Circles on Mars: Evidence of a Dynamic, Living Planet

In the battered landscape of the Martian Protonilus Mensae, there are circles made of stone that shouldn't exist—at least not if we assume the Red Planet has been a static, frozen tomb for most of its history. This discovery matters because it shifts our view of Mars from a world that "died" billions of years ago to one with a persistent, oscillating climate.

The Earthly Clue: Sorted Stone Circles

On Earth, these "clastically-sorted circles" are the rhythmic fingerprints of freeze-thaw cycles, where ice pushes stones into rings over centuries of seasonal shifting. What if Mars has been performing this same Earth-like geological dance for over a billion years?

A new geomorphological analysis suggests that instead of a few isolated bursts of activity, Mars has experienced a monumental continuity of glacial and periglacial cycling throughout the Amazonian Epoch.

Core Discovery & Methodology

The Landforms in Question

Researchers utilized HiRISE imagery with a resolution of 25 cm/pixel to identify candidate sorted circles on Mars.

These Martian circles measure 10–20 m in diameter, providing direct morphological comparisons to active periglacial processes on Earth.

A Tale of Two Terrains

The study centered on mapping the contact between two distinct ancient geological units:

The Ancient, Dark Terrain (Unit [eHT]): This surface is estimated to be between ~100 million and ~1 billion years old, based on an analysis of 404 impact craters. Its stability has preserved some of the oldest periglacial landforms ever documented on Mars.
The Younger, Light-Toned Assemblage (Unit [HNt]): This glacial terrain appears much younger, with a crater retention age ranging from ~10 million to 100 million years old.

Implications of Intertwined Ages

A Paradigm Shift for Martian History

The "intertwining" of these ancient and younger terrains is the key finding. It suggests that glaciers on Mars didn't just arrive and vanish once.

They surged and retreated in rhythmic phases over more than a billion years, leaving behind a stratigraphic record of "buried" ice history. This mirrors the cyclical ice ages of our own planet.

Modeling Challenges & Uncertainties

The Ice Evaporation Hypothesis

The team’s 2D plasticity model, using a yield stress of 22 kPa, failed to align with the current elevation profiles of local debris aprons.

This discrepancy indicates one of two possibilities:

Significant amounts of ancient ice have sublimated (evaporated) into the thin Martian atmosphere.
The original glacial foundations are now hidden beneath the billion-year-old dark terrain.

Limitations of the Study

While the evidence for long-term cycling is compelling, the researchers concede certain hurdles:

Imaging Resolution: HiRISE cannot resolve particles smaller than 91 cm, so the presence of fine-grained rocks in the circle margins must be inferred.
Model Assumptions: The 22 kPa model assumes a flat-bed geometry, which may overlook complex mountain-like massifs hidden beneath the ice deposits.

Conclusion: Mars as a Living World

Despite these uncertainties, the study powerfully suggests the Mid to Late Amazonian Epochs on Mars were far more dynamic than previously thought. The planet may have harbored meta-stable liquid water or high-salinity brines far more recently than we dared to imagine.

Source: Soare, R.J., Williams, J.-P., Hepburn, A.J., Butcher, F.E.G., 2022, A billion or more years of possible periglacial/glacial cycling in Protonilus Mensae, Mars, Icarus 385, 115115, https://doi.org/10.1016/j.icarus.2022.115115.