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In this talk, we present highlights from our recent analyses of mid-infrared observations of Uranus and Neptune, and we look ahead to anticipated discoveries from the James Webb Space Telescope. Drawing from a combination of archival and recent ground-based imaging and spectroscopy, we examine the spatial structure and trends of mid-infrared emission from the ice giant atmospheres. We report on surprising temporal variability in the atmosphere of Neptune (see Figure 1) with an unexpected decline in stratospheric temperatures since at least 2003. Recent VLT-VISIR imaging and spectroscopy are presented, revealing how this trend has progressed. In contrast, we show that no evidence yet exists of long-term thermal changes in Uranus’ stratosphere, but mid-IR observations of Uranus are still extremely limited. The observed spatial structure of Uranus’ mid-infrared emission is intriguing (Figure 2), as it is inconsistent with simple models of the atmospheric circulation and/or chemistry and its physical interpretation remains unclear. We share recent observations from VLT-VISIR and express the need for continued ground-based imaging for both Uranus and Neptune. Finally, we discuss how the James Webb Space Telescope MIRI observations will help greatly advance our understanding of the ice giants in the years ahead. In particular, given its supreme sensitivity compared to ground-based observations, JWST-MIRI observations will resolve the nature of Uranus stratospheric thermal/chemical structure and reveal how temperature and chemical abundances change with the seasons in the atmospheres of both ice giants. Figure 1: Observed changes in Neptune’s thermal-infrared brightness, a measure of temperature in Neptune’s atmosphere. The plot shows the relative change in the thermal-infrared brightness from Neptune’s stratosphere with time for all existing images taken by ground-based telescopes. Brighter images are interpreted as warmer. Corresponding thermal-infrared images (top) at wavelengths of ~12 µm show Neptune’s appearance in 2006, 2009, 2018 (observed by the European Southern Observatory’s Very Large Telescope VISIR instrument), and 2020 (observed by Subaru’s COMICS instrument). The south pole appears to have become dramatically warmer in just the past few years. Figure 2: Uranus' 13-micron images from VLT-VISIR in 2009 and 2018, with enhanced emission from high latitudes. From current ground-based observations alone, it is unclear whether the enhanced radiances are due to greater temperatures or enhance acetylene abundances, but JWST will solve this mystery.

We present the primary results from our recent analyses of mid-infrared observations of Neptune and Uranus from ground-based telescopes, including VLT-VISIR, Subaru-COMICS, and Gemini-TEXES. We discuss our recent discovery that Neptune’s stratospheric temperatures appear to be changing dramatically in just the past few years, following decades of cooling. In contrast, we show that no evidence yet exists of long-term thermal changes in Uranus’ stratosphere, but mid-IR observations of Uranus are still extremely limited. We share new observations from VLT-VISIR, express the need for continued ground-based imaging, and discuss how the James Webb Space Telescope MIRI observations will help greatly advance our understanding of the Ice Giants in the years ahead.

We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune’s mid-infrared (∼8–25 μm) emission over time in the years surrounding Neptune’s 2005 southern summer solstice. Images sensitive to stratospheric ethane (∼12 μm), methane (∼8 μm), and CH3D (∼9 μm) display significant subseasonal temporal variation on regional and global scales. Comparison with H2 S(1) hydrogen quadrupole (∼17.035 μm) spectra suggests that these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2009 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper-tropospheric temperatures—inferred from ∼17 to 25 μm imaging—appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce subseasonal variation on both global and regional scales in Neptune’s stratosphere.

International audience

Astrophysics facilities have been of tremendous importance for planetary science. The flagship space observatory Hubble Space Telescope has produced ground-breaking Solar System science, but when launched it did not even have the capability to track moving targets. The next astrophysics flagship mission, the James Webb Space Telescope, included Solar System scientists in its science team from the earliest days, with the result that Webb will launch with a diverse program and capabilities for Solar System exploration. The New Great Observatories, as well as future ground-based facilities, offer the opportunity for a robust suite of observations that will complement, enhance, and enable future Solar System exploration. We encourage the Planetary Science and Astrobiology Decadal Survey to overtly acknowledge the prospects for excellent Solar System science with the next generation of astrophysics facilities. We hope the Planetary Decadal will further encourage these missions to continue to formally involve Solar System scientists in the science working groups and development teams., 8 pages, 2 figures, 1 table; white paper submitted to the Planetary Science and Astrobiology Decadal Survey 2023-2032

Observations of thermal emission from Uranus and Neptune have been made over a broad wavelength range from ground-based platforms, airborne observatories, Earth-proximal spacecraft and from the Voyager-2 flybys in the 1980s. Observations since the Voyager flybys have included long-wavelength observations of disk-averaged radiances from the Infrared Space Observatory and the Herschel Space Observatory covering the far-infrared to millimeter range. We present recent airborne spectra from SOFIA covering 17-35 µm, together with Akari and Spitzer spectroscopy at wavelengths extending down to 7 µm, below which contributions from reflected sunlight and potential auroral emissions may confuse the signature of thermal emission. We also show how these disk-averaged spectra are complemented by ground-based filtered imaging and spectroscopy at 8-10m telescopes, which have enabled spatially resolved measurements, complementing those of Voyager IRIS from several decades ago. The critical insights into the structure, chemistry and dynamics of the atmospheres of these Ice Giants attest to the need for significant parts of this spectral region to be included in the instrument complement to be assigned to spacecraft sent to these planets. A vigorous program of Earth-based observations in the accessible spectral range should accompany the spacecraft capability in order to track potential seasonal and non-seasonal variability of these planets, as is evident in the atmospheres of both Jupiter and Saturn. The latter would include mid-infrared observations from the James Webb Space Telescope.

In its 16 years of scientific measurements, the Spitzer Space Telescope performed a number of groundbreaking and key infrared measurements of Solar System objects near and far. In this second of two Review Articles, we describe results from Spitzer observations of asteroids, dust rings and planets that provide new insight into the formation and evolution of our Solar System. The key Spitzer results presented here can be grouped into three broad classes: characterizing the physical properties of asteroids, notably including a large survey of near-Earth objects; detection and characterization of several dust/debris disks in the Solar System; and comprehensive characterization of ice giant (Uranus and Neptune) atmospheres. Many of these observations provide critical foundations for future infrared space-based observations. In the Spitzer Space Telescope’s 16 years of operation, it observed many Solar System objects and environments. In this second Review Article of a pair, Spitzer’s insight into asteroids, dust clouds and rings and the ice giant planets are summarized.

The field of exoplanetary science has emerged over the past two decades, rising up alongside traditional solar system planetary science. Both fields focus on understanding the processes which form and sculpt planets through time, yet there has been less scientific exchange between the two communities than is ideal. This white paper explores some of the institutional and cultural barriers which impede cross-discipline collaborations and suggests solutions that would foster greater collaboration. Some solutions require structural or policy changes within NASA itself, while others are directed towards other institutions, including academic publishers, that can also facilitate greater interdisciplinarity., 8 pages; white paper submitted to the Planetary Science and Astrobiology Decadal Survey 2023-2032

This white paper, written in support of NASA's 2023-2032 Planetary Decadal Survey, outlines 10 major questions that focus on the origin, evolution, and current processes that shape the atmospheres of Uranus and Neptune. Prioritizing these questions over the next decade will greatly improve our understanding of this unique class of planets, which have remained largely unexplored since the Voyager flybys. Studying the atmospheres of the Ice Giants will greatly inform our understanding of the origin and evolution of the solar system as a whole, in addition to the growing number of exoplanetary systems that contain Neptune-mass planets., Comment: 8 pages, 0 figures, White Paper submitted to the Astrobiology and Planetary Science Decadal Survey

We are building a next-generation laser adaptive optics system, Robo-AO-2, for the UH 2.2-m telescope that will deliver robotic, diffraction-limited observations at visible and near-infrared wavelengths in unprecedented numbers. The superior Maunakea observing site, expanded spectral range and rapid response to high-priority events represent a significant advance over the prototype. Robo-AO-2 will include a new reconfigurable natural guide star sensor for exquisite wavefront correction on bright targets and the demonstration of potentially transformative hybrid AO techniques that promise to extend the faintness limit on current and future exoplanet adaptive optics systems., Comment: 15 pages

Observations of Neptune with the Kepler Space Telescope yield a 49 day light curve with 98% coverage at a 1 minute cadence. A significant signature in the light curve comes from discrete cloud features. We compare results extracted from the light curve data with contemporaneous disk-resolved imaging of Neptune from the Keck 10-m telescope at 1.65 microns and Hubble Space Telescope visible imaging acquired nine months later. This direct comparison validates the feature latitudes assigned to the K2 light curve periods based on Neptune’s zonal wind profile, and confirms observed cloud feature variability. Although Neptune’s clouds vary in location and intensity on short and long timescales, a single large discrete storm seen in Keck imaging dominates the K2 and Hubble light curves; smaller or fainter clouds likely contribute to short-term brightness variability. The K2 Neptune light curve, in conjunction with our imaging data, provides context for the interpretation of current and future brown dwarf and extrasolar planet variability measurements. In particular we suggest that the balance between large, relatively stable, atmospheric features and smaller, more transient, clouds controls the character of substellar atmospheric variability. Atmospheres dominated by a few large spots may show inherently greater light curve stability than those which exhibit a greater number of smaller features.

We report here on our search for excess power in photometry of Neptune collected by the K2 mission that may be due to intrinsic global oscillations of the planet Neptune. To conduct this search, we developed new methods to correct for instrumental effects such as intrapixel variability and gain variations. We then extracted and analyzed the time-series photometry of Neptune from 49 days of nearly continuous broadband photometry of the planet. We find no evidence of global oscillations and place an upper limit of $\sim$5 ppm at 1000 \uhz\ for the detection of a coherent signal. With an observed cadence of 1-minute and point-to-point scatter less than 0.01\%, the photometric signal is dominated by reflected light from the Sun, which is in turn modulated by atmospheric variability of Neptune at the 2\% level. A change in flux is also observed due to the increasing distance between Neptune and the K2 spacecraft, and solar variability with convection-driven solar p modes present., Comment: 31 pages, 6 figure, accepted for publication in AJ

We report observations of Neptune made in H-(1.4-1.8 {\mu}m) and K'-(2.0-2.4 {\mu}m) 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 deg 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., Comment: 31 pages, 13 Figures, 3 Tables, Accepted for publication in Astrophysics and Space Science

The James Webb Space Telescope will enable a wealth of new scientific investigations in the near- and mid-infrared, with sensitivity and spatial/spectral resolution greatly surpassing its predecessors. In this paper, we focus upon Solar System science facilitated by JWST, discussing the most current information available concerning JWST instrument properties and observing techniques relevant to planetary science. We also present numerous example observing scenarios for a wide variety of Solar System targets to illustrate the potential of JWST science to the Solar System community. This paper updates and supersedes the Solar System white paper published by the JWST Project in 2010 (Lunine et al., 2010). It is based both on that paper and on a workshop held at the annual meeting of the Division for Planetary Sciences in Reno, NV in 2012., 52 pages (with figures), 32 figures; More information about JWST Solar System observations is available at http://www.stsci.edu/jwst/science/solar-system

Observations of Neptune with the Kepler Space Telescope yield a 49-day light curve with 98% coverage at a 1-minute cadence. A significant signature in the light curve comes from discrete cloud features. We compare results extracted from the light curve data with contemporaneous disk-resolved imaging of Neptune from the Keck 10-meter telescope at 1.65 microns and Hubble Space Telescope visible imaging acquired 9 months later. This direct comparison validates the feature latitudes assigned to the K2 light curve periods based on Neptune's zonal wind profile, and confirms observed cloud feature variability. Although Neptune's clouds vary in location and intensity on short and long time scales, a single large discrete storm seen in Keck imaging dominates the K2 and Hubble light curves; smaller or fainter clouds likely contribute to short-term brightness variability. The K2 Neptune light curve, in conjunction with our imaging data, provides context for the interpretation of current and future brown dwarf and extrasolar planet variability measurements. In particular we suggest that the balance between large, relatively stable, atmospheric features and smaller, more transient, clouds controls the character of substellar atmospheric variability. Atmospheres dominated by a few large spots may show inherently greater light curve stability than those which exhibit a greater number of smaller features., revised version, submitted to ApJ

Within several days of A. Wesley’s announcement that Jupiter was hit by an object on UT 19 July 2009, we observed the impact site with (1) the Hubble Space Telescope (HST) at UV through visible (225–924 nm) wavelengths, (2) the 10-m W.M. Keck II telescope in the near-infrared (1–5 μm), and (3) the 8-m Gemini-North telescope in the mid-infrared (7.7–18 μm). All observations reported here were obtained between 22 and 25 July 2009. Observations at visible and near-infrared wavelengths show that large (∼0.75-μm radius) dark (imaginary index of refraction m i ∼ 0.01–0.1) particulates were deposited at atmospheric pressures between 10 and 200–300 mbar; analysis of HST-UV data reveals that in addition smaller-sized (∼0.1 μm radius) material must have been deposited at the highest altitudes (∼10 mbar). Differences in morphology between the UV and visible/near-IR images suggest three-dimensional variations in particle size and density across the impact site, which probably were induced during the explosion and associated events. At mid-infrared wavelengths the brightness temperature increased due to both an enhancement in the stratospheric NH 3 gas abundance and the physical temperature of the atmosphere. This high brightness temperature coincides with the center part of the impact site as seen with HST. This observation, combined with (published) numerical simulations of the Shoemaker-Levy 9 impacts on Jupiter and the Tunguska airburst on Earth, suggests that the downward jet from the terminal explosion probably penetrated down to the ∼700-mbar level.

We report Hubble Space Telescope images of Jupiter during the aftermath of an impact by an unknown object in 2009 July. The 2009 impact-created debris field evolved more slowly than those created in 1994 by the collision of the tidally disrupted comet D/Shoemaker-Levy 9 (SL9). The slower evolution, in conjunction with the isolated nature of this single impact, permits a more detailed assessment of the altitudes and meridional motion of the debris than was possible with SL9. The color of the 2009 debris was markedly similar to that seen in 1994, thus this dark debris is likely to be Jovian material that is highly thermally processed. The 2009 impact site differed from the 1994 SL9 sites in UV morphology and contrast lifetime; both are suggestive of the impacting body being asteroidal rather than cometary. Transport of the 2009 Jovian debris as imaged by Hubble shared similarities with transport of volcanic aerosols in Earth's atmosphere after major eruptions.

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

In late 2003, the young eruptive variable star V1647 Orionis optically brightened by over 5 magnitudes, stayed bright for around 26 months, and then decline to its pre-outburst level. In August 2008 the star was reported to have unexpectedly brightened yet again and we herein present the first detailed observations of this new outburst. Photometrically, the star is now as bright as it ever was following the 2003 eruption. Spectroscopically, a pronounced P Cygni profile is again seen in Halpha with an absorption trough extending to -700 km/s. In the near-infrared, the spectrum now possesses very weak CO overtone bandhead absorption in contrast to the strong bandhead emission seen soon after the 2003 event. Water vapor absorption is also much stronger than previously seen. We discuss the current outburst below and relate it to the earlier event., 6 pages, 3 figures

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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.

The 3-13 μm spectra of Deimos and Phobos were obtained on the nights of 2003 August 19, 20, and 21 UT, within a week of opposition (August 28). Spectra in the 1.5-2.5 μm region were taken a few weeks earlier on July 26. Observations were made near greatest elongation for each satellite in order to minimize scattered light from Mars. The L, M, and narrowband N (10.2 μm) magnitudes for Deimos were 7.7 ± 0.1, 4.9 ± 0.1, and 0.79 ± 0.09, respectively. The L, M, and narrowband N (10.2 μm) magnitudes for Phobos were 6.4, 3.8, and -0.52, respectively, all ±0.07. The 5-12 μm color temperatures of Deimos and Phobos were 344 ± 5 and 357 ± 5 K, respectively, significantly higher than the blackbody thermal equilibrium temperature for either a fast rotator at Mars's heliocentric distance (237 K) or a slow rotator (282 K). The high brightness temperatures of Deimos and Phobos in the thermal infrared of 310-330 and 320-340 K, respectively, are not understood, but they are somewhat consistent with the high color temperatures. Within the errors, there was little evidence for spectral features indicative of surface composition, although there were some 1-2 σ variations that warrant further observations.

We present the first published spatially resolved images of Neptune's thermal emission. Our 7.7 and 11.7 μm images, taken on 2005 July 4 and 5 at the Gemini North telescope, show enhanced methane and ethane emission within 30° of the south pole. This bright polar region is the first direct imaging evidence for radiative forcing of Neptune's stratosphere, similar to that seen on Saturn. Enhanced emission from ethane, but not methane, also emerges from the planetary limb, suggesting differing vertical profiles. Stratospheric emissions are uncorrelated with tropospheric clouds seen in reflected sunlight in our near-simultaneous adaptive optics images at 1.6 and 2.2 μm taken with the Keck II telescope on 2005 July 5.

Long-term photometric measurements of Uranus and Neptune through 2005 show variations in brightness. For Uranus, much of the variation can be interpreted as seasonal, i.e., caused by viewing angle changes of an oblate planet. The photometry suggests that if seasonal variations on Uranus are north–south symmetric, then the northern pole should begin to brighten in 2006. However, seasonal “aspect” changes cannot explain all the variation; the Uranus observations require intrinsic atmospheric change. Furthermore, Uranus observations spanning many scale heights in the atmosphere may show similar change. For Neptune, variations in sub-solar latitude may explain the general shape of the long-term light curve, but significant deviations occur that have no explanation at present. Observations are needed over a longer temporal baseline than currently exists to fully characterize both atmospheres.

As the 7 December 2007 equinox of Uranus approached, ring and atmosphere observers produced a substantial collection of observations using the 10-m Keck telescope and the Hubble Space Telescope. Those spanning the period from 7 June 2007 through 9 September 2007 we used to identify and track cloud features, determine atmospheric motions, characterize cloud morphology and dynamics, and define changes in atmospheric band structure. We confirmed the existence of the suspected northern hemisphere prograde jet, locating its peak near 58 N, and extended wind speed measurements to 73 N. For 28 cloud features we obtained extremely high wind-speed accuracy through extended tracking times. 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., 22 pages, 20 figures, 6 tables, on-line data available

We report 8- to 13-micron spectral observations of Neptune and Uranus from the NASA Infrared Telescope Facility spanning more than a decade. The spectroscopic data indicate a steady increase in Neptune's mean atmospheric 12-micron ethane emission from 1985 to 2003, followed by a slight decrease in 2004. The simplest explanation for the intensity variation is an increase in stratospheric effective temperature from 155 +/- 3 K in 1985 to 176 +/- 3 K in 2003 (an average rate of 1.2 K/year), and subsequent decrease to 165 +/- 3 K in 2004. We also detected variation of the overall spectral structure of the ethane band, specifically an apparent absorption structure in the central portion of the band; this structure arises from coarse spectral sampling coupled with a non-uniform response function within the detector elements. We also report a probable direct detection of ethane emission on Uranus. The deduced peak mole fraction is approximately an order of magnitude higher than previous upper limits for Uranus. The model fit suggests an effective temperature of 114 +/- 3 K for the globally-averaged stratosphere of Uranus, which is consistent with recent measurements indicative of seasonal variation., Accepted for publication in ApJ. 16 pages, 10 figures, 2 tables

We present high quality images of the uranian ring system, obtained in August 2002, October 2003, and July 2004 at 2.2 μm with the adaptive optics camera NIRC2 on the Keck II telescope. Using these data, we report the first detection in backscattered light of a ring (which we refer to as the ζ ring) interior to Uranus' known rings. This ring consists of a generally uniform sheet of dust between 37,850 and 41,350 km with an equivalent width EW = 250 ± 20 m (in 2004; or μ EW ≈ 50 m ), and extends inward to 32,600 km at a gradually decreasing brightness. This ring might be related to the Voyager ring R/1986 U 2, although both its location and extent differ. This could be attributed to a difference in observing wavelength and/or solar phase angle, or perhaps to temporal variations in the ring. Through careful modeling of the I / F of the individual rings at each ansa, we reveal the presence of narrow (few 100 km wide) sheets of dust between the δ and e rings, and between rings 4 and α . We derived a typical anisotropy factor g ≈ 0.7 in the scattering behavior of these particles. The spatial distribution and relative intensity of these dust sheets is different than that seen in Voyager images taken in forward scattered light, due either to a difference in observing wavelength, and/or solar phase angle or to changes over time. We may have detected the λ ring in one scan at μ EW ≈ 15 m , but other scans provided upper limits below this value. A single detection, however, would be consistent with azimuthal asymmetries known to exist in this ring. We further demonstrate the presence of azimuthal asymmetries in all rings. We confirm the eccentricity of ∼0.001 in rings 4, 5, 6, which in 2004 are ∼70 km closer to Uranus in the north (near periapse; lower I / F ) than in the south. We find a global optical depth of τ ∼ 0.3 in the main rings, and of τ = 0.25 ± 0.05 in the e ring.

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.

Analysis of Hubble Space Telescope images of Uranus taken between 1994 and 2002 shows evidence for temporal changes in zonal brightness patterns in the south polar region. Between 1994 and 2002, a relatively bright ring developed near 70° S. The pole itself, which was the brightest area of the southern hemisphere in 1994, has become relatively dark. The polar collar at 45° S has also become brighter relative to the rest of the southern polar region. Comparison of images through different filters suggests that the change is occurring at pressures of 2–4 bars in the atmosphere. A change at this depth is consistent with radio measurements which indicate seasonal variability in Uranus' deep atmosphere. Disk-integrated photometry at visible wavelengths also exhibits variability on seasonal (∼ decades) timescales. The observed changes are not predicted by existing dynamical models of Uranus' atmosphere.

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.

On 1995 August 21, Saturn's moon Titan occulted the 13th magnitude star GSC5254-00997. The predawn event was observed at 2.3 μm with the NASA 3.0-m Infrared Telescope Facility. We measured the average atmospheric scale height between latitudes 50°S and 67°S by fitting isothermal models to the immersion portion of the occultation lightcurve. Within the altitude range of 300 to 500 km, we find an isothermal scale height of Titan's sunset atmosphere of 55±9 km. Our derived temperature of 180±30 K is consistent with modeled temperature profiles derived from Voyager 1 IRIS observations (A. Coustenis and B. Bezard 1995, Icarus 115 , 126–140) and the 28 Sagittarii occultation event (W. B. Hubbard et al. 1993, Astron. Astrophys. 269 , 541–563). The atmospheric albedo asymmetry due to low level haze (Caldwell et al. 1992, Icarus 96 , 1–9) is inferred from the difference in Titan's center-of-light (determined from precise astrometric measurements of images of Titan and the star before and after the occultation) and the actual center of Titan (determined from the occultation analysis). We find the southern hemisphere to be brighter than the northern hemisphere, a result consistent with Combes et al. (1997, Icarus 129 , 482–497), Gibbard et al. (1999, Icarus 139 , 189–201), and Smith et al. (1996 Icarus , 119 , 336–349).

Hubble Space Telescope imaging of Uranus in 1994, 1997, 1998, and 2000 revealed 13 cloud features, allowing the first measurements of wind velocities at northern latitudes not accessible to the Voyager cameras and new measurements of southern-latitude wind velocities determined during the 1986 Voyager encounter. Images acquired with the Keck 10-meter telescope adaptive optics system in June 2000 also showed some of the same features. Wind speeds inferred from feature motions—along with additional measurements by Karkoschka (1998, Science 280 ) and Voyager measurements (Smith et al . 1986, Science 246 )—indicate a zonal wind profile that is asymmetric with respect to the equator. Small but consistent differences are seen between the newer data and a profile determined from Voyager data: nearly all the newer measurements have slightly slower velocities than expected. We cannot yet determine whether the source of these differences is a slowly changing Uranian wind profile or subtle latitudinal structure in a temporally constant profile. If Uranus' winds vary with time, this may indicate unusual atmospheric dynamics created by the 98° tilt of the planet's rotation axis.

We present a detailed analysis of Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC 2) images of the first 25 min of the impacts of Comet Shoemaker–Levy 9 fragments A and G with Jupiter and the post impact ejecta patterns observed 1–2 h after each impact event, with a brief analysis of the available observations of the first 25 min of the E and W events. Via a ballistic model we (i) constrain the initial time of impact and the initial velocity and elevation angle of the apex of each observed impactor; (ii) reproduce the shape and the motion of the sunlit A and G ejecta clouds utilizing the initial velocities constrained for the apex of each ejecta cloud and initial elevation angles ranging from 80° to 60°±5°; and (iii) compare the ejecta pattern generated by particle trajectories consistent with the observed ejecta clouds to the observed postimpact ejecta pattern. We conclude that (i) the crescent patterns can be reconstructed from ballistic ejecta traveling on trajectories of elevation angles ranging from 70° to 45° and initial velocity equivalent to that constrained for the apex of each cloud; (ii) the streaks can be reconstructed from ballistic particles propelled up from the channel at velocities less than 8.5±1 km s −1 ; and (iii) the altitude of the self-luminous radiation detected at the onset of the A, G, and W observations is inconsistent with the ballistic trajectories fit to the apex of the sunlit ejecta clouds. We believe that the discrepancy between the trajectories of the initial HST/WFPC 2 detections and the later images is an indication of distinctly different phenomena associated with the impacts of the comet fragments. The timing of these events indicates that the first event recorded in association with the G impact is consistent with the entry of the bolide and that the self-luminous radiation recorded ∼120 s after the ballistically derived impact time for the A and G impacts may be consistent with the excitation of the jovian atmosphere and/or cometary debris in the path of shock fronts accelerating away from the respective impact fireballs. Given the uncertainty in the ballistically derived impact time, the initial detection of the W event may have corresponded to either the incoming bolide or atmospheric and/or cometary debris excitation initiated by the passing of a shock front associated with the impact fireball.

Hubble Space Telescope imaging of Neptune in 1994, 1995, and 1996 reveals changes in atmospheric features. Dark spots at 467 nm near latitude +31° were present in both 1994 and 1995, but absent from images in March 1996. However, a larger data set in August 1996 (Sromovskyet al., Bull. Am. Astron. Soc.28,1077, 1996) revealed a dark spot. The features resemble the Great Dark Spot present during the Voyager encounter in 1989 in size, shape, and contrast. Dark spots occur near the single brightest 619- and 889-nm features visible on the planet at that time. Methane-band clouds in the northern hemisphere were a factor of 6 brighter than the surrounding atmosphere in 1994, but only a factor of 2 brighter in 1995 and 1996. The pattern of zonal bands on Neptune was stable from 1994 to 1996, but differed from that seen by Voyager in 1989 when a prominent dark band from planetographic latitudes −40° to −68° dominated at blue wavelengths. In 1994–1996, this band was weaker and extended only from −55° to −70°, perhaps explaining the 0.06-mag brightening of Neptune between 1989 and 1994. Bright methane clouds generally appear at specific latitudes, but the active latitudes change from −25° in 1989 to −30° and −46° in 1994. The region near +30° was active in both 1989 and 1994–1996. We confirm the general shape and stability of the zonal wind profile measured by Voyager, revising previous measurements with more accurate values and filling in latitudes +38°, −30°, and −31° where no clouds had previously been seen.

This paper presents new results about Triton's atmospheric structure from the analysis of all ground-based stellar occultation data recorded to date, including one single-chord occultation recorded on 1993 July 10 and nine occultation lightcurves from the double-star event on 1995 August 14. These stellar occultation observations made both in the visible and in the infrared have good spatial coverage of Triton, including the first Triton central-flash observations, and are the first data to probe the altitude level 20–100 km on Triton. The small-planet lightcurve model of J. L. Elliot and L. A. Young (1992, Astron. J. 103, 991–1015) was generalized to include stellar flux refracted by the far limb, and then fitted to the data. Values of the pressure, derived from separate immersion and emersion chords, show no significant trends with latitude, indicating that Triton's atmosphere is spherically symmetric at ∼50-km altitude to within the error of the measurements; however, asymmetry observed in the central flash indicates the atmosphere is not homogeneous at the lowest levels probed (∼20-km altitude). From the average of the 1995 occultation data, the equivalent-isothermal temperature of the atmosphere is 47 ± 1 K and the atmospheric pressure at 1400-km radius (∼50-km altitude) is 1.4 ± 0.1 μbar. Both of these are not consistent with a model based on Voyager UVS and RSS observations in 1989 (D. F. Strobel, X. Zhu, M. E. Summers, and M. H. Stevens, 1996, Icarus 120, 266–289). The atmospheric temperature from the occultation is 5 K colder than that predicted by the model and the observed pressure is a factor of 1.8 greater than the model. In our opinion, the disagreement in temperature and pressure is probably due to modeling problems at the microbar level, since measurements at this level have not previously been made. Alternatively, the difference could be due to seasonal change in Triton's atmospheric structure.

During the first few hours after each impact, numerous phenomena were observed with telescopes on Earth, in orbit, and in space. The primary events in that time were: impacts themselves, rise and fall of large plumes of ejected material, and atmospheric waves; also of interest were the characteristic morphologies of fresh sites. Based on timing from Galileo instruments and ground-based observations, the Hubble Space Telescope (HST) recorded actual impact phenomena for fragments G and W, with the A and E impacts occurring just prior to the HST observation window. For these four events, plumes were directly imaged; plume development and collapse correlated with strong infrared emission near the jovian limb, supporting the interpretation that the IR brightness was created by the fall-back of plume material from high altitude (see chapter by Nicholson). For medium-to-large fresh impact sites imaged by HST within a few hours of impact, expanding rings were detected, caused by horizontal propagation of atmospheric waves (see chapters by Ingersoll and Zahnle). Initial site morphology at visible wavelengths was similar for all medium-to-large impacts: a dark streak surrounded by dark material, dominated by a large crescent-shaped ejecta to the southeast. Smaller impact sites typically only showed a dark patch (no ejecta) which dissipated quickly. This chapter summarizes the most recent measurements and interpretations of plumes and fresh impact sites as observed by HST.

Hubble Space Telescope far-ultraviolet images of Jupiter during the Shoemaker-Levy 9 impacts show the impact regions darkening over the 2 to 3 hours after the impact, becoming darker and more extended than at longer wavelengths, which indicates that ultraviolet-absorbing gases or aerosols are more extended, more absorbing, and at higher altitudes than the absorbers of visible light. Transient auroral emissions were observed near the magnetic conjugate point of the K impact site just after that impact. The global auroral activity was fainter than average during the impacts, and a variable auroral emission feature was observed inside the southern auroral oval preceding the impacts of fragments Q1 and Q2.

Images of Neptune obtained in 2006 at ESOs Very Large Telescope (Orton et al.; 2007, Astronomy and Astrophysics 473, L5) revealed a near-polar hot spot near 70°S latitude that was detectable in filters sampling both stratospheric methane (7 μm) and ethane (∼12 μm) emission. Such a feature was not present in 2003 Keck and 2005 Gemini North observations, which showed only a general warming trend toward Neptunes pole that was longitudinally homogeneous. Because of the paucity of longitudinal sampling in the 2003, 2005 and 2006 images, it was not clear whether the failure to see this phenomenon in 2003 and 2005 was simply the result of insufficient longitudinal sampling or whether the phenomenon was truly variable in time. To unravel these two possibilities, we made follow-up observations on large telescopes that were capable of resolving Neptune at thermal-infrared wavelengths: Gemini South in 2007 and 2010 using the T-ReCS instrument, Subaru in 2008 using the COMICS instrument and VLT in 2008 and 2009 using the VISIR instrument. Two serendipitous T-ReCS images of Neptune were also obtained in 2007 using a broad N-band (814 μm) filter, whose radiance is dominated by stratospheric emission from both methane and ethane. The feature was recovered (i) in 2007 with T-ReCS in the broad N-band image and (ii) in 2008 with COMICS in a 12.5-μm image. However, T-ReCS observations in 2010 that covered up to 250° of longitude did not show evidence of an off-polar hot spot. Although we have not definitively ruled out the possibility that various observers have simply missed a semi-permanent feature, it seems statistically very unlikely to be the case. With only 3 sightings in 13 independent observing epochs, it is likely that the phenomenon is ephemeral in time. A possible origin for the phenomenon is a large planetary wave that is dynamically confined to the high-latitude regions characterized by prograde zonal winds. It may be episodically excited by dynamical activity deeper in the atmosphere. This must be coupled with mixing near the poles that destroys or at least substantially attenuates the hot spot over the south pole that leads to an appearance of the typical polar stratospheric hot spot being offset in latitude. © 2011 Elsevier Ltd. All rights reserved.

Cosmic collisions on planets cause detectable optical flashes that range from terrestrial shooting stars to bright fireballs. On 2010 June 3 a bolide in Jupiter's atmosphere was simultaneously observed from the Earth by two amateur astronomers observing Jupiter in red and blue wavelengths. The bolide appeared as a flash of 2 s duration in video recording data of the planet. The analysis of the light curve of the observations results in an estimated energy of the impact of (0.9-4.0) × 1015 J which corresponds to a colliding body of 8-13 m diameter assuming a mean density of 2 g cm-3. Images acquired a few days later by the Hubble Space Telescope and other large groundbased facilities did not show any signature of aerosol debris, temperature, or chemical composition anomaly, confirming that the body was small and destroyed in Jupiter's upper atmosphere. Several collisions of this size may happen on Jupiter on a yearly basis. A systematic study of the impact rate and size of these bolides can enable an empirical determination of the flux of meteoroids in Jupiter with implications for the populations of small bodies in the outer solar system and may allow a better quantification of the threat of impacting bodies to Earth. The serendipitous recording of this optical flash opens a new window in the observation of Jupiter with small telescopes. © 2010 The American Astronomical Society. All rights reserved.

We present the first dynamical solution of the triple asteroid system (45) Eugenia and its two moons Petit-Prince (Diameter~7 km) and S/2004 (45) 1 (Diameter~5 km). The two moons orbit at 1165 and 610 km from the primary, describing an almost-circular orbit (e~6x10-3 and e~7x10-2 respectively). The system is quite different from the other known triple systems in the main belt since the inclinations of the moon orbits are sizeable (9 deg and 18 deg with respect to the equator of the primary respectively). No resonances, neither secular nor due to Lidov-Kozai mechanism, were detected in our dynamical solution, suggesting that these inclinations are not due to excitation modes between the primary and the moons. A 10-year evolution study shows that the orbits are slightly affected by perturbations from the Sun, and to a lesser extent by mutual interactions between the moons. The estimated J2 of the primary is three times lower than the theoretical one, calculated assuming the shape of the primary and an homogeneous interior, possibly suggesting the importance of other gravitational harmonics., 38 pages, 5 Tables, 5 Figures, revised for Icarus Journal