Comparative planetology

Spacecraft have obtained detailed photographic, magnetic, radar, and chemical data from the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, as well as from numerous natural satellites and even from some asteroids and comets. Data have been used to prepare models describing planetary and minor bodies' structure and geological history throughout the solar system.

Overview

Comparative planetology is the study of the broad physical and chemical processes that operate in and on planets over time. It looks for patterns in the similarities and differences displayed by the planets and seeks to provide explanations for them in terms of planetary origins and evolution.

The first successful step in planetary exploration using robotic spacecraft was taken on August 26, 1962, when the National Aeronautics and Space Administration’s (NASA’s) Mariner 2 spacecraft was launched on a flyby mission to Venus. Mariner 4 was sent to Mars on November 28, 1964. Mariner, Pioneer, Pioneer Venus, Venera, Viking, Voyager, Magellan, Galileo, and Pathfinder space probes have sent back information about Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. Long after their primary missions had been completed, some of these spacecraft continued to transmit valuable information back to astronomers on Earth.

The planet Mercury had eluded detailed analysis by astronomers for centuries because of its small size and proximity to the Sun. Mariner 10, launched in 1973 with a dual mission of studying the clouds of Venus and photographing Mercury, made three passes by Mercury and was able to photograph about 45 percent of the surface of the planet. Mariner 10 was, thus, the first spacecraft to take scientific equipment to Mercury. Photographs of Mercury revealed a heavily cratered surface very much like that of Earth’s Moon. Naturally, there are differences between the Moon and Mercury. Since the number of craters per square kilometer varies by as much as a factor of ten, it is believed that some craters may have been covered by a volcanic process. Still, Mercury’s surface shows evidence of less volcanic activity than the Moon’s. Mercury’s largest impact basin, Caloris, has a diameter of thirteen hundred kilometers. It is believed to have been formed when a large asteroid struck the planet. Photographs show that the shock wave from this collision penetrated the planet and altered the surface on the opposite side, an example of antipodal focusing of seismic energy by the planet’s core. Compression scarps (cliffs), which can be as much as three kilometers tall and hundreds of kilometers long, were also found on Mercury. They are younger than the craters and are thought to have formed due to some internal process, such as the cooling of the planet's core. A magnetic field with a strength of about 1 percent of Earth’s magnetic field and a very diffuse atmosphere containing mostly helium were found. Surface temperature ranges from 90 to 948 kelvins.

Venus has been a difficult planet to study from Earth because of its dense atmosphere. Russian Venera probes found a surface temperature of 748 kelvins and an atmospheric pressure of 95 atmospheres. (One atmosphere is the pressure exerted by Earth’s atmosphere at sea level.) Photographs of the Venusian surface revealed some areas with smooth plains while others have rocky terrain. Radar mapping, first by the American Pioneer Venus probes and later, in higher resolution and with fuller coverage, by the Magellan orbiter, shows a surface of 70 percent gently rolling plains, 20 percent lowlands, and 10 percent highlands. The Ishtar Terra highland area of Venus is larger than the United States and includes the mountain Maxwell Montes, which stands about 11.3 kilometers tall. Alpha Regio and Beta Regio are mountainous regions that may contain shield volcanoes. A Russian Vega probe observed lightning discharges in these regions, which could mean the volcanoes are still active. Lowland areas have the appearance of a cracked slab of lava or cemented volcanic ash. The rocks in the plains are probably granitic rocks or potassium-rich basalt.

At the relatively low altitude of twenty-six kilometers, the Venusian atmosphere is clear, with the temperature dropping to 583 kelvins and the pressure to twenty atmospheres. A thick cloud layer, about 80 percent liquid sulfuric acid in the upper portion, exists from twenty-six kilometers to sixty kilometers above Venus’s surface. A sulfuric acid haze exists from sixty kilometers to eighty kilometers altitude. The overall atmosphere comprises 96 percent carbon dioxide, 3.4 percent nitrogen, and trace amounts of several other gases and reflects 76 percent of the light striking it. No planetary magnetic field was found.

Following the Pioneer Venus probes after more than a decade’s hiatus, NASA returned to Venus with the Magellan orbiter. The primary objective of Magellan was to obtain a high-resolution map of nearly the entire globe of Venus using a synthetic aperture radar system. Magellan’s mission was extended to permit more site-specific investigations. In all, Magellan produced a map of Venus’s surface that, in resolution and coverage, exceeded any available maps of Earth’s surface at that time.

Spacecraft such as Mariner, Viking, Pathfinder, and the Mars Exploration Rovers Spirit, Opportunity, and Curiosity found that the surface of Mars contains craters, extensive plains marked by significant sand dune areas, chaotic terrain characterized by irregular ridges and depressions, and many volcanoes. The largest volcano, Olympus Mons, stands twenty-seven kilometers tall, has a base diameter of six hundred kilometers, and is sixty-four kilometers across its summit. Orbiting probes uncovered unmistakable signs of catastrophic floods. Impact craters on Mars are concentrated in the southern hemisphere. The northern hemisphere, where lava flows have smoothed, has fewer craters, and their features tend to be sharper, indicating that they may be younger than those found in the southern hemisphere. Many of the craters on Mars show evidence of significant erosion. Mars Exploration Rover studies of rocks in situ revealed evidence of sedimentary processes requiring water.

Mars’s seasonal polar caps, made of carbon dioxide, extend well into the hemisphere, experiencing winter, but shrink and retreat quickly in early summer. Residual polar caps remain throughout the year, although their size does vary. The southern cap is made of carbon dioxide only. The northern polar cap is larger than the southern cap, has a wider temperature variation, and contains mostly water ice. It may be one of the main storehouses for water on Mars. The Mars Polar Lander was designed to investigate that, but it crashed. In 2008, the Mars Phoenix landed on the northern polar region to continue that search for water. Through sampling and analyzing the subsurface material, it provided direct evidence that water ice was present in significant amounts. Water molecules have been discovered in a soil sample collected by the Curiosity rover, which landed on Mars in August 2012.

Mars’s atmosphere is 95.3 percent carbon dioxide, 2.7 percent nitrogen, and 1.6 percent argon, with a total pressure of 0.01 atmosphere. The atmospheric temperature varies from 243 down to 173 kelvins. Sublimation of the polar ice caps causes the pressure to vary about 20 percent from season to season—fog forms in low areas in the early morning. Clouds have been seen around some of the volcanoes. Winds at least 150 kilometers per hour pick up surface dust and cause global dust storms. It can take six months for all the dust to settle out from one of these storms. Several spacecraft in orbit at Mars have had to wait for months until the dust cleared for their cameras to resume imaging surface features. Long-lived orbital spacecraft have taken images of specific features at widely spaced intervals, showing evidence of wind erosion having altered the surface. Other features have established proof of water slumping of crater walls in modern times.

Jupiter's atmosphere is about 1,000 kilometers thick, with a gaseous composition of 75 percent hydrogen, 24 percent helium, and 1 percent other gases. Pressure at the base of such an atmosphere would be about one hundred atmospheres, and the temperature would be about 813 kelvins. The temperature at the top of the atmosphere is only about 113 kelvins. Colored bands, termed zones and belts, are visible in the atmosphere. Zones are yellow-white and represent high-pressure areas where warm currents are rising. Belts are brown, red, or blue-green, representing low-pressure areas where colder gases are sinking. Colors have their source in the interaction of chemical compounds in the atmosphere with sunlight. Powerful wind currents flow in opposite directions where the belts and zones touch. Bands are stable and have not changed their positions for a hundred years.

Jupiter’s Great Red Spot has been its most prominent feature for over 350 years. It is about twenty-six thousand kilometers from east to west and fourteen thousand kilometers from north to south. This large cyclonic storm wanders in an east-west fashion and may be stable enough to last for many more centuries. Voyager investigations were followed by the Galileo orbiter and its atmospheric probe, which entered the atmosphere at a point where it found relatively little water vapor.

Jupiter’s magnetic field, at least ten times stronger than Earth’s, produces a radiation belt that is strong enough to kill a human quickly. The radiation belt almost ruined some transistors in the Pioneer probes. Jupiter’s radio emissions come from charged particles trapped in the magnetic field. Voyager 1 found a ring system whose main ring is six thousand kilometers wide and thirty kilometers thick. A thin sheet of material extends to the surface of the planet.

Jupiter has sixty-seven known moons. More than a dozen were discovered by Earth-based astronomers, while two were found in Voyager 1 photographs. Others were found by Voyager 2, the Hubble Space Telescope, and the Galileo spacecraft. Active volcanoes were found on the moon Io, Jupiter’s innermost moon. An icy crust on Europa is believed to cover an ocean of liquid water; evidence of crustal movement upon such a water layer was found in 2008. That provided strong evidence for internal heating to drive large-scale movements of the icy crust.

Saturn is the second of the giant planets visited by spacecraft. Its atmospheric structure is much like that proposed for Jupiter, but its composition is more like the Sun’s, with only 11 percent helium. Belts and zones seen on Jupiter are also visible on Saturn, but their colors are not as intense. The outer layer is predominantly hydrogen. The temperature at the bottom of this layer is seventy kelvins.

Saturn has the most highly developed ring system in the solar system. The ring system has a width of 153,000 kilometers and a thickness of two kilometers. There are nine distinct rings, labeled A through G. (Identification of portions of the planet’s ring system retains naming schemes from the early days of telescopic observations before the full complexity of the rings was seen; as a result, six parts of the overall ring system are identified by capital letters, whereas the rest are given names. Unfortunately, letters and names do not necessarily provide information about distance from the outer atmosphere; for example, the C and B rings are outside the D ring but inside the A ring, and the F and G rings are outside the A ring.) The rings are very complex, made up of many ringlets, some of which are only about two kilometers wide. Shepherding moons orbit around the edge of some rings, and their gravity functions to maintain the sharp edge of the rings. The B ring shows dark features that resemble spokes in a wheel. The particle size in the rings varies from a few thousandths of a centimeter to about ten meters. The spokes rotate as if solid and appear to be particles electrostatically raised above the ring plane.

Saturn has at least sixty satellites. The Cassini spacecraft provided images of many satellites detected after the Voyager era. Saturn’s only large satellite, Titan, has received much attention since it can retain a thick atmosphere of nitrogen, methane, and other hydrocarbons. Its surface is obscured due to the thickness of that atmosphere. For that reason, the Cassini orbiter carried a European Space Agency probe named Huygens that was detached from the main spacecraft to land on the surface of Titan. Huygens provided evidence of liquid hydrocarbons at its touchdown site at cryogenic temperatures. Cassini was then able to image ancient shorelines and prove the existence of lakes of liquid methane across the surface of Titan.

In January 1986, Voyager 2 passed by Uranus en route to Neptune. Uranus’s rotational axis is tilted eighty-two degrees from the plane it orbits. Its rotational direction is retrograde. Voyager 2 data established the rotational period of the planet to be seventeen hours and fourteen minutes. The greenish atmosphere of Uranus is unusually free of clouds. The primary components of the atmosphere are hydrogen (84 percent), helium (14 percent), and methane (2 percent). The temperature of the atmosphere where the pressure is 1 atmosphere is about seventy-three kelvins. Voyager 2 observed a tenuous haze around Uranus’s rotational pole. This haze is probably formed by the steady irradiation of the planet’s upper atmosphere by solar ultraviolet light. Uranus is a weak emitter of thermal radiation from deep within the atmosphere. The planet has been found to be warmer than thought, implying a greater atmosphere transparency than models had predicted. The magnetic field of Uranus is inclined at a sixty-degree angle to the axis of rotation. (Earth’s rotational axis and magnetic field are roughly parallel by comparison.)

Voyager 2 found a large spot on Neptune’s southern hemisphere in 1989, similar to Jupiter’s Great Red Spot. The probe also confirmed the presence of three thin, faint rings around the planet and a magnetosphere. The atmosphere is cold, about fifty-three kelvins, and its soft, blue tint comes from the presence of methane in the upper atmosphere. Neptune has fourteen known moons, the largest of which, Triton, is covered with methane and nitrogen ice.

The New Horizons spacecraft was launched in January 2006 to fly by the Pluto-Charon system and complete the initial survey of all major solar system systems. New Horizons was launched when Pluto was still classified as a planet. Later that same year, an identification system adopted by the International Astronomical Union (IAU) demoted Pluto to the status of a dwarf planet. In June 2008, the IAU again redefined Pluto as a plutoid or plutino. Regardless of whether Pluto is a full-fledged planet or a plutoid, New Horizons will provide the first in-depth investigations and closeup images of Pluto and its nearly similar-sized satellite, Charon, sometime in the second decade of the twenty-first century.

Knowledge Gained

Spacecraft data concerning the atmospheric composition and structure of individual planets have provided significant insight into the solar system as a whole. Mercury’s small size and high temperature made it an unlikely candidate for having any measurable atmosphere, yet Mariner 10 found a tenuous atmosphere on the planet. This condition probably arises from the solar wind that bathes Mercury. Venus has a high surface temperature but significantly more mass than Mercury and has retained its atmosphere effectively. Venus’s high temperature prevents the buildup of any significant quantity of water so that carbon dioxide remains in the atmosphere rather than forming carbonates as it can on water-rich Earth. Mars perhaps once had a much denser atmosphere, with large quantities of liquid water—possibly enough to cover the planet to an average depth of ten meters—channels on the surface point to large amounts of flowing liquid. As a result of low temperature and low surface gravity, most of Mars’s atmosphere has been lost. Perhaps water ice is trapped below the surface or in the north polar ice cap. Confirming that was the primary objective of the Mars Phoenix mission in 2008, early results from the lander strongly suggested that white material just underneath the soil was indeed water ice and neither salts nor dry ice. Mars Phoenix was outfitted with a Thermal Evolution and Gas Analyzer (TEGA). Before the end of 2008, TEGA obtained evidence of the presence of water vapor after heating soil samples that were carefully placed within its ovens by a robotic arm equipped with a scoop.

Since the giant planets Jupiter, Saturn, Uranus, and Neptune have much more massive cores and are much colder than the four inner planets, they can effectively retain light gases, such as hydrogen and helium. However, differences exist among these four because the core size differs from planet to planet.

The terrestrial planets, Mercury, Venus, Earth, and Mars, show similar surface features—craters, volcanoes, and mountains. Only Earth has demonstrated active volcanic activity, but lightning discharges around the volcanic mountains on Venus suggest that they may also be active. Volcanism has also been found on Jupiter’s satellite Io, Saturn’s satellite Enceladus, and Neptune’s satellite Triton.

The giant planets all have ring systems, although each differs from the others. Saturn’s rings are incredibly complex, with minor divisions between the rings. Uranus has a set of narrow ribbons separated by large spaces, while Neptune has only partially complete ring arcs. Jupiter has a three-component ring system. The innermost portion is called the Halo Ring. Further out is the Main Ring, which the wispy Gossamer Rings follow. High-resolution images from Galileo at Jupiter, Cassini at Saturn, and Voyager 2 at Uranus and Neptune greatly added to the storehouse of knowledge about diversity in ring system dynamics.

One great hope in the exploration of Mars was that some life form would be discovered. Experiments performed by the Viking landers provided no definitive results. Many astronomers believe Mars’s environment is much too harsh presently to support life as it would exist on Earth. Any primitive non-Earth-like forms of life might be challenging to detect. Life, primitive or otherwise, may also be possible on Europa, Enceladus, or Titan. Few scientists expect to find organisms on the latter two satellites, but some hold out hope that some degree of organized life forms may be swimming in Europa’s ocean under the satellite’s icy crust. Until a Europa lander equipped with a subterranean probe can be sent to this satellite, however, that remains only wishful speculation by exobiologists.

Context

Fascination with outer space is evident when one examines the popularity of space-based science-fiction books, films, and television programs and when one keeps track of the number of Internet hits on NASA websites during high-profile missions, such as Mars Exploration Rover landings or space shuttle flights to refurbish the Hubble Space Telescope. Solar system exploration programs are scientific attempts to satisfy human curiosity about space. One fundamental purpose for planetary exploration is to seek a better understanding of the history and origin of the solar system. While current models meet some of the criteria, many questions remain. Examining planetary atmospheres, magnetic fields, ring systems, satellites, and surfaces allows models to be improved and planetary history to be more accurately recorded. For example, the Jupiter and Saturn systems are large enough for them and their satellites to constitute small-scale solar systems. A study of such smaller systems could reveal significant details about the solar system as a whole.

Humanity also wants to know whether life exists elsewhere besides Earth. Are we alone? Is the vastness of the universe just for us, or is it teeming with life? The chances of detecting life in another star system are remote, even if it does exist. The search for the solar system's planets is much more easily accomplished. In the late twentieth century, both the United States and the former Soviet Union planned uncrewed missions to Mars, including orbiters, landers, balloons, surface-roving vehicles, and a round-trip mission to return soil samples to Earth. Many ambitious plans were delayed considerably. Still, early in the new millennium, an armada of robotic spacecraft orbited around the Red Planet, and several landers were on the surface searching for evidence of water.

Bibliography

Bagenal, Fran, Timothy E. Dowling, and William B. McKinnon, eds. Jupiter: The Planet, Satellites, and Magnetosphere. Cambridge UP, 2004.

Beattie, Donald A. Taking Science to the Moon: Lunar Experiments and the Apollo Program. Johns Hopkins UP, 2003.

Bolles, Dana. "Comparative Planetology." NASA, 20 Sept. 2023, science.nasa.gov/wavelength-topics/comparative-planetology. Accessed 20 Sept. 2023.

Bond, Peter. Exploring the Solar System. Wiley, 2012.

Cole, G. H. A., and M. M. Woolfson. Planetary Science: The Science of Planets around Stars. Taylor, 2013.

Glassmeier, KH. "Solar System Exploration via Comparative Planetology." Nature Communications, vol. 11, no. 4288, 2020. doi.org/10.1038/s41467-020-18126-z.

Greenberg, Richard. Europa the Ocean Moon: Search for an Alien Biosphere. Springer, 2005.

Harland, David M. Cassini at Saturn: Huygens Results. Springer, 2007.

Harland, David M. Water and the Search for Life on Mars. Springer Praxis, 2005.

Hartmann, William K. Moons and Planets. 5th ed. Thomson Brooks/Cole, 2005.

Irwin, Patrick G. J. Giant Planets of Our Solar System: An Introduction. 2nd ed. Springer, 2006.

Lissauer, Jack Jonathan, and Imke De Pater. Fundamental Planetary Science: Physics, Chemistry, and Habitability. Cambridge UP, 2012.

Lovett, Laura, Joan Harvath, and Jeff Cuzzi. Saturn: A New View. Abrams, 2006.

Morrison, David, and Tobias Owen. The Planetary System. 3rd ed. Pearson/Addison-Wesley, 2003.

Squyres, Steve. Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet. Hyperion, 2006.