re: OK, so can we agree that Mars has been dead for 3.8 Ga?
Whoa there, I wasn't saying anything about mars being alive or dead just pointing out the story! I don't think liquid water is anything other than extremely rare and transient on the martian surface, and due to occasional heating of ice by volcanoes or impacts. Back in the deep past when mars had more of both it would make sense that liquid water events were more frequent, and that fits with this evidence nicely.