[ ♪ Intro ]
When scientists try to understand the geology of the solar system,
one of their most powerful tools is Earth.
Our planet shares a lot with some of our neighbors,
from our atmosphere up top all the way down to the spinning core.
Sometimes, though, astronomers find things that don’t look like anything here at home,
places that couldn’t even exist on Earth.
And over the years, we’ve discovered some very unusual mountains.
Take Rheasilvia, a gigantic mountain on Vesta,
the second-biggest object in the asteroid belt.
What makes Rheasilvia so impressive isn’t just how tall it is,
but how tall it is relative to everything else.
At about 22 kilometers high, it’s nearly three times the height of Mt. Everest,
even though Vesta is only 500 kilometers across.
That would be like a mountain 250 kilometers high here on Earth,
tall enough to put you well into space!
Rheasilvia is more than just a mountain: it’s also an impact crater,
which might sound kind of strange since we don’t normally think of holes making mountains.
But the process of an asteroid slamming into the surface is so powerful
that the ground briefly acts like a liquid and splashes back up,
like a droplet of water falling into a pond.
As everything cools down, that splash solidifies into a mountain called the central uplift.
This process happens for basically every crater above a certain size,
and we do have the remnants of some here on Earth.
The impact that formed Rheasilvia, though, must have been astonishingly powerful.
Scientists think its remnants became some of the most commonly-found types of meteorites.
A central uplift isn’t the only way an impact can form a mountain.
Mercury is home to one of the solar system’s most mysterious mountain ranges,
and for decades astronomers have thought it might be the result of an impact,
but on the other side of the planet.
This area doesn’t have an official name, and it’s a far cry from the peak of Rheasilvia.
The tops of these hills only rise from about 0.8-1.8 kilometers above the surface.
What’s more impressive is what’s on the planet’s far side: Caloris Planitia,
an impact crater so large, you could put three Vestas inside it with room to spare.
This is the site of one of the solar system’s most violent collisions,
and its effects were global.
When planetary scientists assembled the first surface maps of Mercury in the 1970s,
they noticed something weird.
These strange, low hills weren’t just on the other side of the planet from Caloris Planitia,
they were exactly opposite, at what’s called the antipodal point.
It’s too perfect to be a coincidence:
the impact must have created the mountain range through the planet.
As impossible as that sounds, it probably happened because of Mercury’s geometry.
Mercury is a virtually-perfect sphere, unlike Earth,
whose fast rotation makes it fatter at the equator and shorter at the poles.
When the Caloris impact occurred, it sent massive shockwaves through the planet.
Because the path to the other side is the same distance in every direction,
these waves converged at the antipode.
Their combined strength deformed Mercury’s surface and created the mountains.
It’s also possible that with less gravity and no atmosphere,
material ejected from the crater traveled halfway around the planet to land on the far side.
Between the shockwaves and all that flying debris,
Mercury ended up with one of the solar system’s most intriguing spots.
Another is found at the other end of the solar system, way out on freezing Pluto.
There, at the edge of its icy heart, lies a peak no taller than the Rocky Mountains,
but much, much cooler.
Like, figuratively and literally.
It’s named Wright Mons after the brothers of airplane fame,
and it’s one of the best candidates for a cryovolcano.
These volcanoes aren’t too different from the ones you’re familiar with,
except for what makes up their lava.
Pluto’s crust is made of water ice, but, in the seriously cold temperatures of the outer solar system,
that ice has many of the same properties as rock here on Earth.
So it only makes sense that a volcano on Pluto would have lava made of liquid water,
instead of liquid rock.
Scientists aren’t certain that Wright Mons, or a nearby mountain that’s also a candidate, are cryovolcanos.
And it will probably be a long time before we get more observations of far-off Pluto.
But cryovolcanoes do seem to be a thing in the solar system.
There’s potential evidence for them on Saturn’s moon Titan and on the asteroid Ceres.
It’s not yet clear how they’re possible in some of these places
and that’s what makes them so interesting.
Every mountain reveals the history of the area around it,
even if you can’t see that history directly.
And in space, that’s a tool for learning about some seriously weird geology
that we might never have known existed otherwise.
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[ ♪ Outro ]