Summary
caltech geologists abdallah-zaki (former postdoc) and michael-lamb (geology professor) report in Nature (April 15, 2026) topographic evidence for a long-dried northern ocean that once covered roughly a third of Mars. The team first “dried up” Earth’s oceans in computer simulations to identify which features survive — finding that continental shelves (flat bands hundreds of km wide, like a bathtub ring) are the most stable ocean-margin landform. They then identified an analogous shelf-like band in Mars orbiter topography in the northern hemisphere, with associated river deltas aligned to it. Because shelves require sustained marine conditions to form and are not produced around lakes, the Martian shelf implies a stable ocean lasting possibly millions of years, billions of years ago. The authors flag Martian coastal sediments as priority targets for future rovers and mars-sample-return because terrestrial coastal sediments are exceptional fossil archives. The paper does not claim biosignatures but reframes where to look for them on Mars — see ancient-mars-ocean.
Key Claims
- A flat topographic band wraps the Martian northern lowlands at a consistent elevation, analogous to Earth’s continental shelf.
- Shelves form only around long-lived oceans, not lakes — so the Martian feature implies an ocean stable for millions of years.
- The dried-up ocean once covered roughly one third of Mars’s surface.
- Inferred river deltas line up with the shelf, reinforcing a coastline interpretation.
- Past “shoreline” claims were ambiguous because they appear at varying elevations; the shelf is a more robust signature.
- The ocean dried up “possibly several billion years ago, more than half the age of the planet.”
- Coastal sediments are the recommended next-mission target as terrestrial-style fossil archives for biosignatures.
- Funded by Caltech, the Swiss National Science Foundation, and UT Austin’s Jackson School.
Notable Quotes
“If Mars did have an ocean, it dried up a long time ago — possibly several billion years ago, more than half of the age of the planet itself. There is hardly anything on Earth that is that old; anything on Mars from that time has been eroded by billions of years of wind blowing, volcanoes erupting, and other disturbances removing subtle features. We wanted to find a better topographic feature than shorelines that could be evidence for an ocean.” — Michael Lamb
“The shelf is a new observation that ties together evidence of what the coastal zone would have looked like. Nobody had really looked for it before. It’s a strong additional piece of evidence supporting a northern ocean on Mars, but there’s plenty of follow-up work to be done for rovers to examine deposits and for further analysis of satellite data.” — Abdallah Zaki