Nitrogen in ancient rocks a sign of early life
Nitrogen is one of the essential nutrients of life on Earth, with some organisms, such as the kinds of microbes found within the roots of legume plants, capable of converting nitrogen gas into molecules that other species can use.
|Scientists in Greenland excavating rocks that may hold 3.8-billion-year-old evidence of life |
[Credit: Laure Gauthiez]
With organisms playing such a crucial role in the chemistry of nitrogen on Earth, scientists are examining nitrogen in billion-year-old rocks to decipher its potential as a bio-signature of life on other planets. New findings in this area of research appeared recently in the paper, "Nitrogen in Ancient Mud: A Biosignature?" in the journal Astrobiology.
"This study identifies nitrogen abundances as a potential tool to detect remnants of life on Mars," said one of the study's authors, Eva Stüeken, an astrobiologist at the University of Washington at Seattle and the University of California at Riverside.
|Soil microbes known as rhizobia supply much-needed nitrogen to legumes such as clover |
(Trifolium species). In return, legumes shelter the rhizobia in nodules on their roots
and provide them with carbon [Credit: Julie McMahon]
Stüeken focused on 3.8 billion-year-old rocks from the Isua Supracrustal Belt in Greenland. Previous researchers interpreted organic carbon molecules in those rocks as biogenic, a controversial finding because the widely accepted scientific date for earliest life has been a younger 3.5 billion years.
"It is possible to make organic carbon even in the absence of life," Stüeken said. "So those studies have been viewed with skepticism by some researchers, and an independent biosignature was needed."
|The coast of Greenland [Credit: Mariusz Kluzniak/Flickr]|
"The nitrogen concentrations are close to what one would find in modern mud," Stüeken said. "So I asked myself if this amount of nitrogen could possibly result from abiotic sources."
In Earth's earliest history, abiotic processes may have been much more important to nitrogen fixation than biological ones, but nobody had tested the hypothesis, Stüeken said. "I was determined to find out when life began to dominate the global nitrogen cycle," she added.
|Deep canyons and over glaciers along the northwest coast of Greenland |
[Credit: NASA/Michael Studinger]
"Under abiotic conditions, it is impossible to accumulate so much nitrogen in sediments," Stüeken said. "Life, on the other hand, can easily accumulate so much nitrogen." As such, "the results provide more evidence of an early origin of life on Earth—before 3.8 billion years ago," Stüeken said.
This research suggests that analyzing nitrogen levels could help detect signs of life on Mars, Stüeken said. "Nitrogen abundances are relatively easy to measure by a rover, making this a useful tool for upcoming Mars missions."
Author: Charles Q. Choi | Source: Astrobiolog NASA [November 30, 2016]