
NASA Earth Observatory
Satellite image of Meteor Crater in Arizona, caused by an impact about 49,000 years ago.
Long imagined by science fiction, the tremendous impact of asteroid collisions with Earth is finally being made clear: 4.4 million pounds per square inch of force, and temperatures hot enough to melt any rock.
For Aaron Cavosie, a visiting professor at the UW-Madison’s Astrobiology Research Consortium, questions still remain about how asteroids shaped life.
“Is life on Earth a one-off special?” he asks. “Or is the development of life more of a general result of astronomical processes, kind of like how smog and garbage dumps are unavoidable where humans tend to concentrate?”
Cavosie leads an international team that’s hoping to find some answers in samples of stone that turned molten 49,000 years ago, when impact with an asteroid formed Meteor Crater in northern Arizona.
“For those interested in addressing the origin of life,” he explains, “the role of meteorite impacts on Earth is a first-order process that influenced establishment of habitable surface conditions that would have provided an environment that allowed life to take hold and ultimately take over.”
On the moon there are thousands of impact craters. On Earth only 190 have been confirmed, but time and weathering are assumed to have obscured evidence of many more.
“A great deal has been learned about how impact events affect surface conditions, in terms that give us indications of what conditions may have been like on the early Earth,” says Cavosie. “However, there remains much that isn’t well known.”
He and his international team investigated grains of zircon found in Meteor Crater sandstone (not to be confused with the “cubic zirconium” gems of television home shopping). Zircons can last billions of years, and are tiny. Eight of them, lined up, would extend the width of a hair.
“The Meteor Crater grains we describe actually started off as small grains of zircon sand,” says Cavosie. “When the meteorite impact happened, they underwent several transformations.”
Magnified, the Meteor Crater zircons look like thousands of tiny beads curiously joined together. Termed “granular” zircons, such groupings have been found at impact craters in South Africa and on the moon. No one knew why.
Using a high-resolution electron beam method called electron backscatter diffraction, the researchers found some clues on how the granular zircon grains form. “It provided a wealth of new information about how to interpret conditions of impact preserved in rocks at terrestrial craters, lunar samples, meteorites and Mars,” says Cavosie.
The team, including graduate student Timmons Erickson and collaborators Justin Hagerty from the U.S. Geological Survey and Fred Horz from NASA, found evidence that the zircons had been subjected to a pressure of at least 300,000 atmospheres and temperatures of more than 3632 degrees Fahrenheit.
“From my point of view, it is remarkable that any mineral can survive the extraordinary conditions that affected the granular zircons,” says Cavosie. “These conditions result in the wholesale melting of any rock in the Earth’s crust, and would likely have melted the zircons as well, if the process had lasted much longer than a few seconds.”
And that process would have been repeated many, many times. More massive than the moon, early Earth’s gravity would have attracted far more impacts, leaving it heavily cratered — and hot.
“Some envision that the surface of the Earth was molten magma as a consequence of this process,” says Cavosie. “Others have documented evidence for early oceans as far back as 4.3 billion years ago. So which is right? A hot, hellish and potentially lifeless world sterilized by molten magma? Or a water world, where environments capable of supporting life existed very early on?”
His opinion is that both scenarios are likely. “Giant impacts may have vaporized regions of the crust and oceans, but probably not all of it. Life is tenacious, and hangs on once it gains a foothold.”
The team’s findings were reported in the July issue of Geology, an online academic journal.