Venus is the second planet in the Solar System in order of distance from the Sun, and the sixth largest in both mass and diameter. It owes its name to the Roman goddess of love.
| Orbital characteristics | |
|---|---|
| Semi-major axis | 108,209,500 km (0.723,336 336 au) |
| Aphelion | 108,943,000 km (0.728 24 au) |
| Perihelion | 107,476,000 km (0.718 43 au) |
| Orbital circumference | 679,892,000 km (4,544 8 au) |
| Eccentricity | 0,006 78 |
| Period of revolution | 224.667 days |
| Synodic period | 583.92 days |
| Average orbital speed | 35.025 71 km/s |
| Maximum orbital speed | 35.264 3 km/s |
| Minimum orbital speed | 34.789 5 km/s |
| Tilt on the ecliptic | 3,394 71° |
| Ascending node | 76,68° |
| Perihelion argument | 54,9° |
| Known satellites | 0 |
| Physical characteristics | |
| Equatorial radius | 6,051.8 ± 1.0 km (0.949 Earth) |
| Polar radius | 6,051.8 ± 1.0 km (0.952 Earth) |
| Volumetric mean radius | 6,051.8 ± 1.0 km (0.950 Earth) |
| Flattening | 0 |
| Equatorial perimeter | 38,025 km (0.949 Earth) |
| Area | 4.60 × 108 km2 (0.902 Earth) |
| Volume | 9.284 3 × 1011 km3 (0.857 Earth) |
| Mass | 4.867 5 × 1024 kg (0.815 Earth) |
| Global density | 5.204 × 103 kg/m3 |
| Surface gravity | 8.87 m/s2 (0.905 g) |
| Release speed | 10.46 km/s |
| Rotation period (sidereal day) | -243.023 days |
| Rotational speed (at the equator) | 6.52 kph |
| Axis tilt | 177,36° |
| Right ascension of the North Pole | 272,76° |
| Declination of the North Pole | 67,16° |
| Visual geometric albedo | 0,689 |
| Bond’s Albedo | 0,77 |
| Solar irradiance | 2,601.3 W/m2 (1,902 Earths) |
| Blackbody equilibrium temperature |
226.6 K (−46.4 °C) |
| Temperature Maximum | 763 K (490 °C) |
| Temperature Average | 737 K (464 °C) |
| Temperature Minimum | 719 K (446 °C) |
| Characteristics of the atmosphere | |
| Atmospheric pressure | 9.3 × 106 Pa |
| Density on the ground | ~ 65 kg/m3 |
| Total mass | 4.80 × 1020 kg |
| Ladder height | 15.9 km |
| Average molar mass | 43.45 g/mol |
| Carbon dioxide CO2 | ~96.5% |
| Dinitrogen N2 | ~3.5% |
| Sulfur dioxide SO2 | 150 ppm |
| Argon Ar | 70 ppm |
| Water vapor H2O | 20 ppm |
| Carbon monoxide CO | 17 ppm |
| Helium He | 12 ppm |
| Neon Ne | 7 ppm |
| Hydrochloric acid HCl | 100 to 600 ppb |
| HF hydrofluoric acid | 1 to 5 ppb |
| Carbonyl sulfide COS | Trail |
| History | |
| Babylonian deity | Ishtar |
| Greek deity | Eosphoros and Hesperos |
| Chinese Name(Related Item) | Jīnxīng 金星 (metal) |
Venus orbits the Sun every 224.7 Earth days. With a rotation period of 243 Earth days, it takes longer to rotate around its axis than any other planet in the Solar System. Like Uranus, it has a retrograde rotation and rotates in the opposite direction to that of the other planets: the sun rises in the west and sets in the east. Venus has the most circular orbit of the planets in the Solar System with almost zero orbital eccentricity and, because of its slow rotation, is almost spherical (flattening considered zero). It does not have a natural satellite.
Venus is one of the four terrestrial planets in the Solar System. It is sometimes called Earth’s “sister planet” because of the relative similarities in their diameters, masses, proximity to the Sun and compositions. In other ways, it is radically different from Earth: its magnetic field is much weaker and it has a much denser atmosphere, composed of more than 96% carbon dioxide. The atmospheric pressure on the surface of the planet is thus 92 times higher than that of the Earth, or about the pressure felt, on Earth, 900 meters under water.
It is by far the hottest planet in the Solar System — although Mercury is closer to the Sun — with an average surface temperature of 462 °C (725 K). The planet is shrouded in an opaque layer of sulfuric acid clouds, highly reflective to visible light, preventing its surface from being seen from space. Although the presence of oceans of liquid water on its surface in the past is assumed, the surface of Venus is a dry, rocky desert landscape where volcanism always takes place. The topography of Venus has few high reliefs and consists mainly of vast plains geologically very young: a few hundred million years.
As the second brightest natural object in the night sky after the Moon, Venus can cast shadows and can sometimes be visible to the naked eye in broad daylight. Since Venus is an inferior planet, it remains close to the sun in the sky, appearing either in the west just after dusk or in the east shortly before dawn. Because of its large apparent magnitude, Venus was the subject of the first astronomical observations and was the first planet whose movements man traced as early as the second millennium BC. It has also been incorporated into many mythologies as the morning star and the evening star, and later inspired writers and poets. It is also known in Western culture as the “shepherd’s star“.
Venus was a prime target for early interplanetary explorations because of its short distance from Earth. It is the first planet visited by a spacecraft (Mariner 2 in 1962) and the first where a space probe has successfully landed (Venera 7 in 1970). The thick clouds of Venus made it impossible to observe its surface in visible light, so the first detailed maps were made from images from the Magellan orbiter in 1991. Projects for rovers and more complex missions have also been considered.
Physical characteristics of Venus
Venus is one of the four terrestrial planets in the Solar System, which means it has a rocky body like Earth. It is comparable to Earth in size and mass, and often described as Earth’s “sister” or “twin”. Its diameter is 95% of that of the Earth, and its mass a little more than 80%. Nevertheless, while its geology is undoubtedly close to that of the Earth, the conditions on its surface differ radically from terrestrial conditions.
Venus is notably the hottest planet in the Solar System because of its atmosphere much denser than the Earth’s atmosphere. The geological phenomena affecting the Venusian crust also seem specific to this planet and are at the origin of geological formations sometimes unique in the Solar System such as coronae, arachnoids and Farra, attributed to atypical manifestations of volcanism.
Atmosphere
composition
Venus has an extremely dense atmosphere. It consists mainly of 96.5% carbon dioxide (CO2) and a small amount of 3.5% dinitrogen. This atmosphere is occupied by thick clouds of sulfur dioxide. The mass of its atmosphere is 93 times greater than that of the Earth, while the pressure on its surface is about 92 times greater than that of the Earth, a pressure equivalent to that felt on Earth at a depth of nearly 900 meters below sea level. The surface density is 65 kg/m3, which is 50 times the density of the Earth’s atmosphere at 293K (20°C) at sea level.
This atmosphere, rich in carbon dioxide, is responsible for the strongest greenhouse effect in the Solar System, creating surface temperatures of about 735K (462°C). Thus, the surface of Venus is warmer than that of Mercury, which has a minimum surface temperature of 53 K (−220°C) and a maximum of 700K (427°C) (for the side exposed to the Sun the longest), although Venus is about twice as far from the Sun and therefore receives only about 25% of Mercury’s solar irradiance according to the inverse-square law.
Studies published in the 2000s first suggested that Venus’ atmosphere previously resembled that surrounding Earth and that there may have been significant amounts of liquid water on its surface. Then, after a period that could extend from 600 million to several billion years, a growing greenhouse effect appeared due to the evaporation of this water originally present and eventually leading to the current critical level of greenhouse gases in the atmosphere. However, in 2021, a new study indicates that the climatic conditions of Venus would never have allowed the formation of oceans of liquid water on its surface.
Lightning
The existence of lightning in the atmosphere of Venus has been controversial since the first suspicions during the Soviet Venera program.
In 2006 and 2007, Venus Express detected plasma waves, the signature of lightning. Their intermittent appearance suggests an association with meteorological activity and, according to these measurements, the lightning rate is at least half that of the Earth. However, other instruments in the mission do not detect lightning. Moreover, the origin of this lightning remains uncertain.
In December 2015, and to a lesser extent in April And May 2016 researchers working on the Akatsuki spacecraft observe arc shapes in the atmosphere of Venus. This is considered evidence of the existence of the largest stationary gravity waves in the Solar System discovered to date.
Layers of the atmosphere
The Venusian atmosphere can be roughly divided into three parts: the lower atmosphere, the cloud layer and the upper atmosphere.
Low atmosphere
The lower atmosphere is between 0 and 48 km above sea level and is relatively transparent.
The composition of the lower atmosphere is described in the table below. Carbon dioxide largely dominates, the secondary gas being nitrogen and all others being minor constituents (~300 ppm in total). At this pressure (9.3 MPa) and temperature (740 K), CO2 is no longer a gas, but a supercritical fluid (intermediate between gas and liquid), with a density of about 65 kg/m3.
| Composition of the lower atmosphere of Venus | |
|---|---|
| Element or molecule | Percentage in the lower atmosphere (below clouds) |
| Carbon dioxide | ~96.5% |
| Dinitrogen | ~3.5% |
| Sulfur dioxide | 150 ppm |
| Argon | 70 ppm |
| Steam | 20 ppm |
| Carbon monoxide | 17 ppm |
| Helium | 12 ppm |
| Neon | 7 ppm |
Thermal effusivity and heat transfer by winds in the lower atmosphere mean that the surface temperature of Venus does not vary significantly between the illuminated and dark hemispheres despite the planet’s very slow rotation. Surface winds are slow, moving at a few kilometers per hour, but due to the high density of the atmosphere on the surface, they exert a significant force against obstacles. This force alone would make it difficult for a human being to move.
Cloud layer
Above the dense layers of CO2 are, between 45 km and 70 km from the surface, layers of thick clouds of sulfuric acid in the form of droplets, formed of sulfur dioxide and water (solid and gaseous state) by a chemical reaction resulting in the hydrate of sulfuric acid. Sulfuric acid droplets are in an aqueous solution, consisting of 75% sulfuric acid and 25% water. The atmosphere also contains about 1% ferric chloride and other possible constituents for the composition of these clouds are iron sulfate, aluminum chloride and phosphorus pentoxide.
These clouds reflect about 90% of sunlight back into space, preventing visual observation of Venus’s surface. These are also the cause of its brightness in the terrestrial sky, giving it a Bond albedo of 0.77. This permanent cloud cover means that although Venus is closer than Earth to the Sun, it receives less sunlight on the ground because only 5% of the rays reach it.
The strong winds of more than 300 km/h that carry the highest clouds circle Venus in four to five Earth days. These winds move up to sixty times the speed of the planet’s rotation; by comparison, the fastest winds on Earth have a speed of only 10 to 20% of the Earth’s rotational speed.
Although surface conditions on Venus are not conducive to this, some speculate on the possibility of life in the upper layers of the clouds of Venus (where temperatures vary between 30 and 80 °C), despite an acidic environment.
If Venus has no seasons as such, astronomers identify in 2019 a cyclical variation in the absorption of solar radiation by the atmosphere, probably caused by opaque particles suspended in the upper clouds. The variation causes observed changes in Venus’ wind speeds and appears to increase and decrease with the Sun’s eleven-year sunspot cycle.
Upper atmosphere
The mesosphere of Venus extends from 65 km to 120 km altitude and the thermosphere begins at about 120 km, probably reaching the upper limit of the atmosphere (exosphere) between 220 and 350 km.
In 2007, the Venus Express probe discovered the existence of a double atmospheric vortex at the south pole, and, in 2011, also discovered the existence of an ozone layer in the upper layers of Venus’s atmosphere. However, since this layer is very weak, it is considered that Venus has no stratosphere.
In January 2013, ESA reports that the ionosphere of Venus trickles outward in a manner similar to that of a comet’s tail.
Geography
The Venusian surface has been the subject of speculation, because of its thick clouds reflecting visible light, until the sending of space probes allowed it to be studied. The Venera missions in 1975 and 1982 returned images of a surface covered with sediments and relatively angular rocks. The area was mapped in detail by Magellan in 1990–91. The soil then shows signs of significant volcanism, and the sulfur found in the atmosphere seems to indicate recent eruptions.
Since Venus has zero flattening, altitudes are defined in relation to the volumetric average radius of the planet, which is 6,051.8 km. It is a planet with relatively uneven relief: four-fifths of its surface is covered with volcanic plains with a low slope. The Venusian surface is mainly occupied to the tune of 70% by vast plains without much relief. Called planitiae in planetary geomorphology, the main ones are Atalanta Planitia, Guinevere Planitia or Lavinia Planitia. They are dotted with craters. These plains, a priori volcanic in nature, deepen in places up to 2,900 m below the mean surface level, at the level of depressions covering about a fifth of the surface of the planet. The remaining 10% of plains are smooth or lobed.
The plateaus (also called Highlands or Highlands), high reliefs sometimes compared to terrestrial continents, thus represent less than 15% of the surface of the planet (unlike the 29% of the surface occupied by continents on Earth). Two are particularly noteworthy for their dimensions, one located in the northern hemisphere of the planet and the other just south of the equator.
The northern continent, near the polar regions, is called Ishtar Terra after Ishtar the Babylonian goddess of love. Its dimensions of 3,700 × 1,500 km are slightly larger than those of Australia. It is a geological ensemble essentially volcanic to the west, including the Lakshmi Planum formation, and orogenic to the east, where Skadi Mons is located, the highest point on the planet at 10,700 m, in the chain of Maxwell Montes, then the immense Fortuna Tessera which is a region of typically Venusian terrain.
The southern continent is called Aphrodite Terra, after the Greek goddess of love. It is three times larger than the previous one, having an area similar to that of South America. Its reliefs are however lower, presenting a network of fragments of plateaus in a set of tesserae extended to the southeast and especially to the northeast by coronae and volcanoes, among which Maat Mons, the highest Venusian volcano.
Other higher regions, of lesser importance, also exist. This is the case of Alpha Regio, a series of basins, ridges, and folds that arrange in all directions with an average altitude of 1 to 2 km; or Beta Regio, remarkable since we would have found high volcanic formations of which some summits, recent, would exceed 5,000 m of altitude. Along with the Ovda Regio and the Maxwell Montes, named after James Clerk Maxwell, these are the only features of the Venusian surface to be named after a male name, before the adoption of the current system by the International Astronomical Union. The current planetary nomenclature is to name the Venusian characteristics after historical and mythological women.
The planet shows few impact craters, indicating that the surface is relatively young, at about 300 to 600 million years old. Venus has unique surface features in addition to the impact craters, mountains, and valleys commonly found on terrestrial planets. Among these are flat-topped volcanic elements called “Farra”, resembling pancakes, and whose diameter varies from 20 to 50 km and the height from 100 to 1,000 meters. There are also concentric fractures resembling cobwebs called “arachnoids” and fracture rings sometimes surrounded by depression, called “coronae”. These characteristics are of volcanic origin.
The longitude of the physical characteristics of Venus is expressed relative to its prime meridian. This was originally defined as crossing a bright spot called Eve, located south of Alpha Regio. Once the Venera missions were completed, the prime meridian was redefined to pass through the central peak of the Ariadne crater. In addition, the surface of the planet is divided between 62 quadrangles mapped at 1:5,000,000.
The surface temperature of Venus varies little according to latitudes and longitudes (it is isothermal). The temperature is constant not only between the two hemispheres but also between the equator and the poles. The tilt of Venus’ very weak axis—less than 3°, compared to 23° on Earth—also minimizes seasonal variations in temperature. Thus, altitude is one of the few factors that can affect the Venusian temperature. The highest point of Venus, Maxwell Montes, is, therefore, the coldest point with a temperature of about 655K (380 °C) and an atmospheric pressure of 4.5 MPa (45 bar).
In 1995, the Magellan space probe took an image of a highly reflective substance at the top of the highest mountain peaks, resembling the snow found at the tops of the Earth’s mountains. This substance was probably formed from a process similar to snow, although this one takes place at a much higher temperature. Too volatile to condense on the surface of the planet, it would have risen in gaseous form at higher altitudes to finally precipitate there due to lower temperatures. The composition of this substance is not known with certainty, but it is assumed that it may be tellurium or galena (lead sulfide).
In addition, emissivity measurements at 1.18 μm carried out in 2008 suggest a relative abundance of granites and other felsic rocks on the highest — which are usually the oldest — terrains on the planet. This would imply the past existence of a global ocean with a mechanism for recycling water into the mantle that may have produced such rocks. Like Mars, Venus may have known, several billion years ago, temperate conditions allowing the existence of liquid water on the surface, water now disappeared — by evaporation and photochemical dissociation in the upper atmosphere.
Surface geology of Venus
Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has many more volcanoes than Earth, including 167 large volcanoes over 100 km in diameter; the only terrestrial volcanic complex with at least this diameter is the Big Island of Hawaii. This is not the consequence of greater volcanic activity on Venus, but especially of the age of its crust. The oceanic crust on Earth is continuously recycled by subduction at the boundaries of tectonic plates and has an average age of about 100 million years while the Venusian surface is estimated at 300–600 million years.
Several elements indicate ongoing volcanic activity on Venus. Sulfur dioxide concentrations in the atmosphere decreased by a factor of ten between 1978 and 1986, then jumped in 2006, before decreasing again by a factor of ten between 2006 and 2012. This may mean that levels had risen as a result of large volcanic eruptions. There remains on Venus a residual volcanism, sometimes leading to the presence of molten lava on the ground. It is also suggested that Venusian lightning could have originated from volcanic activity, and therefore be volcanic lightning. In January 2020, astronomers report evidence suggesting that Venus was currently volcanically active.
In 2008 and 2009, the first direct evidence of ongoing volcanism was observed by Venus Express, in the form of four infrared hot spots located in the Ganis Chasma rift zone, near the 8 km Maat Mons shield volcano. Three of the spots were observed in several successive orbits. Geologists believe that these spots represent lava freshly released by volcanic eruptions. The actual temperatures are not known, as the size of the hot spots could not be measured, but were to be contained in a range of 800 K (526.85 °C) to 1,100 K (826.85 °C) while the normal temperature is estimated at 740 K (466.85 °C).
Other Montes are remarkable, with for example the shield volcano Gula Mons reaching an altitude of 3,000 m in the west of Eistla Regio or Theia Mons and Rhea Mons in the Beta Regio. Separated by 800 km, these last two were formed by the plume of the mantle during the appearance of Devana Chasma.
The Soviet probes Venera 15 and Venera 16 have recorded impact craters on the surface of Venus.
There are nearly a thousand, these being evenly distributed on the planet. On other cratered bodies, such as Earth and the Moon, craters show a range of degradation states. On the Moon, degradation is caused by subsequent impacts, while on Earth it is caused by wind and rain erosion. However, on Venus, about 85% of the craters are in perfect condition. The number of craters, as well as their preserved state, indicates that the planet underwent a global resurfacing event (i.e. the almost complete renewal of its surface) about 300 to 600 million years ago followed by a decrease in volcanism.
Also, while the Earth’s crust is in continuous motion, Venus would be unable to sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise to a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on a large scale, completely recycling the crust.
Venusian craters have a diameter ranging from 3 to 280 km. No crater is smaller than 3 km, due to the dense atmosphere of the planet: objects that do not have enough kinetic energy are so slowed down by the atmosphere that they do not create an impact crater. Thus incoming projectiles with a diameter of less than 50 meters will fragment before reaching the ground.
Internal structure
Without seismic data, little direct information is available on the internal structure and geochemistry of Venus. On the basis of fifteen years of observation in radio waves, it is however estimated in 2021 that its normalized moment of inertia would be 0.337 ± 0.024. Venus resembles the Earth in size (6,051 km radius against 6,378 km for Earth) and density (5.26 against 5.52), several authors assume, however, that the two planets have a comparable internal structure: a core, a mantle and a crust.
Crust
The silicate crust, supposedly 20 to 50 km thick, would be thicker than the Earth’s oceanic crust (average of 6 km) and in the order of magnitude of the Earth’s continental crust (average of 30 km). The size of the Venusian crust was deduced from the numerous lava effusions observed around the impact craters. This crust would represent only 0.34% of the radius of the planet and the analyses made by the various Venera probes have proven that the outer material of Venus is similar to granite and terrestrial basalt (rocks rich in silica and ferromagnesian). The continental plate system would be less complex than on Earth: the more plastic rocks strongly absorb the effects of continental drift. Thus, Venus does not have tectonic plates like those of Earth.
This fundamental difference between the geology of the two most similar terrestrial planets can be attributed to their divergent climatic evolution. Indeed, the Venusian climate prevents water from being preserved on the surface, irreversibly drying out the rocks of the crust. However, pore water from rocks plays a major role in subduction on Earth where it is preserved in its oceans. Terrestrial rocks all contain minimal residual water, which is not the case under the high-temperature climate conditions of Venus.
Mantle
Venus would have a rocky mantle representing about 52.5% of the radius of the planet, composed mainly of silicates and metal oxides. This mantle could still have today (like the Earth for 2 or 3 Ga) a magmatic ocean, 200 to 400 km thick.
Core
Like Earth’s, the Venusian core is at least partially liquid because the two planets have cooled at about the same rate. The slightly smaller size of Venus means that pressures are about 24% lower in its core than in the Earth’s core. The main difference between the two planets is the lack of evidence of plate tectonics on Venus, perhaps because its crust is too hard for there to be waterless subduction to make it less viscous. As a result, heat loss is reduced on the planet, preventing it from cooling. This provides an explanation for its lack of an internal magnetic field. Instead, Venus could mostly reduce its internal heat during major resurfacing events.
The core of Venus would consist of two parts: an outer core made of liquid iron and nickel that would represent about 30% of the radius of the planet; an inner core composed of solid iron and nickel that would represent about 17% of the radius of Venus. But this remains speculative because, unlike Earth, there have been no seismic measurements. It is not impossible that the core of Venus is entirely liquid.
Magnetic field
In 1967, the Venera 4 probe discovered that the magnetic field of Venus is much weaker than that of Earth. This magnetic field is created by an interaction between the ionosphere and the solar wind rather than by an internal dynamo effect as in the Earth’s core. The almost non-existent magnetosphere of Venus offers negligible protection of the atmosphere against cosmic radiation.
The absence of an intrinsic magnetic field to Venus was a surprise at the time of this discovery, the great similarity of the planet with Earth suggesting a dynamo effect in its core. For there to be a dynamo, it is necessary that there is a conductive liquid, rotation and convection. The nucleus is thought to be electrically conductive and, although it is very slow, simulations show that the rotation of Venus is sufficient to produce a dynamo. This implies that convection in the core of Venus is missing to make the dynamo appear.
On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much higher in temperature than the top. On Venus, one of the global resurfacing events may have stopped plate tectonics and led to a decrease in heat flow through the crust. This lower thermal gradient would result in an increase in mantle temperature, thereby reducing heat flow out of the core. As a result, no convection is performed to drive a magnetic field. Instead, heat from the core is used to warm the crust.
Other hypotheses would be that Venus does not have a solid inner core, greatly limiting the separation of the various constituents and impurities, and hence the internal movements of the metallic fluid of the nucleus that generate the magnetic field, or that its core does not cool, so that the entire liquid part of the core is at about the same temperature, preventing convection again. Another possibility is that its core has already completely solidified. The state of the core is highly dependent on its sulfur concentration, which is currently unknown and therefore prevents uncertainties from being removed. Despite its weak magnetic field, auroras have been observed.
The faint magnetosphere around Venus means that the solar wind interacts directly with the upper layers of its atmosphere. Here, hydrogen and oxygen ions are created by the dissociation of neutral molecules by ultraviolet radiation. The solar wind then provides enough energy for some of these ions to reach a speed to escape the gravity field of Venus.
This erosion process leads to a constant loss of low-mass ions (hydrogen, helium and oxygen) in the atmosphere, while higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind likely resulted in the loss of most of Venus’ water during the first billion years after its formation. Erosion also increased the proportion of the isotope deuterium to hydrogen protium without neutron (thus of lower mass and more easily carried away), resulting in a ratio of deuterium to protium in the atmosphere greater than 100 times those found in the rest of the Solar System.
Comparisons
In size and mass, Venus is very similar to Earth and has often been described as the Earth’s “twin sister”. The two planets are similar in their physical aspects, possessing few craters – a sign of a relatively young surface and a dense atmosphere – and having similar chemical compositions. The following table summarizes other physical properties that are relatively close to Earth:
| Comparison of physical properties of Venus and Earth | |||
|---|---|---|---|
| Physical properties | Venus | Earth | Venus/Earth ratio |
| Mass | 4.868 5 × 1024 kg | 5.973 6 × 1024 kg | 0,815 |
| Equatorial radius | 6,051 km | 6,378 km | 0,948 |
| Average density | 5,25 | 5,51 | 0,952 |
| Semi-major axis | 108,208,926 km | 149,597,887 km | 0,723 |
| Average orbital speed | 35.02 km.s−1 | 29.79 km.s−1 | 1,175 |
| Equatorial external gravity | 8.87 m.s−2 | 9.81 m.s−2 | 0,906 |
Some astronomers thought, before sending space probes to its surface, that Venus could be very similar to the Earth under its thick clouds and perhaps even harbor life. Some studies hypothesize that a few billion years ago, Venus would have been much more similar to Earth than it is now. There would have been significant quantities of water on its surface and this water would have evaporated as a result of a significant greenhouse effect.
| Comparison of physical characteristics of terrestrial planets in the Solar System | ||||
|---|---|---|---|---|
| Planet | Equatorial radius | Mass | Gravity | Axis tilt |
| Mercury | 2,439.7 km (0.383 Earth) |
3.301 × 1023kg (0.055 Earth) |
3.70 m/s2 (0.378 g) |
0.03° |
| Venus | 6,051.8 km (0.95 Earth) |
4.8675 × 1024 kg (0.815 Earth) |
8.87 m/s2 (0.907 g) |
177.36° |
| Earth | 6,378.137 km | 5.9724 × 1024 kg | 9.780 m/s2 (0.997 32 g) |
23.44° |
| March | 3,396.2 km (0.532 Earth) |
6.44171 × 1023 kg (0.107 Earth) |
3.69 m/s2 (0.377 g) |
25.19>° |
Orbit and rotation of Venus
Orbit
Venus orbits the Sun at an average distance of about 108 million kilometers (between 0.718 and 0.728 AU) and completes an orbit every 224.7 Earth days, about 1.6 times faster than Earth. Although all planetary orbits are elliptical, the orbit of Venus is the closest to a circular orbit, with an eccentricity of less than 0.01. When it is between the Earth and the Sun in inferior conjunction, Venus is the planet closest to the Earth, at an average distance of about 42 million kilometers. However, she spends most of her time away from Earth. Mercury is therefore on average the closest planet to Earth, due to its shorter distance from the Sun. The planet reaches its lower conjunction on average every 584 days, which is called its synodic period.
Rotation
All the planets in the Solar System revolve around the Sun counterclockwise as seen from the Earth’s north pole. Also, most planets also rotate on their axes counterclockwise/direct. This is not the case of Venus (we can also mention Uranus), which rotates clockwise: we will speak of retrograde rotation. Its rotation period is 243 Earth days—the slowest rotation of any planet in the Solar System. This has only been known since 1962, when radar observations conducted by the Jet Propulsion Laboratory made it possible to observe the surface of the planet through the thick atmosphere.
A Venusian sidereal day, therefore, lasts longer than a Venusian year (243 against 224.7 Earth days). Because of this retrograde rotation, an observer on the surface of Venus would see the Sun rise in the west and set in the east. In practice, the opaque clouds of Venus prevent observation of the Sun from the surface of the planet.
Due to retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day, during 116.75 Earth days, while they are longer for planets with a rotation in the direct direction. A Venusian year therefore represents about 1.92 Venusian solar days and nights and the Venusian days and nights each extend over nearly two Earth months: 58 days 9 am.
Because its rotation is so slow, Venus is very close to a sphere with almost zero flattening. Also, the equator of Venus rotates at 6.52 km / h while that of the Earth rotates at 1,674 km / h. The rotation of Venus slowed down during the 16 years between the visits of the Magellan and Venus Express spacecraft: the Venusian sidereal day increased by 6.5 minutes in this time frame.
Origin of retrograde rotation
The causes of Venus’ retrograde rotation are still poorly understood and the planet may have formed from the solar nebula with a different rotation period and obliquity than it is currently experiencing. The explanation most often put forward is a gigantic collision with another large body, during the formation phase of the planets of the Solar System.
Another explanation involves the Venusian atmosphere which, because of its high density, may have influenced the rotation of the planet. Works by Jacques Laskar and Alexandre C. M. Correia taking into account the effects of atmospheric thermal tides show the chaotic behavior of the obliquity and rotation period of Venus. Venus could therefore have evolved naturally over several billion years towards a retrograde rotation without having to involve a collision with a massive body.
The rotation period observed today could thus be a state of equilibrium between a tidal lock due to the gravitation of the Sun, which tends to slow down the rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere that would accelerate it. However, it is not possible to know if the obliquity of Venus suddenly increased from 0° to 180° during its history or if its rotational speed slowed to zero speed and then became negative. Both scenarios are possible and result in the same current state of equilibrium.
The hypothetical Earth-Venus synchronization
Venus is almost in synchronous rotation with the Earth, so whenever Venus is in inferior conjunction, Venus presents almost exactly the same face to Earth. This is due to the fact that the average interval of 583.92 Earth days between successive close approaches to Earth (synodic period) is almost equal to 5 Venusian solar days (because 583.92/116.75 ≈ 5.0015).
Thus, it was discussed a hypothetical Earth-Venus synchronization. However, this ratio is not exactly equal to 5, while the gravitational locking of the Moon on the Earth (1:1) or that of Mercury’s rotation on its revolution (3:2) are accurate and stabilized. Also, the tidal forces involved in the Venus-Earth synchronization are extremely weak. The hypothesis of a spin-orbit resonance with the Earth has therefore been ruled out, the observed synchronization may be a coincidence only observable in our astronomical epoch.
Absence of satellites
Venus has no natural satellites. It does, however, have several Trojan asteroids: the quasi-satellite 2002 VE 68 with a horseshoe orbit and two temporary Trojans, 2001 CK 32 and 2012 XE133.
In 1645, Francesco Fontana and Giovanni Cassini reported the presence of a moon orbiting Venus, which was later named Neith. Many observations were reported over the next two centuries, including from renowned astronomers such as Joseph-Louis Lagrange in 1761, and Johann Heinrich Lambert calculated its orbit in 1773. However, most of these observations were then correctly attributed to nearby stars or optical illusions in the late nineteenth century and Neith’s quest stopped.
A 2006 modeling study at the California Institute of Technology by Alex Alemi and David Stevenson on the origin of the Solar System shows that Venus probably had at least one moon created by a large cosmic impact billions of years ago. Then, about 10 million years later, according to the study, another impact reversed the planet’s direction of rotation and caused the Venusian moon to accelerate by tidal effect towards Venus until it collided with it. If later impacts created moons, these were also suppressed in the same way. Another explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting inner terrestrial planets, as is also the case for Mercury.
Venus observation
To the naked eye, Venus is the third brightest natural object in the sky (after the Sun and Moon). It appears as a bright white dot with an apparent magnitude ranging from -4.6 to -3.7 (mean -4.14 and standard deviation of 0.31), and an apparent diameter between 9.7 and 66 arcseconds. The brightest magnitude occurs during the crescent phase about a month before or after the inferior conjunction.
The planet is bright enough to be seen in clear daytime skies but is most easily visible when the sun is low on the horizon or setting. Because of its luminosity, it is the only celestial object in the night sky, apart from the moon, to be able to cast a shadow on the Earth’s soil. As the lower planet of the Earth, its elongation (that is, the sharp angle between the planet and the sun in the terrestrial sky) has a maximum value of 47°.
Venus overtakes Earth every 584 days in terms of their orbit around the Sun. In doing so, it changes from the “evening star”, visible after sunset, to the “morning star”, visible before sunrise. Unlike Mercury, the other lower planet which has a maximum elongation of 28° and is often difficult to discern at dusk, Venus is very easily visible, especially when it is at its strongest. Since the astronomical twilight (when the sun is sufficiently below the horizon for there to be a totally dark sky) is about 18°, it can reach an angle of 47-18 = 29° in a dark sky and remain visible until several hours after sunset.
These characteristics contributed to its nickname in Western popular culture of the “shepherd’s star” (although the term “star” is a misnomer since it is a planet) as it can be easily visible in the sky, which historically helped guide herders to and from pasture. As the brightest point object in the sky, Venus is also commonly mistaken for an unidentified flying object.
Phases
During its orbit around the Sun, Venus displays phases like those of the Moon when viewed through a telescope. The planet appears as a small disk called “full” when it is located on the other side of the Sun from the Earth (at higher conjunction). Venus shows a larger disk and a “quarter phase” at its maximum elongations relative to the Sun, and then appears at its brightest in the night sky. The planet has a much larger thin crescent in telescopic view as it passes along the nearby side between the Earth and the Sun. Finally, Venus displays its largest size and its “new phase” when it is between the Earth and the Sun (at lower conjunction). Its atmosphere is visible through a telescope due to the halo of sunlight refracted around it.
Their observation was made for the first time in the early seventeenth century by Galileo using his telescope. They were an argument used by the latter to rally to the heliocentric theory of Copernicus.
Transit of Venus
The transit of the planet Venus between the Earth and the Sun, where the shadow of Venus appears in front of the solar disk, is called “transit of Venus”. Since the Venusian orbit is slightly inclined with respect to the Earth’s orbit, when the planet passes between the Earth and the Sun it usually does not cross the face of the Sun. Thus, the transits of Venus then occur when the inferior conjunction of the planet coincides with its presence in the plane of the Earth’s orbit, more precisely when they cross the intersection line of their orbital planes.
This event is rare on the human time scale because of the criteria necessary for this observation: transits of Venus occur in cycles of 243 years, the current pattern being pairs of transit separated by eight years and occurring at intervals of about 105.5 years or 121.5 years. This model was first discovered in 1639 by the English astronomer Jeremiah Horrocks.
During the transit of Venus, an optical effect called the “black drop phenomenon” appears. On the second contact and just before the third contact, a small black teardrop appears to connect the planet’s disk with the boundary of the solar limb, making it difficult to precisely date the contacts.
Historically, the observation of transits of Venus is important because they allowed astronomers to determine the value of the Earth-Sun distance (the astronomical unit) by the parallax method, as Horroks did first during the 1639 transit. The eighteenth century saw great expeditions by European astronomers to measure the two transits of 1761 and 1769, to which the name of the French astronomer Guillaume Le Gentil remained attached because of bad luck which prevented him from making the observations to which he had devoted years of preparation. Also, Captain Cook’s exploration of the east coast of Australia came after he sailed to Tahiti in 1768 to observe the transit of Venus in 1769.
The next pair of transits occurred in December 1874 and December 1882. The transit of 1874 is the subject of the oldest known chronophotography experiment to measure its precise duration, the Transit of Venus by the French astronomer Jules Janssen.
The last pair of transits occurred on June 8, 2004, and June 5-6, 2012. The transit could be watched live on the Internet from many streams or observed locally with the right equipment and conditions. The next transit will take place on December 11, 2117.
Daytime appearances
Daytime observations of Venus with the naked eye are noted in several anecdotes and recordings.
Astronomer Edmond Halley calculated its maximum brightness with the naked eye in 1716, when many Londoners were alarmed by its appearance in broad daylight. The French Emperor Napoleon Bonaparte witnesses a daytime appearance of the planet during a reception in Luxembourg. Another famous historical daytime observation of the planet takes place during the inauguration ceremony of U.S. President Abraham Lincoln’s second term in Washington, DC, on March 4, 1865. During World War II, the American warship USS Langley (CV-1) fired into the sky on December 12, 1941 (five days after the attack on Pearl Harbor) to attempt to shoot down Venus, mistaking it for an enemy aircraft.
Although the naked-eye visibility of Venus’ phases is disputed, there are records of observations of its crescent.
Pentagram of Venus
The pentagram of Venus is the path that Venus traces as observed from Earth. It results from the fact that the successive inferior conjunctions of Venus repeat very close to a ratio of 13:8 (the Earth making 8 revolutions when Venus makes 13), thus giving a constant angle of 144° on the sequential lower conjunctions, that is to say at each synodic period. This ratio is an approximation: in reality 8/13 is 0.615 38 while Venus orbits the Sun in 0.615 19 Earth years. Since it takes 5 synodic periods of Venus to form the pentagram, this happens every 8 Earth years.
Earthshine
A long-standing mystery of Venus’ observations is its ashy light. It is an evanescent luminous phenomenon that would appear in the form of a diffuse glow barely discernible illuminating the dark part of the disk of Venus when the planet is in the crescent phase. The first claimed observation of ashen light was made in 1643, but the existence of illumination has never been reliably confirmed. Observers hypothesize that this could result from electrical activity in the Venusian atmosphere, but it could also be an optical illusion resulting from the physiological effect of observing a bright crescent-shaped object.
History of its observation
Before telescopes
Venus being the third celestial body in the sky in terms of apparent magnitude, after the Sun and the Moon, it attracted the attention of the first astronomers. Also, Venus is the first planet to have its movements traced in the sky, from the second millennium BC.
However, because the movements of the planet seem to be discontinuous (it can disappear from the sky for several days due to its proximity to the sun) and that it appears sometimes in the morning (morning star) and sometimes in the evening (evening star), many cultures and civilizations first thought that Venus corresponded to two different celestial bodies. Thus, for the ancient Egyptians, the morning star was called Tioumoutiri and the evening star Ouaiti. Similarly, the Chinese have historically called the Morning Venus “the Great White” (Tài-bái 太白) or “the Opener of Brightness” (Qǐ-míng 啟明), and the Evening Venus as “the Excellent West” (Cháng-gēng 長庚).
Nevertheless, a cylinder seal from the period of Djemdet Nasr and the tablet of Ammisaduqa from the first dynasty of Babylon indicate that the Babylonians seem to have understood quite early that the “morning and evening stars” were the same celestial object. Venus was then known as Ninsi’anna (“divine lady, illumination of heaven” because of her brilliance) and later as Dilbat. The first spellings of the name are written with the cuneiform sign si4 (= SU, meaning “to be red”) whose primary meaning could be “divine lady of the redness of the sky”, in reference to the color of dawn and dusk.
The ancient Greeks also thought that Venus were two separate bodies, a morning star and an evening star. They called them respectively Phōsphoros (Φωσφόρος), meaning “light-bringer” (hence the phosphorus element; alternatively Ēōsphoros (Ἠωςφόρος), meaning “dawn”) for the morning star, and Hesperos (Ἕσπερος), meaning “western”, for the evening star. Pliny the Elder attributes the discovery that they were a single celestial object to Pythagoras in the sixth century BCE, while Diogenes Laertius argues that Parmenides was probably responsible for this rediscovery. Later, although the ancient Romans recognized Venus as a single celestial object, the two traditional Greek names continued to be used and also translated into Latin by Lucifer (meaning “bearer of light”) for the morning apparition and Vesper for the evening apparition.
In the second century AD, Ptolemy hypothesized in his astronomical treatise Almagest that Mercury and Venus are located between the Sun and the Earth, within a geocentric system.
At the same time, the Mayans consider Venus to be the most important celestial body after the Sun and the Moon. They call it Chac ek or Noh Ek, meaning “the big star” and know that it is only one celestial body. The cycles of Venus are the subject of a calendar found in the Dresden Codex and the Mayans follow the apparitions and conjunctions of Venus at dawn and dusk. This calendar is based in particular on their observation that five synodic periods of the planet correspond to eight Earth years, cause of the “pentagram of Venus”. Many events in this cycle were associated with evil and wars were sometimes coordinated to coincide with the phases of the cycle.
Al-Khwârizmî, known as Algorismus, mathematician, geographer and astronomer of Persian origin, established in the ninth-century astronomical tables based on Hindu and Greek astronomy. He studied the position and visibility of the Moon and its eclipses, the Sun and the five planets visible to the naked eye. He is the first of a long series of Arab scientists.
In the eleventh century, the Persian astronomer Avicenna claims to have observed a transit of Venus, which will be a confirmation of Ptolemy’s theory for later astronomers. In the twelfth century, the Andalusian astronomer Ibn Bajjah observed “two planets like black spots on the face of the Sun” and Averroes states that the nephew of Sa’d ibn Mu’adh had witnessed a simultaneous transit of Venus and Mercury, announcing that he had calculated their trajectories and that they were in conjunction at that time to support his thesis; this observation will then be quoted by Nicolaus Copernicus in “Des révolutions des sphères célestes”. Qutb al-Din Shirazi, an astronomer of the school of Maragha, also considers in the thirteenth century these observations as transits of Venus and Mercury.
In reality, there was no transit of Venus during Ibn Bajjah’s lifetime and the transits of two planets could not have been simultaneous as described by Averroes. Also, if Avicenna did not note the day on which he would have observed a transit and if there was indeed a transit during his lifetime (the 24 May 1032, five years before his death), it could not be visible to him because of his geographical position.
In general, doubts have been raised by more recent astronomers about the observation of transits by medieval Arab astronomers, these having been potentially confused with sunspots. Thus, any observation of a transit of Venus before telescopes remains speculative.
Telescope research from the seventeenth century
The Italian physicist Galileo invented the telescope in 1609. In May 1610, he used it to observe Venus and found that the planet had phases, like the Moon. He notes that it shows a semi-illuminated phase when it is at the height of its elongation and that it appears as a crescent or a complete phase when it is closest to the Sun in the sky. Galileo deduced that Venus orbited the Sun, which was one of the first observations that clearly contradicted Claudius Ptolemy’s geocentric model according to which the Solar System was concentric and Earth-centered.
The transit of Venus in 1639 was accurately predicted by Jeremiah Horrocks and then observed by him and his friend, William Crabtree, in their respective homes, on December 4, 1639 (i.e. November 24 according to the Julian calendar used at that time). If we consider the observations of Arab astronomers as disputed, they are therefore the first humans to have observed a transit of Venus.
In 1645, a first observation of a supposed satellite of Venus was made by Francesco Fontana, later called Neith. Other observations will be made during the next two centuries (including those of Giovanni Cassini or Joseph-Louis Lagrange), but the existence of Neith is finally refuted at the end of the nineteenth century and observations attributed to errors or optical illusions.
Around 1666, Cassini estimated the rotation period of Venus at just over 23 hours, without being able to identify whether it was really a rotation or a libration. This error compared to the real value now known is due in particular to the marks of motion on the surface of the planet created by its dense atmosphere, whose existence was not known at the time.
Around 1726, Francesco Bianchini observed, or believed to observe, with a particularly powerful telescope, spots on the surface of the planet indicating expanses similar to the lunar seas. He thus realizes the first planisphere of Venus.
For the transits of Venus of 1761 and 1769, large expeditions were organized around the world to make observations to measure the astronomical unit using the parallax method. Names such as those of James Cook and Guillaume Le Gentil remain associated with these expeditions. However, the results of the measurement of DU made in 1771 by Jérôme de Lalande are disappointing because of the poor quality of the observations.
The atmosphere of Venus was also discovered in 1761 by the Russian polymath Mikhail Lomonosov, then observed in 1792 by the German astronomer Johann Schröter. Schröter discovers that when the planet is a thin crescent, its points extend more than 180°; He, therefore, assumes that this is due to the effect of the scattering of sunlight in a dense atmosphere. Later, American astronomer Chester Lyman observed a complete ring around the planet while it was in inferior conjunction, providing further evidence of an atmosphere.
New expeditions were organized for the transits of 1874 and 1882, resulting in better approximations of the DU, studies of the Venusian atmosphere and the oldest known film experiment: the Transit of Venus by the French astronomer Jules Janssen.
The atmosphere, which had previously complicated efforts to determine a rotation period of observers such as Cassini and Schröter, was taken into account in 1890 by Giovanni Schiaparelli and others who then opted for a rotation period of about 225 days, which would then have corresponded to a synchronous rotation with the Sun.
Use of new tools from the twentieth century
Little was discovered on the planet until the twentieth century. Its almost featureless disc giving no indication of its surface due to the thick atmosphere, it was only with the development of spectroscopic, radar and ultraviolet observations that more information was obtained.
Spectroscopic observations in the 1900s gave the first more accurate clues about the Venusian rotation. Vesto Slipher tries to measure the Doppler shift of light from Venus, but finds that he cannot detect any rotation. He deduces that the planet must have a much longer rotation period than previously thought.
The first ultraviolet observations were made in the 1920s, when Frank E. Ross notes that ultraviolet photographs reveal details absent in visible and infrared radiation. He suggests that this is due to a dense, yellow lower atmosphere with high clouds of cirrus clouds.
Later work in the 1950s shows that rotation is retrograde. Also, radar observations of Venus were made for the first time in the 1960s and provided the first measurements of the rotation period, which were then already close to the exact value known sixty years later. It is also the radio observation that indicates in 1958, well before the landing of the Venera 7 probe in 1970, that the temperature of the ground of the planet is of the order of 500 ° C.
In the 1970s, radar observations revealed details of the Venusian surface for the first time. Pulses of radio waves are broadcast across the planet using the 300-meter radio telescope at the Arecibo Observatory and the echoes reveal two highly reflective regions, designated Alpha Regio and Beta Regio. Observations also reveal a bright region attributed to a mountain, which is called Maxwell Montes. These three characteristics are now the only ones on Venus not to have a female name because they were named before the standardization of the International Astronomical Union.
Venus exploration
Space probe exploration of Venus begins in the early years 1960, shortly after the first artificial satellite was sent into orbit, Sputnik 1. About twenty of them have since visited the planet, whether for simple flybys, for longer stays in orbit around Venus, or to drop observation modules in the atmosphere and on the surface of Venus. Until the 2000s, the exploration of this planet was carried out solely by the Soviet Union and the United States.
The first mission to send a space probe to Venus, and generally to a planet other than Earth, began with the Soviet program Venera (“Venus” in English) in 1961. However, it was the United States that had the first success with the Mariner 2 mission on December 14, 1962, becoming the first successful interplanetary mission in history, passing 34.833 km above the surface of Venus and collecting data on the planet’s atmosphere and its surface temperature estimated at nearly 700K (427 °C). The probe does not detect a magnetic field in the vicinity of the planet and highlights the virtual absence of water in the Venusian atmosphere. The information sent by Mariner 2 complements radar observations made from the ground the same year, including at the Goldstone Observatory in California, which made it possible to estimate the rotation period of the planet, unknown until then.
In October 1967, the Soviet probe Venera 4 successfully enters the Venusian atmosphere and performs experiments. The probe shows that the surface temperature is warmer than Mariner 2 had calculated (nearly 500°C), determines that the atmosphere is 95% carbon dioxide, and discovers that Venus’ atmosphere is considerably denser than the spacecraft’s designers had predicted. The Venera 4 probe manages to launch a capsule towards the Venusian soil, and it transmits data on the composition of the Venusian atmosphere up to an altitude of 24 km. In parallel, the Americans launched Mariner 5 whose data would be analyzed jointly with those of Venera 4 by a Soviet-American scientific team in a series of symposia during the following year, which was the first example of space cooperation in the midst of the Cold War.
In 1974, Mariner 10 transited Venus during a gravitational assist maneuver that allowed it to move towards Mercury. The probe takes ultraviolet photographs of clouds during the flyby, revealing very high wind speeds in the Venusian atmosphere.
In 1975, the Soviet Venera 9 and 10 landers transmitted the first images of the surface of Venus, which were then in black and white. Venera 9 becomes the first probe of humanity to land on a planet other than Earth, and the first to transmit images of its surface. In March 1982, the first color images of the surface were obtained by the Soviet Venera 13 and 14 landers, launched a few days apart.
NASA obtained additional data in 1978 with the Pioneer Venus project, which included two separate missions: Pioneer Venus Orbiter and Pioneer Venus Multiprobe. The Soviet Venera program realizes in October 1983, when Venera probes 15 and 16 are placed in orbit, a detailed mapping of 25% of the terrain of Venus (from the north pole to 30° north latitude).
Venus is then regularly flown by to perform gravitational assistance maneuvers, notably by the Soviet probes Vega 1 and Vega 2 (1985), which take advantage of their passage around the planet to each drop an atmospheric balloon and a lander, before heading towards Halley’s Comet. However, no landers reached the surface, their parachute having been torn off by the strong winds of the Venusian atmosphere.
Subsequently, Galileo (1990) performs the same type of maneuver before going to Jupiter, as does Cassini – Huygens (1998) before going to Saturn and MESSENGER (2006) before going to Mercury. During its flyby, the Galileo probe makes near-infrared observations.
In orbit for four years around Venus, between 1990 And 1994, the Magellan probe performs a complete and very precise mapping (with a horizontal resolution of less than 100 m) of the surface of the planet. The spacecraft uses radar, the only instrument capable of piercing the thick atmosphere of Venus. An elevation reading is also carried out. This detailed mapping shows a remarkably young soil geologically speaking (on the order of 500 million years), the presence of thousands of volcanoes and an absence of plate tectonics as we know it on Earth but new analyses suggest that the surface is divided into boulders, “softened” by the intense heat of the environment and seem to move between them like the blocks of ice of the Earth’s pack ice.
The European Space Agency’s Venus Express probe (built in cooperation with Roscosmos) is launched in November 2005 and observes Venus from April 2006 until December 16, 2014. It made several important discoveries including possible recent volcanic activity, the slowing down of its rotation speed and the presence of a “magnetic tail”.
In 2007, a European mission Venus Entry Probe is planned to allow the exploration in situ of the Venusian atmosphere thanks to a balloon sailing at an altitude of 55 km, but it finally does not succeed.
In 2014, NASA researchers present the High Altitude Venus Operational Concept project, which aims to establish a human colony installed in airships at an altitude of 50 kilometers where the temperature is only 75 °C and the pressure close to that of the Earth. At the end of the year 1960, NASA had previously studied the possibility of using elements of the Apollo program to perform a manned flyby of Venus with a crew of three astronauts who would have made the round trip in about a year.
In 2016, NASA’s Institute for Advanced Concepts program began studying a rover, the Automaton Rover for Extreme Environments, designed to survive for a long time in the environmental conditions of Venus. It would be controlled by a mechanical computer and powered by wind energy.
Since 2016, a JAXA probe, Akatsuki, has been in a highly elliptical orbit around Venus. Launched in 2010 but arriving five years late due to a thruster failure during its initial insertion, it is the only probe in orbit around Venus in 2020. It aims to better understand what has led the planet to its current state, including its greenhouse effect. The craft made it possible to discover the presence, at 64 km altitude, of a gravity wave 10,000 km long and 65 km wide, stationary relative to the ground and able to last several days (unlike gravity waves on Earth which disappear very quickly). Akatsuki also took infrared pictures of the night side of Venus.
The 19 October 2018, the European BepiColombo probe, built in partnership with Japan’s JAXA, takes off towards the planet Mercury. During its journey, it will carry out two flybys of the planet Venus, during which it will carry out several experiments, serving, in particular, to test the instruments of the probe before its arrival around Mercury in 2025.
Habitability
Living conditions on Venus
Speculation about the existence of life on Venus has diminished dramatically since the early 1960s, when spacecraft began studying the planet and it became clear that conditions on Venus are far more hostile than those on Earth.
With Venus having surface temperatures of nearly 462°C with an atmospheric pressure 90 times higher than that of Earth, the extreme impact of the greenhouse effect makes water-based life as currently known unlikely.
Some scientists have hypothesized the existence of thermoacidophilic extremophile microorganisms in the acidic upper layers at low temperatures of the Venusian atmosphere. Also, in August 2019, astronomers have reported that the new long-term pattern of absorbance and albedo change in the atmosphere of the planet Venus is caused by “unknown absorbers,” which may be chemicals or even large colonies of microorganisms high up in the planet’s atmosphere.
In September 2020, the Atacama Large Millimeter/Submillimeter Array and the James Clerk Maxwell Telescope observed the signature of gaseous phosphine in the spectrum of Venus’ atmosphere, in the absence of known natural abiotic mechanisms for producing sufficient quantities of this molecule on a terrestrial planet. However, the article in Nature remains cautious: “Questions of why hypothetical organisms on Venus might make PH3 are also highly speculative”. NASA as well as various scientific journals call for caution on phosphine detection results as well as its potential origins. In November, the observation itself was disputed, in particular, because of errors in the calibration of the telescope.
Methods of colonization and Exploration
A permanent presence on Venus, as well as on Mars, would be a new step in the conquest of space. Also, different methods of colonization are being considered or have been.
The atmospheric pressure and temperature at fifty kilometers above the surface are similar to those of the Earth’s surface. This led to proposals to use aerostats (balloons lighter than the atmosphere) for initial exploration and, eventually, for permanent “floating cities” in the Venusian atmosphere. Among the many engineering challenges are the dangerous amounts of sulfuric acid at these heights. This approach has been proposed by NASA as part of its High Altitude Venus Operational Concept project.
Another form of colonization on Venus would be its terraforming. Terraforming of Venus would consist in making it habitable for humans, and thus making surface conditions less hostile. Thus, it would be necessary to lower its surface temperature, remove excess carbon dioxide from the atmosphere and accelerate its rotation period in order to achieve a day/night cycle closer to that known on Earth.
Venus in culture
Mythology
Since Venus is a main feature of the night sky, it has gained importance in mythology, astrology and fiction throughout history and in different cultures. Thus, the planet owes its name to the goddess Venus, the goddess of love in Roman mythology (assimilated to the Aphrodite of Greek mythology). Hence also comes the name of the fifth day of the week: Friday (from veneris diem, in Latin, for “day of Venus”).
In Mesopotamian mythology, Ishtar, the goddess of love, is associated with the planet Venus. One of the symbols of the goddess, the eight-pointed star, represents her as the morning or evening star. Moreover, the movements of Ishtar in the myths associated with it correspond to the movements of the planet Venus in the sky.
Christians, using the Roman name Lucifer (“bearer of light”) to designate the “morning star”, associate the “fall” of the planet into the sky with that of an angel. This eventually resulted in the figure of the fallen angel Lucifer.
In Chinese, the planet is called Jīn-xīng (金星), the golden planet of the metallic element. In India, it is called Shukra Graha (“the planet Shukra”), after the deity Shukra and is used in Indian astrology. This name means “clear, pure” or “brightness, clarity” in Sanskrit. Modern Chinese, Japanese, and Korean cultures literally refer to the planet as the “metal star” (金星), based on the five elements (Wuxing). Ancient Japan also refers to the planet Venus as Taihaku, this name has since been reassigned to an asteroid by astronomers: 4407 Taihaku.
Contemporary culture
In Western popular culture, the planet Venus is nicknamed the “Shepherd’s Star” because it can be easily visible in the morning sky (east), before dawn, or in the evening sky (west), after dusk. In modern times, the term “star” is inappropriate because it is known that it is a planet, but for the ancients, it was one of the five so-called “wandering” stars. It was given this name because herders in ancient times took it into account when going to or from pastures. Among the Dogon, a contemporary people of Mali renowned for their cosmogony, it can also be called enegirim tolo, for “shepherd’s star”. Moreover, their cosmogony mentions the existence of a natural satellite around Venus.
The Shepherd’s Star is sometimes confused with the Star of the Magi, or the Star of Bethlehem, although they are different stars. The latter is sometimes mentioned as having been a nova, supernova or Halley’s comet, these hypotheses having been set aside because none of these phenomena took place during the reign of Herod. The current explanation is that the intense light was produced by a conjunction between Jupiter and Saturn.
Classical poets such as Homer, Sappho, Ovid and Virgil spoke of the star and its light, and then pre-romantic and romantic poets such as William Blake, Robert Frost, Letitia Elizabeth Landon, Alfred Lord Tennyson, and William Wordsworth wrote odes about the planet.
Gustav Holst dedicated the second movement of his symphonic poem The Planets (1918) to Venus, where she embodied peace. More recently, Alain Bashung, for his album Bleu Pétrole, composed the song Vénus, dedicated to the planet and its role as a guide.
In painting, the most famous representation of Venus remains that of Vincent Van Gogh in The Starry Night, seen from the room of his asylum of the monastery of Saint-Paul-de-Mausole in Saint-Rémy-de-Provence in May 1889. A study of the sky in the spring of 1889 confirms that it is indeed Venus, surrounded by white at the bottom right of the great cypress. The painter also explicitly indicates the presence of the celestial object in his letters. The planet is also visible in his paintings Road with a Cypress and a Star and The White House, at Night, made in 1890 and shortly before the artist’s death.
Modern fiction
With the invention of the telescope, the idea that Venus was a physical world and a possible destination began to take shape.
Also, it is represented in fiction since the nineteenth century. The strong Venusian cloud cover also leaves science fiction writers free to speculate about living conditions on its surface, especially since early observations have shown that not only is its size similar to that of Earth but it has a substantial atmosphere. Closer to the Sun than Earth, the planet is often described as hotter, but still habitable by humans the writers then imagine extraterrestrials they call the Venusians.
The genus reached its peak between the 1930s and 1950s, at a time when science had revealed some aspects of Venus, but not yet the harsh reality of its surface conditions; we can mention in particular Dans les murs d’Eryx by H.P. Lovecraft in 1939, Parelandra de C. S. Lewis in 1943 and Isaac Asimov’s The Oceans of Venus in 1954. However, after the first robotic exploration missions, the results show that no form of life is possible on the surface. This puts an end to this particular genre based on the hope of an inhabited Venus.
Also, as scientific knowledge of Venus has advanced, science fiction authors are changing the themes addressed. Thus, works conjecturing human attempts to terraform Venus have developed, as in Pamela Sargent’s diptych (Venus of Dreams and Venus of Shadows in 1986 and 1988). An approach of floating cities in the planet’s thick atmosphere in order to experience milder temperatures is also discussed in Geoffrey Landis’ The Sultan of the Clouds (2010). Four years later, NASA researchers proposed a similar project, the High Altitude Venus Operational Concept, to establish a human colony in airships at an altitude of 50 kilometers on Venus.
Symbolism and vocabulary
The astronomical symbol of Venus is the same as that used in biology for the female sex: a circle with a cross pointing downwards. It also corresponds to the ancient alchemical symbol of copper. In modern times, it is still used as an astronomical symbol for Venus, although its use is discouraged by the International Astronomical Union.
The erroneous idea that the symbol represents the mirror of the goddess was introduced by Joseph Justus Scaliger at the end of the sixteenth century. He also invokes the fact that copper was used to make ancient mirrors, thus making the link with the alchemical symbol. In the early seventeenth century, Claude Saumaise established that the symbol actually derives from the first letter of the Greek name of the planet Phōsphoros (Φωσφόρος), like the symbols of the other planets.
Kythera being a Homeric epiclesis of Aphrodite, the adjective “Kytherian” or “Kytherean” is sometimes used in astronomy (especially in asteroid Cytheroner) or science fiction (Kytherians, a race from Star Trek). Also, the adjective “Venusian” is commonly used to define the characteristics of Venus instead of “venereal”, which has taken on a medically induced pejorative connotation as a synonym for sexually transmitted infection.
References (sources)
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