Most rocky planets have pretty consistent surface features.
Like, both hemispheres will generally have some mix of mountain ranges and basins, but
they won’t be fundamentally different.
This is not the case on Mars.
Its northern hemisphere is smooth and almost featureless, but its southern hemisphere is
dark, craggy, and marked with craters and chasms.
The southern highlands are also three to eight kilometers higher than the northern plains,
and the crust is twice as thick.
This fundamental difference is one of the biggest mysteries in Martian geology, and
it even has a name:
the Martian crustal dichotomy.
But understanding it wouldn’t just scratch our itch for knowledge; it would also open
a window onto the planet’s early evolution.
So far, scientists have two mostly-complementary ideas about how this dichotomy happened.
One is that a kind of tectonic activity made the southern highlands.
On Earth, this activity includes stuff like plate tectonics, and it’s powered by the
planet’s hot core, which can sustain huge convection currents in the mantle.
But generally, we don’t see things like this on Mars.
That’s because Mars is about half the size of Earth, so it cooled down much faster — meaning
convection in the mantle has come to a halt over the last four billion years.
But before then, something weird might have happened.
So, in 2012, scientists from the University of Colorado were studying the magnetic fields
frozen into the Martian crust.
These fields were locked into the crust’s volcanic rocks as lava cooled and hardened,
and they’re a record of what Mars’s planet-wide magnetic field used to be like.
They’re basically little time capsules.
And by studying them, we can learn more about when volcanic activity happened and for how
In their study, the team was looking at stripes in the magnetization of the rocks that showed
how Mars’s magnetic field may have periodically swapped its orientation.
The idea here is that Mars’ magnetic field flip-flopped like Earth’s, swapping north
and south poles over thousands or millions of years.
And the map from this study showed something remarkable:
The magnetic stripes in the southern highlands are concentric rings.
If this study is right, that could mean lava was being produced from a single point near
the south pole for a very, very long time — long enough for the magnetic field to
flip-flop several times, and possibly long enough to thicken the crust in the southern
The researchers hypothesize that this ongoing eruption could have been fed by a plume of
magma from the Martian mantle.
These plumes are thought to happen when unusually hot rock from deep in the mantle rises up
and punches through the crust, creating what’s known as a hot spot.
On Earth, mantle plumes are thought to be the mechanism that formed Hawai’i and the
rest of the Emperor Chain of islands.
And on Mars, this single plume could have been all the tectonic activity that the cooling
Martian core was capable of sustaining, leading to the uneven crust.
If true, that could explain a lot.
But it still wouldn’t explain how the northern plains ended up so smooth.
[USGS Astrogeology Science Center
For that, scientists favor a more... destructive explanation.
This hypothesis suggests that a massive collision between Mars and a smaller planet left behind
a crater that spanned the entire northern hemisphere.
This idea was first suggested in 1984, and at first, it had a few problems.
Like, an impact usually leaves behind a circular impression, and the northern plains are elliptical.
And anyways, scientists used to think that a collision big enough to make that kind of
crater would have also made a magma ocean that would have obliterated the crater itself.
But in 2008, a team of scientists ran the numbers again, and they found that there was
a collision that could produce the characteristic terrain.
It would just require a small body, about the size of Pluto, to hit Mars at an angle,
and to hit relatively slowly by planetary standards… at a measly 20 to 30 thousand
kilometers per hour.
Under these conditions, the molten rock made by the impact would have been mostly confined
to the crater itself, making the plains we see today.
This may sound unlikely, considering there aren’t any rogue, Pluto-sized objects barreling
around Mars these days.
But these kinds of collisions were pretty common in the early solar system.
And based on how old Mars’s other craters are, that’s exactly when we think the Martian
Overall, the thicker crust in the south and the smooth plains in the north support both
mantle plumes and an impact, so scientists think the Martian dichotomy was probably caused
by a combination of the two.
It’s even possible that a giant impact in the northern hemisphere could have made the
excess magma that plumed up in the south.
But at this point, we won’t know for sure until we can get rock samples from all over
the Martian crust.
Those will likely be able to tell us exactly what parts were made by mantle plumes, or
impact melting, or if the dichotomy is the result of something else entirely.
But for now, studies like this can help scientists understand the processes that affect planets
in their early evolution, which is an important tool for understanding new worlds.
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