Sometimes, surprises await in your own back yard.
I’m not surprised new rings of Uranus were found when Hubble took another look at the gas giant. After all, it’s 3 billion kilometers away, and the rings are thin and faint. And actually, geometry is favoring them getting easier to see: as Uranus orbits the Sun, the rings get closer to being seen edge-on by us. Since they are so thin, this makes them easier to see, like how a transparent pane of glass gets easier to see as you tilt it.
No, what’s surprising about this new set of observations had to do with the moons seen along with these rings. What’s very interesting is that a moon discovered two years ago, called Mab, shares the same orbit as these rings. Now, you might think that a moon sitting in the middle of a ring would sweep up the dust in the ring, destroying it. But really, the fact that we see a ring at all means it must be coming from that moon, or related to it somehow.
But how? Well, enter surprise #2. These images have allowed scientists to track the orbits of these moons. They determined that the moons’ orbits are chaotic. That means that as the moons pass by each other in their orbits, they affect each other a lot, and it’s very difficult to predict how that will change their orbits in the future (we would need infinitely precise observations to make those predictions, which is of course impossible).
It also means that over millions of years, collisions are likely. The moons must smack into each other. What a sight that would be! The energy released would be awesome to behold!
And this might be the answer to the mystery of the moon in the ring: the moon might be the source of the faint ring, or, more accurately, both are related to the same event.
Imagine, a million years or so ago, as Mab (larger than it is now) orbits Uranus. Looming ahead is another moon… and they are aimed right at each other. They get closer, approaching at thousands of kilometers per hour. Then… kaBLAM! The collision would be more energetic than all the nuclear weapons on Earth combined. The catastrophe easily shatters the moons, creating millions of smaller moons a few meters to kilometers across. Too small for us to see, they would still be there orbiting Uranus today. What we now call Mab is simply the largest of those remaining chunks. The pulverized particles become the ring, and further collisions among the moonlets replenishes it.
When I was a kid, I thought Uranus wasn’t very interesting. I knew it was “lying on its side”, but other than that very little was known by anyone. Ho hum! But I was wrong . There’s a lot going on out there.
All of this, I found, was the result of very short exposures of Uranus taken by Hubble. I’d love to see longer exposures taken (or in reality, more short exposures that can be added together) to see the rings and moons in more clarity. What other surprises await us in the solar system’s back yard?’