Mars Didn't Have Bathtubs, It Had Shelves

Scientists have been debating for decades whether Mars once held a vast ocean covering a large part of its northern face. To prove the idea, they’ve been looking for a “bathtub ring” - a distinct, level shoreline that shows where water once stood. But, despite years of looking, they’ve only been able to find a very distorted potential shoreline whose height deviates by several kilometers - not exactly great evidence of a stable water level. But, according to a new paper in Nature from Abdallah Zaki and Michael Lamb of CalTech, what scientists should have been looking for wasn’t a bathtub ring, but a continental shelf.

So what could cause a distorted shoreline? Existing arguments for what might have caused the massive changes in the observable shoreline ring included true polar wander, where a mass redistribution of the planet itself caused by its spin would change the location of its equatorial bulge and noticeably warp the shorelines on other parts of the planet. Another theory was that the growth of the volcanoes surrounding Tharsis was so massive that it caused the entire planet to “flex”, altering its shape while the oceans were still present. But according to the paper, the answer might be much simpler.

On Earth, the most prominent feature of an ocean isn’t a shoreline - it’s the low slopes of our coastal plains and continental shelves. They just happen to be covered up by water, making it difficult for us to visualize them on our own planet. But the geological difference is clear enough - the researchers found that Earth above sea level typically has a 0.3° slope whereas below sea level it drops off to 0.08°. Turning that same analysis to Mars, they found an almost exact “flat zone” match ranging between -1,800 and -3,800 meters in elevation.

Fraser talks about the history of the search for life on Mars.

To say that the newly discovered shelf was massive is an understatement. It covers 10.2 million square kilometers - almost 7% of the entire Martian surface. And it has several features that confirm the theory that it is, in fact, a continental shelf. Most of the known deltas that rovers and orbiters have already found on the Red Planet sit within that region. Two “Shorelines” known as Arabia and Deuteronilus also sit tucked inside that shelf, and thick groupings of layered rocks and clays are also concentrated in this zone - and they are only formed in long-lived water.

But that doesn’t explain why the shorelines appear so warped. The simple answer appears to be that, without tectonic cycles to “recycle” the crust, Martian deltas and shelves suffered much more extreme sea-level fluctuations than on Earth. Evidence from the Hypanis Valles and Aeolis Dorsa areas show sea-level changes of around 500 to 900 meters - up to eight times larger than Earth’s glacial cycles. But these changes happened over millions of years, “smearing” the shoreline indicators, and leading Mars researchers astray for decades.

But the evidence for this theory isn’t only coming from orbital measurements. The Zhurong Rover is currently exploring Utopia Planitia, where it detected subsurface sediment layers that look exactly like coastal deposits on Earth. Perseverance famously also found traces of beaches and rocks modified by circulating water around Jezero Crater. Soon there might be even more evidence, as ESA plans to land the Rosalind Franklin rover in 2030, which will search Oxia Planum, which is right within this proposed shelf line.

Video discussing how we measure Mars’ ancient ocean. Credit - NASA Goddard YouTube Channel

Stable water was the most likely environment to develop life. Consequently, searching places where we know there was stable water for millions of years is where we are most likely to find evidence of previous living organisms. That’s been a long-term goal of the Martian exploration community for as long as they’ve been looking for bathtub rings, so having additional evidence of such a huge area that could potentially fit the bill is a massive boon to their efforts. It still might be some time before we find Martian life, if we ever do. But now we at least have a better idea of where to look for it.

Learn More:

CalTech - "Bathtub Ring" is New Evidence for an Ancient Ocean on Mars

A.S. Zaki & M.P. Lamb - Identifying the topographic signature of early Martian oceans

UT - New Evidence That An Ancient Martian Ocean Covered Half The Planet

UT - Yes, Mars Once Had an Ocean, says New Research