This episode of Astrum Answers is brought to you by CuriosityStream. The scene is set,
your archrival has got you pinned, gun pointed at you, with your back overlooking the drop
of the tallest cliff face in the solar system, Verona Rupes. This 20km drop is roughly 16
times bigger than the tallest cliff face on Earth, Mt Thor in Canada, coming in at a mere
1,250m. The only advantage you have right now is that you are standing on Miranda, a
tiny 470km wide moon of Uranus, meaning the gravity here is weak.
Do you jump and take your chances with the drop? Or have you got more chance with the
gunman? Well, let’s have a look at a few factors.
Miranda is the smallest of the five circular moons of Uranus. Being only 470km across puts
it right on the edge of being in a hydrostatic equilibrium, or in other words, it’s just
about big enough to be spherical under its own gravity. It is also not very dense, thought
to consist of at least 60% water ices. Its small size and low density means its gravity
is very weak. Just for a comparison, on Earth, gravity accelerates you towards its surface
at 9.8m/s². On our moon, that’s reduced to 1.6m/s². On Miranda, it is just 0.08m/s².
That means, should you jump, you would have a very gradual descent towards the bottom.
But there’s another factor. While falling on Earth, after a certain distance, you would
reach a terminal velocity. Air resistance combats the acceleration of gravity, which
means your speed would eventually cap out. In a lying position this is roughly 200km/h.
Without Earth’s atmosphere, jumping out of a plane
would mean you hit the ground extremely fast as there is nothing to slow you down, and
Earth’s gravity is pretty strong. Miranda has no atmosphere. Again, this means there
would be no air resistance to slow your fall. So, although you are falling very slowly at
first, your speed would begin to pick up with nothing but the base of the cliff to eventually
slow you down. As I mentioned, Verona Rupes is a massive 20 kms tall. Such a tall cliff
can exist on Miranda because its gravity isn’t strong enough to pull it down. We don’t
really know how it formed like this, as Miranda should be too small for plate tectonics. One
theory is that water under the ice crust froze and expanded, forcing the surface to split,
creating canyons hundreds of kilometres long. So, a 20 km drop, with an acceleration of
0.08m/s²? Well you may not be surprised to find out that it will take you a long time
to reach the bottom of the cliff. 12 whole minutes! That means it won’t be a “The
Fugitive” scenario where Harrison Ford quickly jumps off the dam before Tommy Lee Jones can
do anything about it. You archrival will have to be quite slow to allow you to jump off
before he can shoot you. And should that all go well, by the time you finally reach the
base, with no atmosphere to slow you down, you will be travelling at 200km/h! A parachute
won’t help you here, but a really big air bag or little rockets might do the job. So,
there you have it! What would happen if you fell from the tallest cliff in the solar system.
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