Ice Giants are a type of gas giant planet. Gas giants have extensive gaseous atmospheres. Ice Giants are
gas giants with rocky/icy cores and mantles of slushy ices and liquid. In the Solar System, the planets Uranus
and Neptune fall into this category. In these planets, the atmosphere is about 83% hydrogen, 15% helium
and 2% methane and other hydrocarbons. The atmosphere constitutes several times the Earth's mass and
clouds of methane absorb red light, giving these planets a cyan colour. Ice giants may or may not have rings
- both Uranus and Neptune have faint rings. Neptune is 3.883 times the diameter of the Earth and 17.147
times the Earth's mass. Uranus is 4.007 times the Earth's diameter and has 14.536 times the Earth's mass.
The exact structure and composition of these planets remains a mystery, but some models are outlined
Much of this mass (perhaps about 65%) is made up of hot ices, which despite being very hot are probably
kept at least partially solid by the enormous pressures within these massive planets. One model predicts that
the water and ammonia will exist as electrically charged liquids whilst the methane ice may condense and
decompose to form diamond. It is possible that diamonds slowly form inside the atmospheres of these planets
and then rain down onto the solid core. This diamond rain could also account for the heat generated by
Neptune, which generates more heat than it receives from the Sun, as the descent of these diamonds
convert gravitational potential energy into heat. Uranus produces much less internal heat, which possibly
accounts for the lack of obvious clouds in Uranus's atmosphere - clouds are there but they are less turbulent
and so less visible than clouds on Neptune. Experiments in which methane was compressed to 100 000 to
500 000 times atmospheric pressure on Earth and then heated to 2000 to 3000 degrees kelvin with an
infrared laser converts methane into a mixture of solid diamonds in liquid hydrocarbons. These extreme
conditions of immense pressure and high temperature simulate conditions in the atmospheres of Uranus and
Uranus's mantle is some 56% water ice, 36% methane ice and 8% ammonia ice and Neptune is similar.
These ices are probably either liquids or complex mixtures of proton-rich liquids and solid oxygen/nitrogen
crystalline lattices. The semi-molten rocky core (probably comprised of iron, silicon and magnesium oxides,
iron sulphide and liquid metal) and is also surrounded by ices which are probably solid, despite being very
hot, because the immense pressure favour ice formation.
Weather on Ice Giant Planets
Neptune has the fastest winds in the Solar System, reaching 2100 kilometres per hour! These winds are
possibly driven by the flow of heat produced inside Neptune (possibly by diamond rain and/or radioactive
decay or some other mechanism?) and storm systems are readily visible in Neptune's upper atmosphere,
such as the Great Dark Spot - a cyclone seen in 1989 by the Voyager 2 space probe. Being much further
from the Sun than Uranus, Neptune has the colder cloud tops, with temperatures as low as -224 degrees
centigrade in the upper atmosphere. However, deep within the planet the temperatures become very high
due to heat produced within the planet. Uranus also has an internal heat source, but one that generates only
very slightly more heat than the planet receives from the Sun and Uranus' atmosphere appears calm at first
sight as the planet appears to be an almost uniform cyan, but with bands of cloud apparent on closer
Above: a model of Uranus (rendered in Pov-Ray).
Above: the typical structure of an Ice Giant planet of about 15 Earth masses. The outer
atmosphere is visible, with its banded clouds that follow fixed latitudes, and the blue colour due
to methane. The outer atmosphere is comprised mostly of hydrogen and helium with some
methane and other hydrocarbons. Beneath this is the icy mantle, which is probably mostly
water and ammonia dissociated into ionic liquids, but may be slushy lower down, and then in
the centre is the core. The outer core is solid ice, but within this is probably a semi-molten
rocky core partly consisting of molten metal.
Magnetic fields are generated by the movement of ionic or electrically charged fluids (liquids or plasmas). In
the case of ice giants, this liquid could be ionised liquid water and ammonia, or a layer of liquid metallic water
and ammonia which is thought to exist at the mantle-core boundary (but is probably too thin to account for
observed magnetic strengths) or molten liquid inside the rocky core.
Much more research needs to be done before scientists can be confident about the strange and complex
behaviour of materials at the immense pressures inside hot ice giants. It is now possible for labs on Earth to
recreate these extreme conditions and the data obtained can be used to construct computer models which
may recreate the observed characteristics of these planets.