Decades of searches have finally been crowned with success: scientists are close to unraveling where carbon appeared in the soil and atmosphere of the red planet.
Six years after the start of work at Gale Crater on the Martian surface, the Curiosity rover made perhaps the most important discovery in the search for signs of life: the rocky surface of the red planet is teeming with organic molecules, and from time to time gas even seeps into its rarefied atmosphere methane is the simplest of organic molecules. For comparison: on Earth, carbonaceous substances are the basis of life.
Both discoveries were made in the course of analysis of samples collected by Curiosity. At SAM, the rover's miniature chemistry lab, also called an "oven," tiny fragments of air, rock, and soil are "roasted" to be studied at the molecular level. Thus, in the samples of ancient mudstone, a wide variety of organic molecules was found. Another study, which lasted no less than five years, found regular fluctuations in methane in the Martian atmosphere. The peak in emissions was in the Martian summer. The results were published in the journal Science.
However, no matter how they excite the imagination, the conclusions about the past, present and future life on Mars are not yet final - methane is found everywhere in the atmosphere of the gas giants. And this by no means speaks of the presence of life: methane is formed from a banal interaction between running water and heated stones. In addition, other simple organic molecules are known to be found in some meteorites and interstellar gas clouds. “It is extremely difficult to scientifically prove the existence of life on Mars. This requires literally showing a photograph of the fossil,”says Chris Webster, Jet Propulsion Laboratory chemist and lead author of methane research.
Where did the Martian carbon go?
The very presence of organic molecules on Mars is not surprising. Like any other planet in our solar system, Mars regularly receives its share of carbon-rich micrometeorites and cosmic dust. However, NASA's Viking spacecraft, which landed on the red planet in 1976, made a sensational discovery: it turned out that there is even less carbon in Martian soil than in lifeless lunar rocks. "This was a big surprise," explains astrobiologist Caroline Freissinet, co-author of the Curiosity argillite study and at the Laboratory for Atmospheric and Space Research in France. "Unfortunately, this led to the collapse of the entire Martian program."
Since then, scientists have been zealously searching for carbon on Mars - or at least struggling to explain why it isn't found. The key hunch came in 2008 when NASA's Phoenix lander found perchlorate salts, highly reactive chlorine-containing molecules, in soil samples taken near the north pole of Mars. Combined with bright ultraviolet light and cosmic rays from space, perchlorates destroy all organic matter on the surface, leaving no evidence even for the sensitive sensors of Mars rovers. Perhaps, some researchers have suggested, the residual organic matter of Mars - and, therefore, any signs of past or present life - lurk in its depths.
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In 2015, however, Curiosity came close to proving the existence of organic molecules on Mars when, heating soil samples to 800 degrees Celsius in an oven, it discovered traces of chlorine-contaminated carbon compounds. However, at the very beginning of the Martian mission, scientists discovered a leak of carbon-containing chemical reagents from a number of components of the "oven" itself, which could lead to contamination of the samples. To combat pollution, the Curiosity team focused on searching for other samples of chlorine-containing organics, while simultaneously reducing the temperature of the “oven” - during subsequent runs it only heated up to 400 degrees.
Before embarking on a new task, the team made sure that nothing was overlooked this time. After rechecking the level of background pollution, Fressinet and her colleagues "baked" samples of mudstones, dating back three billion years, at a temperature of 500 degrees Celsius - the perchlorates are completely burned with it. Thiophenes, relatively small and simple ring-shaped molecules containing both carbon and sulfur, were found in the ash. The latter appears to come from a sulfur-rich mineral called jarosite. Previously, Curiosity discovered its 3.5 billion-year-old deposits in Gale Crater - apparently, they formed at a time when the still-cold crater was filled with water and was suitable for life. Scientists suspect that the carbon contained in thiophene comes from as yet unidentified, but larger molecules,preserved inside jarosite for billions of years.
Despite the controversy of the discovery, George Cody, a geochemist at the Carnegie Institute of Science who was not involved in the study, believes this is a giant step forward. The presence of these larger molecules, he says, suggests the presence of well-preserved deposits of carbon hidden under the Martian surface. Such prospects, he believes, provide a scientific basis for the upcoming missions to collect samples and return them to Earth. “If this can be done on Mars, just imagine what can be achieved in terrestrial laboratories,” he says.
Seasons and fluctuations in methane
Meanwhile, the Curiosity rover has made what Webster says is the most important methane measurements in history. This carbonaceous gas is critical because most of Earth's methane is produced by methanogenic microbes that survive even in oxygen-poor environments. In addition, methane is quickly destroyed by ultraviolet radiation, so any find on Mars is most likely "fresh" - the gas was released only recently. Using the "oven," Webster and his colleagues found a steady background level of methane in the atmosphere above Gale Crater. Over the past five years, it has been approximately 0.4 parts per billion. And although this amount is barely detectable, astrobiologists are already interested in it. It is noteworthy that the level of methane fluctuates along with the Martian seasons: in sunny summer, its content is three times higher,than a cold and dark winter.
For Webster, this periodicity is perhaps the most exciting of his discoveries. Previously, only evidence of accidental, but not seasonal emissions was found on Mars. “Imagine that your car is junk. Until the problem recurs, you never know what is wrong,”Webster explains. He and his colleagues speculate that methane may come from deep aquifers: in the summer they melt, water is released and fresh gas is formed. According to another version, these substances are ancient and they were formed billions of years ago in the course of various geological and biological processes. Then they froze in matrices of ice and rocks and stand out only when defrosting, from sunlight. And, finally, there is a possibility that the Martian methanogens doze in the bowels of the planet to this day,periodically waking up and producing a characteristic gas by which they can be identified.
Other scientists, who did not take part in the study, assess the significance of the results for the search for life on Mars ambiguously. Michael Mumma, an astrobiologist at the Goddard Space Flight Center, says measurements are critical because they provide direct evidence of his own observations. Earlier, he wrote about Martian methane emissions, which he discovered with the help of terrestrial telescopes - although scientists in circles accepted his discovery with disbelief.
Planetologist Marc Fries, who oversees the collection of cosmic dust at the Lyndon Johnson Space Center, was skeptical about the recent discoveries of Curiosity. Carbon-rich meteorites and cosmic dust entering the Martian atmosphere may be the source of the stated amounts of methane, he said. He also stresses that the seasonal periodicity is not entirely consistent with the Martian seasons. “A rigorous, evidence-based approach based on the available evidence assumes that Mars has always been and remains lifeless,” says Freese. "Even putting forward the opposite hypothesis requires strong evidence." Soon, this hypothesis will be able to be tested by the data of the joint mission of the EU and Russia "Exomars Trace Gas Orbiter". This spacecraft has been orbiting Martian since 2016 and displays concentrations of methane and other gases from above.
Webster, in turn, says that none of the possible explanations is favored until final conclusions are drawn. Moving forward gradually is NASA's approach to Mars exploration, Fressinet notes: "Step by step, mission by mission."
Adam Mann