“Old men and comets have been reverenced for the same reason: their long beards, and pretenses to foretell events.” –Jonathan Swift
And I also showed you a small, somewhat unusual feature that I named “Snakey.”
Snakey isn’t all that impressive, of course. What might strike you as unusual, however, is that it appears to be a chain of craters, found close together, making a (roughly) straight line!
But there is a far better example. Much thanks goes to our reader, Tom Scrace, who was looking at this area of the Moon.
When you zoom in on this area, what do you find?
An amazingly long chain of craters! In fact, this particular crater chain — or Catena — has a name: Davy Crater Chain.
So what made it? Where do features like this come from?
Believe it or not, we think that most of them come from comets!
Comets are, of course, masses of rock and various ices, frozen together. When they get close enough to the Sun, the intense heat starts to melt some of the ices, and the Sun pushes some of the comet’s matter away, forming a spectacular tail.
But every once in a while, the comet in question passes close to a large mass, like a planet or moon. What happens then?
Well, gravity happens. Or, in particular, tidal forces happen. You might think that gravity attracts objects towards one another, but it’s a little more subtle than that. Gravity also cares how far away you are from what’s attracting you. And when you’re just a poor little comet, a planet or moon can pull the “closer” part of you with a greater force than the “farther” part of you.
Shown as it was imaged in 1994, Comet Shoemaker-Levy 9 is no more. While on a collision course with Jupiter, the now-deceased comet fragmented into more than 20 pieces, thanks to these tidal forces.
And although it was the first one we observed doing it, Shoemaker-Levy isn’t the only one.
Comet Schwassmann-Wachmann 73P, imaged also in high resolution by the Hubble Space Telescope, is in the process of disintegrating! These crater chains are formed when a comet, normally torn apart by tides, has many of its fragments collide into the planet or moon at nearly the same time.
And although these are abundant on the Moon, crater chains are found all across the Solar System. They’re also found on Mars, Mercury, and many of the larger moons, including Saturn’s Enceladus, Jupiter’s Callisto, and below, the 120-mile-long Enki Craters on Jupiter’s Ganymede.
And that’s where these great crater chains come from! So each time you find a feature like this, you’re likely finding the burial ground of a comet, brought to its gloomy demise by the massive body it smashed into. It’s absolutely amazing that we can not only find these things, but that we know how the Universe makes them!
It makes you wonder what the next object to get hit will be…