@Trevor_Pott - Questions And Statements, Valid And Otherwise
Look at my original question: "I wonder how sure they are that the sediments could only have been formed by liquid water and not some other liquid."
Now let's look at your post. First you state "there are plenty of geochemical signatures that - to our current knowledge at least - can only occur with Mars having had a thicker atmosphere." And then - just a few words later - you state "This means that we no longer need to prove that it occurred; there is so much evidence that it did we can simply take it for granted.
So in the span of a few short words you've managed to go from the correct idea that scientific theories are provisional and subject to change (in consequence of the acquisition of better data, different interpretations of the data or of the pre-existing theories on which the theory under discussion is built, or simply in consequence of someone having newer and maybe better ideas), to the completely erroneous idea that science has arrived at something which you seem to think is very, very close to an incontrovertible truth.
That you think that we have arrived at a state where we are able to reveal the ultimate truth in this matter is laughable; I would be very surprised if any scientist actually involved in these matters would take the same view.
What you seem to be unaware of, is that a theory built on a series of high-probability statements becomes a lower-probability theory the more higher-probability statements it includes. Here: calculate the probability of a theory being correct if it is based on FIVE statements each with a 90% probability of being correct. Hint: the probability of five statements, each having a 90% probability of being true, all being true is a somewhat underwhelming 59%.
By the way, that's one of the reasons why astrophysicists, astrobiologists, xenobiologists, and their ilk, want more scientific space missions: because the certitude that you have is something that they don't have. You seem to think that "our current knowledge" has enabled us to arrive at The Truth.
To sum up: your vacuous post and pompous disquisition on how science operates (in your limited understanding) really don't answer the question of "how sure they are that the sediments could only have been formed by liquid water and not some other liquid" Some essentially empty babble about "geochemical signatures" and "evidence stacking up" is empty handwaving. And although the earth might indeed be "one hell of a lab" it is not "one hell of a lab" for absolutely whatever needs to be found out - and it might not all that effective for finding out the natural history of Mars. It's possible that extrapolating from the earth to Mars might actually mislead us. This is what needs to be actually be discovered, not merely surmised.
Furthermore, asking how sure scientists really are of their theories is not - as you seem to think - some form of lese majesty against science. What is actually is, is a very valid question.