Editing Venus (section)

== Physical characteristics ==
[[File:Terrestrial planet size comp 2024.png|thumb|upright=1.3|Venus to scale among the [[Inner Solar System]] [[planetary-mass object]]s, arranged by the order of their orbits outward from the Sun (from left: [[Mercury (planet)|Mercury]], Venus,
[[Earth]], the [[Moon]], [[Mars]] and [[Ceres (dwarf planet)|Ceres]])]]
Venus is one of the four [[terrestrial planet]]s in the Solar System, meaning that it is a rocky body like Earth. It is similar to Earth in size and mass and is often described as Earth's "sister" or "twin".<ref name= "Lopes_Gregg_2004"/> Venus is very close to spherical due to its slow rotation.<ref name="Venus"/> It has a diameter of {{convert|12103.6|km|mi|abbr=on}}—only {{convert|638.4|km|mi|abbr=on}} less than Earth's—and its mass is 81.5% of Earth's, making it the third-smallest planet in the [[Solar System]]. Conditions on the surface of Venus differ radically from those on Earth because its dense [[atmosphere]] is 96.5% carbon dioxide, causing an intense [[greenhouse effect]], with most of the remaining 3.5% being [[nitrogen]].<ref name=Darling_Venus/> The surface pressure is {{convert|9.3|MPa|bar|lk=on|abbr=off}}, and the average surface temperature is {{convert|737|K|C F|abbr=on}}, above the [[Critical point (thermodynamics)|critical points]] of both major constituents and making the surface atmosphere a [[supercritical fluid]] of mainly [[supercritical carbon dioxide]] and some supercritical nitrogen.

=== Geography ===
{{Main|Geology of Venus|Geodynamics of Venus|Mapping of Venus|Surface features of Venus}}

[[File:2438_pioneer_venus_map_of_venus.jpg|thumb|upright=1.5|Color-coded elevation map, showing the elevated [[Planetary nomenclature#Terra|terrae]] "continents" in yellow and minor [[Surface features of Venus|features of Venus]]]]
[[File:Venus globe.jpg|thumb| Global view of the [[Mapping of Venus|surface of Venus]], created using data obtained primarily by synthetic aperture radar aboard NASA's 1989 [[Magellan (spacecraft)|Magellan]] mission.]]

The Venusian surface was a subject of speculation until some of its secrets were revealed by probes in the 20th century. ''[[Venera]]'' landers in 1975 and 1982 returned images of a surface covered in sediment and relatively angular rocks.<ref name=Mueller_2014/> The surface was mapped in detail by [[Magellan (spacecraft)|''Magellan'']] in 1990–91. There is evidence of extensive volcanism, and variations in the atmospheric [[sulphur dioxide]] may indicate that there are active volcanoes.<ref name=Esposito_1984/><ref name=Bullock_Grinspoon_2001/>

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.<ref name=Basilevsky_Head_1995/> Two [[highland continent|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 [[Babylon]]ian goddess of love, and is about the size of Australia. The [[Maxwell Montes]] mountain range lies on Ishtar Terra. Its peak is the highest point on Venus, {{convert|7|mi|km|order=flip|abbr=on}} above the Venusian average surface elevation.<ref name="planetology"/> The southern continent is called [[Aphrodite Terra]], after the [[Greek mythological]] 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.<ref name="Kaufmann"/>

There is recent evidence of [[lava]] flow on Venus (2024),<ref>National Geographic [https://www.nationalgeographic.com/science/article/venus-is-volcanically-alive (2024) Venus is volcanically alive]</ref> such as flows on [[Sif Mons]], a shield volcano, and on [[Niobe Planitia]], a flat plain.<ref>The New York Times [https://www.nytimes.com/2024/05/27/science/venus-volcanoes-lava.html (27 May 2024) Rivers of Lava on Venus Reveal a More Volcanically Active Planet]</ref> There are visible [[caldera]]s. The planet has few [[impact crater]]s, demonstrating that the surface is relatively young, at 300–600{{spaces}}million years old.<ref name="Nimmo98" /><ref name="Strom1994" /> Venus has some unique surface features in addition to the impact craters, mountains, and valleys commonly found on rocky planets. Among these are flat-topped volcanic features called "[[Farra (Venus)|farra]]", which look somewhat like pancakes and range in size from {{convert|20|to|50|km|mi|abbr=on}} across, and from {{convert|100|to|1000|m|ft|abbr=on}} high; radial, star-like fracture systems called "novae"; features with both radial and concentric fractures resembling spider webs, known as "[[arachnoid (astrogeology)|arachnoids]]"; and "coronae", circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.<ref name="Frankel"/>

[[File:Surface of Venus from Venera 13.jpg|thumb|upright=1.3|Surface panorama taken by Venera 13]]

Most [[List of geological features on Venus|Venusian surface features]] are named after historical and mythological women.<ref name=Batson_Russell_1991/> Exceptions are Maxwell Montes, named after [[James Clerk Maxwell]], and highland regions [[Alpha Regio]], [[Beta Regio]], and [[Ovda Regio]]. The last three features were named before the current system was adopted by the [[International Astronomical Union]], the body which oversees [[planetary nomenclature]].<ref name="jpl-magellan"/>

The longitude of physical features on Venus is expressed relative to its [[prime meridian]]. The original prime meridian passed through the radar-bright spot at the centre of the oval feature Eve, located south of Alpha Regio.<ref name="Davies_1994"/> After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater [[Ariadne (crater)|Ariadne]] on [[Sedna Planitia]].<ref name=Seidelmann_et_al_2007/><ref name="jpl-magellan2"/>

The stratigraphically oldest [[Tessera (Venus)|tessera terrains]] have consistently lower thermal emissivity than the surrounding basaltic plains measured by ''[[Venus Express]]'' and [[Magellan (spacecraft)|''Magellan'']], indicating a different, possibly a more [[felsic]], mineral assemblage.<ref name="Hashimoto_et_al_2008"/><ref name=Helbert_et_al_2008/> The mechanism to generate a large amount of felsic crust usually requires the presence of a water ocean and [[plate tectonics]], implying that habitable condition existed on early Venus, with large bodies of water at some point.<ref name="Petkowski Seager 2021">{{cite web | last1=Petkowski | first1=Janusz | last2=Seager | first2=Sara | title=Did Venus ever have oceans? | website=Venus Cloud Life – MIT | date=18 November 2021 | url=https://venuscloudlife.com/are-venus-cloud-layers-too-dry-for-life/ | access-date=13 April 2023 | archive-date=13 April 2023 | archive-url=https://web.archive.org/web/20230413112527/https://venuscloudlife.com/are-venus-cloud-layers-too-dry-for-life/ | url-status=live }}</ref> However, the nature of tessera terrains is far from certain.<ref name=Gilmore_et_al_2017/>

Studies reported in 2023 suggested for the first time that Venus may have had [[plate tectonics]] during ancient times and, as a result, may have had a more [[Planetary habitability|habitable environment]], possibly one capable of sustaining [[life forms|life]].<ref name="NYT-20231026">{{cite news |last=Chang |first=Kenneth |title=Billions of Years Ago, Venus May Have Had a Key Earthlike Feature – A new study makes the case that the solar system's hellish second planet once may have had plate tectonics that could have made it more hospitable to life. |url=https://www.nytimes.com/2023/10/26/science/venus-plate-tectonics-life.html |date=26 October 2023 |work=[[The New York Times]] |url-status=live |archive-url=https://archive.today/20231026181052/https://www.nytimes.com/2023/10/26/science/venus-plate-tectonics-life.html |archive-date=26 October 2023 |access-date=27 October 2023 }}</ref><ref name="NA-20231026">{{cite journal |author=Weller, Matthew B. |display-authors=et al. |title=Venus's atmospheric nitrogen explained by ancient plate tectonics |url=https://www.nature.com/articles/s41550-023-02102-w |date=26 October 2023 |journal=[[Nature Astronomy]] |volume=7 |issue=12 |pages=1436–1444 |doi=10.1038/s41550-023-02102-w |bibcode=2023NatAs...7.1436W |s2cid=264530764 |url-status=live |archive-url=https://archive.today/20231027132655/https://www.nature.com/articles/s41550-023-02102-w |archive-date=27 October 2023 |access-date=27 October 2023 |url-access=subscription }}</ref> Venus has gained interest as a case for research into the development of [[Earth analogue|Earth-like planets]] and [[planetary habitability|their habitability]].

==== Volcanism ====
{{Main|Volcanism on Venus}}
[[Image:PIA00084 Eistla region pancake volcanoes.jpg|thumb|left|upright=1.1|Radar mosaic of two [[pancake dome]]s in Venus's Eistla region—both {{Convert|65|km|mi|abbr=on}} wide and less than {{Convert|1|km|mi|abbr=on}} high]]

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 {{convert|100|km|mi|sigfig=1|abbr=on}} across. The only volcanic complex of this size on Earth is the [[Hawaii (island)|Big Island]] of Hawaii.<ref name="Frankel" />{{rp|154}} More than 85,000 volcanoes on Venus have been identified and mapped.<ref>{{cite web |title=A new catalog pinpoints volcanic cones in the best available surface images of Venus – those gathered 30 years ago by NASA's Magellan spacecraft. |url=https://skyandtelescope.org/astronomy-news/85000-volcanoes-mapped-on-venus |website=skyandtelescope.org |date=14 April 2023 |access-date=16 April 2023 |archive-date=16 April 2023 |archive-url=https://web.archive.org/web/20230416223821/https://skyandtelescope.org/astronomy-news/85000-volcanoes-mapped-on-venus/ |url-status=live }}</ref><ref>{{cite journal |last1=Hahn |first1=Rebecca M. |last2=Byrne |first2=Paul K. |title=A Morphological and Spatial Analysis of Volcanoes on Venus |journal=Journal of Geophysical Research: Planets |date=April 2023 |volume=128 |issue=4 |pages=e2023JE007753 |doi=10.1029/2023JE007753 |bibcode=2023JGRE..12807753H |s2cid=257745255 |quote=With the Magellan synthetic-aperture radar full-resolution radar map left- and right-look global mosaics at 75 m-per-pixel resolution, we developed a global catalogue of volcanoes on Venus that contains ~85,000 edifices, ~99% of which are <5 km in diameter. We find that Venus hosts far more volcanoes than previously mapped, and that although they are distributed across virtually the entire planet, size–frequency distribution analysis reveals a relative lack of edifices in the 20–100 km diameter range, which could be related to magma availability and eruption rate.}}</ref> This is not because Venus is more volcanically active than Earth, but because its crust is older and is not subject to the [[erosion]] processes active on Earth. Earth's [[oceanic crust]] is continually recycled by [[subduction]] at the boundaries of tectonic plates, and has an average age of about 100 million years,<ref name=Karttunen_et_al_2007/> whereas the Venusian surface is estimated to be 300–600{{spaces}}million years old.<ref name="Nimmo98" /><ref name="Frankel" />

Several lines of evidence point to ongoing [[volcanic]] activity on Venus. Sulfur dioxide concentrations in the upper atmosphere dropped by a factor of 10 between 1978 and 1986, jumped in 2006, and again declined 10-fold.<ref name="ESA_2012-12-03"/> This may mean that levels were boosted several times by large volcanic eruptions.<ref name=Glaze_1999/><ref name="Marcq2012"/> It has been suggested that Venusian lightning (discussed below) could originate from volcanic activity (i.e. [[volcanic lightning]]). In January 2020, astronomers reported evidence suggesting that Venus is currently volcanically active, specifically the detection of [[olivine]], a volcanic product that would weather quickly on the planet's surface.<ref name="NYT-20200109"/><ref name="SCI-20200103"/>

This massive volcanic activity is fuelled by a hot interior, which models say could be explained by energetic collisions when the planet was young, as well as [[radioactive decay]] as in the case of the earth. Impacts would have had significantly higher velocity than on Earth, both because Venus moves faster due to its closer proximity to the Sun and because high-eccentricity objects colliding with the planet would have high speeds.<ref>{{Cite web |url=https://www.sci.news/space/venus-volcanism-12114.html |title=Early, Energetic Collisions Could Have Fueled Venus Volcanism: Study {{!}} Sci.News |date=20 July 2023 |access-date=21 July 2023 |archive-date=21 July 2023 |archive-url=https://web.archive.org/web/20230721193015/https://www.sci.news/space/venus-volcanism-12114.html |url-status=live }}</ref>

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]],<ref name="USGS_Ganis_Chasma"/>{{refn|group=note|Misstated as "Ganiki Chasma" in the press release and scientific publication.<ref name = "Lakdawalla2015"/>}} 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.<ref name="Lakdawalla2015"/><ref name="ESA_2015-06-18"/> 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 {{convert|800|-|1100|K|C F}} range, relative to a normal temperature of {{convert|740|K|C F}}.<ref name="Shalygin2015"/> In 2023, scientists reexamined topographical images of the Maat Mons region taken by the ''[[Magellan (spacecraft)|Magellan]]'' orbiter. Using computer simulations, they determined that the topography had changed during an 8-month interval, and concluded that active volcanism was the cause.<ref>{{cite web | title=Why the Discovery of an Active Volcano on Venus Matters | last=Kluger | first=Jeffrey | url=https://time.com/6264160/why-volcanoes-on-venus-matter/ | publisher=[[Time (magazine)|Time]] | date=17 March 2023 | access-date=19 March 2023 | archive-date=19 March 2023 | archive-url=https://web.archive.org/web/20230319022005/https://time.com/6264160/why-volcanoes-on-venus-matter/ | url-status=live }}</ref>

==== Craters ====
[[File:PIA00103 Venus - 3-D Perspective View of Lavinia Planitia.jpg|thumb|upright=0.9|alt=The plains of Venus|[[Impact crater]]s on the surface of Venus (false-colour, [[3D projection]] image reconstructed from radar data)]]

There are almost a thousand impact craters on Venus, evenly distributed across its surface. On other cratered bodies, such as Earth and the Moon, craters show a range of states of degradation. On the Moon, degradation is caused by subsequent impacts, whereas on Earth it is caused by wind and rain erosion. On Venus, about 85% of the craters are in pristine condition. The number of craters, together with their well-preserved condition, indicates the planet underwent a global resurfacing event 300–600{{spaces}}million years ago,<ref name="Nimmo98" /><ref name="Strom1994"/> followed by a decay in volcanism.<ref name=Romeo_Turcotte_2018/> Whereas Earth's crust is in continuous motion, Venus is thought to 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 until they reach a critical level that weakens the crust. Then, over a period of about 100{{spaces}}million years, subduction occurs on an enormous scale, completely recycling the crust.<ref name="Frankel" />

Venusian craters range from {{convert|3|to|280|km|mi|0|abbr=on}} in diameter. No craters are smaller than 3{{spaces}}km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain [[kinetic energy]] are slowed so much by the atmosphere that they do not create an impact crater.<ref name=Herrick_Phillips_1993/> Incoming projectiles less than {{convert|50|m|ft|-1|abbr=on}} in diameter will fragment and burn up in the atmosphere before reaching the ground.<ref name=Morrison_Owens_2003/>

=== Internal structure ===
[[File:InteriorOfVenus.svg|thumb|upright=1.05|The [[Planetary differentiation|differentiated]] structure of Venus|alt=Spherical cross-section of Venus showing the different layers]]

Without data from [[reflection seismology]] or knowledge of its [[moment of inertia]], little direct information has been available about the internal structure and [[geochemistry]] of Venus.<ref name="goettel"/> The similarity in size and density between Venus and Earth suggests that they share a similar internal structure: a [[Planetary core|core]], [[Mantle (geology)|mantle]], and [[Crust (geology)|crust]]. Like that of Earth, the Venusian core is most likely at least partially liquid because the two planets have been cooling at about the same rate,<ref name=Faure_Mensing_2007/> although a completely solid core cannot be ruled out.<ref name=Dumoulin2017/> The slightly smaller size of Venus means pressures are 24% lower in its deep interior than Earth's.<ref name=Aitta_2016/> The predicted values for the moment of inertia based on planetary models suggest a core radius of 2,900–3,450&nbsp;km.<ref name=Dumoulin2017/> There is now an estimate of 3,500&nbsp;km from the [[moment of inertia]] based on the rate of [[axial precession]], measured between 2006 and 2020.<ref name=Margot_et_al_2021/><ref name="O'Callaghan_2021"/>

The crust of Venus is estimated to be 40 kilometers thick on average and at most 65 kilometers thick.<ref>{{Cite journal |last1=Semprich |first1=Julia |last2=Filiberto |first2=Justin |last3=Weller |first3=Matthew |last4=Gorce |first4=Jennifer |last5=Clark |first5=Nolan |date=2025-03-25 |title=Metamorphism of Venus as driver of crustal thickness and recycling |url=https://www.nature.com/articles/s41467-025-58324-1 |journal=Nature Communications |language=en |volume=16 |issue=1 |pages=2905 |doi=10.1038/s41467-025-58324-1 |pmid=40133342 |bibcode=2025NatCo..16.2905S |issn=2041-1723|pmc=11937330 }}</ref>

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]].<ref name=Nimmo_2002/> Instead, Venus may lose its internal heat in periodic major resurfacing events.<ref name="Nimmo98"/>

=== Magnetic field and core ===

In 1967, ''[[Venera 4]]'' found Venus's [[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]],<ref name=Eroshenko_et_al_1969/><ref name=Kivelson_Russell_1995/>{{Page needed|date=January 2023}} rather than by an internal [[dynamo theory|dynamo]] as in the Earth's [[Planetary core|core]]. [[Magnetosphere of Venus|Venus's small induced magnetosphere]] provides negligible protection to the atmosphere against [[solar radiation|solar]] and [[cosmic radiation]].

The lack of an intrinsic magnetic field on Venus was surprising, given that it is similar to Earth in size and was expected to contain a dynamo at its core. A dynamo requires three things: a [[Electrical conductor|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.<ref name=Luhmann_Russell_2006/><ref name=Stevenson_2003/> This implies that the dynamo is missing because of a lack of convection in Venus's 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 [[Thermal insulation|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.<ref name="nimmo02"/>

One possibility is that Venus has no solid inner core,<ref name=Konopliv_Yoder_1996/> 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 been completely solidified. The state of the core is highly dependent on the concentration of [[sulphur]], which is unknown at present.<ref name="nimmo02" />

Another possibility is that the absence of a large impact on Venus (''contra'' the Earth's "Moon-forming" impact) left the core of Venus stratified from the core's incremental formation, and without the forces to initiate/sustain convection, and thus a "geodynamo".<ref name="Jacobsen2017">{{cite journal | last1=Jacobson | first1=Seth A. | last2=Rubie | first2=David C. | last3=Hernlund | first3=John | last4=Morbidelli | first4=Alessandro | last5=Nakajima | first5=Miki | title=Formation, stratification, and mixing of the cores of Earth and Venus | journal=Earth and Planetary Science Letters | publisher=Elsevier BV | volume=474 | year=2017 | doi=10.1016/j.epsl.2017.06.023 | page=375| arxiv=1710.01770 | bibcode=2017E&PSL.474..375J | s2cid=119487513 }}</ref>

The weak magnetosphere around Venus means that the solar wind interacts directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by the [[Dissociation (chemistry)|dissociation]] of water molecules due to [[ultraviolet]] radiation. The solar wind then supplies energy that gives some of these ions sufficient speed to escape Venus's 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's water during the first billion years after it formed.<ref name="nature450_7170_629"/> However, the planet may have retained a dynamo for its first 2–3 billion years, so the water loss may have occurred more recently.<ref name="O'Rourke_et_al_2019"/> 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.<ref name=Donahue_et_al_1982/>