We may be routinely orbiting, roving, drilling and lasing Mars, searching for elusive traces of life and reconnoitering sites for future human missions, but that doesn't mean studies of the red planet don't throw up surprises. On the contrary.
Take this March 21, 2013 observation by the High-Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter (MRO) of the southern edge of Acidalia Planitia, a plain located in the planet's northern hemisphere.
These irregular depressions with weird raised rims aren't impact craters and they can't be wind-blown features as the pits contain boulders that could not have been moved by the Martian winds. HiRISE mission scientists don't believe they could be caused by volcanism either.
Scientists believe the Acidalia Planitia region of Mars was once the location of a huge ocean, so it seems plausible that it may have been caused by some fluvial process. They may also have been caused by shallow lenses of water ice that have since sublimated into the atmosphere, leaving these small basins. But that doesn't obviously explain why the depressions have raised rims.
"Ancient glaciation is another possibility, perhaps depositing ice-rich debris next to topographic obstacles," said HiRISE principal investigator Alfred McEwen, planetary geologist at the University of Arizona. "Future images of this region may provide clues, but for now this is a mystery."
A wider-angle view of the mystery pits show them hugging a long, raised ridge:
If I had to place a bet, I'd put my money on some kind of ancient permafrost process -- perhaps the water ice sublimation idea or some more complex subsurface process that caused the slumping of material. The long ridge that the pits seem to be formed along is interesting too, so some kind of tectonic process shouldn't be ruled out.
Acidalia Planitia is a fascinating region. It is, after all, home to the famous "Face on Mars" mystery that captivated the world for years until advanced orbital missions proved that the "head" was actually a mesa (a Mars hill) and the "face" was just a trick of the light.
This little mystery of odd depressions, however, probably won't be solved until we can get a robotic or human mission to view the pits up-close. Until then, we can only speculate.