The Solar System in Close-Up
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Fig. 5.2The Magellan spacecraft showed Venus is covered with extensive lava flows and lava plains. This global view of the surface of Venus was produced by the Solar System Visualization project and the Magellan science team at the JPL Multimission Image Processing Laboratory. The bright areas are the equatorial highlands known as Aphrodite Terra (Credit: NASA).
The Venus Express probe was launched by the European Space Agency on 9 November 2005. On 11 April 2006 the probe went into polar orbit around Venus. At closest approach, the probe was about 250 km above the north pole and 66,000 km above the south pole. The main objectives of the mission include exploring the global circulation of the Venusian atmosphere, chemistry of the atmosphere, surface volcanism and atmospheric loss. Thermal imaging done by the probe showed a thick layer of clouds, located at about 60 km altitude that traps heat radiating from the surface. In December 2014 the probe ran out of fuel and contact was lost. The spacecraft was expected to plunge into the atmosphere of Venus in January 2015, falling in pieces, corroding and melting, toward the searing planetary surface.
On 24 October 2006 NASA’s Messenger probe (while en route to Mercury) made a flyby of Venus at an altitude of 3000 km. During the encounter, Messenger passed behind Venus and entered superior conjunction, a period when Earth was on the exact opposite side of the Solar System, with the Sun inhibiting radio contact. For this reason, no scientific observations were conducted during the flyby. Communication with the spacecraft was reestablished in late November and it performed a deep space maneuver on 12 December, to correct the trajectory to encounter Venus in a second flyby. On 5 June 2007, Messenger performed a second flyby of Venus at an altitude of 338 km, for the greatest velocity reduction of the mission. The encounter provided visible and near-infrared imaging data of the upper atmosphere of Venus. Ultraviolet and X-ray spectrometry of the upper atmosphere were also recorded, to characterize the composition. The ESA’s Venus Express was also orbiting during the encounter, providing the first opportunity for simultaneous measurement of particle-and-field characteristics of the planet (Table 5.2).Table 5.2Recent space probes to Venus
Spacecraft
Country of origin
Date launched
Notes
Vega 1
USA
1984
Flyby with lander
Vega 2
USA
1984
Flyby with lander
Galileo
USA
1989
Flyby
Magellan
USA
1989
Radar mapping 99 %
Messenger
USA
2004
Flyby
Venus Express
Europe
2005
Polar orbiter
Akutsuki
Japan
2010
Flyby; retry 2015
IKAROS
Japan
2010
Flyby, solar sail test
Shin’en
Japan
2010
Flyby, failed
Position and Orbit
Venus orbits the Sun in a nearly circular orbit as shown by its small orbital eccentricity. Its mean distance from the Sun is just over 108 million km and it passes within 40 million km of Earth, closer than any other planet.
Observation of Venus is easy because of its close proximity to Earth, and it is the brightest object in the sky (apart from the Sun and Moon). Venus can be seen either in the eastern sky before sunrise or in the western sky after sunset. It is so bright is can often be seen during daylight and is often mistaken for an Unidentified Flying Object (UFO).
Venus orbits the Sun in about 225 Earth days. Because it is closer to the Sun than Earth, Venus is seen to go through phases just like our Moon. Venus’s size appears to vary according to its phases because of its changing distance from Earth. Venus is in full phase when furthest from Earth, and when close to Earth it is seen as a thin crescent phase. It is possible to view the phases through binoculars or a small telescope.
As Venus and Earth travel around the Sun, Venus can be seen near the opposite side of the Sun about every 584 days. When Venus is moving toward Earth, the planet can be seen in the early evening sky (west). When Venus is moving away from Earth, the planet can be seen in the early morning sky (east).
At rare intervals, observers from Earth can see Venus transit or pass in front of, the Sun. A last transit occurred on 8 June 2004, and again on 6 June 2012. A transit should be viewed by projecting the Sun’s image from a telescope, onto a white screen. The planet can be seen as a black dot slowly moving across the Sun’s image. Care should always be taken when viewing the Sun—never look through a telescope at the Sun with your eyes.
Fig. 5.3The 2006 transit of Venus across the Sun as recorded by the author through a H-alpha solar telescope (Credit: J. Wilkinson).
Density and Composition
Venus is a rocky planet, much like the Earth. Given its similar size, mass, and density to our planet, scientists think that its interior is much like Earth’s own. In addition to a crust significantly older than Earth’s constantly changing surface, Venus probably also has a mantle and a core.
Venus is the third densest planet in the solar system. This high density is due to the fact that Venus probably has a large rocky core made of mostly nickel and iron. The core is about 3340 km in radius and is surrounded by a molten silicate mantle about 2680 km thick. There is also a thin outer layer or crust about 50 km thick similar to the crust on Earth. Recent data from the Magellan probe indicates the crust is stronger and thicker than had previously been assumed. Future spacecraft will deposit seismometers to search for ‘earthquakes’ that can help scientists probe the planet’s interior (see Fig. 5.4).
Fig. 5.4The interior structure of Venus.
The strength of gravity on Venus is slightly less than that on Earth. A 75 kg person on Earth would weigh 735 N, but on Venus they would only weigh 607 N.
The Surface
The surface of Venus cannot be seen from Earth because of the thick clouds surrounding the planet. However, some features have been detected by radar.
In the 1960s, both the USSR and USA began sending space probes to Venus. Reaching the surface proved to be more difficult than anyone thought because the atmospheric pressure was so great that many early craft were crushed.
Instruments on Venera 7 found the surface temperature was 475 °C and the surface pressure 90 atm (about the same as the pressure at a depth of 1 km in Earth’s oceans). The black-and-white photographs returned from the Venera 9 lander showed a rocky terrain with basaltic stones several centimetres across and soil scattered between them. The temperature at the landing site was 460 °C and wind speed was only 2.5 km/h. The terrain around the lander from Venera 10 was more eroded than at the Venera 9 landing site.
Soil analysis by Venera 14 showed the surface rock type to be basaltic, similar to that found at mid-ocean ridges on Earth.
Venera 15 and 16 produced a map of the northern hemisphere from the pole to 30°N and found several hot spots that possibly were caused by volcanic activity. Most of the surface of Venus consists of gently rolling plains with no abrupt changes in topography. There are also several broad depressions called lowlands, and some large highland areas including two named Aphrodite Terra and Ishar Terra. The highland areas can be compared to Earth’s continents, and the lowland areas to its ocean basins. Several canyons and a few rift valleys were also mapped.
There are no small craters on Venus, probably because small meteoroids burn up in Venus’ dense atmosphere before reaching the surface. There are some large craters and these appear to come in bunches suggesting that large meteoroids break up into pieces just before hitting the ground.
The best pictures of the Venusian surface came from the orbiting US spacecraft Magellan, which produced detailed maps of Venus’ surface using radar. Images from Magellan shows that much of the surfac
e is covered by flat rolling plains with several highland regions. The highest mountain on Venus is Maxwell Montes, situated near the centre of Ishtar Terra and rising to around 11 km above the mean surface level. Maxwell Montes is about 870 km long.
Infrared measurements by Venus Express suggest that Venus might have had a system of plate tectonics in the past, as Earth does today, as well as an ocean of liquid water. Other observations indicate that the planet was likely volcanically active as recently as 2.5 million years ago. Venus may still be active volcanically, but only in a few hot spots. There is also evidence of lava flows, volcanic domes, collapsed volcanic craters and volcanic plains. There are also several large shield type volcanoes (similar to those at Hawaii). Two large and possibly active shield volcanoes are Rhea Mons and Theia Mons, which tower 4 km high.
Planetary scientists believe Venus loses heat from its interior via hot spot volcanism rather than via convection as in the case of Mercury. Hot spots produce shield volcanoes and flood volcanism and these features are common on the present day Venusian landscape. The Magellan probe also revealed surface features called arachnoids that look like craters with spider legs radiating from them. These features are thought to have formed when molten magna pushes up from the interior with such force that the surrounding crust gets cracked (Fig. 5.5).
Fig. 5.5False-colour image of the Venusian volcano Sapas Mons as produced by Magellan. Sapas Mons is approximately 400 km across and 1.5 km high (Credit: NASA).
Tectonic movement that has resulted in crustal shortening, stacking of crustal blocks and wrinkle ridges on the lowlands and rolling plains may have also shaped the surface of Venus. Also seen on the lowlands and plains are fractures formed when the crust was stretched or pulled apart. Diana Chasma is the deepest fracture on Venus with a depth around 2 km below the surface and a width of nearly 300 km.
The oldest terrains on Venus are about 800 million years old. Lava flows at that time probably wiped out earlier surface features and larger craters from early in Venus’ history.
Approximately 300–500 million years ago there was massive resurfacing on Venus which may have “turned off” any plate tectonics on the planet, completely solidifying the crust into a single surface.
Venus probably once had large amounts of water but the high temperature boiled all this away, so Venus is now very dry (Fig. 5.6).
Fig. 5.6Aine Corona is the large circular structure near the center of this Magellan radar image. It is approximately 200 km in diameter. Just north of Aine Corona is one of the flat-topped volcanic constructs known as ‘pancake’ domes for their shape and flap-jack appearance. This pancake dome is about 35 km in diameter and is thought to have formed by the eruption of extremely viscous lava. Complex fracture patterns like the one in the upper right of the image are often observed in association with coronae and various volcanic features (Credit: NASA).
The Atmosphere
Many of the probes sent to Venus have provided information about its atmosphere. The clouds of Venus conceal a hostile atmosphere that reaches a height of about 250 km. Most of the atmosphere however is concentrated within 30 km of the surface.
The first probe to be placed directly into the atmosphere and to return data was Venera 4 in 1967. It found that the atmosphere was 90–95 % carbon dioxide with clouds of sulfuric acid droplets. Mariner 5 arrived at Venus 1 day after Venera 4 and passed within 3900 km of the planet’s surface—it also found an atmosphere dominant in carbon dioxide. Venera 5 and 6 reported an atmosphere of 93–97 % carbon dioxide, 2–5 % nitrogen, and less than 4 % oxygen. These two probes returned data to within 26 km and 11 km respectively of the surface before being crushed by the high atmospheric pressure.
From 1978 to 1988 the amount of sulfur dioxide in the atmosphere decreased by 10 %. The reason for this decrease may have been due to a decrease in volcanic activity during this period.
In 1978, the Pioneer Venus 2 probe detected a fine haze in the atmosphere at a height of 70–90 km. Between 10 and 50 km there was some atmospheric convection and below 30 km the atmosphere was clear.
The Venus Express probe found Venus has an unusual super-rotating upper atmosphere, which flies around the planet once every 4 days, in stark contrast to the rotation of the planet itself, 243 days. By tracking the movements of distinct features in the cloud tops over the last 6 years, scientists have been able to monitor patterns in the long-term global wind speeds. Venus Express determined that wind speeds have mysteriously increased from 300 to 400 km/h over a span of 6 years. The probe also discovered a huge double atmospheric vortex at the south pole of the planet, and flashes of lightning that regularly occur in Venus’s sulfuric acid clouds. In 2011 a layer of ozone was detected in the upper atmosphere of Venus (see Fig. 5.7).
Fig. 5.7Venus rotates slowly, yet it has permanent vortices in its atmosphere at both poles. These vortices form a constantly evolving structure on the surface of Venus. Long-term vortices are a frequent phenomenon in the atmospheres of fast rotating planets, like Jupiter and Saturn, for example. These infrared images show the changing shape of the double vortex over the south pole of Venus (Credit: ESA).
Unlike the clouds on Earth, which appear white from above, the cloud tops on Venus appear yellowish or yellow–orange. These colours are thought to be due to sulfur and sulfur compounds in the atmosphere. Evidence suggests these compounds have originated from volcanic activity.
Fig. 5.8Computer generated perspective image of Latona Corona and Dali Chasma, Venus. The image was created by superimposing Magellan radar data on topography vertically exaggerated by a factor of 10. The eastern part of the 1000 km diameter Latona Corona is on the left, and the view is from the northeast looking along the 3 km deep Dali Chasma (Credit: NASA).
In July 2014 Venus Express used aerobraking maneuvers to lower its orbit to within 250 km of Venus’s north pole (just above the top of the atmosphere). The results show that the atmosphere seems to be more variable than previously thought for this altitude range.
Between altitudes of 165 and 130 km, the atmospheric density increases by a factor of roughly a thousand, meaning that the forces and stress encountered by Venus Express were much higher than during normal operations. The probe also experienced extreme heating cycles, with temperatures rising by over 100 °C during several 100 s-long passages through the atmosphere.
Temperature and Seasons
Surface temperatures on Venus can rise to 482 °C, hot enough to melt lead, zinc and tin. The high temperature and pressure were responsible for the failure of many early space probes. Temperature and pressure in the atmosphere decrease with increasing altitude.
The dense atmosphere on Venus allows heat from the Sun to warm the surface but it also traps heat radiated from the surface of Venus. This results in a higher surface temperature than on Mercury (which is closer to the Sun). The trapping of heat by the atmosphere produces a greenhouse effect because the carbon dioxide acts like glass in a greenhouse. Earth has a greenhouse effect in its atmosphere, but Venus is an extreme case. The thick atmosphere of Venus also keeps the night side of Venus at nearly the same temperature as the side facing the Sun (unlike the night side of Mercury where the temperature drops dramatically). Temperatures at the poles of Venus are as hot as those at the equator.
As it orbits the Sun, Venus rotates very slowly on its axis, more slowly than any other planet. It takes 243 Earth days for just one spin, which means that a Venusian day is longer than a Venusian year.
Venus’s rotation axis is tilted more than 177°, compared to Earth’s 23.5° tilt. This means that Venus’s axis is within 3° of being perpendicular to the plane of its orbit around the Sun. Because of this, the planet has no seasons. Neither of the planet’s hemispheres or poles point notably towards the Sun during any part of its orbit.
Magnetic Field
In 1967, Venera 4 found the magnetic field of Venus to be much weaker than that of Earths. This magnetic field is induced by an interaction between the ionosphere and the solar wind,
rather than by an internal dynamo in the core like the one inside Earth. The lack of a strong magnetic field may be due to the slow rotation of Venus on its axis. One possibility is that Venus has no solid inner core, or that its core is not currently cooling, so that the entire liquid part of the core is roughly at the same temperature. Another possibility is that its core has already completely solidified.
Venus also has a weak magnetosphere that provides negligible protection to the atmosphere against incoming cosmic radiation. A weak magnetosphere means that the solar wind is interacting directly with the planet’s atmosphere. Ions of hydrogen and oxygen are being created by the break up of neutral molecules by ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape Venus’s gravitational field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules such as carbon dioxide are more likely to be retained. Atmospheric erosion by the solar wind probably led to the loss of most of Venus’s water during the first billion years after it formed.
The ESA’s Venus Express probe, which followed a near-polar orbit around Venus, also monitored the solar wind and Venusian magnetosphere. Data from the probe suggested Venus has a magnetotail but that it is much smaller than Earth’s.