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Scientists have largely debated whether Mars has ever contained a hospitable atmosphere that provided an environment suitable for life to emerge. Researchers have attempted to understand if this planet possesses organic compounds, and more importantly ventured to seek their source.
According to NASA on their website nasa.gov, there is “clear evidence that in the distant past, Martian climate allowed for liquid water to pool at the surface of the planet”, a direct indicator that life could exist on Mars. With radiation and harsh chemicals in space, the organic matter contained in the Martian surface had begun to break down.
Although recent rover-based discoveries indicate the existence of organic compounds on the surface of the planet, some researches have shifted their focus to Martian meteorites. Also stated by NASA, on nasa.gov, of the 60,000 meteorites found on Earth, only 126 of them were identified as Martian. Studying these meteorites can give insight to the formation and geologic history of Mars. More importantly, Martian meteorites show evidence of interaction with water on the Red Planet, that scientists examine to gauge how suitable Mars was for life (Arizona State University).
Why Nitrogen?
Nitrogen is an incredibly essential element for all living things as it is necessary to construct larger molecules such as DNA and RNA that encode proteins to build structures in cells as well as speed up chemical reactions within. The reason why scientists study nitrogen is because this element is a key “geochemical tracer to reveal the co-evolution of the planetary atmospheres, hydrospheres, lithospheres, and biospheres” (nature.com). Both on Earth and on Mars, nitrogen is found as a gas and nitrogen atoms are only accessible once separated or fixed.
A process known as nitrogen fixation converts nitrogen gas (N2) into ammonia (NH3), a compound of nitrogen and hydrogen or nitrate. The process is broken down into 2 steps: abiotic fixation, where the N2 gas combines with oxygen, and biological fixation, where the nitrogen-bearing bacteria carries out the fixation process. Those microorganisms that reside in Mars’ soil, may indicate the possibility of biotic processes resulting in the formation of nitrogen bearing compounds, but more likely caused by abiotic factors.
Nitrate (a type of fixed nitrogen) is the only biologically useful form of nitrogen on Mars, because of the low levels of nitrogen gas in the atmosphere. Therefore, the presence of nitrate in the Red Planet’s soil is of major astrobiological significance (Jet Propulsion Laboratory). Researchers speculate these nitrates were not formed by living things or biotic processes, but are ancient and likely came from non-biological processes like meteorite impacts.
Back in 1995, a meteorite titled Grave Nunataks 95229 from Antarctica that happened to shower Earth proved to contain 4 grams of powder abundant in ammonia. Professor Sandra Pizzarello who led the research on behalf of Arizona State University, claims that “there are asteroids out there that when fragmented and become meteorites, could have showered the Earth with an attractive mix of components, including a large amount of ammonia". Her research is proof that some meteorites descending from planets contain fixed nitrogen, a component that is necessary for life to evolve, and thus provide a hospitable environment for living organisms.
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Curiosity Rover and the Gale Crater
The recent discovery of 4 billion year old nitrogen-bearing organic materials on the ALH 84001 is substantial evidence that organic compounds can be preserved in carbonates of meteorites. Nevertheless, it's important to take into account the immensely crucial discoveries of the Curiosity Rover on Mars.
This rover was the first to detect nitrogen on the surface of Mars. It was sent to heat Martian sediments, which released nitrogen in the form of nitric oxide, that only occurs with the breakdown of nitrates. Nitrate is a form of nitrogen gas that is only usable by living organisms, further proof of a habitable environment on ancient Mars.
Another discovery made by the Curiosity team was dry riverbeds and minerals that suggest the presence of liquid water and thus, organic matter in the Gale Crater. In 2012, NASA’s Curiosity Rover explored Gale Crater, a large impact basin with a massive, layered mountain in the middle. Closer to 3.7 billion years ago, a large meteor impacted the planet and cracked the rock below, allowing for “groundwater to seep into the crater while rivers fed by rain or melting snow also flowed in, forming a lake” (jpl.nasa.gov).
Each new layer begins to solidify and is buried under layers of wind for millions of years. In gradual time, the crater begins to dry, but the sediment keeps piling up as sand and dust blow into the crater. The microorganisms and organic compounds found in that ancient water are deeply buried by the sediment. The same winds that blanket the crater with gravel, sand, and silt reversibly move the sediment back out. As the heaviness of the sediment is lifted the layers that cover the groundwater crack, which allows the water to flow through the crater again before it dries out.
The Gale Crater allowed the Curiosity team to discover how the Sediment patterns reveal the presence of water, continually over many millions of years. The presence of water is not only an indicator of past life on the Red Planet, but samples taken from the crater show key elements, organic molecules, nutrients and energy sources that microbes could have used. The Curiosity Rover’s travel to Mars and the data they gathered could be argued as the fundamental evidence of past life on Mars, and the existence of complex compounds like water could be proof of other elements existing on the Martian surface.
Nitrogen in Martian Meteorites
A recent discovery of nitrogen-bearing organic material in carbonate minerals, contained in a 4 billion year old Martian meteorite, dates back all the way to Mars’ Noachian age. It is solid evidence of 4-billion year old Martian organics containing nitrogen.
A research team composed of scientist Atsuko Kobayashi from the Earth-Life Science Institute (ELSI) at the Tokyo Institute of Technology, with scientist Mizuho Koike from the Institute of Space and Astronautical Science at Japan Aerospace Exploration Agency (JAXA), believe their findings further “suggest a wet and organic-rich early Mars, [that] could have been habitable and favourable for life to start.”
The team used unique “analytical techniques to study the nitrogen content of the ALH84001 carbonates,” according to the Tokyo Institute of Technology. The meteorite of their focus is named Allan Hills (ALH) 84001, that contains orange-colored carbonate minerals that precipitated from salty liquid water on Mars' near-surface 4 billion years ago. Researchers believe ALH84001 was blasted off Mars by a powerful impact 16 million years ago and came down to Earth much later, about 13,000 years ago (space.com). According to earlier data collected from the ALH84001 meteorite, terrestrial contamination with material from Antarctic ice and snow made it difficult to say if the material in the meteorite was truly Martian. Additionally, technical limitations could not conclude that the meteorite contained traces of nitrogen.
To conduct further experiments on the AL H84001 meteorite, the ELSI and JAXA teams used “silver tape in an ELSI clean lab to pluck off the tiny carbonate grains from the host meteorite. [The team then] prepared these grains further to remove possible surface contaminants with a scanning electron microscope-focused ion beam instrument at JAXA. Another device was used to detect nitrogen- the Nitrogen K-edge micro X-ray Absorption Near Edge Structure (µ-XANES) spectroscopy,” Science Daily concludes. A combination of these devices allowed the researchers to detect the nitrogen in very small amounts and determine what chemical form that nitrogen was in. They also gathered controlled samples from nearby minerals that contained no trace of nitrogen proving that the organic compound was found only inside the carbonate.
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Significance
Their findings proved that organic compounds such as nitrogen could be preserved in carbonates of meteorites for billions of years. The ALH84001 carbonates trapped the minerals for 4 billion years and was able to preserve that nitrogen-bearing organic material until it was delivered to Earth.
The researchers still had more questions about their findings, the main one being where the nitrogen-organic compounds came from. There were two speculations; either the organic compounds formed outside of Mars or on Mars. In the earlier years of the Solar System, Mars was filled with carbon-rich meteorites, comets and dust particles, Kobayashi explains, from which the organic matter could have dissolved into the carbonates and been preserved for billions of years.
These Martian carbonate minerals typically precipitate from the groundwater and according to eos.org are formed when water traps atmospheric carbon dioxide. Therefore, the Red Planet must have contained a wet atmosphere with possible “ancient lakes and rivers” (eos.org). These findings conclude that organic nitrogen on Mars was present before Mars became not habitable, and early Mars was even “Earth-like', less oxidising, wetter, and organic-rich” (Tokyo Institute of Technology).
Conclusion
Discoveries like these are what fueled scientists to focus on something else that can directly carry and store planetary matter for billions of years- meteorites. By taking a closer look at meteorites, researchers have found traces of key elements such as nitrogen, adding on to the evidence that some planets were at one point, capable of facilitating life. Through these expeditions scientists get one step closer to finding out what life was really like for the beginning of these planets.