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（原標題：Ancient Ocean Found on Martian Middle-Latitude Regions）

A team of researchers from the Chinese Academy of Sciences' Institute of Space Information Science, led by Dr. Fang Guang, has made a significant discovery. The study reveals that beneath the祝融著陸區(qū) (Rutong landing area) on Mars' northern Utopia平原 (Utopia Planum), at depths between 10 and 35 meters, there are multiple inclined sedimentary structures. These features closely resemble high-energy coastal sedimentary deposits on Earth, offering the most direct subsurface evidence to date that ancient oceans once existed in the Martian middle-latitude regions. The findings were published in the prestigious journal《National Academy of Sciences Proceedings》(PNAS) on February 25, 2025.

Mars, with its similarities in geological characteristics, seasonal variations, and diurnal rhythms, has long been considered a prime destination for human interplanetary exploration. Over the past several decades, numerous milestone discoveries have been made in Mars exploration, but most have focused on the extreme cold environments of the high latitudes or polar regions. The debate over whether the northern lowlands on Mars ever contained vast oceans has persisted, highlighting the importance of obtaining direct evidence of ancient Martian oceans.

△A hypothetical image of Mars 3.6 billion years ago. The blue areas depict the locations of the now-dis消失的Deuteronilus ancient ocean and its coastlines. The orange asterisks indicate the landing sites of the Rutong rover, while the yellow asterisks mark the landing sites of NASA's Perseverance rover (image source: Robert Citron)

China's first Mars rover, Rutong, successfully landed on the Utopia平原's southern section on May 15, 2021. Equipped with a domestically developed次表層穿透雷達 (subsurface penetrating radar), Rutong is designed to investigate underground structures and potential water ice deposits. Its operational path lies approximately 280 kilometers north of the previously hypothesized ancient ocean coastlines, at an altitude lower than these coastlines by about 500 meters.

△Map of Utopia Planum, Rutong rover landing site, and four potential ancient coastline lines

The research team analyzed the low-frequency channel data from Rutong's subsurface radar, identifying 76 inclined reflection bodies within the 10 to 35-meter depth range along the rover's path. These reflection bodies are spatially distributed evenly and widely, covering more than 1.3 kilometers. All of them exhibit a consistent northern lowland inclination, with tilt angles ranging from 6° to 20°, averaging 14.5°. Multiple parallel layers were observed at the same location across different depths. These layering structures closely resemble the radar signatures of coastal sedimentary deposits on Earth, with similarities in formation mechanisms and physical properties that rule out factors such as wind-blown sand, volcanic melt channels, or river delta deposits. The presence of such large-scale deposits suggests that wind-driven coastal sediment transport provided a stable net sand inflow to the shore, forming pre-deposit layers. Such structures can only form in extensive and stable large water bodies over long periods, not as a result of brief and localized melting events.

This study not only provides critical subsurface evidence that the northern middle latitudes of Mars once contained ancient oceans but also reveals that Mars experienced long-term warm and humid climatic periods. This implies that Mars maintained suitable temperature and atmospheric pressure conditions for liquid water to exist for extended periods. Additionally, the electrical properties of the coastal sediment deposits align with those of fine sand and silt from Earth, further confirming their oceanic nature.

△Comparison of subsurface radar reflections between Earth and Mars (A) Shark Bay, Australia, coastal sedimentary deposits; (B) Subsurface radar剖面圖 of Rutong rover's low-frequency channel data

△Formation process of subsurface deposits at Rutong rover's landing site (A) Sublandar sedimentation under tidal influence; (B) Long-term erosion and chemical weathering after the ancient coastline recedes and liquid water is depleted, resulting in the formation of the Martian surface layer.

本次發(fā)現(xiàn)的最大意義在于，不僅將火星液態(tài)水的證據(jù)從高緯度的極地地區(qū)擴展到了更適宜人類活動的中低緯度地區(qū)，還證實了火星曾經(jīng)是宜居的星球。如果該區(qū)域曾經(jīng)存在過海洋，那么隨著氣候的變化，大量水可能以地下冰的形式被封存，這將為未來火星基地提供水資源利用的可能性，同時大大降低火星基地建設和維護的成本。此外，這些古海洋沉積物不僅保存了火星氣候變化的歷史信息，研究它們還有助于我們理解火星是如何從溫暖濕潤的環(huán)境轉變?yōu)楹涓稍锏?，這將為指導人類如何改造火星環(huán)境、實現(xiàn)火星的長期可持續(xù)居住提供科學依據(jù)。

（總臺央視記者 帥俊全 褚爾嘉）