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. Amongst 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 kilometres) may have an ocean of water some kilometres deep
beneath an icy crust which is perhaps 100 kilometres 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 kilometres, 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.
Triton (2700 km diameter) is one of Neptune's moons. 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 metres 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 kilometres 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. It is a moon of Saturn and is only 505 kilometres 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 vapour, 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 metres across, and troughs and ridges. Vast canyons, up to 200 kilometres long, 5-10 kilometres
wide and one kilometre deep, have also been discovered on Enceladus. The surfaceis 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 all quite
small compared to the Earth (though Titan is larger than the planet Mercury (4879 kilometres diameter)). Recall
that the Earth's equatorial diameter is 12 756 kilometres.
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
Acknowledgments: the maps of Europa and Enceladus were provided courtesy ofNASA JPL and then mapped
onto spheres using Pov-Ray. All other images created de novo in Pov-Ray.