Category Archives: Satellites of Jupiter

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.


And here are the results:


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.


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.