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.
Galilean
Satellites
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)
Other Regularities
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
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)
The Inactivity
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
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)
Atmosphere
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)
The Interior
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
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)
Eruptions
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.
Volatile Species
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
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).
Stratification and
Temperature
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.
Climatic Seasons
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.
Titanic Landscape
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
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