PURDUE UNIVERSITY

GEOS 105-THE PLANETS

Prof. Robert L. Nowack

Lecture 12

Venus

Venus is the closest planet to the Earth at a distance of 0.723 Au from the Sun. It is very similar in size to the Earth and in average density too. Venus has a sidereal year of 224.7 days. However its length of day was first determined by radar in the 1960’s to be 243.08 days in a retrograde sense (clockwise as seen from the North Pole).

Venus Vital Statistics

Radius 6052 km

Surface Area 4.6 x 10⁸ km²

Mass 4.9 x 10²⁴ kg

Density 5.3 gm¤cm³

Local Gravity 8.92 m/s²

Escape Velocity 10.3 km/s

Albedo 0.76

Surface Temperature 740K

Length of Day 243.08 days (Retrograde)

Length of Year 224.7 days

Distance from Sun 0.723 Au

Since Venus rotates very slowly, it was not anticipated to have a significant magnetic field and this was confirmed by different orbiters.

Venus has a very circular orbit. In addition, the inclination of the equator of Venus to its orbit plane is only 2.6° (as opposed to 23.4° for Earth. Thus, Venus experiences no seasons. However, Venus' solar day (time period between successive noons) is 116.67 Earth days. A point on the surface would have almost 60 Earth days of Sun and 60 Earth days of darkness.

Dense clouds completely shroud the planet. Upper atmosphere cloud circulation can be seen in ultraviolet light. However, the surface isn't visible from space.

Daily Pictures of Venus

High altitude winds blow at speeds of 100 m/s (360 mi/hr) from east to west.

Exploration of Venus

In 1967, a Soviet spacecraft, Venera 4, passed through the clouds of Venus. Venera 7 reached the surface of Venus in 1975 and transmitted information back to Earth for 23 minutes before succumbing to the 740 K (900°F) surface temperature and a surface pressure of 90 Earth atmospheres. Landers, such as Venera 13 and 14, successfully photographed the surface. However, surface conditions are very difficult for exploration.

Soviet Space Probe Venera 14 on Venus March 5, 1982

On March 5, 1982, the Venera 14 Lander touched down on Venus at 13^o (south latitude) and 310^o (east longitude) where it survived for 60 minutes before succumbing to the planet’s high surface temperatures. In that time, it radioed to Earth these images of the surface of Venus, which include parts of the lander at bottom (a mechanical arm can be seen in the lower image and a lens cover in the upper image. The landscape appears distorted because Venera 14's wide-angle camera scanned in a tilted, sweeping arc. The horizon appears in the upper left and right corners of each scene and the views are remarkably free of atmospheric haze. Note the dominance of platy rocks, separated by minor amounts of soil. The composition and texture of these rocks is similar to terrestrial basalts.

The Pioneer Venus, U.S. mission in 1978, performed radar mapping from orbit in addition to sending down a probe. Higher resolution radar images have been obtained by the Soviet Venera 15 and 16 spacecraft, as well as the more recent U.S. Magellan orbiter. In addition, the Soviet Vega spacecraft deployed balloons into the upper atmosphere of Venus. Although the cloud tops of Venus are 240 K, the surface temperature is 740 K.

Why is Venus so HOT? Even though the cloud tops of Venus, are highly reflective, sunlight ultimately reaches and warms the surface. The surface then radiates heat back in the infrared.

Sunlight that penetrates to the lower atmosphere and the surface re-radiates back in the infrared wavelength. The atmosphere has a high capacity of holding in this heat. This results in a much higher surface temperature than would be present without the blanketing effect of the atmosphere. However, carbon dioxide (CO₂) in the atmosphere is opaque to infrared and traps this heat from escaping and temperature increases. Note that both oxygen and nitrogen are transparent to infrared radiation. Nevertheless, both carbon dioxide and water vapor, as well as methane, CH₄, are good absorbers of infrared radiation. These are called greenhouse gases. The atmosphere of Venus is principally carbon dioxide and almost no water.

Composition of the Atmosphere of Venus

Thus, carbon dioxide must be the principal greenhouse gas on Venus.

Venus also has a massive atmosphere. Since 10 meters of water on Earth would result in 1 bar of pressure, 90 bars would be the pressure exerted on a submarine at a depth of 900 meters in the ocean. This is similar to the pressure of the atmosphere on the surface of Venus.

What type of droplets make up the clouds of Venus? Since Venus appears to have a low concentrations of water, this was a puzzle until the 1970's. Spectral data from Earth suggested, finally, that the clouds were concentrated sulfuric acid (H₂SO₄). Even the clouds are not pleasant on Venus!

In comparison to Earth, the clouds of Venus are high at about 50 km with a clear CO₂ atmosphere below.

“Standard” Atmospheric Vertical Profile of Venus Compared to Earth

Venus has a much warmer atmosphere near the planet’s surface than the Earth’s, but it is actually colder at high altitudes. On the surface, it would appear like a heavy overcast day with a strong red tint. The weather would be very stable with a temperature of about 740 K and wind speeds of 2 m/s.

Since Venus rotates very slowly, its atmospheric circulation in the troposphere appears to be one large Hadley cell in which warm air rises at the equator, travels toward the poles and sinks. It returns to the equator along the surface. (The Earth's circulation is much more complicated by the Earth's rotation.)

In simple Hadley cell circulation, warm air rises at the equator of a planet and travels toward the poles where it sinks and returns to the equator along the surface.

The Pioneer Venus, U.S. spacecraft, was the first to map the surface of Venus using radar and most recently by U.S. spacecraft, Magellan. (This is similar to aircraft radar mapping in cloudy regions such as the Amazon basin on Earth.)

On August 16, 1990, NASA's Magellan spacecraft went into orbit around Venus after a 15 month journey from Earth. Since Venus is almost completely covered by clouds, Magellan's mission was to use radar to map the entire surface of the planet down to a resolution of less than 1 km (Pioneer Venus had a resolution of 25 km).

Magellan Mapping Venus

Magellan's mapping orbit brought the spacecraft within 300 kilometers of Venus every 3¼ hours. During these times, Magellan bounced short pulses of radio energy through the atmosphere and off the surface for 37 minutes, long enough to cover a nearly pole-to-pole strip of ground 20 to 25 kilometers wide and roughly 15,000 kilometers long. Radar imagery, altimetry and microwave surface emissions were recorded together. Magellan would then turn and relay the data to Earth. Halfway through each playback came an orientation check using the positions of certain stars. Each subsequent mapping swath over-lapped the previous one in longitude, though alternately biased toward the northern and southern hemispheres to increase the coverage at high latitudes. Venus rotated slowly beneath the spacecraft’s orbit allowing Magellan to view the entire globe every 243 days.

Magellan took 24 months to map 98% of the surface of Venus.

In this Mercator-projected view, red corresponds to the highest elevations and blue to the lowest. Maxwell Montes, the highest mountains on Venus rises 12 kilometers above mean elevation. Even though Venus exhibits a range of elevations comparable to that of Earth’s, the two planets have distinct topographies. Earth has many high-standing continents and low-lying ocean floors; whereas Venus has 60% of its terrain within 500 meters of the mean planetary radius (Venus’s sea-level equivalent). The scorpion-shaped feature extending along the equator between 70° and 210° east longitude is Aphrodite Terra, a continent like highland that contains several spectacular volcanoes at its eastern end: Maat Mons, Ozza Mons, and Sapas Mons.

Map of Important Geological Features on Venus

The general topography between Earth and Venus is very different. On Earth, approximately 2/5 of the surface consists of continental highlands and 3/5 consists of deep ocean basins. In contrast, on Venus more than 5/6 of the surface is gently rolling volcanic plains with no major basins. The higher elevation regions are mostly aggregated into 2 large continents, Aphrodite and Ishtar, as well as a number of smaller regions.

Aphrodite is an equatorial continent as big as Africa that stretches 1/3 of the way around the planet. Ishtar is smaller, about the size of Australia, and is located in the northern high latitudes. Ishtar resembles the Himalayan Plateau on Earth and includes the highest mountains on Venus, the Maxwell Mountains, which rise 11 kilometers high, roughly the height of Mt. Everest on Earth.

Comparison of Large-scale Topography between Earth and Venus

VENUS

EARTH

A 2^nd Comparison Figure of Large-Scale Topography on Earth and Venus

One of the unexpected results of the Magellan mission is the discovery that there is a change in surface reflectivity changes with altitude. Essentially all regions higher than 5 kilometers have high radar reflectivity. One suggestion is that this is a chemical change because of the relatively cooler temperatures above 5 km of the iron bearing minerals.

Craters on Venus

While mapping Venus, Magellan revealed over 900 impact craters. On account of the thick atmosphere on Venus, most of the craters exceed 20 kilometers in diameter.

The distribution of 842 impact craters ranging from 1.5 to 280 kilometers in diameter observed by Magellan on 89% of Venus. Areas inside the white lines have not been mapped.

The density of impact craters is considerably lower than Mercury, Mars or the Moon. Also, the distribution of crater sizes is interpreted to mean that the crater retention age of the surface of Venus is about 500 million years old. Also, it has recently been observed that the density of impact craters on large volcanic structures is less than the global average after the major resurfacing event 500 million years ago.

An example of 3 impact craters

The presence of many large, fresh impact craters on Venus is a consequence of the older surface and lower erosion rates on this planet, compared with the Earth. This Magellan radar image shows three craters in the Lavinia region of Venus. The biggest crater has a diameter of 50 kilometers. The rough crater rims and ejecta are excellent radar reflectors and therefore appear bright in a radar image.

Mead is the largest crater on Venus with a diameter of 280 kilometers (similar in size to the largest post-Maria lunar craters)

Presumably, the major agent for resurfacing on Venus wipes away craters in a geologically short time, presumably from large-scale volcanism.

Volcanism on Venus

Much of the volcanic activity on Venus takes the form of basaltic eruptions that flood large areas, similar to Lunar Maria. More familiar are eruptions that produce volcanic mountains, the largest of which are shield volcanoes which erupt highly fluid lavas over a long period of time.

Volcanic Structure Maat Mons

Maat Mons, a 5 kilometer high volcanic edifice, is portrayed as it might appear looking south in a synthesized perspective view that combines Magellan’s radar imagery with altimetry data. Vertical relief has been exaggerated tenfold. Radar-bright lava flows from the Maat Mons complex extend into the foreground and partially cover the margins of Melba, an impact crater 23 kilometers across.

Volcanic Structure Gula Mons

Gula Mons seen in this computer-simulated view of the surface of Venus is located 3 kilometers (1.9 miles) above Eistla Regio. Lava flows extend hundreds of kilometers across fractured plains. The view is to the northeast with Gula Mons appearing at the center of the image. Gula Mons, a 3 kilometer (1.9 mile) high volcano, is located at approximately 22 degrees north latitude, 359 degrees east longitude in western Eistla Regio. Magellan radar imagery and altimeter data produced a three-dimensional map of the surface. Simulated color and a digital elevation map are used to enhance small-scale structure. Simulated hues are based on color images recorded by Soviet Venera 13 and 14 spacecraft.

Several manifestations of volcanism on Venus differ from those on Earth. One of these are called pancake domes and several dozen have been found. Each consists of an almost circular flat dome with steep sides. An average width is about 25 kilometers and height of 2 kilometers. Each pancake dome appears to be made of highly viscous lava erupted rather suddenly from a single vent.

These unusual volcanoes are called pancake domes. Each dome is about 25 kilometers in diameter and about 2 kilometers high. Such circular features are thought to be formed of highly viscous lava that erupted within a short span of time. The Earth has domes of viscous lava too, but they are smaller and less symmetric.

At the other extreme are lava rivers of extremely low viscosity. About 40 of these channels are longer than 100 kilometers.

Another distinctive volcanic feature on Venus are circular structures called coronae. Artemis is the largest with a diameter of 2000 km. Each has a slightly raised interior surrounded by low circular ridges and troughs. Coronae are circular features hundreds of kilometers across characterized by concentric tectonic patterns. Each has a low central dome surrounded by a shallow trough and many concentric cracks. One of the models for coronae is that they result from the ascent of volcanic plume resulting from flow in the interior of Venus.

Selu Corona

Located in Lada Terra, Selu Corona measures 350 kilometers ( 210 miles) in diameter.

A feature related to coronae are Arachnoids (meaning spiders) which are generally circular between 50-200 kilometer across and surrounded by a radial system of ridges.

Arachnoid Located West of Themis Regio

This arachnoid measures 200 kilometers by 380 kilometers across and comprises two linked structures.

Tectonics on Venus

Plate tectonics is the large scale lateral motion of the Earth’s outer surface and is thought to result from large-scale heat-releasing and convection in the Earth’s mantle over a geologic time. It is assumed that Venus must also have a mechanism for releasing heat from its interior. The amount of volcanism on Venus is about the same as on Earth. Yet the scale of convection plumes are much smaller scale than the thousands of kilometers that characterize crustal plates on Earth. Nonetheless, virtually the entire surface of Venus is subject to tectonic forces.

Some have suggested that the existence of water may help to lubricate the process of plate tectonics on Earth. Still, on Venus, there are some concentrated tectonic zones, including the continent of Aphrodite. Within these zones, the crust is compressed and folded. For example, Tesserae are terrains on Venus that have been intensely modified by tectonic processes. They consist of interlacing ridges and valleys.

Alpha Regio Tesserae Tectonic Complex

The complex pattern of intersecting ridges and valleys are called Tesserae. This mosaic of a portion of Alpha Regio covers an area 125 x 150 kilometers.

The Ishtar Continent (including the Maxwell Mountains and broad Lakshmi Plateau) resembles terrestrial mountainous regions.

In summary, Venus and Earth are similar in having high levels of volcanic and tectonic activity – presumably resulting from plumes of hot material rising from the interior. Yet, Venus has not developed the planetary scale plate tectonics that led to the large continent and deep-ocean basins as found on Earth.

How is the atmosphere of Venus different from Earth's? At present, they are very different with the Earth's atmosphere dominated by nitrogen (N₂) and oxygen (O₂), whereas on Venus carbon dioxide (CO₂) dominates. However, if all the carbon dioxide that is locked in rocks on Earth were released, this would result in a more similar composition. One of the differences is caused by life on Earth.

Another basic difference is that Earth is abundant in water (H₂O) while Venus is not. Venus could have simply formed without water. Alternatively, Venus could have started with a similar amount of water as Earth, but somehow lost it. One way Venus could have lost its water is by a runaway greenhouse.

Thought Experiment

(1) Move Earth to Venus’ position from the Sun

(2) Oceans become warmer.

(3) More water evaporates.

(4) Increased water vapor in the Earth’s atmosphere blocks radiation increasing surface temperature.

(5) Cycle repeats until:

(6) Ocean boil away. The atmosphere becomes very hot, and full of water vapor which rises to upper altitudes.

(7) Ultraviolet light breaks water molecules into hydrogen and oxygen at high altitudes.

(8) Hydrogen escapes into outer space. Oxygen remains to chemically react with rock strata. Atmosphere becomes dry and full of carbon dioxide since carbonates cannot form anymore.

(9) Earth now would resemble Venus!

Critical parameters:

(a) Size of the planet

(b) Distance from Sun

If oceans of water have escaped from Venus, then the hydrogen on Venus should be enriched in the slightly heavier Deuterium (with a proton and neutron in the nucleus).

The Pioneer Venus Probe tested for Deuterium, or heavy hydrogen, in water molecules and found an enrichment comparable to Earth. This seems to give evidence for the above scenario for Venus.