The largest volcano in the Solar System may once have been an island in a vast sea, according to a new study.

When Mars was young and wet billions of years ago, the gigantic Olympus may have resembled Mons Stromboli or Savai'i, but on a much larger scale.

A new analysis shows similarities with active volcanic islands on Earth, adding to a growing body of evidence about Mars' watery past.

"Here we show that the Olympus Mons giant volcano shares morphological similarities with active volcanic islands on Earth, where large structural slope breaks systematically occur at the sea-air transition in response to sharp lava viscosity contrasts," writes a team led by geoscientist Anthony Hildenbrand of the University of Paris-Saclay in France.

"We propose that the upper edge of the 6-kilometre-high concentric bedrock surrounding Olympus Mons was most likely formed by lavas flowing into liquid water when the structure was an active volcanic island during the late Noachian-early Hesperian period."

Olympus Mons is a monster, no matter how you measure it. The shield volcano is about 25 kilometres (16 miles) high and spreads over an area roughly the size of Poland (or Arizona, as another point of reference).

It is not only the largest volcano, but also the highest known planetary mountain in the Solar System. But there is something strange at its feet. It does not meet the ground as a clean slope. Rather, at an altitude of about 6 kilometres, for much of its circumference it becomes a prominent cliff or crag that drops sharply into the surrounding landscape below. The origin of this feature is something of a mystery.

Mars today is very dry and dusty. Any surface water is in the form of ice; rivers do not flow, oceans do not fill its vast basins and craters. But evidence continues to emerge that Mars was once rich in liquid water.

Gale Crater, where Curiosity travelled, appears to have been a large lake billions of years ago. Perseverance in Jezero Crater discovers an ancient delta that has dried up.

Hildenbrand and his colleagues used this information to recontextualise Olympus Mons. They looked at similar shield volcanoes on Earth. In particular, they looked at three active volcanic islands: Pico Island in Portugal; Fogo Island in Canada; and Hawaii Island in the USA.

They found that the coasts of these islands have sharp slopes, similar to the slopes surrounding Olympus Mons. On Earth, these cliffs are the result of sharp contrasts in lava viscosity due to differential cooling as it passes from air to water.

"This leads us to think that Olympus Mons was an ancient volcanic island surrounded by liquid water," the researchers write in their paper.

This could give us some clues about Mars' water history. For example, the height of the rock could be the sea level of the long-lost ocean. And the age of the lava flows, dated to about 3.7 to 3 billion years ago, tells us when this ocean might have been there.

The team also detected similar features on Alba Mons, another Martian volcanic mountain located in a vast plain more than 1,500 kilometres (932 miles) away from Olympus Mons.

This could mean that the waters of this ghost ocean filled much of the Martian surface.

The team says the results offer future Mars exploration missions a new avenue of research into the history and evolution of Mars.

"Future spacecraft dedicated to rovers equipped for sample return and/or in situ dating in selected regions of Olympus Mons constitute a promising line of research for the future that could have important implications for the longevity of oceans and the potential fate of early life on Mars," the researchers write.

 

Source: https://www.sciencealert.com/