual groups of rings could m v = 8.8. The Keck AO system includes a small (4 diameter)
not be resolved in these images. field stop in front of the wavefront sensor, which may reduce the
Adaptive optics (AO) techniques on large telescopes have flux from Uranus even more, making it equivalent to a m v = 9–
opened up an entire new area of imagery, previously only feasi- 10 star. When we selected point-source function (PSF) reference
ble from space. While AO techniques on moderately sized tele- stars, as discussed below, we generally selected stars somewhat
scopes already equal the HST spatial resolution at near-infrared dimmer than Uranus itself in an attempt to match AO perfor-
wavelengths, AO on 8–10-m telescopes can reach angular reso- mance. However, in retrospect our selection was not entirely
lutions four times higher than those of HST. In the past few years successful, as explained in the next two paragraphs.
it has become possible to use AO at infrared wavelengths on 8– The Keck AO system uses a 64 × 64 pixel CCD for its wave-
10-m telescopes. The angular resolution obtained by the 10-m front sensor. Since the wavefront must be measured over 289
Keck telescope on a star at a wavelength of 2 µm is typically active subapertures, each subaperture is only allocated 2 × 2
0.05 , similar to that of HST at visible (0.5 µm) wavelengths, pixels—a “quad cell”—for the centroid measurement, with the
and four times higher than that of NICMOS on HST. In this lenslets positioned so that each subaperture image is close to
report we present results on Uranus, using AO techniques on the center of the quad cell, equally illuminating all four pixels.
the W. M. Keck telescope. We obtained an angular resolution on (Individual quad cells are separated by a 1-pixel guard band of
Uranus of ∼0.06 at 1.6 µm and 0.09 at 2 µm. unused pixels.) The pixels on the Keck AO system correspond
to a size on the sky of 2 . Such an arrangement is common in
2. OBSERVATIONS adaptive optics as it also minimizes the effects of readout noise
from the CCD. If the wavefront coming into the wavefront sen-
2.1. Adaptive Optics System sor were perfectly flat and undistorted, each subaperture’s im-
We observed Uranus and its rings on June 17, 2000 UT and age would fall precisely on the center of the quad cell. In a
June 18, 2000 UT with the adaptive optics system on the 10-m perfect AO system, this would be the desired state of corrected
W. M. Keck II telescope (Wizinowich et al. 2000a,b). An adap- images. However, a real AO system often includes optical aber-
tive optics system in its simplest form consists of a wavefront rations that are not common to both the science camera and
sensor—a device to sense the distortions caused by atmospheric the wavefront sensor—for example, aberrations in the optics of
turbulence—and a device to correct these distortions, typically a the wavefront sensor itself. If the AO system would attempt to
deformable mirror. (See Beckers 1993 or Hardy 1998 for details counteract such noncommon path aberrations, it would produce
of AO architecture and performance.) a better image on the wavefront sensor but a worse image on the
The Keck AO system uses a Shack–Hartmann wavefront sen- science camera, since the science camera was originally undis-
sor. Such a sensor consists of many individual lenslets, which torted. To avoid this, the AO system is calibrated such that the
focus light in the form of many separate images on a CCD. By desired state for each subaperture is not to have the subimage
measuring the centroid position of the individual images the av- perfectly centered but slightly offset, with the offsets adjusted
erage slope of the wavefront over each lenslet’s subaperture can to produce the best possible image on the science camera.KECK AO IMAGES OF URANUS 361
Unfortunately, a quad cell of 2 × 2 pixels does not directly
determine the centroid of an object. Instead, its response is lin-
early proportional to the size of the object being measured; a
0.5 image of a star displaced 0.2 from the center of the quad
cell produces the same response as a 1.0 image displaced 0.4 .
Hence, when observing 3.65 Uranus, the true position of the
Uranus subimages must be moved four to five times further than
the image of a star would have to move. The deformable mirror
must thus also move four to five times further, and the noncom-
mon path optical aberrations are therefore “overcorrected” by
a factor of 3–4. As a result, the AO system is no longer trying
to produce a “perfect” image of Uranus on the science cam-
era, but instead an image convolved with the overcorrection of
these noncommon path aberrations. Since the aberrations are
generally very high order and not symmetrical, the point-spread
function of an AO system locked on Uranus will be broader than
expected, with asymmetric artifacts (see, for example, the ghost
outer ring in Fig. 3). The Strehl ratio (the ratio of the observed
peak intensity to that expected with the AO system if there are
no aberrations present) in our Uranus images is in fact closer
to that obtained on a m v ≈ 11 star than a m v ≈ 9 PSF reference
(Section 2.4 and Fig. 9).
b Uranus Viewer Results 2.2. Observations: General
-15 18 32
We used NIRSPEC, a near-infrared spectrometer, behind the
AO system to record both images and spectra. NIRSPEC is
equipped with a 1024 × 1024 InSb ALADDIN array for spec-
troscopy, and also a slit-viewing camera (SCAM) containing a
-15 18 34
256 × 256 HgCdTe PICNIC array (McClean et al. 2000). Behind
the AO system, SCAM has a platescale of 0.017 /pixel (245 km
on Uranus). We obtained a typical resolution (full width at half
maximum; FWHM) of ∼0.05 in the N5 and N6 filters on a
Declination (d m s)
-15 18 36
bright star (apparent magnitude m v ≈ 8–9). Since Uranus itself
was used as the wavefront reference required by the AO sys-
tem, the AO performance on Uranus is more similar to that of
an 11th mag star, as explained in Section 2.1. The FWHM for
such a star was 0.06 in the N5 filter (H band; see Table III
-15 18 38
for filter bandwidths), and 0.09 in the N6 filter (H + K band),
which corresponds to 860–1225 km at Uranus. The field of view
of the slit-viewing camera SCAM is 4.48 , sufficient to encom-
pass Uranus’s entire disk. By shifting the object to the north
-15 18 40
and south, we were able to image the entire ring system in just
two pointings. These were mosaicked together afterward to con-
21 32 53 21 32 52.8 21 32 52.6 struct pictures as displayed in Fig. 1. As a reference point we
Right Ascension (h m s)
used the -ring and its known distance to the center of Uranus at
Time (UTC):
Ephemeris:
18-jun-2000:14:00
#4 (URA027 + URA039 + DE405)
periapse and apoapse (e.g., Yoder 1995). Our observations are
Viewpoint: Earth’s center summarized in Tables I (ephemeris) and II (observations).
Moon selection: Miranda-Oberon
Ring selection: 6-Epsilon
FIG. 1. (a) Keck adaptive optics image of Uranus and its rings in the N6 band (1.558 − 2.315 µm), taken on June 18, 2000 UT. This image has been processed
as decribed in the text and rotated so that the rings are oriented north–south in the picture. Before displaying the image, we subtracted an image (at 60% intensity)
convolved with a gaussian function from the data (Section 2.3). The planet itself was further darkened by a factor of ∼5. These techniques allow both the rings (note
the three fainter rings interior to the ring) and details on the planet (note the south polar cap and the three northern cloud features) to be visible in the same picture.
(b) Geometry of the system at the time of our observations. The location of ring periapse is indicated by the filled circle (available at http://ringmaster.arc.nasa.gov/).362 DE PATER ET AL.
TABLE I Fig. 1 we further interpolated over any remaining gaps to pro-
Ephemeris duce a clean looking image. All analyses, however, were made
on images before interpolating over the gaps.
UT time
The images were reduced in the standard way: flat fielded us-
date RA Dec ro Ang. Obs. Sun Phase
(2000) h min ◦ AU AU diam. lat.◦ lat.◦ angle◦ ing twilight and dome flats, bad pixels were removed (replaced
with the median of neighboring pixels), and the sky was sub-
June 17 21 33 −15 18 19.33 19.40 3.646 −27.74 −30.15 2.38 tracted using a separate image of the sky taken just prior to or
June 18 21 33 −15 18 19.32 19.40 3.648 −27.76 −30.14 2.35 after the Uranus exposure itself. On June 17 we observed the
photometric star HD201941 at airmass ∼1.1 (Elias et al. 1982);
Notes. , geocentric distance; ro , heliocentric distance; Ang. diam., angular
diameter; Obs. lat., observers’ latitude; Sun lat., Sun’s latitude.
Uranus was observed at airmass ∼1.25. On June 18 we ob-
served SJ9182 at airmass ∼1.35 (our primary photometric star;
Persson et al. 1998), and HD207438 (a PSF star) at airmass 1.22,
Since NIRSPEC contains both a spectrometer and slit- while Uranus varied between 1.35 and 1.22. We converted stel-
viewing camera, we acquired spectra simultaneously with the lar magnitudes to flux units using the spectrum of Vega (Colina
images; the spectra will be discussed in a future paper. As a re- et al. 1996a, assumed to be a zero magnitude star at all wave-
sult of this instrument design, any individual image lacks data lengths), as indicated in Table III. These numbers, as well as our
at the position of the slit. We minimized this “gap” by shift- adopted monochromatic solar flux at 1 AU (Colina et al. 1996b),
ing Uranus between exposures. In the final image displayed in were calculated by averaging over the bandpass of our filters,
TABLE II
Observations of Cloud Features
UT time (2000)a N × Tint Cloud Cloud Cloud Cloud Cloud
h:m:s CMLb ◦ Filter c Image N×s longd (rel. int) longd (rel. int) longd (rel. int) longd (rel. int) longd (rel. int)
June 17, 2000 Latitude s6: +21◦ s7: −43◦ s8: −30◦ s5:−27◦
14:49:52 284.9◦ N5 Ring north 5 × 60 −65◦ (1.05) −6◦ (1.06) −60◦ (1.08)
15:07:15 290.9◦ N5 Ring south 5 × 60 +47◦ (1.01)
15:13:55 293.2◦ N5 Ring south 5 × 60 +43◦ (1.03)
15:24:00 296.7◦ N5 Ring north 1 × 60
June 18, 2000 Latitude s1: +42◦ s3: +36◦ s2: +28◦ s4: −38◦ s5: −27◦
12:34:38 19.0◦ N6 South Pole 5 × 60 +28◦ (1.55) +30◦ (1.05) −50◦ (1.02)
12:47:00 23.3◦ N6 South Pole 5 × 60 +25◦ (1.54) +28◦ (1.05) −52◦ (1.04)
12:58:37 27.4◦ N6 Ring north 6 × 60 −29◦ (1.35)
13:12:48 32.3◦ N6 Ring north 6 × 60 −32◦ (1.31)
13:17:38 34.0◦ N6 Disk 1 × 60 +52◦ (1.08) −38◦ (1.25) +15◦ (1.07) +15◦ (1.03)
13:19:51 34.7◦ N6 Disk 1 × 60 +49◦ (1.27) −42◦ (1.15) +13◦ (1.09) +15◦ (1.03)
13:24:20 36.3◦ N6 Ring south 5 × 60 +47◦ (1.36) +9◦ (1.37) +10◦ (1.03)
13:36:08 40.4◦ N6 Ring south 5 × 60 +44◦ (1.37) +6◦ (1.35) +5◦ (1.10)
13:40:22 41.9◦ N5 Disk 1 × 60 +47◦ (1.44) −48◦ (1.05) +6◦ (1.12) 5◦ (1.05) −65◦ (1.0)
13:43:11 42.8◦ N5 Disk 1 × 60 +45◦ (1.38) −50◦ (1.08) +5◦ (1.15) 4◦ (1.06) −67◦ (1.0)
13:44:17 43.2◦ N5 Disk 1 × 60 +42◦ (1.16) −52◦ (1.07) +4◦ (1.27) 3◦ (1.07) −67◦ (1.0)
13:47:00 44.2◦ N5 Disk 1 × 60 +38◦ (1.52) −54◦ (1.11) +3◦ (1.30) 2◦ (1.07) −67◦ (1.0)
13:48:54 44.8◦ K Disk 1 × 120 +35◦ +2◦ (2.2)
13:53:43 46.5◦ K Disk 1 × 120 +29◦ −1◦ (2.0)
13:55:56 47.3◦ K Disk 1 × 120 +30◦ −2◦ (1.7)
14:01:52 49.4◦ K Disk 1 × 120 +30◦ −4◦ (1.9)
14:39:30 62.5◦ N1 Disk 1 × 120 +15◦ (1.11) −17◦
14:44:20 64.1◦ N1 Disk 1 × 120 +15◦ (1.11) −20◦
14:46:40 65.0◦ N2 Disk 1 × 120 +12◦ (1.24) −21◦ (1.07)
14:51:40 66.7◦ N2 Disk 1 × 120 +10◦ (1.23) −22◦ (1.11)
14:56:45 68.3◦ N3 Ring north 5 × 60 −22◦ (1.04)
15:08:00 72.4◦ N3 Ring north 5 × 60 −25◦ (1.07)
15:12:04 73.8◦ N3 Disk 1 × 60 +3◦ (1.21) −31◦ (1.09)
15:14:41 74.7◦ N3 Disk 1 × 60 +2◦ (1.14) −31◦ (1.06)
a Time at the midpoint of the observation.
b The central meridian longitude at the time of the observation (available at the JPL ephemeris: http://ssd.jpl.nasa.gov).
c See Table III for the filter bandwidth.
d + sign refers to increasing planetocentric east longitude, with respect to central meridian longitude. Error in latitude isKECK AO IMAGES OF URANUS 363
TABLE III
Photometric Calibration
Bandwidth Fλ (0 mag)a 1 Count/s in
Filter µm 10−19 W/m2 /µm Starb Magnitudeb W/m2 /µm
N6-band (June 18) 1.558–2.315 6.799 × 10−10 SJ9182 (HD207438) 11.085 (8.09) 1.70 ± 0.08
K -band (June 18) 1.950–2.295 4.435 × 10−10 HD207438 8.09 1.66 ± 0.13
N5-band (June 18) 1.413–1.808 1.177 × 10−9 SJ9182 (HD207438) 11.142 (8.11) 3.4 ± 0.2
N5-band (June 17) 1.413–1.808 1.177 × 10−9 HD201941 6.64 2.94 ± 0.15
N3-band (June 18) 1.143–1.375 2.946 × 10−9 SJ9182 (HD207438) 11.479 (8.17) 10.4 ± 0.5
N2-band (June 18) 1.089–1.293 3.469 × 10−9 HD207438 8.17 11 ± 1
N1-band (June 18) 0.947–1.121 5.521 × 10−9 HD207438 8.20 34 ± 3
aFlux density for Vega based upon the filter function and atmospheric transmission profile (see text).
bNo photometric standards were measured in the N1, N2, and K filters. We derived the magnitude of our PSF HD207438 at J , H , and K bands from SJ9182.
These numbers are given in parentheses. Based upon these numbers we adopted the magnitudes as listed for HD207438 in the N1, N2, and K bands.
and using atmospheric transmission spectra generated with the central meridian longitude), and at the equator, along the central
ATRAN modeling software (Lord 19922 ). The zero magnitude meridian longitude. Since the SCAM field-of-view in AO mode
flux densities are very similar to those published by Tokunaga is only 4.48 , image frames centered on rings and/or Uranus’s
(2000); discrepancies are due to small differences in the filter south pole usually cover only a fraction (50–90%) of the disk.
functions. We estimated the magnitudes for HD207438 from Images of the disk in various bands are shown in Fig. 2. We show
our observations of SJ9182 in order to calibrate our data in the the original images on the left side; on the right we show images
N1, N2, and K filters. Since the calibrators and Uranus were after subtraction of the original image convolved with a gaussian
always observed at similar airmasses A ( A ∼ < 0.15) and the function e−r /a , with the distance r 2 = (xc − x)2 + (yc − y)2 in
2 2
nights were photometric, any calibration errors introduced by pixels, and [xc , yc ] is the pixel at the center of the gaussian.
variations in extinction are less than a few percent. We used a = 15 pixels. This procedure enhances the contrast of
Figure 1 shows our mosaicked image of the uranian system in small features and/or sharp boundaries. The black stripes across
the N6 filter, which covers the combined H + K (1.56–2.31 µm) the images are caused by the spectrometer slit. Our image sharp-
band. Due to the planet’s increased brightness from K to J bands ening procedure introduced a brighter band along the slit. (Note
(in broadband filters), Uranus’s scattered light makes observa- that the figure in panels a and b is a composite of several images).
tions of the rings very difficult in broadband J . One can easily At all wavelength bands, except for K (1.950–2.295), the south
discern the bright asymmetric ring in Fig. 1 (on the viewing polar haze is prominent. This circumpolar band is the methane
geometry sketch in Fig. 1b we indicated the location of ring’s cloud or haze that condenses out near the 1.3 bar level (see be-
periapse). Inside of the ring at least three more rings are visible: low, and West et al. 1991 plus references therein). The pictures
from the outside inward these are: (1) combined δ, γ , η rings, on the right side clearly show that this layer is brightest near
(2) combined β, α rings, and (3) combined 4, 5, 6 rings. Equi- its northern edge (∼ −45◦ ) and around the south pole, while
valent widths and ring particle reflectivities are derived in there is an apparent lack of haze immediately around the south
Section 3.2, and listed in Tables V and VI. In addition to the pole. A similar gradation in brightness with the bright inner and
rings, the image shows the familiar circumpolar haze around outer bands was seen in ADONIS images of Uranus after de-
Uranus’s south pole, two cloud features in the southern hemi- convolution of the data (Conan et al. 2000), and in HST images
sphere (s4, s5), and three cloud features in the planet’s north- (Karkoschka 2001c). This brightness contrast may be caused
ern hemisphere (s1, s2, s3). These features are discussed in by an increase in the cloud column density (g cm−2 , i.e., either
Section 3.1. through an increase in particle number density or particle size)
at these locations.
2.3. Observations: Atmosphere A total of eight cloud features can be distinguished on the
We observed Uranus in several wavelength bands, as speci- disk. Most notable are the three features at northern latitudes
fied in Table II. In Table IV we summarize, for each filter, the (+28◦ , +36◦ , and +42◦ ) on June 18, which stand out particularly
monochromatic solar flux at 1 AU (Colina et al. 1996b) as in- bright compared to the background in the K filter. These cloud
tegrated over the bandpath of the various filters, and different features are visible in all filters, though the contrast relative to the
I/F values for Uranus: a disk-averaged value, and an average background is very low near 1 µm in the N1 and N2 filters. These
I/F per pixel near the south pole (at a latitude of 60◦ S, along the features are probably similar to the clouds seen by NICMOS on
HST (Karkoschka 1998), and the clouds reported by Sromovsky
et al. (2000) from IRTF data; they are not the same features as
2 We obtained the atmospheric transmission profile from Gemini Observa- those seen by NICMOS and IRTF, since we see several of them at
tory’s Web site: http://www.gemini.edu/sciops/telescope/telIndex.html. higher northern latitudes, regions still in darkness when observed364 DE PATER ET AL.
TABLE IV
Solar Flux and Reflectivitiesa for Uranus
Bandwidth Solar flux (π F)b I/F Uranus I/F south pole I/F equator
Filter µm W/mb /µm disk-averagedc average/pixeld average/pixele
N6-band 1.558–2.315 141.5 0.0059 0.0073 0.0046
K -band 1.950–2.295 93.8 0.00016 0.00013 0.00011
N5-band (June 18) 1.413–1.808 235.7 0.0090 0.0145 0.0091
N5-band (June 17) 1.413–1.808 235.7 0.0082 0.013 0.0079
N3-band 1.143–1.375 446.3 0.015 0.020 0.015
N2-band 1.089–1.293 494.8 0.013 0.018 0.013
N1-band 0.947–1.121 679.8 0.036 0.045 0.038
a Typical errors in the reflectivities are ∼10%.
b Monochromatic solar flux at 1 AU, as integrated over the bandpath of the various filters (see text).
c We adopted a Uranus radius of 25559 km (Uranus’ radius at the 1 bar level; Yoder 1995).
d Average I/F per pixel near the central meridian at a latitude of 60◦ S.
e Average I/F per pixel near the central meridian at the equator.
by NICMOS and IRTF. Our spatial resolution is higher than that Interior to the ring three faint rings can be discerned, which
in the NICMOS and IRTF data sets, and still the smallest features consist of multiple ringlets as described above in Section 2.1.
are unresolved in our beam; i.e., these are ∼KECK AO IMAGES OF URANUS 365
observations, we did carry out such tests when we reobserved
Uranus in August.
On August 21, 2000 UT we imaged Uranus’s rings at two
different position angles (PA) of the image rotator, one at PA = 0◦
and one at 90◦ . The results are shown in panels a and b in Fig. 4.
The overall quality of the images is lower than that seen in
the June data due to poorer seeing, which translates into a lower
Strehl ratio, and hence angular resolution. Nevertheless the ring-
loop is clearly visible in panel a, and absent in panel b. Figure 5
shows two images of the satellite Miranda at the same position
angles, while we continued to close the loop on Uranus. In these
observations Miranda appears to have a companion in panel a,
which is nearly absent in panel b. Similar results were obtained
for the satellite Ariel. Such a PSF on rings would translate into a
ring-loop, as observed for Uranus. The double-peaked PSF also
explains the shape of the cloud features on the planet, which
appear double-lobed on many of the images.
The artifacts in the rings complicate the ring analysis: in addi-
tion to the ring-loop, which was clearly discerned in the images,
similar stuctures are present within the ringlets. To clean the im-
age from such artifacts we adopted the following simple method:
we rotated panel b in Figs. 4 and 5 to the position angle of a and
subtracted this from panel a. In Miranda’s case, this results in
an image of the secondary component (considering only posi-
tive values, with a nonzero lower cutoff), and for Uranus of ring
artifacts. When these are subtracted from panels a, the artifacts
get largely removed. Figure 4c shows the result for Uranus, after
the cleaned image from panel a was combined with that in b (to
increase the signal-to-noise ratio).
In June we had data at only one position angle, so this ap-
proach could not work. Instead, we assumed symmetry between
the left and right sides of the rings. Since periapse of the ring is
almost exactly at the southernmost tip of the ring (Fig. 1b), this
assumption is probably quite good. We then flipped the image
over, left to right, and subtracted it from the original image. The
positive artifacts (adopting a positive nonzero lower cutoff) were
then subtracted from the original image to obtain the cleaned im-
age; this image was shown in Fig. 1a (for the N6 filter). The total
intensity in the subtracted artifacts was ∼366 DE PATER ET AL.
FIG. 3. Images of Uranus’s rings in the N5 (taken on June 17, 2000 UT) and N6 (taken on June 18, 2000 UT) filters. The north and south sides are shown
separately. In contrast to Fig. 1, these images have not been processed to remove artifacts.
along the rings: we constructed a model of elliptical rings pro- images, are shown in Fig. 6. The dotted lines are scans toward
jected onto the plane of the sky (simulating Uranus’s ring orbits) the north (PA = 0◦ ; positive toward the East) through the rings,
and averaged the intensity along these ringlets. The results for averaged over 60◦ in azimuth. This scan includes Uranus’s scat-
the N6 (H + K ) and N5 (H ) filters, as obtained from our cleaned tered light. The solid and dashed lines show scans centered at
PA = 0◦ and 180◦ (apo- and periapse of the ring), respectively,
after the scattered light contribution was subtracted.
We determined Uranus’s scattered light contribution as fol-
lows: We took a radial scan through Uranus at PA ≈ 45◦ (i.e.,
away from the rings). Using this scan, we calculated the scattered
light contribution at all positions in the image and subtracted this
from the original image. Note that in reality the amplitude of the
scattered light contribution is not circularly symmetric about
the planet; since the south polar cap is brighter than the northern
hemisphere, we have either over- or underestimated the scat-
tered light contribution, depending on our location on the disk.
We then constructed a radial profile through this cleaned im-
age, averaged over 60◦ in azimuth and centered at PA = 0◦ and
180◦ . The difference between this profile and the original pro-
file (i.e., the dotted line in Fig. 6) represents Uranus’s scattered
light contribution. To minimize the noise introduced via this pro-
cess, we made a polynomial fit through this difference profile.
This smooth curve was then scaled such that when subtracted
from the original ring profile, the resulting profile at a uranian
distance r = 3.5 × 104 and r = 6 × 104 was close to zero. The
results of this process were shown as solid and dashed curves in
Fig. 6.
Superposed on Fig. 6 are the positions of all known ringlets.
FIG. 4. Images of Uranus’s rings taken on August 21, 2000 UT at different
Since the ring’s periapse was near the direction of the ring’s
position angles of the image rotator: (a) PA = 0◦ , (b) PA = 90◦ , and (c) combined south polar axis, the south side of the rings is much weaker
images from (a) and (b) after artifacts had been removed as described in the text. than the north side. This effect has been known for a long timeKECK AO IMAGES OF URANUS 367
and can be attributed to a combination in the variation in ring
width and optical depth, as mentioned above. The ring stands
out clear and bright; the λ ring cannot be discerned, while the
δ, γ , and η rings, combined, result in the peak just inside the
ring, the α and β rings form the second peak inward, and
rings 4, 5, and 6 the third peak. All three peaks inside the
ring are slightly resolved. We remind the reader here that our
images were mosaicked together, and that we used the distance
of the epsilon ring to “glue” the north and south together; thus
the relative ring positions in each hemisphere are properly mea-
sured, but the absolute scale is based upon prior knowlege of the
ring.
3. DISCUSSION
3.1. Atmosphere
We detected a total of eight atmospheric features during our
two-day observing period (Fig. 2, Table II). Fortuitously,
Hammel et al. (2001) observed Uranus with HST on June 16,
2000 UT and June 17, 2000 UT, just prior to our observing dates.
They detected the same features in the HST data. By combining
our data we could derive precise wind velocities for seven of
the eight features. Feature S8 was observed on the limb of the
planet by both telescopes, resulting in too large an uncertainty
on its exact location to allow the derivation of an accurate wind
speed. The results of this work were reported by Hammel et al.
(2001). For completeness we show Hammel et al.’s wind profile
in Fig. 7. The red data points represent the wind velocities as
derived from the combined HST and Keck data. We refer the
reader to Hammel et al.’s paper for a detailed discussion of the
wind profile.
The northern latitude features s1 and s2 were visible in all
FIG. 5. Images of Miranda (intensity is on a log scale) while the AO system
filters and showed a very high brightness contrast in the K filter.
closed the loop on Uranus. (a) Position angle of the image rotator PA = 0◦ and Uranus’s rings pass in front of feature s1; hence it is visible, but
(b) PA = 90◦ . its intensity cannot be determined without extensive modeling.
1 1
(a) (b)
0.8 0.8
0.6 0.6
0.4 0.4
0.2 0.2
0 0
Distance (km) Distance (km)
FIG. 6. Radial scans through the rings toward the north (apoapse) and south (periapse) in the N6 (panel a) and N5 (panel b) filters, after the images were
cleaned as described in the text. The profiles are averaged over 60◦ in azimuth, centered at apo- and periapse. The upper dotted curves are profiles through apoapse
taken straight from the image. The solid lines are the same profiles after Uranus’s scattered light was subtracted. The lower dashed lines are profiles through
periapse after removal of Uranus’s scattered light. The position of the individual ringlets is indicated on each graph. Note that we took images of the north and
south sides separately, and combined the data by forcing the location of the ring at its known distance from Uranus.368 DE PATER ET AL. FIG. 7. Wind velocity profile from Hammel et al. (2001). The red data points are the combined HST and Keck data. Other points are HST data from previous years and Voyager data, as indicated in the figure legend. The curves are various models, as indicated in the legend as well. The reader is referred to Hammel et al. (2001) for more details. Since the south polar methane haze is not visible in the K filter, tion (CIA), and H2 –He CIA were included as sources of opac- it is clear that the northern latitude features are located above ity. If the clouds are represented by a simple reflecting (equally the nominal altitude of any methane haze layer. To get a rough reflecting at all wavelengths involved) layer, a comparison of estimate of the altitude of the northern latitude features, we used the ratio of the transmission curves with the observed I/F val- a simple model of Uranus’s atmosphere, where we calculated ues in various filters can be used to determine the approximate the atmospheric transmission (going into and out of the atmo- altitude of the cloud features. We show a graph of the trans- sphere) as a function of pressure level for each of our filters mission (near the subsolar point) in the N1, N5, and K filters (Roe et al. 2001). Only CH4 , H2 –H2 collision induced absorp- in Fig. 8. In this model we adopted the temperature–pressure
KECK AO IMAGES OF URANUS 369
1 3.2. Rings
N1
Figure 6 showed a radial profile of the rings. To extract qual-
itative information on the rings, we fit models to the data to
0.8 determine the equivalent I/F width (EW), the width (in meters)
the ring would have if its reflectivity I /F = 1, where I/F is the
observed intensity I divided by F, where the incident solar flux
is π F. As mentioned in Section 2.4, PSF artifacts in the images
0.6 complicate a qualitative analysis. To get a good estimate of the
EW of each ring together with a realistic error bar, we applied
three different fitting procedures. Two of these techniques, our
Kp
1-D methods, are based upon an analysis of ring profiles, such
0.4
as shown in Fig. 6, and the third technique, our 2-D method,
N1
involves model fitting the images with simultaneously fitting
the PSF.
For both 1-D analyses, we constructed a simple model where
0.2
N5 each ring is represented by a delta function at its known location.
Convolution with the proper PSF then allows determination of
the relative intensities by matching the modeled profile to the
0 data. Choice/knowledge of the proper PSF is crucial in all three
1000 100 10 1 methods. As mentioned in Section 2.4, the PSF generally con-
Pressure (mbar)
sists of a core and broad low intensity halo (such as seen in
FIG. 8. Curves of Uranus’s atmospheric transmission (sunlight going in Fig. 5), which can usually be determined by observing a star.
and out of the atmosphere; i.e., twice the transmission) averaged over the N1, To construct a PSF appropriate for a radial ring profile, we con-
N5, and K filters. The solid curves were calculated with a CH4 abundance of structed an image with a narrow (∼1-pixel wide) elliptical ring
zero in the stratosphere; for the dashed curves it was set equal to 0.00035. The of unit intensity, shaped like Uranus’s ring. Convolution with
CH4 abundance in the troposphere was equal to 2.3% in all calculations.
the appropriate PSF, followed by an azimuthal averaging proce-
dure, similar to that applied to Uranus’s rings (Fig. 6), results in
the 1-D PSF used in our 1-D analyses.
profile of Lindal (1992) and used a H2 : He number ratio of
85 : 15, with a fraction of 0.80 for H2 in ortho-para equilibrium.
3.2.1. 1-D Analysis I
We assumed a methane abundance of 2.3% in the lower tropo-
sphere (Lindal et al. 1987), while following the saturated vapor In this analysis we tried to match the radial ring profiles shown
pressure curve at higher altitudes. The methane abundance was in Fig. 6, i.e., profiles through our cleaned images. We convolved
set equal to zero above the tropopause for the model with the our model ring with an image of a star (8th mag star HD207438
solid lines. The resulting transmission curves are not very sen- and 11th mag star SJ9182), prior to the azimuthal averaging. The
sitive to small (∼5%) changes in the H2 : He ratio, but the trans- resulting profiles are shown in Fig. 9 (dotted line for convolution
mission in some of the filters is quite sensitive to the methane with HD207438, dashed line for SJ9182). The solid line in this
abundance in the stratosphere. The dashed profiles show the sen- figure is the observed ring profile from Fig. 6a. Since the ring
sitivity of the model if the methane abundance in the stratosphere is unresolved, and there are no known features exterior to the
were increased to 0.00035, the value determined for Neptune’s ring, its core and the right-hand side of the ring should mimic
stratosphere (Baines and Hammel 1994). the PSF. The model profile HD207438 does not match the ring
Since the south polar haze is invisible in the K filter and bright profile at all: as expected from a m v = 8 star, it has a well-defined
in the N5 and N1 filters, one can deduce from Fig. 8 that the top core and low level halo. Typical Strehl ratios for this star were
of the south polar haze layer must be at a pressure level between on the order of 20–25% in the N5 and N6 filters. The Strehl
1.1 and 1.2 bar. This is indeed consistent with the interpreta- ratio in the model profile SJ9182 is considerably lower (Strehl
tion that we are looking down at the CH4 methane cloud which ratios for this star were ∼5–6% in the N5 and N6 filters), and
has its base at a pressure level of 1.23 bar (Lindal et al. 1987). this profile does match the ring better, although the halo is still
The northern latitude clouds have a high brightness contrast in too low. Note that none of the profiles is symmetric, due to the
the K filter, but because the I/F of Uranus in this filter is very azimuthal averaging effect.
low, the I/F of the cloud features in this filter is still very low In Section 2.3 we discussed PSF issues to explain and clean
compared to that in other filters. From the transmission curves it up some of the ring artifacts. The Miranda image (Fig. 5) was
follows that these clouds must be located at altitudes just above an excellent representation of the PSF in August (Miranda’s
the 1 bar level, but below ∼0.5 bar, and therefore below the angular extent is 0.035 ; such a small source is indistinguishable
tropopause. from our PSF). It is a better representation than a conventional370 DE PATER ET AL.
ring very well. We conclude from this exercise that the use of an
200
Comparison PSF Model - Data
extended source as an AO guidestar may not only show multiple
Model with PSF HD207438
peaks, but also show a lower Strehl ratio so that the intensity
Model with PSF SJ9182
ratio between the peak and the halo is decreased. To construct
Model with PSF Miranda a radial PSF profile which best matches the PSF in Fig. 6, we
150 Observed ring profile combined the core of the ring as observed (solid line) with
the wings from our model ring profile after convolution with
Miranda (dash-dot-dash line). To avoid insertion of noise from
our PSF images in the final radial PSF profile, we replaced the
wings of the profile with polynomial fits. The final PSF used in
100
our 1-D analysis is shown by the dashed lines in Fig. 10 (panels
a and b).
In Table V we show the equivalent I/F width in meters for
each ring, which best fits our data. The fits were obtained via
50 a least-squares fitting routine using the AMOEBA algorithm
(Press et al. 1992) as implemented in the IDL programming
environment. We forced rings 4, 5, and 6 to be equal, and hence
fit seven independent variables.
0 3.2.2. 1-D Analysis II
Distance (km)
In this analysis we used ring profiles through images before
FIG. 9. Comparison of our ring profile from Fig. 6a (solid line) with a PSF: they were cleaned. Since the PSF from analysis I did fit the
we convolved a model ring with an image of a star taken during our observations. profile very well, we used the same PSF for analyses I and II.
The profiles shown are averaged over 60◦ in azimuth, centered at PA = 180◦ , Profiles and best fits through the data are shown in Fig. 10, panels
just like for Uranus’s rings. The dotted line shows the profile where the ring was
a and b. The equivalent I/F widths EW derived for the individual
convolved with an 8th mag. star (HD207438); for the dashed line the ring was
convolved with an 11th mag. star (SJ9182), and for the dash-dot-dash line the ringlets are summarized in Table V. As in analysis I, we fit seven
ring was convolved with the Miranda image from August. The ring profiles are independent variables. In addition to listing the EW for each
quite similar, but not exactly the same, as radial cuts through the ring. ring, we show the EW for the combined α + β ring, γ + δ + η
ring, and 4 + 5 + 6 ring. Both 1-D analyses were carried out for
star image, since for Miranda we used Uranus as the wavefront the north side of the rings, where the ring is brightest, and
reference for the AO system, just as for the Uranus observations hence has the best signal-to-noise ratio.
themselves. The dash-dot-dash line in Fig. 9 shows a profile
3.2.3. 2-D Analysis
through our artificial ring after convolution with the Miranda
image. The resulting radial profile of the model ring is much In our 1-D analysis we created an image of the ring to con-
broader; in fact it is too broad in the core of the ring (caused by struct a PSF profile. For our 2-D analysis we created a model
poor seeing), but the halo or wings of the profile do match the of all rings: the ring and its azimuthal brightness gradient,
TABLE V
Equivalent Widths for Uranus’s Rings
EW (in m) EW (in m) EW (in m) EW (in m) EW (in m) EW (in m)
Distancea Widtha Method 1 Method 2 Method 3b Method 1 Method 2 Method 3b
Ring 103 km km τ a (Voyager) (1.6 µm) (1.6 µm) (1.6 µm) (2.0 µm) (2.0 µm) (2.0 µm)
α 44.7 4.8 → 10.0 ∼0.4 277 242 101 286 320 102
β 45.7 6.1 → 11.4 ∼0.3 65 95 170 173 87 184
η 47.2 1.9 → 2.9 <
∼0.4 127 63 28 54 82 34
γ 47.6 3.6 → 4.7 >
∼1.5 225 257 74 263 280 152
δ 48.3 4.1 → 6.1 ∼0.5 15 24 102 89 66 118
51.1 19.7 → 96.4 0.5 → 2.3 2750 2870 532 3670 3790 635
6+5+4 42.3 115 134 82 144 186 68
α+β 45.1 342 337 271 459 407 286
η+γ +δ 47.8 367 344 204 406 428 304
a Yoder (1995).
b Note that Method 3 determined the EW at the longitude of the ring’s periapse, while Methods 1 and 2 used its apoapse.KECK AO IMAGES OF URANUS 371
1 a 1 b
0.8 0.8
0.6 0.6
0.4 0.4
0.2 0.2
0 0
120
c
100
80
Intensity (arbitrary units)
60
40
20
0
-20
0 20 40 60 80
Distance (pixels)
FIG. 10. (a, b) Model fits through the radial ring profiles at apoapse; these profiles were taken from the original images (i.e., before any cleaning algorithm
was applied): (a) best fit through the N6 filter data, (b) best fit through the N5 filter data. The model fit (dotted line) is superposed on the data (solid line). The
dashed line is the PSF for the ring profile. The lower solid line is the residual, after subtracting the model from the data. Largest residuals are at the location of the
ring. (c) A slice through the N6 images of the 2-D model fit (dotted line) and data (solid line). These slices are taken after subtraction of the convolved images
from the original images (see text).
the δ, γ , η, β, α, and combined 456 rings. Each ring is mod- which is assumed to have the same shape as Uranus’s moon
eled as a 1-pixel-wide ellipse with a single brightness (I/F Miranda. We characterize the PSF by three parameters: the
value), at its known location. The brightness of each pixel is brightness ratio between the two lobes, the distance between
scaled by the fraction of the pixel that would be occupied by the centers of the lobes, and the orientation angle of the lobes.
the ring ellipse. We further modeled Uranus as a disk whose We thus have a total of 13 independent variables, which we de-
brightness and brightness gradient were determined by the min- termined via an iterative fitting routine based on AMOEBA in
imization procedure (results are fairly insensitive to these pa- IDL.
rameters since only the portion of the rings away from the As in the 1-D analyses, scattered light from the planet presents
planet was modeled; the presence of the planet is only required a problem and had to be removed before we could fit a model to
in the attempt to remove scattered light as discussed below). the data. To remove the scattered light of Uranus from the ring
This planet + ring model must be convolved with the PSF be- system, we convolved the model and data with a median function
fore comparison with the data. As discussed above, the real with a width of 30 pixels and subtracted these images from
PSF for these observations is unknown, but we do have evi- the originals. This method is analogous to that used to create
dence from images of Uranus’s moons taken in August 2000 the images on the right-hand side of Fig. 2 and enhances the
that the PSF exhibits a double-lobed structure. Therefore we contrast of small and/or narrow features. As in the 1-D analysis,
have constructed a theoretical PSF with two lobes, each of we determined a ring profile by averaging over 30◦ in azimuth372 DE PATER ET AL.
TABLE VI
Ring Particle Reflectivities
Distancea Eb Aν Aν Aν c
Ring 103 km km (Keck, 1.6 µm) (Keck, 2.0 µm) (HST, 0.9 µm)
51.1 ∼15 → 58 0.042 ± 0.006 0.053 ± 0.008 0.044 ± 0.004
6+5+4 42.3 3.0 → 3.2 0.033 ± 0.006 0.037 ± 0.010 0.046 ± 0.029
α+β 45.1 7.5 0.041 ± 0.005 0.048 ± 0.008 0.048 ± 0.011
η+γ +δ 47.8 6.7 0.042 ± 0.008 0.054 ± 0.007 0.040 ± 0.010
a Yoder (1995).
b Equivalent width as determined by French et al. (1986).
c Reflectivities at phase angle 2.82◦ and wavelength 0.9 µm, from Karkoschka (2001a).
and minimized the residuals between the model and data. We (column 6 in Table VI). In general the ring particle reflectivi-
applied this method to the south sides of the rings, where the ties are quite similar from ring to ring, although the particles
brightness contrast between the ring and the other rings is in the inner rings may be slightly less reflective. These rings,
smallest. however, are influenced the most by Uranus’s scattered light,
which may bias the ring reflectivities. Although our ring reflec-
3.2.4. Discussion tivities might be slightly higher at 2 µm compared to a 1.6-µm
wavelength, when including the individual error bars and com-
The equivalent I/F width as listed in Table V varies substan-
paring with Karkoschka’s 0.9-µm results, we conclude that the
tially for individual rings depending on which method is used.
spectra of the ring particles are quite flat over the range of
This is not too surprising, since the I/Fs of adjacent rings are not
0.9–2 µm.
independent due to the extent of the PSF. Hence variations in one
ring’s I/F does influence other rings. This is in particular true
for ringlets in the three clusters: [4, 5, 6], [α, β], and [δ, γ , η]. 4. CONCLUSIONS
As shown, the total EW for each cluster of rings is much better
behaved. Note that analyses I and II were performed on the north We presented adaptive optics images of Uranus and its rings
(bright) side of the rings, while method III was used for the south at an angular resolution of ∼0.06 in H band and ∼0.09 in the
(dim) side. Because the uncertainties in the EW of the individual combined H + K band. We identified eight small cloud features,
rings are quite large, we will focus the following discussion on all of which had also been seen by HST just prior to our obser-
the rings and the three groups of combined rings. vations. Hammel et al. (2001) derived an accurate wind pro-
To facilitate comparison between our own numbers (north and file for Uranus’s atmosphere by combining the HST and Keck
south side of the rings) and with other data, we next calculate the data. Continued observations of these and other such features
ring particle reflectivities. The width and normal optical depth are important for studies of atmospheric dynamics. It would be
(τ ) of the rings as measured by Voyager were summarized in highly valuable to monitor the planet over time scales of weeks—
columns 3 and 4 of Table V (Yoder 1995). In analogy to spectral months to derive an accurate wind profile as a function of lati-
lines, Elliot et al. (1984) introduced equivalent widths (E; not to tude, study its stability over time, and determine the longevity
be confused with our equivalent I/F width EW) and equivalent of cloud features. Are the clouds detected in the northern hemi-
depths for the rings based upon occultation profiles. French et al. sphere short-lived temporary features, or are the same features
(1986) derived a relation between the radial width of a ring visible over many weeks? Karkoschka (1998) saw cloud features
and its equivalent width E for each ring. This parameter does at the same locations over a 100-day time period, but we cannot
not vary with ring inclination angle (Elliot et al. 1984), and exclude the possibility that features disappeared and re-formed
we used French et al.’s values as the true width of the rings, later. Our data show that these features did disappear over year-
i.e., the width of a square-well model to each ring’s occultation long time scales. The northern hemisphere clouds are located at
profile. These numbers (in kilometers) are listed in column 3 of higher altitudes (between 1 and 0.5 bar) than the south polar haze
Table VI. Dividing our measured EW by E results in the particle (1.1–1.2 bar) and southern hemisphere clouds. Why are these
reflectivity, Aν . These numbers are listed in columns 4 and 5 of high altitude features only visible in the northern hemisphere?
Table VI. The numbers obtained via the individual methods were Will they continue to exist even after these regions have been ex-
averaged (method 3 was given double weight), and the error is posed to sunlight for many years? With continued observations
based upon the spread between the results. We also included a we will be able to address questions regarding the similarity and
10% photometric error in the quoted uncertainty. differences between the two hemispheres as the north polar cap
Our reflectivities are close to those derived by Karkoschka rotates into view: will the northern hemisphere be covered by a
(2001a) from HST data at 0.9 µm at a similar phase angle methane cloud/haze just like the south pole? Why/why not?KECK AO IMAGES OF URANUS 373
We derived equivalent I/F widths and ring particle albedos AS 00-AP-009. The authors extend special thanks to those of Hawaiian an-
for the rings. Overall our derived reflectivities (0.04–0.05) agree cestry on whose sacred mountain we are privileged to be guests. Without their
well with the albedos derived from HST data at 0.9 µm at generous hospitality, none of the observations presented would have been
possible.
a similar phase angle. Our results suggest the particles to be
gray, since the spectra appear quite flat. There also does not
appear to be any significant variation in color between rings.
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ACKNOWLEDGMENTS
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LLNL participation was performed under the auspices of the U.S. Depart- Lindal, G. F., J. R. Lyons, D. N. Sweetnam, V. R. Eshelman, D. P.
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