A photograph of water on Mars has been posted on the NASA website!

Most Honorable, allow me to disagree with your highly respected opinion!
Even if it is a "complete candy," between the "complete" part and the water itself, as you correctly noted, there is a LAYER in the form of a glass. Which proves that the use of the preposition "ON" in the source question is false. And, accordingly, the only correct preposition is "ABOVE". :)

VeryHighlyRespected colleague,
as Einstein said: "everything is relative"...
Depending on what point of observation we choose... :p
The glass includes the water.. this is evident from the preposition "in" i.e. water (or alcohol) in a glass, therefore there is no layer... since the water and the glass become inseparable from each other...
Consequently, the statement "there is water on Mars" becomes true!

With very deep respect to my VeryHighlyRespected colleague!