Here is a bit more on how they came to this conclusion.  It is a bit technical:

Dr Cole used a variety of data gathered by several instruments on NASA’s Mars rover Spirit to tease out information from exposures of the ancient bedrock.

The chemical composition of these rocks, as determined by Spirit’s Alpha Proton X-ray Spectrometer, is the same, but the rocks looked different to all of the other instruments.

Dr Cole hypothesizes that the rocks were exposed to acidic water vapor from volcanic eruptions, similar to the corrosive volcanic smog (vog).

“We propose that the knobby protuberances are agglomerations of preexisting grains, formed from the interaction of acid fog and the host rock,” Dr Cole and co-authors explained.

Ok so what does this mean?  Does this indicate that the atmosphere was more similar to ours than previously thought?  Feel free to comment with your thoughts.

thanks to sci-news.com for the great info

thanks to NASA for the pic