Galileo Observations Research Articles

Correction to “Mountains on Io: High‐resolution Galileo observations, initial interpretations, and formation models” by E. P. Turtle et al.

Correction to “Mountains on Io: High‐resolution Galileo observations, initial interpretations, and formation models” by E. P. Turtle et al.

Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites

Data from the recent gravity measurements by the Galileo mission are used to construct wide ranges of interior structure and composition models for the Galilean satellites of Jupiter. These models show that mantle densities of Io and Europa are consistent with an olivine-dominated mineralogy with the ratios of Mg to Fe components depending on mantle temperature for Io and on ice shell thickness for Europa. The mantle density and composition depend relatively little on core composition. The size of the core is largely determined by the core's composition with core radius increasing with the concentration of a light component such as sulfur. For Io, the range of possible core sizes is between 38 and 53% of the satellite's radius. For Europa, there is also a substantial effect of the thickness of the ice layer which is varied between 120 and 170 km on the core size. Core sizes are between 10 and 45% of Europa's radius. The core size of Ganymede ranges between one-quarter and one-third of the surface radius depending on its sulfur content and the thickness of the ice shell. A subset of the Ganymede models is consistent with an olivine-dominated mantle mineralogy. The thickness of the silicate mantle above the core varies between 900 and 1100 km. The outermost ice shell is about 900 km in thickness and is further subdivided by pressure-induced phase transitions into ice I, ice III, ice V, and ice VI layers. Callisto should be differentiated, albeit incompletely. It is proposed that this satellite was never molten at a large scale but differentiated through the convective gradual unmixing of the ice and the metal/rock component. Bulk iron-to-silicon ratios Fe/Si calculated for the inner pair of satellites, Io and Europa, are less than the CI carbonaceous chondrite value of 1.7±0.1, whereas ratios for the outer pair, Ganymede and Callisto, cover a broad range above the chondritic value. Although the ratios are uncertain, in particular for Ganymede and Callisto, the values are sufficiently distinct to suggest a difference in composition between these two pairs of satellites. This may indicate a difference in iron–silicon fractionation during the formation of both classes of satellites in the protojovian nebula.

Geology and origin of Europa's “Mitten” feature (Murias Chaos)

The “Mitten” (provisionally named Murias Chaos by the International Astronomical Union) is a region of elevated chaos‐like terrain in the leading hemisphere of Europa. Its origin had been explained under the currently debated theories of melting through a thin lithosphere or convection within a thick one. Galileo observations reveal several characteristics that suggest that the Mitten is distinct from typical chaos terrain and point to a different formational process. Photoclinometric elevation estimates suggest that the Mitten is slightly elevated with respect to the surrounding terrain; geologic relations indicate that it must have raised significantly from the plains in its past, resembling disrupted domes on Europa's trailing hemisphere. Moreover, the Mitten material appears to have extruded onto the plains and flowed for tens of kilometers. The area subsequently subsided as a result of isostatic adjustment, viscous relaxation, and/or plains loading. Using plate flexure models, we estimated the elastic lithosphere in the area to be several kilometers thick. We propose that the Mitten originated by the ascent and extrusion of a large thermal diapir. Thermal‐mechanical modeling shows that a Mitten‐sized plume would remain sufficiently warm and buoyant to pierce through the crust and flow unconfined on the surface. Such a diapir probably had an initial radius between 5 and 8 km and an initial depth of 20–40 km, consistent with a thick‐lithosphere model. In this scenario the Mitten appears to represent the surface expression of the rare ascent of a large diapir, in contrast to lenticulae and chaos terrain, which may form by isolated and clustered small diapirs, respectively.

Mountains on Io: High‐resolution Galileo observations, initial interpretations, and formation models

During three close flybys in late 1999 and early 2000 the Galileo spacecraft acquired new observations of the mountains that tower above Io's surface. These images have revealed surprising variety in the mountains' morphologies. They range from jagged peaks several kilometers high to lower, rounded structures. Some are very smooth, others are covered by numerous parallel ridges. Many mountains have margins that are collapsing outward in large landslides or series of slump blocks, but a few have steep, scalloped scarps. From these observations we can gain insight into the structure and material properties of Io's crust as well as into the erosional processes acting on Io. We have also investigated formation mechanisms proposed for these structures using finite‐element analysis. Mountain formation might be initiated by global compression due to the high rate of global subsidence associated with Io's high resurfacing rate; however, our models demonstrate that this hypothesis lacks a mechanism for isolating the mountains. The large fraction (∼40%) of mountains that are associated with paterae suggests that in some cases these features are tectonically related. Therefore we have also simulated the stresses induced in Io's crust by a combination of a thermal upwelling in the mantle with global lithospheric compression and have shown that this can focus compressional stresses. If this mechanism is responsible for some of Io's mountains, it could also explain the common association of mountains with paterae.

The Io mass‐loading disk: Model calculations

The observations of ion cyclotron waves up to 0.5 RJ beyond the orbit of Io are best explained by the presence of a thin disk of fast neutrals whose ionization and pickup provide the free energy for the waves. We extend the model of Wilson and Schneider [1999] in order to explain the observed properties of this mass‐loading region, especially the most recent Galileo observations near Io. In the extended model, some of the molecules of sulfur compounds in Io's exobase are first ionized by photoionization, impact ionization, and charge exchange. These charged particles are accelerated in the corotation electric field associated with the motion of the magnetized Io torus plasma that is moving through its exosphere. After a period of acceleration the heavy ions are neutralized by charge exchange with other exospheric neutral particles or combined with local electrons. These newly neutralized particles continue with high velocities similar to those of their former charged state, moving only under the influence of the gravity fields of Jupiter and Io, not affected by the electric and magnetic field. If they do not impact Io or its atmosphere, these neutral particles can propagate large distances across the magnetic field before they are reionized. Eventually, the reionized particles are lost by dissociation. Characteristic Io mass‐loading particle distributions, such as high torus plasma density outside Io's orbit and the lower density inside, and the directional feature of the mass‐loading neutral cloud, are qualitatively reproduced in the model. Meanwhile, the configuration of the mass‐loading region, in which the ion cyclotron waves are observed, is obtained, and the results are consistent with Galileo wave observations. Three parameters are found to control the structure of the neutral and ion loading disks: the characteristic lifetimes of the initially created ions, of the neutral molecules, and of the ions generated by neutral particle reionization. In addition, particle source geometry, gravity, and the near Io field configurations play important roles in the Io mass‐loading process.

Energetic ion dynamics in Jupiter's plasma sheet

A systematic study of energetic ion trajectories in Jupiter's plasma sheet region predicts a significant role for both regular and stochastic types of motion and reveals several aspects of the ion dynamics that aid in the interpretation of Galileo observations. The motion is generally confined to the vicinity of the current sheet in a way that is proportional to the variable current sheet thickness as seen in the particle and field data. The radial extent of the trajectories increases with rigidity and initial radial distance from the planet, explaining the corresponding lack of high‐rigidity ions and decreasing radial gradients at lower rigidity. Ion intensity increases associated with changes in current sheet thickness suggest an acceleration region at ∼25 to 30 Rj. Energy dispersion in ion events at larger radial distances can be explained by such a source combined with elastic ion scattering.

Replication or monopoly? the economies of invention and discovery in galileo's observations of 1610

i propose a revisionist account of the production and reception of galileo's telescopic observations of 1609–10, an account that focuses on the relationship between credit and disclosure. galileo, i argue, acted as though the corroboration of his observations were easy, not difficult. his primary worry was not that some people might reject his claims, but rather that those able to replicate them could too easily proceed to make further discoveries on their own and deprive him of credit. consequently, he tried to slow down potential replicators to prevent them from becoming competitors. he did so by not providing other practitioners access to high-power telescopes and by withholding information about how to build them. this essay looks at the development of galileo's monopoly on early telescopic astronomy to understand how the relationship between disclosure and credit changed as he moved from being an instrument-maker to becoming a discoverer and, eventually, a court philosopher.

Laboratory detection of X-ray fringes with a grazing-incidence interferometer

Starting with Galileo's observations of the Solar System, improvements of an order of magnitude in either the sensitivity or resolution of astronomical instruments have always brought revolutionary discoveries. The X-ray band of the spectrum, where exotic objects can have extremely high surface brightness, is ideally suited for significant improvements in imaging, but progress has been impeded by a lack of optics of sufficiently high sensitivity and quality. Here we present an X-ray interferometer design that is practical for adaptation to astronomical observatories. Our prototype interferometer, having just under one millimetre of baseline, creates fringes at 1.25 keV with an angular resolution of 100 milliarcseconds. With a larger version in orbit it will be possible to resolve X-ray sources at 10(-7) arcseconds, three orders of magnitude better than the finest-resolution images ever achieved on the sky (in the radio part of the spectrum) and over one million times better than the current best X-ray images. With such resolutions, we can study the environments of pulsars, resolve and then model relativistic blast waves, image material falling into a black hole, watch the physical formation of astrophysical jets, and study the dynamos of stellar coronae.

Multispectral observations of Jupiter's aurora

Remote sensing of Jupiter's aurora from x-ray to radio wavelengths has revealed much about the nature of the jovian aurora and about the impact of ionosphere-magnetosphere coupling on the upper atmosphere of Jupiter. As indicated by the combination of x-ray and ultraviolet observations, both energetic heavy ions and electrons energized in the outer magnetosphere contribute to auroral excitation. Imaging with the Hubble Space Telescope in the ultraviolet and with the InfraRed Telescope Facility at infrared wavelengths shows several distinct regions of interaction: 1) a dusk sector where turbulent auroral patterns extend well into the polar cap; 2) a morning sector generally characterized by a single spatially confined auroral arc originating in the outer or middle magnetosphere of Jupiter; 3) diffuse emissions associated with the Io plasma - spectroscopy has provided important information about the thermal structure of Jupiter's auroral atmosphere and the altitude distribution of auroral particle energy deposition, while Lyman alpha line profiles yield clues to the nature of thermospheric dynamical effects. Galileo observations at visible wavelengths on the nightside offer a new view of the jovian aurora with unprecedented spatial information. Infrared observations have added much to the understanding of thermal structure at all latitudes, the dynamics of the thermospheric wind system, and auroral morphology, and may hold the key to understanding the role of Joule heating in Jupiter's thermosphere. ROSAT observations have revealed soft x-ray emissions from Jupiter's lower latitudes as well as from the auroral zones, implying that energetic particle precipitation also occurs at low latitudes in regions magnetically linked to the inner radiation belts. In this review, multispectral observations of jovian auroral emissions are presented within a theoretical/modeling framework that is intended to provide some insight into magnetosphere-ionosphere coupling and its effects on the upper atmosphere.

Substorms at Jupiter: Galileo observations of transient reconnection in the near tail

The magnetic flux content of the Jovian magnetosphere is set by the internal dynamo, but those magnetic field lines are constantly being loaded by heavy ions at the orbit of lo and dragged inexorably outward by the centrifugal force. Vasyliunas has proposed a steady state reconnecting magnetospheric model that sheds plasma islands of zero net magnetic flux and returns nearly empty flux tubes to the inner magnetosphere. The Galileo observations indicate that beyond 40 Rj the current sheet begins to tear and beyond 50 Rj on the nightside explosively reconnects as the tearing site reaches the low density lobe region above and below the current sheet. Small events occur irregularly but on average about every 4 hours and large events about once a day. The magnetic flux reconnected in such events amounts up to about 70,000 Webers/sec and is sufficient to return the outwardly convected magnetic flux to the inner magnetosphere. Since this process releases plasmoids into the jovian tail, as do terrestrial substorms; since this process involves explosive reconnection across the current sheet on the nightside of the planet, as do terrestrial substorms; and since the process is a key in closing the circulation pattern of the magnetic and plasma flux, as it is in terrestrial substorms; we refer to these events as jovian substorms.

Replication or Monopoly? The Economies of Invention and Discovery in Galileo's Observations of 1610

The ArgumentI propose a revisionist account of the production and reception of Galileo's telescopic observations of 1609–10, an account that focuses on the relationship between credit and disclosure. Galileo, I argue, acted as though the corroboration of his observations were easy, not difficult. His primary worry was not that some people might reject his claims, but rather that those able to replicate them could too easily proceed to make further discoveries on their own and deprive him of credit. Consequently, he tried to slow down potential replicators to prevent them from becoming competitors. He did so by not providing other practitioners access to high-power telescopes and by withholding information about how to build them. This essay looks at the development of Galileo's monopoly on early telescopic astronomy to understand how the relationship between disclosure and credit changed as he moved from being an instrument-maker to becoming a discoverer and, eventually, a court philosopher.

The planet Jupiter

The exploration of Jupiter, the closest and biggest giant planet, has provided key information about the origin and evolution of the outer Solar system. Our knowledge has strongly benefited from the Voyager and Galileo space missions. We now have a good understanding of Jupiter's thermal structure, chemical composition and magnetospheric environment. There is still debate about the nature of the heating source responsible for the high thermospheric temperatures (precipitating particles and/or gravity waves). The measurement of elemental abundance ratios (C/H, N/H, S/H) gives strong support to the “nucleation” formation model, according to which giant planets formed from the accretion of an initial core and the collapse of the surrounding gaseous protosolar nebula. The D/H and $^3$ He/ $^4$ He ratios are found to be representative of their protosolar value. The helium abundance, in contrast, appears to be slightly depleted in the outer envelope with respect to the protosolar value; this departure is interpreted as an evolutionary effect, due to the condensation of helium droplets in the liquid hydrogen ocean inside Jupiter's interior. The cloud structure of Jupiter, characterized by the belt-zone system, is globally understood; also present are specific features like regions of strong infrared radiation (“hot spots”), colder regions (“white ovals”) and the Great Red Spot (GRS). Clouds were surprisingly absent at the hot spot corresponding to the Galileo probe entry site, and the water abundance measured there was strongly depleted with respect to the solar O/H value. This probably implies that hot spots are dry, cloud-free regions of subsidence, while “normal” air, rich in condensibles, is transported upward by convective motions. As a result, the Jovian meteorology, still based on Halley-type cells, seems to be much more complex than a simple zone-belt system. The nature of the GRS, a giant anticyclonic storm, colder and higher than its environment, has been confirmed by the Galileo observations, but its internal structure appears to be very complex. Strong winds, probably driven by the Jovian internal source, were measured at deep tropospheric levels. The troposphere might be statically stable at pressures higher than 18 bars, but the extent of this putative radiative layer is still unknown.

Active Volcanism on Io: Global Distribution and Variations in Activity

Io's volcanic activity has been monitored by instruments aboard the Galileo spacecraft since June 28, 1996. We present results from observations by the near-infrared mapping spectrometer (NIMS) for the first 10 orbits of Galileo, correlate them with results from the Solid State Imaging System (SSI) and from groundbased observations, and compare them to what was known about Io's volcanic activity from observations made during the two Voyager flybys in 1979. A total of 61 active volcanic centers have been identified from Voyager, groundbased, and Galileo observations. Of these, 41 are hot spots detected by NIMS and/or SSI. Another 25 locations were identified as possible active volcanic centers, mostly on the basis of observed surface changes. Hot spots are correlated with surface colors, particularly dark and red deposits, and generally anti-correlated with white, SO2-rich areas. Surface features corresponding to the hot spots, mostly calderas or flows, were identified from Galileo and Voyager images. Hot spot temperatures obtained from both NIMS and SSI are consistent with silicate volcanism, which appears to be widespread on Io. Two types of hot spot activity are present: persistent-type activity, lasting from months to years, and sporadic events, which may represent either short-lived activity or low-level activity that occasionally flares up. Sporadic events are not often detected, but may make an important contribution to Io's heat flow and resurfacing. The distribution of active volcanic centers on the surface does not show any clear correlation with latitude, longitude, Voyager-derived global topography, or heat flow patterns predicted by the asthenosphere and deep mantle tidal dissipation models. However, persistent hot spots and active plumes are concentrated toward lower latitudes, and this distribution favors the asthenosphere rather than the deep mantle tidal dissipation model.

Temperatures on europa from galileo photopolarimeter-radiometer: nighttime thermal anomalies

Galileo observations of Europa's thermal emission show low-latitude diurnal brightness temperatures in the range of 86 to 132 kelvin. Nighttime temperatures form an unexpected pattern, with high temperatures on the bright ejecta blanket of the crater Pwyll and an equatorial minimum in temperatures after sunset, uncorrelated with surface albedo or geology. The nighttime anomalies may be due to regional thermal inertia variations of an unknown origin, which are equivalent to a two- to threefold variation in thermal conductivity, or to endogenic heat fluxes locally reaching 1 watt per square meter. Endogenic heat flow at this high level, although consistent with some geological evidence, is theoretically unlikely.

The fluctuating magnetic field in the middle jovian magnetosphere: initial Galileo observations

The region of the jovian magnetosphere from 10–24 planetary radii is a region of strong, nearly dipolar, magnetic field through which the plasma added to the Io torus must pass as it convects and diffuses outward to the magnetodisk region and ultimately to be lost down the tail. Magnetic fluctuations can be used to diagnose the processes present in this plasma. Herein we examine magnetic fluctuations at frequencies below 0.04 Hz observed in the middle magnetosphere on a single pass of the Galileo spacecraft. These frequencies are generally well below the ion gyro-frequencies in this portion of the magnetosphere. Near perijove, about 10.7 R J on this orbit, the equatorial region is not more disturbed than off the equator, but it is quite disturbed at the outer edge of this dipolar region near 24 R J where the magnetic field becomes weak. These fluctuations are quasi-periodic, strongly compressional, and incoherent resembling somewhat the mirror mode. Conversely, the region off the equator is quiet in this outer portion of the middle magnetosphere. In the quasi-dipolar region there are irregular transverse fluctuations everywhere Galileo travels. Compressional transient events are also observed that may mark flux tube interchange. An unusual structure that could be due to the outward convection of Europas wake is also seen. In toto, these fluctuations indicate that the middle magnetosphere is a very dynamic region.

Remanent ferromagnetism and the interior structure of Ganymede

The Galileo spacecraft discovered that Jupiter's moon, Ganymede, has both an internal magnetic field and a strongly differentiated interior. This has prompted new models of Ganymede's interior, featuring an iron core and dynamo action to produce the magnetic field. However, extant dynamo action is not necessary to produce Ganymede's magnetic fields. The rocky material within Ganymede is believed to be rich in iron‐bearing minerals, particularly magnetite, and its outer layer is capable of holding strong remanent magnetism. We investigate two sources of remanent ferromagnetism: Jupiter's magnetic field, especially in the past, when Ganymede was closer to Jupiter due to tidal evolution, and a paleomagnetic field from dynamo action which is no longer active. In this analysis we use a model of remanent magnetization similar to those which have been applied to the Moon, Mars, and Mercury, but we have generalized it to apply to strongly magnetic materials. We find that magnetization by the jovian background field can account for no more than 5.6% of Ganymede's internal field. However, a paleomagnetic field of roughly 10,000 nT or greater (at least 15 times the current field of Ganymede and a third that of the Earth's current field) is capable of producing sufficient remanent magnetization to account for the Galileo observations, given very favorable assumptions about the magnetic properties of the rock layer. For more probable values a paleomagnetic field equal to or greater than the Earth's current field is necessary.

Formation of Ganymede Grooved Terrain by Sequential Extensional Episodes: Implications of Galileo Observations for Regional Stratigraphy

High-resolution Galileo images of grooved terrain on Ganymede have revealed details of the morphology of grooves and their stratigraphic relationships. Based on lower resolution Voyager images, complex areas of grooved terrain in Uruk Sulcus were interpreted in the context of structurally isolated grooved polygons which were resurfaced by cryovolcanism and individually deformed by local processes. In Galileo images of grooved polygon boundaries, the faults comprising the groove lanes between the polygons are observed to truncate features within the polygons, and incipient fractures from these groove lanes modify the edges of the polygons, indicating that these groove lanes are younger than the polygons. In the Uruk Sulcus region, tectonic resurfacing by the formation of younger sets of grooves appears to be obscuring older patterns of deformation. The grooved polygons are thus remnants of terrain affected by older episodes of deformation isolated from each other by areas of more recent deformation. The stratigraphy of grooves can be determined for small areas covered by high-resolution Galileo images (∼100 m/pixel) and then generalized to larger areas imaged by Voyager at lower resolution (∼1 km/pixel). This method has been applied to the regions around Uruk Sulcus and Nippur Sulcus, resulting in two preliminary conclusions about the nature of grooved terrain deformation: (1) the style of deformation has changed through time and (2) the orientation of least compressive stress has changed through time. Throughout the mapped region, the stratigraphically oldest grooves are closely spaced, subdued structures which indicate least compressive stress oriented NW–SE, while the youngest grooves exhibit morphology consistent with tilt block normal faulting, with least compressive stress oriented NE–SW. These observations are consistent over an area of 1.6 million square kilometers and therefore do not favor highly localized or low-strain mechanisms for grooved terrain formation, such as the cooling of cryovolcanic flows or the surface expression of diapirs. Instead, the formation of grooved terrain on Ganymede may be dominated by global expansion due to tidal heating and/or differentiation, possibly organized into coherent regional patterns by transient low-order convection or changes in the satellite's figure.

Plasma injection near Io

Plasma and field observations obtained by the Galileo spacecraft during its first close encounter with Io indicate a much stronger electrodynamic interaction than anticipated. The strength of this interaction can be explained if one assumes that a significant fraction of the Io torus plasma is ionized in the immediate vicinity of the satellite. We adopt a simple model to calculate the strength and pattern of the flow perturbation resulting from a localized equatorial ionization source with dimensions much less than the radial thickness of the torus itself. Comparison with Galileo observations suggests that, when due allowance is made for the oversimplifications in the model, the observations are consistent with the hypothesis that the torus mass loading rate is highly localized near Io, and that Galileo encountered this region of concentrated mass loading at its closest approach altitude of about 0.5 Io radius. This hypothesis may also help explain two otherwise puzzling properties of the torus that were known before the Galileo encounter, the low ion temperature (relative to corotation energy) and the existence of a bright, radially confined “ribbon” feature.

Energetic particle bursts in the predawn Jovian magnetotail

From September to October 1996 the Galileo spacecraft crossed through the distant predawn tail region of the Jovian magnetosphere. The Energetic Particles Detector (EPD) onboard Galileo recorded a series of energetic particle flow bursts in the region beyond 80 RJ to the apojove at 113 RJ. The events are similar in nature to an event observed with the hot plasma instrument (LECP) onboard Voyager 2. The individual events last for several hours and cover the whole energy range from 15 keV to 55 MeV. The majority of them show considerable intensity increases which are most prominent for heavy ions. The events exhibit high radially outward directed anisotropies suggesting strongly collimated radial outflowing ion beams. The Voyager event was observed beyond the corotation boundary within a magnetospheric boundary layer termed the magnetospheric wind region and consequently it was assumed that the underlying process is connected with a boundary layer instability. However, the Galileo observations show the bursts being embedded in a general corotation flow. It is thus suggested that the flow bursts are driven by an internal plasma sheet process possibly associated with a major re‐configuration of the Jovian magnetotail. A series of five very prominent flow bursts observed near apojove of the orbit occurred quasi‐periodically with a repetition period of about 2.6±0.2 days which is presumably related to an internal time constant of the Jovian magnetotail.

Shoemaker-Levy 9: distribution of radiant energy

Multicolor Galileo observations of several SL9 impacts permit derivation of size and temperature information for the fireball phase. Integration of lightcurves yields total fireball radiant energy estimates. Event Q1 has an initial color temperature of 24 000 K. Total radiant fireball energy for G was 8.3 × 10 24 erg. The largest event, L, is estimated to be 10 25 erg. Modelling of total energy radiated during the fallback phase shows a much larger energy release during that period—up to 4 × 10 26 erg for L. The mass required to provide the equivalent kinetic energy is about 1 × 10 13 g, which is a strong lower limit to initial fragment mass. The fallback energy derived for the R impact is 1 × 10 25 erg, with the strongest spectral constraints. Fallback energies for H and Q1 are about 4 × 10 25 erg, derived from only two spectral points. The fallback energies extrapolated from the 2.3 μm lightcurves of the G and K impacts are 7 × 10 25 and 9 × 10 25 erg, respectively. Uncertainties of the fallback energies are up to an order of magnitude. Improvements to these estimates and their uncertainties will derive from the application of more detailed physical models.