Venus is the second planet in the Solar System, second closest to the Sun. It is only about 600 kilometers smaller than Earth in diameter and about 18,5% lighter (less massive). Venus also has a gravity of 8.87 m/s², just a little less than 1 m/s² weaker gravity than that of Earth. Being similar to Earth in appearance, size, mass and gravity, and also called ''Earth's sister'' might suggest equal atmospheric, temperature, habitability and life thriving conditions, but that couldn't be further from the truth.
Venus is also known as the morning star, because it is visible during sunsets and sunrises. It's atmosphere makes the planet shiny, the brightest of all the planets, and thus the third brightest object in the sky, after the Sun and the Moon. Romans named the planet Venus after the goddess of love, because of the planet's brightness. Did you know that Venus shows phases? Through a telescope, it can look like a tiny, featureless waning or waxing moon. At this writing (late February 2020), Venus is in a waning gibbous phases. In other words, telescopic observers are seeing it as more than half lighted, but less than full.
Venus is also the closest planet to Earth, closer than Mars is. Venus has lava and magma on it's surface because of volcanic activity and very high temperature. It's often called the cloudy planet, because it is full of clouds and it's surface can not be seen. Telescopes, technology, satellites and data show how Venus looks like.
To the naked eye, Venus appears as a white point of light brighter than any other planet or star (apart from the Sun). The planet's mean apparent magnitude is −4.14 with a standard deviation of 0.31. The brightest magnitude occurs during crescent phase about one month before or after inferior conjunction. Venus fades to about magnitude −3 when it is backlit by the Sun. The planet is bright enough to be seen in a clear midday sky and is more easily visible when the Sun is low on the horizon or setting. As an inferior planet, it always lies within about 47° of the Sun.
Venus "overtakes" Earth every 584 days as it orbits the Sun. As it does so, it changes from the "Evening Star", visible after sunset, to the "Morning Star", visible before sunrise. Although Mercury, the other inferior planet, reaches a maximum elongation of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported "unidentified flying object".
Human presence and life
Venus is the place of the very first interplanetary human presence, mediated through robotic missions, with the first successful landings on another planet and extraterrestrial body other than the Moon. Venus was at the beginning of the space age frequently visited by space probes until the 1990s. Currently in orbit is Akatsuki, and the Parker Solar Probe routinely uses Venus for gravity assist maneuvers. The reason for the decreased interest in Venus has been called surfacism, shifting space advocacy to other astronomical bodies, like Mars, with more accessible surface conditions, neglecting higher atmospheric altitudes.
The surface of Venus has been mapped from orbit by radar on the US Magellan mission. However, only a few locations on the surface have ever been visited, by the series of Venera missions of Soviet probes in the late 1970s. These probes returned the first – and so far only – images of the Venusian surface. Certainly surface conditions seem utterly inhospitable to any kind of life. The upper atmosphere is a different story however. Certain kinds of extremophile organisms already exist on Earth which could withstand the conditions in the atmosphere at the altitude at which HAVOC would fly. Species such as Acidianus infernus can be found in highly acidic volcanic lakes in Iceland and Italy. Airborne microbes have also been found to exist in Earth’s clouds. None of this proves that life exists in the Venusian atmosphere, but it is a possibility that could be investigated by a mission like HAVOC.
The current climatic conditions and composition of the atmosphere are the result of a runaway greenhouse effect (an extreme greenhouse effect that cannot be reversed), which transformed the planet from a hospitable Earth-like “twin” world in its early history. While we do not currently expect Earth to undergo a similarly extreme scenario, it does demonstrate that dramatic changes to a planetary climate can happen when certain physical conditions arise. By testing our current climate models using the extremes seen on Venus we can more accurately determine how various climate forcing effects can lead to dramatic changes. Venus therefore provides us with a means to test the extremes of our current climate modelling, with all the inherent implications for the ecological health of our own planet. We still know relatively little about Venus, despite it being our nearest planetary neighbour. Ultimately, learning how two very similar planets can have such different pasts will help us understand the evolution of the solar system and perhaps even that of other star systems.
Atmosphere and Climate
The atmosphere of Venus is drastically different from all of its planetary counterparts. Compared to Earth, Venus's atmosphere consists 96.5% of carbon dioxide, 3.4% of nitrogen, sulphuric acid clouds, which are found above the carbon dioxide layer, and tiny amounts of carbon monoxide. The existence of lightning in the atmosphere of Venus has been controversial. The atmosphere of Venus weighs 93 times more than Earth's.  This causes Venus to be highly inhospitable and toxic, and cause Venus to have acidic rains.The atmosphere of Venus is 96.5%, as mentioned previously. Above the dense CO2 layer are thick clouds, consisting mainly of sulfuric acid, which is formed by sulfur dioxide and water through a chemical reaction resulting in sulfuric acid hydrate. Additionally, the atmosphere consists of approximately 1% ferric chloride. Other possible constituents of the cloud particles are ferric sulfate, aluminium chloride and phosphoric anhydride. Clouds at different levels have different compositions and particle size distributions. These clouds reflect and scatter about 90% of the sunlight that falls on them back into space, and prevent visual observation of Venus' surface. The permanent cloud cover means that although Venus is closer than Earth to the Sun, it receives less sunlight on the ground. Strong 300 km/h (185 mph) winds at the cloud tops go around Venus about every four to five Earth days. Winds on Venus move at up to 60 times the speed of its rotation, whereas Earth's fastest winds are only 10–20% rotation speed. Despite the harsh conditions on the surface, the atmospheric pressure and temperature at about 50 km to 65 km above the surface of the planet is nearly the same as that of the Earth, making its upper atmosphere the most Earth-like area in the Solar System, even more so than the surface of Mars. Due to the similarity in pressure and temperature and the fact that breathable air (21% oxygen, 78% nitrogen) is a lifting gas on Venus in the same way that helium is a lifting gas on Earth, the upper atmosphere has been proposed as a location for both exploration and colonization. Recently, NASA has found possible biological evidence and potential life-forming compositions in the upper atmosphere of Venus. The pressure in the upper atmosphere of Venus is very similar to that of Earth and similar evidence of biological connections and potential micro-life also exist in the upper clouds of Venus, as NASA said. The temperature in this region is the lowest known on Venus, at 70 °C, high for humans but maybe not for micro-organisms, that likely float and glide on the clouds of Venus.
Unlike Earth, Venus lacks a magnetic field. Its ionosphere separates the atmosphere from outer space and the solar wind. This ionized layer excludes the solar magnetic field, giving Venus a distinct magnetic environment. This is considered Venus's induced magnetosphere. Lighter gases, including water vapor, are continuously blown away by the solar wind through the induced magneto-tail. It is speculated that the atmosphere of Venus up to around 4 billion years ago was more like that of the Earth with liquid water on the surface. A runaway greenhouse effect may have been caused by the evaporation of the surface water and subsequent rise of the levels of other greenhouse gases.
The atmosphere contains a range of compounds in small quantities, including some based on hydrogen, such as hydrogen chloride (HCl) and hydrogen fluoride (HF). There is carbon monoxide, water vapour and atomic oxygen as well. Hydrogen is in relatively short supply in the Venusian atmosphere. A large amount of the planet's hydrogen is theorised to have been lost to space, with the remainder being mostly bound up in sulfuric acid (H2SO4). The loss of significant amounts of hydrogen is proven by a very high D–H ratio measured in the Venusian atmosphere. The ratio is about 0.015–0.025, which is 100–150 times higher than the terrestrial value of 1.6×10−4. According to some measurements, in the upper atmosphere of Venus D/H ratio is 1.5 higher than in the bulk atmosphere.
In June 2021, NASA selected the DAVINCI+ mission to send an atmospheric probe to Venus in the late 2020s. DAVINCI+ will measure the composition of Venus’ atmosphere to understand how it formed and evolved, as well as determine whether the planet ever had an ocean. The mission consists of a descent sphere that will plunge through the planet’s thick atmosphere, making measurements of noble gases and other elements to understand Venus’ climate change. This will be the first U.S.-led mission to Venus’ atmosphere since 1978.
In September 2020, it was announced that phosphine, a potential biomarker indicating the presence of life, had been detected in the atmosphere of Venus. No known abiotic source present on Venus could produce phosphine in the quantities detected. The re-analysis of Pioneer Venus data in 2020 has found part of chlorine and all of hydrogen sulfide spectral features are instead phosphine-related, meaning lower than thought concentration of chlorine and non-detection of hydrogen sulfide. In a preprint made available in October 2020, a re-analysis of archived infrared spectral measurements in 2015 did not reveal any phosphine in Venusian atmosphere, placing an upper limit for phosphine concentration at 5 parts per billion by volume—a quarter of the spectroscopic value reported in September). In late October 2020, the review of data processing used in original publication of September 2020, has revealed an interpolation error resulting in multiple spurious lines, including the spectral feature of phosphine. Re-analysis of data with the fixed algorithm either do not result in the detection of the phosphine or detected it with much lower concentration of 1ppb.
Missions to observe the atmosphere of Venus include the following. In December 2015, and to a lesser extent in April and May 2016, researchers working on Japan's Akatsuki mission observed bow shapes in the atmosphere of Venus. This was considered direct evidence of the existence of perhaps the largest stationary gravity waves in the solar system. In 2007, Venus Express discovered that a huge double atmospheric vortex exists at the south pole. Venus Express also discovered, in 2011, that an ozone layer exists high in the atmosphere of Venus. On 29 January 2013, ESA scientists reported that the ionosphere of Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions." In 2006–07, Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. According to these measurements, the lightning rate is at least half of that on Earth, however other instruments have not detected lightning at all. The origin of any lightning remains unclear, but could originate from the clouds or volcanoes. Expectadly, this has caused multiple problems and complications during missions, as some rovers can survive only a couple hours on Venus, with others last for only about 1 hour or less.
Magnetic field and Core
In 1967, Venera 4 found Venus' magnetic field to be much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo as in the Earth's core. Venus' small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. The lack of an intrinsic magnetic field at Venus was surprising, given that it is similar to Earth in size and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in Venus' core. 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, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This insulating effect would cause the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat from the core is reheating the crust.
One possibility is that Venus has no solid inner core, or that its core is not cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already completely solidified. The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.
The weak magnetosphere around Venus means that the solar wind is interacting directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by the dissociation of water molecules from ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape Venus' gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind could have led to the loss of most of Venus' water during the first billion years after it formed. However, the planet may have retained a dynamo for its first 2–3 billion years, so the water loss may have occurred more recently. The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the atmosphere 100 times compared to the rest of the solar system.
Geology and Geography
The surface of Venus has been heavily changed by volcanic activity. Due to the age of Venus' crust, the amount of volcanoes skyrockets, with over a hundred being 100km+ in diameter. Any craters located on Venus can only be 3 km or larger, as the atmosphere prohibits anything smaller than 3 km to crash into it.The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the 20th century. Venera landers in 1975 and 1982 returned images of a surface covered in sediment and relatively angular rocks. The surface was mapped in detail by Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate that there have been recent eruptions.
About 80% of the Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains. Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km (7 mi) above the Venusian average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area. Several lines of evidence point to ongoing volcanic activity on Venus. Sulfur dioxide concentrations in the atmosphere dropped by a factor of 10 between 1978 and 1986, jumped in 2006, and again declined 10-fold. This may mean that levels had been boosted several times by large volcanic eruptions. It has also been suggested that Venusian lightning (discussed below) could originate from volcanic activity (i.e. volcanic lightning). In January 2020, astronomers reported evidence that suggests that Venus is currently volcanically active.
Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes as Earth, and it has 167 large volcanoes that are over 100 km (60 mi) across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. This is not because Venus is more volcanically active than Earth, but because its crust is older and is not subject the same erosion process. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about a hundred million years, whereas the Venusian surface is estimated to be 300–600 million years old.
Several lines of evidence point to ongoing volcanic activity on Venus. Sulfur dioxide concentrations in the atmosphere dropped by a factor of 10 between 1978 and 1986, jumped in 2006, and again declined 10-fold. This may mean that levels had been boosted several times by large volcanic eruptions. It has also been suggested that Venusian lightning (discussed below) could originate from volcanic activity (i.e. volcanic lightning). In January 2020, astronomers reported evidence that suggests that Venus is currently volcanically active, specifically the detection of olivine, a volcanic product that would weather quickly on the planet's surface. In 2008 and 2009, the first direct evidence for ongoing volcanism was observed by Venus Express, in the form of four transient localized infrared hot spots within the rift zone Ganis Chasma, near the shield volcano Maat Mons. Three of the spots were observed in more than one successive orbit. These spots are thought to represent lava freshly released by volcanic eruptions. The actual temperatures are not known, because the size of the hot spots could not be measured, but are likely to have been in the 800–1,100 K (527–827 °C; 980–1,520 °F) range, relative to a normal temperature of 740 K (467 °C; 872 °F).
Sadly, though, we know close to little about the internal composition of Venus, we can only hope to gather somewhat correct data by comparing Venus to Earth, due to how similar it is in age and size, and hope that the data extrapulated is correct. Sometimes, we know this data has to be correct, for example, we know that the core of Venus is liquid, because it started to cool down at the same rate and time as Earth. The slightly smaller size of Venus means that pressures are 24% lower in its deep interior than Earth's. The predicted values for the moment of inertia based on planetary models suggest a core radius of 2,900–3,450 km. The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field. Instead, Venus may lose its internal heat in periodic major resurfacing events. It's is not clear how Venus will lose it's internal heat, but what is certain is that it will lose it quicker than Earth's, since Venus is less massive, so it's core is less pressured, causing the heat of Venus's interior to decay faster than that of Earth, so scientists, NASA, ESA and National Geographic estimate that the interior heat of Venus will have cooled down at around 3 billion years, shorter period of time than Earth's 3,5 billion years.