NEEP602 Course Notes (Fall 1996)
Resources from Space

Venus

The Bringer of Peace

Venus Facts

Venus is the second planet from the Sun and the sixth largest:
distance from Sun: (0.72 AU)
diameter: 12,103.6 km (0.0008AU)
mass: 4.869e24 kg (compare w/ 5.97e24kg Earth)
density: 5.25 gm/cc (compare w/ Earth 5.52)
no magnetic field

Venus's orbit is the most nearly circular of that of any planet, with an eccentricity (0.007) less than 1%

rotation rate is 243 Earth days/ retograde
revolution rate around the axis of the planet is 224.7 days
obliquity to the plane of the ecliptic is 178 degrees

Venus "quasi" facts

surface may be as little as 500 My old from crater counts per unit area & random distribution of craters
no cold (trap) region in the present atmosphere to condense water and prevent its loss (especially hydrogen from dissociation of water) into space (escape velocity Venus 10.4 km/sec, Earth 11.2km/sec), hence very little water now
absence of water may make similar rocks stronger than on Earth by factor of 10
crude estimate of heat flow, if like Earth, 1-1.5 cal/cm2/sec mainly from radio active decay but no plate tectonics (unlike Earth, unimodal topography vs elevation) to make escape of heat easy.
no life now

The planet is so named probably because it is the brightest of the planets known to the ancients.

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon.

Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of the Earth. Galileo's observation of this phenomenon was important evidence in favor of Copernicus's heliocentric theory of the solar system.

The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. Most recently, the orbiting US spacecraft Magellan produced detailed maps of Venus's surface using radar.

Venus's rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus's year) and retrograde. In addition, the periods of Venus's rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach.

Venus is sometimes regarded as Earth's sister planet. In some ways they are very similar:

Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).
Both have few craters indicating relatively young surfaces.
Their densities and chemical compositions are similar.

Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth.

The pressure of Venus's atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view surface (picture 2). This dense atmosphere produces a run-away greenhouse effect that raises Venus's surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus's surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

There are strong (350 kph) winds at the cloud tops (picture 6) but winds at the surface are very slow, no more than a few kilometers per hour.

Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Most of Venus's surface consists of gently rolling plains with little relief. There are also several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highest mountains on Venus including the enormous Maxwell Montes.

Data from Magellan's imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons (picture 3). Recently announced findings indicate that Venus is still volcanically active, but only in a few hot spots (picture 4); for the most part it has been geologically rather quiet for the past few hundred million years.

There are no small craters on Venus because small meteors burn up in Venus's dense atmosphere before reaching the surface. Craters on Venus seem to come in bunches (picture 11) indicating that large meteors that do reach the surface usually break up in the atmosphere.

The oldest terrains on Venus seem to be <500million years old. Extensive volcanism at that time wiped out the earlier surface including any large craters from early in Venus's history.

Magellan's images show a wide variety of interesting and unique features including pancake volcanoes which seem to be eruptions of very thick lava (picture 8) and coronae which seem to be collapsed domes over large magma chambers (picture 9).

The interior of Venus is probably very similar to that of Earth: an iron core about 3000 km in radius, a largely molten rocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus's lithosphere (crust and upper mantle) is stronger and thicker than had previously been assumed. Unlike Earth, convection in the defrming lower mantle produces stress on the surface which is relieved in many relatively small regions eposodically instead more continually at plate boundaries as is the case on Earth.

Venus has no magnetic field, perhaps because of its slow rotation.

Venus is usually visible with the naked eye. Mike Harvey's planet finder charts show the current position of Venus (and the other planets) in the sky.

Pictures

(above) False color high-res image a whole hemisphere 342k gif; 3000k gif
Visible light view from Galileo 73k gif
Sif Mons 156k gif; 270k jpg
Recent Volcanism 123k gif; 22k jpg
Large Impact Crater 131k gif; 495k jpg
Image from Pioneer Venus 15k jpg
Gula Mons and Cunitz crater 250k gif; 178k jpg
7 pancake volcanoes, alpha region 227k gif; 327k jpg
Pandora Corona, Lada Terra 42.5 deg S 6 lon 177k gif; 186k jpg
Vires-Akka Chasma, Denitsa Region 130k gif; 337k jpg
Crater Farm, Lavinia Region, small 107k gif; 147k jpg
Volcano, 3 mile diameter, paragon chasma, 9.4S by 247.5 long 282k gif; 72k jpg
Golubkina crater, 60.5 Deg N, 287.2 Long (computer generated Perspective) 107k gif; 149k jpg
Alpha Regio, showing hills and craters 237k gif
UV image from HST (false color) 23k jpg; 99k gif; 1200k tiff
New Magellan global mosaics html
Surface photographs from the Soviet Venera 9 and 10 spacecraft
NASA PDS Magellan Image Browser html
... more Venus images

Movies

Venus animation 1542K fli; 1239K pics; 405K quicktime; 444K B&W pics
Venus Globe Animation 303k mpeg
Earth/Venus Rotation Movie 1000k AVI
Magellan - Mapping the planet Venus 10000k AVI
Flight over Western Atla Regio 7000k AVI; (caption)
Flight over Artemis 11000k AVI; 23000k AVI; (caption)
Flight over Alpha Regio 8000k AVI; (caption)
Flight over Western Eistla Regio - 3300k AVI; 7600k AVI; 15000k AVI; (caption)
A dramatic view the the moon with Venus in the distance 82k mpeg

More about Venus

from ASU
from LANL
from JPL
from RPIF
from RGO
from NSSDC
from NASA Spacelink
Venus papers and info from MIT
Magellan mission home page
The Face of Venus: surface of the planet Venus through hypertext documents and the interactive databases of craters and coronae. (Very nice!)
Venusian Craters and Venusian Volcanic Features from LANL
Magellan Venus Explorer's Guide from JPL; about Magellan and Venus prior to the Magellan data
Guide to Magellan Image Interpretration from JPL
Magellan Highlights of Venus (ExInEd Hypercard stack)
Dr. Cain's class notes
The Magellan Spacecraft at Venus by Andrew Fraknoi of ASP
Venus Nomenclature Table

Open Issues

There is some evidence of spreading and folding on Venus's surface and of recent volcanic flows. But there is no evidence of plate tectonics as seen on Earth. Is this a result of the higher surface temperature?
The greenhouse effect is much stronger on Venus than Earth because of Venus's dense carbon dioxide atmosphere. But why did Venus evolve so differently from Earth?

... Sun ... Mercury ... Venus ... Earth ...

Bill Arnett; original source: 1995 August 12

Revised for 602/376 by R. P. Meyer, February 19, 1996

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