By Will Dunham
WASHINGTON, April 15 (Reuters) - There is a lot of evidence from orbiting satellites and surface rovers indicating that liquid water was present long ago on the surface of Mars in the form of ponds, lakes and rivers. But the idea that Mars once had a huge and long-lived ocean on its northern plains has remained a matter of debate.
Scientists have now detected what might be the contours of this putative ocean on Earth's planetary neighbor, using data collected by a NASA probe that orbited
Mars for a decade. Their research indicates the presence of the Martian equivalent of the continental shelf that defines the boundaries of Earth's oceans.
They compared it to a huge "bathtub ring" showing where water may have once met land on Mars. Because Mars does not possess continents and lacks the geological process called plate tectonics that led to the formation of Earth's continents, the researchers are calling the Martian landforms a coastal shelf.
Like Earth and the solar system's other planets, Mars formed roughly 4.5 billion years ago. Early in its history, Mars was warmer and wetter than the cold and arid place it is today.
"The Martian ocean could have existed when the planet had an active hydrological cycle, with flowing rivers and lakes, nearly 3.7 billion years ago, and it likely disappeared as the planet became dry. Exactly where the water went remains highly debated," said University of Texas planetary scientist Abdallah Zaki, lead author of the research published on Wednesday in the journal Nature.
A coastal shelf could have formed on Mars over millions of years due to rivers dumping sand and mud into an ocean, waves spreading those sediments around, and the sea level rising and falling over time. Looking from above, the sedimentary rock making up this shelf might superficially resemble the ring that remains after a bathtub has been drained.
"A 'bathtub ring' on Mars means that if an ocean filled the northern lowlands, it may have left behind a shoreline, or shelf-like boundary, marking the water level," Zaki said.
"If we sent a rover, we would expect to see sedimentary rocks and structures similar to those found on Earth's continental shelves, including layering, sloping surfaces called clinoforms and textures produced by waves and currents," Zaki said.
The researchers examined topographical data for Mars that was obtained by NASA's Mars Global Surveyor, and identified a band suggestive of oceanic contours. Previous research has indicated that this hypothesized ocean in the Martian northern hemisphere covered approximately a third of the planet's surface, equivalent to about 13% of the total area of Earth's oceans.
The study builds on earlier evidence of a bygone Martian ocean including studies identifying features resembling an ancient shoreline. Ground-penetrating radar data obtained by China's Zhurong rover, described in a study published last year, detected evidence suggesting sandy beaches from a Martian shoreline now buried underground.
Scientists also have identified multiple landforms that appear to be remnants of ancient river deltas, where rivers may have flowed into a larger body of water.
"The northern ocean on Mars, if it existed, dried up a long time ago, and there have been billions of years of volcanic activity and wind abrasion on Mars, so interpreting ancient landforms is not straightforward," Caltech planetary scientist and study senior author Michael Lamb said.
"Nonetheless, our study shows some telltale signs of a coastal shelf. And the shelf occurs in the same zone as the deformed shorelines and where scientists have previously mapped river deltas and other landforms and deposits characteristic of a land-ocean transition," Lamb said.
Taken together, Lamb said, the evidence points toward a long-lived ancient ocean.
"This means that Mars once looked much more like the Earth than it does today," Lamb said.
The existence of an ocean would be an important factor in understanding whether Mars was ever able to give rise to life.
"If Mars once had an ocean covering roughly one third of the planet over geologic timescales, that would suggest that a large part of its surface had sustained water, which is a key ingredient for habitability," Zaki said.
"That does not mean Mars was inhabited, but it does suggest that potentially habitable environments may have been more widespread and longer-lived than if water had only existed briefly or locally," Zaki said.
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
