ASTR 1210 (O'Connell) Study Guide


Radar map of Venus' surface, from the Magellan Mission. The red color is artificial,
intended to represent the effects of Venus' thick clouds. Click for enlargement.

A. The "Inferior" Planets

Mercury and Venus are called "inner" or "inferior" planets because they are closer to the Sun than is Earth.

Both revolve around the Sun in shorter times than the Earth (88 and 225 days, respectively).

Planet-Sun Configurations

Image of the Caloris Basin on Mercury taken by the MESSENGER spacecraft.
Color coding is for different mineral types.

B. Mercury

Mercury is hard to observe from Earth because it is above the nighttime horizon only for brief periods.

It has been less well studied than most other planets. Until 2007 there had been only 2 spacecraft visits, both flybys, in contrast to Venus, which has been a major destination of space missions. Here is hemispheric view of Mercury from Mariner 10 (1974).

Mercury has a high average density of 5.4 grams/cc, like Earth, but Mercury's mass (& therefore gravity compression) is smaller. That implies Mercury is more rich in heavy elements than Earth

Mercury's surface is similar to Earth's Moon (impact-driven terrain), but with important differences (e.g. shallower craters) due to slower cooling and higher gravity. See the image at right and compare to Earth's Moon topography here.

Mercury's orbit is an important test of General Relativity, the revised interpretation of gravity proposed by Einstein in 1915. Mercury's perihelion (the closest point to the Sun in its elliptical orbit) shifts 43 arc-seconds/century more than predicted for Newtonian gravity. This is only 1/10 millionth of its total orbital motion per orbit, but it can be measured highly accurately over many orbits. The extra shift is predicted exactly by Einstein's GR theory.


Venusian cloud layers in UV/optical bands (image from Mariner 10, 1974)

C. Venus: Introduction

Venus is a near "twin" of Earth in global properties: diameter (95%); mass (82%); distance from Sun (0.7 AU)

But unlike Earth, thick cloud layers completely obscure its surface. See image above (click for enlargement).

USSR & USA space missions to Venus started in 1961 and have included flybys, orbiters, atmospheric probes, and short-survival landers.

Results from these missions, as well as Earth-based radio-wave observations, quickly demolished the notion of a Venusian tropical paradise:

D. Venus: Surface/Topography

For Venus, the only feasible surface mapping technique was to use radar to penetrate the thick clouds.

Top: elevation maps of Venus and Earth compared.
Bottom: Radar map of Venus with main features labeled. (Pioneer Mission, 1981)

The image above is a relief map of Venus derived from radar observations with the Pioneer mission. Best mapping coverage was from the Magellan Mission (radar orbiter, 1990-94).

The overall topography is flatter than Earth's. There are only two "continent"-like features (Ishtar and Aphrodite in the map above).

Vast lava flows cover 85% of the surface, but there are no large basins, either impact (like the Moon's marias) or tectonic-related (like Earth ocean beds). Most flow regions are smooth. There is little current eruptive activity (no changes in the surface features, for example).

There are many dormant volcanoes, from 500 km diameter to tiny vents; 3000 over 20 km diameter; 100,000 altogether! Over 160 larger than the largest volcano on Earth (Hawaii).

DanilovaCrater There are also many impact craters, but fewer per unit area than on the Moon or Mercury. This implies a younger surface than those planets.

Surprisingly, Venus shows a uniform distribution of craters across its surface.

E. Venus: Atmosphere

Venus' atmosphere is dense, hot, dry, and corrosive. At surface levels, it is entirely hostile to Earth-like life.

Despite the dense and corrosive atmosphere, there is little weathering of surface features on Venus because windspeeds are very low (and the sulfuric acid rain evaporates at high altitude before reaching the ground).

The Greenhouse Effect

Venus would be warmer than the Earth simply because it is nearer the Sun. But the extraordinarily high Venusian temperature is not caused only by higher solar input. Instead, it is produced by the
Greenhouse Effect, an atmospheric process which inhibits surface cooling.

F. Venus and Earth

Venus is a sobering lesson in comparative planetology.

The incredible differences between terrestrial and Venusian conditions were a great shock to astronomers. How can the atmospheres of Venus and Earth, despite their similarities in size and mass be so different? The culprit is probably the seemingly small difference in distance to the Sun (30%), as we will see in Guide 19.

Spaceman Spiff zooms past Venus on his way to Mars --- next lecture.

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Last modified January 2023 by rwo

Venus images copyright © 1997, Calvin J. Hamilton. Atmosphere profile copyright © Harcourt, Inc. Greenhouse effect drawing copyright © Toby Smith. Text copyright © 1998-2023 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.