Heat from a cosmic crash could have nurtured ancient organisms

By Erin Wayman

Web edition: March 1, 2013
Print edition: April 6, 2013; Vol.183 #7 (p. 9)

Meteorites smacking into the early Earth could have created warm, watery environments favorable to primordial life. A new study of an impact crater in Finland suggests that such hydrothermal activity could have lasted up to 1.6 million years — at least 10 times longer than theory suggested, providing plenty of time for life to emerge and spread.

Ancient impact craters on Mars were probably also home to hydrothermal activity, making them good places to search for signs of life, the team reports online February 19 in Geochimica et Cosmochimica Acta.

The work is “quite exciting,” says Gordon Osinski, a planetary geologist at the University of Western Ontario in Canada. “One of the big unknowns has been how long do these hydrothermal systems last.”

Because hydrothermal systems house life’s most ancient lineages, many biologists think that the first organisms arose there. Volcanoes drive most hydrothermal activity today, such as the hot springs and geysers of Yellowstone. But when life evolved about 3.8 billion years ago, frequent impacts pummeling the planet were the largest source of hydrothermal activity. Energy from such events melted rock and heated water circulating through the Earth’s crust. These hydrothermal environments would have been cozy, protective habitats where life could have emerged, or at least thrived and evolved, Osinski says.

Geologic activity has erased most of the planet’s craters and left the remaining ones poorly preserved, says planetary scientist David Kring of the Lunar and Planetary Institute in Houston. But scientists have managed to estimate that the roughly 250-kilometer-wide Sudbury Crater in Canada hosted hydrothermal activity for a million years or longer after it formed about 1.85 billion years ago.

Smaller impacts, leaving behind holes 20 to 30 kilometers wide, are 10 times as common. So these medium-sized impacts could have played a more important role than big ones in the origins of life, says study coauthor Martin Schmieder, a geologist at the University of Western Australia in Crawley. But theoretical calculations had indicated these craters would have cooled too quickly to sustain hydrothermal activity for more than a few tens of thousands of years—probably not long enough for life to have gotten its start there. 

Schmieder and coauthor Fred Jourdan of Curtin University in Perth, Australia, didn’t intend to measure the cooling time of a medium-sized crater. But that’s what happened when they dated Finland’s 23-kilometer-wide Lappajärvi Crater. Using rocks from the crater, the pair determined that the impact occurred about 76.2 million years ago.

But some samples were as much as 1.6 million years younger. Those samples were grains of potassium-feldspar, which is one of the last minerals to crystallize when rock melted by an impact cools. The difference in age between the older rocks and the potassium-feldspar represents the period when the crater was hot enough to support a hydrothermal environment, Schmieder and Jourdan say.

Similar studies of other craters will help determine whether long-lived hydrothermal activity is common to all medium-sized impacts or unique to Lappajärvi, says planetary scientist Jay Melosh of Purdue University in West Lafayette, Ind. Schmieder and Jourdan plan to look at well-preserved craters in Germany or Australia, where they will also investigate properties that influence how long a crater takes to cool.

“I’m happy, if not ecstatic, that people are going to these [smaller] impact craters to collect the data,” Kring says. That will help researchers improve computer simulations of hydrothermal systems in impact craters on Earth and on Mars.