We observed the evolution of Jupiter's polar cyclonic structures over two years between February 2017 and February 2019, using polar observations by the Jovian InfraRed Auroral Mapper, JIRAM, on the ...Juno mission. Images and spectra were collected by the instrument in the 5‐μm wavelength range. The images were used to monitor the development of the cyclonic and anticyclonic structures at latitudes higher than 80° both in the northern and the southern hemispheres. Spectroscopic measurements were then used to monitor the abundances of the minor atmospheric constituents water vapor, ammonia, phosphine, and germane in the polar regions, where the atmospheric optical depth is less than 1. Finally, we performed a comparative analysis with oceanic cyclones on Earth in an attempt to explain the spectral characteristics of the cyclonic structures we observe in Jupiter's polar atmosphere.
Plain Language Summary
The Jovian InfraRed Auroral Mapper (JIRAM) is an instrument on‐board the Juno NASA spacecraft. It consists of an infrared camera, for mapping both Jupiter's auroras and atmosphere, and a spectrometer. In February 2017, the complex cyclonic structures that characterize the Jupiter's polar atmospheres were discovered. Here, we report the evolution of those cyclonic structures during the 2 years following the discovery. We use for this purpose infrared maps built by the JIRAM camera images collected at wavelengths around 5 μm. The cyclones have thick clouds that obstruct most of the view of the deeper atmosphere. However, some areas, near the cyclones, are only covered by thin clouds allowing the spectrometer to see deeper in the atmosphere. In those areas, the instrument was able to detect spectral signatures that permitted estimation of abundances of water vapor, ammonia, phosphine, and germane. Those gases are minor but significant constituents of the atmosphere. Finally, the dynamics of the Jupiter's polar atmosphere are not well understood and are still under study. Here, to suggest possible mechanisms that governs the polar dynamics, we attempted a comparative analysis with some Earth oceanic cyclones that show similarities with the Jupiter ones.
Key Points
The Jupiter's polar cyclonic structures did not change much in two years of observations from February 2017 to February 2019
Abundances of some atmospheric minor constituents measured in the hottest spots of the polar regions, higher values registered in the south
Earth oceanic cyclones analogies suggest a well‐mixed upper boundary layer on Jupiter's Poles
The origin of the irregular satellites of the giant planets has been long debated since their discovery. Their dynamical features argue against an in situ formation suggesting that they are captured ...bodies, yet there is no global consensus on the physical process at the basis of their capture. In this paper, we explore the collisional capture scenario, where the actual satellites originated from impacts occurred within Saturn's influence sphere. By modelling the inverse capture problem, we estimated the families of orbits of the possible parent bodies and the specific impulse needed for their capture. The orbits of these putative parent bodies are compared to those of the minor bodies of the outer Solar system to outline their possible region of formation. Finally, we tested the collisional capture hypothesis on Phoebe by taking advantage of the data supplied by Cassini on its major crater, Jason. Our results presented a realistic range of solutions matching the observational and dynamical data.
ABSTRACT
The dynamical features of the irregular satellites of the giant planets argue against an in situ formation and are strongly suggestive of a capture origin. Since the last detailed ...investigations of their dynamics, the total number of satellites has doubled, increasing from 50 to 109, and almost tripled in the case of Saturn system. We have performed a new dynamical exploration of Saturn system to test whether the larger sample of bodies could improve our understanding of which dynamical features are primordial and which are the outcome of the secular evolution of the system. We have performed detailed N‐body simulations using the best orbital data available and analysed the frequencies of motion to search for resonances and other possible perturbing effects. We took advantage of the hierarchical Jacobian symplectic algorithm to include in the dynamical model of the system also the gravitational effects of the two outermost massive satellites, Titan and Iapetus. Our results suggest that Saturn's irregular satellites have been significantly altered and shaped by the gravitational perturbations of Jupiter, Titan, Iapetus and the Sun and by the collisional sweeping effect of Phoebe. In particular, the effects on the dynamical evolution of the system of the two massive satellites appear to be non‐negligible. Jupiter perturbs the satellites through its direct gravitational pull and, indirectly, via the effects of the Great Inequality, i.e. its near‐resonance with Saturn. Finally, by using the hierarchical clustering method we found hints to the existence of collisional families and compared them with the available observational data.
The electromagnetic coupling between the Galilean satellites at Jupiter and the planetary ionosphere generates an auroral footprint, which is detected with high spatial resolution in the infrared L ...band by the Jovian InfraRed Auroral Mapper (JIRAM) onboard the Juno spacecraft. We report the JIRAM data acquired since 27 August 2016 until 23 May 2022, which are used to compute the average position of the footprint tracks of Io, Europa and Ganymede. The result of the present analysis help to test the reliability of magnetic field models, to calibrate ground‐based observations and to highlight the variability in the footprint positions, which can be used to probe the plasma environment at the orbit of the satellites. The determination of the plasma properties around the moons is particularly relevant to complement the Juno flybys of the moons during its extended mission, and to support the future Juice and Europa Clipper missions. Lastly, we report no clear evidence of the auroral footprint of Callisto, which is likely due to a combination of its low expected brightness and its position very close to the main Jovian aurora.
Plain Language Summary
The Jovian InfraRed Auroral Mapper onboard the Juno spacecraft around Jupiter has now been gathering 6 years of observations. Here, we report the position of the auroral infrared emission associated with the orbital motion of Io, Europa and Ganymede. The position of this emission ‐ called footprint ‐ carries information on the magnetic field geometry and the distribution of charged particles along the magnetic field. Therefore, the footprint tracks provided here can be used to test and constrain magnetic field models, and to improve the calibration of ground based observations of Jupiter: this can help better understand the source region of the main Jovian aurora and its variations. Lastly, by surveying the data acquired over 40 Juno orbits, we point out variations in the footprint position, which reflect the variability in the plasma conditions near the moons: this monitoring may help determine the mass loading of the magnetosphere, which affects the intensity of the main aurora. The possibility of investigating the plasma environment at the orbit of the satellites is important to complement the satellite flybys performed during the extended mission of Juno and to support the future Juice and Europa Clipper missions, which are dedicated to the Galilean moons.
Key Points
The position of the Io, Europa and Ganymede footprints based on Juno‐JIRAM observations are reported with unprecedented spatial resolution
The positions of the footprints support the Juno‐based magnetic field models and the calibration of ground‐based observation
The transversal shift of the Ganymede footprint suggests variations of the plasmadisk; the shift appears to be correlated with local time
The spatial distribution of water, ammonia, phosphine, germane, and arsine in the Jupiter's troposphere has been inferred from the Jovian Infrared Auroral Mapper (JIRAM) Juno data. Measurements allow ...us to retrieve the vertically averaged concentration of gases between ~3 and 5 bars from infrared‐bright spectra. Results were used to create latitudinal profiles. The water vapor relative humidity varies with latitude from <1% to over 15%. At intermediate latitudes (30–70°) the water vapor maxima are associated with the location of cyclonic belts, as inferred from mean zonal wind profiles (Porco et al., 2003). The high‐latitude regions (beyond 60°) are drier in the north (mean relative humidity around 2–3%) than the south, where humidity reaches 15% around the pole. The ammonia volume mixing ratio varies from 1 × 10−4 to 4 × 10−4. A marked minimum exists around 10°N, while data suggest an increase over the equator. The high‐latitude regions are different in the two hemispheres, with a gradual increase in the south and more constant values with latitude in the north. The phosphine volume mixing ratio varies from 4 × 10−7 to 10 × 10−7. A marked minimum exists in the North Equatorial Belt. For latitudes poleward 30°S and 30°N, the northern hemisphere appears richer in phosphine, with a decrease toward the pole, while the opposite is observed in the south. JIRAM data indicate an increase of germane volume mixing ratio from 2 × 10−10 to 8 × 10−10 from both poles to 15°S, with a depletion centered around the equator. Arsine presents the opposite trend, with maximum values of 6 × 10−10 at the two poles and minima below 1 × 10−10 around 20°S.
Key Points
Horizontal variations of gases are dominated by latitudinal components; longitudinal variations are relatively more important for water
Phosphine and germane abundances fit well the model of disequilibrium species transported upward from deep troposphere by vertical mixing
Strong upturn of arsine at polar latitudes seen by JIRAM cannot be explained by the diffusion‐kinetics model
ABSTRACT
The Martian satellite Phobos has been observed on 2007 February 24 and 25, during the pre‐ and post‐Mars closest approach (CA) of the ESA Rosetta spacecraft Mars swing‐by. The goal of the ...observations was the determination of the surface composition of different areas of Phobos, in order to obtain new clues regarding its nature and origin. Near‐ultraviolet, visible and near‐infrared (263.5–992.0 nm) images of Phobos's surface were acquired using the Narrow Angle Camera of the OSIRIS instrument onboard Rosetta. The six multi‐wavelength sets of observations allowed a spectrophotometric characterization of different areas of the satellite, belonging respectively to the leading and trailing hemisphere of the anti‐Mars hemisphere, and also of a section of its sub‐Mars hemisphere. The pre‐CA spectrophotometric data obtained with a phase angle of 19° have a spectral trend consistent within the error bars with those of unresolved/disc‐integrated measurements present in the literature. In addition, we detect an absorption band centred at 950 nm, which is consistent with the presence of pyroxene. The post‐CA observations cover from NUV to NIR a portion of the surface (0° to 43°E of longitude) never studied before. The reflectance measured on our data does not fit with the previous spectrophotometry above 650 nm. This difference can be due to two reasons. First, the OSIRIS observed area in this observation phase is completely different with respect to the other local specific spectra and hence the spectrum may be different. Secondly, due to the totally different observation geometry (the phase angle ranges from 137° to 140°), the differences of spectral slope can be due to phase reddening. The comparison of our reflectance spectra, both pre‐ and post‐CA, with those of D‐type asteroids shows that the spectra of Phobos are all redder than the mean D‐type spectrum, but within the spectral dispersion of other D‐types. To complement this result, we performed an investigation of the conditions needed to collisionally capture Phobos in a way similar to that proposed for the irregular satellites of the giant planets. Once put in the context of the current understanding of the evolution of the early Solar system, the coupled observational and dynamical results we obtained strongly argue for an early capture of Phobos, likely immediately after the formation of Mars.
The Jovian InfraRed Auroral Mapper (JIRAM) onboard the NASA Juno mission monitored the evolution of Jupiter’s polar cyclones since their first observation ever in February 2017. Data acquired by ...JIRAM have revealed cloudy cyclones organized in a complex, yet stable geometrical pattern at both poles. Several studies have investigated the dynamics and the structure of these cyclones, to understand the physical mechanisms behind their formation and evolution. In this work, we present vorticity maps deduced from the wind fields for the region poleward of ∼−80°, which has been extensively covered over the last four years of observations. The cyclonic features related to the stable polar cyclones are embedded in a slightly, but diffused anticyclonic circulation, in which short‐living anticyclones emerge with respect to the surroundings. Although the general stability of both the cyclones and the whole system is strongly confirmed by this work, variations in the shape of the vortices, as well as changes in the local structures, have been observed.
Plain Language Summary
The Jovian InfraRed Auroral Mapper is the instrument onboard the NASA Juno spacecraft that has provided observations of Jupiter’s poles since February 2017. These data have shown cyclones organized in snowflake‐like structures. The Jupiter’s polar cyclones are long‐lasting features, which did not disappear or merge during 4 years of observations. In general, the analysis of the winds is important in the study of the cyclones. In this work, we focus on the vorticity, a quantity derived by the winds, that gives information on the magnitude and direction of the rotation of the cyclones. We focused on the southern polar region, which has a better coverage in time, with respect to the northern counterpart. The general pattern of the southern polar cyclones is preserved along the observations.
Key Points
The vorticity field of Jupiter’s southern polar cyclones is evaluated for different orbits
The temporal variability of the vorticity field of the central polar cyclone is analyzed
We found extremely long stability of the morphology of circumpolar cyclones both in terms of clouds and winds
The regular polygons of circumpolar cyclones, discovered by Juno in 2017, are one of the most puzzling features of Jupiter. Here we show new recent global pictures of the North polar cyclones' ...structure. These are the first simultaneous images of the whole structure since 2017, and we find that it remained almost unperturbed, just like the South one. The observation of these long‐lasting structures poses questions regarding the formation mechanism of cyclones, and on their vertical structure. Data by Juno/JIRAM infrared camera collected over the last 5 years show that cyclones migrate around what may seem like equilibrium positions, with timescales of a few months but, aside from that, the cyclones systems are very stable. Our analysis of the observations shows that the motion of cyclones around their equilibrium position is uncorrelated with their position if a barotropic approximation (β‐drift) is assumed. Thus, a different dynamical explanation than the barotropic β‐drift is needed to explain the stability of the observed features. Each cyclone has a peculiar morphology, which differs from the others and is stable over the observed lapse of time in most cases.
Plain Language Summary
In 2017, Juno discovered that the poles of Jupiter are occupied by regular polygons of cyclones. Here we report 5 years of observations of these cyclones by JIRAM, the infrared imaging spectrometer on board Juno. In particular, we show the latest observations of the North Pole cyclones structure. In fact, this structure has only been partially observed since its discovery 5 years ago. One important question is how these structures of cyclones form, and if they are stable. We find that both remained almost unperturbed. Hence, we show that these cyclones may have very long lifetimes. Cyclones migrate around what may appear to be equilibrium positions. The scale time is a few months. We analyzed the movement of cyclones and compared it with a model where winds don't change with depth (a barotropic model). We found that the motion of the cyclones is not related to their position according to this model. We conclude that a different model is needed to explain some of the observed characteristics. Each cyclone has a peculiar morphology, which differs from the others and is stable over the observed time span in most cases.
Key Points
For the first time after 5 years, we show a global picture of the North polar cyclones' structure, which has remained almost unperturbed
Each cyclone has a peculiar morphology, which differs from the others and it is stable over the observed lapse of time
Beta‐drift is not responsible for the motion of the vortices on timescales of months
Asteroid 21 Lutetia, seen by the Rosetta spacecraft, plays a crucial role in the reconstruction of primordial phases of planetary objects. Its high bulk density and its primitive chondritic crust ...suggest that Lutetia could be partially differentiated. We developed a numerical code, also used for studying the geophysical history of Vesta, to explore several scenarios of internal evolution of Lutetia. These scenarios differ in the strength of their radiogenic sources and in their global post-sintering porosity. The only significant heat source for partial differentiation is super(26)Al; the other possible sources ( super(60)Fe, accretion, and differentiation) are negligible. In scenarios in which Lutetia completed its accretion in less than 0.7 Myr from the injection of super(26)Al in the solar nebula and for post-sintering values of macroporosity not exceeding 30% by volume, the asteroid experienced only partial differentiation. The formation of the proto-core, a structure enriched in metals and also containing pristine silicates, requires 1-4 Myr and the size of the proto-core varies from 6-30 km.
Evolution of Icy Satellites Schubert, G.; Hussmann, H.; Lainey, V. ...
Space Science Reviews,
06/2010, Letnik:
153, Številka:
1-4
Journal Article, Book
Recenzirano
Evolutionary scenarios for the major satellites of Jupiter, Saturn, Neptune, and Pluto-Charon are discussed. In the Jovian system the challenge is to understand how the present Laplace resonance of ...Io, Europa, and Ganymede was established and to determine whether the heat being radiated by Io is in balance with the present tidal dissipation in the moon. In the Saturnian system, Enceladus and Titan are the centers of attention. Tidal heating is the likely source of activity at the south pole of Enceladus, although the details of how the heating occurs are not understood. An evolutionary scenario based on accretion and internal differentiation is presented for Titan, whose present substantial orbital eccentricity is not associated with any dynamical resonance. The source and maintenance of methane in Titan’s present atmosphere remain uncertain. Though most attention on the Saturnian moons focuses on Titan and Enceladus, the mid-size satellites Iapetus, Rhea, Tethys, and the irregular satellite Phoebe also draw our interest. An evolutionary scenario for Iapetus is presented in which spin down from an early rapidly rotating state is called upon to explain the satellite’s present oblate shape. The prominent equatorial ridge on Iapetus is unexplained by the spin down scenario. A buckling instability provides another possible explanation for the oblateness and equatorial ridge of Iapetus. Rhea is the only medium-size Saturnian satellite for which there are gravity data at present. The interpretation of these data are uncertain, however, since it is not known if Rhea is in hydrostatic equilibrium. Pluto and Charon are representative of the icy dwarf planets of the Kuiper belt. Did they differentiate as they evolved, and do either of them have a subsurface liquid water ocean? New Horizons might provide some answers when it arrives at these bodies.
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