Venus is an Earth-like planet in the Solar System with an average distance of about 0.72 AU (108 million kilometers) from the Sun and an orbital period of 224.71 days. Venus rotates from east to west and has the longest rotation period (243 days) of the eight planets. Among the eight planets in the solar system, Venus is known as the "twin star" of the Earth, and due to the close size and density of the Earth, it is speculated that the initial material composition of Venus is also similar to that of the Earth, but based on the only detection data, it is not possible to draw conclusions.
The size of Venus versus Earth
Now, The surface environment of Venus is very different from that of Earth: the atmospheric pressure is 92 times that of the Earth's surface; the ground temperature is about 465 ° C, and the world is more uniform; the car will melt on the surface of Venus, which is very unsuitable for life. Venus has a thick atmosphere, consisting mainly of carbon dioxide (96.5%) and nitrogen (3.5%). A variety of corrosive acid gases are present in the clouds, which are about 50 kilometers high.
However, 3.6 billion years ago, Venus was also in the habitable zone of the solar system, did Venus ever exist in the ocean and give birth to life? What caused Venus to embark on a very different evolutionary path from Earth and become such a purgatory as it is now? Is Venus Earth's past or future? These questions are the most critical scientific questions in the exploration of Venus.
The surface of Venus is wrapped in a dense atmosphere, and it is difficult to see the true appearance of the earth from the orbit
Probe Venus
Human access to extraterrestrial planets began with Venus. From the beginning of the First Soviet Exploration of Venus in 1961 to the end of 2021, countries around the world have launched a total of 43 Venus probes, including 33 (15 successful), 8 (successful 6) by the United States, 1 (successful) by the European Union, and 1 (successful) by Japan. The mission types are dominated by Venus flyby, orbit and atmospheric entry missions (33), with relatively few landing missions (8) and floating missions (2). Missions focused on the U.S.-Soviet space race of the 1960s and 1970s, with only two missions since the 21st century, the ESA Venus Express (2006-2014) and The Japanese Akatsuki (orbited so far in 2015). Dawn is currently the only Venus probe in orbit.
There are two main types of exploration of Venus: ground-based observations and Venus probes. Ground-based observations are an important complement to the probe's mission, observing atmospheric composition, atmospheric dynamics, and Venus geology. The Venus probe is divided into three types: orbital exploration, aerial in-situ exploration and ground-based in-situ exploration missions. Orbital exploration is relatively the most technologically mature, and orbiters account for the absolute majority of existing Venus missions. Aerial in-place detection is carried out through aerial platforms (e.g. floating air balloons) and descending probes/sondesons. Ground-based in-situ detection on the surface of Venus can obtain information such as the composition of The surface material of Venus, the interaction between the atmosphere and the surface, and earthquakes. But the harsh environment on Venus's surface is a major constraint on the probe's survival time. Based on the survival time, the ground-based in-situ detector is also divided into short-life workstations (24 hours), movable Venus vehicles (> 24 hours) and other types.
Dawn Venus probe
The advantage of the Venus orbiter is that it can observe the atmosphere and space environment of Venus for a long time and at large scales. Since the 21st century, thanks to the success of ESA's Venus Express and Japan's Dawn, people have obtained a large amount of valuable Venus remote sensing data and gained a deeper understanding of The atmosphere and climate of Venus.
The Venus Express is mainly focused on clouds and the middle and upper atmosphere. It adopted a different orbital design from the Pioneer-Venus Orbiter, allowing it to enter different regions of the Venus-induced magnetosphere and obtain many new observations and discoveries. Some of the highlights of the Venus Express include: the construction of a model of the global atmospheric circulation of Venus; the mapping of the global surface temperature of Venus, the thermal profile and thermal structure of the atmosphere of Venus, the profile of the chemical composition of the atmosphere (CO, SO2, OCS, D/H ratio, etc.), the discovery of new atmospheric components (O3 and OH); the discovery of the continuous increase in average wind speed in the low latitude regions of Venus; the presence of cryogenic layers in the upper atmosphere; the characteristics of atmospheric gravitational waves caused by the terrain and the characteristics of deep clouds. The Venus Express also found some suspected volcanic "hot spots" that were active. Overall, however, the Venus Express's detection range and accuracy of the earth's surface are limited.
"Venus Express" Venus probe
The detection target of Japan's Dawn is concentrated in the atmosphere and space environment of Venus, and the cloud top to the deep cloud has been carefully probed. Dawn depicted the morphology of 35-50 km deep clouds; found the presence of large-scale bow-shaped features in the atmosphere of Venus; the presence of horizontal jet currents in the low and middle clouds above the equatorial region; the radio occultation experiment obtained atmospheric temperature distribution above 40 km; the discovery of small particles of thick stratus near the transition zone between the upper and middle clouds; and the observation and simulation of the cloud top boom found a mechanism for maintaining the super rotation of the atmosphere.
Schematic diagram of the vertical structure of the atmosphere of Venus
Under the atmosphere of Venus
Although the surface of Venus is surrounded by a thick atmosphere, electromagnetic waves of certain wavelengths can still penetrate the atmosphere and make observations of the surface of Venus.
Schematic diagram of electromagnetic wave bands that can penetrate planetary clouds
To date, the most comprehensive and highly accurate topographic map of Venus is derived from the radar detection of the Magellan orbiter in the United States.
Topographic features of Venus
The surface of Venus is relatively flat , and the landform can be divided into three categories of geological units : lowlands , plains , and highlands. About 80% of The surface of Venus is covered by smooth volcanic plains, of which 70% have folded ridges and 10% of them are either smooth or fractured. Two highlands occupy the remaining 20% of Venus' surface, one in the Northern Hemisphere (Ishtar Terra) and the other south of the equator (Aphrodite Terra). Maxwell Montes, the highest mountain range on Venus (whose highest peak is more than 11 km above the average radius of Venus) is located within the Ishtar Terra range.
Typical morphological features of Venus's surface include radial rock walls, shield volcanoes, lava flows, and spider-reticulated formations
Only about 940 impact craters remain on the surface of Venus. Interestingly, there are very few impacts of Venus craters smaller than 30 km and near missing ones of less than 5 km. At the same time, the surface of Venus lacks large impact craters. Based on existing impact craters, it can be inferred that Venus underwent a global surface reshaping event around 750 million years ago. The whole process took about 100 million years. The reshaping event erased an earlier geological record. Whether this global remodeling event will be cataclysmic or slowly balanced is unclear.
Distribution map of impact craters on the surface of Venus
(Background: pink - loose sediment; brown - sedimentary or weathered rock; green - volcanic rock; blue - low dielectric constant material)
In addition , there are a variety of tectonic features on the surface of Venus. Active volcanic activity creates multi-scale tectonic deformations that eventually form a global tectonic network associated with volcanoes. The most special of these is a type of geological unit called a mosaic mass (tessera). They are isolated blocks of tens of kilometers surrounded by plains, shaped like mosaic floors. Parallel ridges, fractures, and graben crossings in different directions within the mosaic plot, accompanied by a small amount of volcanic activity. Mosaic plots are the oldest possible bodies of land on the current surface of Venus and may also be related to the action of water. In the future, mosaic plots will be an important target for Venus exploration.
Rift valleys resemble mid-Ocean ridges; ridges are mainly distributed in the lowlands; mosaic plots span highly deformed regions and may resemble the Earth's continental crust in composition
Compared with remote sensing, landing detection is more difficult, and the data is less and more precious. The record for the duration of the Venus landing is still maintained by the Soviet Union's Venera series of probes, with a maximum record of 127 minutes.
Artistic hypothetical image of the Soviet "Venus" lander landing on the surface of Venus (lightning and sulfuric acid rain)
All the landers landed on the volcanic plains of Venus. From the photos transmitted back, it can be seen that there is no liquid water or vegetation on the surface of Venus, only scattered rocks. The lander measured the composition of matter on the surface, which was not only extremely limited and error-prone, but even lacked data on certain key elements (such as sodium), but these measurements remained the main basis for the composition of Venus' material, especially in the absence of Venus meteorites or the return of samples.
Surface image of the Landing Site of Venus taken by the Soviet 9 and 13 Missions
Is there "life" there?
Due to the potential similarity with Earth, the presence of living matter on the surface or in the atmosphere of Venus is a long-standing concern of the international academic community. There are two related hypotheses. One hypothesis is that the surface of early Venus had a mild climate and liquid oceans until the greenhouse effect spiraled out of control and all the water evaporated into the atmosphere and escaped. There is no evidence to support this hypothesis; models suggest that Venus may never have had a liquid ocean. One hypothesis is that there is a habitable zone in the clouds of modern Venus, where there are suitable temperature and pressure conditions (~60 ° C, 1 atmosphere), and micron-sized aerosols have a shielding effect on cosmic rays or ultraviolet rays to protect the existence of life.
The hypothesis of a habitable environment on the surface of Venus
(Left) Schematic diagram of the hypothesis that Venus could be a habitable planet before the greenhouse effect spirals out of control.
Schematic diagram of the thermophilic-superacphilic microbial cycle hypothesis that survives between the haze layers of Venus clouds.
In September 2020, a research team published a paper in the journal Nature Astronomy announcing that the use of ground-based radio telescopes to detect phosphine (PH3) at a certain height in the Clouds of Venus may be indirect evidence of the existence of life, causing a huge sensation and controversy. The focus is on the polysolutability of the observational data, and even if the signal is actually derived from phosphine, other non-inanimate sources cannot be ruled out. But in any case, these new explorations and controversies mark that Venus has become a new hotspot for the exploration and research of international planetary and space life, and has become an important field of international scientific and technological competition.
Although Venus exploration has gone through many years, it is still in the stage of accumulating key data, and there are many observation gaps that need to be filled urgently. For example, 99% of the mass of Venus's atmosphere is concentrated in the troposphere, especially below 28 km, but the deep atmosphere of Venus from the surface to 12 km altitude currently lacks direct detection data. Radar-based detection of the topography of Venus has been in a stagnant state after the Magellan mission, and the existing Radar detection resolution of Venus is in the order of 100 meters, and the accuracy is only equivalent to the Mars mission in the 1970s, which cannot achieve more subtle identification and classification of Venus's landforms, especially the analysis and study of the geological process scale on the surface of Venus, which seriously restricts the understanding of the key areas of the surface of Venus and the geological evolution of Venus. The in situ detection of the atmosphere of Venus (especially the isotope measurement of atmospheric elements) and the fine remote sensing detection of mosaic plots and even in situ detection have put forward clear and urgent needs for future Venus exploration missions.
The achievable spatial resolution of radar detection for future Venus missions is compared to Magellan
Go, go to Venus
In June 2021, NASA and ESA approved new missions to Venus, the VERITAS mission, the DAVINCI+ mission, and the EnVison mission, respectively. In addition, Russia and India have also proposed and actively promoted their respective Venus exploration missions. International Venus exploration and research is about to usher in a new round of enthusiasm, and the mystery of the Earth twin will be gradually unveiled.
The Truth mission and the Da Vinci Plus are two highly complementary missions that are scheduled to launch around 2030. The full name of "Truth" is the mission "Venus emissivity, radio science, interferometric synthetic aperture radar, topography and spectroscopy". The main scientific goal is to generate high-resolution global topographic maps and images of Venus, resulting in a series of global atlases of Venus, including deformation, surface composition, thermal emission, and gravity field maps. Attempts have been made to detect whether Venus has an ancient water environment and whether current volcanic activity is limited to the mantle pillar region or has a wider distribution. The full name of the "Da Vinci+" mission is the "Rare Gases, Chemistry and Imaging of The Deep Atmosphere of Venus" mission, through the descending detector, in the process of 63 minutes of descent, directly measure the composition of the atmosphere of Venus, measure the composition of rare gases, trace gases and their isotopes, and measure the temperature, pressure and wind speed of the atmosphere of Venus. Before reaching the surface, the probe will also take images of Venus's mosaic massifs to explore its origins and tectonic, volcanic and weathering history.
Scheduled to launch in 2032, EnVison is an orbit-based mission for high-resolution radar mapping and atmospheric research on the surface of Venus. The scientific objectives are to find active geological processes, measure surface temperature changes related to active volcanic action, characterize regional and local geological features, determine crustal support mechanisms and constrain mantle and core characteristics, orbit-based high-resolution radar mapping and atmospheric research missions on the surface of Venus, be able to detect centimeter-level surface changes, characterize volcanic and tectonic activity, and estimate the rate of weathering and surface alteration. The underground radar sounder will map faults, strata and weathering within about 100 m of the subsurface of the area, identifying structural relationships and geological history.
The concept of russia's "Venera-D" mission is in the preparatory stages, marking Russia's determination to return to Venus. The concept of the Venus D mission has been modified several times, and the current baseline mission consists of an orbiter and a short-lived (2-3 hours) Vega lander. In addition to the baseline mission, there are a number of potential elements under demonstration, such as balloons, sub-satellites, long-lived (24-hour) ground stations, etc. It is also proposed that it will be returned from Three Samples from Venus in 2029-2034.
In addition, other interplanetary missions can also detect Venus during the gravitational assist phase of Venus. These missions include NASA's Parker Solar Probe, ESA's Solar Orbiter, and Jupiter's Ice Moon Probe (JUICE).
Q&A session
The report mentions that the Venus lander has only survived for about two hours at most. The temperature and pressure on the surface of Venus do not appear to be very extreme values. Is the main reason for the short survival time of the lander being temperature and pressure? Are there any other reasons?
A: It is mainly the reason for the temperature and pressure on the surface of Venus. The environment on the surface of Venus is a bit like a pressure cooker. Electronic devices and scientific instruments must first cool down when working to ensure that they are not too hot, but it is very difficult in the Venus environment. Some long-working instruments during mars exploration usually choose to work at night without the need for additional cooling methods. In contrast, Venus's persistent high temperatures place higher demands on both electronic devices and circuit designs.
Venus is about the same size as Earth, why does Earth have a moon but Venus doesn't?
A: That's a good question, but we don't know it yet. The exact cause of the Moon is still unknown, but it is thought to be more acceptable that the origin of the Great Collision was that in the early days of the formation of the primordial Earth, it was hit by a celestial body as large as Mars, and then re-evolved to form the current Earth and Moon. Venus may not have undergone such a process, which may also have a great influence on the evolution of Venus. In contrast to Earth, Venus may be a negative example of whether no large natural satellite would go down another evolutionary path.
When did the surface of Venus get so hot? Why did Venus have no oceans in the beginning?
A: This question is also something we don't know, and it's all speculation at the moment. We speculate that Venus may have resembled Earth when it first formed (the first 100 million years). But whether this is actually the case or not, it is not clear. From the present surface of Venus we see no trace of the ocean. But it's also possible that the global magma cover of about 700 million years erased some traces of the early oceans. This is also the problem that future Venus exploration missions want to solve.
Is the mainland preparing a Plan for the Venus Exploration?
A: We are now doing some preliminary research, which is to help future exploration plans, if China wants to explore Venus, what should it probe and what preparations should be made. Mainly scientific scenarios. But a specific Venus mission has not yet been undertaken.
Is there a tendency, that is, a lander or a floating platform?
A: With today's technology, landing and floating platforms will be more difficult. Keeping the lander and the scientific instruments it carries alive and working, the design requirements for the lander and the scientific payload are very high. If it is an aerial platform, there are also many requirements for the aerostat or the aircraft itself. Therefore, in the next decade, if China wants to detect Venus, it may still consider orbiters, but it can consider carrying some distinctive detection instruments to do detection that other countries have not done or done.
Author: Zhao Yuyi
Editor: En Lijuan Huai Chen
Copyright: International Institute of Space Sciences - Beijing (ISSI-BJ)
Biography of the Rapporteur
Dr. Zhao Yuyan, Associate Professor, Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Center for Comparative Planetology Excellence, Chinese Academy of Sciences, and member of the pre-scientific research team for China's first Mars exploration mission. He has long been engaged in the study of the evolution of planetary geological environment. In recent years, he has paid attention to the detection of Venus and participated in the pre-research and demonstration of scientific problems in The detection of Venus.
[This article is compiled from the lecture series "Tianfang Night Lake", and the main speakers are outstanding female scientists in the field of space science.] The lecture is hosted by the International Institute of Space Science - Beijing (ISSI-BJ), and welcomes your attention. ]
This article is reprinted from the National Astronomical Corporation of China (ID: chineseastronomy) with permission, please contact the original author for secondary reprinting.