page_title: Geology of solar terrestrial planets
section_title: Venus
hierarchical_section_title: Geology of solar terrestrial planets / Surface geology of inner solar planets / Venus
caption_attribution_description: English: Magellan radar image of the "crater farm", showing the craters (clockwise from top left) Danilova, Aglaonice, and Saskia centered at 27 S, 339 E. Aglaonice is 65 km in diameter. This image is from C1-MIDR 30S333, CD-ROM MG_0002.
context_page_description: The geology of solar terrestrial planets mainly deals with the geological aspects of the four terrestrial planets of the Solar System – Mercury, Venus, Earth, and Mars – and one terrestrial dwarf planet: Ceres. Earth is the only terrestrial planet known to have an active hydrosphere.
  Terrestrial planets are substantially different from the giant planets, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets have a compact, rocky surfaces, and Venus, Earth, and Mars each also have an atmosphere. Their size, radius, and density are all similar.
  Terrestrial planets have numerous similarities to plutoids, which also have a solid surface, but are primarily composed of icy materials. During the formation of the Solar System, there were probably many more, but they have all merged with or been destroyed by the four remaining worlds in the solar nebula.
  The terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks a substantial iron core.
context_section_description: The surface of Venus is comparatively very flat. When 93% of the topography was mapped by Pioneer Venus, scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 kilometres (8 mi), while on the Earth the distance from the basins to the Himalayas is about 20 kilometres (12.4 mi).
  According to the data of the altimeters of the Pioneer, nearly 51% of the surface is found located within 500 metres (1,640 ft) of the median radius of 6,052 km (3760 mi); only 2% of the surface is located at greater elevations than 2 kilometres (1 mi) from the median radius.
  Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved impact craters has been utilized as a dating method to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). Dates derived are the dominantly in the range ~500 Mya–750Mya, although ages of up to ~1.2 Gya have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent. There are almost 1,000 impact craters on Venus, more or less evenly distributed across its 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. 
  Crater counts give an important estimate for the age of the surface of a planet. Over time, bodies in the Solar System are randomly impacted, so the more craters a surface has, the older it is. 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 agree: there are very few impact craters with a diameter less than 30 kilometres (19 mi), and data from Magellan show an absence of any craters less than 2 kilometres (1 mi) in diameter. However, there are also fewer of the large craters, and those appear relatively young; they are rarely filled with lava, showing that they happened after volcanic activity in the area, and radar shows that they are rough and have not had time to be eroded down.
  Much of Venus' surface appears to have been shaped by volcanic activity. Overall, Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over 100 kilometres (62 mi) across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, while Venus' surface is estimated to be about 500 million years old.
  Venusian craters range from 3 kilometres (2 mi) to 280 kilometres (174 mi) in diameter. There are no craters smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater.
caption: Danilova, Aglaonice and Saskja craters
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