Using advanced techniques, scientists have detected nitrogen-bearing organic material in meteorites that were ejected from the surface of Mars some 15 million years ago, proving that evidence of early life can be preserved and detected today.
The scientists that found nitrogen with organic content of carbonate minerals in a Martian meteorite include Atsuko Kobayashi of the Earth-Life Science Institute (ELSI) at the Institute of Technology in Tokyo, Japan, and Mizuho Koike of the Institute for Space and Astronautical Science of the Aerospace Exploration Agency of Japan.
The scientists consider that it is very likely that this organic material has been conserved for 4 billion years since the so-called Noachian age on Mars. Because carbonate minerals typically precipitate from the groundwater, this finding – published in Nature Communications – suggests an early, humid, organic-rich Mars that might have been habitable and favorable for life as we know it.
For decades, scientists have tried to understand whether there are organic compounds on Mars and, if so, what their source is. Although recent studies from rover-based Mars exploration have detected strong evidence for Martian organic compounds, little is known where they came from, how old they are, how widely distributed and conserved they may be, or what their possible relationship with biochemical activity could be.
Martian meteorites are pieces of the surface of Mars that were thrown into space by meteorite impacts and ultimately crashed on Earth, after failing to burn up in the atmosphere.
These alien space rocks provide important insights into Martian history. One particular meteorite, called the Allan Hills (ALH) 84001, named for the Antarctic region that was found in 1984, is especially important. It contains orange-colored carbonate minerals, which precipitated from salty liquid water on the surface of the red planet approximately 4 billion years ago. As these minerals record the early watery environment of Mars, many studies have tried to understand their unique chemistry and whether they could provide evidence of ancient life on Mars.
However, previous studies suffered contamination with terrestrial material from Antarctic snow and ice, making it difficult to say how much organic material in the meteorite was actually Martian, and how much belonged to Earth.
In addition to carbon, nitrogen (N) is an essential element for terrestrial life and a useful marker for the evolution of the planetary system. However, due to previous technical limitations, nitrogen had not yet been measured in ALH84001. This new research by the joint ELSI-JAXA team used state-of-the-art analytical techniques to study the nitrogen content of ALH84001 carbonates, and the team is now confident that they have found the first solid evidence of 4 billion-year-old Martian organic matter that contains nitrogen.
Ground pollution is a big problem for studies of extraterrestrial materials. To avoid such contamination, the team developed new techniques to prepare the samples. In this occasion, the scientists reportedly made us of silver-tape in a clean ELSI lab to pluck the tiny carbonate grains, which are about the width of a human hair, from the host meteorite. The team further prepared the grains in order to remove potential contaminants from the surface with an electron beam focused ion beam scanning instrument at JAXA.
Scientists used a technique called nitrogen x-ray absorption spectroscopy that allowed them to detect the nitrogen present in very small amounts and to determine what chemical nitrogen was in. Nearby igneous minerals gave no detectable nitrogen, showing that the organic molecules were only in the carbonate.
After careful contamination checks, the team determined that the detected organics were truly Martian. They also determined that the contribution of nitrogen in the form of nitrate, one of the strong oxidants on present-day Mars, was negligible, suggesting that early Mars probably did not contain strong oxidants, and as scientists suspected, was less oxidizing than is today.
The current surface of Mars is too hard for most organics to survive. However, scientists predict that organic compounds could be conserved in near-surface environments for billions of years. This appears to be the case for the nitrogen-containing organic compounds the team found in ALH84001 carbonates, which appear to have been trapped in the minerals 4 billion years ago and conserved for long periods before finally being delivered to Earth.