Comparative planetology of Venus, Earth, and Mars

The rocky planets Venus, Earth, and Mars are similar in size, mass, and proximity to the Sun, yet they have evolved in three very different directions. Venus has a thick atmosphere of carbon dioxide and a surface temperature hot enough to melt lead. Earth’s atmosphere is mostly nitrogen, with only a trace amount of carbon dioxide. Mars has a very thin atmosphere of carbon dioxide and quite cold surface temperatures.

Overview

When the planets formed in the primordial solar system, they existed primarily as gases coalesced by gravity. The four inner planets, because of their proximity to the Sun, lost most of their primitive atmosphere of hydrogen and helium, retaining only a molten rocky core rich in heavier elements. As these planets cooled, a secondary atmosphere was created by gases ejected from the mantle by many active volcanoes on the geologically unstable surface. These gases included ammonia, carbon dioxide, water vapor, and nitrogen. Mercury, being the smallest planet with the greatest intensity of sunlight, quickly lost its entire atmosphere.

More massive and farther from the Sun than Mercury, Venus retained its secondary atmosphere. This caused the surface to remain hot enough to prevent the water vapor from condensing into liquid. As the concentration of CO₂ increased, the heat, in conjunction with ultraviolet light from the Sun, dissociated the water vapor into free hydrogen and oxygen. The hydrogen dissipated into space, while the very reactive oxygen combined with rock minerals, thus disappearing from the atmosphere. Estimates made from Venus’s rocks indicate that initially, there was enough water vapor on Venus to have covered the surface with an ocean at least 9.14 meters deep, on average. The continued volcanic outgassing of CO₂ continued to heat the surface until equilibrium was established. When visible light strikes a planetary surface, much of the energy is converted into heat, which radiates from the surface as infrared radiation. Carbon dioxide will absorb outgoing infrared radiation and reemit it uniformly in all directions, thus heating the surface in proportion to the amount of CO₂ present in the atmosphere. Because Venus’s atmosphere is 95 percent carbon dioxide, the surface temperature is even hotter than the surface of the planet Mercury due to what is referred to as a runaway greenhouse effect. Venus, therefore, is now a stifling inferno smothered by a thick carbon dioxide (CO₂) atmosphere. The surface temperature is about 750 kelvins, distributed uniformly across the planet due to atmospheric carbon dioxide.

Because Earth is 43 percent farther from the Sun, the solar intensity is only half that of Venus, while the gravitational attraction is slightly greater than Venus’s. Consequently, water vapor was retained and able to condense as the planet cooled, thereby forming the oceans. Carbon dioxide readily dissolves in warm water. It was progressively removed from the atmosphere, eventually forming carbonate rocks. Relatively early in Earth’s history, primitive life formed in the oceans and evolved into small-shelled sea creatures. These primitive animals formed their shells from the dissolved carbon dioxide and calcium, thus further removing CO₂ from the water and leaving shell deposits that eventually agglomerated into calcium carbonate (CaCO₃). As CO₂ was removed from the water, more could enter the atmosphere until almost all was gone. Analysis shows that the carbonate rocks on Earth’s crust contain about the same amount of CO₂ as Venus’s atmosphere. Although oxygen was present in Earth’s primitive atmosphere, most reacted with metals, such as iron, to form oxides. Life further altered the atmosphere with photosynthesis commencing about 2.5 billion years ago, eventually reducing the concentration of atmospheric CO₂ to its present 0.04 percent and creating most of the free oxygen.

Mars is 1.5 times farther from the Sun than Earth. Being considerably smaller and less massive, Mars had less internal heating and consequently considerably less outgassing. Mars, therefore, has an extremely thin atmosphere, 95 percent of which is CO₂. This is an amount insufficient to create any significant greenhouse warming.

The Vikingspacecraft showed that although Mars originally had a denser atmosphere and running water on the surface, most of the atmosphere leaked into space because of the low gravity. The atmospheric pressure, less than 1 percent of Earth’s sea-level pressure, is below the pressure where water can exist in the liquid state. Although a core of water ice is present in the polar caps, the ice in those caps consists primarily of solid carbon dioxide. Surface temperatures on Mars vary from a maximum of 300 kelvins at the warmest spot at the warmest moment of the warmest day of the Martian year to typical nighttime temperatures of 155 kelvins. At a location midway between the equator and the poles, the maximum daytime temperature barely exceeds the freezing point of water. Much of the remaining water is assumed to be tied up below the surface as permafrost. The original CO₂ is now either carbonate rocks or dissipated into space. The polar caps consist of small permanent water ice caps and solid CO₂ (dry ice), which sublimates directly into its vapor form when the temperature increases during summer.

Although the atmospheres of Venus, Earth, and Mars appear very different, when the elements bound up in rocks and permafrost are included, the inventories of water, carbon dioxide, and nitrogen are remarkably similar when adjusted for the differing planetary masses.

The topography of the three planets is a result of their size and evolution. Earth, the largest, is dominated by rolling seafloor plains interrupted by continents and mountain ranges where continental plates have collided. All traces of Earth’s primeval crust have been destroyed by basalts that erupted to form much of the seafloor crust and by plate tectonics that broke up and recycled the original surface.

On Venus, basaltic volcanism dominated and covered most of the planet with lava flows. Only a small part of the primordial surface remained as protocontinents projecting several miles above the plains. Perhaps because of its smaller size, there was not enough internal energy to drive plate tectonic crustal motions. Some volcanoes may still be active. The scarcity of meteorite impact craters indicates that the entire crust has been replaced within the past half million years—recently, in geologic terms.

Being considerably smaller than Venus, Mars lacked sufficient tectonic energy to destroy its original cratered features. The deepest surface depressions are ancient impact craters, but the highest Martian mountains are simply masses of volcanic lava surmounted by the volcanic caldera. As a rule of thumb, small worlds preserve their ancient surfaces formed by meteorite bombardment, but volcanic forces break through and resurface parts of the planet. Larger planets, on the other hand, have surfaces dominated by the internal forces of volcanism and plate tectonics.

Knowledge Gained

It has been known since the eighteenth century that Venus has an atmosphere. Although it was first assumed to be composed of water vapor, in 1932, spectroscopic studies indicated that Venus’s atmosphere was primarily CO₂. Thermal radiation measurements in the 1960s indicated a surface temperature close to 750 kelvins. This temperature was confirmed when, in 1967, the Russian probe Venera 4 crashed into the Venusian surface. In 1970, the Venera 7 successfully landed on Venus’s surface and transmitted data for twenty-three minutes, verifying the surface temperature and an extremely dense CO₂ atmosphere. The American spacecraft Pioneer (1978) discovered clouds of sulfuric acid droplets positioned about 48 kilometers above the surface. This mission also included an orbiter that mapped the surface features by using radar. Russian landings later sent back panoramic photographs of a haze-free surface imbued with a reddish hue from atmospheric filtered sunlight, strewn with boulders on gravel and fine, rocky soil. In 1985, two balloons dropped by Russian probes floated in the sulfuric acid clouds for forty-six hours, measuring hurricane-force winds of 240 kilometers per hour but temperatures and pressures similar to Earth’s surface. When the sulfuric acid particulates reach a sufficient size, they begin to fall out of the cloud deck as sulfuric acid rain. Unlike rain on Earth, however, they never reach the ground. The rapidly increasing temperature causes them to evaporate. Throughout the early twentieth century, several missions funded by Japanese and European Space Agencies, as well as the Parker Solar Probe, launched by NASA in 2018, have continued exploration of Venus. Several more were planned throughout the 2020s.

Telescopic observations of Mars during the eighteenth and nineteenth centuries revealed clear seasonal changes. In summer the polar cap shrinks, while dark markings, once assumed to be vegetation, darken and grow more prominent. This assumption was disproved in 1965 when Mariner 4 returned the first close-up pictures revealing a desolate, red-colored desert. Three Russian probes reached the surface between 1971 and 1974, but all failed to return useful data. The first successful landing was by Viking 1 on July 20, 1976, followed in September by its sister craft, Viking 2. The Viking landers photographed a surface strewn with boulder-sized fragments of lava flows and meteorite impact craters in different stages of erosion. Soil analyses provided chemical evidence that Mars’s atmosphere was once almost as dense as Earth’s, which would have enabled water to flow. Earlier, Mariner 9 had photographed channels looking like dry riverbeds containing tributaries and sedimentary deposits, providing additional evidence that water once flowed on Mars.

Indisputable evidence of Mars’s water was provided during the summer of 2008, when the National Aeronautics and Space Administration’s (NASA’s) Mars Phoenix lander dug into Martian soil and scooped up what appeared to be ice. When melted, it proved to be water vapor. Three spacecraft landed between 1997 and 2004 to photograph and analyze the surface using robotic rovers. The Martian surface consists of two parts: a heavily cratered southern highlands and northern smooth lowlands. Evidence from the Odyssey spacecraft (2001) suggests these lowlands were once filled by an ocean of liquid water, now present in permanent ice caps and subterranean permafrost. Mars’s early dense atmosphere may have been depleted by changes in the polar climate. If solar radiation striking the poles were 10 percent higher in the past, the ice caps would have been much smaller, and most of the now-frozen CO₂ would have existed in a gaseous state. Exploration of Mars continued in the twenty-first century with three rovers in the active process of exploration in 2023: CuriosityPerseverance, and Zhurong.

Context

Although Venus, Earth, and Mars are similar in mass and distance from the Sun, enough minor differences existed to compel each to have evolved along very different paths. The atmospheres of Venus and Mars have been thoroughly analyzed, and their surface features have been carefully mapped, photographed, and subjected to chemical analysis. Nevertheless, several questions remain unanswered. How much water was initially present on Venus? Why was there not more plate tectonic activity during Venus’s formative years? Why does Earth’s atmosphere contain an abundance of nitrogen, and why is it scarce on the other planets? Why did Mars have a much warmer surface in the past, and where is the once prevalent water? Did the formation of the gigantic volcano, Olympus Mons, cause a change in the tilt of the Martian axis that instigated a planetary cooling?

There is still much to learn about the comparative planetology and evolution of these three planets. Future interplanetary spacecraft that land on Venus and Mars will continue to advance the understanding of the dynamics of their surfaces and, in particular, the manner in which subterranean features influenced their respective planetary evolution.

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