Key Ideas

Motions of Mercury, Venus, and Mars in Earth’s Sky: Mercury and Venus can be seen in the morning or evening sky only, while it is possible to see Mars at any time of night depending on its position in its orbit.

• At their greatest eastern and western elongations, Mercury is only 28° from the Sun and Venus is only 47° from the Sun.

Rotation of Mercury, Venus, and Mars: Poor telescopic views of Mercury’s surface led to the mistaken impression that the planet always keeps the same face toward the Sun (1-to-1 spin-orbit coupling).

• Radio and radar observations revealed that Mercury in fact has 3-to-2 spin-orbit coupling: The planet rotates on its axis three times every two orbits.
• Venus rotates very slowly in a retrograde direction. Its rotation period is longer than its orbital period.
• Mars rotates at almost the same rate as Earth, and its rotation axis is tilted by almost the same angle as Earth’s axis.

Mercury’s Surface, Interior, and Magnetic Field: Mercury’s surface is pocked with craters, but there are extensive smooth plains between these craters.

• Long cliffs called scarps meander across the surface of Mercury. These probably formed as the planet’s crust cooled, solidified, and shrank.
• Mercury has an iron core with a diameter equal to about 83% of the planet’s diameter. By contrast, the diameter of Earth’s core is only slightly more than one-half of Earth’s diameter.
• Mercury has a weak magnetic field, which indicates that at least part of the iron core is liquid. However, the field is off-center by 20%, which remains unexplained.
• The high abundance of volatile elements that should have “boiled off” from Mercury’s material during formation presents a mystery considering the planet’s close proximity to the Sun.

Comparing Venus and Mars: Most of the surface of Venus is at about the same elevation, with just a few elevated regions. On Mars, the southern highlands rise several kilometers above the northern lowlands.

• Venus has a thick atmosphere and a volcanically active surface. Mars has a very thin atmosphere and little or no current volcanism.
• There is no evidence of plate tectonics on Venus, but Valles Marineris shows there are at least two plates on Mars. On Venus, there is vigorous convection in the planet’s interior, but the crust is too thin to move around in plates; instead, it wrinkles and flakes. On Mars, the planet’s smaller size means the crust cooled long ago and became too thick to allow widespread plate tectonic activity.
• Volcanoes on both Venus and Mars were produced by hot spots in the planet’s interior.
• The entire Venusian surface is about 500 million years old and has relatively few craters. By contrast, most of the Martian surface is cratered and is probably billions of years old. The southern highlands on Mars are the most heavily cratered and hence the oldest part of the planet’s surface.

The Atmospheres of Venus and Mars: Both planetary atmospheres are more than 95% carbon dioxide, with a few percent of nitrogen.

• The pressure at the surface of Venus is about 90 atmospheres. The greenhouse effect is very strong, which raises the surface temperature to 460°C. The pressure at the surface of Mars is only 0.006 atmosphere, and the greenhouse effect is very weak.
• The permanent high-altitude clouds on Venus are made primarily of sulfuric acid. By contrast, the few clouds in the Martian atmosphere are composed of water-ice and carbon dioxide ice.
• The circulation of the Venusian atmosphere is dominated by two huge convection currents in the cloud layers, one in the northern hemisphere and one in the southern hemisphere. The upper cloud layers of the Venusian atmosphere move rapidly around the planet in a retrograde direction, with a period of only about 4 Earth days.
• Weather on Mars is dominated by the north and south flow of carbon dioxide from pole to pole with the changing seasons. This can trigger planetwide dust storms.

Evolution of Atmospheres: Earth, Venus, and Mars all began with relatively thick atmospheres of carbon dioxide, water vapor, and sulfur dioxide.

• On Earth, most of the carbon dioxide went into carbonate rocks and most of the water into the oceans. Ongoing plate tectonics recycles atmospheric gases through the crust.
• On Venus, more intense sunlight and the absence of plate tectonics led to a thick carbon dioxide atmosphere and a runaway greenhouse effect.
• On Mars, a runaway icehouse effect resulted from weaker sunlight and a lack of strong plate tectonic activity.

Water on Mars: Liquid water cannot exist on present-day Mars because the atmosphere is too thin and cold. But there is evidence for frozen water at the polar ice caps and beneath the surface of the regolith.

• Geological evidence from unmanned rovers shows that much of the Martian surface has been dry for billions of years, but some regions had substantial amounts of liquid water in the past.

The Moons of Mars: Mars has two small, football-shaped satellites that move in orbits close to the surface of the planet. They may be captured asteroids or may have formed in orbit around Mars out of solar system debris.