(via)
Comets are living glimpses of the solar system’s rumspringa. The European Space Agency says comets are “probably the most primitive bodies in the Solar System, preserving the earliest record of material from the nebula out of which our Sun and planets were formed.” While much of the solar system’s primordial swirling dust and gas got together into planets, settled into nice steady orbits, and maybe had a moon or two, comets never gave up the old whipping around.
Some comets have orbits that take them out to their homeland in the Oort Cloud–50,000 times farther away from the Sun than the Earth–over hundreds of thousands of years. Some comets come barreling through the inner solar system, smashing into planets and, scientists think, forming our oceans. Some comets, called Kreutz Sungrazers, plunge through the Suns corona. Some don’t make it back out.
Like all rebels, comets can’t keep it up forever. Some comets burn out and fade away, some just burn out and become asteroids. And while Neil Young’s chestnut of wisdom “Once you’re gone, you can’t come back” might be true in rock and roll, it turns out it isn’t true for comets.
A comet is defined by its coma—the hazy cloud of ice sublimating into gas and dust, that surrounds it and grows brighter as the comet gets closer to the Sun. It’s this coma, when hit with solar wind, that forms the comet’s tail. When a comet runs out of volatiles that form the coma, it falls dark, no better than an asteroid. In recent years, scientists have discovered that many asteroids were once bad-ass comets.
Comet or asteroid? Comet Hartley 2, in fact. Look at the coma there at the back end (via)
But researchers from the University of Antioquia in Colombia have discovered that even comets who seemed done with the comet life can get their grooves back. In a study published in Monthly Notices of the Royal Astronomical Society, researchers have found that dead-comets-gone-asteroid occasionally have second acts. It’s just a matter of getting closer to the Sun and getting those volatiles sublimating again.
The paper suggests that the asteroid belt between Mars and Jupiter is positively a graveyard of old comets—some are extinct, which means they have no more gases left to sublimate. And some are merely dormant, meaning they have volatiles left, but the ice is deep under a layer of carbon compounds and dust and insulated from the solar energy that causes the characteristic glowing sublimation.
Given a chance to get closer to the Sun, radiation can reach that latent ice, and bring the comet back to life. All it takes is a fortuitous collision with an asteroid (or other extinct or dormant comet) that sends the dormant comet closer to the Sun.
The researchers looked at comets that had done this very thing, and what they found most interesting was that it wasn't the interplanetary collision knocking the insulating crust loose that kick-started the comets again; it was the proximity to the Sun. They cited a pair of examples of dormant comets that didn't turn back on until after making their closest pass by the Sun. Judging from how long the rejuvination took, they were able to calculate how deep down the volitales were buried.
The study's findings cast further doubt on just how different asteroids and comets are in essence, or if the distinction is more arbitrary than we thought.