Water on Mars

According to NASA, one of the most important things they found out about Mars was discovering rocks that form in liquid water. They believe that the planet once had water oceans and rivers. 

Picture drawn by AI on behalf of Alan Booth

Another important discovery was finding rocks with magnetic minerals that were aligned to the poles of the planet. Although the magnetic fields of Mars have faded away, in the past they existed. Mars once had a strong magnetic field protecting its atmosphere from solar wind particles.