This is an old debate, and one that was tainted early on by hysterical pronouncements from the anomalists of giant "macrobes" on the surface of Mars. At one point Arthur C. Clarke allowed as how they might be lifeforms, and Hoagland & Co. ran with that and declared that Clarke "believed" in these macrobes.

I suppose it's not entirely impossible, when you look at terrestrial analogs such as kilometer-sized fungi found underground, and underwater formations like coral. But both those examples highlight the problem of structural strength in any living thing that size: These real examples require support from the surrounding soil or the water to prevent the organism from collapsing under its own weight. I don't know if 1/3 gravity on Mars makes enough of a difference, or possibly the low air pressure allowing for an "inflated" structure to remain rigid. Then there's the problem of temperature, though in that case the large ratio of volume to surface area might be advantageous. I just don't know.

Given all those disclaimers, now I can go way out on a limb of pure speculation. Suppose a martian organism evolved to live near a vent of some kind, to use its heat and chemical spew. Suppose further that the organism "learned" to cap the vent during a quiescent phase with some kind of flexible, expandable membrane ("learn" in the evolutionary sense of making better and more efficient use of the environment; it was a major leap forward when terrestrial organisms evolved to enclose a small sphere of water and nutrients for their own internal use, and this speculation is based on the same idea writ large). When the vent becomes active, the membrane might trap gases and heat, expand like a balloon, and perhaps grow to great size. A sudden eruption would burst the membrane, but it's possible there are "slow" vents that outgas slowly enough to give the membrane time to grow. Or, I'd allow that eventual bursting could be part of the natural life cycle of such an organism, as a mechanism to propogate spores of some kind. One can go further, and postulate that the interior of such a natural dome would be populated by opportunistic or parasitic organisms, to form an enclosed habitat. Perhaps hypothetical life on Mars took a fundamentally different path--instead of an open, porous biosphere such as Earth's, it evolved in the direction of a myriad of enclosed ecosystems.

We tend to look at the atmosphere of Mars and wonder how in the heck life could possibly have evolved in such a non-nutritional mix; and why we don't see metabolic byproducts if there's life on Mars. But maybe we should approach it from a different perspective: Is the current atmosphere of Mars a consequence of some biological strategy that sequesters the good stuff for itself and shuns the atmosphere? Earth's higher gravity and consequentially thicker air supported the open system of the first oxygen-metabolism life, but on Mars that would likely be a suicidal (or a non-starter) strategy. Life would have had to find another way.

"Suppose"..."perhaps"..."might"..."possible"...fun to imagine it, anyway. Entirely a construct of imagination: I don't "believe" it in any sense.

On a more concrete level, a while back I found two overlapping images taken in 1999 that show what I think is, in actuality, just the spring thaw and preferential melting of darker areas of the surface. I set up a web page with several "blink-comparators" to show the before-and-after of the process in the two images. I find it kind of fascinating to flip between the images and see the changes, because change of any kind on Mars is so rare (on human timescales). It says that Mars is, at least at the most basic level, still somewhat "alive". Stuff happens.

--Robert