Describe the current atmosphere on Mars. What evidence suggests that it must have been different in the past?

The evidence of water that once was on mars

Explanation:

The martian atmosphere consists mostly of carbon dioxide, but is very thin, less than 1% of Earth's atmosphere. However, the strong evidence of water on Mars in the past that our missions have found means that the atmosphere must have been thicker and warmer, or water would have evaporated away very quickly.