Subsurface ice-rich layered deposits west of Utopia Planitia: SHARAD evidence from an unexplored longitude band on Mars

Abstract

The northern mid-latitudes of Mars preserve extensive subsurface ice deposits whose distribution and composition record the planet’s Late Amazonian glacial history. In western Utopia Planitia (75-90E, 40-50N), Stuurman et al (2016) identified a subsurface reflector spanning approximately 400,000km2 and interpreted it as an ice-rich layered unit (periglacial unit ABp; Kerrigan, 2013) 80-170m thick with a bulk dielectric permittivity ɛ’=2.8±0-8, consistent with a 50-85% water ice mixture. To the west, in the longitude band approximately 50-75E, no dedicated orbit -level SHARAD survey has been published. This band encompasses the southwestern margin of Utopia Planitia, the northern boundary of Nilosyrtis Mensae, and the Pyramus Fossae tectonic system. This terrain shares the periglacial morphologies of the Utopia type area yet remains uncharacterized in terms of subsurface reflector thickness, lateral extent or dielectric properties.

Data and methods

We present a manual SHARAD reflector-picking analysis of five study areas distributed across this longitude band, based on CO-SHARPS SQA radargram products. Approximately 700 SHARAD radargrams were reviewed in JMARS; 49 orbits across five areas met acceptance criteria including clutter-simulation cross-validation, lateral continuity over no-less than 15 traces, and geometric consistency with surface topography. A total of 1,058 surface-reflector pick pairs were accepted. Two-way travel time differences Δt were converted to thickness at different reference dielectric values (ɛ’=2.8, 3.0, 3.15, 4.0) spanning the published range for mid-latitude ice-bearing material. 

Surface morphology was characterized by CTX imagery and integrated with SHARAD results in ArcGIS Pro.

Results and observations

Four of the five study area (A1-4) displays three key morphological features of Stuurman et al.’s (2016) ABp unit. These are; layered scarps, scalloped depressions, and polygonal terrain - in combination with coherent, clutter validated SHARAD subsurface reflectors. Mean bulk thicknesses at ɛ’=2.8 range from 60m to 72m with maximum thicknesses of 118-158m, all within the 80-170m range of the Stuurman type area.

The spatial distribution of Δt across each area is consistent with lens-shaped deposits thickening toward the unit interior. The four areas span a longitude range of approximately 9-16 west of Stuurman et al.’s western survey boundary at 75E, extending the known or inferred distribution of the ABp unit by at least 600km to the west. 

The westernmost area (Area 3, 59.1E) provides the morphologically strongest match to the type-area, with all three ABp key features present coupled with layering being exposed thorough interior degradation depressions consistent with Stuurman et al.’s (2016) description of interior gaps within the type-area mesa unit.

Area 1 shows a spatially coherent lens shaped depth distribution with coincident fading of surface morphological indicators and subsurface reflector amplitude at the unit’s western margin, providing two independent lines of evidence for a bounded deposit.

A fifth area (A5, 66.5E) is morphologically and geometrically distinct. It displays substantially greater apparent thickness and is associated with a ~28km diameter degraded impact crater at i’s southern margin. Surface polygons are present but show an immature, north-south oriented linear expression on the unit, transitioning to further developed networks on the adjacent crater floor. No single favored dielectric is adopted for Area 5.

Discussion

The convergence of ABp key feature morphology, lens-shaped subsurface geometry, and Stuurman-range thicknesses across four study areas strongly supports the interpretation that the ice-rich layered mesa unit of western Utopia Planitia extends west into the 50-75E longitude band. The consistency of bulk thickness and dielectric properties with the type area suggests compositional continuity of the unit across at least 600km of additional longitude.

The elongated north-south geometry of Area 2 and the alignment of several areas with the Pyramus Fossae trend, raises the possibility of structural control on unit distribution or preservation which warrants further investigation.

Area 4’s greater apparent depth, weaker ABp morphological expression, and crater-margin associations are interpreted as consistent with a crater-confined deposit of lower ice content than the four ABp areas. Following the polygon-maturity explained by Levy et al. (2009), the transition from immature polygons on the unit to better developed polygons on the crater floor is interpreted as reflecting higher ice content in the crater interior. A tentative bulk dielectric of ɛ’=3.5 is proposed, giving a mean thickness of ~119m which lies within the Stuurman thickness range but with a composition intermediate between clean ice and debris-rich material.

Conclusion

Together, these results provide a novel orbit-resolved SHARAD characterization of subsurface ice-bearing material in a previously under-surveyed sector of the northern mid-latitudes and suggest that the ice-rich layered unit of Utopia Planitia extends substantially further west.

 

References

Kerrigan (2013), UWO ETD 1101. Levy et al. (2009), JGR 114, E01007. Levy et al. (2010), Icarus 206, 229. Mellon (1997), JGR 102, 25617. Stuurman et al. (2016), GRL 43, doi:10.1002/2016GL070138. Morgan et al. (2021), Nature Astronomy 5, 2030. Andres & Smith (2025), JGR Planets 130, e2024JE008830.