| 1. |
- Coustenis, A., et al.
(författare)
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TandEM : Titan and Enceladus mission
- 2009
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 23:3, s. 893-946
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Tidskriftsartikel (refereegranskat)abstract
- TandEM was proposed as an L-class (large) mission in response to ESA's Cosmic Vision 2015-2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini-Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini-Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (MontgolfiSre) and possibly several landing probes to be delivered through the atmosphere.
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| 2. |
- Jones, G. H., et al.
(författare)
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The dust halo of Saturn's largest icy moon, Rhea
- 2008
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 319:5868, s. 1380-1384
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Tidskriftsartikel (refereegranskat)abstract
- Saturn's moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturn's magnetosphere. The Cassini spacecraft's in situ observations reveal that energetic electrons are depleted in the moon's vicinity. The absence of a substantial exosphere implies that Rhea's magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.
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| 3. |
- Marzok, A., et al.
(författare)
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Mapping the Brightness of Ganymede's Ultraviolet Aurora Using Hubble Space Telescope Observations
- 2022
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Ingår i: Journal of Geophysical Research - Planets. - : American Geophysical Union (AGU). - 2169-9097 .- 2169-9100. ; 127:6
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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.
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| 4. |
- Roth, Lorenz, et al.
(författare)
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Constraints on an exosphere at Ceres from Hubble Space Telescope observations
- 2016
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:6, s. 2465-2472
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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.
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| 5. |
- Roth, Lorenz, et al.
(författare)
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Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 121:3, s. 2143-2170
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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.
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| 6. |
- Saur, J., et al.
(författare)
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The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals
- 2015
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 120:3, s. 1715-1737
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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
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| 7. |
- Szalay, J.R., et al.
(författare)
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Oxygen production from dissociation of Europa’s water-ice surface
- 2024
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Ingår i: Nature Astronomy. - : Springer Nature. - 2397-3366. ; 8:5, s. 567-576
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Tidskriftsartikel (refereegranskat)abstract
- Jupiter’s moon Europa has a predominantly water-ice surface that is modified by exposure to its space environment. Charged particles break molecular bonds in surface ice, thus dissociating the water to ultimately produce H2 and O2, which provides a potential oxygenation mechanism for Europa’s subsurface ocean. These species are understood to form Europa’s primary atmospheric constituents. Although remote observations provide important global constraints on Europa’s atmosphere, the molecular O2 abundance has been inferred from atomic O emissions. Europa’s atmospheric composition had never been directly sampled and model-derived oxygen production estimates ranged over several orders of magnitude. Here, we report direct observations of H2+ and O2+ pickup ions from the dissociation of Europa’s water-ice surface and confirm these species are primary atmospheric constituents. In contrast to expectations, we find the H2 neutral atmosphere is dominated by a non-thermal, escaping population. We find 12 ± 6 kg s−1 (2.2 ± 1.2 × 1026 s−1) O2 are produced within Europa’s surface, less than previously thought, with a narrower range to support habitability in Europa’s ocean. This process is found to be Europa’s dominant exogenic surface erosion mechanism over meteoroid bombardment.
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| 8. |
- Vigren, E., et al.
(författare)
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On the thermal electron balance in Titan's sunlit upper atmosphere
- 2013
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 223:1, s. 234-251
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Tidskriftsartikel (refereegranskat)abstract
- The Cassini mission has investigated Titan's upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N2 and CH4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions' dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan's upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan's upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan's upper atmosphere.
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