Yes, you are correct, there are great difficulties in getting liquids on Mars without some heat engine present. And once the liquid is present, it shouldn't stay around for long. However, there have been volcanoes evidently not in the extreme past. There are many features that were seemingly carved by flows that at least partially contained water. Also, I am not ready to write off the entire planet of Mars as to be without any heat sources whatsoever. Obviously, impacts could provide local heating in the past as well, but it is not too relevant today as the planetary bombardment by asteroids is rather sporadic these days! While I'm certainly not thinking in terms of Earth like oases, still it is tantalizing to think that some Mars oases may be present in underground caverns. It would only take one anomaly that conducted heat from deep underground closer to the surface to provide for some potentially interesting phenomena and environments. We've got a lot more exploring to do, before Mars reveals its secrets.