arXiv subjects

I. de Pater

Publications and source records attributed to I. de Pater.

At least 19 recordsLinked to original sources

arXiv2026

Detection of stratospheric HCN and tropospheric CO in Uranus and the implication for their sources

Uranus belongs to the category of ice giants that are common in our Galaxy. However, Uranus is one of the least explored and understood planets in our Solar System. This work aims to constrain the deep oxygen abundance of Uranus to better understand its formation. Another goal concerns the origin of exogeneous species, such as CO and HCN, found in the upper stratospheres of giant planets. We used spectral mapping observations of the CO (J=3-2) and HCN (J=4-3) rotational submillimeter lines obtained with ALMA in 2022 and 2024. We combined them with radiative transfer and thermochemical modeling to determine the tropospheric abundance of CO and the deep O/H ratio of Uranus. We used radiative transfer simulations with physical models of various sources of external CO and HCN to constrain the vertical and meridional distributions of these species and narrow down the nature of their external sources. We also applied a wind retrieval algorithm to search for zonal winds in the stratosphere of Uranus at the levels probed by the CO and HCN lines. We unambiguously detect tropospheric CO for the first time with a mole fraction of 5.8$\pm$0.3 ppb and stratospheric HCN with a mole faction of (1.8$\pm$0.2)$\times10^{-11}$ restricted to pressures lower than 0.2 mbar. Thermochemical calculations suggest that the deep interior of Uranus is enriched in oxygen with respect to the protoplanetary nebula by at least a factor of 52$^{+30}_{-20}$. We also find that the stratospheric CO is rather uniform over the observed latitudes and that the CO lines are best fit by an old comet impact model, in which a large comet hit the planet several centuries ago. CO therefore has a dual origin in Uranus. Finally, we do not detect stratospheric winds from these data, but the CO data indicate that zonal winds in the 10$^\circ$S-10$^\circ$N latitudinal range are likely retrograde at submillibar pressures.

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arXiv2025

Spectral trends across the rings and inner moons of Uranus and Neptune from JWST NIRCam images

JWST NIRCam images provide low-resolution spectra of the rings and inner moons orbiting Uranus and Neptune. These data reveal systematic variations in spectral parameters like the strength of the strong OH absorption band around 3 microns and the spectral slopes at continuum wavelengths. Neptune's rings show an extremely weak 3-micron band, which is likely due to the small particle sizes in these dusty rings. Neptune's small inner moons also have weaker 3-micron bands and redder continua than Uranus' small inner moons, indicating that Neptune's moons have a lower water-ice fraction. There are also clear spectral trends across the inner Uranian system. The strength of the 3-micron band clearly increases with distance from Uranus, with the rings having a noticeably weaker 3-micron band than most of the small inner moons, which have a weaker 3-micron band than the larger moons like Miranda. While the rings and most of the small moons have neutral spectra between 1.4 microns and 2.1 microns, the outermost small moon Mab exhibits a blue spectral slope comparable to Miranda, indicating that Mab's surface may also be relatively water-ice rich. The next moon interior to Mab, Puck, exhibits a stronger 3-micron band and bluer continuum slope than any of the moons orbiting interior to it, perhaps indicating that it is being covered by water-ice-rich material derived from Mab via the mu ring. Finally, the small moon Rosalind has a redder spectral slope than its neighbors, possibly due to being coated with material from the dusty nu ring.

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arXiv2025

Observed Latitudinal, Longitudinal and Temporal Variability of Io's Atmosphere Simulated by a Purely Sublimation Driven Atmosphere

How much of Io's SO$_2$ atmosphere is driven by volcanic outgasing or sublimation of SO$_2$ surface frost is a question with a considerable history. We develop a time dependent surface temperature model including thermal inertia and the exact celestial geometry to model the radiation driven global structure and temporal evolution of Io's atmosphere. We show that many observations can be explained by assuming a purely sublimation driven atmosphere. We find that a thermal diffusivity $α=2.41\times10^{-7}$ m$^2$s$^{-1}$ yields an averaged atmospheric SO$_2$ column density decreasing by more than one order of magnitude from the equator to the poles in accordance with the observed spatial variations of Io's column densities. Our model produces a strong day-night-asymmetry with modeled column density variations of almost two orders of magnitude at the equator as well as a sub-anti-Jovian hemisphere asymmetry, with maximum dayside column densities of $3.7\times10^{16}$ cm$^{-2}$ for the sub-Jovian and $8.5\times10^{16}$ cm$^{-2}$ for the anti-Jovian hemisphere. Both are consistent with the observed temporal and large-scale longitudinal variation of Io's atmosphere. We find that the diurnal variations of the surface temperature affect the subsurface structure up to a depth of 0.6m. Furthermore, we quantify seasonal effects with Io having a northern summer close to perihelion and a northern winter close to aphelion. Finally, we found that at Io's anomalous warm polar regions a conductive heat flux of at least 1.2 Wm$^{-2}$ is necessary to reach surface temperatures consistent with observations.

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arXiv2025

Mass supply from Io to Jupiter's magnetosphere

Since the Voyager mission flybys in 1979, we have known the moon Io to be both volcanically active and the main source of plasma in the vast magnetosphere of Jupiter. Material lost from Io forms neutral clouds, the Io plasma torus and ultimately the extended plasma sheet. This material is supplied from Io's upper atmosphere and atmospheric loss is likely driven by plasma-interaction effects with possible contributions from thermal escape and photochemistry-driven escape. Direct volcanic escape is negligible. The supply of material to maintain the plasma torus has been estimated from various methods at roughly one ton per second. Most of the time the magnetospheric plasma environment of Io is stable on timescales from days to months. Similarly, Io's atmosphere was found to have a stable average density on the dayside, although it exhibits lateral and temporal variations. There is potential positive feedback in the Io torus supply: collisions of torus plasma with atmospheric neutrals are probably a significant loss process, which increases with torus density. The stability of the torus environment may be maintained by limiting mechanisms of either torus supply from Io or the loss from the torus by centrifugal interchange in the middle magnetosphere. Various observations suggest that occasionally the plasma torus undergoes major transient changes over a period of several weeks, apparently overcoming possible stabilizing mechanisms. Such events are commonly explained by some kind of change in volcanic activity that triggers a chain of reactions which modify the plasma torus state via a net change in supply of new mass. However, it remains unknown what kind of volcanic event (if any) can trigger events in torus and magnetosphere, whether Io's atmosphere undergoes a general change before or during such events, and what processes could enable such a change in the otherwise stable torus.

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arXiv2024

Water-Ice Dominated Spectra of Saturn's Rings and Small Moons from JWST

JWST measured the infrared spectra of Saturn's rings and several of its small moons (Epimetheus, Pandora, Telesto and Pallene) as part of Guaranteed Time Observation program 1247. The NIRSpec instrument obtained near-infrared spectra of the small moons between 0.6 and 5.3 microns, which are all dominated by water-ice absorption bands. The shapes of the water-ice bands for these moons suggests that their surfaces contain variable mixes of crystalline and amorphous ice or variable amounts of contaminants and/or sub-micron ice grains. The near-infrared spectrum of Saturn's A ring has exceptionally high signal-to-noise between 2.7 and 5 microns and is dominated by features due to highly crystalline water ice. The ring spectrum also confirms that the rings possess a 2-3% deep absorption at 4.13 microns due to deuterated water-ice previously seen by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. This spectrum also constrains the fundamental absorption bands of carbon dioxide and carbon monoxide and may contain evidence for a weak aliphatic hydrocarbon band. Meanwhile, the MIRI instrument obtained mid-infrared spectra of the rings between 4.9 and 27.9 microns, where the observed signal is a combination of reflected sunlight and thermal emission. This region shows a strong reflectance peak centered around 9.3 microns that can be attributed to crystalline water ice. Since both the near and mid-infrared spectra are dominated by highly crystalline water ice, they should provide a useful baseline for interpreting the spectra of other objects in the outer solar system with more complex compositions.

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arXiv2024

Subsurface Thermophysical Properties of Europa's Leading and Trailing Hemispheres as Revealed by ALM

We present best-fit values of porosity -- and the corresponding effective thermal inertiae -- determined from three different depths in Europa's near-subsurface (~1-20 cm). The porosity of the upper ~20 cm of Europa's subsurface varies between 75-50% ($Γ_{eff}\approx50-140$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) on the leading hemisphere and 50-40% ($Γ_{eff}\approx140-180$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) on the trailing hemisphere. Residual maps produced by comparison with these models reveal thermally anomalous features that cannot be reproduced by globally homogeneous porosity models. These regions are compared to Europa's surface terrain and known compositional variations. We find that some instances of warm thermal anomalies are co-located with known geographical or compositional features on both the leading and trailing hemisphere; cool temperature anomalies are well correlated with surfaces previously observed to contain pure, crystalline water ice and the expansive rays of Pwyll crater. Anomalous regions correspond to locations with subsurface properties different from those of our best-fit models, such as potentially elevated thermal inertia, decreased emissivity, or more porous regolith. We also find that ALMA observations at ~3 mm sound below the thermal skin depth of Europa (~10-15 cm) for a range of porosity values, and thus do not exhibit features indicative of diurnal variability or residuals similar to other frequency bands. Future observations of Europa at higher angular resolution may reveal additional locations of variable subsurface thermophysical properties, while those at other wavelengths will inform our understanding of the regolith compaction length and the effects of external processes on the shallow subsurface.

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arXiv2024

An Enduring Rapidly Moving Storm as a Guide to Saturn's Equatorial Jet's Complex Structure

Saturn has an intense and broad eastward equatorial jet with a complex three-dimensional structure mixed with time variability. The equatorial region experiences strong seasonal insolation variations enhanced by ring shadowing and three of the six known giant planetary-scale storms have developed in it. These factors make Saturn's equator a natural laboratory to test models of jets in giant planets. Here we report on a bright equatorial atmospheric feature imaged in 2015 that moved steadily at a high speed of 450 ms-1 not measured since 1980-81 with other equatorial clouds moving within an ample range of velocities. Radiative transfer models show that these motions occur at three altitude levels within the upper haze and clouds. We find that the peak of the jet (latitudes 10\degree N to 10\degree S) suffers intense vertical shears reaching +2.5 ms-1 km-1, two orders of magnitude higher than meridional shears, and temporal variability above 1 bar altitude level.

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arXiv2023

Composition and thermal properties of Ganymede's surface from JWST/NIRSpec and MIRI observations

JWST NIRSpec IFU (2.9-5.3 mu) and MIRI MRS (4.9-28.5 mu) observations were performed on both the leading and trailing hemispheres of Ganymede with a spectral resolution of ~2700. Reflectance spectra show signatures of water ice, CO2 and H2O2. An absorption feature at 5.9 mu is revealed and is tentatively assigned to sulfuric acid hydrates. The CO2 4.26-mu band shows latitudinal and longitudinal variations in depth, shape and position over the two hemispheres, unveiling different CO2 physical states. In the ice-rich polar regions, which are the most exposed to Jupiter's plasma irradiation, the CO2 band is redshifted with respect to other terrains. In the leading northern polar cap, the CO2 band is dominated by a high wavelength component at ~4.27 mu, consistent with CO2 trapped in amorphous water ice. At equatorial latitudes (and especially on dark terrains) the observed band is broader and shifted towards the blue, suggesting CO2 adsorbed on non-icy materials. Amorphous ice is detected in the ice-rich polar regions, and is especially abundant on the leading northern polar cap. In both hemispheres the north polar cap ice appears to be more processed than the south polar cap. A longitudinal modification of the H2O ice molecular structure and/or nano/micrometre-scale texture, of diurnal or geographic origin, is observed in both hemispheres. Ice frost is observed on the morning limb of the trailing hemisphere, possibly formed during the night from the recondensation of water subliming from the warmer subsurface. Reflectance spectra of the dark terrains are compatible with the presence of Na-/Mg-sulfate salts, sulfuric acid hydrates, and possibly phyllosilicates mixed with fine-grained opaque minerals, having an highly porous texture. Mid-IR brightness temperatures indicate a rough surface and a very low thermal inertia of 20-40 J m-2 s-0.5 K-1, consistent with a porous surface.

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arXiv2023

Examining Uranus' zeta ring in Voyager 2 Wide-Angle-Camera Observations: Quantifying the Ring's Structure in 1986 and its Modifications prior to the Year 2007

The zeta ring is the innermost component of the Uranian ring system. It is of scientific interest because its morphology changed significantly between the Voyager 2 encounter in 1986 and subsequent Earth-based observations around 2007. It is also of practical interest because some Uranus mission concepts have the spacecraft pass through the inner flank of this ring. Recent re-examinations of the Voyager 2 images have revealed additional information about this ring that provide a more complete picture of the ring's radial brightness profile and phase function. These data reveal that this ring's brightness varies with phase angle in a manner similar to other tenuous rings, consistent with it being composed primarily of sub-millimeter-sized particles. The total cross section of particles within this ring can also be estimated from these data, but translating that number into the actual risk to a spacecraft flying through this region depends on a number of model-dependent parameters. Fortunately, comparisons with Saturn's G and D rings allows the zeta-ring's particle number density to be compared with regions previously encountered by the Voyager and Cassini spacecraft. Finally, these data indicate that the observed changes in the zeta-ring's structure between 1986 and 2007 are primarily due to a substantial increase in the amount of dust at distances between 38,000 km and 40,000 km from Uranus' center.

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arXiv2022

Phosphine in the Venusian Atmosphere: A Strict Upper Limit from SOFIA GREAT Observations

The presence of phosphine (PH$_3$) in the atmosphere of Venus was reported by Greaves et al. (2021a), based on observations of the J=1-0 transition at 267 GHz using ground-based, millimeter-wave spectroscopy. This unexpected discovery presents a challenge for our understanding of Venus's atmosphere, and has led to a reappraisal of the possible sources and sinks of atmospheric phosphorous-bearing gases. Here we present results from a search for PH$_3$ on Venus using the GREAT instrument aboard the SOFIA aircraft, over three flights conducted in November 2021. Multiple PH$_3$ transitions were targeted at frequencies centered on 533 GHz and 1067 GHz, but no evidence for atmospheric PH$_3$ was detected. Through radiative transfer modeling, we derived a disk-averaged upper limit on the PH$_3$ abundance of 0.8 ppb in the altitude range 75-110 km, which is more stringent than previous ground-based studies.

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arXiv2020

Frontiers in Planetary Rings Science

We now know that the outer solar system is host to at least six diverse planetary ring systems, each of which is a scientifically compelling target with the potential to inform us about the evolution, history and even the internal structure of the body it adorns. These diverse ring systems represent a set of distinct local laboratories for understanding the physics and dynamics of planetary disks, with applications reaching beyond our Solar System. We highlight the current status of planetary rings science and the open questions before the community to promote continued Earth-based and spacecraft-based investigations into planetary rings. As future spacecraft missions are launched and more powerful telescopes come online in the decades to come, we urge NASA for continued support of investigations that advance our understanding of planetary rings, through research and analysis of data from existing facilities, more laboratory work and specific attention to strong rings science goals during future mission selections.

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arXiv2019

Jupiter's Ammonia Distribution Derived from VLA Maps at 3--37 GHz

We observed Jupiter four times over a full rotation (10 hrs) with the upgraded Karl G. Jansky Very Large Array (VLA) between December 2013 and December 2014. Preliminary results at 4-17 GHz were presented in de Pater et al. (2016); in the present paper we present the full data set at frequencies between 3 and 37 GHz. Major findings are: (i) the radio-hot belt at 8.5--11$^\circ$N latitude, near the interface between the North Equatorial Belt (NEB) and the Equatorial Zone (EZ) is prominent at all frequencies (3--37 GHz). Its location coincides with the southern latitudes of the NEB (7--17$^{\circ}$ N). (ii) Longitude-smeared maps reveal belts and zones at all frequencies at latitudes $\lesssim |20^\circ|$. The lowest brightness temperature is in the EZ near a latitude of 4$^\circ$N, and the NEB has the highest brightness temperature near 11$^\circ$N. The bright part of the NEB increases in latitudinal extent (spreads towards the north) with deceasing frequency, i.e., with depth into the atmosphere. In longitude-resolved maps, several belts, in particular in the southern hemisphere, are not continuous along the latitude line, but broken into small segments as if caused by an underlying wave. (iii) Model fits to longitude-smeared spectra are obtained at each latitude. These show a high NH$_3$ abundance (volume mixing ratio $\sim 4 \times 10^{-4}$) in the deep ($P>8-10$ bar) atmosphere, decreasing at higher altitudes due to cloud formation (e.g., in zones), or dynamics in combination with cloud condensation (belts). In the NEB ammonia gas is depleted down to at least the 20 bar level with an abundance of $1.75 \times 10^{-4}$. (iv) Using the entire VLA dataset, we confirm that the planet is extremely dynamic in the upper layers of the atmosphere, at $P<$2--3 bar, i.e., at the altitudes where clouds form. [Abridged]

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arXiv2019

A Fast Radio Burst with frequency-dependent polarization detected during Breakthrough Listen observations

Here, we report on the detection and verification of Fast Radio Burst FRB 180301, which occurred on UTC 2018 March 1 during the Breakthrough Listen observations with the Parkes telescope. Full-polarization voltage data of the detection were captured--a first for non-repeating FRBs--allowing for coherent de-dispersion and additional verification tests. The coherently de-dispersed dynamic spectrum of FRB 180301 shows complex, polarized frequency structure over a small fractional bandwidth. As FRB 180301 was detected close to the geosynchronous satellite band during a time of known 1-2 GHz satellite transmissions, we consider whether the burst was due to radio interference emitted or reflected from an orbiting object. Based on the preponderance of our verification tests, we find that FRB 180301 is likely of astrophysical origin, but caution that anthropogenic sources cannot conclusively be ruled out.

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arXiv2017

Neptune long-lived atmospheric features in 2013-2015 from small (28-cm) to large (10-m) telescopes

Since 2013, observations of Neptune with small telescopes have resulted in several detections of long-lived bright atmospheric features that have also been observed by large telescopes such as Keck II or Hubble. The combination of both types of images allows the study of the long term evolution of major cloud systems in the planet. In 2013 and 2014 two bright features were present on the planet at southern mid latitudes. These may have merged in late 2014, possibly leading to the formation of a single bright feature observed during 2015 at the same latitude. This cloud system was first observed in January 2015 and nearly continuously from July to December 2015 in observations with telescopes in the 2 to 10 meter class and in images from amateur astronomers. These images show the bright spot as a compact feature at 40.1 deg South planetographic latitude well resolved from a nearby bright zonal band that extended from 42 deg South to 20 deg South. Tracking its motion from July to November 2015 suggests a longitudinal oscillation of 16 deg in amplitude with a 90 day period, typical of dark spots on Neptune and similar to the Great Red Spot oscillation in Jupiter. The limited time covered by high-resolution observations only covers one full oscillation and other interpretations of the changing motions could be possible. HST images in September 2015 show the presence of a dark spot at short wavelengths in the southern flank of the bright cloud observed throughout 2015.

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arXiv2017

Retrieving Neptune's aerosol properties from Keck OSIRIS observations. I. Dark regions

We present and analyze three-dimensional data cubes of Neptune from the OSIRIS integral-field spectrograph on the 10-m Keck telescope, from July 2009. These data have a spatial resolution of 0.035"/pixel and spectral resolution of R~3800 in the H and K broad bands. We focus our analysis on regions of Neptune's atmosphere that are near-infrared dark- that is, free of discrete bright cloud features. We use a forward model coupled to a Markov chain Monte Carlo algorithm to retrieve properties of Neptune's aerosol structure and methane profile above ~4 bar in these near-infrared dark regions. Using a set of high signal-to-noise spectra in a cloud-free band from 2-12N, we find that Neptune's cloud opacity is dominated by a compact, optically thick cloud layer with a base near 3 bar and composed of low albedo, forward scattering particles, with an assumed characteristic size of ~1$μ$m. Above this cloud, we require a vertically extended haze of smaller (~0.1 $μ$m) particles, which reaches from the upper troposphere (~0.6 bar) into the stratosphere. The particles in this haze are brighter and more isotropically scattering than those in the deep cloud. When we extend our analysis to 18 cloud-free locations from 20N to 87S, we observe that the optical depth in aerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- and high-southern latitudes relative to low latitudes. We also consider Neptune's methane (CH$_4$) profile, and find that our retrievals indicate a strong preference for a low methane relative humidity at pressures where methane is expected to condense. Our preferred solution at most locations is for a methane relative humidity below 10% near the tropopause in addition to methane depletion down to 2.0-2.5 bar. We tentatively identify a trend of lower CH$_4$ columns above 2.5 bar at mid- and high-southern latitudes over low latitudes.

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arXiv2017

Jovian Temperature and Cloud Variability during the 2009-2010 Fade of the South Equatorial Belt

Mid-infrared 7-20 $μ$m imaging of Jupiter demonstrates that the increased albedo of Jupiter's South Equatorial Belt (SEB) during the `fade' (whitening) event of 2009-2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB ($7-17^\circ$ S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool quiescent band of high aerosol opacity (the SEB zone at $11-15^\circ$ S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of $1.0\pm0.5$ K), as well as the increased aerosol opacity at 4.8 and 8.6 $μ$m, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free `brown barges' (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles from deeper levels (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air. [Abridged]

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arXiv2016

Jupiter's Para-H$_2$ Distribution from SOFIA/FORCAST and Voyager/IRIS 17-37 $μ$m Spectroscopy

Spatially resolved maps of Jupiter's far-infrared 17-37 $μ$m hydrogen-helium collision-induced spectrum were acquired by the FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) in May 2014. Spectral scans in two grisms covered the broad S(0) and S(1) absorption lines, in addition to contextual imaging in eight broad-band filters (5-37 $μ$m) with spatial resolutions of 2-4". The spectra were inverted to map the zonal-mean temperature and para-H$_2$ distribution ($f_p$, the fraction of the para spin isomer with respect to the ortho spin isomer) in Jupiter's upper troposphere (the 100-700 mbar range). We compared these to a reanalysis of Voyager-1 and -2 IRIS spectra covering the same spectral range. Para-H$_2$ increases from equator to pole, with low-$f_p$ air at the equator representing sub-equilibrium conditions (i.e., less para-H$_2$ than expected from thermal equilibration), and high-$f_p$ air and possible super-equilibrium at higher latitudes. In particular, we confirm the continued presence of a region of high-$f_p$ air at high northern latitudes discovered by Voyager/IRIS, and an asymmetry with generally higher $f_p$ in the north than in the south. We note that existing collision-induced absorption databases lack opacity from (H$_2$)$_2$ dimers, leading to under-prediction of the absorption near the S(0) and S(1) peaks. There appears to be no spatial correlation between para-H$_2$ and tropospheric ammonia, phosphine and cloud opacity derived from Voyager/IRIS at mid-infrared wavelengths (7-15 $μ$m). We note, however, that para-H$_2$ tracks the similar latitudinal distribution of aerosols within Jupiter's upper tropospheric and stratospheric hazes observed in reflected sunlight, suggesting that catalysis of hydrogen equilibration within the hazes (and not the main clouds) may govern the equator-to-pole gradient. [Abridged]

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arXiv2015

High S/N Keck and Gemini AO imaging of Uranus during 2012-2014: New cloud patterns, increasing activity, and improved wind measurements

We imaged Uranus in the near infrared from 2012 into 2014, using the Keck/NIRC2 camera and Gemini/NIRI camera, both with adaptive optics. We obtained exceptional signal to noise ratios by averaging 8-16 individual exposures in a planet-fixed coordinate system. noise-reduced images revealed many low-contrast discrete features and large scale cloud patterns not seen before, including scalloped waveforms just south of the equator. In all three years numerous small (600-700 km wide) and mainly bright discrete features were seen within the north polar region (north of about 55°N). Over 850 wind measurements were made, the vast majority of which were in the northern hemisphere. These revealed an extended region of solid body rotation between 62°N and at least 83°N, at a rate of 4.08$\pm0.015$°/h westward relative to the planet's interior (radio) rotation of 20.88°/h westward. Near-equatorial speeds measured with high accuracy give different results for waves and small discrete features, with eastward drift rates of 0.4°/h and 0.1°/h respectively. The region of polar solid body rotation is a close match to the region of small-scale polar cloud features, suggesting a dynamical relationship. While winds at high southern latitudes (50°S - 90°S) are unconstrained by groundbased observations, a recent reanalysis of 1986 Voyager 2 observations by Karkoschka (2015, Icarus 250, 294-307) has revealed an extremely large north-south asymmetry in this region, which might be seasonal. Greatly increased activity was seen in 2014, including the brightest ever feature seen in K' images (de Pater et al. 2015, Icarus 252, 121-128). Over the 2012-2014 period we identified six persistent discrete features. Three were tracked for more than two years, two more for more than one year, and one for at least 5 months and continuing.

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