It was once Earth's "twin star", but went on to a completely different fate

Venus is an Earth-like planet in our solar system with an average distance from the Sun of about 0.72 AU (108 million km) and a rotation 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 called the "twin" of Earth, and because of its close size and density to Earth, it is assumed that the initial material composition of Venus is also similar to that of Earth, but based on the only detections available, no conclusions can be drawn yet.

It was once Earth's "twin star", but went on to a completely different fate

Size of Venus compared to Earth

Now, the surface environment of Venus is very different from that of Earth: the atmospheric pressure is 92 times higher than that of the Earth's surface; the surface temperature is about 465°C, and is more homogeneous globally; cars also melt on the surface of Venus, which is very unsuitable for the survival of life. Venus has a thick atmosphere, composed mainly of carbon dioxide (96.5%) and nitrogen (3.5%). A variety of corrosive acidic gases are present in the clouds at a height of about 50 km.

However, 3.6 billion years ago, Venus was also in the habitable zone of the solar system, did oceans exist and nurture life at that time? What caused Venus to follow a different evolutionary path than Earth and become the purgatory that it is now? Is Venus the past or the future of the Earth? These are the most critical scientific questions in Venus exploration.

It was once Earth's "twin star", but went on to a completely different fate

The surface of Venus is wrapped in a dense atmosphere, making the surface difficult to see from orbit

/ Exploring Venus

Human exploration of extraterrestrial planets began with Venus. Since the first Soviet Venus exploration in 1961 until the end of 2021, 43 Venus probes have been launched worldwide, including 33 by the Soviet Union (15 successful), 8 by the United States (6 successful), 1 by the European Union (successful), and 1 by Japan (successful). The mission types are mainly Venus flyby, orbit and atmosphere entry exploration missions (33), with relatively few landing missions (8) and floating missions (2). Missions were concentrated during the U.S.-Soviet space race in the 1960s and 1970s, and since the 21st century, only two missions have been conducted by ESA's Venus Express (Venus Express; 2006-2014) and Japan's Dawn (Akatsuki; successfully orbited in 2015 to date). Dawn is the only Venus probe currently operating in orbit.

The two main types of exploration of Venus are ground-based observations and Venus probes. Ground-based observations are an important complement to the rover missions, which can observe atmospheric components, atmospheric dynamics, and Venusian geology. Venus probes are divided into three types of missions: orbital probes, airborne in-situ probes, and ground-based in-situ probes. Orbital exploration is relatively the most mature technology, and orbiters account for the absolute majority of the existing Venus missions. Airborne in-situ surveys are carried out by airborne platforms (e.g., floating balloons) and descending probes/sounders. Ground-in-place surveys land on the surface of Venus to obtain information on the composition of Venusian surface materials, atmosphere-surface interactions, and earthquakes. However, the harsh environment of the Venus surface is a major constraint on the survival time of the probe. Based on the survival time, ground-in-place probes are also classified into short-lived workstations (< 24 hours), long-lived workstations (> 24 hours), and movable Venus rovers (> 24 hours).

It was once Earth's "twin star", but went on to a completely different fate

Dawn Venus Explorer

Since the 21st century, thanks to the success of ESA's Venus Express and Japan's Dawn, a large amount of valuable Venus remote sensing data has been obtained. In the 21st century, thanks to the success of ESA's Venus Express and Japan's Dawn, a large amount of valuable data from remote sensing of Venus has been obtained, and a deeper understanding of Venus' atmosphere and climate has been gained.

The Venus Express probe is focused on the clouds and the middle and upper atmosphere. It has adopted a different orbital design than Pioneer-Venus Orbiter, allowing it to access different regions of Venus' induced magnetosphere and to make many new observations and discoveries. Some of the highlight results of Venus Express include: the construction of a model of Venus' global atmospheric circulation; the mapping of Venus' global surface temperature, the thermal profile and thermal structure of Venus' atmosphere, the atmospheric chemical composition profile (CO, SO2, OCS, D/H ratio, etc.), and the discovery of new atmospheric components (O3 and OH); the discovery of a continuous increase in the mean wind speed in Venus' low-latitude regions; the presence of a cryosphere in the high-altitude atmosphere; the topography caused atmospheric gravity wave features and deep cloud features, etc. Venus Express has also identified a number of suspected active volcanic "hot spots". Overall, however, Venus Express has limited range and accuracy in detecting the surface.

It was once Earth's "twin star", but went on to a completely different fate

"Venus Express Venus Rover

The Japanese DAWN survey focused on the atmosphere and space environment of Venus, and conducted a detailed survey of cloud tops to deep clouds. DAWN has characterized the morphology of 35-50 km deep clouds, found large scale bow features in the Venusian atmosphere, the existence of horizontal rush winds in the low and middle cloud layers above the equatorial region, the temperature distribution of the atmosphere above 40 km altitude obtained by radio occultation experiment, the existence of small particles in the thick layer of clouds near the transition zone between the upper and middle cloud layers, and the observation and simulation of the heat wave at the cloud tops have revealed the maintenance mechanism of the heat wave on the atmospheric super rotation. The observations and simulations of cloud-top heat waves have revealed the mechanism of heat wave maintenance on atmospheric super rotation.

It was once Earth's "twin star", but went on to a completely different fate

Diagram of the vertical structure of Venus' atmosphere

/ Under the atmosphere of Venus

Although the surface of Venus is wrapped in a thick atmosphere, some electromagnetic waves of specific wavelengths can still penetrate the atmosphere and make observations of the surface of Venus.

It was once Earth's "twin star", but went on to a completely different fate

Schematic diagram of the electromagnetic waveband that can penetrate planetary clouds

The topographic map of Venus with the most complete coverage and highest accuracy to date originated from the radar survey of the U.S. Magellan orbiter.

Features of Venus surface topography

The surface of Venus is relatively flat, and the landscape can be divided into three types of geological units: lowlands, plains and highlands. About 80% of the surface of Venus is covered by smooth volcanic plains, 70% of which have folded ridges and 10% of which 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). The highest mountain on Venus, Maxwell Montes (the highest peak is more than 11 km above the mean radius of Venus), is located within the Ishtar Terra.

It was once Earth's "twin star", but went on to a completely different fate

Typical surface features of Venus, including radial walls, shield volcanoes, lava flows, and spider web structures

Only about 940 impact craters remain on the surface of Venus. Interestingly, there are few impact craters smaller than 30 km and nearly none smaller than 5 km. Also, there is a lack of large impact craters on the surface of Venus. Based on the existing impact craters, it can be inferred that Venus underwent a global surface remodeling event around 750 million years ago. The whole process took about 100 million years. The remodeling event erased the earlier geological record. It is not clear whether this global remodeling event was cataclysmic or slow equilibrium.

It was once Earth's "twin star", but went on to a completely different fate

Distribution of impact craters on the surface of Venus

(Background: pink - sediment sparseness; brown - sedimentary or weathered rocks; green - volcanic rocks; blue - low dielectric constant material)

In addition, there are diverse tectonic features on the surface of Venus. Active volcanism has created multi-scale tectonic deformation, culminating in a global tectonic network associated with volcanoes. The most unusual of these is a geologic unit known as a tessera (mosaic block). They are isolated blocks of several tens of kilometers surrounded by plains, shaped like mosaic floors. Within the tessera, parallel ridges, fractures, and grabens cross in different directions, accompanied by a small amount of volcanic activity. The mosaic is the oldest body that may be preserved on the surface of Venus at present, and may also be associated with the action of water. In the future, mosaics will be an important target for Venus exploration.

It was once Earth's "twin star", but went on to a completely different fate

Rift valleys are similar to Earth's mid-ocean ridges; wrinkle ridges are mainly in lowlands; mosaic massifs span areas of high deformation and may be similar in composition to Earth's continental crust

Compared with remote sensing exploration, landing exploration is more difficult and the data is less and more precious. The current record for the length of time a Venus landing has survived is still held by the Soviet Venera series of probes, with a maximum record of 127 minutes.

It was once Earth's "twin star", but went on to a completely different fate

Artistic imagery of the Soviet "Venus" lander landing on the surface of Venus (lightning and sulfuric acid rain)

All landers landed in the volcanic plain region of Venus. The photos returned show no liquid water or vegetation on the surface of Venus, only scattered outcrops of rock. The landers measured the composition of the surface material, which is extremely limited and inaccurate, and even lacks data for some key elements (e.g. sodium), but these measurements are still the main basis for the composition of the material on Venus, especially in the absence of Venusian meteorites or return samples.

It was once Earth's "twin star", but went on to a completely different fate

Surface images of the Venus landing site taken by the Soviet Venus 9 and 13 missions

/ Is there "life" there?

The presence of living material on the surface or in the atmosphere of Venus has been a long-standing concern of the international academic community because of the potential similarity to Earth. 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 gradually got out of control and all the water evaporated into the atmosphere and escaped. This hypothesis is not yet supported by evidence; some models suggest that Venus may never have had a liquid ocean. One hypothesis is that there is a habitable zone in the modern Venusian clouds, where there are suitable temperature and pressure conditions (~60°C, 1 atmosphere) and where micron-sized aerosols shield against cosmic rays or ultraviolet light to protect life.

Hypothesis of a habitable environment on the surface of Venus

(Left) Schematic diagram of the hypothesis that Venus may have been a habitable planet before the runaway greenhouse effect.

(Right) Schematic diagram of the thermophilic-hyperacidic microbial cycle hypothesis surviving between the haze layers of Venusian clouds.

In September 2020, a team of researchers caused a huge stir and controversy when they published a paper in the journal Nature Astronomy announcing the detection of phosphine (PH3) at some altitude in the clouds of Venus using ground-based radio telescopes, possibly as indirect evidence for the existence of life. The focus has been on the polysynthetic nature of the observations and the fact that even if the signal does originate from phosphine, other non-living sources cannot be ruled out. But in any case, these new detections and controversies mark Venus as a new hot spot in international planetary and space life detection and research, and an important area of international scientific and technological competition.

Although Venus has been explored for many years, it is still in the stage of accumulating key data and there are many observation gaps that need to be filled. For example, 99% of the mass of Venus' atmosphere is concentrated in the troposphere, especially below 28 km, but there is a lack of direct data on the deeper atmosphere of Venus from the surface to 12 km altitude. The radar-based detection of Venusian topography has been stagnant after the Magellan mission, and the resolution of the existing Venusian radar detection is in the order of 100 meters, which is only equivalent to the Mars mission in the 1970s, and cannot achieve more detailed identification and classification of Venusian topography, especially the analysis and study of the surface of Venus at the scale of geological processes, which seriously restricts the understanding of the key regions of the surface of Venus and the geological evolution of Venus. geological evolution of Venus. In-situ detection of the Venusian atmosphere (especially atmospheric elemental isotope measurements) and fine remote sensing detection of mosaics or even in-situ detection have put forward clear and urgent needs for future Venus exploration missions.

It was once Earth's "twin star", but went on to a completely different fate

Spatial resolution achievable by radar surveys on future Venus missions compared to Magellan

/ Go, go to Venus

In June 2021, NASA and ESA approved new missions to Venus - "VERITAS", "DAVINCI+ " mission and "Envision (EnVison)" mission. In addition, Russia and India have also proposed and actively promoted their own Venus exploration missions. International Venus exploration and research will soon usher in a new round of enthusiasm, and the mystery of Earth's twin star will be gradually unveiled.

"The Truth and Da Vinci+ missions are two highly complementary missions scheduled for launch around 2030. The full name of Truth is "Venus Emissivity, Radio Science, Interferometric Synthetic Aperture Radar, Topography, and Spectroscopy" mission. The primary science goal is to generate high-resolution topographic maps and images of Venus' global surface, and to produce a series of global maps of Venus, including maps of deformation, surface composition, thermal emission, and gravity fields. It will attempt to detect whether Venus has an ancient water environment and whether current volcanism is limited to the mantle column region or has a wider distribution. "The full name of the Da Vinci+ mission is "Venus Deep Atmosphere Rare Gas, Chemistry, and Imaging", which directly measures the composition of Venus' atmosphere during 63 minutes of descent with a descending probe, measuring rare gases, trace gases, and their isotopic composition. It will measure the temperature, pressure, and wind speed of the Venusian atmosphere. Before reaching the surface, the probe will also take images of Venus' mosaic to explore its origin and its tectonic, volcanic, and weathering history.

"EnVison, scheduled for launch in 2032, is an orbit-based, high-resolution radar mapping and atmospheric research mission on the surface of Venus. The science goals are to search for active geologic processes, measure surface temperature changes associated with active volcanism, characterize regional and local geologic features, determine crustal support mechanisms and constrain mantle and core properties, and an orbit-based Venus surface high-resolution radar mapping and atmospheric research mission capable of detecting centimeter-scale surface changes, characterizing volcanic and tectonic activity, and estimating rates of weathering and surface alteration. The subsurface radar bathymetry will map faults, stratigraphy, and weathering within ~100 m depth of regional subsurfaces, identifying structural relationships and geologic history.

The Russian Venus D (Venera-D) mission concept is in the preparatory stage, signaling Russia's determination to return to Venus. "The Venus D mission concept has been modified several times, and the baseline mission currently consists of an orbiter and a short-lived (2-3 hours) Vega-type lander. In addition to the baseline mission, a series of potential elements are being demonstrated, such as balloons, subsatellites, and a long-lived (24-hour) ground station. Three sample returns from Venus are also proposed for 2029-2034.

In addition, other interplanetary exploration missions are available to explore Venus during its gravity-assisted phase. These missions include NASA's Parker Solar Probe, ESA's Solar Orbiter and Jupiter Icy Moon Explorer (JUICE).

Question and Answer Session /

1. The report mentions that the Venus lander survived for a maximum of about two hours. The temperature and pressure on the surface of Venus do not seem to be very extreme values. Are temperature and pressure the main reasons for the short survival time of the lander? Are there any other reasons?

A: The main reason is the temperature and pressure of the surface of Venus. The environment on the surface of Venus is a bit like a pressure cooker. And electronic devices and scientific instruments in the work of the first to cool down to ensure that it does not overheat, but in the Venusian environment is very difficult. When exploring Mars some instruments that work for long periods of time usually choose to work at night and do not need additional cooling methods. In contrast, the constant high temperature of Venus places higher demands on the design of both electronics and wiring.

2. Venus is about the same size as Earth, so why does Earth have a moon but Venus does not?

A: That's a good question, but at this point we don't know. The exact cause of the Moon is still unclear, but it is considered more acceptable to have a large collisional origin - an impact by an object the size of Mars at the beginning of the formation of the primitive Earth, followed by re-evolution to form the present Earth and Moon. Venus may not have undergone such a process, which may also have had a significant impact on its evolution. In contrast to the Earth, Venus may be a counter-example of whether the absence of large natural moons would have led to another evolutionary path.

3. When did the surface of Venus become so hot? Why was there no ocean at the beginning of Venus?

A: This issue is also unknown to us, it is all still speculation. We speculate that when Venus was first formed (the first 100 million years) it may have been very similar to Earth. But it is not clear whether this is actually the case. We see no trace of oceans on the surface of Venus now. But it is also possible that the global magma cover around 700 million years ago erased some traces of the early days when there were oceans. This is something that future Venus exploration missions will want to address.

4. Is our country preparing a Venus exploration program?

A: We are now doing some preliminary research, these studies are hoped to help the future exploration plan, if China wants to go to Venus exploration, should go to explore what, what to do to prepare. Mainly the scientific aspects of the idea. But the specific Venus exploration mission is not yet.

5. Is there a preference for a lander or a floating platform?

A: With the current technology, landing and floating platforms will be a little more difficult. Keeping the lander and the scientific instruments it carries alive and working is very demanding on the design of the lander and the scientific payload. In the case of an airborne platform, there are similarly many requirements for the floatplane or vehicle itself. So in the next ten years, if China wants to explore Venus, it may still consider the orbiter as the main instrument, but it can consider carrying some characteristic exploration instruments to do the exploration that other countries have not done or done.

Rapporteur Profile /

Dr. Yu-Qiu Zhao is an Associate Researcher at the Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, and the Center of Excellence for Innovation in Comparative Planetology, Chinese Academy of Sciences; a member of the pre-science research team for China's first Mars exploration mission. He has long been engaged in research on the evolution of planetary geology and environment. In recent years, he has focused on the exploration of Venus, and participated in the pre-study and demonstration of the scientific problems of Venus exploration.

It was once Earth's "twin star", but went on to a completely different fate

[This article is compiled from the lecture content of the "Tianfang Night Pool" series, and the speakers are all outstanding female scientists in the field of space science. The lecture is sponsored by the International Space Science Institute - Beijing (ISSI-BJ), and your attention is welcome.]

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