Astronomy college course/Venus

extracted from https://en.wikipedia.org/w/index.php?title=Geology_of_Venus&oldid=594616239

Magellan studies the geology of Venus
Launched May 4, 1989 aboard the space shuttle Atlantis, the Magellan probe was first placed into low Earth orbit, before firing its upper stage motor to send it on a trajectory toward Venus. The first images of Venus were received on August 16, 1990.

Topography
The surface of Venus is comparatively flat. The total distance from the lowest point to the highest point on the entire surface was about 13 km, while on the Earth the distance from the basins to the Himalayas is about 20 km.

The topography of the planet is divided into three provinces: lowlands, deposition plains, and highlands.
 * Highlands covers about 10% of the planet's surface, with altitudes greater than 2 km. The two most important provinces of the highlands are Aphrodite Terra and Ishtar Terra, but a number of other less important highland regions also have been named.
 * Deposition plains have altitudes averaging 0 to 2 km and cover more than half of the planet's surface.
 * The rest of the surface is lowlands and generally lies below zero altitude. 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).

Impact craters


Compared to Mercury, the Moon and other such bodies, Venus has very few craters. This seems to be due in part to the dense atmosphere of Venus, which prevents small objects from striking the surface, as well as volcanism, which fills craters with lava.
 * 1) In part, this is because Venus's dense atmosphere burns up smaller meteorites before they hit the surface. As a result of this dense atmosphere there are very few impact craters with a diameter less than 30 km, and an even more striking absence of any craters less than 2 km in diameter. The small craters are irregular and appear in groups, thus pointing to the deceleration and the breakup of impactors.
 * 2) 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.

Taken together, this evidence suggests that the surface of Venus is 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.

A young surface all created at the same time is a different situation compared with any of the other terrestrial planets.

Global resurfacing event
It is hypothesized that Venus underwent some sort of global resurfacing about 300–500 million years ago, though no Venusian rock has ever been dated.

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. However, 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.

There are several other attributes of Venus that this model can help explain. Venus's lack of a magnetic field is puzzling, as Venus is similar to Earth in size, and presumably composition. However, it can be explained by a core that is not losing heat. Also, Venus has a much higher deuterium to hydrogen ratio in its atmosphere than do the Earth or comets. Atmospheric escape is one of the very few processes that differentiate between the deuterium and hydrogen. The extremely high ratio implies that large amounts of water have recently been in Venus's atmosphere. This water might have been released in volcanic eruptions.

Volcanoes


The surface of Venus is dominated by volcanism. Although Venus is superficially similar to Earth, it seems that the tectonic plates so active in Earth's geology do not exist on Venus. About 80% of the planet consists of a mosaic of volcanic lava plains, dotted with more than a hundred large isolated shield volcanoes, and many hundreds of smaller volcanoes and volcanic constructs.Volcanoes less than 20 km in diameter are very abundant on Venus and they may number hundreds of thousands or even millions.

On Earth, volcanos are mainly of two types: shield volcanoes and composite or stratovolcanoes. The shield volcanoes, for example those in Hawaii, eject magma from the depths of the Earth in zones called hot spots. The lava from these volcanos is relatively fluid and permits the escape of gases. Composite volcanos on Earth are associated with tectonic plates. In this type of volcano, the oceanic crust of one plate slides beneath the other in a subduction zone, together with an inflow of seawater, producing a gummier lava that restricts the exit of the gases, and for that reason, composite volcanoes tend to erupt more violently.

On Venus, where there are no tectonic plates or seawater, volcanoes are of the shield type.

Tectonic activity
Despite the fact that Venus appears to have no tectonic plates as such, the planet's surface shows various features usually associated with tectonic activity. Features such as faults, folds, large mountains and rift valleys are caused on Earth by plates moving over relatively weak parts of the planet's interior. These structures are less abundant on Venus than on Earth.



Lava flows and channels
Lava flows on Venus are often much larger than Earth's, up to several hundred kilometres long and tens of kilometres wide. It is still unknown why these lava fields or lobate flows reach such sizes, but it is suggested that they are the result of very large eruptions of basaltic, low-viscosity lava spreading out to form wide, flat plains.

Although the shape of these channels is highly suggestive of fluid erosion, there is no evidence that they were formed by water. In fact, there is no evidence of water anywhere on Venus in the last 600 million years. While the most popular theory for the channels' formation is that they are the result of thermal erosion by lava, there are other hypotheses, including that they were formed by heated fluids formed and ejected during impacts.

Surface processes
Water is almost nonexistent on Venus, and thus the only erosive process to be found (apart from thermal erosion by lava flows) is the interaction produced by the atmosphere with the surface. This interaction is present in the ejecta of impact craters expelled onto the surface of Venus. The material ejected during a meteorite impact is lifted to the upper atmosphere, where winds transport the material toward the west. As the material is deposited on the surface, it forms parabola-shaped patterns. This type of deposit can be established on top of various geologic features or lava flows. Therefore, these deposits are the youngest structures on the planet. Images from Magellan reveal the existence of more than 60 of these parabola-shaped deposits that are associated with crater impacts.

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. In the regions covered by ejection deposits one may find wind lines, dunes, and yardangs. The wind lines are formed when the wind blows ejection material and volcano ash, depositing it on top of topographic obstacles such as domes. As a consequence, the leeward sides of domes are exposed to the impact of small grains that remove the surface cap. Such processes expose the material beneath, which has a different roughness, and thus different characteristics under radar, compared to formed sediment.

The dunes are formed by the depositing of particulates that are the size of grains of sand and have wavy shapes. Yardangs are formed when the wind-transported material carves the fragile deposits and produces deep furrows.

The line-shaped patterns of wind associated with impact craters follow a trajectory in the direction of the equator. This tendency suggests the presence of a system of circulation of Hadley cells between medium latitudes and the equator. Magellan radar data confirm the existence of strong winds that blow toward the east in the upper surface of Venus, and meridional winds on the surface.

Meteor impacts on Venus have occurred for the last hundreds of millions of years. The superposition of lava flows can be noted. Radar reflection from the oldest lava flows, covered by the newest flows, present distinct intensities. The oldest flows reflect less than the plains that surround the flows. Data from Magellan show that the most recent flows are similar to &#x02BB;a&#x02BB; and pāhoehoe. However, the oldest lava flows are darker and look like deposits in arid regions of the Earth that have suffered meteor impacts.

Chemical and mechanical erosion of the old lava flows is caused by reactions of the surface with the atmosphere in the presence of carbon dioxide and sulfur dioxide (see carbonate-silicate cycle for details). These two gases are the planet's first and third most abundant gases, respectively; the second most abundant gas is inert nitrogen. The reactions probably include the deterioration of silicates by carbon dioxide to produce carbonates and quartz, as well as the deterioration of silicates by sulfur dioxide to produce anhydrate calcium sulfate and carbon dioxide.

One of the most interesting characteristics of radar images is the diminishing of reflection at high altitudes, exhibiting extremely low values beyond a radius of 6,054 km. This change is related to the diminishing of emission and temperature at high altitudes.

There are various hypotheses for the unusual characteristics of Venus' surface. One idea is that the surface consists of loose ground with spherical hollows that produce an efficient reflection of radar. Another idea is that the surface is not smooth and is covered by material that has an extremely high dielectric constant. Yet another theory says that the layer one metre above the surface is formed by sheets of a conductive material such as pyrite. Last, a recent model supposes the existence of a small proportion of ferroelectric mineral.

Ferroelectric minerals exhibit a unique property at high temperatures: the dielectric constant increases abruptly, yet as the temperature increases further, the dielectric constant returns to its normal values. The minerals that could explain this behaviour on the surface of Venus are perovskite and pyrochlores.

Despite these theories, the existence of ferroelectric minerals on Venus has not been confirmed. Only in situ exploration will lead to an explanation of such unresolved enigmas.

Venus/Quiz/Quizbank