In this appendix we briefly summarize aspects of observational planetary science. Techniques specific to extrasolar planets are discussed in §12.2. References to more extensive treatments are provided in the Further Reading section at the end.

E.1 Photometry

Photometry is a photon-counting technique, wherein the brightness of an object is measured. Time series of brightness measurements can be combined into a photometric lightcurve that shows, e.g., the variations in a body's brightness as the object rotates around its axis (Fig. 9.4).

Graphs of an asteroid's brightness in reflected sunlight as a function of phase angle, ø, usually show an abrupt increase in intensity at ø ≲ 2°, referred to as the opposition effect. The opposition effect for the Moon, shown in Figure E.1, is very large; the intensity increases by ∼20% from ø ∼ 2° down to ø = 0°. This is why a ‘full Moon’ can appear to be much brighter than a nearly full gibbous Moon. Part of the opposition effect can be attributed to the hiding of shadows when the Sun and observer are located in the same direction as seen from the object. Laboratory simulations of this phase angle effect, however, show that this is not the complete story. In any particulate material, multiple reflections diffusely scatter the incoming waves in all directions. At zero phase angle the waves interfere constructively, and the reflected intensity can be amplified considerably. This process is known as the coherent-backscatter effect. Both the shadow-hiding and coherent-backscatter contribute to the opposition effect. For the Moon, the coherent-backscatter effect results in the narrow peak near opposition at phase angles ø < 2°, whereas the broader component at ø < 20° can be explained by the shadow-hiding theory.

E.2 Spectroscopy

Spectroscopy pertains to the dispersion of light as a function of wavelength. Spectra can be used to derive, e.g., the composition of gaseous and solid objects, the temperature and pressure in an atmosphere (§4.3), and the radial velocity of an object via the Doppler shift (§12.2.2).

Most planets are surrounded by huge magnetic structures, known as magnetospheres. These are often more than 10–100 times larger than the planet itself, and therefore form the largest structures in our Solar System, other than the heliosphere. The solar wind flows around and interacts with these magnetic ‘bubbles’. A planet's magnetic field can either be generated in the interior of the planet via a dynamo process (Earth, giant planets, Mercury), or induced by the interaction of the solar wind with the body's ionosphere (Venus, comets). Large-scale remanent magnetism is important on Mars, the Moon, and some asteroids.

The shape of a planet's magnetosphere is determined by the strength of its magnetic field, the solar wind flow past the field, and the motion of charged particles within the magnetosphere. Charged particles are present in all magnetospheres, though the density and composition varies from planet to planet. The particles may originate in the solar wind, the planet's ionosphere, or on satellites or ring particles whose orbits are partly or entirely within the planet's magnetic field. The motion of these charged particles gives rise to currents and large-scale electric fields, which in turn influence the magnetic field and the particles' motion through the field.

Although most of our information is derived from in situ spacecraft measurements, atoms and ions in some magnetospheres have been observed from Earth through the emission of photons at ultraviolet and visible wavelengths. Accelerated electrons emit photons at radio wavelengths, observable at frequencies ranging from a few kilohertz to several gigahertz. Radio emissions at ∼10 MHz were detected from Jupiter in the early 1950s, and formed the first evidence that planets other than Earth might have strong magnetic fields.

The Interplanetary Medium

7.1.1 Solar Wind

The presence of corpuscular radiation from the Sun, the solar wind, was first suggested by L. Biermann in 1951 based on the observation that cometary ion tails always point away from the Sun.

In the previous two chapters, we discussed the atmospheres and surface geology of planets. Both of these regions of a planet can be observed directly from Earth and/or space. But what can we say about the deep interior of a planet? We are unable to observe the inside of a planet directly. For the Earth and the Moon we have seismic data, revealing the propagation of waves deep below the surface and thereby providing information on the interior structure (§6.2). The interior structure of all other bodies is deduced through a comparison of remote observations with observable characteristics predicted by interior models. The relevant observations are the body's mass, size (and thus density), its rotational period and geometric oblateness, gravity field, characteristics of its magnetic field (or absence thereof), the total energy output, and the composition of its atmosphere and/or surface. Cosmochemical arguments provide additional constraints on a body's composition, while laboratory data on the behavior of materials under high temperature and pressure are invaluable for interior models. Quantum mechanical calculations are used to deduce the behavior of elements (especially hydrogen) at pressures inaccessible in the laboratory.

In this chapter we discuss the basics of how one can infer the interior structure of a body from the observed quantities. As expected, there are large differences between the interior structure of the giant planets, the terrestrial planets, and the icy moons. Moreover, even within each of these groupings there are noticeable differences in interior structure.

Modeling Planetary Interiors

Key observations used to extract information on the interior structure of a body are its mass, size, and shape. The mass and size together yield an estimate for the average density, which can be used directly to derive some first order estimates on the body's composition.

A meteorite is a rock that has fallen from the sky. It was a meteoroid (or, if it was large enough, an asteroid) before it hit the atmosphere and a meteor while heated to incandescence by atmospheric friction. A meteor that explodes while passing through the atmosphere is termed a bolide. Meteorites that are associated with observations prior to or of the impact are called falls, whereas those simply recognized in the field are referred to as finds.

The study of meteorites has a long and colorful history. Meteorite falls have been observed and recorded for many centuries (Fig. 8.1). The oldest recorded meteorite fall is the Nogata meteorite, which fell in Japan on 19 May 861. Iron meteorites were an important raw material for some primitive societies. However, even during the Enlightenment it was difficult for many people (including scientists and other natural philosophers) to accept that stones could possibly fall from the sky, and reports of meteorite falls were sometimes treated with as much skepticism as UFO ‘sightings’ are given today. The extraterrestrial origin of meteorites became commonly acknowledged following the study of some well-observed and documented falls in Europe around the year 1800. The discovery of the first four asteroids, celestial bodies of sub-planetary size, during the same period added to the conceptual framework that enabled scientists to accept extraterrestrial origins for some rocks.

Meteorites provide us with samples of other worlds that can be analyzed in terrestrial laboratories. The overwhelming majority of meteorites are pieces of small asteroids, which never grew to anywhere near planetary dimensions. Primitive meteorites, which contain moderate abundances of iron, come from planetesimals that never melted. Most of the iron-rich meteorites presumably come from the deep interiors of planetesimals that differentiated (§5.2.2) prior to being disrupted, while iron-poor meteorites are samples from the outer layers of differentiated bodies. As small objects cool more rapidly than do large ones, the parent bodies of most meteorites either never got very hot, or cooled and solidified early in the history of the Solar System.

Each of the four giant planets in our Solar System is surrounded by flat, annular features known as planetary rings. Planetary rings are composed of vast numbers of small satellites, which are unable to accrete into large moons because of their proximity to the planet.

When Galileo Galilei first observed Saturn's rings in 1610, he believed them to be two giant moons in orbit about the planet. However, these ‘moons’ appeared fixed in position, unlike the four satellites of Jupiter which he had previously observed. Moreover, Saturn's ‘moons’ had disappeared completely by the time Galileo resumed his observations of the planet in 1612. Many explanations were put forth to explain Saturn's ‘strange appendages’, which grew, shrank, and disappeared every 15 years (Fig. 11.1a). In 1656, Christiaan Huygens finally deduced the correct explanation, that Saturn's strange appendages are a flattened disk of material in Saturn's equatorial plane, which appear to vanish when the Earth passes through the plane of the disk (Fig. 11.1b).

For more than three centuries, Saturn was the only planet known to possess rings. Although Saturn's rings are quite broad, little structure within the ring system was detected from Earth (Fig. 11.2). Observational and theoretical progress towards understanding the physics of planetary rings was slow. But then, in March of 1977, an occultation of the star SAO 158687 revealed the narrow opaque rings of Uranus (Fig. 11.3) and launched a golden age of planetary ring exploration. The Voyager spacecraft first imaged and studied the broad but tenuous ring system of Jupiter in 1979 (§11.3.1). Pioneer 11 and the two Voyagers obtained close-up images of Saturn's spectacular ring system in 1979, 1980, and 1981 (Fig. 11.4; §11.3.2). Neptune's rings, whose most prominent features are azimuthally incomplete arcs, were discovered by stellar occultation in 1984. Voyager 2 obtained high-resolution images of the rings of Uranus in 1986 (§11.3.3) and the rings of Neptune in 1989 (§11.3.4).

In addition to the eight known planets, countless smaller bodies orbit the Sun. These objects range from dust grains and small coherent rocks with insignificant gravity to dwarf planets that have sufficient gravity to make them quite spherical in shape. Most are very faint, but some, the comets, release gas and dust when they approach the Sun and can be quite spectacular in appearance (Fig. 10.1); comets are discussed in Chapter 10. In this chapter, we describe the orbital and physical properties of the great variety of non-cometary small bodies ranging in radius from a few meters to over 1000 km that orbit the Sun. We refer to these bodies collectively as minor planets.

Minor planets occupy a wide variety of orbital niches (see Fig. 1.2). Most travel in the relatively stable regions between the orbits of Mars and Jupiter (known as the asteroid belt), exterior to Neptune's orbit (the Kuiper belt), or near the triangular Lagrangian points of Jupiter (the Trojan asteroids). The Kuiper belt is by far the most massive of these reservoirs, and contains the largest objects. However, the largest members of the asteroid belt appear much brighter than any Kuiper belt objects (KBOs) by virtue of their proximity to both the Earth and the Sun (see eq. 10.3 with ζ = 2).

Smaller numbers of minor planets are found in unstable regions. Most of these cross or closely approach the orbits of one or more of the eight planets, which control their dynamics. Those that come near our home planet are known as near-Earth asteroids (NEAs); those orbiting among the giant planets are called centaurs.