Image by NASA/JPL/MSSS
Some contain clay, which has suggested to some observers that they formed under water. Rather than having that prove that Mars had water, I'd prefer to have it suggest those meteorites are possibly not Martian. They were found on Earth, after all, and could have come from anywhere, even here, though that is pure heresy.
Maybe something hit some old rocks here, a long time ago, and up they went and back they flopped. Maybe they are from some other planet, or an asteroid. As we so far have not one single rock from Mars brought here by a method we can be sure of, namely a spacecraft launched by us, we have no rocks to compare these meteorites with to confirm their past Martian identity. The isotope ratios of minute amounts of material can be misleading.
I may be in error. In personal communication, Dr Tilman Spohn, of the Institut fur Planetologie, recently offered another view of these rocks. He emailed: "I personally have no severe problems with water on Mars but from my background (interior structure, dynamics and evolution, stated above PSR) I can supply you with a thought that has always puzzled me.
I understand from my friends from cosmochemistry (e.g. Heinrich Wanke) that the Martian interior as witnessed by the SNC meteorites must be as dry as a bone, at least today, or more precisely, at the time of formation of the SNC rocks, which is relatively recent.
If that is so, then the water on Mars must be exogenic in origin, I would maintain. It is very difficult if not impossible to completely degas a planetary interior as I have learned from my modelling of interior out-gassing.
To say it more clearly: If the Martian water is thought to have originated from interior outgassing then the interior cannot be dry! I find it a bit difficult to understand how an exogenically produced hydrosphere can be stable for such an extended time.
However, I have to say that at least some atmospheric scientists do not seem to see too much of a problem here. I am not sure how well this problem has been studied. I do not know if that really helps you since you believe that the SNC's are not from Mars. So you may find it difficult to integrate into your picture."
This is very helpful. I am always only too ready to bow to better-informed insights, particularly if they support my main contention. So if I may, I will have two bob each way on these rocks. I will remain sceptical, but will be delighted if they are in fact Martian and Drs Spohn and Wanke are correct about their implications.
Past coverage of as much as 15 percent of the surface of Mars by oceans has been suggested. If ever there were large amounts of surface water on Mars, there would by definition have been extensive shorelines.