Venus is the brightest natural object in the night sky except for the moon. Like Mercury, it is within the orbit of the Earth. Among the eight planets in the solar system, Venus is the second closest to the Sun, so it can shine brightly for a period of time before the Sun rises or after it sets. Venus was called "Taibai" in ancient China. When it appears in the eastern sky in the early morning, it is called "Qiming"; and when it is in the western side of the sky at dusk, it is called "Changgeng".

Morning Star Rising

Fantasy Paradise

From the names given to Venus by the ancient Chinese, we can see that people have beautiful imaginations about this bright yellowish planet. In the West, the ancient Romans called Venus "Venus", the goddess in Western mythology. Whether in the East or the West, Venus is shrouded in a layer of romanticism. In his book "Lucky Star and the Sea of ​​Venus", the American science fiction master Asimov imagined a sea covering the entire Venus, as well as rich aquatic plants and animals.

Asimov's "Lucky Star and the Sea of ​​Venus"

This beautiful fantasy has existed in the astronomical community for quite a long time. Many astronomers have placed high hopes on the existence of life on Venus. However, people's fantasies about Venus are not like those about Mercury, where people are misled by various dramatic coincidences and draw wrong conclusions, because Venus is, to a certain extent, a true "sister planet" of the Earth.

The radius of Venus is about 6052 kilometers, and the mass is about 4.9×1024 kilograms, both of which are relatively close to the Earth. In addition, the radius of Venus's orbit is also relatively close to that of the Earth, which means that the material composition of the two is also likely to be similar, because the material composition of a planet is related to its position on the planetary disk, and planets in similar positions also have similar material compositions. It is precisely these similarities between Venus and the Earth that once made astronomers believe that Venus and the Earth have similar surface environments.

Size comparison of Venus and Earth

In fact, after the heliocentric theory was proposed, the status of the Earth suddenly dropped from the center of the universe to an ordinary planet orbiting the sun. This made many people feel that the existence of life on Earth was also a common phenomenon. With the gradual deepening of observations of planets and understanding of life, we have ruled out the possibility of life on most planets. However, the fantasy about Venus persisted until the middle of the 20th century.

The thick atmosphere of Venus protects this fantasy, which covers the mysterious face of the "goddess" like a veil. It was first noticed in 1761, when a famous transit of Venus occurred. Russian scientist Lomonosov observed the transit of Venus and found that the light near the intersection was abnormal at the beginning and end of the transit (when the edges of Venus and the sun seemed to intersect). He believed that this was caused by the refraction of sunlight by the atmosphere of Venus. The atmosphere of Venus prevents people from observing its surface in detail, and it can explain one of the reasons why Venus is bright - the planet relies on reflecting sunlight for us to see it, and the thick atmosphere makes the reflectivity of sunlight higher, so it appears brighter. At the same time, people imagined that due to the reflection of the atmosphere, Venus might not be abnormally hot because it is closer to the sun than the earth - this added another piece of evidence to the fantasy of heaven.

James Cook and Charles Greene's 1769 transit of Venus

Another interesting proof of these fantasies is related to the rotation period of Venus. We usually measure the rotation period of a planet by observing a landmark on the planet. However, the dense atmosphere of Venus prevents us from finding a landmark on Venus, and the atmosphere of Venus itself does not have obvious signs for us to observe, which makes it difficult for us to measure the rotation period of Venus. But is our imagination so easy to give up? Some people have "measured" the rotation period of Venus based on the imaginary landforms of Venus, and a considerable number of the results are concentrated around 24 hours (yes, this familiar number!). This makes people more imaginative.

The fantasy about Venus has lasted for such a long time, and people were once moved and excited by it. Unfortunately, with further exploration, all these fantasies have been shattered.

Hellish reality

In 1962, astronomers measured the rotation period of Venus in the same way as they did with radar waves. The results showed that all the previous "measurements" of Venus' rotation period were wrong. Not only were the values ​​wrong, but the directions were wrong. Venus is the only planet in the solar system that rotates in the opposite direction to its revolution. Its rotation period is about 243 days, which is also the longest rotation period among the planets in the solar system. Combined with its revolution period of about 225 days, we can deduce that Venus' "average solar day" is about 117 days. With such a long day and night cycle, if the atmospheric insulation effect is not good, Venus will not be able to escape the fate of "ice and fire" like Mercury, not to mention the stable existence of liquid water!

During the US-Soviet space race, the two countries ambitiously launched a large number of explorations of celestial bodies in the solar system - launching probes was naturally the most important means. Venus has so many wonderful fantasies, so it is naturally worth a lot of money and has become one of the important targets of exploration. The Soviet Union once had a considerable advantage in the exploration of Venus, and several probes successfully made soft landings, sending back a lot of first-hand information for us. It's a pity that the environment on Venus is so harsh that we have only conducted specific research on a small part of it so far. The Soviet Union was also pitted by such "low cost performance". In contrast, the United States, which placed its bets on Mars, gained more.

The detection results of dozens of detectors show that the temperature on the surface of Venus is as high as about 460 degrees Celsius. This should be attributed to an "old friend" we are familiar with - carbon dioxide. The culprit of the greenhouse effect that worries us every day is showing its strength on Venus! 96.5% of Venus' atmosphere is carbon dioxide. In contrast, the proportion of carbon dioxide in the Earth's atmosphere is only 0.04%. If it were not for the greenhouse effect, Venus would probably not be too hot because it is closer to the sun than the Earth - because its atmosphere has a high reflectivity to sunlight - and it would even be cooler than the Earth. Carbon dioxide "warms" Venus, which is one of the reasons why Venus' atmosphere is so dense.

Venus's dense atmosphere

Moreover, Venus is not "ice and fire" as mentioned before, because there is no place or time that is slightly "cool"! Whether it is day or night, whether it is the equator or the poles, it is all furnaces! The angle between the rotation axis and the revolution axis of Venus is less than 3°, which means that the change of seasons is difficult to detect on Venus. The most important thing is still the atmosphere. The atmospheric pressure on the surface of Venus is about 92 Earth atmospheres, and the atmospheric density is as high as 67 kg/m3 (about 55 times the density of air on the surface of the earth). Although the wind speed on Venus is very slow, about 2 meters/second, which can only be regarded as a breeze, but such a dense atmosphere is still enough to fly sand and rocks. More importantly, its heat transfer ability is unique, almost turning the entire surface of Venus into an isothermal hell!

As for the scenery on Venus, it gives people an oppressive feeling like hell. There are clouds on Venus, and there may be rain and even snow, but don't think of any poetic scenery! The clouds on Venus are yellow, and the main component is sulfuric acid. The rain under the sulfuric acid clouds is naturally sulfuric acid rain, and these raindrops will evaporate as early as the falling process; snow may be condensed from minerals such as sulfur metal compounds. However, these are only assumptions and have not been confirmed. In addition, the probe has found evidence of lightning activity, and it may be very frequent. The sky on Venus is yellow and red, and the sun cannot be seen with the naked eye; at night, the sky will also glow with a faint red light, and the moon cannot be seen-Mercury and Venus are the only two planets in the solar system without natural satellites.

Venus surface data sent back by Venus 13 and 14 probes

Suffocating Venus

I have to say that everything on Venus gives people a sense of hell.

Unremitting pursuit

Searching for extraterrestrial life is a long-cherished wish of mankind. Although the surface environment of Venus is very harsh, the hope of life's existence is not necessarily in vain. The emergence of life is complementary to the evolution of the environment. Survival of the fittest. The evolution of Earth's creatures makes its existence more adaptable to the Earth's environment, while also transforming the Earth's environment into a better home. In this case, the life forms nurtured by different environments may be very different. Undersea hot springs, craters, or ice and snow are hellish for humans, but lush biomes can also develop near them. The existence of extraterrestrial life is more likely to be far beyond people's imagination.

The Three-Body Problem describes how the Trisolarans evolved the ability to "dehydrate" to adapt to the unpredictable "era of chaos". It is also possible that unique life forms were born on Venus. We are not Venusians, so how do we know that Venus is not their paradise?

With this hope, scientists have never stopped looking for extraterrestrial life. Unfortunately, current technology does not allow us to travel to and from other planets at will and explore freely. We can only use chemical substances to track possible life signals. So if there is a substance that is difficult to produce naturally, biological activity is its only known source, and it is not easy to be confused with other substances during detection, we can stay at home and use spectroscopy to detect the life situation of other celestial bodies. Phosphine (PH3) is such a substance. On Earth, it always appears in places related to human or microbial activities. At the same time, it is easily oxidized into other substances-only when life activities continuously produce it can phosphine exist stably in considerable amounts.

Phosphine (PH3) in the atmosphere of Venus

Jane Greaves, a professor at Cardiff University in the UK, and his colleagues discovered phosphine in the clouds of Venus. Although the surface of Venus is a "hell on earth", its clouds have a mild environment. Moreover, the clouds of Venus are acidic, which is exactly the condition required for biological activities that produce phosphine. Preliminary models of biochemistry also show that the atmosphere of Venus is suitable for such biological activities. Greaves' team estimated the phosphine content in the clouds of Venus and analyzed non-life activity mechanisms such as gas reactions and photochemical reactions, which could not explain the relatively high phosphine content in the atmosphere of Venus. This may mean the existence of life on Venus. Observations also show that phosphine only exists on Venus below a latitude of about 60°, which is roughly consistent with the mid-latitude Hadley circulation cell, which is a habitable zone for life previously envisioned by scientists.

Schematic diagram of the existence of phosphine in the Hadley cell

In addition to phosphine, there are other possible evidences for the existence of life. Just as rainforests on Earth can make the Earth look green from space, if there is life on Venus, they may also change the color of Venus. Japan's Akatsuki Venus probe is monitoring changes in the transparency of Venus' atmosphere to ultraviolet rays - this may be the "coloring" of Venus' atmosphere by biological particles. On the other hand, computer simulations of Venus' geology and climate change indicate that about 700 million years ago, liquid water may have existed on the surface of Venus, providing a habitable environment for life. It is very likely that life on Venus had already appeared at that time, and as the surface environment deteriorated, they gradually "migrated" to the atmosphere. Of course, we don't know all this now.

Although there are many signs pointing to the existence of life on Venus, they are all too indirect. Scientists need to put these fragments together like suspenseful reasoning, eliminate all impossibilities, and piece together the complete truth.