Water ice and organic molecules have been discovered on an asteroid's surface for the first time. Researchers glimpsed the ice on 24 Themis, a frosty rock that could be the key to understanding how Earth became the blue planet.
"What we've found suggests that an asteroid like this one may have hit Earth and brought our planet its water," said astronomer Humberto Campins of the University of Central Florida, the lead of one of the two separate teams that reported similar findings April 28 in Nature.
While there is plenty of debate around how Earth got its oceans, this new evidence suggests some of the water came from extraterrestrial sources. Here's how it may have happened: More than four billion years ago, after a massive collision between Earth and another large object created the moon, our planet was completely dessicated. Then, during the Late Heavy Bombardment period that followed, during which lots of asteroids hit Earth, the ice that the objects carried became our store of water.
"The more we find in our asteroid belt objects that do have water, the more convinced we are that that was a possible process to rehydrate the earth," said NASA astrobiologist Mary Voytek.
The ice on Themis 24 could be a sort of time capsule from the early solar system and could be similar to the ice that may have arrived on Earth from asteroids during the Heavy Bombardment.
"The ice that we see there, right now, is sort of related to the ice that could have come from the main asteroid belt that hit us about 4 billion years ago," astronomer Henry Hsieh of Queen's University Belfast told NPR. "It gives us a way to kind of probe the cousins of the asteroids that hit us and probably gave us water in the early stages of the Earth's formation." Hsieh wrote a commentary that accompanied the stories in Nature.
The presence of ice and organic molecules on the surface of an asteroid is the latest in a string of discoveries that collectively indicate water ice is a more common substance than we might have thought. In just the past few years, scientists have confirmed the presence of ice at the moon's north pole as well as beneath the surface of Mars.
It had previously been thought that asteroids were too warm to retain water ice on their surfaces. The exact method for how they do so remains unclear.
Image: Artist's conception of asteroid 24 Themis and two small fragments of this dynamic family, which resulted from a large impact more than one billion years ago. One of the small fragments is inert (as most asteroids are), and the other has a comet-like tail, produced by the sublimation of water ice from its surface.
Gabriel Pérez/Servicio MultiMedia, Instituto de Astrofisica de Canarias, Tenerife, Spain