Geology of Venus
Much of the ground surface is exposed volcanic bedrock, some with thin and patchy layers of soil covering, in marked contrast with Earth, the Moon, and Mars.
[1] 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.
1 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.
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.
Radar reflectivity data suggest that at a centimeter scale these areas are smooth, as a result of gradation (accumulation of fine material eroded from the highlands).
[14] 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.
Subsequent to this period of extreme activity, process rates declined and impact craters began to accumulate, with only minor modification and resurfacing since.
One hypothesis is that Venus underwent some sort of global resurfacing about 300–500 million years ago that erased the evidence of older craters.
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.
These are geological features believed to be almost unique to Venus: huge, ring-shaped structures 100–300 kilometers (62–186 miles) across and rising hundreds of meters above the surface.
It is believed that they are formed when plumes of rising hot material in the mantle push the crust upwards into a dome shape, which then collapses in the centre as the molten lava cools and leaks out at the sides, leaving a crown-like structure: the corona.
The second type of volcanic activity is not radial or centralized; flood basalts cover wide expanses of the surface, similar to features such as the Deccan Traps on Earth.
[citation needed][note 4] These pancake dome volcanoes are fairly round features that are less than 1-kilometre (0.62 mi) in height and many times that in width.
These dikes form a symmetrical network around the central point where the lava emerged, where there may also be a depression caused by the collapse of the magma chamber.
Arachnoids are so named because they resemble a spider's web, featuring several concentric ovals surrounded by a complex network of radial fractures similar to those of a nova.
The active volcanism of Venus has generated chains of folded mountains, rift valleys, and terrain known as tesserae, a word meaning "floor tiles" in Greek.
The apparent absence of this layer on Venus suggests that the deformation of the Venusian surface must be explained by convective movements within the planet's mantle.
The effects of extensive tectonism are shown by the presence of normal faults, where the crust has sunk in one area relative to the surrounding rock, and superficial fractures.
Radar imaging shows that these types of deformation are concentrated in belts located in the equatorial zones and at high southern latitudes.
These belts are hundreds of kilometers wide and appear to interconnect across the whole of the planet, forming a global network associated with the distribution of volcanoes.
The rifts of Venus, formed by the expansion of the lithosphere, are groups of depressions tens to hundreds of meters wide and extending up to 1,000 km (620 mi) in length.
These highlands seem to be the result of enormous mantle plumes (rising currents of magma) which have caused elevation, fracturing, faulting, and volcanism.
The highest mountain chain on Venus, Maxwell Montes in Ishtar Terra, was formed by processes of compression, expansion, and lateral movement.
[7]: 1729 Since Venus is a terrestrial planet, it is presumed to have a core, made of semisolid iron and nickel with a radius of approximately 3,000 kilometres (1,900 mi).
Venus is expected to have an electrically conductive core of similar composition, and although its rotation period is very long (243.7 Earth days), simulations show that this is adequate to produce a dynamo.
Two planetary astronomers from the University of Wollongong in Australia, Dr Graeme Melville and Prof. Bill Zealey, researched these lava tubes, using data supplied by NASA, over a number of years and concluded that they were widespread and up to ten times the size of those on the Earth.
This category includes rills similar to those found on the Moon, and a new type, called canali, consisting of long, distinct channels which maintain their width throughout their entire course.
The ejection material, transported by the wind, is responsible for the process of renovation of the surface at speeds, according to the measurements of the Venera soundings, of approximately one metre per second.
Given the density of the lower Venusian atmosphere, the winds are more than sufficient to provoke the erosion of the surface and the transportation of fine-grained material.
NASA's Goddard Institute for Space Studies and others have postulated that Venus may have had a shallow ocean in the past for up to 2 billion years,[27][28][29][30][31] with as much water as Earth.
