There is evidence that liquid water existed on Mars at some time in the past. What is the evidence found indicating the presence of liquid water? Why isn’t it possible to have liquid water on the surface today?

There mag be water on Mars but it’s not very easy to spot, but that is besides the point,

Mars is a smaller planet then Earth, thus it has less gravity, meaning when water evaporates into the atmosphere of the planet the water slowly escapes into space, so there is less and less water on the planet.

Hope this helps!