Olympus Mons is one of the many reasons I have an abiding fascination with Mars. Located to the northwest of the Tharsis Montes (Tharsis Mountains), a chain of super volcanoes marching across the planet’s northern hemisphere, Olympus Mons is the largest of all the volcanoes so far discovered in the solar system and boasts some incredible statistics.
For example, it rises a huge 26 km above the surrounding plains, or 21.9 km above datum for the planet, marking it as being around twice the height on Hawaii’s Mauna Kea as it rises from the sea bed. It is over 600km, covering an area almost the size of Poland. The volcano’s peak comprises a series of nested caldera craters which all speak to a violent volcanic past, and which at their widest measure some 60 km x 80 km and are up to 3.2 km deep.
So broad is the volcano that its slopes would not be at all mountain-like, but rather a continuous incline rising for the most part at an angle of just 5% from the horizontal; outside of the base escarpment that is. The latter, running around the volcano forms a near-continuous set of cliffs rising up to 8 km from the plains on which it sits.
Precisely how Olympus Mons formed has been open to some debate. While it and the three volcanoes of the Tharsis Montes – Arsia Mons, Pavonis Mons, and Ascraeus Mons (all of which are as impressive as Olympus Mons, if each somewhat smaller) – formed in the same period of Mars’ early history some 3.7 to 3 billion years ago, Olympus Mons is potentially the eldest. Now a team led by Anthony Hildenbrand of Université Paris-Saclay in France believe they can show that a major contributing factor in the formation of Olympus Mons was water.
Using data from a range of missions in orbit around Mars, the team has carried out an extensive comparative study between Olympus Mons and volcanic island chains such as the Azores, the Canary Islands and the Hawaiian islands. In doing so, they have found evidence which strongly supports the idea of the escarpment around Olympus Mons were laid over thousands of years through the interaction of lava from the volcano and a surround ocean.
That an ocean once existing in the northern lowland of Mars – called the Vastitas Borealis – has long been known. However, given the elevation at which Olympus Mons sits, it had long been assumed it was above this ancient ocean. However, in their work, Hildenbrand’s team suggest Olympus Mons actually grew out of the ocean, rising through successive eruptions in much the same way as, say, Mauna Kea, until it broke the surface of the sea, and the interaction of the hot lava and cold water giving rise to the escarpment as the volcano contained to rise.
In support of this, the team found evidence that the flanks of Alba Mons, another huge, but much flatter – a mere 6.8 km in elevation – volcano further north along the edge of Vastitas Borealis and much older than Olympus Mons, suffered a series of violent tsunamis. These were likely the result of the violence of the eruptions which raised Olympus Mons.
If Hildenbrand’s team are correct in piecing their evidence together, it could help explain one of the many mysterious of Mars. The edge of Vastitas Borealis has two shorelines differing substantially in elevation. Until this study, it had been widely accepted that the two shorelines were the result of two different oceans having once occupied the lowlands. The first, much higher (and older) shoreline marked a time very early on in Mars’ history when Vastitas Borealis was home to a broad, deep ocean which, due to climatic changes was almost completely lost.
Then, as volcanism again took hold, warming the planet again a few hundred million years later, a new, much shallower sea formed within Vastitas Borealis, evening rise to the younger shoreline at the lower elevation. However, this idea has always had its problems; in particular, it seems unlikely a vast, globe-circling ocean would form, and then almost complete recede, only to return again, even during Mars’ somewhat cyclical warm, wet period of history.
Instead, Hildenbrand’s work suggests that both shorelines belonged to the same ocean, one which was continuously present on Mars for perhaps close to a billion years. What changes was that in that period, the massive volcanic activity that gave rise to first Alba Mons and then to Olympus Mons and the Tharsis Montes and Tharsis Bulge, pushed up the overall elevation of the northwest quadrant of the planet to a far greater extent than thought.
Again, if this theory is correct, and Mars likely had a single, continuous northern ocean directly interacting with the volcanic activity in the region, it would have had a significant impact on the development of the planet’s climate and environment, including the development of any life which may have also developed.
The volcanic shorelines proposed in our paper may be an unambiguous witness for past sea level, where research for traces of early life (organic matter) could be targeted. More generally speaking, knowing where and when past Martian oceans may have been has significant implications for climatic models, because this would give decisive constraints on the initial amount of stable liquid water, the physical conditions for the persistence of a stable atmosphere, until when magmatic degassing associated with major planet activity may have occurred.
– Anthony Hildenbrand
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