Volcanology of Venus

The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System. It has a surface that is 90% basalt, and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide, with a density that is 90 times that of Earth's atmosphere.

Even though there are over 1,600 major volcanoes on Venus, none are known to be erupting at present and most are probably long extinct. However, radar sounding by the ''Magellan'' probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank. Although many lines of evidence suggest that Venus is likely to be volcanically active, present-day eruptions at Maat Mons have not been confirmed. Nevertheless, other more recent studies, in January 2020, suggests Venus is currently volcanically active.

Types of volcanoes

Venus has shield volcanoes, widespread lava flows and some unusual volcanoes called pancake domes and "tick-like" structures which are not present on Earth. Pancake dome volcanoes are up to in diameter and less than in height and are 100 times the size those formed on Earth. They are usually associated with coronae and ''tesserae'' (large regions of highly deformed terrain, folded and fractured in two or three dimensions, which are unique to Venus). The pancakes are thought to be formed by highly viscous, silica-rich lava erupting under Venus's high atmospheric pressure.

The "tick-like" structures are called scalloped margin domes. They are commonly called ''ticks'' because they appear as domes with numerous ''legs''. They are thought to have undergone mass wasting events such as landslides on their margins. Sometimes deposits of debris can be seen scattered around them.

On Earth, volcanoes 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, such as Mount St. Helens and Mount Pinatubo, are associated with tectonic plates. In this type of volcano, the oceanic crust of one plate is sliding underneath 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 mostly of the shield type . Nevertheless, the morphology of volcanoes on Venus is different: on Earth, shield volcanoes can be a few tens of kilometres wide and up to high in the case of Mauna Kea, measured from the sea floor. On Venus, these volcanoes can cover hundreds of kilometres in area, but they are relatively flat, with an average height of . Large volcanoes cause the Venusian lithosphere to flex downward because of their enormous vertical loads, producing flexural moats and/or ring fractures around the edifices. Large volcano edifice loading also causes magma chambers to fracture in a sill-like pattern, affecting magma propagation beneath the surface.

Other unique features of Venus's surface are ''novae'' (radial networks of dikes or grabens) and arachnoids. A nova is formed when large quantities of magma are extruded onto the surface to form radiating ridges and trenches which are highly reflective to radar. 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. It is not known whether the 250 or so features identified as arachnoids actually share a common origin, or are the result of different geological processes.A New Episode of Volcanism on Venus. ESA: Science and technology. December 2, 2012

Recent volcanic activity

Volcanism on Venus has taken place within the last 2.5 million years; however, there is no absolute proof that any volcano on Venus has erupted recently. Recent radar imagery shows more than 1,000 volcanic structures and evidence of possible periodic resurfacing of the planet by floods of lava. In addition to the radar images, there is supporting evidence that volcanism has taken place, including an unusual change in the amount of sulphur dioxide gas in the upper atmosphere. Sulphur dioxide is an important component of volcanic outgassing. However, the sulphur dioxide in the lower atmosphere remains stable. This could mean that a change in the global atmosphere caused the sulphur dioxide concentration to increase above the clouds. Even though the change in the atmosphere may be evidence that there have been volcanoes that erupted in Venus, it is difficult to determine whether they occurred or not. In March 2014, the first direct evidence for ongoing volcanism was located, in the form of infrared "flashes" over the edges of rift zone Ganis Chasma, near the shield volcano Sapas Mons. These flashes were detectable during two or three consecutive Earth days in 2008 and 2009 and are thought to be caused either by hot gases or lava released from volcanic eruptions. Scientists suspect that there are four volcanoes that may be active: Maat Mons, Ozza Mons, Sapas Mons and Idunn Mons.

In 2020, a study by University of Maryland supported by Swiss National Science Foundation and NASA discovered that 37 of Venus coronae show signs of ongoing activity. Maryland professor Laurent Montesi said, "we are able to point to specific structures and say ‘Look, this is not an ancient volcano but one that is active today, dormant perhaps, but not dead..." The active coronae are clustered near each other, so positioning geologic survey instruments would now be easier.

Lightning

Lightning on Venus may serve as a diagnostic of volcanism or atmospheric convection, so some effort has been devoted to detecting possible lightning on Venus.. The Japanese orbiter ''Akatsuki'' is currently searching for visible lightning on Venus, among other science objectives.Hunt for optical lightning flash in Venus using LAC onboard Akatsuki spacecraft
Takahashi, Yukihiro; Sato, Mitsuteru; Imai, Masataka. 19th EGU General Assembly, EGU2017, proceedings from the conference held 23–28 April 2017 in Vienna, Austria., p.11381.

Atmospheric phosphine

In 2020, Greaves et al. detected phosphine levels of 1-5 PPB in Venus' atmosphere using ALMA and JCMT. Historic data from Pioneer Venus also shows the possible detection of phosphine. Phosphine (PH₃) is derived from Phosphide (P^3-) through the following interaction with sulfuric acid in Venus’ atmosphere:

2 P^3- + 3H_sSO₄ = 2PH₃ + 3SO₄^2-

Phosphide comes from metals such as iron and magnesium, which should exist in great quantities in Venus’ mantle. The phosphines were detected at a height of 70km, which implies a volcanic eruption on the explosive scale of Krakatau or Yellowstone on Earth. The implication of this is not only that Venus has experienced recent volcanism, but that it is capable of explosive eruptions despite the lack of hydrated melts created at subduction zones on Earth. It is thought that Venus may have primordial water in the mantle that could be concentrated through fractionation.

Biological activity has been suggested as an alternate explanation for the phosphines in Venus’ atmosphere, but this is unlikely due to the absence of any other biosignatures. Another hypothesis states that the phosphine could be produced in Venus’ clouds, but this process requires water which is generally unavailable on Venus. Some scientists question that the phosphine levels found are truly as high as indicated. If the phosphine is present in amounts of 1-5 PPB and can be determined to originate in the mantle, it will imply a deep mantle plume system which contains enough volatiles to produce explosive volcanism.

Exploration

In April 2010, Suzanne E. Smrekar ''et al.'' published that ''Venus Express'' observed three volcanoes that have had eruptions about 250,000 years ago or less, which suggests that Venus is periodically resurfaced by lava flows. She has proposed two missions to Venus to elucidate the planet: Venus Origins Explorer (VOX), and VERITAS. Meanwhile, the Japanese spacecraft ''Akatsuki'' has been orbiting Venus since December 2015 and one of its goals is to scan for active volcanism using its infrared cameras, although the infrared detector that was supposed to do this failed in December 2016 after a relatively short period of observations.Initial products of Akatsuki 1-μm camera
''Earth, Planets and Space''. 2018, vol. 70, nbr. 6.

Identifying volcanoes on Venus

Locating volcanoes on Venus became possible during the Magellan mission in 1990, which mapped over 95% of the surface of Venus. The surface of Venus is hidden by clouds but surface features can be mapped using synthetic aperture radar. Some images created by this mapping can give a perspective view of the elevation of the surface of Venus, which assists in the identification of volcanoes. Volcanic features discovered include flood lavas, edifice clusters, shield volcanos, volcanic cones, and volcanic domes. Since the Magellan mission, more than 1,660 volcanic landforms have been identified on the surface of Venus.

After the surface of Venus was mapped, the California Institute of Technology created an algorithm for automatically identifying volcanoes from the mapping images. Certainty that all the identified features are volcanoes is not possible but a system of categories was developed that label the confidence of whether a surface feature is a volcano or not. The algorithm examines images of a 30 km x 30 km area of the surface of Venus and areas considered to be volcanoes are reshaped into a vector and processed through a series of equations. This algorithm has been used to identify multiple volcanoes in different mapping images from Venus.

Scientists are also able to determine the age of volcanoes on Venus using images from the Magellan mission, for example by examining wrinkle ridges on regional plains; if the flanking slopes of a volcano do not have wrinkles ridges, then they would be considered young.

Examples

Sif Mons

The Sif Mons volcano is 350 km in diameter, 2km height, and is in the Western Eistla Regio Rise. Based on the mapping of the volcano, the area around the central caldera is mostly flat with many chain pits surrounding the area. On eastern parts of the volcano, lava has flooded from the main caldera to smaller calderas nearby. Evidence suggests that there were many flank eruptions at this volcano. Most of the flow fields around this volcano are sheet flow fields.

Gula Mons

The Gula Mons volcano is 460 km in diameter, 3.2 km high and is in the Western Eistla Regio Rise. Gula Mons is considered to be a shield volcano. This volcano has a central edifice that is surrounded by the peaks of the volcano. Mapping suggests that there are multiple caldera pits in this volcano that are partially filled with lava.

Kunapipi Mons

The Kunapipi Mons volcano has a diameter of 580 km, is 2.5 km high and is on the Juno Chasma rift. The summit of the volcano is a long plateau region. The main edifice of this volcano consists of many short flows and most of these flows are sheet flows.

See also

* Volcanism on Io
*Volcanism on Mars

References

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