Martian Organic Molecules Came From Natural “Batteries”

A close-up look at minerals in Martian meteorites shows that complex organic compounds formed not from life, but from electrochemical reactions similar to the ones that occur in a battery.

In 2012, NASA’s Curiosity rover scooped up a handful of Martian soil and analyzed its contents to find several hydrocarbons [1]. That same year, millions of miles away on Earth, researchers discovered similar organic compounds in 10 meteorites, blasted off of the red planet by asteroid or comet impacts, which then fell to Earth [2]. Now, the researchers think they know how these compounds originally formed on Mars.

DCO members Andrew Steele (Carnegie Institution for Science, USA), Erik Hauri (deceased, formerly of Carnegie Institution for Science, USA), Richard Wirth (GFZ German Research Centre for Geosciences, Germany), Marc Fries (NASA Johnson Space Center, USA), and Karyn Rogers (Rensselaer Polytechnic Institute, USA) and colleagues, used high-resolution transmission electron microscopy to take a nano-scale look at three of those 10 Martian meteorites [3]. Based on the mineral patterns, they propose that salty fluids on Mars reacted with the mineral magnetite. The resulting electrochemical reactions transformed dissolved inorganic carbon in the fluids into complex organic molecules. These reactions occur independently of life, but they may be one possible way to generate organic compounds that support the first life forms on potentially habitable planets. 

“Even if life doesn’t exist on Mars, this study is telling us a huge amount of information on how life potentially started on Earth,” said Steele. “This reaction is probably typical to early Earth, Europa, Enceladus and other solar system bodies.”

Steele and his colleagues looked at three Martian meteorites, Nakhla, Tissint, and
NWA 1950, using multiple imaging and spectroscopy techniques. They saw complex organic compounds attached to layers of volcanic minerals and within tiny cracks in the rocks. Based on the minerals associated with the organic carbon and the presence of chlorine compounds (likely from salts), the researchers suspect that the organics formed when brine percolated through the rocks, causing corrosion. 

Martian Meteorite
This high-resolution Transmission Electron Micrograph shows a grain from a Martian meteorite. Layers of organic carbon (black) have formed in between layers of titanium-rich minerals (white), through a natural corrosion reaction between salty brines and volcanic rocks. Credit: Andrew Steele

In the Nakhla meteorite, the erosion reaction is particularly clear and looked just like a natural battery. A grain of the meteorite had fingers of a titanium-rich mineral, like the tines of a comb, interspersed with iron-rich layers. Parts of the iron-rich phase had eroded away, to be replaced by organic compounds. Assuming that the titanium-rich phase acted as the cathode in a battery and the iron-rich phase as the anode, this reaction could have generated enough energy to convert inorganic compounds dissolved in the brine into organic molecules. The size of the layers is on the scale of nanometers, making them too small to hold microbes, further supporting the idea that the compounds formed abiotically.

To understand the exact conditions that led to these corrosion-powered batteries, Steele is attempting to recreate the process on the benchtop. “It’s going to be difficult to show this in the laboratory setting,” said Steele, “but I’m going to give it a try. Preliminary experiments have shown the release of methane from a carbonated seawater.” He also will continue using high-resolution microscopy to examine the complex organic compounds in the remaining seven meteorites.

The study is part of larger efforts to catalog the organic compounds present on Mars, to determine whether they are evidence of life, potential building blocks for cells, or “background” molecules on the planet that formed from abiotic, chemical processes. A comprehensive understanding of these background organics will help researchers to focus their search for extraterrestrial life on the red planet and elsewhere in the solar system. These reactions may also occur in Earth’s mantle, creating deep reservoirs of organic carbon and methane.

Some of the analyses from the paper occurred as a proof of concept for an instrument in development that combines multiple detection capabilities into a single vacuum chamber to prevent contamination of the Martian samples. The DCO provided some funding to Steele, who worked with former postdoctoral researcher Sandra Siljeström (now at RISE, Sweden) to perform the Time-of-Flight Secondary Ion Mass Spectrometry analyses of the chemical species in the meteorites. The proof-of-concept instrument is still under development. 

The authors dedicated this paper to the memory of co-author Erik Hauri, who passed away before publication. Hauri was an Earth scientist in the Department of Terrestrial Magnetism at the Carnegie Institution for Science and a Co-Chair of the Scientific Steering Committee of the DCO Reservoirs and Fluxes Community. Read more about Erik Hauri, his work, and his legacy in a tribute on the DCO website.
 

Main image: This self-portrait of NASA's Curiosity rover, taken 15 June 2018, shows the rover in front of the Gale Crater, where it discovered chlorine compounds and hydrocarbons in the soil. Credit: NASA/JPL-Caltech/MSSS
 

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