One of the great mysteries of planetary science is how Earth got so wet. By the time our planet formed about 4.5 billion years ago, the sun's heat had driven most of the solar system's complement of water out toward the edges. Most of it is still there, frozen solid in, among other things, the rings of Saturn, Jupiter's moon Europa, the bodies of Neptune and Uranus and billions upon billions of comets.
But the Earth has plenty of water as well, and scientists have wondered for years how it got here. One leading theory: it came from a fusillade of comets that came screaming back in toward the sun a half-billion years or so after our planet formed. That idea got a big boost last week with the discovery that some comets, at least, have the same chemical signature as the water found on Earth.
Before the ink could even dry on that study, astronomers have come in with another key piece of evidence to support the theory and it comes from nearly 400 trillion miles away. To be precise, it comes from Eta Corvi, a bright star in the northern hemisphere, where, says lead researcher Carey Lisse, of the Johns Hopkins University, "we're seeing a storm of primordial comets smashing into something relatively close to the star."
What Lisse and his colleagues actually spotted, as they'll describe in an upcoming issue of the Astronomical Journal, is the infrared signature of dust grains at about three astronomical units three times the Earth-sun distance from the central star. A detailed examination of those grains with the infrared-sensitive Spitzer Space Telescope shows that they come from forceful collisions with some massive rocky body.
"We're seeing nanodiamonds and amorphous silica," says Lisse, "which suggests that the comets collided with an object at least as big as [the asteroid] Ceres, and up to several times the size of Earth." If the comets had just run into each other, says Lisse, "it would have been more like colliding powder puffs."
The observations are by no means conclusive, and Lisse concedes that instead of a storm of smallish comets, he could be seeing the debris from a single big one. "We're not sure," he says. "All we know is that a lot of material got sprayed around."
It's not just any material that caught the eye of Lisse and his colleagues. It's the particular type of material, which also included grains of ice and organic chemicals just what you'd expect from a pulverized comet. Beyond that, the chemical signature of those faraway dust grains are a good match for the Almahata Sitta meteorite, which struck Sudan in 2008 and which probably originated in the Kuiper Belt just beyond Neptune, where billions of comets lurk (as do the dwarf planets Pluto and Eris, which are essentially gigantic comets themselves).
Put it all together and you have what appears to be a real-world echo of the process that hydrated Earth and maybe even brought us the building blocks of life, a billion years after our solar system was born. Since the Eta Corvi system is about a billion years old itself, this raises the obvious question of whether life could be in the cards there as well. On first blush, you might think not: the planet where the comet or comets came to grief is farther out than Mars, in an orbit where water would be permanently frozen.
It would in our solar system, anyway. But Eta Corvi is significantly brighter than the sun, so its habitable zone, where life-sustaining liquid water can exist, is correspondingly farther out. "There's a hotter lightbulb at the center of the system," says Lisse, "so you have to back off." Another obvious question is whether there's any evidence of comparable comet storms and therefore conditions favorable to life in other young solar systems. The short answer: not yet.
"We've looked at about a thousand," says Lisse, "and this is the only one that looked anything like this." That doesn't mean the evidence doesn't exist elsewhere, he hastens to add. "Right now, it looks as though it's rare." The James Webb Space Telescope, which could launch as early as 2018 if Congress permits it, will be much more sensitive and could turn up still more tantalizing clues. It's too early, in other words, to conclude that life on Earth was an unlikely accident.