C J L MARR
The Martian Dichotomy
an article written in 2012
There is a remarkable difference between the northern hemisphere of Mars with its uniformly smooth, flat and low topography, and the southern hemisphere with its high ground pock-marked by thousands of craters, as shown in the topographical image above. It is this dichotomy that I seek to explain, but this theory is just one of many that might explain the dichotomy — it just happens to be the one theory that I prefer.
It started because I wondered, if there was once an ocean of liquid water in the northen hemsphere, which would explain the flatness, smoothness and lack of impact craters, what caused the ocean to be there? On Earth, the ocean basins were formed as a result of the lighter and less-dense granitic rock that makes up the continents being separated from the heavier and denser basalts that make up the ocean floor. The heavier material sank to create natural depressions in the Earth’s crust and water eventually filled these depressions to create the oceans. This separation was a direct result of movements in the Earth’s crust — tectonic plate activity. The dense heavy plates are those that now make up the ocean floors. So could the same activity have caused the creation of the Martian “ocean”?
The main evidence to support the theory of tectonic plates here on Earth comes from magnetic striping. New rock appearing from below at a mid-ocean ridge is strong in magnetite, and as it cools this material points in the direction of the Earth’s magnetic field. Once solidified this will indicate which direction was magnetic north at the time it cooled. Studies during the 1950s found that this material was “striped” – one line of material pointing towards our current north pole, the next “stripe” pointing toward our south pole and so on. This confirmed that Earth’s magnetic polarity reverses every few thousand years, but since the rocks grow steadily older the further they get from the mid-ocean ridge, and since the newest rocks all have “normal” polarity, it also confirmed that the plates are moving apart. So is there magnetic “striping” on Mars that could indicate tectonic plate activity?
The answer is yes, but it runs east-to-west, not north-to-south, and it is also ancient. Where the large impacts at Hellas Planetia and Schiaparelli have interrupted the stripes, there has been no further activity, so the stripes that we are now detecting were created before those impacts. There are no signs of recent striping on Mars, and therefore it must be assumed that if there was tectonic plate activity, it ended long ago. This assumption is reinforced by the enormity of Olympus Mons, the largest volcano in the solar system that rises 22 kilometres above the surrounding plains. If there were active tectonic plates on Mars, this enormous structure probably wouldn’t have formed, as the plate would have kept moving beyond the hot spot that created the original volcano. Instead it has remained static and the volcano has continued erupting at the same spot over many thousands (possibly millions) of years to create the monstrous mountain we see today. Another notable fact is that there is no striping in the area of that Martian ocean.
Now assuming that the magnetic poles of a planet tend to be oriented very close to the axis about which it rotates, why would the striping on Mars indicate a magnetic field in an east-to-west direction? Well could it be that Mars was once at a very different orientation and something caused its axis to tilt around 90 degrees? That would certainly explain the odd striping.
It is known that planets adopt a stable rotation when the more massive objects, like mountain ranges, are at or near the equator, and that is the situation on Mars now. But suppose there was once a great deal more mass in the area of the Martian ocean. Back then the axis (and therefore the north-to-south magnetic field) was running at around 90 degrees from the current axis. When new molten material emerged from the centre of the planet and cooled, it adopted the north-to-south magnetic field of the day. Now further suppose that a glancing impact from a large body tore off a massive portion from the side of Mars. The planet would now be most unstable having lost a large part of its mass in one area, so it would tilt until it regained stability in the orientation that we see today.
Thus, the glancing impact from a large body would explain why the ancient magnetic striping now flows east-to-west, why there is no magnetic striping at all in the area of the Martian ocean (the rock containing the stripes was torn off by the impact and blown away), and would also explain why the northern hemisphere is so low relative to the rest of the planet. The crater resulting from such an impact would be the lowest ground around and any surface water would naturally fill the area creating an ocean. The ocean’s water would prevent most of the falling meteorites from impacting the ocean floor, thereby keeping it relatively crater-free, and any craters that were formed would soon be smoothed over by the constant eroding action of currents and tides. To me, the grazing impact theory works well and satisfactorily explains the Martian dichotomy. What do you think?