4) Why is Mars less dense, on average, than Earth?
(a) Mars has smaller moons
(b) Mars has less iron in its core
(c) Mars has weaker gravity, so it is less compressed
(d) Mars has ice under its surface
(e) Mars is colder
(b) and (c) are correct. Iron is denser than the rocks (and water) which are near
the surface of both planets, so the more iron there is in the core, the more dense
the planet will be on average. Strong gravity tends to compress materials into a
smaller space, thus increasing their density a bit. On both counts, Earth should
have a higher density than Mars. The size of the moons has no effect on the density
of the planet. There is ice under the surface of Mars, but it would take a huge
amount of it to make a difference to the planet's density. Mars' being colder is
definitely incorrect, since if anything it would tend to make the planet slightly
denser (since cold objects contract, thus packing the same mass into a smaller
volume).