We present the most comprehensive analysis to date of the basal unit, a vast, ice-rich deposit beneath Mars' North Polar Layered Deposits. We mapped the internal structure of the basal unit across more than 600 radar profiles acquired by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express, leveraging new processing techniques and extensive coverage of the north pole of Mars.

The results reveal that the rupēs subunit—previously poorly understood—extends widely across the western polar region and comprises more than half of the basal unit’s total volume. By analyzing the radar reflections and reconstructing the electromagnetic properties of the rupēs unit, we found that it has a distinct composition compared to other polar units: significant amounts of sediments make up this unit, such as minerals derived from basalt alteration like oxidized iron and gypsum.

These findings suggest the materials may be the result of water-based alteration during warmer periods of Mars' climate history. Together, the results offer a new, deeper look into Mars' climate evolution and sedimentary history over the last 3 billion years, with implications for water-related processes during past warmer periods.