The geology of Venus is the scientific study of the surface, crust, and interior of the planet Venus. Within the Solar System, it is the one nearest to Earth and most like it in terms of mass, but has no intrinsic magnetic field or recognizable plate tectonics. About 75% of the surface is composed of bare rock, predominantly volcanic bedrock, some with thin and patchy layers of regolith. This is in marked contrast with Earth, the Moon, and Mars. Some impact craters are present, but the vast majority of the surface is uncratered. This is due in part to the thickness of the Venusian atmosphere disrupting small impactors before they strike the ground, but the paucity of large craters may be due to volcanic re-surfacing, possibly of a catastrophic nature. Volcanism appears to be the dominant agent of geological change on Venus. Some of the volcanic landforms appear to be unique to the planet, such as arachnoids and pancake domes. There are shield and composite volcanoes similar to those found on Earth, although these volcanoes are significantly shorter than those found on Earth or Mars. Given that Venus has approximately the same size, density, and composition as Earth, it is plausible that volcanism may be continuing on the planet today, as demonstrated by recent studies.
Most of the Venusian surface is relatively flat; it is divided into three general topographic units: lowlands, highlands, and plains. In the early days of radar observation the highlands drew comparisons to the continents of Earth, but modern research has shown that this is superficial and the absence of plate tectonics makes this comparison misleading. Tectonic features are present to a limited extent, including linear "deformation belts" composed of folds and faults. These may be caused by mantle convection. Many of the tectonic features such as tesserae (large regions of highly deformed terrain, folded and fractured in two or three dimensions), and arachnoids (those features resembling a spider's web) are associated with volcanism.
Eolian landforms are not widespread on the planet's surface, but there is considerable evidence the planet's atmosphere causes the chemical weathering of rock, especially at high elevations. The planet is remarkably dry, with only a chemical trace of water vapor (20 ppm) in the Venusian atmosphere. No landforms indicative of past water or ice are visible in radar images of the surface. The atmosphere shows isotopic evidence of having been stripped of volatile elements by off-gassing and solar wind erosion over time, implying the possibility that Venus may have had liquid water at some point in the distant past; no direct evidence for this has been found. Much speculation about the geological history of Venus continues today.
The surface of Venus is not easily accessible because of the extreme conditions and permanent cloud cover, a thick atmosphere of over 90 bar and temperatures reaching severely limit the lifespan of any probe to land on the surface. Much of what is known about it stems from orbital radar observations, because the surface is permanently obscured in visible wavelengths by cloud cover. In addition, a number of landers have returned data from the surface, including images from the Venera probes.
Studies reported in October 2023 suggest for the first time that Venus may have had plate tectonics during ancient times and, as a result, may have had a more habitable environment, possibly once capable of harboring life forms.
Topography
The surface of Venus is comparatively flat. When 93% of the topography was mapped by Pioneer Venus Orbiter, scientists found that the total distance from the lowest point to the highest point on the entire surface was about , about the same as the vertical distance between the Earth's ocean floor and the higher summits of the Himalayas. This similarity is to be expected as the maximum attainable elevation contrasts on a planet are largely dictated by the strength of the planet's gravity and the mechanical strength of its lithosphere, these are similar for Earth and Venus.
According to data from the Pioneer Venus Orbiter altimeters, nearly 51% of the surface is located within of the median radius of ; only 2% of the surface is located at elevations greater than from the median radius. This gives Venus a very sharp hypsometry.
The altimetry experiment of Magellan confirmed the general character of the landscape. According to the Magellan data, 80% of the topography is within of the median radius. The most important elevations are in the mountain chains that surround Lakshmi Planum: Maxwell Montes (11 km, 6.8 mi), Akna Montes (7 km, 4.3 mi) and Freya Montes (7 km, 4.3 mi). Despite the relatively flat landscape of Venus, the altimetry data also found large inclined plains. Such is the case on the southwest side of Maxwell Montes, which in some parts seems to be inclined some 45°. Inclinations of 30° were registered in Danu Montes and Themis Regio.
Highlands
Highlands cover about 10% of the planet's surface, with highlands being defined as areas greater than in elevation. The largest provinces of highlands are Aphrodite Terra, Ishtar Terra, and Lada Terra, as well as the regions Beta Regio, Phoebe Regio and Themis Regio. The regions Alpha Regio, Bell Regio, Eistla Regio and Tholus Regio are smaller regions of highlands.
Some of the terrain in these areas is particularly efficient at reflecting radar signals, appearing bright in synthetic aperture radar imagery. This is possibly analogous to snow lines on Earth and is likely related to temperatures and pressures there being lower than in the other provinces due to the higher elevation, which allows for distinct mineralogy to occur. It is thought that high-elevation rock formations may contain or be coated by minerals that have high dielectric constants. The high dielectric minerals would be stable at the ambient temperatures in the highlands, but not on the plains that comprise the rest of the planet's surface. Pyrite, an iron sulfide, matches these criteria and is widely suspected as a possible cause; it would be produced by chemical weathering of the volcanic highlands after long-term exposure to the sulfur-bearing Venusian atmosphere. The presence of pyrite on Venus has been contested, with atmospheric modelling showing that it might not be stable under Venusian atmospheric conditions. Other hypotheses have been put forward to explain the higher radar reflectivity in the highlands, including the presence of a ferroelectric material whose dielectric constant changes with temperature (with Venus having a changing temperature gradient with elevation). It has been observed that the character of the radar-bright highlands is not consistent across the surface of Venus. For example, Maxwell Montes shows the sharp, snow line-like change in reflectivity that is consistent with a change in mineralogy, whereas Ovda Regio shows a more gradual brightening upwards trend. The brightening upwards trend on Ovda Regio is consistent with a ferroelectric signature, and has been suggested to indicate the presence of chlorapatite.
Deposition plains Deposition plains have elevations averaging 0 to 2 km and cover more than half of the planet's surface.
Lowlands The rest of the surface is lowlands and generally lies below zero elevation. Radar reflectivity data suggest that at a centimeter scale these areas are smooth, potentially as a result of sedimentary accumulation over longer timespans.
Surface observations Ten spacecraft have successfully landed on Venus and returned data; all were flown by the Soviet Union. Venera 9, 10, 13, and 14 had cameras and returned images of regolith and rock. Spectrophotometry results showed that these four missions kicked up dust clouds on landing, which means that some of the dust particles must be smaller than about 0.02 mm. The rocks at all four sites showed fine layers, some layers were more reflective than others. Experiments on rocks at the Venera 13 and 14 sites found that they were porous and easily crushed (bearing maximum loads of 0.3 to 1 MPa) these rocks may be weakly lithified sediments or volcanic tuff. Spectrometry found that the surface materials at the Venera 9, 10, 14 and Vega 1 and 2 landing had chemical compositions similar to tholeiitic basalts, while the Venera 8 and 13 sites chemically resembled alkaline basalts.
Impact craters and age estimates of the surface
Earth-based radar surveys made it possible to identify some topographic patterns related to craters, and the Venera 15 and Venera 16 probes identified almost 150 such features of probable impact origin. Global coverage from Magellan subsequently made it possible to identify nearly 900 impact craters.
Compared to Mercury, the Moon and other such bodies, Venus has very few craters. In part, this is because Venus's dense atmosphere burns up smaller meteorites before they hit the surface. The Venera and Magellan data are in agreement: there are very few impact craters with a diameter less than , and data from Magellan show an absence of any craters less than in diameter. The small craters are irregular and appear in groups, thus pointing to the deceleration and the breakup of impactors. However, there are also fewer of the large craters, and those appear relatively young; they are rarely filled with lava, showing that they were formed after volcanic activity in the area ceased, and radar data indicates that they are rough and have not had time to be eroded down.
Compared to the situation on bodies such as the Moon, it is more difficult to determine the ages of different areas of the surface on Venus, on the basis of crater counts, due to the small number of craters at hand. However, the surface characteristics are consistent with a completely random distribution, implying that the surface of the entire planet is roughly the same age, or at least that very large areas are not very different in age from the average.
Taken together, this evidence suggests that the surface of Venus is geologically young. The impact crater distribution appears to be most consistent with models that call for a near-complete resurfacing of the planet. Subsequent to this period of extreme activity, process rates declined and impact craters began to accumulate, with only minor modification and resurfacing since.
Global resurfacing event Age estimates based on crater counts indicate a young surface, in contrast to the much older surfaces of Mars, Mercury, and the Moon. For this to be the case on a planet without crustal recycling by plate tectonics requires explanation. One hypothesis is that Venus underwent some sort of global resurfacing about 300–500 million years ago that erased the evidence of older craters.
One possible explanation for this event is that it is part of a cyclic process on Venus. On Earth, plate tectonics allows heat to escape from the mantle by advection, the transport of mantle material to the surface and the return of old crust to the mantle. But Venus has no evidence of plate tectonics, so this theory states that the interior of the planet heats up (due to the decay of radioactive elements) until material in the mantle is hot enough to force its way to the surface. The subsequent resurfacing event covers most or all of the planet with lava, until the mantle is cool enough for the process to start over.