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 470 °C (878 °F) 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 13 kilometres (8.1 mi), 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 500 meters (1,600 feet) of the median radius of 6,052 km (3,761 mi); only 2% of the surface is located at elevations greater than 2 kilometres (1.2 mi) 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 1 km (0.62 mi) 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
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