NASA
A new study that Mars’ distorted coastline was caused by a continental shelf and not a “bathtub ring.”
For decades, scientists have debated whether Mars was once home to a vast ocean that covered much of its north face. To prove this hypothesis, they have been looking for a “bathtub ring” — a distinct, level shoreline that indicates where the water once was. But despite years of searching, they were only able to find a possible, highly distorted shoreline that varies in height by several kilometers — which isn’t exactly great evidence of a stable water level.
However, according to a new paper published in the journal Nature by Caltech’s Abdallah Zaki and Michael Lamb, what scientists should have been looking for was not a bathtub ring, but rather a continental shelf.
So what could cause a distorted shoreline? Existing arguments to explain the drastic changes in the observable coastal ring include true polar drift, where a mass redistribution of the planet itselfcaused by its rotation, would change the location of its equatorial bulge and visibly deform coastlines in other parts of the planet.
Another theory was that the growth of the volcanoes around Tharsis was so massive that it caused the entire planet would “flex”changing its shape while the oceans were still present. But, according to the article, the answer could be much simpler.
On Earth, the most prominent feature of an ocean is not the coastline, but the gentle slopes of our coastal plains and continental shelves. It turns out that they are covered in water, which makes it difficult to see them on our planet. But the geological difference is quite clear: researchers have found that the Earth above sea level has generally a slope of 0.3°while below sea level it drops to 0.08°. Applying this same analysis to Mars, they found an almost exact match in the “flat zone,” ranging from -1,800 to -3,800 meters in altitude.
To say the newly discovered platform was huge would be an understatement. Copper 10.2 million square kilometers — almost 7% of the entire Martian surface. And it has several characteristics that confirm the theory that it is, in fact, a continental shelf. Most of the known deltas already found by rovers and orbiters on the Red Planet are located in this region. Two “shorelines”, known as Arabia and Deuteronomy, are also located within this platform, and dense groupings of layered rocks and clays are also concentrated in this zone – and only form in long-lived waters.
But that doesn’t explain why the coastlines look so distorted. The simplest answer seems to be that without tectonic cycles to “recycle” the crust, Martian deltas and shelves have experienced much more extreme sea level fluctuations than on Earth. Evidence from the Hypanis Valles and Aeolis Dorsa areas shows changes in sea level of around 500 to 900 meters – up to eight times longer than Earth’s glacial cycles. But these changes occurred over millions of years, “distorting” coastline indicators and leading Mars researchers to wrong conclusions for decades.
But the evidence for this theory doesn’t just come from orbital measurements. The Zhurong rover is currently exploring Utopia Plainiawhere he detected layers of underground sediments that exactly resemble coastal deposits on Earth. The Perseverance rover also found traces of beaches and rocks modified by water circulation around Jezero Crater. We may soon have even more proof, as ESA plans to land the Rosalind Franklin rover in 2030, which will explore Oxia Planum, located exactly within this proposed continental shelf line.
Stable water was the most likely environment for life to develop. Consequently, looking in places where we know there has been stable water for millions of years is where we are most likely to find evidence of earlier living organisms. This has been a long-term goal of the Mars exploration community since they started looking for bathtub markings, so having additional evidence from such a vast area that potentially fits this description is a huge boon to their efforts. It may still be some time before we find life on Mars, if we ever do. But now at least we have a better idea of where to look.
