Kasei Vallis on Mars, courtesy ESA Mars Express:

Do you think this is what it appears to be: a rent in the floor of the valley, from which fluid of some sort was expelled relatively recently, fluid which flowed away in torrents (it did not quickly sublime at any rate but could it have been boiling?) both to the left and to the right?

Whatever the fluid was -- sulfuric acid? acidic brine? water? something else? -- appears to have come from the interior, directly beneath the channel's floor, seems to have done some considerable erosion and then disappeared.

Could there be subsurface circulation of fluids that basically parallels the surface channels?

Or is everything frozen to depths of tens of kilometers and more?