Terrestrial planets

The terrestrial planets are the four inner planets of the solar system. Mercury, Venus, Earth, and Mars are respectively the closest to the farthest from the Sun. These planets are composed of mostly silicate minerals and a dense inner core composed mostly of iron.

Overview

In broad terms, the planets of the solar system can be divided into two basic types: terrestrial and gas giants. The terrestrial planets (Mercury, Venus, Earth, and Mars) are, by and large, rocky, with varying degrees of atmospheric envelopes and different amounts of water, ranging from very little to a great deal. The gas giants (Jupiter, Saturn, Uranus, and Neptune), as the name implies, are larger than the terrestrial planets and have tremendous gas envelopes surrounding their cores, which remain hidden from direct observation.

Mercury is the smallest of the terrestrial planets, 4,879 kilometers in diameter. The density of Mercury is relatively high, 5.427 grams per cubic centimeter, implying that it has a bigger iron core than that of Earth. Mercury also has an active magnetic field, though its strength and origin remain unclear. Various models of this large core have been proposed to account for Mercury's odd magnetic field. Models developed since the early 1990s have posited a liquid or part-liquid iron core, and in 2008, laboratory scientists hypothesized that mantle convection might account for the fairly low iron content on Mercury's surface. In 2019, however, data analyzed at NASA’s Goddard Space Center elicited evidence that Mercury’s core is solid. The surface of Mercury contains many impact craters, so it looks much like the surface of Earth’s moon. These craters were probably formed by many meteorites (planetesimals) bombarding Mercury early in the formation of the solar system. In 2008, data from the MESSENGER orbiter showed volcanic vents near the Caloris Basin and evidence of effusion (lava floods), suggesting that volcanism played a significant role in shaping the planetary surface some three to four billion years ago and filling in earlier craters. Mercury has almost no atmosphere. The trace of hydrogen or helium in the atmosphere is most likely derived from the Sun. Temperatures vary from about 700 kelvins in sunlight down to 100 kelvins in darkness.

Venus has a diameter very similar to that of Earth (12,104 kilometers). The density of Venus is high enough (5.243 grams per cubic centimeter) to suggest that it also has a dense iron-rich core. Predictions of the planet’s heat flow suggest that this iron-rich core ought to be molten like that of Earth, which on Earth produces a strong magnetic field. Venus, however, has no magnetic field, and the reason for this lack has not been determined. The surface of Venus is sparsely cratered, with about one thousand randomly distributed impact craters. The reason for the distribution of these few craters is that large lava flows of basalt were extruded over most of the surface within the past five hundred million years, covering craters formed earlier, so only the most recent impact craters are exposed at the surface. Many other surface features have been observed on Venus. For example, highlands and lowlands form 30 percent of the surface, and midlands form 70 percent of the surface. Each of these areas includes features such as folds, fractures, lava flows, volcanoes, and features resulting from weathering. The composition of the rocks is similar to those of basalts found on Earth. The atmosphere of Venus has about ninety times the pressure at the surface as that on Earth. The main constituent in the atmosphere is carbon dioxide (97 percent), with lesser amounts of nitrogen and sulfur dioxide. The surface temperature of Venus is 735 kelvins, a result of extreme greenhouse warming due to the carbon dioxide–rich atmosphere, which absorbs infrared radiation. If the atmosphere were similar to that of Earth, the temperature would likely be about 348 kelvins.

The diameter of Earth is 12,756 kilometers. Earth has a density of 5.513 grams per cubic centimeter, and it has a dense, iron-rich core. The planet’s inner core is believed to be solid, and the outer core is believed to be liquid. The moving liquid iron is believed to produce Earth’s strong magnetic field. This magnetic field helps to protect life by deflecting many of the charged particles (solar wind) coming from the Sun. Few impact craters can be found on Earth because its crust is continuously reformed as a result of plate tectonics and weathering. Earth’s crust consists of about twenty plates of varying sizes, averaging around one hundred kilometers thick. These plates typically move laterally at less than ten centimeters per year from areas where material is being formed in volcanic mountain ranges to regions where one plate is being subducted under another plate. Other volcanism occurs in the subduction zones. In contrast to the other terrestrial planets, Earth contains abundant liquid water, which covers about 70 percent of the surface, mostly in the oceans. Certain organisms in the water have helped to remove much of the carbon dioxide from the atmosphere by forming carbonate rocks containing carbonate minerals. Removal of the carbon dioxide from Earth’s atmosphere has helped to avoid the absorption of infrared energy from the Sun, so the planet’s temperature has remained cool and stable relative to that of Venus. The Earth’s atmospheric pressure is, by definition, one atmosphere at sea level, and it contains 78 percent nitrogen and 21 percent oxygen. There are small amounts of other constituents, including varying amounts of water vapor. Surface temperatures vary from 185 to 331 kelvins.

The diameter of Mars is 6,792 kilometers, which is considerably less than that of Earth or Venus. The density of Mars (3.934 grams per cubic centimeter) is considerably less than that of the other terrestrial planets. The core of Mars still consists mostly of iron, but it appears to have fewer dense elements, such as sulfur. Mars has no magnetic field; this suggests that the core may be solid or that it may be molten but cooling and solidifying over time, thus losing much of its dynamo. Older surface rocks in the southern hemisphere of Mars are magnetized, suggesting that the core was liquid early in the planetary history. Some of the surface of Mars, especially in the southern hemisphere, contains impact craters. Huge volcanoes have formed in places, mostly from the buildup of lava. There are large plains that are composed of lava flows, sedimentary rocks, and material ejected from volcanoes through the air. The surface also displays large fractures in the rocks in places. Although there is no evidence that horizontal plate tectonics occur on Mars as on Earth, vertical tectonic movement seems to have occurred. The polar regions of Mars contain water ice, and in some locations, sinuous valleys and volcanic rock containing aqueous minerals suggest that running water may have existed on the surface in the distant past. If liquid water did exist on Mars, some form of microbial life might also have existed. However, no evidence has yet been found that life currently exists on Mars. The main obstacles to the current existence of life on Mars arise from its low atmospheric pressure (only 1 percent that of Earth) and an atmosphere composed mostly of carbon dioxide. Also, air temperatures are cold, ranging from only 186 to 268 kelvins from night to day. The low atmospheric pressure of Mars means that no liquid water can currently be stable on the Martian surface; any water in solid form would quickly sublimate (change from solid directly to gas). Thus, the sinuous valleys could have been produced in the ancient past only when the atmospheric pressure was much higher.

Knowledge Gained

Much of the information about Mercury, Venus, and Mars comes from observations from spacecraft that have flown by or landed on these planets. Also, some observations from Earth have been important.

Much of the information about Mercury came from the Mariner 10 spacecraft, which made three flybys in 1974 and 1975. Mariner 10 obtained information about the lack of an atmosphere, indication of a magnetic field, images of the abundant impact craters, and great variations in temperature.

Venus, with its continuous and thick cloud cover, has been difficult to study from Earth-based observatories. Mariner 2 flew by Venus in 1962, and the surface temperature and the lack of a magnetic field were detected. The Soviet Union’s Venera missions determined that the atmosphere was mostly carbon dioxide. Venera 7 and 8 landed on the surface of Venus in 1970 and 1972, respectively. They measured the high temperature and pressure of the atmosphere and analyzed some of the surface rocks. Because of these conditions, however, the Venera spacecraft ceased to function within a few hours of landing. Later Venera spacecraft sent back color images of the Venusian surface, obtained better chemical analyses of the rocks, and made some high-resolution radar images of the surface. The Magellan spacecraft from the United States obtained almost complete high-resolution radar images of the surface, and they collected data on the gravity field. In 2010, the Japanese Space Agency launched the Akatsuki spacecraft, which flew into Venus’s orbit. The Parker Solar Probe, launched in 2018 with the mission of producing images of the Sun, has also taken valuable pictures of Mercury and Venus as it flew past.

Earth and its atmosphere have been studied in great detail by geologists, geophysicists, and atmospheric scientists for many centuries, so vast amounts of information have been collected compared to the other terrestrial planets. Geologists have mapped Earth’s surface geology in great detail, and they have determined the mineralogy and chemical composition of the rocks at and near the surface of Earth. Geophysicists have used seismic waves from earthquakes and data on variations in gravity and magnetism to estimate the composition of the interior of Earth. Atmospheric scientists have determined the composition and variations of the present and ancient atmosphere of Earth.

Initially, Mars was viewed by telescope from Earth, and some astronomers believed that they saw “canals” (or, from the Italian, “channels”) on the surface. As a result, some nineteenth- and early twentieth-century astronomers speculated that Mars could have life. When Mariner 4 flew near Mars in 1965, it revealed that the southern hemisphere of Mars had impact craters and no canals. It also found that Mars had no magnetic field. Mariners 6 and 7 also flew by Mars in 1969, and they obtained more surface images. Mariner 9 orbited Mars in 1971. Detailed images of the surface revealed such features as large volcanoes and sinuous canyons. Vikings 1 and 2 landed on Mars in 1976, and they sent back information about the soil and air and searched for life until 1982 when they ceased to function. Numerous spacecraft since the late 1990s have extended the examination of Mars’s surface and atmosphere. These have included the Mars Climate Orbiter, Mars Polar Lander, Mars Odyssey, Mars Pathfinder, Mars Express, Mars Reconnaissance Orbiter, Mars Global Surveyor, and Mars Phoenix, as well as the rovers Sojourner, Spirit , Opportunity, Curiosity, and Perserverence.

Context

Many questions about the terrestrial planets that need to be answered remain and should at least partially be answered by current and future space missions. For instance, the MESSENGER mission, launched on August 3, 2004, reached Mercury in 2008. The spacecraft had to slow down considerably to be able to orbit Mercury, so the National Aeronautics and Space Administration (NASA) had it fly by Venus, Mercury, and Earth several times to slow it sufficiently to facilitate orbit of Mercury in March 2011. As a result, MESSENGER flew by Venus in June 2007 and took pictures to calibrate its cameras. It flew by Mercury on January 14, 2008, on October 6, 2008, and again on September 29, 2009, taking a number of pictures of the surface. MESSENGER's objectives included taking more pictures of the surface to help determine the planet’s geologic history and processes such as mantle convention, studying the magnetic field to help determine its origin and the nature of the core, and conducting experiments to determine the composition of some of the rocks and atmosphere in order to gain better understanding of their formation. Its primary mission lasted from March 2011 to March 2012, a duration of four Mercurian years (or one Earth year), and its extended orbital mission began in March 2013. During this time, the MESSENGER mission revealed unprecedented information about the planet's volcanic activity, surface features, upper atmosphere, and magnetic field, as well as evidence for water ice in the planet's shadowed polar regions.

Another mission to Mercury, a joint effort between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), is the BepiColombo space probe, launched in 2018 and expected to arrive in Mercury's orbit 2025. BepiColombo includes two separable orbiters that will orbit at different distances, allowing more detailed observations of the surface of Mercury.

The ESA's Venus Express, launched in November 2005, entered orbit around Venus in April 2006. Designed to study the dynamics of and chemical interplay between the Venusian atmosphere, clouds, and surface, it has revealed more recent volcanic activity on the planet than previously believed; discovered vortices and lightning in the atmosphere; and recorded wind, temperature, and other atmospheric data. JAXA's Akatsuki (Planet-C) orbiter, launched in May 2010, is designed to use infrared imaging technology to collect meteorological data and to determine whether volcanic activity is ongoing; the spacecraft missed its scheduled December 2010 entry into Venus's orbit but entered successfully in 2015.

Mars is undergoing continual intense investigation through both orbiting spacecraft and surface experiments conducted by rovers. In early 2009, Earth-based observations led to the discovery of a methane signature in the Martian atmosphere. The Mars Science Laboratory, launched in November 2011, landed on August 6, 2012, and its rover Curiosity is collecting and analyzing rock samples in order to determine whether microbial life previously existed on the Red Planet. In February 2021, the Mars rover Perseverance joined.

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