Since British pop icon David Bowie first posed the question in 1971,
“Does life exist on Mars?”
NASA has successfully landed five rovers on Mars. The
Curiosity
rover
landed in Gale Crater in 2012, unveiling rocks formed by a shallow lake approximately 3.6 billion years ago, hinting at a once habitable environment.
Curiosity
continues its mission, while in 2021, the
Perseverance
rover
was launched to explore Jezero Crater, where traces of past life may be found in sediments from a lake dating back 3.7 billion years.
Both
Curiosity
and
Perseverance
have uncovered evidence of
complex carbon-containing molecules
within Martian lake rocks. As all life on Earth is composed of similar organic molecules, astrobiologists speculate that these Martian compounds could lend credence to the existence of ancient life on Mars. However, it is crucial to note that organic molecules can also be formed through non-biological processes, implicating the need for further concrete evidence to definitively identify ancient Martian life.
Researchers at the Center for Astrobiology in Madrid, Spain, are investigating whether
DNA
can serve as a biomarker in Martian rocks. They argue that DNA is utilized by all life forms on Earth and is “the most critical biological molecule for life,” uniquely formed by living organisms. Additionally, factors that typically accelerate DNA degradation on Earth—such as water, heat, and microorganisms—are absent in Mars’
cold, dry climate
.
The greatest challenge in locating ancient DNA on Mars stems from the planet’s surface, which is consistently bombarded by intense
cosmic
and
solar radiation
that can rapidly degrade DNA and other organic molecules. Past studies have shown that DNA is more likely to endure radiation damage when
protected within rock
, prompting researchers to test whether Mars-like rocks could shield DNA from radiation levels akin to those on the planet for about 100 million years.
Direct access to Martian lake rocks is anticipated through future sample return missions such as NASA/ESA’s
Mars Sample Return
or China’s
Tianwen-3
mission. Researchers collected rocks from various geological ages formed in lakes and shallow marine environments globally. They specifically targeted rocks containing remnants of an ancient microbial community known as
microorganisms
and exhibited
total organic carbon concentrations
comparable to those identified in Martian geological samples, including lake microbial rocks from Mexico aged 2,800 years, shallow-water microbial rocks from Morocco aged 541 million years, and iron-rich rocks from Ontario, Canada, aged 2.93 billion years, with characteristics similar to those in Jezero Crater on Mars.
The team crushed the rocks, dividing them into six samples sealed in glass containers. They exposed three samples from each set to radiation levels reflective of 136 million years on the Martian surface, retaining the remaining three for comparison. The DNA was extracted from each sample and analyzed using
nanopore sequencing
, a method that effectively identifies short DNA fragments while assigning a quality score based on the reliability of the sequences.
The analysis indicated that unirradiated samples, presenting higher organic carbon content, also contained a greater abundance of DNA fragments. The findings suggest that the DNA originates from modern microbial communities that recently inhabited the rocks, while the organic carbon represents remnants from ancient microbes. Enhanced availability of nutrients correlates with increased microbial growth, solidifying the view that organic-rich sites such as ancient crater lakes are prime candidates for life-detection missions.
In the irradiated samples, DNA quality diminished and became fragmented from radiation exposure. For instance, the irradiated samples of Mexican lake microorganisms exhibited average quality scores 53% lower and DNA reads 85% shorter than unirradiated samples. However, the research team successfully identified which microorganisms contributed an estimated 2% to 9% of the DNA in these irradiated samples.
The researchers concluded that identifiable DNA fragments could potentially persist in Martian rocks for over 100 million years. They advocate for the application of this sensitive sequencing technology in forthcoming Mars rovers to search for evidence of past life and evaluate the planet’s biological safety. While the results are promising for astrobiologists, some caveats remain. Martian rocks may harbor
toxic salts
that could harm DNA integrity. Furthermore, scientists voice concerns regarding
pollution
from terrestrial life. The research team recommends that future investigations develop stringent protocols for eliminating salts from Martian rock samples and assessing possible external contamination.
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Source: sciworthy.com
