16. The high surface temperature of Venus is due to [ the greenhouse effect / volcanic heat release ]. 17. In planetary sciences, a [ chasma / corona ] is an oval-shaped feature.

18. Because of the
[ high temperature / low escape velocity ], atoms in Mercury’s atmosphere just bounce around and eventually disappear into space.

19. Mercury’s period of rotation is exactly [ one third / two thirds ] of its orbital period.

20. Mercury is [ tidally coupled / antipodal ] to the Sun, thus affecting its rotation.