Comparative Planetology of the Inner Planets

7-15 Comparisons of planetary features provide new insights

Now that we have examined the terrestrial planets individually, it can be useful to see how their various features compare to each other and to Earth. Table 7-1 The Inner Planets: A Comparison summarizes much of this material.

TABLE 7-1 The Inner Planets: A Comparison

Size and Mass

Earth is the largest and most massive of all four terrestrial planets. In this regard, Venus is almost the sister planet to Earth, with nearly 95% of Earth’s diameter and 82% of Earth’s mass. Although Mars is most similar to Earth in other ways, such as its history of surface water and rotation rate, it is only about half (53%) of Earth’s diameter and 11% of Earth’s mass. Mercury, with 38% of Earth’s diameter and a scant 5.5% of Earth’s mass, is much closer in size to Earth’s Moon than it is to the other terrestrial planets; indeed, Mercury is only 1.4 times bigger than the Moon.

Atmosphere

Venus has by far the densest atmosphere of the terrestrial planets, with about 92 times as much gas as the air we breathe. Furthermore, Venus’s atmosphere is composed primarily of carbon dioxide, with a minor component of nitrogen. The thick atmosphere has protected Venus’s surface from all but the most massive infalling space debris. Venus’s atmosphere is most similar in chemical composition to the air around Mars, although Venus’s air is over 15,000 times denser than the atmosphere of Mars. Although Mars’s thin atmosphere has enabled many pieces of space debris to strike the planet and form craters, these craters are slowly being eroded, primarily by wind.

Earth’s atmosphere, once very similar in density and composition to that of Venus, was transformed by water and life into the lower-density nitrogen–oxygen atmosphere we have today. As with Venus, Earth’s atmosphere protects the surface from most impacts. Many craters have been removed by our planet’s plate tectonic motion, which apparently does not occur today on any other planet. Mercury’s gravity is too low to hold any gases as a permanent atmosphere. That planet is surrounded by a very thin atmosphere of transient gases from the Sun, the planet’s interior, and possibly from polar ice. As these gases drift into space, they are replaced by fresh gas. Because its atmosphere is so thin and its surface has been unchanged by internal activity for billions of years, Mercury is the most heavily cratered of the terrestrial planets.

Surface Features

Although all four terrestrial planets have craters, only Mercury has them in large numbers, similar to what we see on our Moon. The thick atmosphere of Venus prevented many craters from forming. It has erased most of those that did form by melting them or covering them with magma. Mars has removed many of its craters as a result of erosion and weather. Earth has removed most of its craters by tectonic plate motion and weathering.

Like Earth, Venus has continentlike plateaus and ocean bottomlike lowlands. Mars has a northern cap that is lower than the rest of the planet, but no real continentlike regions. Mercury has relatively uniform height. While all four terrestrial planets have volcanoes, those of Mercury and Mars are extinct. Water erosion of surface features occurred just on Mars and Earth.

Interior

The interior chemistries of the four terrestrial planets are similar, with cores consisting primarily of iron surrounded by rock. Mercury, Venus, Earth, and Mars all have partially molten cores. Although Mercury is the smallest terrestrial planet, it has the highest density, meaning that it has the highest percentage of iron of these (and, in fact, of any) planets. This high percentage is likely caused by Mercury losing more of its outer, rocky layer, probably as a result of impacts, than did any other terrestrial planet.

Water

Earth contains by far the highest percentage of water of the terrestrial planets. Mars contains water frozen near its surface, and we have yet to determine whether it has any liquid water deep inside. Venus contains very little water compared to either Earth or Mars because Venus is so hot that it has evaporated surface water into its atmosphere, and water in its interior has probably been mostly ejected through volcanoes or when the surface periodically melts. Mercury has some water (very little compared to Earth or Mars) frozen at its poles, the result of collisions with water-rich comets. Because its interior is so iron rich, the water-bearing layers were probably blasted into space by impacts early in Mercury’s existence.

Temperature

Some of the temperatures on the terrestrial worlds are surprising at first glance. Mercury, which is closest to the Sun, has a hot daytime surface of about 700 K (800°F). Its lack of atmosphere and long day-night cycle allow a lot of this heat to escape at night, bringing its nighttime temperature down to a frigid 100 K (−280°F), much colder than on any other terrestrial planet. Venus’s thick atmosphere creates a greenhouse effect that keeps that planet at 750 K (890°F), even hotter than Mercury. Earth’s surface temperature ranges from about 330 K (140°F) to 180 K (−130°F), and Mars is colder, with temperatures ranging from 310 K (95°F) down to 133 K (−220°F).

Rotation and Magnetic Fields

All of the terrestrial planets rotate, with Earth’s solar day being shortest at 24 hours. This motion, combined with Earth’s liquid iron core, creates a strong magnetic field that surrounds our planet. Mars has virtually the same solar day, of 24 hours and 39 minutes, but its molten iron core is much smaller than that of Earth and it has no global magnetic field, only local magnetic fields. The solar days of Venus (117 Earth days) and Mercury (176 Earth days) are both extremely long by Earth standards, and, indeed, a solar day on Mercury is 2 Mercurian years long! Only a transient magnetic field created in its atmosphere by lightning has been detected around Venus. Mercury has a weak global field that apparently results from the extremely high amount of iron it contains.

7-16 Frontiers yet to be discovered

All three terrestrial planets orbiting the Sun with us have much to reveal. What is the chemical composition of Mercury’s surface rocks? What is that planet’s cooling history, and how much of its core is molten? What will the planet’s internal structure reveal?

Which volcanoes on Venus are active? Are they the source of sulfur compounds in Venus’s atmosphere? Can we find observational evidence to support the conjecture that Venus’s rotation axis has flipped over? Likewise, can we find further evidence that its surface periodically undergoes significant resurfacing?

Some of the most intriguing questions about our solar system focus on Mars. Did life ever exist there? Is there liquid water under the red planet’s surface today? If so, how far along did life evolve in that water? What are the similarities and differences between such life and life on Earth? Significant similarities might imply a common origin. Furthermore, what is the surface water history of Mars? What causes its local magnetic fields? What does its interior look like? Where did its moons come from? It is likely that we will have answers to many of these questions in the coming decades.