Our journey in the land of neutrino cosmology is almost over. We have seen how neutrinos have silently influenced the evolution of the universe: how they perhaps may be responsible for the production of the baryon density we observe today, and the way they leave their signature in the nuclear abundances during primordial nucleosynthesis, in the CMB anisotropies and in structure formation.

We cannot leave the patient reader, who has kindly followed us till this point, without offering some final considerations about a simple question he or she might have thought about since the very beginning: can we directly detect the neutrino background in a laboratory experiment as Penzias and Wilson did for the CMB radiation?

There are two major obstacles one must overcome to achieve such a goal. Neutrinos are elusive particles because they interact only weakly. Cross sections are typically small, far smaller than electromagnetic ones, which were exploited by Penzias and Wilson. Furthermore, relic neutrinos today are quite cold particles with an average momentum on the order of c pν ~ 3.15 Tν,0 ~ 5 × 10−4 eV, so reaction rates are further suppressed. For the best interaction process candidate to date, neutrino capture on β-unstable nuclei such as 3H, we have 〈σν/c〈 ~ 10−44 cm2.

Preface

Planetary geoscience had its inception with the birth of the Space Age in the early 1960s. In the ensuing decades, it has evolved into a discipline that is recognized by sections of professional organizations such as the Geological Society of America and the American Geophysical Union, as well as being taught at the university level. Much of our understanding of the geology of extraterrestrial objects is derived from remote sensing data – primarily images that portray planetary surfaces. In fact, discoveries such as the dry river beds on Mars, the tectonic deformation of Venus, and the actively erupting volcanoes on Jupiter’s moon Io all came from pictures taken from spacecraft. Thus, the focus of this book is on the geomorphology of solid-surface objects in our Solar System and the interpretations of the processes that led to the diverse landforms observed. Geomorphology, however, must be analyzed in the context of broader geoscience; consequently, in the chapters on the individual planetary systems, the geophysics and interior characteristics are reviewed along with our current understanding of surface compositions and the general geologic histories. Of course, our knowledge of the Solar System is far from uniform from one planet to another, dependent upon the numbers and types of spacecraft that have returned data. Thus, the chapters on the Moon and Mars are more detailed than those on the outermost planet systems, Uranus and Neptune, because dozens of successful spacecraft have visited our nearest planetary neighbors, in contrast to the limited data returned from “flybys” of the Voyager spacecraft to the planets beyond Saturn.

Our journey to explore the geomorphology of the Solar System begins with introductory chapters that introduce the planets and other objects of planetary geoscience interest, discuss the methods used in studying extraterrestrial objects, and review the fundamental geomorphic processes on Earth that can be compared with what we see on other planets and satellites.

Introduction

Earth is a dynamic planet. That simple statement can be supported by our own direct observations. Earthquakes, river banks collapsing during flooding, erupting volcanoes – all are experienced or documented on the news every year and show that our planet is everchanging. These examples represent three of the four fundamental processes that shape Earth’s surface: tectonism, gradation, and volcanism.

The fourth fundamental process, impact, which is generally less often observed, is also documented, sometimes in quite newsworthy events as when a meteoroid plunges through the roof of a house. As the geologic record shows, the history of Earth can be profoundly altered by impacts, such as the well-known Chicxulub structure in the Yucatan peninsula of Mexico. This structure, now buried beneath a kilometer of sediments, has been mapped by geophysical methods and drill-holes to be more than 80 km in diameter and is estimated to have formed from an impact that released the energy equivalent of some 10 billion tons of TNT. The resulting fireball ignited world-wide fires, generated enormous amounts of CO2 from the vaporization of limestone present at the impact site, and triggered tsunamis throughout the Gulf of Mexico and adjacent waters. As is now widely accepted, these catastrophic events led to mass extinctions, including that of the dinosaurs, and marked the boundary between the Cretaceous Period and the Tertiary Period 65 million years ago. It was not so much the direct impact that led to extinctions, but the effects on the surface environment, including firestorms, enhanced greenhouse processes, and disruption of the food chain.

Introduction

Few objects in the sky hold the fascination in the public mind as much as Mars. Easily seen with the naked eye, the “Red Planet” has been linked with various gods of war through the ages. The late 1800s and early 1900s saw both serious and not-so-serious writings on martian life, including the presence of advanced civilizations, and culminating in the infamous radio broadcast of H. G. Wells’s fictional War of the Worlds, in which martian spacecraft land on Earth. This broadcast filled many a family with terror as the story unfolded with the destruction of whole cities.

Building on public support for the exploration of Mars, the Red Planet has been visited by more spacecraft than any other object except Earth’s Moon. Along with Europa and possibly Titan, Mars is a favorable planet in the exploration for possible present-day or past life.

With a diameter of 6,779 km and a mass of 6.4 × 1023 kg, Mars gravity is 0.37 that of Earth. The total surface area of Mars is just about equal to the land surface on Earth above sea level. One Mars year is 686.98 Earth days, while one Mars day is 24 hr, 39m, 35.2 s. Its present-day spin axis is inclined 25.19° (slightly more than Earth), which leads to distinctive seasons. The seasons are defined by Mars’ position in orbit and described by aerocentric longitudes (Ls) of the Sun in degrees. Ls is the angle between the Mars–Sun line and the line of equinoxes. Ls of 0° is set at the martian equinox for the beginning of winter in the northern hemisphere (Ls 0° to 90°), with northern spring (Ls 90° to 180°), northern summer (Ls 180° to 270°), and northern autumn (Ls 270° to 360°).

Introduction

Planetary geosciences are advanced primarily through new data and are stimulated by physical and computational modeling, theoretical studies, and field studies of terrestrial analogs in support of planetary data analysis. As shown in Fig. 1.11, missions are currently in flight for Mercury, Venus, the Moon, Mars, Jupiter, Saturn, and Pluto, as well as for a host of asteroids and comets. Particularly noteworthy is the New Horizons mission, which will give us our first close-up views of Pluto in 2015. Spacecraft for these missions carry sophisticated scientific payloads, including imaging systems that will provide additional coverage, higher resolution, or first ever views of planetary objects of geoscience interest.

Most planetary missions are flown by NASA, some in partnership with the European Space Agency, which also flies planetary missions independently of NASA. In addition, the space agencies of Japan, India, and China are becoming increasingly important, especially in lunar exploration and the eventual return of humans to the Moon. (After a hiatus of many years, in 2012 Russia attempted to resume planetary exploration with a mission to one of the moons of Mars, Phobos, but that mission failed shortly after launch.) Although NASA had planned lunar exploration by humans early in the twenty-first century, such plans have been deferred because of the economic climate. In its place, considerations are being given to sending humans to one or more asteroids because of the lower costs (it is easier to return to Earth from these low-gravity bodies), the high scientific potential of asteroids, and the need to assess asteroids as hazards to Earth.

The early part of the twenty-first century saw the completion of the reconnaissance of the Solar System by spacecraft. With the launch of the New Horizons spacecraft to Pluto in early 2006 and its expected arrival in 2015, spacecraft will have been sent to every planet, major moon, and representative asteroid and comet in our Solar System. With the return of data taken by spacecraft of these objects, the study of planetary surfaces passed mostly from the astronomer to the geologist and led to the establishment of the field of planetary geology. The term geology is used in the broadest sense to include the study of the solid parts of planetary objects and includes aspects of geophysics, geochemistry, and cartography. Much of our knowledge of the geologic evolution of planetary surfaces is derived from remote sensing, in situ surface measurements, geophysical data, and the analysis of landforms, or their geomorphology, the primary subject of this book.

In this chapter, an overview of Solar System objects is given, the objectives of Solar System exploration are outlined, and the strategy for exploration by spacecraft is discussed. In the following chapters, the approach used in understanding the geomorphology of planets is presented, including the types and attributes of various data sets. The principal geologic processes operating on planets are then introduced, and the geology and geomorphology of each planetary system is described in subsequent chapters. The book ends with a discussion of future missions and trends in Solar System exploration.

Introduction

Jupiter, one of the brightest objects in the sky, was named after the mightiest of the Roman gods because of its dominance. More massive than the other planets combined, Jupiter with its rings and satellites has been likened to a “miniature Solar System.” More than 400 years of telescopic observations and, more importantly, flights of the Pioneer, Voyager, Galileo, Cassini, and New Horizons spacecraft have yielded images and other data for the Jovian system that are among the most spectacular in the Solar System.

Exploration

Scientific exploration of the Jupiter system was begun in 1610 by Galileo Galilei. He had been waiting many days for the night-time skies to clear so that he could try out a new, technically advanced instrument. But it was January in the town of Padua in northern Italy where he worked and winter skies were frequently cloudy. Then, on January 7, a break in the weather brought a sparkling clear night, and Galileo was able to use the new invention to discover a fascinating set of worlds. These discoveries not only brought Galileo much acclaim but also led to a series of military contracts to put the invention to other uses. The invention was the telescope, and, although Galileo did not invent it, he was probably the first to use the telescope to study the heavens, leading to his discovery of the four large moons of Jupiter.

Introduction

For many years, the study of the geomorphology of the Earth was primarily descriptive. In the middle of the twentieth century, the emphasis shifted to a more process-oriented approach, with the goal of understanding the reasons behind a landform’s appearance. The analysis of planetary surfaces has gone through a similar history. When the first close-up images of the Moon and planets were obtained, their surfaces were described, and some attempts were made to interpret their origin and evolution. Unfortunately, some of these attempts were rather immature. Planetary scientists with a geology background drew on their experiences with Earth, taking a simplified “analog” approach; i.e., if it looks like a volcanic crater, it must be of volcanic origin. Scaling the sizes of features and considerations of planetary environments took a back seat to the simple “look alike” answer.

As the Apollo program drew to a close in the early 1970s and the exploration of the full Solar System emerged, planetary geomorphology became more process-oriented, with attempts to take differences in planetary environments into account, while maintaining fundamental geologic principles.

In this chapter, the following question will be addressed: how can one study the geology of a planet or satellite without actually going there? This will include the approaches used in planetary geomorphology and the types of data that are commonly available for the study of planetary surfaces.

Introduction

Saturn is an enormous planet, second in diameter only to Jupiter. From the discovery of rings around Saturn nearly 400 years ago until the last three decades when rings were found around the other giant planets, Saturn was thought to be unique in the Solar System. Saturn has at least 62 satellites, including Titan, which has global clouds and an atmosphere denser than that of Earth, and the moon Enceladus with its actively spewing geysers. Titan is the only outer planet moon on which a spacecraft has landed. Consequently, the Saturn system holds a special place in our view of the Solar System.

Exploration

When Galileo viewed Saturn through his telescope for the first time in 1610, he apparently thought he was seeing three separate objects, but later observations led to his publishing a sketch in 1616 that clearly showed Saturn and its ring system. Rapid improvements in telescopes and their application to planetary observations resulted in more detailed descriptions of Saturn and prompted wide speculation on the origin and characteristics of its system of rings.

Exploration of the Saturn system by spacecraft began with the Pioneer 11 flyby in 1979, followed by Voyager 1 and Voyager 2 in 1980 and 1981, respectively. Pioneer 11 data yielded new insight into the magnetic field generated by the planet and enabled the discovery of the F Ring. The Voyager spacecraft returned the first clear images that revealed the great geologic diversity of Saturn’s satellites.

To Arianna, Carmen, Isabelle, and María José

When neutrinos first came on the scene in 1930, their father, Wolfgang Pauli, confessed to his colleague, the astronomer Walter Baade, that to save energy conservation in β-decays (quoted in Hoyle, 1967),

This was perhaps the only time Pauli was mistaken. Less than 30 years later, neutrinos were discovered by Reines and Cowan.

Since then, we have learned so many things about neutrinos that Pauli himself would be very surprised. More than this, understanding neutrino properties has always brought new insights into the whole field of fundamental interactions, and new theoretical paradigms.

Today we know quite accurately how to describe their feeble interactions with matter, from the very first attempts of Fermi to the succesful Standard Model of electroweak interactions. Many pieces of information have been collected in laboratory experiments, the traditional setting of particle physics. The study of neutrino interactions has been pursued at accelerators and reactors and, more recently, by sending neutrino beams produced at accelerators to underground laboratories. Accelerator experiments have also confirmed that there are only three generations of light neutrinos which are weakly interacting.

Introduction

Throughout the history of humankind, other than the Sun, no other planetary object has held our attention as much as the Moon. The Moon figures prominently in mythology and literature, with notions of vampires and werewolves that were driven by the phases of the Moon. The very term “lunatic” derives from the idea that mentally unstable individuals are influenced by the Moon. Aside from these aspects, scientifically, the Moon holds much for study, especially in terms of planetary geomorphology. Even with the naked eye, we can see that its surface is not uniform. Some areas are dark and circular (the “eyes” of the Man in the Moon) and other areas are very bright. These characteristics led to the terms maria (Latin for seas) for the dark areas for their fanciful resemblance to water areas and terrae (Latin for land), or highlands, for the notion that there were continents surrounding the seas.

At 3,476 km, the diameter of the Moon is nearly the width of the United States; its surface area of 3.79 × 107 km2 is about the same as the land area of Africa and Australia combined. In many ways, the Earth–Moon system is unique in the Solar System, and, because the Moon is comparatively so large, some planetary scientists view Earth–Moon as a “binary” planet. As is true with many natural satellites, our Moon is locked in synchronous rotation in its orbit around Earth, meaning that it always shows the same “face,” termed the near side, toward Earth and hides the far side from direct viewing (Fig. 4.1). Librations, or “wobbles,” in the Moon’s movement enable slightly more than a hemisphere to be seen in both polar areas and on the eastern and western sides, or limbs, of the Moon.

A Tale of Three Numbers:1, 3 and ∞. This might be a good subtitle for this chapter, which is about the problem of baryogenesis in the early universe.

We have already mentioned in Chapter 1 that observations of the light nuclear yields produced during primordial nucleosynthesis and the features of the CMB anisotropies single out a definite value for 1 parameter, the baryon-to-photon density ratio ηB ~ 6 × 10−10 at low temperatures, much below the nucleon mass. If the universe's expansion were starting with symmetric initial conditions, i.e., a zero initial baryon number per comoving volume, and no baryon-violating interactions were at work at all stages of this expansion, ηB would be expected to be much smaller, as we will show in the following. Moreover, we should detect a comparable number of antibaryons in the solar system or on larger scales, such as our galaxy (30 kpc) or the Local Group (3 Mpc). However, this is not what we do observe. Baryogenesis is a collection of several theoretical ideas on how the tiny value of ηB might be dynamically produced at some stage of the universe's history. The number of models which have been proposed is quite large, although maybe not really∞! Despite their different particular properties, they all share some common features, because they should all fullfill three basic conditions which were first put forward by Sakharov quite long ago (Sakharov, 1967).