liquid forming valleys on Mars hundreds of active volcanoes on Io rock and ice forming the core of Neptune ice covering the entire surface of Europa
I gave my students this multiple choice question: Scientists study the current conditions of many solar system objects to gain insight into conditions early in the history of Earth. Which phenomenon most closely models current theories of early Earth?

Only one answer is correct among these 4 choices:
- liquid forming valleys on Mars
- hundreds of active volcanoes on Io
- rock and ice forming the core of Neptune
- ice covering the entire surface of Europa

Could you tell me which one is correct?

hundreds of active volcanoes on Io