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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
long.
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
hemisphere.
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
dichotomy formed.
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.
Thanks for watching this episode of SciShow
Space!
If you enjoyed this episode, you might also
like our video about why landing on Mars is
so hard.
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