| 1. |
- Becker, Tracy M., et al.
(författare)
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HST UV Observations of Asteroid (16) Psyche
- 2020
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Ingår i: The Planetary Science Journal. - : American Astronomical Society. - 2632-3338. ; 1:3
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Tidskriftsartikel (refereegranskat)abstract
- The Main Belt Asteroid (16) Psyche is the target object of the NASA Discovery Mission Psyche. We observed the asteroid at ultraviolet (UV) wavelengths (170–310 nm) using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope during two separate observations. We report that the spectrum is very red in the UV, with a blue upturn shortward of ∼200 nm. We find an absorption feature at 250 nm and a weaker absorption feature at 275 nm that may be attributed to a metal-oxide charge transfer band. We find that the red-sloped, relatively featureless spectrum of (16) Psyche is best matched with the reflectance spectrum of pure iron; however, our intimate mixture models show that small grains of iron may dominate the reflectance spectrum even if iron only comprises up to 10% of the material on the surface. We also stress that there is a limited database of reflectances for planetary surface analogs at UV wavelengths for comparison with the spectrum of (16) Psyche. The mid- and far-UV spectra (<240 nm) are markedly different for each of the four asteroids observed at these wavelengths so far, including ones in the same spectral class, indicating that UV observations of asteroids could be used to better understand differences in the composition and processing of the surfaces of these small bodies.
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| 2. |
- Becker, Tracy M., et al.
(författare)
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HST UV Observations of Asteroid (16) Psyche
- 2020
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Ingår i: The Planetary Science Journal. - : Institute of Physics Publishing (IOPP). - 2632-3338. ; 1:3
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Tidskriftsartikel (refereegranskat)abstract
- The Main Belt Asteroid (16) Psyche is the target object of the NASA Discovery Mission Psyche. We observed the asteroid at ultraviolet (UV) wavelengths (170-310 nm) using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope during two separate observations. We report that the spectrum is very red in the UV, with a blue upturn shortward of similar to 200 nm. We find an absorption feature at 250 nm and a weaker absorption feature at 275 nm that may be attributed to a metal-oxide charge transfer band. We find that the red-sloped, relatively featureless spectrum of (16) Psyche is best matched with the reflectance spectrum of pure iron; however, our intimate mixture models show that small grains of iron may dominate the reflectance spectrum even if iron only comprises up to 10% of the material on the surface. We also stress that there is a limited database of reflectances for planetary surface analogs at UV wavelengths for comparison with the spectrum of (16) Psyche. The mid- and far-UV spectra (<240 nm) are markedly different for each of the four asteroids observed at these wavelengths so far, including ones in the same spectral class, indicating that UV observations of asteroids could be used to better understand differences in the composition and processing of the surfaces of these small bodies.
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| 3. |
- Becker, Tracy M., et al.
(författare)
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Mid-ultraviolet Hubble Observations of Europa and the Global Surface Distribution of SO2
- 2022
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Ingår i: The Planetary Science Journal. - : IOP Publishing Ltd. - 2632-3338. ; 3:6
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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.
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| 4. |
- Giono, Gabriel, et al.
(författare)
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An Analysis of the Statistics and Systematics of Limb Anomaly Detections in HST/STIS Transit Images of Europa
- 2020
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Ingår i: Astronomical Journal. - : IOP PUBLISHING LTD. - 0004-6256 .- 1538-3881. ; 159:4
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Tidskriftsartikel (refereegranskat)abstract
- Several recent studies derived the existence of plumes on Jupiter's moon Europa. The only technique that provided multiple detections is the far-ultraviolet imaging observations of Europa in transit of Jupiter taken by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). In this study, we reanalyze the three HST/STIS transit images in which Sparks et al. identified limb anomalies as evidence for Europa's plume activity. After reproducing the results of Sparks et al., we find that positive outliers are similarly present in the images as the negative outliers that were attributed to plume absorption. A physical explanation for the positive outliers is missing. We then investigate the systematic uncertainties and statistics in the images and identify two factors that are crucial when searching for anomalies around the limb. One factor is the alignment between the actual and assumed locations of Europa on the detector. A misalignment introduces distorted statistics, most strongly affecting the limb above the darker trailing hemisphere where the plumes were detected. The second factor is a discrepancy between the observation and the model used for comparison, adding uncertainty in the statistics. When accounting for these two factors, the limb minima (and maxima) are consistent with random statistical occurrence in a sample size given by the number of pixels in the analyzed limb region. The plume candidate features in the three analyzed images can be explained by purely statistical fluctuations and do not provide evidence for absorption by plumes.
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| 5. |
- Roth, Lorenz, et al.
(författare)
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An attempt to detect transient changes in Io's SO2 and NaCl atmosphere
- 2020
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 350
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Tidskriftsartikel (refereegranskat)abstract
- Io's atmosphere is predominately SO2 that is sustained by a combination of volcanic outgassing and sublimation. The loss from the atmosphere is the main mass source for Jupiter's large magnetosphere. Numerous previous studies attributed various transient phenomena in Io's environment and Jupiter's magnetosphere to a sudden change in the mass loss from the atmosphere supposedly triggered by a change in volcanic activity. Since the gas in volcanic plumes does not escape directly, such causal correlation would require a transient volcano-induced change in atmospheric abundance, which has never been observed so far. Here we report four observations of atmospheric SO2 and NaCl from the same hemisphere of Io, obtained with the IRAM NOEMA interferometer on 11 December 2016, 14 March, 6 and 29 April 2017. These observations are compared to measurements of volcanic hot spots and Io's neutral and plasma environment. We find a stable NaCl column density in Io's atmosphere on the four dates. The SO2 column density derived for December 2016 is about 30% lower compared to the SO2 column density found in the period of March to April 2017. This increase in SO2 from December 2016 to March 2017 might be related to increasing volcanic activity observed at several sites in spring 2017, but the stability of the volcanic trace gas NaCl and resulting decrease in NaCl/SO2 ratio do not support this interpretation. Observed dimmings in both the sulfur ion torus and Na neutral cloud suggest rather a decrease in mass loading in the period of increasing SO2 abundance. The dimming Na brightness and stable atmospheric NaCl furthermore dispute an earlier suggested positive correlation of the sodium cloud and the hot spot activity at Loki Patara, which considerably increased in this period. The environment of Io overall appears to be in a rather quiescent state, preventing further conclusions. Only Jupiter's aurora morphology underwent several short-term changes, which are apparently unrelated to Io's quiescent environment or the relatively stable atmosphere.
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| 6. |
- Roth, Lorenz, et al.
(författare)
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Constraints on Europa's Water Group Torus from HST/COS Observations
- 2023
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Ingår i: The Planetary Science Journal. - : American Astronomical Society. - 2632-3338. ; 4:5
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Tidskriftsartikel (refereegranskat)abstract
- In situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa's atmosphere and consists primarily of molecular hydrogen, but the neutral gas torus has not yet been measured directly through emissions or in situ. Here we present observations by the Cosmic Origins Spectrograph (COS) of the Hubble Space Telescope (HST) from 2020 to 2021, which scanned the equatorial plane between 8 and 10 planetary radii west of Jupiter. No neutral gas emissions are detected. We derive upper limits on the emissions and compare these to modeled emissions from electron impact and resonant scattering using a Europa torus Monte Carlo model for the neutral gases. The comparison supports the previous findings that the torus is dilute and primarily consists of molecular hydrogen. A detection of sulfur ion emissions radially inward of the Europa orbit is consistent with emissions from the extended Io torus and with sulfur ion fractional abundances as previously detected.
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| 7. |
- Roth, Lorenz, et al.
(författare)
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Ganymede’s tenuous atmosphere
- 2024. - 1
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Ingår i: Ganymede. - Cambridge, UK : Cambridge University Press.
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Bokkapitel (refereegranskat)abstract
- Ganymede possesses a tenuous water-based atmosphere, thought to be produced by the solar and Jovian plasma irradiation of its icy surface. Observational evidence for Ganymede's atmosphere is provided by far-ultraviolet (FUV) emission from the atomic oxygen (O) and hydrogen (H) constituents. The relative strengths of the two observed FUV atomic oxygen emission multiplets, which are produced by electron-impact on the atmosphere, allow inference of molecular O2 and H2O components and their relative abundances. Minor species besides O and H have not been detected and the observational constraints can be considered loose overall, partly due to the inherent ambiguity and difficulty in relating the electron-excited emissions to atmospheric properties. Much of our understanding of the configuration of Ganymede’s atmosphere is based on modelling efforts. The modelling studies suggest that O2 is produced primarily through radiolysis and sputtering by the plasma precipitation in the icy polar regions. Since it does not condense, O2 is yet abundant everywhere and likely the dominant species over most of the surface. Ice sublimation increases the abundance of H2O near the sub-solar point, locally exceeding the O2 abundance. The high altitudes, in contrast, are expected to be primarily populated by the lighter H2 molecules everywhere. Inferred column densities for the primary molecules, O2 and H2O, are on the order of 1014 cm-2 to 1015 cm-2. In addition, models suggest that the atmosphere is collisional only up to a few tens of kilometers above the surface, particular in the dense sub-solar region.
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| 8. |
- Roth, Lorenz, et al.
(författare)
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Probing Ganymede's Atmosphere with HST Ly alpha Images in Transit of Jupiter
- 2023
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Ingår i: The Planetary Science Journal. - : American Astronomical Society. - 2632-3338. ; 4:1, s. 12-
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Tidskriftsartikel (refereegranskat)abstract
- We report results from far-ultraviolet observations by the Hubble Space Telescope of Jupiter's largest moon, Ganymede, transiting across the planet's dayside hemisphere. Within a targeted campaign on 2021 September 9 two exposures were taken during one transit passage to probe for attenuation of Jupiter's hydrogen Ly alpha dayglow above the moon limb. The background dayglow is slightly attenuated over an extended region around Ganymede, with stronger attenuation in the second exposure, when Ganymede was near the planet's center. In the first exposure, when the moon was closer to Jupiter's limb, the effects from the Ganymede corona are hardly detectable, likely because the Jovian Ly alpha dayglow is spectrally broader and less intense at this viewing geometry. The obtained vertical H column densities of around (1-2) x 10(12) cm(-2) are consistent with previous results. Constraining angular variability around Ganymede's disk, we derive an upper limit on a local H2O column density of (2-3) x 10(16) cm(-2), such as could arise from outgassing plumes in regions near the observed moon limb.
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| 9. |
- Saur, Joachim, et al.
(författare)
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Alternating North-South Brightness Ratio of Ganymede's Auroral Ovals : Hubble Space Telescope Observations Around the Juno PJ34 Flyby
- 2022
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 49:23
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Tidskriftsartikel (refereegranskat)abstract
- We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m(-2) are required for its OI 1,356 angstrom emission making Ganymede a very poor auroral emitter.
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| 10. |
- Trumbo, Samantha K., et al.
(författare)
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A New UV Spectral Feature on Europa : Confirmation of NaCl in Leading-hemisphere Chaos Terrain
- 2022
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Ingår i: The Planetary Science Journal. - : American Astronomical Society. - 2632-3338. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent visible-wavelength observations of Europa's surface obtained with the Hubble Space Telescope (HST) revealed the presence of an absorption feature near 450 nm that appears spatially correlated with leading-hemisphere chaos terrain. This feature was interpreted to reflect the presence of irradiated sodium chloride ultimately sourced from the interior. Here, we use ultraviolet spectra also collected with the HST to detect an additional previously unseen absorption near 230 nm, which spatially correlates with the 450 nm feature and with the same leading-hemisphere chaos terrain. We find that the new ultraviolet feature is also well matched by irradiated sodium chloride at Europa-like conditions. Such confirmation of sodium chloride within geologically young regions has important implications for Europa's subsurface composition.
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