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.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more


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

Evolution of the Interior of Icy Moons

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

Pluto's Interior

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 Spiral of the IMF

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

Spiral Path of Material

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 SAR Arc

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

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA