This week the team operating the rover Curiosity made the announced the discovery of evidence that fresh waster once existed in Gale Crater. They said the Rock called "John Kein" shows chemical evidence of having been deposited in fresh water. they seem to be ignoring an aspect of the data that goes against habitability at least long-term habitability.
It is true that the clays found in the area do indicate fresh water but the sulfur dioxide, and hydrogen sulfide suggest the past presence of sulfuric acid. The amounts suggest about 3% and thus a Ph of about 0.5, which is quite acidic. The water seems to have been originally fresh when it erupted from underground but then turned acidic from the effects of large amounts of volcanic activity on the planet as described in Catastrophic Model of Martian geology.
It needs to be noted that no actual evidence of any life or organic compounds have been found, only compounds that life could use if present.
------ Charles Creager Jr.