Constraints on Shallow Subsurface Water Ice in Southern Utopia Planitia from Mars Rover Penetrating Radar

The global distribution and stratigraphy of Martian near-surface water ice are critical for understanding the planet’s hydrological evolution and for planning future in-situ resource utilization. While ice is well-documented in polar and high-latitude regions, its presence and stability at lower latitudes remain a key open question. China’s Tianwen-1 mission, which landed the Zhurong rover in southern Utopia Planitia, provides a unique opportunity to investigate this. The landing site lies within a region bearing extensive geomorphological evidence (e.g., lobate debris aprons, polygons) suggestive of a complex aqueous and glacial past, making it a prime candidate for detecting preserved subsurface volatiles.
The Zhurong rover is equipped with the Mars Rover Penetrating Radar (RoPeR), a low-frequency ground-penetrating radar operating in the 15-95 MHz range. By analyzing the propagation delay, amplitude, and frequency dispersion of reflected signals, RoPeR can reconstruct subsurface stratigraphy and constrain the electromagnetic properties of buried materials. During its traverse, Zhurong’s RoPeR detected a distinct, laterally continuous layer at a depth of several meters. This layer is approximately 7 meters thick and is characterized by remarkably low electromagnetic signal attenuation, bounded above and below by materials exhibiting higher losses.
Quantitative analysis of the radar signals yields a low loss tangent of 0.0030 ± 0.0018 and a dielectric constant of ~3.86 for this discrete layer. These values are inconsistent with dry, porous regolith or typical basaltic rocks but align closely with the expected properties of low-loss, low-density water ice or ice-rich regolith at Martian conditions. To test this interpretation, we performed extensive forward modeling to simulate radar wave propagation through various plausible subsurface scenarios. We constructed models with differing lithologies (dry sediments, porous rocks), stone abundances, bulk densities, and volumetric ice contents.
Our simulations demonstrate that models of a layer composed of "dirty ice", a mixture of water ice (estimated at 30-70% by volume) with dispersed lithic fragments and regolith, best reproduce the observed radar signal’s amplitude, two-way travel time, and low attenuation characteristics. Alternative models involving very dry, highly porous materials or layered fractured rock fail to simultaneously match the derived electromagnetic parameters and layer geometry.
The identification of this potential ice-bearing layer at low-to-mid latitudes (~25°N) in Utopia Planitia has significant implications. It suggests that remnant ice from past climatic obliquity cycles may be preserved at shallow depths in specific, protected geological settings. If confirmed, such a reservoir would represent a highly accessible resource for future crewed exploration, potentially simplifying mission architectures. This finding from RoPeR provides the first direct geophysical evidence pointing at substantial, localized subsurface ice reserves outside Mars’s classical high-latitude ice stability zones, offering a new target for understanding the planet’s water inventory and stability history.