Diversity of the Planets

By about 4.4 billion years ago, in less than 200 million years since its origin, Earth had become a fully differentiated planet. The core was still hot and mostly molten, but the mantle was fairly well solidified, and a primitive crust and continents had begun to develop. Oceans and atmosphere had formed, and the geologic processes that we observe today had been set in motion. But what about the other terrestrial planets? Did they experience a similar early history? Information transmitted from space probes indicates that the four terrestrial planets have all undergone gravitational differentiation into layered structures with an iron-nickel core, a silicate mantle, and an outer crust (Table 9.1).

Mercury has a thin atmosphere consisting mostly of helium. The atmospheric pressure at its surface is less than a trillionth of Earth’s atmospheric pressure. There is no surface wind or water to erode and smooth the ancient surface of this innermost planet. It looks like the Moon: it is intensely cratered and covered by a layer of rock debris, the fractured remnants of billions of years of meteorite impacts. Because it is located close to the Sun and has essentially no atmosphere to protect it, the planet warms to a surface temperature of 470°C during the day and cools to −170°C at night—the largest temperature range for any planet.

Mercury’s average density is nearly as great as Earth’s, even though it is a much smaller planet. Accounting for differences in interior pressure (remember, higher pressures increase density), scientists have surmised that Mercury’s iron-nickel core must make up about 70 percent of its mass, a record proportion for solar system planets (Earth’s core is only one-third of its mass). Perhaps Mercury lost part of its silicate mantle in a giant impact. Alternatively, the Sun could have vaporized part of its mantle during an early phase of intense radiation. Scientists are still debating these hypotheses.

Venus developed into a planet with surface conditions surpassing most descriptions of hell. It is wrapped in a heavy, poisonous, incredibly hot (475°C) atmosphere composed mostly of carbon dioxide and clouds of corrosive sulfuric acid droplets. A human standing on its surface would be crushed by the atmospheric pressure, boiled by the heat, and eaten away by the sulfuric acid. At least 85 percent of Venus is covered by lava flows. The remaining surface is mostly mountainous—evidence that the planet has been tectonically active (Figure 9.7). Venus is close to Earth in mass and size, and its core seems to be about the same size as Earth’s, with both liquid and solid portions. How it could develop into a planet so different from Earth is a question that intrigues planetary geologists.

Mars has undergone many of the same geologic processes that have shaped Earth (see Figure 9.7). The Red Planet is considerably smaller than Earth, with only about one-tenth of Earth’s mass. However, the Martian core, like the cores of Earth and Venus, appears to have a radius of about half the planet’s radius, and, like Earth’s, it may have a liquid outer portion and a solid inner portion.

Mars has a thin atmosphere composed almost entirely of carbon dioxide. No liquid water is present on its surface today; the planet is too cold, and its atmosphere is too thin, so any water on its surface would either freeze or evaporate. Several lines of evidence, however, indicate that liquid water was abundant on the surface of Mars before 3.5 billion years ago, and that large amounts of water ice may be stored below the surface and in polar ice caps today. Life might have formed on the wet Mars of billions of years ago and could exist today as microorganisms below the surface.

Most of the surface of Mars is older than 3 billion years. On Earth, in contrast, most surfaces older than about 500 million years have been obliterated through the combined activities of the plate tectonic and climate systems. Later in this chapter, we will compare surface processes on Earth and Mars in more detail.

Other than Earth itself, the Moon is the best-known body in the solar system because of its proximity to Earth and the manned and unmanned programs that have been designed to explore it. In bulk, its materials are lighter than Earth’s, probably because the heavier matter of the giant impacting body remained embedded in Earth after the collision that formed it. The lunar core is therefore small, constituting only about 20 percent of the lunar mass.

The Moon has no atmosphere, and is mostly bone dry, having lost most of its water in the heat generated by the giant impact. There is some new evidence from spacecraft observations that water ice may be present in small amounts deep within sunless craters at the Moon’s north and south poles. The heavily cratered lunar surface we see today is that of a very old, geologically dead body, dating back to a period early in the history of the solar system when crater-forming impacts were very frequent. Once topography is created on any planetary body, plate tectonic and climate processes will work to “resurface” it, as they have on Venus and Mars. However, in the absence of these processes, the planet will remain pretty much the way it was just after its formation. Thus, the heavily cratered terrains of little-studied planetary bodies, such as Mercury, indicate that they lack both a convecting mantle and an atmosphere.

The giant gaseous outer planets—Jupiter, Saturn, Uranus, and Neptune—are likely to remain a puzzle for a long time. These huge gas balls are so chemically distinct and so large that their formation must have followed a course entirely different from that of the much smaller terrestrial planets. All four of the giant planets are thought to have rocky, silica-rich and iron-rich cores surrounded by thick shells of liquid hydrogen and helium. Inside Jupiter and Saturn, the pressures become so high that scientists believe the hydrogen turns into a metal.

Exactly what lies beyond the orbit of Neptune, the most distant giant planet, remains a mystery. Tiny Pluto, once regarded as the ninth planet, is a strange frozen mixture of gases, ice, and rock with an unusual orbit that sometimes brings it closer to the Sun than Neptune. Pluto, along with “2003 UB313” and two other bodies that share its attributes—tiny size, unusual orbit, rock-ice-gas composition—is now known as a dwarf planet. The dwarf planets lie within a belt of icy bodies that is the source region for the comets that periodically pass through the inner solar system. Other dwarf planet–sized objects are likely to be found as we explore the outer regions of the solar system. A spacecraft called New Horizons will visit Pluto beginning in 2015.