Chapter 6: The Terrestrial Planets

Planets of the Solar System (Sizes to scale; distances and illumination not to scale)

The inner Solar System has four terrestrial planets composed of rocky and metallic materials. Mercury is small and appears similar to the Earth's Moon. Its surface is heavily cratered, and its gravity is too weak to retain a substantial atmosphere. Venus is roughly Earth's size. Our "sister planet" has experienced significant geological activity, which has resurfaced the planet and concealed most of the evidence of bombardment from space. Venus has an extremely dense atmosphere of CO₂ that causes an intense greenhouse effect, which heats the barren, volcanic surface to hundreds of degrees.

 

Mars has the most earthlike environment, but even that's a cold, frozen desert with only a thin atmosphere of mainly CO₂. Mars has abundant water, but it's trapped in three forms: polar ice, underground permafrost, and water molecules inside hydrated minerals. In spite of the fact that there is no liquid water now, dry riverbeds and lakeshore features show that ancient Mars — at least temporarily — had a climate that allowed liquid water to flow across the surface. Two mysteries about Mars remain unsolved: the cause of the climate change, and the question of whether life ever evolved on Mars. Possible evidence of ancient microbes was reported in a meteorite from Mars in 1996, but this evidence turned out to be inconclusive.

Differences among the terrestrial planets can be explained using some general physical principles. These principles should apply to other planetary systems as well. Larger planets hold heat longer because they are more insulated, and they have more energy produced by radioactive decay in their dense cores. Therefore, they will have more active volcanism, younger surface features, and fewer impact craters. Planets that are more massive also have stronger gravity, so they are able to retain more atmospheric gases. Smaller planets are more likely to be geologically dead and have older, heavily cratered surfaces. On smaller planets, the gravity is weak and atmospheric gases can escape into space more easily. The volcanic gases emitted from larger planets are rich in CO₂ and H₂O. On Venus, the H₂O evaporated and was lost; it remained liquid on Earth and froze on Mars. CO₂ became the main atmospheric gas on all three planets, although on Earth, most of it dissolved in the oceans, and plant life added oxygen to the atmosphere.

The gas giants against the Sun's limb, at 1 px = 1 Mm. The diameters are to scale. The limb of the Sun is in the background. From left to right, Jupiter, Saturn, Uranus and Neptune.

The four giant planets — Jupiter, Saturn, Uranus, and Neptune — and their satellite systems also show some systematic similarities and differences. All four of the giant planets have dense, cloudy atmospheres, which range from 63% to 93% hydrogen by mass, with the rest being mostly helium. All have rings composed of billions of icy or rocky particles ranging in scale from microscopic to house-sized. All have systems of satellites. Most of the satellites contain abundant ice as well as darker soils because this part of the Solar System is so cold that ices were the main constituent of the material available for building moons and planets.

There are also important differences between the gas giants. The more remote planets are colder and have hazier, less colorful cloud patterns. Uranus and Neptune are also smaller than Jupiter and Saturn and have a blue color caused by methane-rich hazes, instead of the reddish-brown and tan cloud forms of the larger bodies. Uranus is tipped so that its equator and satellite orbits lie nearly perpendicular to the plane of the Solar System. The planets also differ in the structure and composition of their ring systems. Rings are probably composed of tiny pieces of satellites that were fragmented by impacts with interplanetary bodies. The appearance of ring systems may depend on the time elapsed since the latest satellite disruption, and the composition of that satellite.

Only a generation ago, the many satellites of the giant planets were assumed to be geologically dead balls of rock and ice. Direct exploration by spacecraft has shown much more variety. On some of these satellites, tidal heating is important in driving geological activity, including active volcanoes and lava lakes. Generally, the satellites that are large and close to their planets experience the most intense tidal forces. Some of the most distinctive satellites include Jupiter's satellite Europa, which may have an ocean of liquid water beneath the frozen surface, Jupiter's Io, with erupting volcanoes, Saturn's Titan, with its dense, smoggy, nitrogen-rich atmosphere, and Neptune's Triton, which has smoking vents and a thin atmosphere.

Illustration of relative sizes, colours and albedos of the large trans-Neptunian objects.

Pluto, until recently classified as the ninth planet, may only be the largest of many interplanetary bodies in that region. It is smaller than our Moon, and it is unique in having a satellite about half its own size. The New Horizons probe has shown that Pluto is far from being a dull, frozen rock. It has geological activity and underground water and a terrain that is young and constantly evolving.

The diversity of worlds in the outer Solar System is fascinating, but it can also be bewildering. However, simple physical principles govern their behavior. The most important is gravity. Gravity causes the vertical structure of the giant planet atmospheres, where gas compresses into liquid at great depths, and eventually into a slushy solid. The uneven action of gravity over an extended body explains why some satellites have fractured surfaces and hot interiors. This tidal force also explains why ring systems are only found close to a planet. Moons and ring particles all orbit their planets in miniature applications of Kepler's laws, which are just an extension of Newton's laws of motion and gravity. These principles will also apply to other solar systems that astronomers are discovering in other parts of the galaxy.