Summary of Key Ideas

All four inner planets are composed primarily of rock and metal, and thus they are classified as terrestrial.

Mercury

• Even at its greatest orbital elongations, Mercury can be seen from Earth only briefly after sunset or before sunrise.
• The Mercurian surface is pocked with craters like the Moon’s, but extensive, smooth plains lie between these craters. Long cliffs meander across the surface of Mercury. These scarps formed as the planet cooled, solidified, and shrank.
• The long-ago impact of a large object formed the huge Caloris Basin on Mercury and shoved up jumbled hills on the opposite side of the planet. Several similar, but smaller, impact features also exist on Mercury and on our Moon.
• Mercury has an iron core, which fills more of its interior than Earth’s core fills Earth.
• Mercury has a weak, global magnetic field that partially shields it from the solar wind.

Venus

• Venus is similar to Earth in size, mass, and average density, but it is covered by unbroken, highly reflective clouds composed primarily of sulfuric acid that conceal its other features from observers using visible-light telescopes.
• Although most of Venus’s atmosphere is carbon dioxide, its clouds are composed of sulfuric acid mixed with yellowish sulfur dust. Active volcanoes on Venus are likely to be a constant source of this sulfurous veil.
• Venus’s exceptionally high temperature is caused by the greenhouse effect, as the dense carbon dioxide atmosphere traps and retains heat emitted by the planet. The surface pressure on Venus is 92 atm, and the surface temperature is 750 K. Both temperature and pressure decrease as altitude increases.
• The surface of Venus is surprisingly flat and mostly covered with gently rolling hills. It has very few impact craters compared to Mercury or our Moon. There are two major “continents” and several large volcanoes. The surface of Venus shows evidence of local tectonic activity, but not the large-scale motions that play a major role in continually reshaping Earth’s surface.

Mars

• Earth-based observers found that the Martian solar day is nearly the same as that of Earth, that Mars has polar ice caps of carbon dioxide snow that expand and shrink with the seasons, and that the Martian surface undergoes seasonal color changes.
• The Martian surface has many flat-bottomed craters, several huge volcanoes, a vast equatorial canyon, and dried-up riverbeds—but no canals formed by intelligent life. River deltas and dry riverbeds on the Martian surface indicate large amounts of water once flowed there.
• Liquid water would quickly boil away in Mars’s thin present-day atmosphere, but the planet’s polar ice caps contain significant quantities of frozen water, and a layer of permafrost exists beneath parts of the regolith.
• The Martian atmosphere is composed mostly of carbon dioxide. The surface pressure is less than 0.01 atm.
• Mars has no global magnetic fields, but local fields pierce its surface in many places.
• Mars has two irregularly shaped moons, Phobos and Deimos. Both are in synchronous rotation with Mars. How they came into orbit is still under investigation.