Since British pop legend David Bowie posed the question in 1971, “Does life exist on Mars?”, NASA has successfully landed five rovers on the Red Planet. The Curiosity rover, which touched down in Gale Crater in 2012, uncovered rocks formed in a shallow lake approximately 3.6 billion years ago, indicating a once habitable environment. In 2021, the Perseverance rover began exploring Jezero Crater, where traces of ancient life may be found at the base of a lake dating back 3.7 billion years.

Both Curiosity and Perseverance have discovered evidence of complex carbon-containing molecules within Martian lakebed rocks. Organisms on Earth consist of similar organic molecules, leading astrobiologists to speculate that these Martian compounds might indicate past life. However, it’s important to note that organic molecules can also arise from non-biological processes, such as interactions between gases and minerals at high temperatures. Thus, more conclusive evidence is needed to confirm the existence of ancient Martian life.

A recent study by researchers at the Center for Astrobiology in Madrid, Spain, explored whether DNA could function as a potential biomarker in Martian rocks. They posited that DNA is universal among Earth’s life forms and deemed it “the most crucial biological molecule for life.” Only life forms create this molecule. Furthermore, many conditions that degrade DNA quickly on Earth—such as the presence of water, heat, and microorganisms—are absent in the cold, dry climate of Mars.

One major obstacle in detecting ancient DNA on Mars is the planet’s surface, which is constantly bombarded by intense shock waves. Cosmic and solar radiation can rapidly degrade DNA and organic molecules. Prior research has indicated that DNA is more likely to survive radiation damage when protected within rock. Hence, the researchers aimed to examine whether Mars-like rocks could shield DNA from radiation levels equivalent to around 100 million years of exposure on the planet’s surface.

Scientists will not gain direct access to Martian lake rocks until future sample return missions, such as NASA/ESA’s Mars Sample Return or the Chinese Astronomy-3 mission, are conducted. The researchers collected samples from various rock ages formed in lakes and shallow marine environments worldwide. They specifically targeted rocks with remnants of an ancient microbial community known as microorganisms and a total organic carbon concentration similar to that of Martian rocks. The samples included 2,800-year-old lake rocks from Mexico, 541-million-year-old shallow-water rocks from Morocco, and 2.93-billion-year-old iron-rich rocks from Ontario, Canada, featuring minerals akin to those in Jezero Crater on Mars.

The team crushed the rocks, dividing them into six samples each, sealed in glass bottles. They exposed three samples to radiation levels equivalent to 136 million years on the Martian surface, while leaving the other three unexposed for comparison. DNA was extracted from each sample and examined using a technique that enables reliable identification of short DNA fragments known as nanopore sequencing. This method also generates quality scores for each DNA fragment to assess the accuracy of specific DNA sequences.

The analysis revealed that unirradiated samples contained higher quantities of DNA fragments, correlating with a greater presence of organic carbon. This suggests that the DNA originated from contemporary microbial communities residing in the rocks, while the organic carbon was derived from long-deceased microbes. Thus, the researchers inferred that modern microbes were consuming ancient organisms; the more food available, the larger the microbial populations grow. These findings support the proposition that rich organic carbon sites like ancient crater lakes are prime targets for future life-detection missions.

In irradiated samples, DNA quality diminished and fragmented due to radiation exposure. For instance, the DNA from irradiated samples of Mexican lake microorganisms exhibited quality scores that were, on average, 53% lower, with DNA reads averaging 85% shorter compared to unirradiated samples. Nevertheless, the research team managed to identify microorganisms that contributed around 2% to 9% of the DNA in the irradiated samples, despite significant degradation.

The researchers concluded that identifiable DNA fragments could persist in Martian rocks for over 100 million years. They proposed that this sensitive sequencing approach should be implemented in future Mars rovers to search for evidence of past life and evaluate the planet’s biological viability. While these results are promising for astrobiologists, challenges remain, such as the presence of toxic salts that could further degrade DNA and concerns regarding pollution from terrestrial life. The research team recommended developing stringent protocols for decontaminating Martian rock samples and addressing external contamination.

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