Two automatic probes were launched to what is now a slightly more familiar planet than what was first believed, to search for living organisms on a planet other than the Earth. These are undoubtedly two of the most complex objects that ever descended on another planet: Viking 1 and Viking 2, which left Cape Canaveral on August 20 and September 9, 1975. Both consisted of two parts: the first, an "orbiter", was designed to enter and remain in orbit around Mars; the second, a "lander", was designed to land, take photographs and perform tests on the soil. After a voyage of ten months, Viking 1 entered orbit around Mars on June 19, 1976 while Viking 2 arrived on August 7. The first task of the television cameras in the orbiting modules was to check if the landing sites chosen for the landers were flat and safe enough. Such caution paid off because both sites were found to be absolutely unsuitable for a safe landing. Thus, new sites had to be found and finally, on July 20 and September 3, 1976, the two landing modules descended to the surface of the planet: the first at 22 degrees, the second at 48 degrees north of the equator. The landers were very sophisticated machines. Their primary purpose was to provide a general overview of the existing surface conditions of the planet and to analyze its soil. Both modules were equipped with a self-moving arm to penetrate into the ground, collect a sample and place it in a small on-board laboratory which could perform three different types of chemical and biological tests - and all fully automated. The results of these tests, which were gradually sent back to Earth, are still being tenaciously debated even today. In fact, if one of the experiments provided completely negative results regarding the presence of some form of life in the Martian soil, the other two instead provided ambiguous indications which may give rise to very different interpretations. The most interesting of these tests was the so-called labelled release experiment: the soil had to be mixed with a certain amount of nutrient substances, such as sugars, containing radioactive carbon. If there were living organisms in the soil that were somewhat similar to our own, they would have been nourished by these sugars and would then have released carbon dioxide, as the final product of their metabolism, which would contain the radioactive carbon. A special instrument was used to measure the level of radioactivity before, during and after the experiment. The initial results made the Nasa technicians jump off their chairs: after several hours, the level of radioactivity began to rise from a background rating of 400 up to a very high level of 10 000. Sterilization was used to prove that living organisms were involved and not just a strange chemical reaction: without any other living organism the reaction should disappear. The sample was sterilized and in fact the reaction disappeared. However, there still remained the doubt that this involved an anomalous (and not a biological) chemical reaction, which was blocked by the sterilization procedures. So, the next sample was sterilized at a temperature of 120 degrees instead of 300 like with the previous sample. The idea was that, no matter what chemical reaction may have produced the result, it would still occur. If instead living organisms were involved, the reaction would have disappeared or would have been slowed down to a great extent. And this is where the results became truly mysterious. The chemical reaction did clearly slow down, making the chemical hypothesis rather unlikely. But it began to behave in a very strange way: the radioactive carbon was emitted and reabsorbed by the soil in a daily cycle. Just like what happens with plants, which absorb carbon during the day and emit it during the night. Based on these results, which could have made scientists think of the presence in the ground of some microscopic organisms, attention shifted to the chemical analysis of the soil: if these organisms truly existed, then there should be some organic molecules, of which we know how to create the base structure of microorganisms on the Earth. But this was not the case. The experiment produced completely negative results. And so still today we do not know for certain if something is alive or not in the Martian soil.
Two automatic probes were launched to what is now a slightly more familiar planet than what was first believed, to search for living organisms on a planet other than the Earth. These are undoubtedly two of the most complex objects that ever descended on another planet: Viking 1 and Viking 2, which left Cape Canaveral on August 20 and September 9, 1975. Both consisted of two parts: the first, an "orbiter", was designed to enter and remain in orbit around Mars; the second, a "lander", was designed to land, take photographs and perform tests on the soil. After a voyage of ten months, Viking 1 entered orbit around Mars on June 19, 1976 while Viking 2 arrived on August 7. The first task of the television cameras in the orbiting modules was to check if the landing sites chosen for the landers were flat and safe enough. Such caution paid off because both sites were found to be absolutely unsuitable for a safe landing. Thus, new sites had to be found and finally, on July 20 and September 3, 1976, the two landing modules descended to the surface of the planet: the first at 22 degrees, the second at 48 degrees north of the equator. The landers were very sophisticated machines. Their primary purpose was to provide a general overview of the existing surface conditions of the planet and to analyze its soil. Both modules were equipped with a self-moving arm to penetrate into the ground, collect a sample and place it in a small on-board laboratory which could perform three different types of chemical and biological tests - and all fully automated. The results of these tests, which were gradually sent back to Earth, are still being tenaciously debated even today. In fact, if one of the experiments provided completely negative results regarding the presence of some form of life in the Martian soil, the other two instead provided ambiguous indications which may give rise to very different interpretations. The most interesting of these tests was the so-called labelled release experiment: the soil had to be mixed with a certain amount of nutrient substances, such as sugars, containing radioactive carbon. If there were living organisms in the soil that were somewhat similar to our own, they would have been nourished by these sugars and would then have released carbon dioxide, as the final product of their metabolism, which would contain the radioactive carbon. A special instrument was used to measure the level of radioactivity before, during and after the experiment. The initial results made the Nasa technicians jump off their chairs: after several hours, the level of radioactivity began to rise from a background rating of 400 up to a very high level of 10 000. Sterilization was used to prove that living organisms were involved and not just a strange chemical reaction: without any other living organism the reaction should disappear. The sample was sterilized and in fact the reaction disappeared. However, there still remained the doubt that this involved an anomalous (and not a biological) chemical reaction, which was blocked by the sterilization procedures. So, the next sample was sterilized at a temperature of 120 degrees instead of 300 like with the previous sample. The idea was that, no matter what chemical reaction may have produced the result, it would still occur. If instead living organisms were involved, the reaction would have disappeared or would have been slowed down to a great extent. And this is where the results became truly mysterious. The chemical reaction did clearly slow down, making the chemical hypothesis rather unlikely. But it began to behave in a very strange way: the radioactive carbon was emitted and reabsorbed by the soil in a daily cycle. Just like what happens with plants, which absorb carbon during the day and emit it during the night. Based on these result
