To the students, and future scientists, of the Great Planet Debate:

Before I explain to you the deep connection between the Earth and Uranus, I'd like you to keep in mind, as you are debating which planet(s) to observe with your 3 HST orbits, that you are choosing among some of the most important scientific issues and questions facing the astronomical community today. The study of other planets is revealing, mind-expanding, and vital in understanding the connection between the Earth, the only habitable planet in our solar system, and its sister planets, including the gaseous giants beyond the asteroid belt. You and I are in the fortunate and privileged position to be able to answer these questions as no other humans have been able before. We have the proven capability of spaceflight, and we have sophisticated light-gathering instruments like the Hubble Space Telescope at our service.

Some of these fundamental questions, which underlie almost all scientific investigations of the planets and their moons and rings, are: What was so special about the Earth that life arose here and no where else in our solar system? Is this in fact true, that life, even primitive life, appeared on no other planet? And what lucky set of circumstances permitted the emergence and ultimate dominance of intelligent beings like ourselves, capable of love, reason, and great technological accomplishment, on our small fragile blue world? A planet needn't be colorful or pretty to hold precious insights into such questions. Even a planet as shrouded in haze and deep mystery as distant Uranus can bestow valuable secrets into a matter as close to home as the origin of Earthly life.

How? Let's do our `human' thing, use our highly evolved cerebral cortexes, and think about it.

We only have to look at the objects with visible solid surfaces in the solar system to see that there was a time, billions of years ago, when a great many bodies, carrying enormous amounts of energy, were careening around the solar system and smashing, headlong, into the planets and satellites as they were forming. These collosal cosmic collisions did a great deal to build and sculpt the solar system and make it look like it does today. All throughout the solar system, these collisions were responsible, first and foremost, for allowing the planets to grow to their present sizes. Uranus itself, like the Earth and Venus, like Jupiter and Neptune, grew out of an enormous number of `planetesimals' -- large comets in the case of Uranus -- which rained down on the already-growing planet and made it even larger. (The giant planets -- Jupiter Saturn, Uranus and Neptune -- also swept up a lot of gas as they were forming, which is why they are `gaseous' and the Earth, for example, is not.)

But these ancient collisions were very destructive too. They scarred the planets and satellites, giving us giant basins on Mars, craters later filled with water in the middle of Canada here on Earth, huge cracks in the surfaces of some of the satellites of Saturn, very crater-covered surfaces like that of Jupiter's satellite Callisto, and much more. Some of the very large planetesimals that were flying around billions of years ago were, as I've said already, responsible for tilting planets like Uranus and forming its extensive satellite system. One of these large bodies, about the size of the planet Mars, collided with the Earth, causing a great deal of the Earth's outer portion to be pulverized and thrown into Earth-orbit, from which debris subsequently formed our Moon. Imagine that: a collision between the Earth and another Mars formed the Moon! Collisions obviously did remarkable things to our solar system long long ago.

The number of objects screaming around our solar sytem today is very much smaller than it was billions of years ago. (That's because most of that stuff has already collided with a planet or satellite, and so there is very little left.) But there are certainly objects the size of large mountains, the leftovers from those ancient times, still in orbit around the sun. And, computer calculations show that if you waited long enough, these could eventually find themselves on a collision course with a planet or satellite.

Are such collisions capable of doing remarkable things today? Yes, indeed!

Take as an example the rings around the giant outer planets. You may not be aware that these systems are believed to be the result of a catastrophic collisional breakup of a satellite orbiting close to the planet. How big a satellite would you have to smash in order to get a full-blown ring system like Saturn's? You'd have to break up a Mimas, one of Saturn's satellites, which is about 400 km across. To get another set of Uranian rings, you'd have to break up a few of the 10 inner Uranian satellites, which are about 20 km across. To produce the present-day rings, these events must have happened about a hundred million years ago, about the same time that dinosaurs roamed the Earth. To a planetary scientist or geologist, that's relatively very recent. And while we're on the subject, here's something for you to think about: If today you smashed up the satellites that are orbiting within the ring region around Neptune, you'd get a ring system as striking and nearly as massive as Saturn's. Imagine that: Neptune surrounded by Saturn's rings!

Other `recent' collisions probably smashed up the Uranian satellite Miranda. The natural tendency for material in orbit around a planet to collect through gravity to form a single body caused Miranda to re-form, only to be smashed up again when another sufficiently large body crossed its path. This may have happened at least half a dozen times to Miranda, explaining why it looks like a badly-assembled jigsaw puzzle.

And of course, we saw the effects of a rather small collision in July 1994 when comet Shoemaker-Levy crashed into Jupiter. A larger comet would have an even more dramatic effect.

What does this have to do with the origin of life on Earth? Possibly, lots!

First, there are some scientists who believe that the materials from which life-forms on Earth developed came from the outer solar system, delivered to the Earth in the form of comets which long ago collided with the Earth. So, the same stuff which makes up the planet Uranus *might* be the same stuff that you and I are made of. Totally awesome, you say? Second, and even more awesome, a growing number of scientists are becoming convinced that it was a collision between the Earth and a mountain-sized asteroid some 65 million years ago which so changed the Earth's climate that the majority of all the creatures alive at the time, including the dinosaurs, perished and became extinct. This asteroid was about the same size, and arrived at Earth roughly the same time (geologically speaking), as the object which caused the catastrophic formation of the Uranian rings. And many paleontologists (i.e., scientists who study fossils of ancient life) believe that it was the demise of the dinosaurs which cleared the way for the eventual development and successful evolution of the primates, of which humans are the culmination. That is to say, it would have been difficult to invent calculus with a Tyrannosaurus Rex breathing down your neck!

So, in choosing among the 4 planets Jupiter, Uranus, Neptune, and Pluto, think about the collosal events, all of the same ilk, which tilted Uranus on its side, formed its rings, smashed Miranda to bits, brought life to the Earth, death to the dinosaurs, and ultimately permitted you and me to discuss such strange and wondrous happenings over that hallmark of human achievement, the Internet.

Good luck!

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