How to prove this density result?

With Earth having a density of 5.5 gm/cc, what is the density of Pluto? And what is the likely composition of?

• From watching the motion of Charon around Pluto, we can measure its orbital radius and period, and then from Kepler's Law we can determine that Pluto is 0.002 times the mass of Earth. From timing the eclipses of Pluto by Charon we measure that Pluto is 18% of the size of our Earth. (The volume of a spherical planet scales as the cube of the radii. The density is the mass divided by the volume.) With Earth having a density of 5.5 gm/cc, what is the density of Pluto? And what is the likely composition of Pluto? A. 0.95 gm/cc, all ice B. 0.95 gm/cc, mixed rock and ice C. 1.9 gm/cc, all ice (little rock) D. 1.9 gm/cc, comparable fractions of rock and ice E. 2.8 gm/cc, iron core, rocky mantle, thick ice over outer half Look up the story of the prediction of volcanoes on Io by Peale, Cassen, and Reynolds (for example, in the textbook). What does this successful prediction say about how science really works? Below are four truths learned from this episode plus one item not learned. Which is the false item? A. Testing predictions is the heart-and-sole of science B. Had they made their 'prediction' a month after the discovery, people would be a lot less impressed C. The successful test of the prediction provides a powerful increase in confidence in the basic idea of Peale, Cassen, and Reynolds D. The successful prediction does not prove their idea correct, as in principle there might be other ideas that also predict volcanoes on Io E. The success of the prediction adds considerable strength to the idea that volcanoes on Earth are powered by tidal stresses from our Moon

• Answer:

  According to the search engine WolframAlpha, the density of Pluto is 2 grams per cubic centimeter. "Through this information, one would be able to calculate that the former planet’s interior must be made up of 50-70% rock and 30%-50% ice. There are also decaying radioactive elements within it that would heat the dwarf planet’s interiors thus allowing both rock and ice to continuously move around." In addition, "there are just too many tidal maximums and too many volcanoes to base predictions on tidal cycle alone. In the Hawai'i example of 52 eruptions since January 1832, there have been nearly 3,900 tidal maximums, of which roughly 3,850 of them went by without causing an eruption. Statistically, this is about a one percent chance that any tidal maximum will affect the start of an eruption." I hope this helps, Kelly UMD, Librarian-to-be