‘Bathtub ring’ offers new evidence for ancient Mars ocean

Key Points:

• New research suggests Mars may have once hosted an ocean covering one-third of its surface, evidenced by a broad, flat "coastal shelf" feature similar to Earth's continental shelves, identified using data from NASA’s Mars Orbiter Laser Altimeter.
• This coastal shelf, wider and more erosion-resistant than previously proposed shorelines, likely formed from sediment deposits by ancient rivers and changing sea levels, providing stronger evidence of a large, long-lived ocean on Mars.
• Past Viking missions hinted at shorelines, but inconsistencies and lack of a stable elevation profile left the ocean hypothesis debated; the new study’s shelf concept offers a more robust geological marker for ancient Martian seas.
• Experts note differences between Earth’s and Mars’ geology, such as the absence of plate tectonics on Mars, which complicates direct comparisons, but upcoming missions like ESA’s Rosalind Franklin rover aim to provide definitive data to confirm or refute the ocean hypothesis.
• Confirming a vast ancient ocean would deepen understanding of Mars’ climate evolution, water history, and potential habitability, addressing key questions about how and why Mars transformed from a wetter past to its current cold, dry state.