The Structure and Surfaces of Planets

Since the 1960's a vast amount of data have been collected using spacecraft, either during a flyby, orbiting a body, or directly landing on the surface. This gives a great advantage compared to other astronomical observations. We may even speak of revolution: the solar system bodies have turned from astronomical objects to geophysical ones. Many methods traditionally used in various sibling branches of geophysics can now be applied to planetary studies.

The shape and irregularities of the gravitation field generated by a planet reflects its shape, internal structure and mass distribution. Also the surface gives certain indications on internal structure and processes.

The perturbations in the orbit of a satellite or spacecraft can be used in studying the internal structure of a planet. Any deviation from spherical symmetry is visible in the external gravitational field.

The IAU planet definition states that planets are bodies in hydrostatic equilibrium. Gravity of a body will pull its material inwards, but the body resist the pull if the strength of the material is greater than the pressure exerted by the overlying layers. If the diameter is larger than about 800-1000 km, gravity is able to deform rocky bodies into spherical shape. Smaller bodies than this have irregular shapes. On the other hand, e.g. icy moons of Saturn are spherical because ice is more easily deformed than rock.

Hydrostatic equilibrium means that the surface of the body approximately follows an equipotential surface of gravity. This is true e.g. on the Earth, where the sea surface very closely follows the equipotential surface called the geoid. Due to internal strength of rocks, continents can deviate from the geoid surface by a few kilometers but compared to the diameter of the Earth the surface topography is negligible.

A rotating planet is always flattened. The amount of flattening depends on the rotation rate and the strength of the material; a liquid drop is more easily deformed than a rock. The shape of a rotating body in hydrostatic equilibrium can be derived from the equations of motion. If the rotation rate is moderate, the equilibrium shape of a liquid body is an ellipsoid of revolution. The shortest axis is the axis of rotation.

If Re and Rp are the equatorial and polar radii, respectively, the shape of the planet can be expressed as

The dynamical flattening, denoted by f is defined as f=

Because Re > Rp, the flattening f is always positive.

The giant planets are in practise close to hydrostatic equilibrium, and their shape is determined by the rotation. The rotation period of Saturn is only 10.5 h, and its dynamical flattening is 1 /10 which is easily visible.

Asteroids and other minor bodies are so small that they are not flattened by rotation. However, there is an upper limit for a rotation rate of an asteroid before it breaks apart due to centrifugal forces. If we assume that the body is held together only by gravity, we can approximate the the maximum rotation rate by setting

the centrifugal force equal to the gravitational force:

where m is a small test mass on the surface at a distance of R from the center of the body. Substituting the rotation period P ,

R3 GM

4nGp

If we substitute the density p with the mean density of terrestrial rocks, i.e. 2700 kg m-3, we get for the minimum rotation period P ~ 2 hours.

The structure of the terrestrial planets (Fig. 7.8) can also be studied with seismic waves. The waves formed in an earthquake are reflected and refracted inside a planet like any other wave at the boundary of two different layers. The waves are longitudinal or transversal (P and S waves, respectively). Both can propagate in solid materials such as rock. However, only the longitudinal

Fig. 7.8. Internal structure and relative sizes of the terrestrial planets. The percentage shows the volume of the core relative to the total volume of the planet. In the case of the Earth, the percentage includes both the outer and the inner core wave can penetrate liquids. One can determine whether a part of the interior material is in the liquid state and where the boundaries of the layers are by studying the recordings of seismometers placed on the surface of a planet. Naturally the Earth is the best-known body, but quakes of the Moon, Venus, and Mars have also been observed.

The terrestrial planets have an iron-nickel core. Mercury has the relatively largest core; Mars the smallest. The core of the Earth can be divided into an inner and an outer core. The outer core (2900-5150 km) is liquid but the inner core (from 5150 km to the centre) is solid.

Around the Fe-Ni core is a mantle, composed of silicates (compounds of silicon). The density of the outermost layers is about 3000 kg m-3. The mean density of the terrestrial planets is 3500-5500 kg m-3.

The internal structure of the giant planets (Fig. 7.9) cannot be observed with seismic waves since the planets do not have a solid surface. An alternative is to study the shape of the gravitational field by observing the orbit of a spacecraft when it passes (or orbits) the planet. This will give some information on the internal structure, but the details depend on the mathematical and physical models used for interpretation.

Jupiter

Atmosphere' Molecular hydrogen' Metallic hydrogen-Ices Rocks-

Earth

Jupiter

Earth snueifi

Fig. 7.9. Internal structure and relative sizes of the giant planets. Differences in size and distance from the Sun cause differences in the chemical composition and internal structure. Due to smaller size, Uranus and Neptune do not have any layer of metallic hydrogen. The Earth is shown in scale

Fig. 7.8. Internal structure and relative sizes of the terrestrial planets. The percentage shows the volume of the core relative to the total volume of the planet. In the case of the Earth, the percentage includes both the outer and the inner core snueifi

Fig. 7.9. Internal structure and relative sizes of the giant planets. Differences in size and distance from the Sun cause differences in the chemical composition and internal structure. Due to smaller size, Uranus and Neptune do not have any layer of metallic hydrogen. The Earth is shown in scale v

Time before present [10 9 a]

Fig. 7.10. Ages of the surfaces of Mercury, the Earth, the Moon and Mars. The curve represents the fraction of the surface which existed at a certain time. Most of the surface of the Moon, Mercury and Mars are more than 3500 million years old, whereas the surface of the Earth is mostly younger than 200 million years

Time before present [10 9 a]

Fig. 7.10. Ages of the surfaces of Mercury, the Earth, the Moon and Mars. The curve represents the fraction of the surface which existed at a certain time. Most of the surface of the Moon, Mercury and Mars are more than 3500 million years old, whereas the surface of the Earth is mostly younger than 200 million years

The mean densities of the giant planets are quite low; the density of Saturn, for example, is only 700 kg m-3. (If Saturn were put in a gigantic bathtub, it would float on the water!) Most of the volume of a giant planet is a mixture of hydrogen and helium. In the centre, there is possibly a silicate core, the mass of which is a few Earth masses. The core is surrounded by a layer of metallic hydrogen. Due to the extreme pressure, hydrogen is not in its normal molecular form H2, but dissociated into atoms. In this state, hydrogen is electrically conducting. The magnetic fields of the giant planets may originate in the layer of metallic hydrogen.

Closer to the surface, the pressure is lower and hydrogen is in molecular form. The relative thickness of the layers of metallic and molecular hydrogen vary from planet to planet. Uranus and Neptune may not have any layer of metallic hydrogen because their internal pressure is too low for dissociation of the hydrogen. Instead, a layer of "ices" surround the core. This is a layer of a water-dominant mixture of water, methane and ammonia. Under the high pressure and temperature the mixture is partly dissolved into its components and it

Fig. 7.11. An example of resurfacing. Two volcanic plumes on Jupiter's moon Io observed by Galileo spacecraft in 1997. One plume was captured on the bright limb or edge of the moon (inset at upper right), erupting over a caldera named Pillan Patera. The plume is 140 kilometers high. The second plume, seen near the terminator, is called Prometheus. The shadow of the 75 km high airborne plume can be seen extending to the right of the eruption vent. (NASA/JPL)

Fig. 7.12. The number of meteorite impact craters is an indicator of the age of the surface and the shapes of the craters give information on the strength of the material. The upper row shows Mercury (left) and the Moon, and the second row, the Jovian moons Europa (left), Ganymede (centre) and Cal-listo. The pictures of the Jovian moons were taken by the Galileo orbiter with a resolution of 150 metres/pixel. Europa has only a few craters, there are areas of different ages on the surface Ganymede and the surface of Callisto is the oldest.

Fig. 7.12. The number of meteorite impact craters is an indicator of the age of the surface and the shapes of the craters give information on the strength of the material. The upper row shows Mercury (left) and the Moon, and the second row, the Jovian moons Europa (left), Ganymede (centre) and Cal-listo. The pictures of the Jovian moons were taken by the Galileo orbiter with a resolution of 150 metres/pixel. Europa has only a few craters, there are areas of different ages on the surface Ganymede and the surface of Callisto is the oldest.

Note the grooves and ridges that indicate different geological processes. IN the bottom there are two volcanic plumes on Jupiter's moon Io observed by Galileo spacecraft in 1997. One plume was captured on the bright limb or edge of the moon (inset at upper right), erupting over a caldera named Pillan Patera. The plume is 140 kilometers high. The second plume, seen near the terminator, is called Prometheus. The shadow of the 75 km high airborne plume can be seen extending to the right of the eruption vent.(NASA/JPL and DLR)

behaves more like a molten salt and it is also electrically conductive like the metallic hydrogen.

On top of everything is a gaseous atmosphere, only a few hundred kilometres thick. The clouds at the top of the atmosphere form the visible "surface" of the giant planets.

The interior temperatures of the planets are considerably larger than the surface temperatures. For example, the temperature in the Earth's core is about 4500-5000 K, and in the core of Jupiter about 30,000 K.

A part of that heat is the remnant of the released potential energy from the gravitational contraction during the formation of planets. Decay of radioactive isotopes also releases heat. Soon after the formation of planets intense meteorite bombardment was an important source of heat. Together with heat from short-lived radioactive isotopes this caused melting of terrestrial planets. The planets were differentiated: the originally relatively homogeneous material became segregated into layers of different chemical composition. The heaviest elements sank into centre thus forming the Fe-Ni core.

The material of the giant planets is differentiated as well. In Saturn the differentiation may still be going on. Saturn is radiating about 2.8 times the heat it gets from the Sun, more than any other planet. This heat is suspected to originate from the separation of hydrogen and helium, where the heavier helium is gradually sinking toward the centre of the planet.

Planetary surfaces are modified by several geological processes. These include continental drift, volcanism, meteorite impacts and climate. The Earth is an example of a body whose surface has been renewed many times during past aeons. The age of the surface depends on the processes and thus implies the geological evolutionary history of the planet Figs. 7.10, 7.11,7.12).

Continental drift gives rise, for example, to mountain formation. The Earth is the only planet where plate tectonics is active today. On other terrestrial planets the process has either ceased long ago or has never occurred.

Volcanism is a minor factor on the Earth (at least now), but the surface of the Jovian moon Io is changing rapidly due to violent volcanic eruptions (Fig. 7.11). Volcanoes have been observed on Mars and Venus, but not on the Moon.

Lunar craters are meteorite impact craters, common on almost every body with a solid surface. Meteorites are bombarding the planets continuously, but the rate has been diminishing since the beginnings of the solar system. The number of impact craters reflects the age of the surface (Fig. 7.12).

The Jovian moon Callisto is an example of a body with an ancient surface which is not fully inactive. Lack of small craters indicates some resurfacing process filling and degrading the minor surface features. The Earth is an example of a body, whose atmosphere both protects the surface and destroys the traces of impacts. All smaller meteorites are burned to ashes in the atmosphere (one need only note the number of shooting stars), and some larger bodies are bounced back to outer space. The traces on the surface are destroyed very quickly by erosion in less than a few million years. Venus is an even more extreme case where all small craters are missing due to a thick protective atmosphere.

Climate has the greatest influence on the Earth and Venus. Both planets have a thick atmosphere. On Mars, powerful dust storms deform the landscape, too, often covering the planet with yellowish dust clouds.

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