There are many instances in planetary science where the same material can come in different forms. An example of one such instance is shown in this picture. A diamond is one form of pure carbon, and graphite (what goes into pencil lead) is another form of the same material. They are both made of the identical substance (carbon), but they have a very different size and structure. The difference in molecular construction is referred to as the "phase" of carbon. Graphite is a different phase of carbon than that of diamond.
One might be able to imagine two different moons made out of the same amount of carbon, but one moon made of graphite and the other made of diamond. The two imaginary moons would have very different sizes! Then one might imagine one moon evolving into the other, and having to change sizes as it does so. This phenomena explains the evolution of the icy moons. Ice comes in nine different phases, and all of them have a different molecular construction. In it's evolution, an icy moon may change many times from one phase to another, or include many different phases in it's interior.
Two other materials, from the earth's interior, which have similar phase differences are olivine and spinel.
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Olivine looks like little green crystals. It is typically found in some igneous and metamorphic rocks. Often the crystals are so small that you need to use your hand lens or magnifying glass to see them...more
Ice can change itself into many different forms. The forms are called phases. Phases can be as different as diamond and graphite. As can be seen in these pictures, diamond has a very different size and...more
The diagram to the left shows a cutaway of the possible interior structure of Pluto. The composition of Pluto is mostly ice, therefore there is probably a small core of some rocky material buried inside,...more
AU stands for Astronomical Units. It is a useful way to measure the distances in interplanetary space. It is the distance between the Earth and the Sun, which is about 93 million miles. For reference,...more
The solar wind is formed as the Sun's top layer blows off into space, carrying magnetic fields still attached to the Sun. Gusts form in the solar wind associated with violent events on the Sun. Particles...more
For a planet to be affected by a blob of material being ejected by the sun, the planet must be in the path of the blob, as shown in this picture. The Earth and its magnetosphere are shown in the bottom...more
The aurora we are most familiar with is the polar aurora. This is what people are talking about when they say the northern or southern lights. But there are other less-known aurora, such as SAR arcs....more