The fourth rock from the sun, Mars has long captured both popular imagination and scientific interest. For decades, robots exploring the red planet have been beaming back pictures of a strange world full of breathtaking beauty.

With mountains three times higher than Everest and canyons five times longer than the Grand Canyon, Mars is an adventure traveller’s paradise. And with its dusty atmosphere, polar caps that change with the seasons, and roughly 24-hour days, Mars is Earthlike enough to beckon human visitors.

As NASA’s next big mission, the InSight lander, prepares to touch down in late November, take a look at some of the biggest mysteries about Mars yet to be solved—including some things we may never know until humans