Upper Atmosphere Of Jupiter Research Articles

Jupiter Thermospheric General Circulation Model (JTGCM): Global structure and dynamics driven by auroral and Joule heating

A growing multispectral database plus recent Galileo descent measurements are being used to construct a self‐consistent picture of the Jupiter thermosphere/ionosphere system. The proper characterization of Jupiter's upper atmosphere, embedded ionosphere, and auroral features requires the examination of underlying processes, including the feedbacks of energetics, neutral‐ion dynamics, composition, and magnetospheric coupling. A fully 3‐D Jupiter Thermospheric General Circulation Model (JTGCM) has been developed and exercised to address global temperatures, three‐component neutral winds, and neutral‐ion species distributions. The domain of this JTGCM extends from 20‐μbar (capturing hydrocarbon cooling) to 1.0 × 10−4 nbar (including auroral/Joule heating processes). The resulting JTGCM has been fully spun‐up and integrated for ≥40 Jupiter rotations. Results from three JTGCM cases incorporating moderate auroral heating, ion drag, and moderate to strong Joule heating processes are presented. The neutral horizontal winds at ionospheric heights vary from 0.5 km/s to 1.2 km/s, atomic hydrogen is transported equatorward, and auroral exospheric temperatures range from ∼1200–1300 K to above 3000 K, depending on the magnitude of Joule heating. The equatorial temperature profiles from the JTGCM are compared with the measured temperature structure from the Galileo ASI data set. The best fit to the Galileo data implies that the major energy source for maintaining the equatorial temperatures is due to dynamical heating induced by the low‐latitude convergence of the high‐latitude‐driven thermospheric circulation. Overall, the Jupiter thermosphere/ionosphere system is highly variable and is shown to be strongly dependent on magnetospheric coupling which regulates Joule heating.

Giant Planet Ionospheres and Thermospheres: The Importance of Ion-Neutral Coupling

Planetary upper atmospheres — coexisting thermospheres and ionospheres — form an important boundary between the planet itself and interplanetary space. The solar wind and radiation from the Sun may react with the upper atmosphere directly, as in the case of Venus. If the planet has a magnetic field, however, such interactions are mediated by the magnetosphere, as in the case of the Earth. All of the Solar System’s giant planets have magnetic fields of various strengths, and interactions with their space environments are thus mediated by their respective magnetospheres. This article concentrates on the consequences of magnetosphere-atmosphere interactions for the physical conditions of the thermosphere and ionosphere. In particular, we wish to highlight important new considerations concerning the energy balance in the upper atmosphere of Jupiter and Saturn, and the role that coupling between the ionosphere and thermosphere may play in establishing and regulating energy flows and temperatures there. This article also compares the auroral activity of Earth, Jupiter, Saturn and Uranus. The Earth’s behaviour is controlled, externally, by the solar wind. But Jupiter’s is determined by the co-rotation or otherwise of the equatorial plasmasheet, which is internal to the planet’s magnetosphere. Despite being rapid rotators, like Jupiter, Saturn and Uranus appear to have auroral emissions that are mainly under solar (wind) control. For Jupiter and Saturn, it is shown that Joule heating and “frictional” effects, due to ion-neutral coupling can produce large amounts of energy that may account for their high exospheric temperatures.

Constraints on Jupiter's hydrogen corona from Galileo UVS observations

During Galileo orbits C10 and E11, the Ultraviolet Spectrometer (UVS) obtained H Lyα scans at low latitudes across the disk of Jupiter from a phase angle of 90–109° and a distance of 18–21 jovian radii. Jupiter's nightside Lyα brightness was about 0.5kR, approximately half as bright as interplanetary space. From the nightside to the dayside, the Lyα brightness rose to a peak of ∼15kR near the sub-solar limb and then dropped sharply to interplanetary brightness levels. On orbit C10 three disk scans were obtained (near 30°N latitude, 20°S latitude, and along the equator), while a single equatorial scan was made on orbit E11. The steep falloff in Lyα brightness at the sub-solar point provides a strong constraint on the distribution of atomic hydrogen in the upper atmosphere. All disk scans are reasonably well fit by a resonance line radiative transfer model which assumes a total H column density of ∼1017cm−2 and a temperature profile consistent with the Galileo probe results with an exospheric temperature of 940K. However, we find that the inclusion of a small amount of “hot” hydrogen, with an abundance described by a Chapman profile, can substantially improve the fit to the UVS limb scan data. Exactly such a hot hydrogen component is expected in the upper atmosphere of Jupiter, and has been previously invoked to explain observations of the jovian Lyα line profile made from Earth orbit. The best fits to the UVS data occur with a hot H column density that is 0.1% of the ambient H column, with an effective temperature of 25,000K and a topside scale height of 1000km (in our empirical representation, the hot H scale height is unrelated to the hot H temperature). Other combinations of column abundance and the effective temperature can also provide good fits to the data. As long as the hot H is located above the homopause, there is no strong dependence on the height of the Chapman profile.

Cassini-VIMS at Jupiter: solar occultation measurements using Io

We report unusual and somewhat unexpected observations of the jovian satellite Io, showing strong methane absorption bands. These observations were made by the Cassini VIMS experiment during the Jupiter flyby of December/January 2000/2001. The explanation is straightforward: Entering or exiting from Jupiter's shadow during an eclipse, Io is illuminated by solar light which has transited the atmosphere of Jupiter. This light, therefore becomes imprinted with the spectral signature of Jupiter's upper atmosphere, which includes strong atmospheric methane absorption bands. Intercepting solar light refracted by the jovian atmosphere, Io essentially becomes a “mirror” for solar occultation events of Jupiter. The thickness of the layer where refracted solar light is observed is so large (more than 3000 km at Io's orbit), that we can foresee a nearly continuous multi-year period of similar events at Saturn, utilizing the large and bright ring system. During Cassini's 4-year nominal mission, this probing technique should reveal information of Saturn's atmosphere over a large range of southern latitudes and times.

Energetic neutral atoms from a trans-Europa gas torus at Jupiter.

The space environments--or magnetospheres--of magnetized planets emit copious quantities of energetic neutral atoms (ENAs) at energies between tens of electron volts to hundreds of kiloelectron volts (keV). These energetic atoms result from charge exchange between magnetically trapped energetic ions and cold neutral atoms, and they carry significant amounts of energy and mass from the magnetospheres. Imaging their distribution allows us to investigate the structure of planetary magnetospheres. Here we report the analysis of 50-80 keV ENA images of Jupiter's magnetosphere, where two distinct emission regions dominate: the upper atmosphere of Jupiter itself, and a torus of emission residing just outside the orbit of Jupiter's satellite Europa. The trans-Europa component shows that, unexpectedly, Europa generates a gas cloud comparable in gas content to that associated with the volcanic moon Io. The quantity of gas found indicates that Europa has a much greater impact than hitherto believed on the structure of, and the energy flow within, Jupiter's magnetosphere.

Carbon Monoxide on Jupiter: Evidence for Both Internal and External Sources

Thirteen lines of the CO band near 4.7 μm have been observed on a jovian hot spot at a resolution of 0.045 cm −1. The measured line profiles indicate that the CO mole fraction is 1.0±0.2 ppb around the 6-bar level and is larger in the upper troposphere and/or stratosphere. An external source of CO providing an abundance of 4 +3 −2×10 16 molecules cm −2 is implied by the observations in addition to the amount deposited at high altitude by the Shoemaker–Levy 9 collision. From a simple diffusion model, we estimate that the CO production rate is (1.5–10)×10 6 molecules cm −2 s −1 assuming an eddy diffusion coefficient around the tropopause between 300 and 1500 cm 2 s −1. Precipitation of oxygen atoms from the jovian magnetosphere or photochemistry of water vapor from meteoroidal material can only provide a negligible contribution to this amount. A significant fraction of the CO in Jupiter's upper atmosphere may be formed by shock chemistry due to the infall of kilometer- to subkilometer-size Jupiter family comets. Using the impact rate from Levison et al. (2000, Icarus 143, 415–420) rescaled by Bottke et al. (2002, Icarus 156, 399–433), this source can provide the observed stratospheric CO only if the eddy diffusion coefficient around the tropopause is 100–300 cm 2 s −1. Higher values, ∼700 cm 2 s −1, would require an impact rate larger by a factor of 5–10, which cannot be excluded considering uncertainties in the distribution of Jupiter family comets. Such a large rate is indeed consistent with the observed cratering record of the Galilean satellites (Zahnle et al. 1998, Icarus 136, 202–222). On the other hand, the ∼1 ppb concentration in the lower troposphere requires an internal source. Revisiting the disequilibrium chemistry of CO in Jupiter, we conclude that rapid vertical mixing can provide the required amount of CO at ∼6 bar for a global oxygen abundance of 0.2–9 times the solar value considering the uncertainties in the convective mixing rate and in the chemical constants.

Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter.

Io leaves a magnetic footprint on Jupiter's upper atmosphere that appears as a spot of ultraviolet emission that remains fixed underneath Io as Jupiter rotates. The specific physical mechanisms responsible for generating those emissions are not well understood, but in general the spot seems to arise because of an electromagnetic interaction between Jupiter's magnetic field and the plasma surrounding Io, driving currents of around 1 million amperes down through Jupiter's ionosphere. The other galilean satellites may also leave footprints, and the presence or absence of such footprints should illuminate the underlying physical mechanism by revealing the strengths of the currents linking the satellites to Jupiter. Here we report persistent, faint, far-ultraviolet emission from the jovian footprints of Ganymede and Europa. We also show that Io's magnetic footprint extends well beyond the immediate vicinity of Io's flux-tube interaction with Jupiter, and much farther than predicted theoretically; the emission persists for several hours downstream. We infer from these data that Ganymede and Europa have persistent interactions with Jupiter's magnetic field despite their thin atmospheres.

Interaction of Gravity Waves with Ionospheric Plasma: Implications for Jupiter's Ionosphere

The J0-ingress radio occultation of the Galileo orbiter by Jupiter exhibits a system of well-defined, regularly spaced electron layers in the altitude range where the presence of gravity waves have been previously inferred. Based on the terrestrial analog of sporadic E and spread F ionospheric layers, we argue that the observed layers are a result of dynamical processes rather than chemistry. We consider the impact of upward propagating gravity waves on the plasma distribution in a H + dominated ionosphere. The relevant physics is discussed and illustrated with an analytic, small-amplitude model. A time-dependent, 2D, large-amplitude model is developed to simulate the observed large excursions in the J0-electron density profile. We show that gravity waves with parameters consistent with the thermal structure of Jupiter's upper atmosphere are capable of creating large peaks in the electron density similar to the observed ones. The ionospheric response is extremely anisotropic with respect to the direction of wave propagation. We demonstrate that the location of the J0-ingress radio occultation on Jupiter favors large ionospheric response for waves propagating along the magnetic meridian. A wave driven plasma flux results in plasma removal above the altitude of maximum ionospheric response and plasma deposition in the region below, significantly modifying the initial steady state electron density profile.

A self‐consistent model of the Jovian auroral thermal structure

A one‐dimensional (1‐D) model coupling a two‐stream electron transport model of energy deposition with a 1‐D thermal conduction model has been developed. It is applied to investigate the links between auroral heat input and the vertical temperature of Jupiter's upper atmosphere. Two energy distributions meant to reproduce the emissions of a diffuse and a discrete aurora are used to evaluate the importance of the energy spectrum of the incident electrons for the thermal balance of Jupiter's auroral thermosphere. The values of observable quantities such as the altitude of the H2 emission peak, thermal infrared (IR), ultraviolet (UV) emissions, and temperatures associated with various optical signatures are used to constrain the parameters of these distributions. It is shown that the high‐energy component of these energy distributions heats a region of the homosphere between 10−4 and 10−6 bar and mainly controls the H2 temperature and the far‐UV (FUV) emission. A 3‐keV soft electron component is necessary to heat the region directly above the homopause, between 10−6 and 10−9 bar. It has a large influence on the H2 and H3+ temperatures and on the H3+ near‐IR (NIR) emission. It is used in conjunction with a weak 100 eV component which is responsible for heating the thermosphere, from 10−9 to 10−12 bar and exerts a control on the exospheric temperature. The calculated temperatures, UV, and IR emissions suggest that the model probably misses a nonparticle heat source in the 10−5 bar region, that is expected to balance the strong hydrocarbon cooling. Sensitivity tests are performed to evaluate the importance of the parameters of the energy distributions. They show that the FUV color ratio increases with the characteristic energy (or high‐energy cutoff) of the high‐energy component, while the H2 rovibrational temperature varies inversely. A trade‐off is therefore necessary for these two parameters to simultaneously meet their observational constraints. Further tests demonstrate the essential thermostatic role played by H3+, which regulates the net heating in the thermosphere. An increased eddy diffusion reproduces the effect of a possible auroral upwelling of methane but gives rise to an H2 temperature smaller than the observed value.

Enhancement of Deuterated Ethane on Jupiter

We report laboratory measurements of cross sections of CH3D and C2H5D in the extreme ultraviolet. The results are incorporated in a photochemical model for the deuterated hydrocarbons up to C2 in the upper atmosphere of Jupiter, taking into account the fast reactions for exchanging H and D atoms between H2 and CH4, H + HD ↔ D + H2, CH3 + D ↔ CH2D + H. Since there is no reliable kinetics measurement for the reaction, CH2D + H → CH3 + D, we use Yung et al.'s estimate for its rate constant. The strong temperature dependence for this reaction leads to large isotopic fractionation for CH3D and C2H5D in the upper atmosphere of Jupiter, where their production rates depend on the abundance of deuterated methyl radical. The model predicts that the D/H ratio in deuterated ethane is about 15 times that of the bulk atmosphere. A confirmation of this result would provide a sensitive test of the photochemistry of hydrocarbons in the atmosphere of Jupiter.

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.

Observation of short‐term variation of Jupiter's synchrotron radiation

Detailed observations of Jovian decimetric radiation (DIM) from relativistic electrons in the inner Jovian radiation belt was carried out to seek the existence of a short‐term variation which has information on the electromagnetic disturbances in the inner Jovian magnetosphere. Results demonstrated the DIM flux enhancement continuing for a few days for the case of November 1996. It was shown that the relativistic electrons were highly anisotropic during the DIM flux enhancement. The correlation between the DIM flux enhancement and the solar F10.7 flux enhancement was also detected. A model simulation suggested that the observed DIM flux variation was caused by the enhanced radial diffusion due to the intense solar UV/EUV event which heated the Jupiter's upper atmosphere.

The Galileo Probe Doppler Wind Experiment: Measurement of the deep zonal winds on Jupiter

During its descent into the upper atmosphere of Jupiter, the Galileo probe transmitted data to the orbiter for 57.5 min. Accurate measurements of the probe radio frequency, driven by an ultrastable oscillator, allowed an accurate time history of the probe motions to be reconstructed. Removal from the probe radio frequency profile of known Doppler contributions, including the orbiter trajectory, the probe descent velocity, and the rotation of Jupiter, left a measurable frequency residual due to Jupiter's zonal winds, and microdynamical motion of the probe from spin, swing under the parachute, atmospheric turbulence, and aerodynamic buffeting. From the assumption of the dominance of the zonal horizontal winds, the frequency residuals were inverted and resulted in the first in situ measurements of the vertical profile of Jupiter's deep zonal winds. A number of error sources with the capability of corrupting the frequency measurements or the interpretation of the frequency residuals were considered using reasonable assumptions and calibrations from prelaunch and in‐flight testing. It is found that beneath the cloud tops (about 700 mbar) the winds are prograde and rise rapidly to 170 m/s at 4 bars. Beyond 4 bars to the depth at which the link with the probe was lost, nearly 21 bars, the winds remain constant and strong. Corrections for the high temperatures encountered by the probe have recently been completed and provide no evidence of diminishing or strengthening of the zonal wind profile in the deeper regions explored by the Galileo probe.

Secular and local time dependence of Jovian X ray emissions

Jovian soft X rays are believed to be line emissions from energetic oxygen and sulfur ions generated through charge exchange and stripping reactions when the ions enter Jupiter's upper atmosphere. The X rays are brightest in the polar regions (generally about twice as bright in the north as in the south) but also have a large component from low latitudes. Observations of Jupiter's X ray emissions at several epochs by the high‐resolution imager (HRI) instrument on the ROSAT satellite show a general decrease in overall brightness by ∼50% from 1994 to 1996 and reveal an interesting dependence of brightness on local time. The emissions are not of uniform brightness across the disk but are strongly weighted toward one limb of the planet. The X ray limb can be on either the duskside or dawnside of Jupiter, and so far it correlates with position of the subsolar point relative to the sub‐Earth point (i.e., the X ray limb is the bright visible limb). Except during the Shoemaker‐Levy 9 impacts, the total X ray emissions are not strongly correlated with System III central meridian longitude. However, a composite map of the emissions shows that they are generally located in regions of low surface magnetic field strength at low latitudes, and in regions of large horizontal surface magnetic field strength gradients at high latitudes.

Thermal Structure of Jupiter's Upper Atmosphere Derived from the Galileo Probe

Temperatures in Jupiter's atmosphere derived from Galileo Probe deceleration data increase from 109 kelvin at the 175-millibar level to 900 ± 40 kelvin at 1 nanobar, consistent with Voyager remote sensing data. Wavelike oscillations are present at all levels. Vertical wavelengths are 10 to 25 kilometers in the deep isothermal layer, which extends from 12 to 0.003 millibars. Above the 0.003-millibar level, only 90- to 270- kilometer vertical wavelengths survive, suggesting dissipation of wave energy as the probable source of upper atmosphere heating.

Evidence for supersonic turbulence in the upper atmosphere of Jupiter.

Spectra of the hydrogen Lyman alpha (Ly-alpha) emission line profiles of the jovian dayglow, obtained by the Goddard High Resolution Spectrograph on the Hubble Space Telescope, appear complex and variable on time scales of a few minutes. Dramatic changes occur in the Ly-alpha bulge region at low latitudes, where the line profiles exhibit structures that correspond to supersonic velocities of the order of several to tens of kilometers per second. This behavior, unexpected in a planetary atmosphere, is evidence for the particularly stormy jovian upper atmosphere, not unlike a star's atmosphere.

Observational constraints on the composition and nature of comet D/Shoemaker-Levy 9

What did the break-up of comet Shoemaker-Levy 9 (SL9) and its subsequent impact on Jupiter teach us about the nature and constitution of this comet? The break-up of the comet apparently triggered activity of the fragments. Although a dust coma was continuously present around the fragments that orbited Jupiter, spectroscopic observations did not reveal any sign of gas. The impact itself was so energetic that most molecules of the impactor were dissociated and that any chemical memory was lost. Ultraviolet and visible spectroscopy of the impact sites revealed emission lines from several atoms, giving potential information on elemental abundances. However, the fact that both neutral and ionized atoms are emitting, and that both fundamental and inter-system lines are present, suggest that the medium is out-of-equilibrium and that emitting mechanisms other than simple resonance fluorescence are at work. Ultraviolet, infrared, and radio spectroscopy revealed lines of several molecular species, in emission and/or absorption, that are not normally present in Jupiter's upper atmosphere. In the visible, dark spots due to aerosols developed at the impact sites. It is not clear at the present time which part of this material is coming from preserved impactor material, from the recombination of the dissociated impactor material, from reactions between the impactor's and Jupiter's material, or from material coming from the lower layers of Jupiter's atmosphere. Realistic modelling of the impacts and of the following chemical reactions will be necessary to address all these issues. In this chapter, we will review the observational clues to the composition and structure of SL9 in the context of our knowledge of the composition and structure of comets and asteroids.

Changing weather report on Jupiter

With two months to further refine their data sets according to corrections in instrument calibrations, principal investigators for the Galileo probe of Jupiter are revising their report on the chemistry and weather of the gas giant.The composition of Jupiter's upper atmosphere is more Sun‐like than scientists suggested at a January presentation of probe results (Eos, January 30, 1996). A more comprehensive analysis of data from the helium abundance detector—as well as correlation with the neutral mass spectrometer—reveals that the percentage of helium in the Jovian atmosphere is roughly 24%. That figure is significantly higher than the 13.7% announced in January and is quite close to the composition of the Sun.

Hydrocarbon Photochemistry in the Upper Atmosphere of Jupiter

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2species are replaced in importance by cycles involving C–C4species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 Â and H at 1216 Â are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3and higher hydrocarbons in the atmospheres of the jovian planets.

Structure of Jupiter's upper atmosphere: Predictions for Galileo

The Voyager mission to the outer solar system discovered that the thermospheres of all the giant planets are remarkably hot. To date, no convincing explanation for this phenomenon has been offered; however, there are a number of recent observational results which provide new information on the thermal structure of Jupiter's upper atmosphere that bear on this outstanding problem. We present an analysis of Jupiter's thermal structure using constraints from H3+ emissions, Voyager UVS occultation data, ground‐based stellar occultation data, and the properties of the Jovian UV dayglow. Although the initial, separate analysis of these data sets produced contradictory results, our reanalysis shows that the observations are consistent and that the temperature profile in Jupiter's upper atmosphere is well constrained. We find that the data demand the presence of a large temperature gradient, of order 3–10 K/km, near a pressure of 0.3 μbar. Analysis of the temperature profile implies that an energy source of roughly 1 erg cm−2 s−1 is required to produce the high thermospheric temperature and that this energy must be deposited in the 0.1–1.0 μbar region. It is also necessary that this energy be deposited above the region where diffusive separation of CH4 occurs, so that the energy is not radiated away by CH4. We show that dissipation of gravity waves can supply the energy required and that this energy will be deposited in the proper region. Moreover, because the turbulent mixing caused by gravity waves determines the level at which diffusive separation of CH4 occurs, the location of the energy source (dissipation of waves) and the energy sink (radiation by CH4) are coupled. We show that the gravity waves will deposit their energy several scale heights above the CH4 layer; energy is carried downward by thermal conduction in the intervening region, causing the large temperature gradient. Thus dissipation of gravity waves appears to be a likely explanation for the high thermospheric temperature. Our arguments are general and should apply to Saturn, Uranus, and Neptune, as well as Jupiter. The model temperature profiles presented here and the relationship between the gravity wave flux and thermospheric temperature are directly testable by the Atmospheric Structure Instrument carried by the Galileo probe.