Introduction

The planets and the Sun together form a coupled system, the so-called solar system, which is located in a s piral arm of the Milky Way galaxy. The solar system has existed for 4.6 billion years. Its formation took only between 50 and 100 million years (Chapter 3). According to the nebular hypothesis, a large cloud of gas started to contract under self-gravity. Conservation of angular momentum led to a rotating disk. In the center of this disk mass concentrated into a so-called proto-Sun which grew larger and larger. After reaching a temperature of about 15 million K in the core, nuclear fusion processes started turning hydrogen into helium.

In the inner part of the disk, small planetesimals were formed, which by aggregating more mass became the terrestrial planets (Mercury, Venus, Earth, and Mars). The release of potential energy and the impact of particles produced molten spheres causing a chemical differentiation with denser material sinking to the center and with a loss of volatile components. In the outer disk, lower temperatures prevailed allowing the aggregation of volatile matter such as ices and gases. The result was several larger planets with lower densities (Jupiter, Saturn, Uranus, and Neptune). For a more detailed discussion of the formation and evolution of stars and their planets we refer to Chapter 3.

Waves in planetary atmospheres are interesting in their own right, but their importance lies mostly in the effects they have on the background atmosphere. Gravity waves may transport momentum, heat, and minor constituents through wave fluxes and may mix the atmosphere through the turbulence they induce when they break down (Lindzen, 1981; Fritts, 1984; Garcia and Solomon, 1985; Walterscheid, 1981, 1995, 2001; Walterscheid and Schubert, 1989). Planetary waves transport heat, momentum, and constituents (e.g. ozone) latitudinally and play a significant role in the heat, momentum, and ozone budgets (Holton and Wehrbein, 1980; O'Sullivan and Salby, 1990; Fusco and Salby, 1999).

Atmospheric waves

Planetary atmospheres admit a rich variety of waves. These waves involve to varying degrees the rotational, compressional, and buoyant properties of a fluid in motion. Many wave disturbances arise in instabilities, including those that give rise to weather systems. Other motions arise as free waves or waves forced by agents external to the atmosphere (e.g. planetary topography and s olar heating). These comprise two broad classes of waves in planetary atmospheres: Rossby waves and gravity waves. Rossby waves are the comparatively low-frequency waves that dominate the ultra-long wave field in the lower atmosphere. Rossby waves are rotational waves where latitudinal displacements are opposed by the latitudinal gradient of planetary rotation. Gravity waves, in contrast, are comparatively high-frequency divergence waves where vertical displacements are opposed by pressure forces induced by gravity.

Introduction

Early investigation of the terrestrial ionosphere through its effect on radio waves resulted in description by means of layers, principally the D, E, and F layers, the latter subdivided into F1 and F2 (see Vol. I, Fig. 12.1). This terminology continues to influence our current concept of the nature of energy deposition in atmospheres, although the misleading term “layer” has given way to “region”. The term “layer” arose from the observation of systematic variation in the height at which the critical frequency of reflection occurs in ionospheric radio sounding; this method cannot detect ionization above the peak of a region, which explains the appearance of layers. Radar and spacecraft measurements now give a more complete picture of peaks and valleys and reveal the complex morphology of the ionosphere. Chamberlain and Hunten (1987) provide a referenced discussion of the historical literature.

An overview of the altitude dependence and variability of Earth's ionosphere is given in Fig. 13.1, showing the diurnal and solar-cycle changes and the locations of the named regions. Space and planetary exploration has also found that the Earth's ionosphere is unique, just as its atmosphere is unique, and for some of the same reasons as we shall touch upon in this chapter.

An additional historical artifact in terminology is the word ionosphere itself. Because the atmospheric ionization was discovered before the neutral thermo sphere in which it is contained, anything above the stratosphere is often referred to as the ionosphere, resulting in a common misconception that this region of the atmosphere is mostly ionized.

Introduction

This chapter discusses general concepts of planetary habitability as well as major events in Earth's history that relate to habitability over its full past and future evolution. The Sun plays a determining factor for habitability in the solar system as other stars also are critical to habitability in other planetary systems. Stars provide well-known benefits to life, but they also cause life-ending processes such as the loss of oceans and the loss of planetary atmospheres.

Environmental limits for life as we know it

Because life has not been detected anywhere but on Earth, the nature of extraterrestrial life remains completely unknown. In light of this famous shortcoming we can still use environmental requirements for terrestrial organisms to estimate where organisms similar to life-as-we-know-it might plausibly exist elsewhere. While this can be criticized as being overly provincial, the Earth-biased approach provides a practical means to access the potential habitability of other worlds. For life based on complex interactions of compounds analogous to the biomolecules of life on Earth, it is relatively straightforward to set general constraints on environments that might support life similar to life-as-we-know-it.

Many of the environmental constraints are influenced by the central star in a planetary system, as the Sun does in our solar system. The Sun provides warmth and energy for photosynthesis, but it also influences many of Earth's fundamental atmospheric, oceanic, and biological processes.

Introduction

The heliosphere is a vast spheroidal cavity in the local interstellar plasma, some 150–200 AU in size, created by a supersonic flow of plasma called the solar wind, which flows radially outward from the Sun in all directions (Fig. 9.1). The scale of the heliosphere is determined by both the solar atmosphere and the pressure of the surrounding interstellar plasma and magnetic field. Far enough from the Sun the solar wind is spread over such a large volume that it can no longer push back the interstellar plasma. Because the wind is flowing out much faster than waves can propagate inward, the solar wind flow ends at a s pheroidal shock wave, which is called the termination shock, where the supersonic flow changes suddenly to a subsonic outward flow.

The interstellar plasma is moving at about 26 km/s relative to the heliosphere, pushing it in on one side, as shown in Fig. 9.1. Beyond the termination shock, the solar plasma continues to flow outward, but it is deflected and eventually turns to flow in the same direction as the interstellar plasma, forming a large, trailing, heliospheric tail (see Vol. II, Fig. 7.1b). The interstellar medium also contains neutral atoms, and while these play a role in the interaction, they may be neglected in the lowest order.

Introduction

One could write an interesting essay on the twists and turns in Sun–Earth climate science, addressing both the instrumental record and the much longer interval of paleoclimate records. The conclusion at the time of this writing with respect to the importance of low-frequency solar variability in the most recent decades, and perhaps up to centuries, might be “Perhaps, but probably small”. The main reasons why uncertainties persist regarding this issue include:

(i) The ˜ 150-year instrumental record is too short to draw definitive statistical conclusions about the connection of any relation existing on the multi-decadal time scale.

(ii) Forcing from anthropogenic greenhouse gases represents a significant overprint on trends since about 1850 CE. Because to first order the trends in proxies for solar activity indices and in greenhouse gas concentrations are similar, there is a statistical degeneracy that leads to ambiguous, and thus potentially misleading, conclusions unless great care is taken.

(iii) A similar problem of statistical degeneracy applies to the Little Ice Age interval of cool conditions during the last millennium (main phase about 1450–1850 CE), when mountain glaciers advanced in many regions and planetary temperatures were about 0.5°C lower (e.g. Jones and Mann, 2003; Hegerl et al., 2007). During the Little Ice Age, solar activity, as inferred from changes in radiogenic isotopes such as 14C and 10Be, appears to have varied similarly to pulses in volcanism and slightly lower carbon dioxide (CO2) levels (further discussed below).

Magnetism with enthusiasm

As discussed in many chapters throughout these volumes, stars bristle with magnetic energy. They inherit some of their natural magnetism from their parent molecular clouds as they contract from protostellar cores (Chapter 3). However, stars are far from passive. After they ignite and enter the main sequence, much of their magnetism comes from within, bred by active hydromagnetic dynamos (Chapter 2; Vol. I, Chapter 3). Emerging magnetic flux influences the star's evolution and shapes its environment. It is the dissipation of magnetic energy in the solar corona that powers the solar wind, and the wind in turn carves out the heliosphere, a planetary cloister within the surrounding interstellar medium where the Sun holds sway. Magnetic fields originating in the solar interior permeate the heliosphere, weaving an intricate web with planetary magnetospheres and linking the Sun to the planets. The web changes continually as coronal mass ejections send sporadic bursts of magnetized plasma coursing through the heliosphere, restructuring fields and flows as they go.

Stars build magnetic flelds by tapping the energy in their own corporeal constitution. Thermonuclear fusion in their cores converts matter into thermal energy and electromagnetic radiation which, in the Sun, is transported outward via the diffusion of photons. In the solar envelope, the plasma becomes more opaque as the temperature drops, which inhibits radiative diffusion and steepens the temperature gradient relative to the adiabatic temperature gradient (Section 5.2).

Introduction

The possible link between solar variability and climate remains an intriguing and controversial issue. Solar physicists have shown that the total solar irradiance (S0 = L⊙/4πd2⊙), whose value is close to 1366 W m−2 (Fröhlich, 2004), varies typically by 1.5 W m−2 (slightly more than 0.1%) over an 11-year solar cycle (see Chapter 10 and Fig. 10.9). This change is considerably smaller than the radiative forcing produced by enhanced concentrations of greenhouse gases since the beginning of the industrial era. Figure 16.1 (from the Fourth Assessment conducted by the Intergovernmental Panel on Climate Change, or IPCC) highlights a possible longer term trend in the solar irradiance, but, unless some amplification mechanisms occur, this forcing remains small compared, for example, to the effect of carbon dioxide and other radiatively active gases, whose atmospheric concentrations have increased as a result of human activity.

Different mechanisms have been proposed to explain the relations between the state of the atmosphere and the 11-year solar cycle. One of them is the absorption of short-wave solar radiation by ozone in the stratosphere with possible effects on the diabatic heating, temperature, and the general circulation of the atmosphere. Another mechanism refers to the impact of galactic cosmic rays on the formation of cloud condensation nuclei and hence on cloudiness and surface temperature. The cosmic-ray intensity in the atmosphere is anti-correlated with solar activity (Chapters 9 and 11).

Introduction

Over four centuries ago it was realized that the time-averaged direction of a compass needle is not affected by a force emanating from the sky, but by a magnetic field that is intrinsic to the Earth. The basic structure of the geomagnetic field and its slow variation with time was characterized long before magnetic fields were detected on other celestial bodies. By the middle of the twentieth century, the study of remanent magnetization of natural rocks had firmly established that the principal dipole component of the Earth's magnetic field had reversed its direction many times in the past.

Our understanding of the origin of the field by a dynamo process in the Earth's core has developed at a much slower pace, basically in parallel with that of astrophysical dynamos in general. Aside from understanding the intricate details of how a magnetic field is generated by a dynamo, we must ascertain that some fundamental requirements are fulfilled inside our planet. Geophysical observations have shown that one condition, namely the existence of an electrically conducting fluid region, is met inside the Earth, which has an outer core consisting of a liquid iron alloy. It is likely, but not completely certain, that all big planets have conducting fluid cores (see Fig. 7.5). However, some planets may not conform with another basic condition for a dynamo, namely sufficiently fast motion in the fluid layer.