The ancient climate of Mars is a mystery to scientists. Even with all we’ve learned about Mars, it’s still difficult to explain how lakes and rivers existed.

from https://www.universetoday.com/141879/rivers-on-mars-flowed-for-more-than-a-billion-years/
https://www.universetoday.com/wp-content/uploads/2015/04/NASA_martianocean.jpg