Melissa A. McGrath

The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede

Far-UV spectra of Europa and Ganymede, acquired by the Hubble Space Telescope Goddard High Resolution Spectrograph, indicate that, in addition to faintly reflected sunlight, both satellites emit O I 1304 A and O I 1356 A airglow radiation. The observed brightnesses of the reflected solar C II 1335 A feature indicate that the disk-averaged albedos of Europa and Ganymede are about 1.5% and 2.6%, respectively. Airglow emissions from both satellites are characterized by the flux ratio F(1356 A)/F(1304 A) of roughly 1-2, diagnostic of dissociative electron impact excitation of O2. Inferred O2 vertical column densities are in the range (2.4-14) × 1014 cm-2 for Europa and (1-10) × 1014 cm-2 for Ganymede. The observed double-peaked profile of Ganymedes O I 1356 A feature indicates a nonuniform spatial emission distribution that suggests two distinct and spatially-confined emission regions, consistent with the satellites north and south poles.

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 On-Orbit Performance of the Space Telescope Imaging Spectrograph

The Space Telescope Imaging Spectrograph (STIS) was successfully installed into the Hubble Space Telescope (HST) in 1997 February, during the second HST servicing mission, STS-82. STIS is a versatile spectrograph, covering the 115-1000 nm wavelength range in a variety of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. In the months since launch, a number of performance tests and calibrations have been carried out and are continuing. These tests demonstrate that the instrument is performing very well. We present here a synopsis of the results to date.

Saturn: Search for a missing water source

[1] The origin of the large hydroxyl radical (OH) cloud near the inner moons of Saturn, indicative of a surprisingly large water-vapor source, has represented a puzzle since its discovery in 1992. A new set of Hubble Space Telescope measurements is used to constrain the OH spatial densities and to pinpoint the source region. Our model indicates that the vast majority of the water vapor (>80%) originates from Enceladus’s orbital distance. This may indicate the presence of a dense population of small, as of yet unseen, bodies concentrated near Enceladus; collisions between these fragments are the suggested mechanism for producing the necessary amounts of water vapor. We show that collisions between plasma ions and neutral molecules substantially inflate the OH cloud, and increase the OH loss rate, requiring a water source three times larger than previous estimates. INDEX TERMS: 6275 Planetology: Solar System Objects: Saturn; 2756 Magnetospheric Physics: Planetary magnetospheres (5443, 5737, 6030); 6280 Planetology: Solar System Objects: Saturnian satellites; 6213 Planetology: Solar System Objects: Dust. Citation: Jurac, S., M. A. McGrath, R. E. Johnson, J. D. Richardson, V. M. Vasyliunas, and A. Eviatar, Saturn: Search for a missing water source, Geophys. Res. Lett. , 29(24), 2172, doi:10.1029/2002G L015855, 2002.

HST spectroscopic observations of Jupiter after the collision of comet Shoemaker-Levy 9

Ultraviolet spectra obtained with the Hubble Space Telescope identified at least 10 molecules and atoms in the perturbed stratosphere near the G impact site, most never before observed in Jupiter. The large mass of sulfur-containing material, more than 10(14) grams in S2 alone, indicates that many of the sulfur-containing molecules S2, CS2, CS, H2S, and S+ may be derived from a sulfur-bearing parent molecule native to Jupiter. If so, the fragment must have penetrated at least as deep as the predicted NH4SH cloud at a pressure of approximately 1 to 2 bars. Stratospheric NH3 was also observed, which is consistent with fragment penetration below the cloud tops. Approximately 10(7) grams of neutral and ionized metals were observed in emission, including Mg II, Mg I, Si I, Fe I, and Fe II. Oxygen-containing molecules were conspicuous by their absence; upper limits for SO2, SO, CO, SiO, and H2O are derived.

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.

The Pele Plume (Io): Observations with the Hubble Space Telescope

In July 1996, with the Hubble Space Telescope (HST), we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27µm, the first definitive observation of an Io plume from Earth. The height, 420 ± 40 km, was greater than any plume observed by Voyager. The plume had significantly smaller optical depth at 0.34 and 0.41µm, where it was not detected. The wavelength dependence of the optical depth can be matched by a plume either of fine dust, with minimum mass of 1.2 × 109 g and maximum particle size of 0.08µm, or of SO2 gas with a column density of 3.7 × 1017 cm−2 and total mass of 1.1 × 1011 g. Our models suggest that early Voyager imaging estimates of the minimum mass of the Loki plume [Collins, 1981] may have been too large by a factor of ∼ 100. We may have detected the Pele plume in reflected sunlight, at 0.27µm, in July 1995, but did not see it 21 hours earlier, so the plume may be capable of rapid changes.

Detection of SO2 on Callisto with the Hubble Space Telescope

We have detected SO2 in ultraviolet spectra of Callisto obtained with the Hubble Space Telescopes Faint Object Spectrograph. An absorption band centered at 280 nm appears in the spectrum of Callistos leading hemisphere, but is not apparent in the spectrum of the trailing hemisphere. The band is similar to the SO2 band on Europas trailing hemisphere. Callistos leading hemisphere spectrum can be well fit with models that include SO2 ice absorption with N(SO2) ≥ 6 × 1016 cm−2. Callistos leading hemisphere is modified by impacts with micrometeorites; this may directly or indirectly be a source of sulfur dioxide.

Fluorescent hydroxyl emissions from Saturn's ring atmosphere.

Just before Earth passed through Saturns ring plane on 10 August 1995, the Hubble Space Telescope Faint Object Spectrograph detected ultraviolet fluorescent emissions from a tenuous atmosphere of OH molecules enveloping the rings. Brightnesses decrease with increasing distance above the rings, implying a scale height of about 0.45 Saturn radii (RS). A spatial scan 0.28RS above the A and B rings indicates OH column densities of about 1013 cm−2 and number densities of up to 700 cm−3. Saturns rings must produce roughly 1025 to 1029 OH molecules per second to maintain the observed OH distribution.

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.