In 2020, the fate of the earth was ill-fated, but the disaster did not stop mankind from exploring space. UAE July 20th? Hope? Take the lead, China? Tian Wen No.1? American? Perseverance? (Perseverance) will soon follow, with the beautiful expectations of human scientists, to the red planet 55 million kilometers away.
Investigating the traces of life on Mars is one of the important scientific research tasks in this exploration. Although not on Mars? Martians? However, whether there is a low-level or primitive life form and whether it can provide ideas for explaining the origin of life has always been a problem that scientists strive for. In the history of Mars life exploration, the National Aeronautics and Space Administration (NASA) is the most experienced, and they have not given a particularly clear statement before. However, some scientists always insist on saying yes.
This all started half a century ago? Viking mission?
Since the 1960s, NASA has focused on this vigorous development? God of war? Planet. From Mariner 4 in 1964, Mariner 6 and Mariner 7 in 1969 to Mariner 9 in 197 1 and 1972, NASA has been exploring Mars step by step. Actually, yes? Space race? Later, NASA's next goal is to land astronauts on Mars.
The time is 1975, and NASA is officially online? Viking mission? Viking mission, also known as pirate mission. Just like in the Middle Ages, ambitious vikings set sail from Scandinavia and traveled all over the vast territory of the European continent and the Arctic with amazing courage and perseverance, creating? Viking era? . Two Viking ships carry the wishes of the earth people. Found it? God of war? After all, in addition to understanding the climate and geological properties of Mars, another goal is to find traces of life.
That summer, two spaceships went to Mars one after another. After a year's flight, Viking 1 landed on the west slope of Chryse Planitia on July 20th, 1976, while Viking 2 landed in Utopia Planitia, 4000 miles away on September 3rd.
Viking? This title seems to give this mission unparalleled tenacity? Viking spirit? Originally planned for a 90-day exploration mission, the service life of the orbiter and lander far exceeded the design expectation. Especially the Viking 1 orbiter, in the orbit of Mars? Diligence? Worked for four years. With the gradual depletion of gas fuel, the whole exploration mission officially came to an end on May 2 1, 1983.
The Viking lander consists of five basic parts: the main body of the lander; Biological protective cover and base; Fuselage; Bottom cover and parachute system; And a lander subsystem. The instruments carried are used for the main scientific research purposes of the lander: biological research, chemical composition analysis (organic and inorganic), meteorology, seismology, geomagnetism and geomorphology, Mars surface and atmospheric physics. 丨 Image source: National Aeronautics and Space Administration
Have high hopes? Viking mission? biotic experiment
During the mission, the scientific instruments on the lander obtained more data than expected. Biological experiments include meteorological mass spectrometry (GCMS), gas exchange experiment (GEX), label release experiment (LR) and thermal interpretation experiment (PR). In addition, many excellent photos were taken during the pirate mission, including 4500 photos taken by the lander and 52000 photos taken by the orbiter.
Among the above four biological experiments, biologists have high hopes for the experimental project of label release (LR). The specific experimental process is not complicated. Viking- 1 and Viking-2 landers collect Martian soil samples, then inject a drop of diluted nutrient solution into the samples, and then detect whether metabolic by-products will appear in the air above the soil. Before injection, nutrients have been labeled with radioactive carbon 14. If microorganisms in soil metabolize nutrients, they should release radioactive by-products, such as labeled carbon dioxide or methane.
Before the launch of the Viking spacecraft, the researchers tested the experimental scheme in extreme environments around the world, in different soils from Death Canyon to Antarctica. Every experiment is a positive result. At the same time, the researchers also did the control group experiments one by one, heating the samples to the high temperature of 160℃ to kill all life forms, and then repeating the experimental operation, each experiment was negative. In order to confirm that the experimental process will not produce false positive results, the researchers also used known sterile soil samples, such as rock samples from the moon and soil samples from the volcanic island of Satoshi near Iceland, and finally produced the expected negative results.
Researchers are looking forward with confidence to the experimental results of Viking I and Viking II on Mars. What they didn't expect, however, was that this experiment turned out to be the most controversial Mars experiment in history, which also made NASA keep silent about this experiment for the next 50 years.
From the most promising experiment to the most controversial experiment.
The main researcher of LR experiment is Dr Gilbert Levin. Guibert Levin has a rich life experience. During World War II, he served in the US Navy, worked as a public health engineer in the Maryland Department of Health, and founded his own biotechnology company, Biospherics Research Inc (now Aikido Pharmaceutical Company). From 65438 to 0963, he worked in John? Hopkins University School of Engineering received a doctorate in environmental engineering. His invention won many patents, especially the innovative method of detecting microorganisms attracted the attention of NASA, so the latter signed a series of contracts with him in the early 1970 s, inviting him to develop an experimental scheme for detecting extraterrestrial life suitable for space missions.
Gilbert Levin's colleague Patricia Ann Stella is also from John? Hopkins University. Strath is an assistant professor at Hopkins University, studying molecular biology and enzyme systems. Straat watched 1969 landing on the moon on TV, hoping to participate in NASA's grand space exploration program. At that time, the experiment designed by Levin had just been selected by NASA and became? Viking mission? In one of the biological experiments, after two like-minded scientists communicated, Straat was deeply attracted by the LR experiment and immediately joined the team.
She still can't forget watching the Viking land with the whole team at the JPL laboratory in California. Half the people present at that time thought that the lander might crash, but suddenly a loud voice came: We landed safely? Everyone is so happy. ? She recalled. Then the final result of LR experiment made them more excited.
In the paper published in Science on 1976, Gilbert Levin and Patricia Ann Straat reported all the results of life exploration experiments on Mars in detail. It is mentioned in the paper that after adding nutrients containing radioactive carbon 14 to the Martian sample, soil tests at two landing sites 4000 miles apart detected similar radioactive gas release. In order to eliminate the false positive results that may be produced by the strong ultraviolet radiation of Mars, the lander also specially selected soil samples buried under rocks, and also obtained positive results. Moving in the soil? The reaction was stable at 18℃, and the activity was weakened when it was heated to 50℃ after 3 hours, while the sterile Martian soil samples at 160℃ showed negative results as the control group.
However, apart from LR experiment, other experiments in the same period, such as gas mass spectrometry experiment, did not find traces of organic chemistry near the landing site, which led to controversy. The final official report of NASA said:? There is no clear evidence that there are living microorganisms in the soil near the landing site. ? In addition, scientists in NASA's own laboratory can't reproduce Levin's LR experimental results.
In addition, according to NASA's explanation, Mars has the function of self-sterilization. Biologists involved in the mission believe that the combination of solar ultraviolet radiation, extremely dry soil and the oxidation characteristics of soil chemistry will prevent the formation of life in Martian soil. So the conclusion that there is life on Mars is still controversial.
According to NASA, the simulated life activities found in LR experiments may be unknown chemical reactions. Levin and Straat also thought that the results of LR might not be clear at that time, so they said at the end of the report:? At present, there is no chemical experiment that can quantitatively repeat LR Mars data. Although there are (abiotic) hypothesis theories, the most reasonable possibility is that we have observed life activities on Mars. ?
In the next few decades, with NASA launching more Mars exploration missions and using more advanced technical means, scientists' understanding of this planet became clearer, including liquid water and methane. Especially in 20 14, Curiosity not only detected high concentration of methane in the atmosphere, but also confirmed for the first time that the Martian rock powder contains organic molecules such as organic carbon, which can be said to be one of the most important discoveries of NASA's Mars exploration in the past half century. At the same time, this result also allows Levin to adhere to the LR experimental results of that year.
In fact, since 1997, Levin has brought up the past again, and frequently voiced his support for the conclusion of microbial activities in LR experiment. On 20 16, Levin and Straat jointly published an opinion article on astrobiology again. According to the new discoveries on Mars in recent years and the viewpoint that inorganic substances imitate metabolic activities, the LR experiment in the 1970s was discussed again. People think that abiotic substances can't fully explain the results of LR experiment, but the best explanation is Martian microorganisms.
The Viking- 1 injection experiment showed that after the first injection, the soil samples showed strong radioactivity * red (,indicating that there was metabolic activity inside, while the soil samples in the control group did not respond (blue). Source: Levin and Straat, 1977, Biosystems. elsevier
Levin said in an interview with the media:? Since I first firmly put forward that life was detected by LR experiment in 1997, most review journals refused to contribute our articles. Straat and I present papers at the international conference of the Society of Optical Engineering (SPIE, including astronomy) every year, but most of them are ignored by astrobiologists. ? He can only put these articles on his personal website.
As a staunch supporter, Straat was interviewed by Scientific American on 20 19. She said that she didn't completely agree with Levin at first, but as more and more evidence was discovered by NASA on Mars, she began to think that they did discover microbial life that year. ? It is true that some complex organic molecules have been found on Mars, but no simple organic molecules such as alanine and glycine have been found? , she preached. She is looking forward to future Mars exploration experiments, hoping to finally confirm or falsify their experimental findings.
In this regard, Lu Se Biegel, a planetary scientist at the JPL Laboratory of NASA, believes that the truth is actually very simple: the Viking program did not find life on Mars. ? They did a lot of experiments and got many results that they couldn't understand. ? In view of NASA's limited understanding of Martian soil and atmosphere at that time, he said that we should start with geological and chemical experiments instead of jumping to the biological metabolic activity detection experiment. The results of LR experiment are actually ambiguous.
Amazing words: Did it grow on Mars? Mushrooms? ?
In the process of Mars exploration, official institutions and scientists have always been the mainstay, but NASA will publish their photos, which gives other researchers the opportunity to study.
1On October 27th, NASA posted a color photo taken by the Mars probe Opportunity during its mission near Yingkeng in 2004 on its official website. The photo shows that a round object appears on the surface of Mars, giving off a gray-blue luster. What's the official nickname? Mars blueberry? In fact, this is a kind of mineral stone rich in hematite, which proves that there was an ancient water environment in this area of Mars.