Uranus at equinox: Cloud morphology and dynamics

As the 7 December 2007 equinox of Uranus approached, collaboration between ring and atmosphere observers in the summer and fall of 2007 produced a substantial collection of ground-based observations using the 10-m Keck telescope with adaptive optics and space-based observations with the Hubble Space Telescope. Both near-infrared and visible-wavelength imaging and spatially resolved near-infrared spectroscopic observations were obtained. We used observations spanning the period from 7 June 2007 through 9 September 2007 to identify and track cloud features, determine atmospheric motions, characterize cloud morphology and dynamics, and define changes in atmospheric band structure. Atmospheric motions were obtained over a wider range of latitudes than previously was possible, extending to 73°N, and for 28 cloud features we obtained extremely high wind-speed accuracy through extended tracking times. We confirmed the existence of the suspected northern hemisphere prograde jet, locating its peak near 58°N. The new results confirm a small N-S asymmetry in the zonal wind profile, and the lack of any change in the southern hemisphere between 1986 (near solstice) and 2007 (near equinox) suggests that the asymmetry may be permanent rather than seasonally reversing. In the 2007 images, we found two prominent groups of discrete cloud features with very long lifetimes. The one near 30°S has departed from its previous oscillatory motion and started a significant northward drift, accompanied by substantial morphological changes. The complex of features near 30°N remained at a nearly fixed latitude, while exhibiting some characteristics of a dark spot accompanied by bright companion features. Smaller and less stable features were used to track cloud motions at other latitudes, some of which lasted over many planet rotations, though many could not be tracked beyond a single transit. A bright band has developed near 45°N, while the bright band near 45°S has begun to decline, both events in agreement with the idea that the asymmetric band structure of Uranus is a delayed response to solar forcing, but with a surprisingly short delay of only a few years.     

Episodic bright and dark spots on Uranus

The northern mid-latitudes of Uranus produce greater episodes of bright cloud formation than any other region on the planet. Near 30°N, very bright cloud features were observed in 1999, 2004, and 2005, with lifetimes of the order of months. In October 2011, Gemini and HST observations revealed another unusually bright cloud feature near 23°N, which was subsequently identified in July 2011 observations and found to be increasing in brightness. Observations obtained at Keck in November 2011 revealed a second bright spot only 2°N of the first, but with a substantially different drift rate (?9.2°E/day vs ?1.4°E/day), which we later determined would lead to a close approach on 25 December 2011. A Hubble Target of Opportunity proposal was activated to image the results of the interaction. We found that the original bright spot had faded dramatically before the HST observations had begun and the second bright spot was found to be a companion of a new dark spot on Uranus, only the second ever observed. Both spots exhibited variable drift rates during the nearly 5 months of tracking, and both varied in brightness, with BS1 reaching its observed peak on 26 October 2011, and BS2 on 11 November 2011. Altitude measurements based on near-IR imaging in H and Hcont filters showed that the deeper BS2 clouds were located near the methane condensation level (≈1.2 bars), while BS1 was generally ～500 mb above that level (at lower pressures). Large morphological changes in the bright cloud features suggest that they are companion clouds of possibly orographic nature associated with vortex circulations, perhaps similar to companion clouds associated with the Great Dark Spot on Neptune, but in this case at a much smaller size scale, spanning only a few degrees of longitude at their greatest extents.     

The altitude of Neptune cloud features from high-spatial-resolution near-infrared spectra

We report on observations of Neptune from the 10-meter W.M. Keck II Telescope on June 17-18 (UT) 2000 and August 2-3 (UT) 2002 using the adaptive optics (AO) system to obtain a spatial resolution of 0.06 arcseconds. With this spatial resolution we can obtain spectra of individual bright features on the disk of Neptune in a filter centered near 2 microns. The use of a gas-only, simple reflecting layer radiative transfer model allows us to estimate the best fit altitudes of 18 bright features seen on these 4 nights and to set a constraint on the fraction of hydrogen in ortho/para equilibrium. On these nights there were three main types of features observed: northern hemisphere features in the range from +30 to −45 degrees; southern hemisphere features in the range from −30 to −50 degrees; and small southern features at −70 degrees. We find that the altitudes of the northern features are in the range from 0.023-0.064 bar, which places them in Neptune's stratosphere. Southern features at −30 to −50 degrees are mainly at altitudes from 0.10 to 0.14 bars. The small features at −70 degrees are somewhat deeper in the upper troposphere, at 0.17 and 0.27 bars. This pattern of features located at higher altitudes in the northern hemisphere and lower altitudes in the south has also been noted by previous observers. The best fits for all the observed spectra give a value of 1.0 for the fraction of hydrogen in ortho/para equilibrium; the value of the helium fraction is less well constrained by the data at 0.24. We suggest that the southern mid-latitude features are methane haze circulated up from below, while the −70° features may be isolated areas of upwelling in a general area of subsidence. Northern bright features may be due to subsidence of stratospheric haze material rather than upwelling and condensation of methane gas. We suggest that convection efficiently transports methane ice clouds to the tropopause in the Southern mid latitudes and thus plays a key role in the stratospheric haze production cycle.     

Restored methane band images of Uranus and Neptune

Charge-coupled device images of Uranus and Neptune taken in the 8900-Å absorption band of methane are presented. The images have been digitally processed by means of nonlinear deconvolution techniques to partially remove the effects of atmospheric seeing. The restored Uranus images show strong limb brightening consistent with previous observations and theoretical models of the planet's atmosphere. The computer-processed images of Neptune show discreted cloud features similar to those reported previously by B. A. Smith, H. J. Reitsema and S. M. Larson (1979 Bull. Amer. Astron. Soc.11, 570). A time series of the restored Neptune images shows a continuous variation which may be due to the planet's rotation.     

Multispectral imaging observations of Neptune’s cloud structure with Gemini-North

Observations of Neptune were made in September 2009 with the Gemini-North Telescope in Hawaii, using the NIFS instrument in the H-band covering the wavelength range 1.477–1.803 μm. Observations were acquired in adaptive optics mode and have a spatial resolution of approximately 0.15–0.25″.The observations were analysed with a multiple-scattering retrieval algorithm to determine the opacity of clouds at different levels in Neptune’s atmosphere. We find that the observed spectra at all locations are very well fit with a model that has two thin cloud layers, one at a pressure level of ∼2 bar all over the planet and an upper cloud whose pressure level varies from 0.02 to 0.08 bar in the bright mid-latitude region at 20–40°S to as deep as 0.2 bar near the equator. The opacity of the upper cloud is found to vary greatly with position, but the opacity of the lower cloud deck appears remarkably uniform, except for localised bright spots near 60°S and a possible slight clearing near the equator.A limb-darkening analysis of the observations suggests that the single-scattering albedo of the upper cloud particles varies from ∼0.4 in regions of low overall albedo to close to 1.0 in bright regions, while the lower cloud is consistent with particles that have a single-scattering albedo of ∼0.75 at this wavelength, similar to the value determined for the main cloud deck in Uranus’ atmosphere. The Henyey-Greenstein scattering particle asymmetry of particles in the upper cloud deck are found to be in the range g ∼ 0.6–0.7 (i.e. reasonably strongly forward scattering).Numerous bright clouds are seen near Neptune’s south pole at a range of pressure levels and at latitudes between 60 and 70°S. Discrete clouds were seen at the pressure level of the main cloud deck (∼2 bar) at 60°S on three of the six nights observed. Assuming they are the same feature we estimate the rotation rate at this latitude and pressure to be 13.2 ± 0.1 h. However, the observations are not entirely consistent with a single non-evolving cloud feature, which suggests that the cloud opacity or albedo may vary very rapidly at this level at a rate not seen in any other giant-planet atmosphere.     

The methane abundance and structure of Uranus' cloud bands inferred from spatially resolved 2006 Keck grism spectra

Grism spectra of Uranus obtained at the Keck Observatory in 2006, using the NIRC2 instrument and adaptive optics, provide new constraints on the vertical structure of Uranus' cloud bands and on the volume mixing ratio of methane. The best model fits to H-band spectra (1.49-1.635 μm) are found for a methane volume mixing ratio of 1.0 ± 0.25% for latitudes near 43° S and 1-1.6% for latitudes of 12° S and 33° N. Analysis of the J-band spectra are confused by discrepancies between short-wave and long-wave sides of the 1.28 μm window region. The short-wave side of the window (1.23-1.30 μm) is best fit with 1.6% CH₄, but if the fitted spectral range is extended to include the long-wave side of the window (1.2-1.34 μm), the best fit CH₄ mixing ratio is 4% or more, although many small scale spectral features are poorly fit over this range even at high methane mixing ratios, suggesting that models of methane opacity may be inconsistent in this spectral region. Most of the latitudinal variability of the H-band spectra can be fit with clouds near 2-3 and 6-8 bar, with cloud reflectivity of the deeper layer increasing from ∼2% at 33° N to 3-4% in the southern hemisphere. This layer is most likely made of H₂S particles and appears weakly reflective because it is optically thin and possibly also contaminated by absorbing materials. The reflectivity of the 2-3-bar cloud increases from 0.5% at 33° N to ∼1% at the bright band centered near 43° S, where the upper cloud is a little higher (pressure is 10% lower) and ∼25% more reflective than at nearby latitudes. The bright band is also associated with lowering of the deep cloud pressure, by ∼1.4 bar. The bright band parameters are roughly consistent with those obtained from 1975 disk-averaged spectra, obtained when the southern hemisphere was more exposed to the Sun. The lack of significant cloud particle contributions near 1.2 bar, where occultation results suggested a methane cloud, is confirmed by both spectra and HST imaging observations.     

Dynamics, evolution, and structure of Uranus' brightest cloud feature

The brightest cloud feature ever observed on Uranus at near-infrared wavelengths was detected on 14 and 15 August 2005, in images obtained with the NIRC2 instrument and adaptive optics (AO) at the 10-m Keck II telescope. The feature has been tracked forward and backward in time, and appears to have existed almost certainly from 5 November 2004 (possibly as early as 11 July 2004) through 29 October 2005. It appears to exhibit two modes of oscillation in latitude and longitude. The slow oscillation period is too long to be completely characterized by the observations; its period is most likely near 448 days, but might be as long as 753 days. The slow oscillation is consistent with the zonal mean wind profile when a superimposed more rapid oscillation is accounted for. The slow oscillation, possibly associated with a Rossby wave, was centered at 30.2° N and had a latitude amplitude of 0.6°–0.7°. Its rapid oscillation had an amplitude of 1.2° in latitude and a likely period near 0.68455 days, which is consistent with an inertial oscillation at the observed latitude. The multi-component structure of the bright features has evolved over time, as has its vertical structure. Its brightness maximum was due to a combination of cloud particles being lofted to higher altitudes, some rising from 400–500 to 300 mb, and by its effective cloud fraction (or equivalent cloud area) increasing by a factor of 5 or more. In the K′ band (2.2 μm) the differential integrated brightness due to this bright complex increased to 13% of the total light reflected by Uranus on 15 August 2005, rising from about 2% a month earlier and declining to 0.7% two months later. It has not been seen in 2006 observations.     

Temporal variability of ultraviolet cloud features in the Venus stratosphere

Statistics on the temporal variability of uv cloud features on Venus during 66 days of nominal mission imaging by the Pioneer Venus Orbiter Cloud Photopolarimeter reveal at least five types of systematic variability on large scales: (1) a low-latitude global-scale wave of period 3.94 ± 0.1 days corresponding to longitudinal motion of the dark equatorial band and propagating westward relative to the mean flow; (2) a midlatitude wave of period 5.20 ± 0.2 days corresponding to wavenumber 1 oscillations of the latitude of the bright polar bands and propagating eastward relative to the mean flow; (3) ～2- to 3-week fluctuations in the slope of longitudinal cloud brightness power spectra at intermediate wavenumbers manifested by variations in the intensity of large bow-shaped features; (4) ～2-month variations in polar region brightness consistent with polar brightening episodes observed from Earth; and (5) a monotonic decrease in the disk-integrated brightness of Venus during the nominal mission which may be either a true time variation or a solar-locked longitudinal dependence of brightness. Small-scale features appear to correlate with large-scale albedo patterns. Specifically, cellular features exist primarily where large-scale dark material is present, while the orientation of streak features with respect to latitude circles oscillates with the same ～4-day period as the large-scale features at low latitudes. The wide range of time scales present in the data suggests the complexity of Venus stratospheric dynamics. Extended observations over many years may be becessary to define the general circulation.     

Dynamics of cloud features on Uranus

Near-infrared adaptive optics imaging of Uranus by the Keck 2 telescope during 2003 and 2004 has revealed numerous discrete cloud features, 70 of which were used to extend the zonal wind profile of Uranus up to 60° N. We confirmed the presence of a north-south asymmetry in the circulation [Karkoschka, E., 1998. Science 280, 570-572], and improved its characterization. We found no clear indication of long term change in wind speed between 1986 and 2004, although results of Hammel et al. [Hammel, H.B., Rages, K., Lockwood, G.W., Karkoschka, E., de Pater, I., 2001. Icarus 153, 229-235] based on 2001 HST and Keck observations average ∼10 m/s less westward than earlier and later results, and 2003 observations by Hammel et al. [Hammel, H.B., de Pater, I., Gibbard, S., Lockwood, G.W., Rages, K., 2005. Icarus 175, 534-545] show increased wind speeds near 45° N, which we do not see in our 2003-2004 observations. We observed a wide range of lifetimes for discrete cloud features: some features evolve within ∼1 h, many have persisted at least one month, and one feature near 34° S (termed S34) seems to have persisted for nearly two decades, a conclusion derived with the help of Voyager 2 and HST observations. S34 oscillates in latitude between 32° S and 36.5° S, with a period of ∼1000 days, which may be a result of a non-barotropic Rossby wave. It also varied its longitudinal drift rate between −20°/day and −31°/day in approximate accord with the latitudinal gradient in the zonal wind profile, exhibiting behavior similar to that of the DS2 feature observed on Neptune [Sromovsky, L.A., Limaye, S.S., Fry, P.M., 1993. Icarus 105, 110-141]. S34 also exhibits a superimposed rapid oscillation with an amplitude of 0.57° in latitude and period of 0.7 days, which is approximately consistent with an inertial oscillation.     

Seeing double at Neptune’s south pole

Keck near-infrared images of Neptune from UT 26 July 2007 show that the cloud feature typically observed within a few degrees of Neptune’s south pole had split into a pair of bright spots. A careful determination of disk center places the cloud centers at −89.07 ± 0.06° and −87.84 ± 0.06° planetocentric latitude. If modeled as optically thick, perfectly reflecting layers, we find the pair of features to be constrained to the troposphere, at pressures greater than 0.4 bar. By UT 28 July 2007, images with comparable resolution reveal only a single feature near the south pole. The changing morphology of these circumpolar clouds suggests they may form in a region of strong convection surrounding a neptunian south polar vortex.     

The rotational periods of Uranus and Neptune

Observations of tilts of spectral lines in the spectrum of Uranus and Neptune yield the following rotational periods: “Uranus,” 24 ± 3 hr; “Neptune,” 22 ± 4 hr. Neptune is confirmed to rotate in a direct sense. The position angle of the pole of Uranus, projected onto the plane of the sky, is found to be 283 ± 4°. The value for Neptune is 32 ± 11°. These results agree with the direction of the pole of Uranus inferred from the common plane of its four brightest satellites and with the direction of the pole of Neptune as inferred from the precession of Triton's orbit. The rotational period of Uranus is found to be consistent with modern values of its optical and dynamical oblateness and the theory of solid-body rotation with hydrostatic equilibrium. This is barely the case for the period derived for Neptune and we suspect that future observations made under better seeing conditions may lead to a shorter rotation period between 15 and 18 hr. Because of a substantial difference between our results and those of earlier spectroscopic and photometric investigations we include an assessment of several previously published photometric studies and a new reduction of the original Lowell and Slipher spectroscopic plates of Uranus [Lowell Obs. Bull. 2, 17–18, 19–20 (1912)]. The early visual photometry of Campbell (Uranus) and Hall (Neptune) is found to be more satisfactorily accounted for by periods of 21.6 and 23.1 hr, respectively, than by the periods originally suggested by the observers. Our reduction of the Lowell and Slipher Uranus plates yields a period near 33 hr uncorrected for seeing. This value is consistent with the results based on the 4-m echelle date.     

The rotation period of Neptune's upper atmosphere

Significant variations in the near-infrared brightness of Neptune during July and August 1980 were observed. These observations show a well-defined, large-amplitude variation in Neptune's J-K color, with a period of 17.73 ± 0.1 hr and are interpreted as diurnal variations resulting from the 17.73-hr rotation period of the upper atmosphere of Neptune in the presence of inhomogeneous weather. These results qualitatively corroborate those of D. P. Cruikshank (1978, Astrophys. J.220, L57-L59) in an earlier study using similar techniques. In addition, variations were observed in the 5-μm spectral region which are in phase with the variations seen at shorter wavelengths. A new 5-μm measurement of Uranus is also reported.     

Morphology of the cloud tops as observed by the Venus Express Monitoring Camera

Since the discovery of ultraviolet markings on Venus, their observations have been a powerful tool to study the morphology, motions and dynamical state at the cloud top level. Here we present the results of investigation of the cloud top morphology performed by the Venus Monitoring Camera (VMC) during more than 3 years of the Venus Express mission. The camera acquires images in four narrow-band filters centered at 365, 513, 965 and 1010 nm with spatial resolution from 50 km at apocentre to a few hundred of meters at pericentre. The VMC experiment provides a significant improvement in the Venus imaging as compared to the capabilities of the earlier missions. The camera discovered new cloud features like bright “lace clouds” and cloud columns at the low latitudes, dark polar oval and narrow circular and spiral “grooves” in the polar regions, different types of waves at the high latitudes. The VMC observations revealed detailed structure of the sub-solar region and the afternoon convective wake, the bow-shape features and convective cells, the mid-latitude transition region and the “polar cap”. The polar orbit of the satellite enables for the first time nadir viewing of the Southern polar regions and an opportunity to zoom in on the planet. The experiment returned numerous images of the Venus limb and documented global and local brightening events. VMC provided almost continuous monitoring of the planet with high temporal resolution that allowed one to follow changes in the cloud morphology at various scales.We present the in-flight performance of the instrument and focus in particular on the data from the ultraviolet channel, centered at the characteristic wavelength of the unknown UV absorber that yields the highest contrasts on the cloud top. Low latitudes are dominated by relatively dark clouds that have mottled and fragmented appearance clearly indicating convective activity in the sub-solar region. At ～50° latitude this pattern gives way to streaky clouds suggesting that horizontal, almost laminar, flow prevails here. Poleward from about 60°S the planet is covered by almost featureless bright polar hood sometimes crossed by dark narrow (～300 km) spiral or circular structures. This global cloud pattern can change on time scales of a few days resulting in global and local “brightening events” when the bright haze can extend far into low latitudes and/or increase its brightness by 30%. Close-up snapshots reveal plenty of morphological details like convective cells, cloud streaks, cumulus-like columns, wave trains. Different kinds of small scale waves are frequently observed at the cloud top. The wave activity is mainly observed in the 65–80° latitude band and is in particular concentrated in the region of Ishtar Terra that suggests their possible orographic origin. The VMC observations have important implications for the problems of the unknown UV absorber, microphysical processes, dynamics and radiative energy balance at the cloud tops. They are only briefly discussed in the paper, but each of them will be the subject of a dedicated study.     

UBV photometry of Triton

UBV measurements of Triton relative to Neptune were obtained with an area-scanning photometer on 11 nights during the 1977 apparition. Observed orbital brightness variation shows that Triton is locked in synchronous rotation around Neptune. Its leading side, seen at greatest western elongation, is found to be 0.06 mag brighter than its trailing hemisphere.     

Mercury: Radar images of the equatorial and midlatitude zones

Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (λ12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: “A,” an 85-km-diameter crater (348° W, 34° S) whose radar ray system may be the most spectacular in the Solar System; “B,” a 95-km-diameter crater (343° W, 58° N) with a very bright halo but less distinct ray system; and “C,” an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246° W, 11° N). Due south of “C” lies a “ghost” feature (242° W, 27° S) that resembles “A” but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and “Skinakas” basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images.     

Uranus in 2003: Zonal winds, banded structure, and discrete features

Imaging of Uranus in 2003 with the Keck 10-m telescope reveals banded zonal structure and dozens of discrete cloud features at J and H bands; several features in the northern hemisphere are also detectable at K′. By tracking features over four days, we extend the zonal wind profile well into the northern hemisphere. We report the first measurements of wind velocities at latitudes −13°, +19°, and northward of +43°, the first direct wind measurements near the equator, and the highest wind velocity seen yet on Uranus (+218 m/s). At northern mid-latitudes (+20° to +40°), the winds appear to have accelerated when compared to earlier HST and Keck observations; southern wind speeds (−20° to −43°) have not changed since Voyager measurements in 1986. The equator of Uranus exhibits a subtle wave structure, indicated by diffuse patches roughly every 30° in longitude. The largest discrete cloud features on Uranus show complex structure extending over tens of degrees, reminiscent of activity seen around Neptune's Great Dark Spot during the Voyager encounter with that planet. There is no sign of a northern “polar collar” as is seen in the south, but a number of discrete features seen at the “expected” latitudes may signal the early stages of development of a northern collar.     

Io's sodium emission cloud and the voyager 1 encounter

Io's neutral sodium emission cloud was monitored during the period of Voyager 1 encounter from two independent ground-based sites. Observations from Table Mountain Observatory verified the continued existence of the “near-Io cloud” (d < 1.5 × 10⁵ km, for 4πI > 1 kR; R denotes Rayleigh) while those from Wise Observatory showed a deficiency in the weaker emission at greater distances from Io. The sodium cloud has been monitored from both observatories for several years. These and other observations demonstrate that the behavior of the cloud is complex since it undergoes a variety of changes, both systematic and secular, which can have both time and spatial dependencies. The cloud also displays some characteristics of stability. Table Mountain images and high-dispersion spectra (resolution $\text{～0.2}\text{A}\text{?}$) indicate that the basic shape and intensity of the “near cloud” have remained relatively constant at least since imaging observations began in 1976. Wise Observatory low-dispersion spectra (resolution $\text{～1}\text{A}\text{?}$) which have been obtained since 1974 demonstrate substantial variability of the size and intensity of the “far cloud” (d ? 1.5 × 10⁵ km) on a time scale of months or less. Corresponding changes in the state of the plasma associated with the Io torus are suggested, with the period of Voyager 1 encounter represented as a time of unusually high plasma temperature and/or density. Dynamic models of the sodium cloud employing Voyager 1 plasma data provide a reasonable fit to the Table Mountain encounter images. The modeling assumptions of anisotropic ejection of neutral sodium atoms from the leading, inner hemisphere of Io with a velocity distribution characteristic of sputtering adequately explain the overall intensity distribution of the “near cloud”. During the Voyager 1 encounter period there appeared a region of enhanced intensity projecting outward from Io's orbit and inclined to the orbital plane. This region is clearly distinguished from the sodium emission normally aligned with the plane of Io's orbit. The process responsible for this phenomenon is not yet understood. Similar but less pronounced features are also present in several Table Mountain images obtained over the past few years.     

A Prominent Apparition of Neptune's South Polar Feature

During the last week of June 2001, a bright apparition of Neptune's South Polar Feature (SPF) at 70°S was observed to develop and decay in less than 30 hours, displaying contrast of ∼2.5 at 619 nm. Assuming that the same SPF was observed on two consecutive rotations of Neptune, the feature moved eastward at 3.2±1.8° hr⁻¹ (130±80 m s⁻¹). The SPF made no obvious appearances during eight other Hubble Space Telescope (HST) observations of Neptune between July 2000 and June 2001, although there was a faint feature at 70°S in one image in October 2000. A prominent SPF was present in near-IR Keck Telescope images in August 2000. Bright SPFs are seen on ∼10% of the HST images of Neptune obtained since 1994, and a fainter SPF is visible on another ∼10%. An SPF bright enough to be visible at HST resolution was present around half the time during the last week of Voyager's approach to Neptune in August 1989, with one prominent brightening, suggesting that the SPF is less visible now than in 1989.     

Dispersion in Neptune’s zonal wind velocities from NIR Keck AO observations in July 2009

We report observations of Neptune made in H-(1.4–1.8 μm) and K’-(2.0–2.4 μm) bands on 14 and 16 July 2009 from the 10-m W.M. Keck II Telescope using the near-infrared camera NIRC2 coupled to the Adaptive Optics (AO) system. We track the positions of 54 bright atmospheric features over a few hours to derive their zonal and latitudinal velocities, and perform radiative transfer modeling to measure the cloud-top pressures of 50 features seen simultaneously in both bands. We observe one South Polar Feature (SPF) on 14 July and three SPFs on 16 July at ～65?°S. The SPFs observed on both nights are different features, consistent with the high variability of Neptune’s storms. There is significant dispersion in Neptune’s zonal wind velocities about the smooth Voyager wind profile fit of Sromovsky et al. (Icarus, 105:140, 1993), much greater than the upper limit we expect from vertical wind shear, with the largest dispersion seen at equatorial and southern mid-latitudes. Comparison of feature pressures vs. residuals in zonal velocity from the smooth Voyager wind profile also directly reveals the dominance of mechanisms over vertical wind shear in causing dispersion in the zonal winds. Vertical wind shear is not the primary cause of the difference in dispersion and deviation in zonal velocities between features tracked in H-band on 14 July and those tracked in K’-band on 16 July. Dispersion in the zonal velocities of features tracked over these short time periods is dominated by one or more mechanisms, other than vertical wind shear, that can cause changes in the dispersion and deviation in the zonal velocities on timescales of hours to days.     

Solar Control of Martian Polar Caps?

Regression of polar caps of Mars for the period 1905–1988 shows a highly significant anticorrelation with sunspot numbers. This behaviour is abnormal since the regression exhibited a significant correlation for the period 1862–1912. Combined with such unusual behaviour of Jupiter and Neptune, reported earlier, this may imply a common cause outside the solar system, viz., variation of the density of the local interstellar medium.