At first sight it looks like science fiction. But this is a
real photograph of Saturn's innermost moon. What could have created such a huge crater?
How did the moon survive the impact? And what would happen if something so big
hit our planet. Would we survive? Find out as we uncover the secrets of Mimas:
Saturn's Death Star moon. Before we go on,
be sure to hit like and subscribe, to keep up to date with the latest discoveries.
Mimas has to be one of the oddest-looking objects in our solar system. It's the smallest of Saturn's
major moons, the closest to the planet and one of the earliest to be discovered.
It was first observed from Earth in 1789 by the great astronomer William Herschel. He used what
was then the world's largest telescope. But it wasn't until spacecraft flew past Saturn that
humans first saw the massive crater taking up nearly one third of it's surface. A huge scar
over 80 miles across and up to six miles deep. In comparison to the size of Mimas itself it's
one of the biggest craters discovered anywhere in our solar system. Seen here in 3D, there was
really only one name it could be given: Herschel Crater - in honour of the moon's discoverer.
Though Pioneer 11 had flown by Saturn in 1979... it wasn't until 1980 that Voyager 1 came close
enough to see Mimas clearly. These incredible images were the first ever captured... showing
a battered surface peppered with thousands upon thousands of smaller craters and the unbelievable
sight of Herschel Crater itself. Coming just three years after the release of Star Wars, these images
instantly earned Mimas a new nickname: The Death Star Moon . Voyager 1 was followed by Voyager 2
in 1981. Both made many discoveries in their brief flybys of Saturn. Scientists had long been puzzled
by the regularity of Saturn s famous rings and the gaps between them. Voyager's images showed it was
the moons that kept the rings in check. Some, they even christened shepherd moons . Mimas itself,
seen here against the rings, was responsible for clearing one of the largest gaps in the
rings. Acting as a cosmic snowplough, it keeps open the so-called A-B Gap or Cassini Division.
Perhaps this might explain some of the craters on Mimas, but surely not the huge impact that created
Herschel. And why was Mimas so much more heavily cratered than many of the other moons? It wasn't
until 2004 that we began to get some answers. Unlike previous probes, the Cassini-Huygens
spacecraft wasn't designed to simply fly past Saturn. Instead, it would go in orbit.
Launched in 1997, it's epic mission lasted over 19 years. More than 13 of those exploring the planet
and it's dozens of moons. Its visits to Mimas gave scientists a wealth of new information.
Mimas was a tiny world less than 250 miles in diameter. Only just big enough to form itself
into a roughly spherical shape. It was primarily made of water ice. And whatever
created Herschel crater had hit the moon so hard that it had nearly shattered completely.
Huge cracks, or chasmata, ran across the surface. The shock waves had travelled
right through the moon - even the face opposite to Herschel crater was broken and deformed. The
whole tiny world had been pummelled almost to destruction by a truly gigantic collision.
Even stranger discoveries were made: Mimas was wobbling on it's orbit.
"The data suggest that something is not right, so to speak, inside Mimas," said Radwan Tajeddine
a Cassini researcher. One explanation could be an underground ocean, as there's thought to be on two
of Saturn's other moons Titan and Enceladus. That could even open up the possibility of there being
life on Mimas. But the clues just didn't add up. According to astronomer Francis Nimmo from the
University of California - An ocean discovery would be exciting. But... it s really hard to
understand how an ocean could survive for billions of years inside something as small as Mimas.
And there were just no signs of an underground ocean; none of the tell tale surface cracks
caused by tidal forces. No geysers of water... as seen here erupting from the surface of Enceladus.
Cassini did detect strange variations in temperature across the moons surface.
Scientists speculate that this is caused by bombardment from Saturn's magnetosphere
affecting the moon's surface. But even so Mimas seemed to be frozen solid. Far
too cold for liquid to exist. And that in itself was a mystery. As the closest moon to Saturn,
Mimas should be warmer. It's slightly elliptical orbit should lead to more tidal heating. There
should be more chance of an ocean on Mimas than Enceladus. Puzzled by the findings,
scientists invented the Mimas test . Any theory that explains how Enceladus can be warm and wet
must also be able to explain why Mimas is cold and frozen. As yet no theory has passed that test.
But if there's no ocean: what is causing the moon to wobble? One theory is that a
remnant of the object that created the massive Herschel crater still remains buried there.
During the Apollo flights, it was discovered that the gravitational field of Earth's moon
varied; with spacecraft being pulled from their expected orbits. It's thought heavy, dense rock,
perhaps the remnants of ancient asteroids, mean that some regions of our Moon exert
more of a gravitational pull than others. Maybe something similar is happening on Mimas a huge
dense lump of rock buried on the tiny, icy world could be literally pulling it off balance.
But the puzzling, deep-frozen environment of Mimas does explain one mystery. Because it's so cold,
it has remained literally frozen in time. Heated from within, or racked by tectonic
activity, the surfaces of many of Saturn's moons melt and change. Any craters that occur
are smoothed away or hidden under new deposits. On Mimas, that resurfacing hasn't happened. Perhaps
not since Herschel crater itself was formed. It, along with the thousands of other craters that
litter the surface, remain as they first formed. Millions, perhaps billions of years ago.
Compare these images captured by Cassini and the difference is clear. The relatively smooth
surface of Enceladus. Shaped by internal heat and tidal forces from it's underground ocean.
And the ancient, barren landscape of Mimas. Dead and unchanging. But what was the object
that caused such devastation almost shattering the moon into pieces? It may have been a stray
asteroid or even a comet, captured by Saturn's huge gravitational pull. Being so close to the
planet, Mimas is in the firing line. In July 1994 astronomers watched as fragments
of Comet Shoemaker-Levy 9 slammed into Saturn's neighbour, Jupiter. No doubt similar collisions
have occurred at Saturn itself. Perhaps Mimas just got in the way of one of these objects.
Another possibility suggested by researchers though, is that Mimas may have been struck by
a fragment from an even more ancient moon. One that had itself been destroyed in a collision.
Saturn has more moons than any other known planet an astonishing 82 discovered so far.
With such a complex dance of gravitational forces, it's very possible that some of those worlds have
collided. These collisions would have blasted out fragments, that would have hit more moons, causing
more fragments..... .like a massive game of cosmic pinball. Much of the debris may have ended up
as part of Saturn's spectacular ring system. Or perhaps as mini moons in their own right.
Some may have re-formed to create new worlds. But it's possible another of these fragments slammed
into Mimas, causing the huge impact crater we see today. A frozen reminder of the most violent day
in it's history. Hitting the moon so hard it was shattered cracked from side to side.
With only just enough gravitational pull to hold together, wobbling wildly, Mimas gradually
froze into one again. Marked forever by the huge impact that almost destroyed it. Mimas, it seems,
experienced one of the most violent impacts that any object in our solar system has survived.
But what would an object of relatively similar size do to Earth if it hit? Would we survive?
To scale, a crater the size of Herschel on Earth would be over 2500 miles wide: bigger
than Australia. Scientists have developed software which simulates the effects of asteroid strikes.
Such as the Earth Impact Effect Programme created by Imperial College London and Purdue University.
Using these, we can get some idea of what that size of collision would do to our planet.
So let's imagine the day that a chunk of rock nearly 500 miles wide screams into our atmosphere.
For all of us it would be the last day. With a velocity of 10 miles a second and
an energy equivalent to 1,390,000,000 megatons of TNT, it wouldn't matter where the object hit.
Within ten minutes a fireball would have engulfed almost the entire planet.
Grass, trees, clothing and homes would all burst into flame. Around 30 minutes later,
a seismic shock wave would sweep around the globe. Reaching 13.4 on the Richter scale,
greater than any Earthquake in recorded history, destroying buildings, bridges, roads....
After ten hours or so, an air blast would have struck even the furthest point from the impact
site. Winds of over 3000 miles an hour would destroy almost anything that remained. Much of
the planet would be buried, like Pompeii, under a layer of ejecta hundreds of feet thick. Perhaps
littered with craters as larger debris rained down. Depending on the exact direction and
location of the strike, the length of Earth's day could be altered by over 6 minutes.
If you were lucky enough to be on the very far side of the planet and well protected underground,
you might survive. For a short time. But very quickly, life as we know it would end.
Perhaps leaving behind a planet that for a while at least, looked a lot
like this... Such a collision with Earth is incredibly unlikely. But relatively speaking,
that is the scale of impact that Mimas survived - just. So this tiny, battered survivor
can even teach us about our own planet. And who knows? Perhaps there are more secrets yet
to be discovered, more lessons to learn, from Mimas - the extraordinary Death Star Moon.
If you enjoyed this video be sure to check out the next video about the time Apollo 12 was struck by
lightning, showing up on screen now. Thanks for watching ElderFox.
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