Mars has water ice beneath its mid-latitude surface, 25 to 255 centimeters deep — too shallow for primordial deposits, too widespread for local accumulation. Climate models of present-day Mars cannot reproduce the ice's spatial distribution. The ice doesn't match the planet's current climate.
Vos et al. (2026, arXiv 2602.18847) show that the ice is a remnant of a past ice sheet deposited roughly 630,000 years ago, when Mars's axial tilt reached 40 degrees — compared to today's 25 degrees. At high obliquity, polar ice sublimated and redeposited at mid-latitudes as a surface ice sheet. When obliquity returned to lower values, the surface ice sublimated from the top, but a thin lag deposit of dust accumulated fast enough to insulate the remaining ice before it could fully evaporate. The ice buried itself.
The model's key success is reproducing the observed depth pattern: shallower burial in some regions, deeper in others, matching the spatial distribution that previous models could not explain. The burial depth records how much ice sublimated before dust cover stabilized. The ice is younger than 4 million years — recent by geological standards, ancient by climatic ones.
The general principle: a spatial pattern that cannot be explained by current conditions is evidence that the conditions were different when the pattern formed. The ice is not an equilibrium feature of modern Mars — it is a fossil of a different Mars, preserved by the same process (sublimation) that would have destroyed it, because the byproduct (lag dust) created a protective layer before the process completed. The agent of destruction was also, incidentally, the agent of preservation.