| 1. |
- Marzok, A., et al.
(författare)
-
Mapping the Brightness of Ganymede's Ultraviolet Aurora Using Hubble Space Telescope Observations
- 2022
-
Ingår i: Journal of Geophysical Research - Planets. - : American Geophysical Union (AGU). - 2169-9097 .- 2169-9100. ; 127:6
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze Hubble Space Telescope observations of Ganymede made with the Space Telescope Imaging Spectrograph between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1,356 angstrom. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with longitude. To quantify this variation around Ganymede, we investigate the brightness averaged over 36 degrees-wide longitude corridors centered around the sub-Jovian (0 degrees W), leading (90 degrees W), anti-Jovian (180 degrees W), and trailing (270 degrees W) central longitudes. In the northern hemisphere, the brightness of the auroral oval is 3.7 +/- 0.4 times lower in the sub-Jovian and anti-Jovian corridors compared to the trailing and leading corridors. The southern oval is overall brighter than the northern oval, and only 2.5 +/- 0.2 times fainter on the sub- and anti-Jovian corridors compared to the trailing and leading corridors. This demonstrates that Ganymede's auroral ovals are strongly structured in auroral crescents on the leading side (plasma downstream side) and on the trailing side (plasma upstream side). We also find that the brightness is not symmetric with respect to the 270 degrees meridian, but shifted by similar to 20 degrees towards the Jovian-facing hemisphere. Our map will be useful for subsequent studies to understand the processes that generate the aurora in Ganymede's non-rotationally driven, sub-Alfvenic magnetosphere.
|
|
| 2. |
- Roth, Lorenz, et al.
(författare)
-
Constraints on an exosphere at Ceres from Hubble Space Telescope observations
- 2016
-
Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:6, s. 2465-2472
-
Tidskriftsartikel (refereegranskat)abstract
- We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived brightnesses at the oxygen lines at 1304 Å and 1356 Å are consistent with zero signals within the 1σ propagated statistical uncertainties. The OI 1304 Å brightness of 0.12 ± 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 ± 13.4) × 1010 cm-2. Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 × 1026 molecules s-1 was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of ∼1% in the 1300 Å to 1400 Å range.
|
|
| 3. |
- Roth, Lorenz, et al.
(författare)
-
Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations
- 2016
-
Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 121:3, s. 2143-2170
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze a large set of far ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawnside with only few exceptions, which cannot be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near-surface OI] 1356 Å to OI 1304 Å emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O2 prevails at least up to altitudes of ∼900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the near-surface O2 atmosphere of ∼5% (upstream) and ≲1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.
|
|
| 4. |
- Saur, J., et al.
(författare)
-
The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals
- 2015
-
Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 120:3, s. 1715-1737
-
Tidskriftsartikel (refereegranskat)abstract
- 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 Jupiter's 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 Ganymede's 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. Key Points New technique to search for a subsurface ocean in Ganymede with a telescope Ocean affects auroral oscillation caused by time-varying external magnetic field HST observations reveal weak auroral oscillation and imply existence of ocean
|
|
| 5. |
- Fletcher, Leigh N., et al.
(författare)
-
Jupiter Science Enabled by ESA's Jupiter Icy Moons Explorer
- 2023
-
Ingår i: Space Science Reviews. - : Springer Nature. - 0038-6308 .- 1572-9672. ; 219:7
-
Forskningsöversikt (refereegranskat)abstract
- ESA's Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cassini, and Juno missions, alongside ground- and space-based observatories. We focus on remote sensing of the climate, meteorology, and chemistry of the atmosphere and auroras from the cloud-forming weather layer, through the upper troposphere, into the stratosphere and ionosphere. The Jupiter orbital tour provides a wealth of opportunities for atmospheric and auroral science: global perspectives with its near-equatorial and inclined phases, sampling all phase angles from dayside to nightside, and investigating phenomena evolving on timescales from minutes to months. The remote sensing payload spans far-UV spectroscopy (50-210 nm), visible imaging (340-1080 nm), visible/near-infrared spectroscopy (0.49-5.56 & mu;m), and sub-millimetre sounding (near 530-625 GHz and 1067-1275 GHz). This is coupled to radio, stellar, and solar occultation opportunities to explore the atmosphere at high vertical resolution; and radio and plasma wave measurements of electric discharges in the Jovian atmosphere and auroras. Cross-disciplinary scientific investigations enable JUICE to explore coupling processes in giant planet atmospheres, to show how the atmosphere is connected to (i) the deep circulation and composition of the hydrogen-dominated interior; and (ii) to the currents and charged particle environments of the external magnetosphere. JUICE will provide a comprehensive characterisation of the atmosphere and auroras of this archetypal giant planet.
|
|
| 6. |
- Plainaki, C., et al.
(författare)
-
Towards a Global Unified Model of Europa’s Tenuous Atmosphere
- 2018
-
Ingår i: Space Science Reviews. - : Springer Netherlands. - 0038-6308 .- 1572-9672. ; 214:1
-
Forskningsöversikt (refereegranskat)abstract
- Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.
|
|
| 7. |
- Retherford, K. D., et al.
(författare)
-
Io's Atmosphere Silhouetted by Jupiter Ly alpha
- 2019
-
Ingår i: Astronomical Journal. - : IOP PUBLISHING LTD. - 0004-6256 .- 1538-3881. ; 158:4
-
Tidskriftsartikel (refereegranskat)abstract
- We report results from a new technique for mapping Io's SO2 vapor distribution. The Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope observed Io during four Jupiter transit events to obtain medium resolution far-UV spectral images near the Ly alpha wavelength of 121.6 nm. Jupiter's bright Ly alpha dayglow provides a bright, mostly uniform background light source for opacity measurements, much like during a stellar occultation or transiting exoplanet event. Peaks in the photoabsorption cross-sections for sulfur dioxide occur near 122 nm, with resulting absorptions raising the altitude where a tangential line-of-sight opacity of similar to 1 occurs. This method of measuring column densities along lines of sight above the limb uses detailed image simulations and complements Ly alpha reflectance imaging and other methods for measuring Io's SO2 gas on the disk. Our reported near-terminator limb observations with STIS confirm the findings from previous Ly alpha disk reflectance imaging that Io's polar SO2 density is an order of magnitude lower than found at the equator. We provide constraints for additional attenuation by atmospheric hydrogen atoms produced by charge exchange reactions between magnetospheric protons and Io's atmosphere. Searches for plume-related features provided no definitive enhancements within the signal quality, ruling out unusually high levels of activity for Pele and Tvashtar.
|
|
| 8. |
- Roth, Lorenz, et al.
(författare)
-
A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations
- 2021
-
Ingår i: Nature Astronomy. - : Springer Nature. - 2397-3366. ; 5:10, s. 1043-1051
-
Tidskriftsartikel (refereegranskat)abstract
- Ganymede’s atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the O i 1,356 Å and O i 1,304 Å oxygen emissions were used to infer sputtered molecular oxygen (O2) as an atmospheric constituent, but an expected sublimated water (H2O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H2O in Ganymede’s atmosphere. The relative intensity of the oxygen emissions requires contributions from the dissociative excitation of water vapour, indicating that H2O is more abundant than O2 around the subsolar point. Away from the subsolar region, the emissions are consistent with a pure O2 atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H2O/O2 ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~1015 molecules of H2O per cm2 are consistent with sublimation of the icy surface as source.
|
|
| 9. |
- Becker, Tracy M., et al.
(författare)
-
Mid-ultraviolet Hubble Observations of Europa and the Global Surface Distribution of SO2
- 2022
-
Ingår i: The Planetary Science Journal. - : IOP Publishing Ltd. - 2632-3338. ; 3:6
-
Tidskriftsartikel (refereegranskat)abstract
- We present spatially resolved reflectance spectra of Europa's surface in the wavelength range of 210-315 nm obtained by the Hubble Space Telescope Imaging Spectrograph in 2018 and 2019. These data provide the first high-quality, near-global spectral observations of Europa from 210 to 240 nm. They show that the reflectance of Europa's leading, trailing, anti-Jovian, and sub-Jovian hemispheres is similar to 5% near 210 nm, with varying spectral slopes across the mid-UV. This low albedo, even on the more "pristine" leading hemisphere, indicates a lack of the signature far-UV spectral edge characteristic of water ice. We detected and mapped a strong absorption feature at 280 nm that is consistent with an S-O bond that has previously been attributed to SO2 on the surface, hypothesized to be formed through radiolytic processing of Iogenic sulfur ions that have been preferentially emplaced on Europa's trailing hemisphere by Jupiter's magnetic field. Our models show that small inclusions of SO2 (0.1%) within the water ice are sufficient to produce the 280 nm feature without producing a feature at 4.07 mu m, which has not been observed in ground-based spectral observations of Europa. This data set is the first to produce a spatially resolved, near-global map of the assumed SO2 feature, which is primarily concentrated near the apex of the trailing hemisphere and correlated with large-scale darker regions in both the visible and the ultraviolet. This distribution is consistent with "cold" exogenic sulfur ion bombardment on Europa.
|
|
| 10. |
- Becker, T. M., et al.
(författare)
-
The Far-UV Albedo of Europa From HST Observations
- 2018
-
Ingår i: Journal of Geophysical Research - Planets. - : AMER GEOPHYSICAL UNION. - 2169-9097 .- 2169-9100. ; 123:5, s. 1327-1342
-
Tidskriftsartikel (refereegranskat)abstract
- We present an analysis of Europa's far-UV spectral albedo using observations during the 1999-2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk-integrated observations show that the far-UV spectrum in the similar to 130 to 170-nm range is relatively flat or slightly blue (increasing albedo with decreasing wavelength) for the studied hemispheres: the leading, trailing, and anti-Jovian hemispheres. At Lyman- (121.6nm), the albedo of the trailing hemisphere continues the blue trend, but it reddens for the leading hemisphere. Also at this wavelength, the albedo of the leading hemisphere, which is higher than the trailing hemisphere at near-UV and visible wavelengths, is lower than the trailing hemisphere, exhibiting spectral inversion. We find no evidence of a sharp water-ice absorption edge at 165nm on any hemisphere of Europa, which is intriguing since such an absorption feature has been observed on the icy Saturnian satellites. Plain Language Summary We used observations spanning from 1999 to 2015 obtained by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to study the surface reflectance of Europa at far-ultraviolet (UV) wavelengths. We find that Europa has a low reflectance in the UV and that there is little variation in the surface brightness at most of the UV wavelengths. When observed at visible wavelengths, one of Europa's hemispheres is brighter than the other, but at the UV wavelength of 121.6nm, the hemisphere brightness is reversed. We also find that Europa looks different from the icy moons of Saturn at far-UV wavelengths.
|
|
