Category Archives: Satellites of Jupiter

Resurfacing Ganymede

Hi there! After Europa last week, I tell you today on the next Galilean satellite, which is Ganymede. It is the largest planetary satellite in the Solar System, and it presents an interesting surface, i.e. with different terrains showing evidence of past activity. This is the opportunity for me to present you Viscous relaxation as a prerequisite for tectonic resurfacing on Ganymede: Insights from numerical models of lithospheric extension, by Michael T. Bland and William B. McKinnon. This study has recently been accepted for publication in Icarus.

The satellite Ganymede

Ganymede is the third, by its distance to the planet, of the 4 Galilean satellites of Jupiter. It was discovered with the 3 other ones in January 1610 by Galileo Galilei. These are indeed large bodies, which means that they could host planetary activity. Io is known for its volcanoes, and Europa and Ganymede (maybe Callisto as well) are thought to harbour a global, subsurfacic ocean. The table below lists their size and orbital properties, which you can compare with the 5th satellite, Amalthea.

Semimajor axis Eccentricity Inclination Radius
J-1 Io 5.90 Rj 0.0041 0.036° 1821.6 km
J-2 Europa 9.39 Rj 0.0094 0.466° 1560.8 km
J-3 Ganymede 14.97 Rj 0.0013 0.177° 2631.2 km
J-4 Callisto 26.33 Rj 0.0074 0.192° 2410.3 km
J-5 Amalthea 2.54 Rj 0.0032 0.380° 83.45 km

We have images of the surface of Ganymede thanks to the spacecraft Voyager 1 & 2, and Galileo. These missions have revealed different types of terrains, darker and bright, some impacted, some pretty smooth, some showing grooves… “pretty smooth” should be taken with care, since the feeling of smoothness depends on the resolution of the images, which itself depends on the distance between the spacecraft and the surface, when this specific surface element was directed to the spacecraft.

Dark terrain in Galileo Regio. © NASA
Dark terrain in Galileo Regio. © NASA
Bright terrain with grooves and a crater. © NASA
Bright terrain with grooves and a crater. © NASA

A good way to date a terrain is to count the craters. It appears that the dark terrains are probably older than the bright ones, which means that a process renewed the surface. The question this paper addresses is: which one(s)?

Marius Regio and Nippur Sulcus. © NASA
Marius Regio and Nippur Sulcus. © NASA

Resurfacing a terrain

These four mechanisms permit to renew a terrain from inside:

  • Band formation: The lithosphere, i.e. the surface, is fractured, and material from inside takes its place. This phenomenon is widely present on Europa, and probably exists on Ganymede.
  • Viscoelastic relaxation: When the crust has some elasticity, it naturally smooths. As a consequence, craters tend to disappear. Of course, this phenomenon is a long-term process. It requires the material to be hot enough.
  • Cryovolcanism: It is like volcanism, but with the difference that the ejected material is mainly composed of water, instead of molten rock. Part of the ejected material falls on the surface.
  • Tectonics: Extensional of compressional deformations of the lithosphere. This is the phenomenon, which is studied here.

Beside these processes, I did not mention the impacts on the surface, and the erosion, which is expected to be negligible on Ganymede.

The question the authors addressed is: could tectonic resurfacing be responsible for some of the actually observed terrains on Ganymede?

Numerical simulations

To answer this question, the authors used the numerical tool, more precisely the 2-D code Tekton. 2-D means that the deformations below the surface are not explicitly simulated. Tekton is a viscoelastic-plastic finite element code, which means that the surface is divided into small areas (finite elements), and their locations are simulated with respect to the time, under the influence of a deforming cause, here an extensional deformation.

The authors used two kinds of data, that we would call initial conditions for numerical simulations: simulated terrains, and real ones.
The simulated terrains are fictitious topographies, varying by the amplitude and frequency of deformation. The deformations are seen as waves, the wavelength being the distance between two peaks. A smooth terrain can be described by long-wavelength topography, while a rough one will have short wavelength.
The real terrains are Digital Terrain Models, extracted from spacecraft data.

The authors also considered different properties of the material, like the elasticity, or the cohesion.

A new scenario of resurfacing

It results from the simulations that the authors can reproduce smooth terrains with grooves, starting from already smooth terrains without grooves. However, extensional tectonics alone cannot remove the craters. In other words, if you can identify craters at the surface of Ganymede, after millions of years of extensional tectonics you will still observe them. To make smooth terrains, you need the assistance of another process, the viscoelastic relaxation of the lithosphere being an interesting candidate.

This pushed the authors to elaborate a new scenario of resurfacing of Ganymede, involving different processes.
They consider that the dark terrains are actually the eldest ones, having remaining intact. However, there was indeed tectonic resurfacism of the bright terrains, which formed grooved. But the deformation of the lithosphere was accompanied by an elevation of the temperature (which is not simulated by Tekton), which itself made the terrain more elastic. This elasticity itself relaxed the craters.

Anyway, you need elasticity (viscoelasticity is actually more accurate, since you have energy dissipation), and for that you need an elevation of the local temperature. This may have been assisted by heating due to internal processes.

In the future

Ganymede is the main target of the ESA mission JUICE, which should orbit it 2030. We expect a big step in our knowledge of Ganymede. For this specific problem, we will have a much better resolution of the whole surface, the gravity field of the body (which is related to the interior), maybe a magnetic field, which would constrain the subsurface ocean and the depth of the crust enshrouding it, and the Love number, which indicates the deformation of the gravity field by the tidal excitation of Jupiter. This last quantity contains information on the interior, but it is related to the whole body, not specifically to the structure. I doubt that we would have an accurate knowledge of the viscoelasticity of the crust. Moreover, the material properties which created the current terrains may be not the current ones; in particular the temperature of Ganymede is likely to have varied over the ages. We know for example that this temperature is partly due to the decay of radiogenic elements shortly after the formation of the satellite. During this heating, the satellite stratifies, which alters the tidal response to the gravitational excitation of Jupiter, and which itself heats the satellite. This tidal response is also affected by the obliquity of Ganymede, by its eccentricity, which is now damped… So, the temperature is neither constant, nor homogeneous. There will still be room for theoretical studies and new models.

The study and its authors

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Plate tectonics on Europa?

Hi there! Jupiter has 4 large satellites, known as Galilean satellites since they were discovered by Galileo Galilei in 1610. Among them is Europa, which ocean is a priority target for the search for extraterrestrial life. Many clues have given us the certainty that this satellite has a global ocean under its icy surface, and it should be the target of a future NASA mission, Europa Clipper. Meanwhile it will also be visited by the European mission JUICE, before orbital insertion around Ganymede. Since Europa presents evidences of tectonic activity, the study I present you today, i.e. Porosity and salt content determine if subduction can occur in Europa’s ice shell, by Brandon Johnson et al., wonders whether subduction is possible when two plates meet. This study has been conducted at Brown University, Providence, RI (USA).

Subduction on Earth

I guess you know about place tectonics on Earth. The crust of the Earth is made of several blocks, which drift. As a consequence, they collide, and this may be responsible for the creation of mountains, for earthquakes… Subduction is a peculiar kind of collision, in which one plate goes under the one it meets, just because their densities are significantly different. The lighter plate goes up, while the heavier one goes down. This is what happens on the west coast of South America, where the subduction of the oceanic Nazca Plate and the Antarctic Plate have created the Andean mountains on the South America plate, which is a continental one.

Even if our Earth is unique in the Solar System by many aspects, it is highly tempting to use our knowledge of it to try to understand the other bodies. This is why the authors simulated the conditions favorable to subduction on Europa.

The satellite Europa

Europa is the smallest of the four Galilean satellites of Jupiter. It orbits Jupiter in 3.55 days at a mean distance of 670,000 km, on an almost circular and planar orbit. It has been visited by the spacecraft Pioneer 10 & 11 in 1973-1974, then by Voyager 1 & 2 in 1979. But our knowledge of Europa is mostly due to the spacecraft Galileo, which orbited Jupiter between 1995 and 2003. It revealed long, linear cracks and ridges, interrupted by disrupted terrains. The presence of these structures indicates a weakness of the surface, and argues for the presence of a subsurface ocean below the icy crust. Another argument is the tidal heating of Jupiter, which means that Europa should be hot enough to sustain this ocean.
This active surface shows extensional tectonic feature, which suggests plate motion, and raises the question: is subduction possible?

Numerical simulations of the phenomenon

To determine whether subduction is possible, the authors performed one-dimensional finite-elements simulations of the evolution of a subducted slab, to determine whether it would remain below another plate or not. The equation is: would the ocean be buoyant? If yes, then the slab cannot subduct, because it would be too light for that.

The author considered the time and spatial evolution of the slab, i.e. over its length and over the ages. They tested the effect of

  1. The porosity: Planetary ices are porous material, but we do not know to what extent. In particular, at some depth the material is more compressed, i.e. less porous than at the surface, but it is not easy to put numbers behind this phenomenon. Which means that the porosity is a parameter. The porosity is defined as a fraction of the volume of voids over the total volume investigated. Here, total volume should not be understood as the total volume of Europa, but as a volume of material enshrouding the material element you consider. This allows you to define a local porosity, which thus varies in Europa. Only the porosity of the icy crust is addressed here.
  2. The salt content: the subsurface ocean and the icy crust are not pure ice, but are salty, which affects their densities. The authors assumed that the salt of Europa is mostly natron, which is a mixture essentially made of sodium carbonate decahydrate and sodium bicarbonate. Importantly, the icy shell has probably some lateral density variations, i.e. the fraction of salt is probably not homogeneous, which gives room for local phenomenons.
  3. The crust thickness: barely constrained, it could be larger than 100 km.
  4. The viscosity: how does the material react to a subducting slab? This behavior depends on the temperature, which is modeled here with the Fourier law of heat,
  5. The spreading rate, i.e. the velocity of the phenomenon,
  6. The geometry of the slab, in particular the bending radius, and the dip angle.

And once you have modeled and simulated all this, the computer tells you under which conditions subduction is possible.

Yes, it is possible

The first result is that the two critical parameters are the porosity and the salt content, which means that the conditions for subduction can be expressed with respect to these two quantities.
Regarding the conditions for subduction, let me quote the abstract of the paper: If salt contents are laterally homogeneous, and Europa has a reasonable surface porosity of 0.1, the conductive portion of Europa’s shell must have salt contents exceeding ~22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of 0.1 and overall salt contents of ~5%.

A possible subduction does not mean that subduction happens. For that, you need a cause, which would trigger activity in the satellite.

Triggering the subduction

The authors propose the following two causes for subduction to happen:

  1. Tidal interaction with Jupiter, enhanced by non-synchronous rotation: Surface features revealed by Galileo are consistent with a crust which would not rotate synchronously, as expected for the natural satellites, but slightly faster, the departure from supersynchronicity inducing a full rotation with respect to the Jupiter-Europa direction between 12,000 and 250,000 years… to be compared with an orbital period of 3.55 days. So, this is a very small departure, which would enhance the tidal torque of Jupiter, and trigger some activity. This interpretation of the surface features as a super-synchronous rotation is controversial.
  2. Convection, i.e. fluid motion in the ocean, due to the variations of temperature.

No doubt Europa Clipper and maybe JUICE will tell us more!

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

The composition of Himalia, Elara, and Carme

Hi there! Today I tell you on 3 irregular satellites of Jupiter, you know, these small bodies which orbit very far from the planet. Himalia, Elara and Carme have been observed in the Near-InfraRed (NIR), and this gave Composition of Jupiter irregular satellites sheds light on their origin, by M. Bhatt et al., which has been recently accepted for publication in Astronomy and Astrophysics.

The irregular satellites of Jupiter

Jupiter has 69 known satellites, which we can divide into 3 groups:

  1. The 4 Galilean satellites Io, Europa, Ganymede and Callisto. These are large bodies, discovered in 1610 by Galileo Galilei,
  2. The 4 inner satellites Amalthea, Metis, Adrastea, and Thebe. These are small bodies, orbiting inside the orbit of Io,
  3. The irregular satellites, which orbit very far from Jupiter. These are small bodies as well, which are usually thought to have been captured, i.e. they probably not formed in the protojovian nebula.

Contrary to the inner and the Galilean satellites, the irregular satellites have pretty eccentric and inclined orbits. Their eccentricities may exceed 0.4, and most of them are retrograde, i.e. with an inclination larger than 90°. In fact, plotting their inclination vs. their semimajor axes reveals clustering.

Semimajor axes and inclinations of the irregular satellites of Jupiter. The inclinations are given with respect to the ecliptic.
Semimajor axes and inclinations of the irregular satellites of Jupiter. The inclinations are given with respect to the ecliptic.

At least 4 dynamical groups have been defined, all of them being named after the largest of their members:

  1. The Himalia group is made of prograde bodies, with inclinations between 26.6° and 28.3°, eccentricities between 0.11 and 0.25, and semimajor axes between 159 and 176 Jupiter radii (while Callisto orbits at 27 Jupiter radii),
  2. The Ananke group is composed of bodies with inclinations between 145.7° and 154.8°, eccentricities between 0.02 and 0.28, and semimajor axes between 250 and 305 Jupiter radii,
  3. The Pasiphase group is made of bodies with inclinations between 144.5° and 158.3°, eccentricities between 0.25 and 0.43, and semimajor axes between 320 and 350 Jupiter radii,
  4. The Carme group is made of bodies with inclinations between 164.9° and 165.5°, eccentricities between 0.23 and 0.27, and semimajor axes between 329 and 338 Jupiter radii

The clustering among these bodies suggests a common origin, i.e. a group of objects would have a unique progenitor. It is also interesting to notice that some groups are more dispersed than others. In particular, the dispersion of the Carme group is very limited. This could tell us something on the date of the disruption of the progenitor. Another clue regarding a common origin is the composition of these bodies.

Before addressing our 3 objects of interest, i.e. Himalia, Elara (member of the Himalia group), and Carme, I would like to mention Themisto and Carpo, which seem to be pretty isolated, and so would not share a common origin with the other bodies. Their dynamics might be affected by the Kozai-Lidov mechanism, which induces a correlated periodic evolution of their eccentrities and inclinations.

Himalia, Elara, and Carme

These 3 bodies are the ones addressed in this study. You can find below their relevant characteristics.

Semimajor axis Eccentricity Inclination Discovery Radius Albedo
Himalia 163.9 Rj 0.16 27.50° 1904 70-80 km 0.04
Elara 167.9 Rj 0.22 26.63° 1905 43 km 0.04
Carme 334.7 Rj 0.25 164.91° 1938 23 km 0.04

These were among the first known irregular moons of Jupiter. The inclinations are given with respect to the ecliptic, i.e. the orbital plane of the Earth. As a member of the Himalia group, Elara has similar dynamical properties with Himalia. We can also notice the small albedo of these bodies, i.e. of the order of 4%, which means that only 4% of the incident Solar light is reflected by the surface! In other words, these bodies are very dark, which itself suggests a carbonaceous composition. Spectroscopic observations permit to be more accurate.

Spectroscopic observations

These bodies were observed in the near infrared, at wavelengths between 0.8 and 5.5 μm. The observations were made at the IRTF (InfraRed Telescope Facility), located on the Mauna Kea (Hawai’i), with the SpeX spectrograph, during 4 nights, in 2012 and 2013. In measuring the light flux over a specific range of the spectrum, one can infer the presence of some material, which would absorb the light at a given wavelength. For that, we need to be accurate in the measurements, while the atmospheric conditions might alter them. This difficulty is by-passed by the presence of a star in the field, which serves as a reference for the measured light flux.

Detection of minerals

Once a spectrum reflectance vs. wavelength is obtained, it needs to be interpreted. In this study, the authors assumed that the observed spectra were a mixture of the spectra given by different minerals, which have been obtained in laboratories. They disposed of a database of 30 minerals, and fitted mixtures involving 4 of them, to the obtained spectra. This is an optimization algorithm, here named Spectral Mixture Analysis, which fits the relative proportion of the minerals. 4 minerals is actually the best they could obtain, i.e. they failed to produce a significantly better fit in adding a 5th mineral.

In other words, from the absorption spectrum of such a body, you can guess its 4 main components… at least of the surface.

Himalia and Elara are alike, Carme is different

Well, the title contains the conclusion. This is not very surprising, since Himalia and Elara belong to the same group. We can say that the composition confirms the guess that they should have a common origin. Previous studies gave the same conclusions.

In this specific case, Himalia and Elara have a peak of absorption centered around 1.2 μm, and their spectra are similar to C-type, i.e. carbonaceous, asteroids (52) Europa and (24) Themis, of the outer asteroid belt. The best match for Himalia is obtained with a mixture of magnetite and ilmenite, both being iron oxides, with minnesotaite, which is a ferric phyllosilicate. Elara seems to have a similar composition, but the match is not that good. In particular, the spectrum is more dispersed than for Himalia, and a little redder.

Carme has a different spectrum, with a peak of absorption centered around 1.6 μm, and is probably composed of black carbon, minnesotaite, and ilmenite. Another study has proposed that Carme could have a low-level cometary activity, but that would require to observe it at shorter wavelengths. Out of the scope of this study.

The study and the authors

That’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

Periodic volcanism on Io

Hi there! Today’s post addresses the volcanic activity of Io, you know, this very active large satellite of Jupiter. It appears from long-term observations that this activity is somehow periodic. This is not truly a new result, but the study I present you enriches the database of observations to refine the measurement of the relevant period. This study is entitled Three decades of Loki Patera observations, by I. de Pater, K. de Kleer, A.G. Davies and M. Ádámkovics, and has been recently accepted for publication in Icarus.

Io’s facts

Io is one the Galilean satellites of Jupiter. it was discovered in 1610 by Galileo Galilei, when he pointed its telescope to Jupiter. It is the innermost of them, with a semimajor axis of 422,000 km, and a orbital period of 1 day and 18 hours. Its mean radius is 1,822 km.

Io has been visited by the spacecrafts Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini and New Horizons, Galileo being the only one of these missions to have orbited Jupiter. The first images of the surface of Io are due to Voyager 1. As most of the natural satellites in our Solar System, it rotates synchronously, permanently showing the same face to a fictitious jovian observer.

Its orbital dynamics in interesting, since it is locked in a 1:2:4 three-body mean motion resonance (MMR), with Europa and Ganymede. This means that during 4 orbits of Ganymede, Europa makes exactly two, and Io 4. While two-body MMR are ubiquitous in the Solar System, this is the only known occurrence of a three-body MMR, which is favored by the significant masses of these three bodies.

Three full disk views of Io, taken by Galileo in June 1996. Loki Patera is the small black spot appearing in the northern hemisphere of the central image. The large red spot on the right is Pele. Credit: NASA.

Such a resonance is supposed to raise the orbital eccentricity, elongating the orbit. Nevertheless, it appears that the eccentricity of Io is small, i.e. 0.0041, on average. How can this be possible? Because there is a huge dissipation of energy in Io.

Volcanoes on Io

This energy dissipation appears as many volcanoes, which activities can now be monitored from the Earth. When active, they appear as hot spots on infrared images. More than 150 volcanoes have been identified so far, among them are Loki, Pele, Prometheus, Tvashtar…

This dissipation has been anticipated by the late Stanton J. Peale, who compared the expected eccentricity from the MMR with Europa and Ganymede with the measured one. This way, he predicted dissipation in Io a few days before the arrival of Voyager 1, which detected plumes. This discovery is narrated in the following video (credit: David Rothery).

Dissipation induces geological activity, which another signature is tectonics. Tectonics create mountains, and actually Io has some, with a maximum height of 17.5 km.

But back to the volcanoes. We are here interested in Loki. The Loki volcano is the source of Loki Patera, which is a 200-km diameter lava lake. This feature appears to be actually very active, representing 9% of the apparent energy dissipation of Io.

The observation facilities

This study uses about 30 years of observations, from

  • the Keck Telescopes: these are two 10-m telescopes, which constitute the W.M. Keck Observatory, based on the Mauna Kea, Hawaii. This study enriches the database of observations thanks to Keck data taken between 1998 and 2016.
  • Gemini: the Gemini Observatory is constituted of two 8.19-m telescopes, Gemini North and Gemini South, which are based in Hawaii and in Chile, respectively.
  • Galileo NIMS: the Galileo spacecraft was a space mission which was sent in 1989 to Jupiter. It has been inserted into orbit in December 1995 and has been deorbited in 2003. NIMS was the Near-Infrared Mapping Spectrometer.
  • the Wyoming Infrared Observatory (WIRO): this is a 2.3-m infrared telescope operating since 1977 on Jelm Mountain, Wyoming.
  • the Infrared Telescope Facility (IRTF): this is a 3-m infrared telescope based on the Mauna Kea, Hawaii.
  • the European Southern Observatory (ESO) La Silla Observatory: a 3.6-m telescope based in Chile.

All of these facilities permit infrared observations, i.e. to observe the heat. In this study, the most relevant observations have wavelengths between 3.5 and 3.8 μm. Some of these observations benefited from adaptive optics, which somehow compensates the atmospheric distortion.

Results

And here are the results:

Periodicity

The authors notice a periodicity in the activity of Loki Patera. More particularly, they find a period between 420 and 480 days between 2009 and 2016, while a period of about 540 days was estimated for the activity before 2002. Moreover, Loki Patera appears to have been pretty inactive between 2002 and 2009, and the propagation direction of the eruptions seems to have reversed from one of these periods of activity to the other one.

Temperature

The authors show variations of temperature of the Loki Patera, in estimating it from the infrared photometry, assuming the surface to be a black body, i.e. which emission would only depend on its temperature. They analyzed in particular a brightening event, which occurred in 1999. They showed that it consisted in the emergence of hot magma, at a temperature of 600 K.
On the whole dataset, temperatures up to 1,475 K have been observed, which correspond to the melting temperature of basalt.

Resurfacing rate

This production of magma renews the surface. The observations of such events by different authors suggest a resurfacing rate between 1,160 and 2,100 m2/s, while the surface of Loki Patera is about 21,500 km2, which means that the surface can be renewed in between 118 and 215 days. At this rate, we would be very lucky to observe impact craters on Io… we actually observe none.

A perspective

The authors briefly mention the variation of activity of Pele, Gilbil, Janus Patera, and Kanehekili Fluctus. The intensity of the events affecting Loki Patera makes it easier to study, but similar studies on the other volcanoes would probably permit a better understanding of the phenomenon. They would reveal in particular whether the cause is local or global, i.e. whether the same periods can be detected for other volcanoes, or not.

To know more

That’s all for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter.

Hinting the interior of planetary satellites from energy dissipation

Hi there! Today I will present you a paper that has recently been accepted for publication in Celestial Mechanics and Dynamical Astronomy, entitled Constraints on dissipation in the deep interiors of Ganymede and Europa from tidal phase-lags. This study has been conducted in Germany, at the DLR, by Hauke Hussmann.

The idea is here to get some clues on the interior of the satellites of Jupiter Ganymede and Europa, from two different signatures of the tides raised by Jupiter.

The tidal Love numbers h2 and k2

I have recently presented the tidal Love number k2 in a post on Mercury. In a nutshell: it represents the amplitude of variation of the gravity field of the satellite, at the orbital frequency. Please note that contrary to Mercury, only the orbital frequency is to be considered in the periodic variations of the gravity field. The reason for that is in the rotational dynamics: the main satellites of Jupiter rotate synchronously, showing the same face to their planet like our Moon, while Mercury is in a 3:2 spin-orbit resonance.
The tidal Love number h2 represents the amplitude of the tidal deformation of the topography of the satellite. Something remarkable on these 2 numbers is that h2 is mostly sensitive to the surface, while k2 is the response of the whole body. The idea of this study is to compare the two numbers, to get clues on the interior.

The satellites of Jupiter

At this time, 67 natural satellites are known for Jupiter. They can be classified into 3 groups:

  • The inner satellites Metis, Adrastea, Amalthea and Thebe. These are small bodies, their mean radii being between 8 and 85 km. They orbit at less than 3 Jupiter radii.
  • The Galilean satellites Io, Europa, Ganymede and Callisto. These are pretty large bodies, which were discovered in 1610 by Galileo Galileo. They orbit between 6 and 25 Jupiter radii. They contain almost of the mass of the satellites of Jupiter, which make them particularly interesting. For instance, their large masses is responsible for an interesting 3-bodies mean-motion resonance involving Io, Europa, and Ganymede. Basically, Io makes 4 revolutions around Jupiter while Europa makes 2 and Ganymede exactly one. This configuration is known as Laplacian resonance. Moreover the sizes of the 4 Galilean satellites, combined with the tides raised by Jupiter, are also responsible for internal differentiation. In particular, these 4 bodies are all considered to harbor global internal fluid layers.
  • The irregular satellites. These are small bodies orbiting far much further from Jupiter. They are probably former asteroids which were trapped by the gravity field of Jupiter. Contrary to the two other groups, which have pretty circular and coplanar orbits, the irregular satellites can have highly eccentric and inclined orbits. Some of them are even retrograde.

The next space missions JUICE and Europa Multiple Flyby

Ganymede and Europa are the main targets of the next two missions to the system of Jupiter. These two missions are the ESA mission JUICE, and the NASA Europa Mission.

JUICE, for JUpiter ICy moons Explorer, is planned to be launched in 2022 and to orbit Jupiter in 2030. Then, it will make flybys of Europa and Callisto, before becoming a satellite of Ganymede. Ganymede is thus the main target. Among the 11 instruments constituting JUICE, let us focus on two of them: GALA and 3GM.

GALA, for GAnymede Laser Altimeter, will measure the topography of the planet, while 3GM, for Gravity and Geophysics of jupiter and the Galilean Moons, is the radioscience experiment. It will in particular measure the gravity field of the body. The connection with the study I am presenting you is that h2 is expected from GALA, while k2 is expected from 3GM. Another connection is that Hauke Hussmann is both the first author of this study, and the principal investigator of GALA.

The NASA Europa Mission, also known as Europa Multiple-Flyby Mission, and previously Europa Clipper, will obviously target Europa. It should be launched in the 2020’s, and the nominal mission plans to perform 45 flybys of Europa.

One of the motivations to explore these bodies is the search for extraterrestrial life. Europa and Ganymede are known to harbor a subsurface ocean, and we wonder whether these oceans contain the ingredients for bacteriological life. These two missions will give us more information on the interior, from gravity data, analysis of the topography, imagery of the surface, measurements of the magnetic field… bringing new constraints on the oceans, like their depths, density, or viscosity…

This study

The idea of these studies is to compare the Love number h2, from the topography, and k2, from the gravity field, to constrain the interior. For that, the authors have considered several models of interior of Europa and Ganymede, and simulated the resulting Love numbers.

These interior models have to be realistic, which means being consistent with our current knowledge of these bodies, i.e. their total mass and their shapes, and being physically relevant. This implies that their densities increase radially, from the surface to the center. So, the surface is assumed to be made of ice coating a water ocean. Below the ocean is another ice layer, which itself surrounds a denser core. The ocean tends to decouple the icy shell from the action of the interior.

The authors particularly focus on the phase difference between h2 and k2. Basically, the Love numbers are complex numbers, the imaginary part representing the dissipation, while the real part is related to a purely elastic tide. From their simulations, they show that these phase differences should be of several degrees. Their possible measurements should constrain the viscosity of the ice shell coating the core of Ganymede, and the temperature of the mantle of Europa.

Some perspectives

Of course, the most interesting perspective is the future measurements of these phase differences by JUICE and NASA Europa Mission. The information they will provide will be supplemented by better constraints on the gravity field, on the magnetic field, on the rotation…

The authors assumed the rotations of these satellites to be synchronous, as suggested by the theory. But features at the surface of Europa suggest that the rotation of its surface could be actually slightly super-synchronous. This is something that the dynamical theories still need to understand, but this would probably affect the tidal action of Jupiter on Europa.

 

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