Question

The closest planet to the Sun, Mercury, has a diameter that is a little more than 1/3 that of the Earth, yet the density of Mercury is nearly equal to Earth’s (98.4% of Earth’s density).

Question 31: [1] If Mercury is so much smaller than Earth yet has such a similar density, what does this indicate about the interior of Mercury?

Question 32: [2] Why is it that the metallic core of a rocky body such as Earth is primarily composed of iron and not some denser material such as gold? Think back to what you learned in Module 1 about the elemental composition of the Solar System.

Answer 1

31. Since the density of Mercury is very similar to that of Earth, it indicates that the interior of Mercury is made of similar materials to Earth's interior. Since the Earth's interior is made of metallic silicates and a metallic core, the same must be the composition of Mercury. This is because the density of the object is independent of the size of the object. Thus, even though Mercury is smaller, if it is made of similar materials, it will have the same density.

32. Gold is extremely rare in the solar system, while iron and nickel are common in the rocky terrestrial planet of the solar system. It should also be noted that elements up to iron and nickel are formed via nucleosynthesis in stars, but heavier elements are more difficult to form (they are believed to form in supernova explosions). Thus, since iron and nickel are more easily formed, it is more common in the solar system and makes up the core of rocky planets like Earth.