terrestrial planets and other Earth-like bodies

terrestrial planets and other Earth-like bodies The Earth is just one of many planets and planet-sized bodies orbiting the Sun. Information from space missions, most spectacularly in the form of close-up pictures, has removed these planetary bodies from the realm of astronomy into the purview of geologists and meteorologists. Other Earth-like bodies display all the processes with which we are familiar on the Earth, but blended in different proportions and therefore having different net results, and their study helps considerably in understanding our own planet. By way of analogy, suppose you were trying to study the life cycle of a tree. Imagine how poor and incomplete an appreciation you would have of this tree in particular, and of trees in general, if you only ever looked at a single specimen—say an oak growing in a sheltered meadow—and did not realize that its way of growth would be entirely different were it on a windy hillside or in a dense forest. Consider also how wrong you would be to assume that essential attributes of all trees were (like the oak) to shed their leaves in autumn and to have no defence against browsing animals, because you had never looked at an evergreen pine tree or a thorn bush.

The bodies most like the Earth are referred to as the terrestrial planets. Traditionally there are four of these. In sequence outwards from the Sun they are Mercury, Venus, Earth, and Mars. These planets share the fundamental characteristic of being large, rocky bodies with a dense, iron-rich, core. The Moon, although a satellite of the Earth, and therefore not a planet in the astronomical sense, has so much in common with them that geologically it can be regarded as a fifth terrestrial planet. Io, the innermost large satellite of Jupiter, belongs to the same class as the Moon. The tally of Earth-like rocky bodies thus stands at six (Table 1).

The giant planets (Jupiter, Saturn, Uranus, and Neptune) are sufficiently unlike Earth to be discussed in a separate entry (see giant planets), but there are 19 other bodies in the solar system large enough to show (to varying degrees) Earth-like attributes in terms of internal differentiation into core and mantle, and surface processes of volcanism, fault movements, and cratering. These bodies tend to be found further from the Sun than the terrestrial planets and have icy mantles surrounding rocky cores. Although compositionally different from the Earth, the geological processes in their icy outer layers are close analogues to those in the terrestrial planets. Most of these icy bodies are satellites of the giant planets, but Pluto and its moon Charon also belong to this class (Table 2).

The other solid matter in the Solar System consists of objects with insufficient gravity to pull themselves into spherical shapes and generally too small to show internal differentiation. These are rocky asteroids, ranging from 900 km across downwards, and icy bodies less than about 400 km across. They, too, are sufficiently unlike the Earth to be described in a separate entry (see asteroids and comets).

The Moon

The most appropriate Earth-like body with which to begin a review is the Moon, which is the closest body to the Earth and the only one from which samples have been collected. Like the Earth, the Moon's main source of internal heat is the decay of radioactive elements. The total rate of heat production within a rocky planetary body depends on its mass, but the rate of heat loss depends on its surface area. The smaller the body, the greater the ratio of its surface area to its mass, and so the faster it loses its heat. Having less than an eightieth of the Earth's mass, the Moon has lost so much of its internal heat that its interior is rigid down to a depth of about 1000 km, and there has been little volcanism or other geological activity on its surface for the past 3 billion years. Consequently, the Moon retains a record of ancient events whose traces have been almost entirely erased from the Earth's surface by erosion, volcanism, deformation, and burial.

Table 1. Earth-like rocky bodies

Name	Distance from Sun	Diameter (km)	Mass relative to Earth	Density (tonnes m−3)
	(millions of kilometres)
The Earth's mass is 5.98 × 1024 kg. The Moon is the Earth's only natural satellite, and Io is a satellite of Jupiter.
Mercury	57.9	4878	0.0553	5.43
Venus	108.2	12104	0.815	5.25
Earth	149.6	12756	1.000	5.52
Moon	149.6	3476	0.0123	3.34
Mars	227.9	6786	0.11	3.95
Io	778.3	3630	0.0149	3.57

Table 2. Earth-like icy bodies

Name	Satellite of	Diameter (km)	Mass relative to Moon	Density (tonnes m−3)
With the exception of Pluto these are all planetary satellites, and it is convenient to compare their masses with that of the Moon (which is 7.35 ×1022kg). Europa has only a thin icy shell and is intermediate in character between the typical icy bodies and the rocky bodies listed in Table 1.
Europa	Jupiter	3138	0.653	2.97
Ganymede	Jupiter	5264	2.02	1.94
Callisto	Jupiter	4800	1.47	1.86
Mimas	Saturn	396	0.00052	1.17
Enceladus	Saturn	502	0.00109	1.24
Tethys	Saturn	1048	0.0104	1.26
Dione	Saturn	1108	0.0143	1.44
Rhea	Saturn	1524	0.0339	1.33
Titan	Saturn	5150	1.83	1.88
Iapetus	Saturn	1436	0.026	1.21
Miranda	Uranus	472	0.00102	1.35
Ariel	Uranus	1158	0.0184	1.66
Umbriel	Uranus	1192	0.0173	1.51
Titania	Uranus	1580	0.0474	1.68
Oberon	Uranus	762	0.0397	1.58
Proteus	Neptune	418	0.00054	1.1
Triton	Neptune	2700	0.291	2.08
Pluto	(Sun)	2320	0.191	2.1
Charon	Pluto	1270	0.015	1.3

The oldest regions of the Moon's surface are the lunar highlands. These occupy just over half of the Earth-facing hemisphere and most of the far side. The most obvious feature of the highlands is that they are completely covered by impact craters, ranging downwards in size from hundreds of kilometres across. Once thought to be volcanic in origin, it has now been proved that virtually all lunar craters are the result of impacts by meteoritic debris hitting the surface at speeds of a few tens of kilometres per second. In such an event, a shock wave is generated at the point of impact, which propagates radially to excavate a crater with a diameter about thirty times that of the impacting body, and distributes fragmentary ejecta over the surrounding area.

The Moon has several multi-ringed impact basins up to 1000 km or so in diameter that were caused by particularly large impacts. All those on the Earth-facing hemisphere have been flooded by basaltic lava flows. However, the largest one, the South Pole–Aitken basin (which is on the far side), escaped this fate. The lava-covered regions, which in places have spilled beyond the limits of the basins, make the familiar dark patches on the face of the Moon and are referred to (inappropriately) as the lunar seas or maria (Latin, sea; singular mare).

That the maria are younger than the surrounding highlands can be demonstrated by two simple observations. The first relies on the basic geological principle of superposition: the lavas of the maria can be seen to overlap on to the highlands. The second is that any given area of maria has considerably fewer craters than an equivalent area of highlands, and (because impact cratering is essentially a random process) a region with a consistently lower crater density must be younger than a region with a higher density of craters.

Radiometric dating of samples from the Moon has made it possible to put the above deductions on a more refined basis. The lunar maria were created by series of lava eruptions lasting about half a billion years and terminating about 3.1 billion years ago. The lunar highland surfaces, despite having much greater densities of impact craters, are not much older than the oldest maria; they date back to about 3.9 billion years ago. It is believed that the cratering we can see on the lunar highlands records the tail end of the cataclysmic bombardment inherited from the birth of the Solar System (4.6 billion years ago), which obliterated the traces of the highland's previous history. The craters that appear on the lunar maria (and the youngest craters in the highlands) result from occasional impacts by small asteroids and comets that hit the Moon (and, inevitably, the Earth) during subsequent time. On bodies other than the Moon, where we have no radiometric dates, counting the number of craters in a given area and relating these to the calibrated lunar catering timescale is the best way we have of estimating the age of the surface.

The Moon has no atmosphere to shield it from smaller cosmic debris, and as a result it is cratered on all scales down to the microscopic. Because there is no erosion to remove the ejecta thrown out during crater formation it remains where it falls until redistributed by the arrival of a fresh impactor. Consequently, in most places the lunar bedrock is buried by at least several metres of fragmental and dusty debris constituting the lunar ‘soil’ or ‘regolith’ in which the astronauts left their footprints during the Apollo Moon landings of 1969–72.

As well as bringing samples back, the astronauts were able to deploy various geophysical instruments on the Moon's surface; in particular, seismometers to record the vibrations from ‘moonquakes’ that have given us a clearer picture of the Moon's interior than we have of any planet other than the Earth. From these data it has been deduced that the Moon's core occupies no more than about 2 per cent of its volume.

Mercury

Mercury is superficially the most Moon-like of the other terrestrial planets, but is particularly dense for its size, implying that it has a relatively large iron core occupying about 40 per cent of its volume. Only one space probe has visited Mercury, Mariner 10, which made three fly-bys of the planet in 1974 and 1975. The pictures sent back by this mission show a heavily cratered surface, which, as we know from lunar experience, shows that there has been little geological activity to erase the scars of the intense cratering that occurred during the first half-billion years of the history of the Solar System.

Venus

In contrast, Venus is the most Earth-like of the other planets, in that its size, mass, and density are each only slightly lower than those of the Earth. Its history has, however, been very different. The most obvious difference is that Venus's atmosphere has evolved in an entirely different manner from that of the Earth. It is mostly carbon dioxide, with a surface pressure more than 90 times that at sea level on the Earth. Sulphur dioxide droplets high in Venus's atmosphere hide the planet's surface behind a perpetual cloud layer that reflects sunlight so strongly that Venus outshines the brightest star in Earth's sky, when it is visible as the evening or morning ‘star’. Although Venus's atmosphere reflects most of the sunlight, it traps enough solar heat by the greenhouse effect to give the planet a surface temperature of a searing 460 °C.

The surface of Venus has been seen by a series of seven Soviet probes that soft-landed on Venus between 1985 and 1992. Pictures from these landers show a rather slabby surface. Crude chemical analyses identify the rock type as resembling the Earth's ocean floor rather than the Earth's continents. More important than these isolated snapshots are the detailed images of virtually the entire globe obtained by radar from spacecraft in orbit about the planet, notably the American probe Magellan (1990–93). These show that Venus has a bewildering variety of landforms, such as Hawaii-like shield volcanoes; vast fields of lava flows; long, narrow channels apparently carved by flowing lava; crumpled belts of fold mountains resembling the Appalachians on the Earth; fields of sand dunes blown by dense but sluggish winds; and regions of intensely fractured terrain.

No present-day internally driven activity has been observed on Venus. Moreover, impact craters are not uncommon on Venus, and are distributed randomly across the globe in numbers suggesting an age of about 500–800 million years. There are no identifiable tracts of younger terrain with significantly lower crater density. The entire surface is therefore relatively old compared to that of the Earth, where ocean floor is continually being created by sea-floor spreading and destroyed at subduction zones, and where areas of continental crust are resurfaced by volcanic eruptions, erosion of actively uplifting mountains, and burial by deposition in low-lying areas. This contrast between the two planets is surprising, because they should have virtually the same radiogenic heat supply and be losing heat at roughly the same average rate. Outward heat transfer in the Earth is ultimately responsible for all the processes listed above, and yet they seem not to be occurring today on Venus. One explanation is that Venus behaves in an episodic fashion. Most of the time (as at present), heat is trapped below the planet's rigid outer shell (or lithosphere). As the interior part of the mantle warms up, it becomes progressively more buoyant in relation to the colder and denser exterior. Eventually the entire surface founders in a cataclysmic event, leading to volcanic resurfacing on a global scale, followed by tectonic deformation of some areas as the new surface settles down. If this last happened 500–800 million years ago, it would explain what we see today on Venus, with the youngest volcanic features representing the waning phase of activity as the new lithosphere thickened and strengthened. Perhaps we shall have to wait only about another 100 million years before it all happens again.

Mars

Although small, Mars displays clear signs of a wide range of geological processes. Its southern hemisphere is mostly ancient, heavily cratered terrain, whereas its northern hemisphere consists of lower-lying plains that are probably of volcanic origin but are much modified by the deposition of sediment derived from the southern highlands. Although the theory that the northern hemisphere was occupied by a shallow sea some billions of years ago remains controversial, it is undeniable that torrents of water once debouched into it from the southern highlands, fed by channels that are clearly visible today.

At a mere 6 millibars, the present atmospheric pressure on Mars is now too low to permit liquid water to exist, and the martian channel systems are the clearest evidence in the Solar System for a major global climatic change. The only known water on Mars today resides in the residual ice caps at either pole, which are surrounded by seasonal carbon dioxide ice that condenses out of the atmosphere in autumn and sublimes again in spring. There may, however, be an even greater volume of invisible water ice trapped in a permafrost layer below the surface in many parts of the globe. Indeed, although some of the martian channels form dendritic networks characteristic of having been fed by surface run-off resulting from rainfall, others have morphologies consistent with headwards sapping by melting of subsurface ice.

Water also occurs in hydrous minerals, such as the alteration products found within rare groups of meteorites whose martian origin is now well established. The extreme fractionation of stable isotopes of carbon that is consistent with biological activity in parts of these meteorites, the array of organic molecules associated with these, and ambiguous bacteria-like shapes seen in scanning electron microscope images provide tentative evidence that primitive life once existed on Mars. As Mars clearly once had much more water than today and (inevitably) hot, mineral-rich springs associated with volcanic activity, and bearing in mind that life on Earth seems to have begun in comparable conditions, the proposition that life could have originated on Mars is entirely reasonable. This being so, bacteria-like organisms could have survived there to this day within the polar caps or in the subsoil.

Turning to less controversial aspects of martian geology, sediment transport today is largely by wind. Seasonal dust storms frequently obscure our view of the surface, and there are extensive fields of dunes. These are particularly widespread around the northern polar cap, where longitudinal dunes predominate, although crescent-shaped barchan dunes are also found.

Major volcanoes occur in several regions of Mars. The oldest (dated at about 3.1–3.7 billion years ago on the basis of impact crater counting) are very gently sloping structures called paterae, hundreds of kilometres in diameter but only a few kilometres high, and usually with a caldera at the summit a few tens of kilometres in diameter. They are most likely to be products of explosive volcanism, and their flanks are interpreted as deposits of volcanic ash. Younger volcanoes have forms similar to shield volcanoes on the Earth and have clearly been constructed mainly by eruption of basaltic lavas. Most of them are concentrated in the Tharsis volcanic province, an upwards bulge in the martian crust some 3000km across that locally obscures the distinction between the high southern and low northern hemispheres. The youngest of the Tharsis shield volcanoes is Olympus Mons, which with a height of 24 km and a diameter of over 600 km is the largest volcano in the Solar System (Earth's largest is Hawaii, whose summit is 9 km above the floor of the Pacific Ocean). Olympus Mons is hard to date, but may have most recently been active only about 200 million years ago.

Another remarkable feature on Mars is Valles Marineris, a canyon system 4000 km long that runs eastwards from the Tharsis bulge before draining northwards into the northern hemisphere lowlands. The Valles Marineris system probably owes its origin to tectonic fracturing, but has subsequently been modified by water flow and landslips. It reaches 7 km in depth and over 200 km in width and would dwarf the Earth's Grand Canyon as thoroughly as Olympus Mons would dwarf Hawaii.

Io

Io's size, mass, and density (Table 1) would lead one to expect it to be a Moon-like body. However, whereas volcanic activity ceased on the Moon several billion years ago, Io still has about a dozen volcanoes erupting at any one time. This is because of tidal heating, caused by Jupiter's powerful gravitational field, which raises tidal bulges several kilometres high on Io's Jupiter-facing and anti-Jupiter hemispheres. The changing stresses on these tidal bulges as Io orbits Jupiter heats Io's interior about a hundred times faster than does radioactive decay. Io has a strong magnetic field, suggesting that heating of its interior has not merely made it possible for Io to develop a fully differentiated structure but also that its iron-rich core is fluid and therefore molten.

No impact craters are known on Io, which is to be expected on a body where the global average rate of resurfacing by volcanic deposits is apparently well in excess of 1 mm a year. Instead, Io's surface is dominated by volcanic features, notably gentle shield volcanoes with summit calderas several tens of kilometres across and surrounded by radial lava flows. The strong yellowish colour of Io was formerly thought to indicate that many of these flows had been formed by molten sulphur, but it is now accepted that these flows are conventional silicate flows (perhaps basaltic) and that the colours are a result of surface coatings by sulphur and sulphur oxides. A major contrast with volcanism on the Earth is that whereas on Earth the most abundant volatile escaping from erupting volcanoes is water vapour, on Ió this role is taken by sulphur dioxide and sulphur. Violent escapes of volatiles at eruption sites on Ió drive pyroclastic eruptions in the form of eruption plumes that reach up to 300 km in height and spread, umbrella-like, over regions in excess of 1000 km across.

Jupiter's other satellites

Jupiter has three other large satellites, orbiting beyond Io. The closest is Europa, which is transitional in nature between the terrestrial and icy classes of bodies. It has a cold, icy surface, but its high density shows that the ice can be no more than about 100 km thick. Below this is a thick icy mantle, probably with a small iron-rich core in the centre.

Europa's exceptionally complex surface is marked by multiple generations of fractures and curved ridges that appear to indicate episodes when the icy shell was broken into numerous jostling platelets. Unlike Jupiter's other icy-surfaced satellites, Europa has few impact craters, which means that its surface must be very young. This is consistent with the amount of tidal heating to be expected in the upper part of Europa's rocky mantle. It is uncertain whether the ice today is solid all the way to its base, or whether it floats on a layer of water. Local, if not global, melting at depths of only a few tens of kilometres in recent times seems highly likely in view of the young surface and the identification of overlapping flow-like features probably formed by the extrusion of partly crystallized icy brines.

Speculation abounds about the interactions that could occur within Europa where the ice or water is in contact with the underlying rock. In view of the tidal heating, some kind of hydrothermal circulation seems inevitable, with water being drawn through the rock and expelled at nearby warm vents as a brine charged with chemicals dissolved from the rock. On Earth's deep ocean floors, comparable hydrothermal vents such as black smokers are known to host self-contained ecosystems, and it is probable that heat-tolerant bacteria that derive their energy through chemical reactions could survive if transplanted from a black smoker to a vent site below Europa's ice. Whether life has actually begun there remains unanswered as yet, but the chances are good given that the origin of life on Earth is now attributed to hot vents.

Ganymede and Callisto are true icy bodies, with ice extending to depths of more than half their radius. Magnetic and gravity measurements suggest that Ganymede's rocky interior has a fluid iron core within it. Callisto, the outermost of Jupiter's large satellites, has a heavily cratered surface bearing no clear traces of internally driven geological processes. On the other hand, Ganymede has tracts of dark, heavily cratered ancient terrain broken by belts of paler, younger terrain. The pale terrain displays a surprising pattern of ridges and grooves, occurring on a variety of scales, and apparently recording some blend of tectonism and icy volcanism. Cross-cutting relationships demonstrate that the pale, grooved terrain was developed in multiple stages, but the large number of impact craters on even the youngest parts show that these events concluded long ago (probably 3–4 billion years ago). With surface temperatures considerably below −100 °C, the ice is far too cold to flow like a glacier, it behaves to great depth mechanically like the Earth's lithosphere, and large impact craters retain their shapes without slumping.

Other icy satellites

The large satellites of Saturn, Uranus, and Neptune are essentially icy bodies with small, rocky cores. Many are heavily cratered, but those that have experienced significant heating (presumably tidal in origin) at times during the past 4 billion years have younger surfaces and are therefore less heavily cratered and preserve evidence of internally driven geological processes. An important factor governing the volcanology of these icy bodies is that their ice is believed to be contaminated with ammonia, methane, carbon monoxide, and other species that would have condensed out of the solar nebula at the low temperatures prevailing in the outer Solar System. These mixed ices melt in exactly the same way as mixtures of silicate minerals found in ordinary rock. Mineralogically, an ice formed from water and ammonia will consist of crystals of two distinct compositions: pure water-ice (H₂O) and ammonia hydrate ice (H₂O.NH₃). On heating, this ice will begin to melt at −97 °C, well below the temperature at which melting would occur in either pure water-ice or pure ammonia hydrate ice, yielding a melt consisting of about two parts water and one part ammonia. Unless the starting material was particularly rich in ammonia, the solid residue would be crystals of pure water-ice. Melting behaviour of this type is described as partial melting and is important because it allows melts to be generated at low temperatures and, by separating the first-formed melt from the solid residuum, it is a way of generating an evolved crust that is chemically distinct from the underlying mantle. Extrusions of viscous ammonia– water melts are most clearly seen in the form of icy lava flows (long-since solidified) on Miranda.

There are many features of note other than icy volcanism. Several icy satellites are marked by fracture systems demonstrating phases of tectonic activity. Two have atmospheres. Titan, a particularly large satellite of Saturn, has a dense atmosphere consisting mostly of nitrogen but made opaque by clouds of methane and heavier hydrocarbons, not greatly dissimilar, perhaps, to the atmosphere of the primitive Earth. Triton, Neptune's largest satellite, has a much more rarified atmosphere composed essentially of nitrogen that sublimes from the seasonally sunlit polar cap of nitrogen ice (which overlies the methane-rich permanent surface) and condenses on the opposite polar cap in darkness.

Pluto and Charon

Most icy satellites probably formed in orbit around their planets, but Triton's retrograde orbit suggests that it formed separately and was captured by Neptune later. Pluto appears to be a Triton-like body that has evaded capture, although it has its own satellite, Charon. As a pair, Pluto and Charon are much more evenly matched than the Earth and Moon, and are the closest approximation in the Solar System to a double planet.

David A. Rothery

Bibliography

Beatty, J. K., Petersen, C. C., and Chaikin, A. (eds) (1999) The new Solar System (4th edn). Sky Publishing Corporation and Cambridge University Press.
Rothery, D. A. (1999) Satellites of the outer planets (2nd edn). Oxford University Press, New York and Oxford.
Rothery, D. A. (2000) Teach yourself planets. (2nd edn). Modder and Stoughton, London.