Water

What happened to water on Mars 

In the past there was water in liquid state on and in Mars' surface. We know it because we can see minerals that can appiare only with water (as opal).Where is this water now? Approximately 3.7 billion years ago there was much water on Mars. But cosmic radiation gradually destroyed magnetic field of Mars and then its atmosphere. The water molecules were broken down into oxygen and hydrogen (O and H2) because of radiation. Oxygen combined with iron to form iron oxides on the surface (rust).  Hydrogen escaped into space.

Is there water on Mars now

Yes there is water on Mars now. For example sometimes there is fog in valles Marineris or water ice on poles. But is there liquid water? How we already know on the Mars' poles there is water ice but most likely there is no liquid water on Mars. There is only hypothesis that  beneath an icy layer of South Pole at the depth 1.5 kilometers there is lake 20 kilometres in diaminre with liquid salt water. And it was discovered by Mars Express (Launched ESA) in 2018. But now many scientists think it can be some thing else.