Galilean Satellites and Titan
Jupiter, Saturn and Uranus display mini-planetary systems composed by satellites of appreciable dimensions, among which we may emphasize the 4 Galilean satellites of Jupiter and Titan, in Saturn.
The Galilean system is analogue to the inner solar system in what concerns to some regularities, namely the inverse proportion of the average densities to the distance of the satellites from Jupiter and the observance of the law of Tycho-Bode.
Distribution of the Average Densities
The most noticeable regularity is the reduction of the satellites' average densities as the distance between them and the planet increases. That means that the inner bodies (Io and, in a lesser degree, Europa) are essentially made by rocky materials, contrarily to the outer bodies (Ganymede and Callisto), which are essentially made by ice.
The main responsible factor for that shall be the more intense tidal forces present in the interior of the system, which behave as generators of heat in the centre of the satellites. That heat probably determined the evaporation of the volatile materials, as water.
On the other hand, the primitive nebula that gave birth to the system could have been more intensely heated in the internal regions, through the radiation emitted by Jupiter. This would have also narrowed the survival of the volatile elements to the outer zones of the system.
Comparison between the interiors (metallic and rocky) of Io and (rocky and icy) of Callisto (NASA - JPL)
Another curious regularity is the distribution of the satellites, which observes the law of Tycho-Bode. This law is also applicable to the locations of the planets in the solar system and, more precisely, to their distance from the central body.
Finally, it shall be mentioned that the orbits of the satellites are oriented according to the direction of the Jupiter's rotation, they are almost circular and they are located at the equatorial plane of this giant planet, in an analogue way to what happens with the planets of the Solar System.
Callisto, the galilean satellite which lies farther away from Jupiter, is the second largest moon of this planet and the one that displays the largest amount of craters in the whole solar system. It is therefore estimated that its surface must be very old, with an age of about 4 billion years (the epoch of the most intense meteoritic activity).
On average, the relief on Callisto is flat, which may be due to its icy composition (in spite of the considerable hardness that the ice displays at the local temperatures).
The relief of the largest craters (which seldom go above 1000 metres) was erased by the flow of icy crust proceeding from the zones that were “drilled” during the impacts.
The largest ones, characterized for being structures of concentric rings, are Valhalla (the heaven of the Vikings) and Asgard. These formations were originated by the symmetrical spread towards the exterior of the powerful expansive waves created by gigantic explosions, which were a consequence of the collisions, themselves.
A surface riddled by craters (Calvin J. Hamilton)
The Dark Cover
The surface of Callisto is covered, in a large extent, by dark layer of rocks and dust. Possibly, this thin layer was gradually accumulated on Callisto as a consequence of the small meteorites’ impacts, which may have provoked the evaporation of the ice and have left residues of dark carbonic materials.
The dark surface of Callisto (Arizona State University)
What determines that Callisto is an almost inactive satellite is the absence of inner heat, which is maybe due to the weak in rocky material comparing to the ice rich supply (therefore, there is a lower prevalence of unstable radioactive isotopes, whose disintegration would generate heat) and to the greater distance to Jupiter (which excludes tidal phenomena similar to those that affect Io).
It’s possible that the interior of Callisto has always cold enough in order to prevent a clear differentiation between a rocky core and an icy mantle. This way, the transition between these two regions is probably gradual and not sudden.
Ganymede is the largest satellite of the solar system and, like Callisto, it’s thought that is composed by a rocky core and an ice mantle, which is in turn covered by a mixed crust that may contain an ocean of liquid water.
Global view over Ganymede (NASA)
Ganymede is covered by a very tenuous atmosphere of ozone produced by the impact of charged particles (caught by the magnetic field of Jupiter) on the icy surface of the satellite.
Bright Regions vs. Dark Regions
Bright and dark regions, the latter ones presenting a high density of craters, stain Ganymede, revealing its ancient origin. In the darker regions, whose material was probably deposited there through a process similar to the Callisto’s, there seem to be traces of concentric rings similar to those found in Valhalla and Asgard craters of Callisto, under the form of curved strips. The bright zones are grooved by streams of low-relief strips.
The Formation of the Crust
The original crust of the satellite, after its formation, would have been fractured as a consequence of the global heating and perhaps also by a process of global expansion, allowing that liquid water jets (equivalent to the lava on Earth) emerged to the surface and froze in linear patterns of valleys and glaciers that ended up forming complex crossed patterns. The explanation for these patterns may also be based in the presence of an ocean of liquid water under the surface, which may reach a thickness of 200 km.
There are many footprints indicating that in remote times a tectonic process similar to the Earth’s may have been started, although it would have been much less efficient and dynamic than here. It looks like that the frozen surface lithosphere would have been fractured into plates that, nevertheless, didn’t move a lot from each other.
Uruk Sulcus: detail of the surface of Ganymede (NASA)
Ice vs. Contamination
Ganymede presents two polar caps, which is probably due to the preferential condensation on the poles of the water vapour emitted by the fractures of the crust. As in Callisto, in Ganymede many young craters are also characterized by their extremely bright colour and by the surrounding brilliant radial lines of material expelled during their formation. The impacts that formed them would have destroyed the surface dark layer and exposed the originally buried clean ice.
However, there are also many other craters, particularly smaller sized ones, which are surrounded by radial dark lines. That is eventually due to the contamination of the ejected material by the projectile itself or by the subsurface material of Ganymede.
Europa is the smallest of the galilean satellites. It almost doesn’t show any crater and presents an icy surface with quite tenuous relief. It is thought that Europa is internally active, which is due to the fact that it is subject to tidal forces with 1/10 the intensity of those that dominate Io.
Global View over Europa (DLR)
A Subsurface Ocean
Below a thin ice crust (5 km), it is thought that Europa may hold an ocean of liquid water, perhaps more than 50 km thick, which may hypothetically host some sort of life. Some analogies between the surfaces of Europa and Ganymede suggest that the latter may also possess a subsurface ocean, as it was already mentioned. Even in the case of Callisto, this hypothesis is not excluded, since its remarkable magnetic field may be explained by the presence of a subsurface layer of liquid water, electricity conductive. However, the heat that fuses the water in Callisto and Ganymede shall be of purely radioactive origin, opposite to what happens in Europa, where the heat provoked by the tidal forces shall be a prevailing factor.
Formation of the Crust
The features observed on the surface of Europa are probably the result of global expansion phenomena, in regions where the crust was fractured and filled by water contaminated with rocky material, which was frozen further.
Some plates confined between these fractures seem to have been rotated and displaced, which may have been made easier by the lubrication generated by the warm ice or the liquid water existent under the surface.
Detail of the surface of Europa (NASA - JPL)
Opposing to what happens in Ganymede and Callisto (which are bodies essentially frozen but covered with a thin layer of dark rocky material), Europa is thought to be an essentially rocky satellite (composed, namely, by silicates), but covered with a thin layer of water. This fact is revealed by the average density of Europa, higher than those observed in the other two mentioned satellites.
Io is the innermost galilean satellite, essentially composed by rocky material and some iron, being strongly heated by the tidal forces induced by Jupiter, Europa and Ganymede, besides the internal radioactive sources.
O colourful Io, apparently navigating over the clouds of Jupiter (NASA)
For these mentioned reasons Io is, beyond Earth, the sole body in the solar system that displays phenomena of active vulcanism. Although the average temperature in Io is only 130 ºK (-143 ºC), on the hot zones associated to volcanic features can be reached temperatures as high as 290 ºK (17 ºC). These features lie over hot spots where lava pours in lakes and replaces the previous surface.
Composition of the Crust
The conservation of considerable relief in Io (up to 10 km) reveals that the hot crust must be composed by a material strong enough to support them, quite likely dominated by silicates.
However, many of the observed volcanic flows seem to be composed by molten sulphur rather than silicate lava, as it happens on Earth. Therefore, there must be also vast deposits of sulphur and sulphur dioxide (SO2) in the vicinities of the surface if this satellite.
At a determined deepness, the heat from the silicate magma (which lies deeper) will heat these deposits, transforming them into lakes and flows of molten sulphur. If these materials get into direct contact with the magma, then the geyser-type eruptions will be the most likely outcome.
Scheme of the volcanic process in Io (T. Johnson)
Morphology of the Volcanoes
The high fluidity of the expelled volcanic material must be the main responsible factor for the absence of big lava constructions (as, for example, in Hawaii) and for the big dimensions of the volcanic flows that extend for hundreds of kilometres.
A volcanic caldera in Io (NASA - JPL)
That low viscosity (originated by the fluid nature of the mentioned sulphuric materials), along with the low gravity of the satellite (comparable to the Moon’s) and the absence of a considerable atmosphere, is also responsible for the high speed, height and shape (mushroom-shaped) of the volcanic eruptions.
The absence of water, carbon dioxide and other volatile elements means that these materials were probably driven from the interior of Io and expelled from the surface a long time ago. Only the heavier volatile species, as sulphur and sulphur dioxide would have survived.
The Cycle of the Sulphur Dioxide
Io bears a tenuous atmosphere of sulphur dioxide, permanently supplied by the volcanic phenomena. The SO2 solidifies further on the surface and, most noticeably, at the polar regions. The quantity of SO2 is, therefore, regulated through this condensation process.
The Escape of the Material
Io is placed in a zone of intense radiation of electrons and ions captured by the magnetic field of Jupiter. As the field rotates with the planet, it strips 1 ton of material from Io each second, which in turn are condensed in a thorus of ions, particularly sulphur and oxygen ions.
The speed of the eruptions is high but it’s not enough for the elements existing there to be able to overcome the satellite’s gravity. Besides that, the escape resulting from the heating of the sulphur dioxide is not enough to justify the density of the material observed in the magnetosphere. So, it is thought that another mechanism – the sputtering – shall be the mechanism that ejects material from Io into the magnetosphere. The ions of the magnetosphere (O and S) collide against the surface of the satellite and eject material into the atmosphere, which in turn will be thrown into the outer space after a new collision with these particles.
The ion belt expands and, along with it, the magnetosphere of Jupiter is forced to expand to twice the original size. Some sulphur and oxygen ions travel to the atmosphere of Jupiter through the magnetic field, provoking the appearance of aurorae.
Titan is one of the largest icy objects of the outer solar system, along with Ganymede and Callisto (Jupiter), Triton (Neptune) and the system Pluto-Charon. It is the biggest satellite of Saturn and the only one in the solar system to possess an appreciable atmosphere.
An image of the surface of Titan under the thick atmospheric layer, taken by the Cassini probe. A possible crater can be seen in the upper half of the image. (NASA-JPL-University of Arizona)
The Saturn’s System
During the primitive times of the solar system, Jupiter shined and emitted heat, which provoked a differentiation between the physical characteristics observed of its main satellites and particularly the distinction between the essentially rocky bodies of the interior (given the impossibility of survival of the most volatile species inside that hot zone) and the essentially icy bodies of the exterior.
In the Saturn’s system, given the lower mass of this planet, there was less heat produced and, therefore, the differences verified between inner and outer bodies are more subtle. There isn’t a distinction between rocky bodies in the interior and icy bodies in the exterior. Nevertheless, certain volatile molecules, like methane (CH4), ammonia (NH3), nitrogen, carbon monoxide (CO) and argon would have survived in the external region of the system, but not in the inner region.
The Origin of Titan’s Atmosphere
The low temperatures of the Saturn system, comparing to those of the Jupiter system, explain why does Titan possess an atmosphere, while Callisto or Ganymede don’t possess it, even if they hold similar dimensions.
If the ice that accounts for the composition of Callisto and Ganymede solidified at very low temperatures but was further heated above its critical value, its gaseous content would have been strongly depleted. However, the environment where Titan was formed was kept always well below 135 ºK and the gases were not released until the complete formation of the satellite.
The heat generated during the accretion process released the gases trapped inside the ice. It may have also been the cause for the reactions between them, favouring the formations of new species. These gases were converted in the primordial atmosphere of the satellite.
Additional heat was generated later from the decay of radioactive elements of the rocky component of Titan. The interior of Titan was then molten and differentiated, having been formed a dense rocky core, surrounded by an icy mantle. Titan obtained then a secondary atmosphere produced through volcanic type processes, which also dominated the formation of the atmospheres of the inner planets.
Titan, covered by its thick atmosphere (Calvin J. Hamilton)
The layers of opaque material in the atmosphere of Titan spread around the whole satellite.
The dominant gas is molecular nitrogen (N2), as on Earth, and the surface atmospheric pressure is about 1,5 times the terrestrial sea level pressure. The methane is a minor compound (1 to 6%) and the argon, although it hasn’t been observed yet, may reach 10 to 15% of the atmospheric volume.
A rich variety of other compounds, like the hydrocarbons – ethane (C2H6) and acetylene (C2H2) – and the nitriles – hydrogen cyanide (HCN) – were also found in the atmosphere of Titan. Some traces of CO2 and a minor quantity of CO are also observed.
The molecular hydrogen is also a molecule in the atmosphere of Titan, although it very easily escapes from the satellite’s gravity. That means that this species is continuously replaced by some kind of process. For instance, it’s thought that the methane (which possesses four atoms of hydrogen) is continuously supplied from an ocean and, later, it is photochemically split (under the action of the solar light).
The atmospheric temperature decreases with the altitude, from the surface up to the tropopause, 60 km above. From that height, the temperature rises again with the altitude, reaching a peak of 175 ºK, 80 ºK above the surface temperature. This heating is provoked by the ultraviolet radiation proceeding from the Sun, which is responsible for the photochemical processes that take place there. There are two hazy regions in the atmosphere of Titan: one extends up to a altitude of 200 km and a dimmer one lies 100 km above. Apart from the hydrogen and carbon monoxide (CO), all the minor gases condense at the tropopause’s temperature.
The orange colour of the atmosphere suggests the action of additional chemical processes, which transform simple molecules into more complex stuff, like the polymers – one of the foundations of biology.
Clouds, Precipitation and Oceans
The liquid particles of the photochemical haze, as they grow, become heavier and precipitate on the surface of the satellite. It is thought that thin clouds of methane crystals are formed at the surface, as well as vast clouds of ethane, the most important subproduct of the photochemical split of the methane. Consequently, it is thought that Titan may be covered by a vast ocean of ethane, with minor quantities of dissolved methane and nitrogen.
One of the visible characteristics of the atmosphere of Titan is the difference of colour between the summer and the winter hemispheres, which results from different photolysis rates (the dissociation provoked by the sunlight) favoured by the divergent solar light incidence in both hemispheres.
The surface of Titan must be very cold and dark, given that only a small portion of the dim solar light is able to cross the cloud layers down to the ground. The Sun must look as bright as a full moon and is shown as an orange haze. The atmosphere shall be clean at the surface, since the aerosol haze is located at higher altitudes, where it is produced. The Huygens probe will land on this surface on 15th of January, 2005 and will send unprecedented data about this still mysterious world.
The sea pounding its waves at the shore of a beach in Titan