Chapter 9: Earth-Like Planets: Venus and Mars

Venus, Earth, and Mars stand in stark contrast to Mercury and the Moon. We've already discussed the Earth. Venus and Mars are similar in that they are geologically active, and have atmospheres and weather that influence the planet's surface. But for the existence of life, it's like the story of "Goldilocks and the Three Bears": Venus is too hot, Mars is too cold, and Earth is just right.

9.1 The Nearest Planets: An Overview

Venus and Mars are the closest planets to Earth, and, after the Sun and Moon, are the brightest objects in the sky.


Venus is very bright. Because its orbit is closer to the Sun than Earth, it lies close to the Sun in the sky. Venus can't be seen during daylight hours. Venus is visible after sunset or before sunrise, in the same region of the sky as the sun sets or rises. With a modest telescope, the phases of Venus can be seen. Venus is shrouded in a thick atmosphere that obscures the surface from even our best telescopes (in the visible spectrum). [Mention the NYTimes article about Lowell and the spokes on Venus.]

Mars has a much thinner atmosphere, with only partial cloud cover, allowing us to see the surface from Earth. Mars appears reddish, which we now know to be from large amounts of iron oxide in the soil. The red color is visible with the naked eye, and may have contributed to the association of Mars with war (blood).

While Mars has little liquid water, it does have substantial polar ice caps. The ice caps are visible with a telescope, and grow and shrink with the changing seasons on Mars.

In the early 20th century, Percival Lowell popularized the belief of life on Mars. He believed he saw canals on the surface of Mars -- supposedly to irrigate crops in the milder latitudes using water melting from the polar ice caps. His theory is now clearly invalidated. What he actually saw is still a mystery. His promotion of the theory of live on Mars is largely responsible for many science fiction story lines, and the public's view that Mars could be inhabited. [See first figure included with NYTimes article which shows a newspaper story from 1911 entitled "Martians Build Two Immense Canals in Two Years"!]

Rotation of the Planets

The rotation of Mars is determined by watching the motion of surface features. We know with great accuracy that its period of rotation is about 40 minutes longer than Earth's (sidereal period). Mars is tilted at 25°, also comparable to the Earth's tilt. Therefore, Mars experiences seasons comparable to Earth's, but, because Mars orbits the Sun in 1.88 Earth years, the seasons last about 6 months rather than 3 months.

The rotation of Venus cannot be directly measured by with a telescope due to the thick atmosphere that obscures the surface. To determine its rate of rotation, we used radar, as done for Mercury. Mercury's rotation was determined from the doppler shift of the returning radar signal. For Venus, we use the radar to image the surface and observe the motion of surface features. The result is rather surprising.

Venus rotates backward with a sidereal period of 243 days. First, the rotation period is longer than the orbital period by 19 days. The length of the sidereal period means that the length of a "day" on Venus (noon to noon) is 117 days, very different from the sidereal period. Second, Venus is the only planet that rotates in the opposite sense to its orbit. The leading theory for why Venus rotates differently from the other planets is that it suffered an impact with a large object, probably during the early period of the solar system.

Basic Properties of Venus and Mars

In physical size, Venus is much like Earth, almost the same diameter, density, and surface gravity. Venus also has considerable geological activity. But the atmosphere of Venus is very different from Earth's. It's composed primarily of carbon dioxide, the surface pressure is 90 times higher than Earth's (90 bar, roughly equivalent to the pressure 1km under the ocean), and the surface temperature is 730K (850°F).

Mars is smaller than Earth, about half the diameter and 11% of Earth's mass. Mars has considerable geological activity. It's atmosphere is similar in composition to Venus, but is very thin, with a surface pressure of 0.007 bar (0.7% of Earth's atmosphere, equivalent to about 100km altitude on Earth). But there is evidence that long ago Mars may have had a thick atmosphere with surface water, and possibly even life.

9.2 The Geology of Venus

Spacecraft Exploration of Venus

Most of what we know about Venus has come from spacecraft sent to the planet. More spacecraft have been to Venus than any other planet, and many of those were from the Soviet Union.

Probing Through the Clouds

Craters and the Age of the Surface

Volcanos on Venus

Tectonic Activity

On Venus' Surface

9.3 The Massive Atmosphere of Venus

Composition and Structure

Surface Temperature

9.4 The Geology of Mars

Spacecraft Exploration of Mars

Global Properties

Volcanos on Mars

Cracks and Canyons

The View on the Surface

Martian Samples

9.5 Martian Polar Caps and Climate

Clouds on Mars

The Polar Caps

Channels and Floods

Climate Change on Mars

The Search for Life on Mars

9.6 Divergent Planetary Evolution

© Robert Harr 2002