“what is believed to be liquid water”
They are looking at it through “ice” of some sort (not frozen water) with radar. I’m pretty sure that any liquid would show up as a strong reflector of the radar waves. It is probably whatever is frozen - but the same crap in liquid form.
Of course that begs the question of why is it liquid? Increased pressure of thicker “ice”?
Heat source within the planet (I don’t think plate tectonics have been proven on Mars.)
There is an old clip from the X_files TV show where some guy is running Ground Penetrating Radar over a basement slab.
“So that will tell you where the body is?”
“Well, no, but a trained operator can differentiate between the signals, but it may be a body, a rock, or a void.”
“Differentiates? So he guesses?”
“Well - it’s an educated guess.”
Looks like I was wrong. Our rovers measured daytime temperatures as high as 43F, and summer at the equator can apparently get up to 70.
Though that plunges to -100 or more at night.