Ancient ocean coastal deposits imaged on Mars

Abstract

The northern lowlands of early Mars could have contained a significant quantity of liquid water. However, the ocean hypothesis remains controversial due to the lack of conclusive evidence from the Martian subsurface. We use data from the Zhurong Rover Penetrating Radar on the southern Utopia Planitia to identify subsurface dipping reflectors indicative of an ancient prograding shoreline. The reflectors dip unidirectionally with inclinations in the range 6° to 20° and are imaged to a thickness of 10 to 35 m along an uninterrupted 1.3 km northward shoreline-perpendicular traverse. The consistent dip inclinations, absence of dissection by fluvial channels along the extended traverse, and low permittivity of the sediments are consistent with terrestrial coastal deposits-and discount fluvial, aeolian, or magmatic origins favored elsewhere on Mars. The structure, thickness, and length of the section support voluminous supply of onshore sediments into a large body of water, rather than a merely localized and short-lived melt event. Our findings not only provide support for the existence of an ancient Martian ocean in the northern plains but also offer crucial insights into the evolution of the ancient Martian environment.

Keywords: Mars; Zhurong rover; ancient ocean; ground penetrating radar; sedimentary deposits.

Fig. 1.

Zhurong rover landing site and proposed shorelines in Utopia Planitia. (A) Map of Utopia Planitia, showing the landing site of the Zhurong rover and four proposed ancient shorelines (9, 11, 22, 23). The Zhurong landing site is ~280 km north of and ~500 m lower in elevation than the northern hypothesized shorelines (6). (B) HiRISE image (ESP_073225_2055) with the traverse of the Zhurong rover from Sol 11 to Sol 325. The color marked along the traverse indicates the dip angles identified by the RoPeR data. Image credit of HiRISE: NASA/JPL/University of Arizona.

Fig. 2.

B-scans of the RoPeR low-frequency channel data revealing buried coastal sedimentary deposits. (A and B) Processed radargrams of RoPeR low-frequency channel from 12 to 288 m along the Zhurong rover traverse. The radar profile is limited to a depth of 10 to 35 m to highlight the dipping reflectors. Black lines in (B) denote the dipping reflectors identified as the coastal sedimentary deposits. The corresponding interpretive geologic cross-section at the same scale as (A and B) is shown in SI Appendix, Fig. S5. (C) Comparison of dip angles of coastal sedimentary deposits on the Earth and Mars. Some data are presented as mean values ± SD (–48). The Zhurong landing site data are based on measurements from this study. (D) Sketch map illustrating the formation of multilayer dipping sedimentary deposits in a coastal environment.

Fig. 3.

Schematic model of the formation process for tilted sedimentary terrain at the Zhurong landing Site. (A) Stratified structures formed under tidal sedimentation, (B) with the ancient shoreline regressed, liquid water disappeared and sedimentation ceased. Then long-term physical and chemical weathering altered the properties of the rocks and minerals, leading to the formation of a Martian surface layer. Consequently, the sedimentary deposits were covered by the current Martian surface soil.