Kurt D. Retherford

Response of Jupiter's and Saturn's auroral activity to the solar wind

[1] While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth’s magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter and Saturn are driven primarily by internal processes, with the main energy source being the planets’ rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter’s aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn.

Transient Water Vapor at Europa’s South Pole

Europas Plumes Jupiters moon Europa has a subsurface ocean and a relatively young icy surface. Roth et al. (p. 171, published online 12 December 2013; see the Perspective by Spencer) analyzed spectral images taken by the Hubble Space Telescope that show ultraviolet emissions from the moons atmosphere, and report a statistically significant emission signal extending above the satellites southern hemisphere. This emission is consistent with two 200-km-high plumes of water vapor. Tidal stresses likely play a role in opening and closing fractures at the surface. Hubble Space Telescope images of Jupiter’s moon Europa reveal emission consistent with transient water vapor plumes. [Also see Perspective by Spencer] In November and December 2012, the Hubble Space Telescope (HST) imaged Europa’s ultraviolet emissions in the search for vapor plume activity. We report statistically significant coincident surpluses of hydrogen Lyman-α and oxygen OI 130.4-nanometer emissions above the southern hemisphere in December 2012. These emissions were persistently found in the same area over the 7 hours of the observation, suggesting atmospheric inhomogeneity; they are consistent with two 200-km-high plumes of water vapor with line-of-sight column densities of about 1020 per square meter. Nondetection in November 2012 and in previous HST images from 1999 suggests varying plume activity that might depend on changing surface stresses based on Europa’s orbital phases. The plume was present when Europa was near apocenter and was not detected close to its pericenter, in agreement with tidal modeling predictions.

The atmosphere of Pluto as observed by New Horizons

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto’s atmosphere is cold, rarefied, and made mostly of nitrogen and methane, with layers of haze. INTRODUCTION For several decades, telescopic observations have shown that Pluto has a complex and intriguing atmosphere. But too little has been known to allow a complete understanding of its global structure and evolution. Major goals of the New Horizons mission included the characterization of the structure and composition of Pluto’s atmosphere, as well as its escape rate, and to determine whether Charon has a measurable atmosphere. RATIONALE The New Horizons spacecraft included several instruments that observed Pluto’s atmosphere, primarily (i) the Radio Experiment (REX) instrument, which produced near-surface pressure and temperature profiles; (ii) the Alice ultraviolet spectrograph, which gave information on atmospheric composition; and (iii) the Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC), which provided images of Pluto’s hazes. Together, these instruments have provided data that allow an understanding of the current state of Pluto’s atmosphere and its evolution. RESULTS The REX radio occultation determined Pluto’s surface pressure and found a strong temperature inversion, both of which are generally consistent with atmospheric profiles retrieved from Earth-based stellar occultation measurements. The REX data showed near-symmetry between the structure at ingress and egress, as expected from sublimation driven dynamics, so horizontal winds are expected to be weak. The shallow near-surface boundary layer observed at ingress may arise directly from sublimation. The Alice solar occultation showed absorption by methane and nitrogen and revealed the presence of the photochemical products acetylene and ethylene. The observed nitrogen opacity at high altitudes was lower than expected, which is consistent with a cold upper atmosphere. Such low temperatures imply an additional, but as yet unidentified, cooling agent. A globally extensive haze extending to high altitudes, and with numerous embedded thin layers, is seen in the New Horizons images. The haze has a bluish color, suggesting a composition of very small particles. The observed scattering properties of the haze are consistent with a tholin-like composition. Buoyancy waves generated by winds flowing over orography can produce vertically propagating compression and rarefaction waves that may be related to the narrow haze layers. Pluto’s cold upper atmosphere means atmospheric escape must occur via slow thermal Jeans’ escape. The inferred escape rate of nitrogen is ~10,000 times slower than predicted, whereas that of methane is about the same as predicted. The low nitrogen loss rate is consistent with an undetected Charon atmosphere but possibly inconsistent with sublimation/erosional features seen on Pluto’s surface, so that past escape rates may have been much larger at times. Capture of escaping methane and photochemical products by Charon, and subsequent surface chemical reactions, may contribute to the reddish color of its north pole. CONCLUSION New Horizons observations have revolutionized our understanding of Pluto’s atmosphere. The observations revealed major surprises, such as the unexpectedly cold upper atmosphere and the globally extensive haze layers. The cold upper atmosphere implies much lower escape rates of volatiles from Pluto than predicted and so has important implications for the volatile recycling and the long-term evolution of Pluto’s atmosphere. MVIC image of haze layers above Pluto’s limb. About 20 haze layers are seen from a phase angle of 147°. The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. For example, white arrows on the left indicate a layer ~5 km above the surface, which has descended to the surface at the right. Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto’s atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto’s atmosphere to space. It is unclear whether the current state of Pluto’s atmosphere is representative of its average state—over seasonal or geologic time scales.

LRO-LAMP observations of the LCROSS impact plume.

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) sent a kinetic impactor to strike Cabeus crater, on a mission to search for water ice and other volatiles expected to be trapped in lunar polar soils. The Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO) observed the plume generated by the LCROSS impact as far-ultraviolet emissions from the fluorescence of sunlight by molecular hydrogen and carbon monoxide, plus resonantly scattered sunlight from atomic mercury, with contributions from calcium and magnesium. The observed light curve is well simulated by the expansion of a vapor cloud at a temperature of ~1000 kelvin, containing ~570 kilograms (kg) of carbon monoxide, ~140 kg of molecular hydrogen, ~160 kg of calcium, ~120 kg of mercury, and ~40 kg of magnesium.

The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals

We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiters time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0 ◦ ± 1.3 ◦ . Our model calculations predict a significantly stronger oscillation by 5.8 ◦ ± 1.3 ◦ without ocean compared to 2.2 ◦ ± 1.3 ◦ if an ocean is present. Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymedes dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.

Sunlit Io atmospheric [O I] 6300 Å emission and the plasma torus

A large database of sunlit Io [O I] 6300 A emission, acquired over the period 1990–1999, with extensive coverage of Io orbital phase angle ϕ and System III longitude λIII, exhibits significant long-term and short-term variations in [O I] 6300 A emission intensities. The long-term average intensity shows a clear dependence on λIII, which establishes conclusively that the emission is produced by the interaction between Ios atmosphere and the plasma torus. Two prominent average intensity maxima, 70° to 90° wide, are centered at λIII ≈ 130° and λIII ≈ 295°. A comparison of data from October 1998 with a three-dimensional plasma torus model, based upon electron impact excitation of atomic oxygen, suggests a basis for study of the torus interaction with Ios atmosphere. The observed short-term, erratic [O I] 6300 A intensity variations fluctuate ∼20–50% on a timescale of tens of minutes with less frequent fluctuations of a factor of ∼2. The most likely candidate to produce these fluctuations is a time-variable energy flux of field-aligned nonthermal electrons identified recently in Galileo plasma science data. If true, the short-term [O I] intensity fluctuations may be related to variable field-aligned currents driven by inward and outward torus plasma transport and/or transient high-latitude, field-aligned potential drops. A correlation between the intensity and emission line width indicates molecular dissociation may contribute significantly to the [O I] 6300 A emission. The nonthermal electron energy flux may produce O(1D) by electron impact dissociation of SO2 and SO, with the excess energy going into excitation of O and its kinetic energy. The [O I] 6300 A emission database establishes Io as a valuable probe of the torus, responding to local conditions at Ios position.

Lyman‐α imaging of the SO2 distribution on Io

Imaging spectroscopy of Io in the ultraviolet (1160–1720 A) was carried out with the Space Telescope Imaging Spectrograph on HST on three dates in October 1997 and August 1998. Among the initial results was the observation of concentrated regions of Hi Lyman-α flux near the poles of Io that exhibited a morphology and temporal variability different from those of the atomic oxygen and sulfur emission regions seen near the equatorial limbs. We examine the suggestion that the primary source of Lyman-α emission is surface reflected solar radiation that penetrates the thin polar atmosphere, but is strongly absorbed by the thicker SO2 atmosphere near Ios equator. Spectral and spatial analyses lead to derived SO2 column densities that are in good agreement with those derived from earlier HST observations of Ios albedo in the 2000–2300 A wavelength range. The Lyman-α images clearly illustrate features of Ios atmosphere that have been deduced from previous observations and theoretical modeling: a non-uniformity with respect to the sub-solar point dominated by a freezing out of the SO2 near the poles and variation with both longitude and time due to the variability of the sources of the atmospheric gas. Lyman-α imaging is demonstrated to be an extremely powerful and direct way to globally map the dynamic atmosphere of Io.

Io's equatorial spots: Morphology of neutral UV emissions

The first observations of Io with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) showed that the brightest ultraviolet emissions come from localized regions near Ios equator, designated “equatorial spots.” This paper presents a detailed study of the location, shape, and brightness of the equatorial spots in near-monochromatic images obtained using STIS in the first-order long-slit spectroscopy mode. This study provides evidence that the equatorial emissions are linked to the interaction between the Jovian magnetosphere and Ios atmosphere. The morphology of the equatorial spots reported here provides additional information on the nature of this complex electrodynamic interaction. We find the following principal results: the locations of the equatorial spots are correlated with the Jovian magnetic field orientation at Io, but with a relation that is not 1:1; the equatorial spots are centered 10°–30° longitude downstream from Ios sub-Jovian longitude; the brightness of the emissions in this data set is correlated with Ios distance from the plasma torus centrifugal equator; and the anti-Jovian equatorial spots are ∼20% brighter than the sub-Jovian equatorial spots.

Emission profiles of neutral oxygen and sulfur in Io's exospheric corona

Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) observations of Io acquired in 1997 [Roesler et al., 1999] provided the first simultaneous spatially resolved measurements of emission from neutral sulfur and oxygen, the dominant atomic species in Ios exospheric corona. Previous measurements of Ios corona relied primarily on sunlight resonantly scattered from sodium, a trace element in Ios atmosphere, and required measurement during mutual satellite eclipses to obtain the necessary spatial resolution. We present here spatial profiles of Ios extended emissions derived from observations spanning the time period from October 1997 to February 2000. The STIS Far Ultraviolet Multi-Anode Microchannel Array (FUV-MAMA) detector permits measurement of the emissions with a spatial resolution of ∼0.05 Io radii out to distances of ∼20 Io radii. Useful measurements are limited to ∼10 Io radii owing to the low signal-to-noise ratio of the extended emission features. The coronal emission profiles vary considerably in slope and intensity and are generally brighter for Io west (duskside) of Jupiter. Emission profiles obtained near western elongation are relatively symmetric about Io; profiles obtained in other orbital positions display varying degrees of asymmetry, with enhanced emissions and generally steeper slopes in the downstream direction relative to the plasma flow. The downstream-upstream profile asymmetry is thought to be caused by higher electron densities in Ios plasma wake. While the coverage of the data is limited in both Jovian System III coordinates and geocentric phase, the intensities of emission from regions both near Io and in the extended corona vary with System III longitude in a near-simultaneous fashion, suggesting local torus electron density as the probable source of this modulation. The observed ratio of oxygen to sulfur emission is relatively constant in time, perhaps reflecting the stoichiometric ratio of the SO 2 source molecules. Eclipse and posteclipse observations on February 25, 2000, show a dramatic increase in profile emission brightness and slope, suggesting a dynamic response by a sublimation-supported component of Ios SO 2 atmosphere and associated atomic species.

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Significance Images of Europa’s UV aurora taken by the Hubble Space Telescope in December 2012 have revealed local hydrogen and oxygen emissions in intensity ratios that identify the source as electron impact excitation of water molecules. The existence of water vapor plumes as a source for the detected localized water vapor and the possible accessibility of subsurface liquid water reservoirs at these locations have important implications for the exploration of Europa’s potentially habitable environments. The observations reported here tested whether orbital position near the apocenter is an essential requirement for plume activity. Only an upper limit on the amount of water vapor was obtained. Orbital position is therefore not a sufficient condition for detecting plumes and they may be episodic events. We report far-ultraviolet observations of Jupiter’s moon Europa taken by Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) in January and February 2014 to test the hypothesis that the discovery of a water vapor aurora in December 2012 by local hydrogen (H) and oxygen (O) emissions with the STIS originated from plume activity possibly correlated with Europa’s distance from Jupiter through tidal stress variations. The 2014 observations were scheduled with Europa near the apocenter similar to the orbital position of its previous detection. Tensile stresses on south polar fractures are expected to be highest in this orbital phase, potentially maximizing the probability for plume activity. No local H and O emissions were detected in the new STIS images. In the south polar region where the emission surpluses were observed in 2012, the brightnesses are sufficiently low in the 2014 images to be consistent with any H2O abundance from (0–5)×1015 cm−2. Large high-latitude plumes should have been detectable by the STIS, independent of the observing conditions and geometry. Because electron excitation of water vapor remains the only viable explanation for the 2012 detection, the new observations indicate that although the same orbital position of Europa for plume activity may be a necessary condition, it is not a sufficient condition. However, the December 2012 detection of coincident HI Lyman-α and OI 1304-Å emission surpluses in an ∼200-km high region well separated above Europa’s limb is a firm result and not invalidated by our 2014 STIS observations.