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The Small Satellites of the Outer Solar System


All the giant planets of the solar system are surrounded by "mini-planetary systems". We can emphasize the 4 galilean satellites (Jupiter), Titan (Saturn) and Triton (Neptune), analysed in other sub-chapters. 12 middle sized satellites (Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus in Saturn; Miranda, Ariel, Umbriel, Titania and Oberon in Uranus; Proteus in Neptune) and at least 39 small dimension satellites (12 in Jupiter, 11 in Saturn, 10 in Uranus and 6 in Neptune) also orbit these planets.



The System of Jupiter


4 of the 12 smallest satellites of Jupiter orbit in the interior of its system, while the other 8 orbit in the exterior.

The latter ones can be sub-divided  in a less external group and in a more external group, each comprising  4 satellites. Of those, the ones belonging to the more external group are characterized by their clockwise orbits (opposite to the counter-clockwise rotation of Jupiter). These 8 bodies may probably have their roots in 2 bodies formed far away from the planet, which would have been captured by it in a later time. Even later, the collisions with other asteroids or comets would have provoked a fragmentation of these objects, giving birth to the two detected groups.

In the interior of the system, the satellites would have presumably been born close the planet but their proximity to this and the resulting tidal forces would have prevented their gathering into larger bodies.



The System of Saturn


Mimas (with a diameter of only 390 km, slightly more than enough to give it a spherical shape) and Enceladus (with a diameter of 500 km and slightly farther away from Saturn) are the innermost and the smallest of the intermediate sized satellites of Saturn. Tethys (with a diameter of 1050 km), Dione (1120 km) and Rhea (1530 km), are the following ones, as we go away from the planet. Iapetus (1140 km), lies far beyond the orbit of Titan, and is by far the most distant of the intermediate satellites. All the intermediate satellites of Saturn are bodies composed essentially by water ice. However, while Mimas, Tethys, Dione, Rhea and Iapetus are densely covered by craters, they seem to have been almost completely erased in Enceladus.



Enceladus apparently went through a phase of intense geological transformation, which is suggested by the degradation conditions of the craters’ rims and by the complex linear features that frequently superimpose over the craters.

The origin of this activity, revealed in the form of water vulcanism, should have its roots in tidal forces (just like Io and Europa), determined by the interactions with Saturn and Dione, which has an orbital period approximately twice that of Enceladus, meaning that it is almost resonant. This resonance with Dione, given the assumption that it was accurate in the past, would have thrown the satellite into an eccentric orbit and, therefore, would have been the responsible factor for the mentioned tidal forces.


Enceladus (A. Tayfun Oner)



The largest crater of Mimas (Herschel), with a diameter extending for more than 1/3 of the satellite’s has dimensions that are close to the critical value beyond which it would have been broken into pieces after the collision.

It’s quite possible that Mimas was, in an earlier time, destroyed by one of those impacts and the fragments of the ring that resulted from them accumulated again to form the body that we know today.


Two hemispheres of Mimas: to the left, the crater Herschel (Calvin J. Hamilton)


Tethys and Dione: Geological Activity

Tethys and Dione, less than Enceladus, appear to have been affected by resurfacing processes, unveiled by canyons crossing long distances.

Tethys would have been mainly liquid in the beginning and, later, would have entered a phase of crust expansion as a consequence of the freezing of its interior (the water expands when it freezes, contrarily to what happens with most materials). So, its surface would have been fractured, originating part of the detected features.

The largest crater of Tethys, Odysseus, seems to have been flattened and stripped of remarkable reliefs, which would be explainable if it had been formed in an early phase of the solidification process, during which the geological features would easily fade.

In Dione it is remarkable a brilliant region, located in the trailing hemisphere (the back side of the satellite concerning its movement), which is the centre of a light grooves and ridges’ system. This region seems to have been, in the past, the centre of some geological activity.


Tethys (Calvin J. Hamilton)


Dione (Calvin J. Hamilton)


Dione and Rhea: Micro-Cratering

Dione and Rhea are characterized by a remarkable asymmetry of the reflectivity (albedo) between the leading (the front side of the satellite concerning to its orbital movement around Saturn) and the trailing hemispheres (the back side, facing to where it was just before). The leading hemispheres display more or less uniform reflectivities, while the trailing hemispheres are characterized by structures of bright features lying over a dark background.

The leading hemispheres may have been optically uniformed by a more intense process of micro-cratering, which would result from the movement of these satellites concerning to the rainfall of tiny projectiles (meteors), orbiting around Saturn. That movement would have then favoured the impacts against the leading hemispheres of both.

The observations also suggest the existence of a population of small and middle-sized meteorites, orbiting around Saturn and that would have intensely hit some of Dione’s regions in a period of late bombardment. This population could have been the result of the destruction of one or more large dimensions objects.


Detail of Rhea’s surface (Calvin J. Hamilton)


Rhea: Tranquillity

Contrarily to Tethys or Dione, in Rhea there aren’t big signals of evident inner activity or fractures resulting from the freezing and expansion. Neither it is reasonable to take into account any “phoenix bird” hypothesis, according to which this satellite could be a body that was re-built from the fragments of the destruction of previous bodies (as it probably happened in Mimas or, less likely, in the case of the other two bodies mentioned above). The biggest mass and gravity of Rhea may explain why, i this particular case, this hypothesis would have been made particularly difficult.


Iapetus: Black and White

Iapetus is remarkable for the huge reflectivity difference between the leading and the trailing hemispheres. The satellite is almost entirely composed by ice, but in the leading hemisphere its luminosity is as dark as coal, while in the trailing hemisphere it is as bright as snow.

It’s possible that the origin of the black colour of the leading hemisphere is related with Phoebe, a very dark satellite that orbits Saturn at a higher distance. The interaction with the radiation may release small particles from its surface, which further travel fast in a backward direction, until some of them intercept Iapetus.

The difference between the brightness of both hemispheres can also have an internal origin, because the borders between the bright and dark zones are sharp and irregular, contrarily to what should happen if the phenomenon could be explained by external processes. The dark bottom of the craters in the bright hemisphere (trailing) of Iapetus also suggests this hypothesis may be the most reasonable one.


Iapetus: to the left – the dark hemisphere, to the right – the bright hemisphere (Calvin J. Hamilton)



Phoebe, the possible source of the particles that dye the leading hemisphere of Iapetus, is a small satellite (with a diameter of 200 km) with a nearly spherical shape and that may probably have been captured by Saturn in a remote time. Its long permanence in the system suggests that it may have been braked by the friction with the gaseous environment that surrounded Saturn during the time of its formation.


Image of Phoebe taken by the Cassini probe in June, 2004 (NASA-JPL)



Hyperion, which is satellite that is placed between the orbits of Titan and Iapetus, appears to be a fragment of a big object destroyed in a violent impact during the primordial phases of the solar system history. That hypothesis is favoured by the exaggerated irregularity of the satellite given its big dimensions (shorter axis: 230 km, longer axis: 410 km) and by its eccentric orbit.

The object that was in the origin of Hyperion may also be responsible for the already mentioned population of small and middle-sized meteorites that long ago would have orbited around Saturn.


Hyperion (Calvin J. Hamilton)



5 small inner satellites of Saturn are directly linked to the shape patterns of the rings, limiting their dimensions. Two other satellites orbit Saturn at the same distance of Tethys, forming with the latter an angle that the celestial mechanics defines as equilibrium point and that allows them to stay in that position for a very long period. The satellite Helene is placed at an orbit close to Dione’s, also in an equilibrium point in relation with this satellite and Saturn.



The System of Uranus


The main satellites of Uranus – Miranda (with a diameter of 470 km), Ariel (1160 km), Umbriel (1170 km), Titania (1580 km) and Oberon (1520 km) – seem to be richer in rocky material than the satellites of Saturn.


Miranda: The Exotics

Miranda, although it is the smallest of these satellites, displays an extraordinary variety of geological features like hills, fractures, faults, canyons, parallel strips’ systems and mountain chains with diverse morphology.

The most ancient regions of Miranda are intensely covered by craters which, given the properties of the surface materials, hold relatively shallow reliefs. In other zones, the crust seems to have been deformed and stretched, forming deep folds and leaving uncovered materials with different optical properties.

There are two hypotheses that try to explain the peculiar characteristics of Miranda:

    1. One states that Miranda, just like Enceladus, may have been recently affected by resonance and tidal heating phenomena. This would have been enough for stirring up extensive tectonic and volcanic processes;
    2. Another hypothesis advocates that Miranda, like other bodies in the solar system, may be an object that was reaccumulated from the fragments proceeding from another body previously destroyed by an impact. That would have produced a mosaic surface, overlaying regions of different fragments, some proceeding from the surface (rich in ice) of the parent body and others proceeding from its interior (with a much more significative rocky component). After the reaccumulation, a differentiation process could have been stirred up, favouring the sinking and melting of the rocky regions. The energy produced through this process would have been, according to this hypothesis, the responsible factor for the fractures and volcanic phenomena.



Miranda (A. Tayfun Oner)



Ariel displays a less exotic surface than Miranda’s, but displays clear signals of geological activity. The largest craters seem to have been erased from the surface, and the largest survivor of them has a flattened bottom, which may have been caused by the slackness of the material under the effect of the gravity.

The cratered terrains are crossed, interrupted and sometimes seem to have been displaced by a global system of faults and fractures, which could have resulted from the freezing of the water that would have provoked the expansion of the crust. Very viscous material, expelled from the interior during the expansion phase, would have been laid in successive phases over the most ancient terrains.

Ariel seems to be composed by water ice mixed with ammonia and, eventually,  some methane and carbon monoxide. The rocky material existing in Ariel, along with the eventual resonances and tidal forces, may have been the main factor that caused the heating of the satellite, given its primordially radioactive properties.


Ariel (Calvin J. Hamilton)


Umbriel: The Darkness

Umbriel is a very dark satellite and displays a uniform brightness. It has a surface intensely crowded with craters. The cause of the uniformity of the surface brightness may have been the occurrence of an impact that expelled a large quantity of debris from the satellite, which in turn would have been intercepted and collected again by Umbriel. The debris would have then covered Umbriel with a thin layer of dust.


Umbriel (Calvin J. Hamilton)


Titania and Oberon

Just like Ariel, Titania and Oberon display regions of diverse reflectivity, the darkest ones being probably rich in carbon composites, similarly to the ones that are found in the carbonic chondrites.

Titania is densely populated with craters, but it displays flatter zones that may have been covered by aqueous volcanic material whose expulsion from the interior, like in the Moon, would have been stirred up by the energy released by the largest impacts. The system of fractures and cliffs, as in Ariel, may have been caused by the expansion of the crust provoked by the freezing of the water.

In Oberon, as it happens on the bright hemisphere of Iapetus, the bottom of the craters is dark and probably rich in carbon. This material may have been deposited there through the ocurrence of eruptions provoked by the impacts, in a similar way to what happened in Titania. Oberon seems, nevertheless, to be deprived of structures that might reveal processes of surface renewal.


To the left: Titania, to the right: Oberon (Calvin J. Hamilton)



Such as the rings of Saturn, the rings of Uranus are also confined by the disturbance caused by the "shepherd satellites", namely in the case of the Epsilon ring, whose edges are imposed by the trajectories of the Cordelia and Ophelia satellites.



The System of Neptune



Proteus, with a diameter of 420 km, is a satellite with dimensions comparable to those of Mimas, Enceladus and Miranda. However, its shape is somewhat irregular, contrarily to the spherical shapes that characterize these 3 satellites. It’s thought that it stands at an intermediate point between the high mass spherical bodies and the low mass tri-axial bodies (with 3 axis of different dimensions).

The high density of craters reveals the old age of its surface. The largest of them, Pharos, covers more than a half of the satellites’ diameter. As it clearly exceeds the critical point at which a regular satellite would have been destroyed by the impact, it is thought that the surface layers are composed by porous rather than compact material, plenty of internal cavities.


Proteus: above, the Pharos crater (Calvin J. Hamilton)



Out of the 6 small satellites of Neptune, the largest – Nereid, holds the most eccentric orbit of the solar system (it approaches Neptune to a distance of only 1,4 million km during the perihelion and gets away from it up to a distance of 9,7 million km during the aphelion). Besides that, it has a reflectivity (albedo) that exceeds twice the reflectivity of the other small satellites. Therefore, it is thought that it is a body that was captured by the system in a time posterior to the formation of the system of Neptune.

Out of the remaining 5 satellites, 4 are placed inside the system of the thin rings of Neptune and it is thought that they were formed from the debris of 2 larger satellites that were fragmented after collisions with asteroids or comets. The 5th satellite, Larissa, is placed slightly farther away, between the orbits of Galatea and Proteus. The satellites that, during the ancient times, presumably existed in the zone of Triton were probably swept by the gravitational disturbances that would have been provoked on them immediately after the capture of that large satellite by the system of Neptune.



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