Deep in the heart of
Greenland, in an area recently laid bare by melting ice, lies a controversy: A
rocky outcrop that some scientists say contains the oldest known signs of life
on Earth. Others disagree. So a handful of scientists recently headed to the
site to study it together.
It’s not easy to identify biological
traces within rocks that have been churned and chewed by tectonic pressure and
heat over billions of years. But figuring out how best to identify such traces
on Earth could help scientists spot those signs on other worlds, such as Mars.
The Greenland outcrop, which dates to between 3.7 billion and 3.8 billion years ago, contains strange squiggles just a few centimeters tall. One team of scientists has suggested the wavy pattern was shaped by microbes living in ancient shallow pools (SN: 8/31/16). The microbes shifted sediments until they formed thinly layered structures called stromatolites, geologist Allen Nutman of the University of Wollongong in Australia and colleagues reported in Nature in 2016.
Other scientists have rebutted that idea, based on geologic and chemical evidence from samples of the Greenland outcrop (SN: 10/17/18). Astrobiologist Abigail Allwood of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who led another study published in Nature in 2018, says that visiting the site herself in 2016 and getting a fuller, 3-D picture of the pattern within the entire outcrop was key to her conclusion.
With the two camps at an
impasse, Allwood says she had an idea: What if a handful of scientists,
including members of both teams, studied the outcrop together and compared
notes? So in August, Allwood, Nutman and about 10 more geologists, astrobiologists
and other specialists helicoptered in to the remote site.
Nutman introduced the group to the outcrop and laid out his evidence, and the researchers spent a day and a half observing and debating, says Dawn Sumner, an geobiologist at the University of California, Davis.
“A number of us went in with
an open mind,” Sumner says. But by the end of the trip, she says, most
expedition members concluded that the pattern’s peaks probably weren’t shaped
by microbes. Seeing the structures in context, rather than in published images,
was crucial to that result, Sumner says. “One of the things we came away with
is how difficult it is to get a sense [of what formed the pattern] without
actually being there.”
Nutman and his team remain committed
to the microbial explanation. But all parties agreed on one thing: Building a
three-dimensional picture of a site could be the key to accurately identifying
signs of life.
Now, expedition members are doing
just that, using aerial drone photographs, stereoscopic images and scans taken
by light detection and ranging equipment, or lidar, during the trip. The goal
is for other researchers to be able to study the site from afar, Sumner says —
much like, perhaps, scientists might be able to someday squint at mysterious
structures in Martian rocks.