“Springtime on Earth has lots of trickling as water ice gradually melts. But on Mars, everything happens with a bang,” said Serina Diniega of NASA’s Jet Propulsion Laboratory. That “bang” is not just a poetic flourish; it’s the soundless drama of carbon dioxide frost sublimating directly into gas, reshaping the Red Planet’s surface and hinting at its climatic past.
Among the most curious actors in this seasonal performance are the so‑called kidney bean-shaped dunes in Mars’ northern hemisphere. Captured in exquisite detail by NASA’s High-Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter, these dunes are mantled with CO₂ frost during the long Martian winter. The frost locks the dunes in place, halting their migration until spring sunlight triggers sublimation. This transition from solid to gas bypasses the liquid phase entirely, a process driven by Mars’ low atmospheric pressure currently about 0.6% of Earth’s. When the frost vanishes, winds resume their work, shuttling sand from one side to the other, slowly advancing the dunes across the landscape.
The amount of frost deposited in each winter is strongly coupled to the axial tilt or obliquity of Mars. While Earth’s tilt is stabilised by its massive Moon, the tiny Martian moons Phobos and Deimos exert negligible gravitational influence. Mars’ tilt thus wobbles from nearly upright to more than 80°, wildly changing climate patterns. Orbital simulations reveal that these changes occur in 2.5‑million‑year cycles, with shorter-term oscillations layered on top. At high tilt, polar regions receive intense summer sunlight, which releases vast stores of CO₂ and water vapor into the atmosphere. Both gases are strong greenhouse agents that thicken the atmosphere, raise the surface pressure, and create conditions under which liquid water might persist-and that might have supported life.
Such climate swings are evidenced not only by dune frost but also by buried CO₂ deposits. Ground-penetrating radar has revealed a reservoir near Mars’ south pole containing 80% of the CO₂ currently in the atmosphere. If this reservoir were released during times of high obliquity, it would give rise to a substantial increase in atmospheric pressure, as much as 75% above that today, due to increased wind and possibly more frequent dust storms. Thicker atmospheres trap more heat, and the absence of a strong magnetic field allows solar wind to strip the gases away over geological timescales in a complex feedback loop.
The HiRISE observations of frost cycles complement measurements from NASA’s MAVEN mission, which has been measuring the steady loss of atmospheric CO₂ to space. However, carbonate-rich sedimentary rocks discovered by the Curiosity and Perseverance rovers imply another significant sink. In Gale Crater, Curiosity drilled into strata bearing 5–11 wt% carbonates, likely created when atmospheric CO₂ interacted with liquid water. These carbonates sequester CO₂ permanently and draw down atmospheric pressure toward the point at which liquid water becomes unstable. The inclusion of carbonate formation in models of climate evolution demonstrates that Mars may have been able to support oases of surface water intermittently for hundreds of millions of years beyond the loss of its global habitability.
Seasonal frost dynamics also provide a window into contemporary atmospheric behaviour. As CO₂ frost sublimates in the spring, it drives localised jets of gas that can erode surface material, carve “swiss cheese” depressions in polar ice, and set dunes in motion. These changes are not uniform: from year to year, variations in frost thickness and sublimation rates reflect subtle shifts in Mars’ orbital parameters and seasonal energy balance. Over decades of observation, scientists have been able to correlate these frost cycles with broader climatic patterns, which might offer clues into how Mars’ atmosphere responds to orbital forcing. To planetary scientists, the kidney bean dunes are more than whimsical shapes-they are natural climate loggers.
By tracking their frost cover and migration rates, researchers can infer seasonal CO₂ budgets, and by extension, reconstruct aspects of Mars’ obliquity history. Each frosty winter and explosive spring adds another data point to the long record of a planet whose climate has swung between extremes, sometimes thick and warm enough for rivers and lakes, now thin and cold enough that water can only exist as ice or vapour.
