For many billions of years, life on Earth has been well disguised from outside observers, and we, in turn, might accidentally miss life on other planets for similar reasons: it is still too simple.
Researchers from the United States considered the question of at what point in the history of the Earth a hypothetical alien observer could detect life on it. According to their calculations, for almost 90 percent of the planet's history, it should have looked uninhabited to outside observers. The corresponding article is sent for publication in Astrobiology, and its text can be found on the preprint server of Cornell University.
Scientists wondered how the ratio of oxygen, ozone and methane in the atmosphere changed throughout the history of the planet. These gases are often called biomarkers, because now on Earth they are formed biogenically, that is, as a result of the activity of living organisms. According to this group of researchers, in the past of the Earth, they were also produced biogenically. It turned out that the ratio of oxygen, ozone and methane in the Earth's atmosphere varied according to very complex laws, difficult to detect by external astronomical observation. In particular, the production of oxygen by terrestrial life began hundreds of millions of years before the appearance of free oxygen in the atmosphere. For a very long time, all of it was spent on the oxidation of dead organic matter left by past generations of bacteria that had not yet produced oxygen.
The methane content in the atmosphere has changed dramatically throughout the history of the planet. Before large amounts of oxygen appeared, there might not be an ozone layer that protects methane from ultraviolet radiation. Therefore, the lifetime of the molecules of this gas at that time was very short. After the appearance of oxygen, it increased, as did the concentration of methane. However, as soon as the oxygen content reached a few percent, the methane concentration dropped again. Oxygen and water vapor contributed to the formation of • OH - hydroxyl radicals, which rapidly destroy methane molecules. As the authors note, from the outside, without landing on the planet or obtaining extremely accurate spectra, it is difficult to understand which of the above phases is the local atmosphere.
The authors come to the conclusion that up to 2.5 billion years ago, the detection of biogenic oxygen and ozone in the gas shell of the Earth from the outside was excluded, and 2.5–0.5 billion years ago, it was very difficult. Despite the appearance of oxygen, its concentration still remained low: it continued to oxidize the organic matter of past eras. But methane after the appearance of oxygen, that is, all the last 2.5 billion years, would often be unrealistic to detect at interstellar distances. However, even having discovered it before this event, it would be difficult to confidently declare the biogenic origin of this gas. The solar system, at least on Titan, contains methane and similar hydrocarbons. But it is believed that this gas is formed there in an inorganic way.
Separately, researchers focus on biospheres, which are located mainly in the oceans. They note that, before land development, the processes of binding biogenic methane and oxygen largely take place inside the hydrosphere and are not reflected in the atmosphere in any way. Oxygen is bound by organic waste in the water, and methane is consumed by living ocean microorganisms. This scenario reigned on Earth until the Cambrian period, that is, for billions of years. The authors classify such a biosphere as a camouflage one. Using telescopes that are technologically available to mankind today, it is extremely difficult to detect it even at a distance of a few tens of light years.
Many thousands of exoplanet candidates have been discovered in recent years through the use of space telescopes. Dozens of them already have data on the composition of the atmosphere. The next generation of space telescopes, such as the James Webb or TESS, will be operational in the coming years. They will be able to obtain data on the composition of the atmospheres of fairly small earth-like planets. Previously, it was believed that immediately after this, due to the detection of biomarker gases, it would be possible to confidently find out whether there is life in neighboring stellar systems.
Critics of this approach pointed to Mars, where methane appears locally every summer, but it has not yet been possible to understand whether it is biogenic or not. As they note, it is naive to believe that we will be able to understand whether there is life within light years from us, if we cannot say anything for sure about its existence only two or three light minutes from Earth. The new work shows that telescopes in the near future can only confidently detect highly advanced life, like the one that has been present on Earth for the past 500 million years. Other options, such as bacteria or protozoa, are still inaccessible to them. Judging by our planet, this may mean that only a small part of all existing biospheres in nearby stellar systems will be revealed in the foreseeable future.
Promotional video: