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Ice Planet
Above: Europa: a 3D model made using an image map courtesy of NASA.
Ice
planets
appear to be a common type of terrestrial planet. (For ice giants,
click here). They are common in the Solar System as secondary
planets (satellites) and dwarf planets such as Pluto probably fall
into this category. Among the Moons of Jupiter, Europa is an ice
planet, as are Ganymede and Callisto. These bodies are thought to
comprise rocky cores covered with ice and at least some of them
appear to have subsurface oceans beneath the thick icy crust,
between the crust and the rocky core, forming a liquid mantle.
Evidence indicates that Europa
(diameter
3122 kilometers) may have an ocean of water some kilometers deep
beneath an icy
crust
which is perhaps 100 kilometers deep. Although the surface of Europa
is enormously cold, due to its great distance from the Sun, the subsurface ocean is insulated by the thick
ice and also probably heated from below by hydrothermal vents (ocean
floor volcanic activity). Although no sunlight can penetrate these
thick icy crusts, life on Earth is known to exist near hydrothermal vents on the ocean floor, with
whole communities drawing their energy requirements from the
volcanic vents rather than from the Sun. Thus, it is possible that
life could exist in an ocean on an icy planet. There is no
atmosphere to speak of Europa, so the icy crust is the layer it
presents direct to outer space.
Europa exhibits a number of features that indicate the presence of a
subsurface ocean. First of all, notice the fault-line like features
- the brown lines that crisscross the smooth icy crust of Europa.
These seem to contain hydrocarbons and probably form when
the icy crust cracks, causing hydrocarbon-rich water from the
underlying ocean to fill the cracks and freeze in place. There are
also ice rafts which appear to have been once mobile icebergs that
froze in place - perhaps after a meteorite impact temporarily thawed
a region the icy crust, causing the rafts of ice to float upon the
liquid ocean until the surface froze over again.
Titan, Saturn's largest
satellite with a diameter of 5150 kilometers, is possibly a planet
of this type, although Titan has a thick atmosphere and surface
lakes of hydrocarbons, it may also have a subsurface ocean. The
crust of Titan also appears to be at least partially made of ice
(water and hydrocarbon ice), if not entirely. It is possible that
hydrocarbons erupt onto the surface of Titan from cryovolcanoes,
or that caverns connect to subterranean reservoirs. On Titan, liquid
hydrocarbons play the role that water does on Earth, and water ice
plays the role of rocks.
Click to learn more about Titan
Click to learn more about Pluto and Charon
Click to learn more about Triton
Triton (2700 km diameter) is one
of Neptune's moons. Above is a simulated view from Triton to Neptune
on the horizon. Triton is an ice planet with a difference. Triton is
so far from the Sun and so cold that it is doubtful that it contains
any subsurface liquid water and any surface hydrocarbons are totally
frozen. Indeed, nitrogen, which is still a gas in the very cold
atmosphere of Titan, is a frozen solid on Triton! The surface of
Triton is unimaginably cold at about minus
235 degrees
centigrade!! The surface consists mostly of nitrogen ice, but water
and methane, carbon monoxide and carbon dioxide ices also occur, and
possibly ammonia ice too.
However, the weak rays from the distant Sun appear to shine through
thin layers (about 2 meters deep) of overlying transparent nitrogen
ice. These panels of ice could act as solar panels, turning the
Sun's rays into heat, much as the glass of a greenhouse does,
heating the underlying solid/liquid nitrogen just enough to vaporise
some of it. This heating may be assisted by black particles of
carbon in the ice. This would cause a build-up of pressure as
nitrogen gas accumulates beneath the surface. Eventually the
nitrogen gas seeps through to a nearby vent and explodes out as a
fountain or geyser
of nitrogen gas and liquid nitrogen
and other materials that may be carried with it, such as carbon
particles. Due to the very low surface gravity on Triton, these
geysers form spectacular plumes that may soar to over 150 kilometers
above the surface! These geysers would be a major tourist
attraction!
In addition to the geysers, there is evidence for another form of cryovolcanism on Triton, at least in its
past. There appear to have been outpourings of slushy ice from ice
volcanoes. Thus ice plays the role n Triton that molten rock plays
on earth, and liquid nitrogen plays the role that water does on
Earth.
Thus, we have looked at three principle types of ice planet:
1. Ice planets with subsurface oceans of liquid water and icy crusts
and no substantial atmosphere, e.g. Europa.
2. Ice planets with liquid hydrocarbon lakes and nitrogen
atmospheres, e.g. Titan.
3. Ice planets with frozen nitrogen crusts, subsurface liquid
nitrogen, and no substantial atmosphere, e.g. Triton.
Enceladus is a very interesting case;
above: a model of Enceladus using an image map courtesy of NASA. It is
a moon of Saturn and is only 505 kilometers in diameter. Being so
small and very far from its star, the Sun, it was expected that
Enceladus would be completely frozen and inert. However, it has
recently been discovered by the Cassini space probe, that Enceladus is
currently venting vast amounts of water vapor, from a warm subsurface
sea or ocean of liquid water, directly into space. These water
cryovolcanoes also erupt hydrocarbons with the water. Models predict
that this subsurface water has existed since the formation of
Enceladus billions of years ago and could therefore contain life that
may have evolved there. Heat released by the decay of radioactive
materials inside Enceladus are thought to have maintained this warm
temperature (radiogenic heating), coupled with the insulating icy
crust.
There is evidence of tremendous tectonic activity on Enceladus, with
planes strewn with boulders (presumably ice) 10-100 meters across, and
troughs and ridges. Vast canyons, up to 200 kilometers long, 5-10
kilometers wide and one kilometer deep, have also been discovered on
Enceladus. The surface is also covered with cracks and smooth plains.
The smooth plains are probably the result of resurfacing from water
pouring out of water
volcanoes
and freezing. The surface temperature is only about minus 198 degrees
centigrade.
So
far, the ice planets we have looked at have all been secondary
planets (satellites or moons) and a dwarf planet in the case of
Pluto, and all quite small compared to the Earth (though Titan is
larger than the planet Mercury (4879 kilometers diameter)). Recall
that the Earth's equatorial diameter is 12 756 kilometers.
In the section on Seraf-9 we look at a hypothetical ice planet about
the size of the Earth with very deep oceans and explore the
possibility of life on this planet.
Seraf-9 - click to examine the possibilities of life on ice planets with subsurface oceans.
Acknowledgments: the maps of Europa and Enceladus were provided courtesy of NASA JPL and then mapped onto spheres using Pov-Ray. The map of Pluto was provided by NASA New Horizons. All other images created de novo in Pov-Ray.