As a result of our limited lifespans as human beings, we tend to view the rest of the macroscopic Universe around us as static, immobile, and unchanging. The Earth itself also seems changeless, with its land and sea masses carved in stone as something rigid and enduring over the eons. But if examined over the scale of geologic time, our home planet reveals its true nature as a lively, ever-developing and dynamic world. The same holds true for the rest of the planets in the Solar System as well, which, thanks to the more than 50 years of robotic planetary exploration since the start of the Space Age, have been revealed as equally unique and constantly evolving worlds in their own right.
One of them, Mars, has captivated scientists and non-scientists alike for more than a century due to its promise as one of the best possible habitable environments in the Solar System beyond Earth. The results of a study by an international team of astronomers now provide an entirely new picture for the Red Planet’s geophysical evolution early on in its history which, if ultimately confirmed, could provide revolutionary insights into Mars’ entire geological history, as well as its potential habitability as a planet and our search for life on it.
Of all the terrestrial planets in the Solar System, Earth is the only one where the process of plate tectonics, also known as continental drift, has been proven to occur. As the Earth gradually cooled following its formation approximately 4.5 billion years ago, its interior differentiated into layers, forming an inner and outer core, mantle, and crust. The crust and upper mantle, which together constitute the lithosphere, were further broken into several bigger and many smaller pieces, called tectonic plates. Our planet’s cooling and dissipation of internal heat, which continues to this day, causes among other things a constant moving of these plates, which “float” above the mantle as they are driven by convection currents underneath. Plate tectonics has been the primary force behind the constant re-shaping of the Earth’s surface and re-distribution of our planet’s continental mass over the course of hundreds of millions of years in the past, a process that is expected to continue for at least 1 billion years still in the future.
A second process that is believed to have played a role on the re-shaping of the Earth’s land masses throughout geologic time, albeit in much smaller extend, is known as “true polar wander.” Simply put, the latter is the change in latitude of land masses relative to our planet’s rotation axis. Not to be confused with the more familiar concept of “precession,” which is the slow shift in the orientation of our planet’s rotation axis itself, caused by the gravitational influences of the Sun and Moon, true polar wander refers to the actual shifting and rearrangement of the lithosphere in order to correct an imbalance caused by the accumulation over geologic time of excess weight on the crust itself and align it with the planet’s rotation axis.
More specifically, if a land mass of sufficient weight like a massive mountain ridge or a supervolcano were to form far from the Earth’s equator it would create a weight imbalance, causing the planet’s actual force of rotation to relocate it over time as much away as possible from the rotation axis and toward the equator as a balancing measure. The results according to the theory could be as profound as having our planet’s outer layers tilt as much as several degrees over the course of a few dozen million years, causing entire continents to move from the Arctic toward the equatorial regions and vice versa.
Since the study of the Earth’s geological history has been focused on the effects caused by the much more dominant process of plate tectonics, the theory of true polar wander has remained a somewhat obscured one over the years. Nevertheless, a series of studies during the last two decades have provided compelling evidence in support of true polar wander as an actual geophysical process occurring concurrently with continental drift, causing scientists to rethink their current understanding of the Earth’s overall geodynamics and evolution.
One such study that was published back in 2006 by a team led by Adam Maloof, now an associate professor of geosciences at Princeton University, found evidence for true polar wander in a series of magnetised sediments that were uncovered at the Norwegian archipelago of Svalbard in the Arctic Ocean. Since many rocks and sediments can accurately preserve a record of the strength and direction of the Earth’s magnetic field from the time of their formation, scientists can use them as a tool for unraveling the processes that have shaped our planet’s surface throughout its history.
Maloof’s team studied such sediments at Svalbard from 1999 to 2004 in order to determine the archipelago’s changes in latitude during the Neoproterozoic era between 1 billion and 540 million years ago. Sure enough, the researchers found evidence for two rapid true polar wander events which took place approximately 800 million years ago and were characterised by an abrupt, polar motion between 54 and 270 cm per year relative to the Earth’s magnetic field axis.
“The sediments we have recovered from Norway offer the first good evidence that a true polar wander event happened about 800 million years ago,” says Maloof. “We found just such anomalies [of abrupt change in the direction of the sediments’ magnetic field relative to the Earth’s axis] in the Svalbard sediments. We made every effort to find another reason for the anomalies, such as a rapid rotation of the individual crustal plate the islands rest upon, but none of the alternatives makes as much sense as a true polar wander event when taken in the context of geochemical and sea level data from the same rocks.”
The conclusions by Maloof’s team regarding true polar wander were supported by a different theoretical study a few years later, which showed that the famous volcanic hotspot at the Hawaiian islands in the Pacific Ocean has gradually shifted over the eons relative to the Earth’s rotation axis.
Could similar processes be taking place on other planets as well? Even though the rest of the terrestrial planets—Mercury, Venus, and Mars—show no readily identifiable signs of plate tectonics activity, planetary scientists had long suspected that the geomorphology of Mars, as well as other rocky objects in the outer Solar System like Jupiter’s moon Europa, could have been largely influenced by true polar wander.
A new science paper, published last week in the journal Nature by a research team led by Sylvain Bouley, an associate professor at the Université Paris-Sud in France, is the most comprehensive one to date showing just how such a process could have unfolded on early Mars, while also explaining some long-standing enigmas concerning the orientation of several surface features on the Red Planet.
Based on gravity and topography data that were gathered from Mars orbiting spacecraft, Bouley’s team run a series of computer simulations in order to study the evolution of the planet’s geomorphology early on its history, centered around the Tharsis region, a vast volcanic plateau that is located near the Mars’ equator. Spanning 5,000 km across and up to 10 km high, Tharsis is home to a sprawling canyon system known as Valles Marineris and to four giant shield volcanoes, one of which Olympus Mons extending more than 25 km above the Martian surface is three times taller than Mount Everest. In fact, both Valles Marineris and Olympus Mons are the largest formations of their kind in the entire Solar System. And with a total mass of 1021 kg, Tharsis itself is as massive as dwarf planet Ceres, which is the largest celestial body in the asteroid belt.
Tharsis is believed to have formed very soon after the formation of Mars, during the Noachian Period between 4.1 and 3.7 billion years ago. One of the Red Planet’s defining topographical characteristics, the so-called Mars dichotomy between the lowlands of the northern hemisphere and the highlands of the southern hemisphere, was also largely in place by that time. Since the Tharsis region is such a massive and dominant feature on the Martian surface, Bouley’s team wanted to see what the planet’s overall evolution would be with and without the expansive volcanic plateau.
The researchers’ simulations produced some profound results, showing that as Tharsis’ immense mass was slowly accumulated over the course of hundreds of millions of years due to volcanic eruptions, it caused the martian surface to tilt in the range of 20 to 25 degrees relative to the planet’s axis in order to account for the growing weight imbalance, thus repositioning entire areas of the planet from their original polar latitudes towards the equator. For comparison, were such a thing to occur on Earth, the U.S. city of Tucson, Ariz., would be shifted where present-day Calgary is located in Canada, and Paris would end up at near the north pole!
Similarly, Tharsis was relocated to its present location at Mars’ equatorial regions, as a balancing measure in order for the planet to evenly redistribute its own weight. “Any major shift of planetary mass —on the surface or within the mantle – could cause a shift with respect to the spin axis, because a spinning body is most stable with its mass farthest from its spin axis,” says Itsamu Matsuyama, an assistant professor of planetary sciences at the University of Arizona and member of Bouley’s team.
This large-scale face-lift of the Martian surface had profound implications for the planet’s hydrological evolution as well. It was long thought that the many thousands of Mars’ river valleys followed the formation of the Tharsis region, which influenced and defined their direction of flow with its presence. Yet, what the simulations by Bouley’s team showed was that these river valleys networks were formed at least concurrently with Tharsis, if not earlier instead. This realisation helped to address several standing enigmas relating to the present-day direction of the river valleys on the martian surface as well as the presence of past water activity in areas of the planet where it should be none.
More specifically, the river valleys as seen today on present Mars are concentrated within a tilted ring relative to the equator with a north to south direction, from the cratered southern highlands to the plain northern lowlands. When Bouley’s team removed the Tharsis region from their early-Mars simulations, the ring of river valleys developed as well, but in a parallel line just south of the equator. Furthermore, the absence of Tharsis from primordial Mars meant that the planet originally had polar ice caps at very different latitudes than present-day Mars, meaning that the surface shift caused by the formation of the Tharsis region dramatically changed the orientation of the river valleys as well as altering the planet’s overall climate and weather, while the original polar ice caps changed their position and mostly melted away leaving behind pockets of ice in areas that have been observed by orbiting spacecraft.
“The present distribution of valley network contains large variations in longitude that are difficult to explain without the tilting scenario, because with the current orientation of Mars relative to its spin axis, you see evidence of precipitation where you should not see it,” says Matsuyama. “The extensive networks of valleys and channels change latitude as you move in longitude.”
Present-day Mars is a vastly different planet than what is generally thought to have been during its early years, ultimately transitioning from a warm, wet and potentially habitable world that was dominated by liquid water billions of years ago, to the cold, arid, and dry place that we see today, due to the loss of its atmosphere and water into space sometime in its history.
Could the surface shifting that was brought on by the effects of true polar wander as evidenced in the study by Bouley’s team have any effects in this dramatic environmental change? That remains a huge unknown at this point, yet, as the researchers point out, their picture of a dramatically changed landscape brought on by true polar wander should be taken into account in future studies of Mars’ ancient climate as well as present-day searches for life on the planet.
“This study upsets our picture of the surface of Mars as it must have been 4 billion years ago and modifies the timing of events profoundly”, write the researchers at a press release. “According to the new scenario, the period of liquid water stability permitting the formation of fluvial valleys is contemporaneous with and, most likely, a consequence of the volcanic activity of the Tharsis dome. The great tilt that Tharsis provoked happened after fluvial activity ended (3.5 billion years ago) and then gave Mars the face we know today. From now on, when we study the earliest days of Mars, we must learn to think with this new geography.”