Introduction
Abstract
Our views of the solar system are ever changing. As observations are made, and data are collected, new ideas come to light. Old paradigms shift and change into the visions of the future. This ongoing process requires planetary scientists to continually re-evaluate their perceptions of the solar system in order to move the field forward. Exploration and investigation of a wide variety of bodies has caused a similar shift in the way we view rocky, airless surfaces. The scientific returns from missions such as Dawn (Vesta/Ceres), LRO (Moon), Hayabusa (Itokawa), and MESSENGER (Mercury) along with other data sets have created a new conception of the common processes that affect their surfaces. Such new conceptions are helping us understand the places we have not sent dedicated missions, such as Phobos and Deimos and untold small asteroids. “Airless Bodies” in the inner solar system have become a class of objects unto themselves.
Keywords
Exploration; Investigation; Observations; Spacecraft; Airless bodies
Motivation
Our views of the solar system are ever changing. As observations are made, and data are collected, new ideas come to light. Old paradigms shift and change into the visions of the future. This ongoing process requires planetary scientists to continually re-evaluate their perceptions of the solar system in order to move the field forward.
Exploration and investigation of a wide variety of bodies has caused a similar shift in the way we view rocky, airless surfaces. The scientific returns from missions such as Dawn (Vesta/Ceres), LRO (Moon), Hayabusa (Itokawa), and MESSENGER (Mercury) along with other data sets have created a new conception of the common processes that affect their surfaces. Such new conceptions are helping us understand the places we have not sent dedicated missions, such as Phobos and Deimos and untold small asteroids. “Airless Bodies” in the inner solar system have become a class of objects unto themselves.
While a fuller story of the exploration of the rocky, airless bodies will unfold throughout this book, it is clear that the last two decades have seen a deluge of new data from in situ missions as well as telescopic observations (Fig. 1.1). Our moon has seen several visitors from the United States, Japan, India, and China, including orbiters, impactors, a lander, and a rover. NASA's Lunar Reconnaissance Orbiter (LRO) has operated since 2009 and continues to return detailed images and data from its instruments. China's Chang’E series of missions are building toward an anticipated far side sample return in coming years. India's Chandrayaan-1 orbiter was one of three missions to cooperatively return widely accepted evidence that water and/or hydroxyl (OH in minerals) is present on the lunar surface, while NASA's LCROSS impactor exhumed material thought to contain polar ice.
Similarly, flyby, orbiter, and sample return missions have flown to several asteroids, with more in the plans and in operation. NEAR Shoemaker and Hayabusa revolutionized our conception of asteroid geology, with the latter mission returning particles confirming a compositional link between the most common NEO spectral type and the most common meteorite type seen to fall, a link that was a matter of controversy for decades. Dawn spent a year orbiting Vesta, the second-most-massive object in the asteroid belt, finding evidence for water and/or hydroxyl on an object where few expected it (much like Chandrayaan-1 et al. found for the Moon). Dawn is currently orbiting Ceres, where it has found enigmatic high-albedo spots rich in carbonate minerals while searching for evidence of water ice. Hayabusa's successor, Hayabusa-2, is already en route to the low-albedo asteroid Ryugu, while the NASA OSIRIS-REx mission launched in 2016 to the low-albedo asteroid Bennu. Both missions plan to return samples of their target asteroids to Earth. In addition to robotic missions, Earth-based remote sensing including radar experiments has detected a wide variety of asteroid shapes and a surprisingly high fraction of binary objects, particularly where the larger component is in the ~ 1–10 km size range. The interpretation of these asteroid shapes has been influenced by ongoing theoretical work, including simulations of nongravitational forces and cohesive forces in regolith, both of which are of particular importance in the low gravity found at km-sized objects.
While Mercury has only been visited by one spacecraft in the past decade, the MESSENGER mission was seen as a complete success. The end of the mission was sufficiently recent that important work is ongoing at this writing, but of obvious importance is the finding of water ice near Mercury's poles. Unlike the Moon, Mercury appears to have widespread ice in places where it is stable enough to exist. Curious “hollows” were found on Mercury's surface, suggestive of volatile sublimation but not thought to be related to ice per se. The BepiColombo mission, ESA's follow-up to MESSENGER, is expected to launch in the coming years.
Finally, while Phobos and Deimos have not had a successful dedicated mission, American and European missions studying Mars have also measured the properties of these satellites. The most recent findings suggest that water/hydroxyl is present on their surfaces, though the overall composition for these objects and their origin is still a matter of current research. Some possible origins, like as captured outer-belt asteroids, are consistent with the water/OH being indigenous. Others, like ejected martian material, would suggest the water/OH may be delivered or created, like what is seen on the Moon and Vesta. An upcoming Japanese mission, MMX, intends to reprise the success of Hayabusa at Phobos, and return samples of that object to Earth.
While spacecraft visits to airless bodies have provided some of the most high-profile data for the changing paradigms of the last decade or so, measurements made from Earth's orbit and its surface have also played a major role. Radar transmitters and receivers in North America and the Caribbean provided critical evidence for the presence of ice in permanently shadowed regions on the Moon and Mercury and the seminal papers on these subjects included both a spacecraft and radar component, demonstrating the interdisciplinary nature of planetary science. Radar measurements are also central to our understanding of small body shapes, which we suspect are strongly influenced by tiny nongravitational forces that, in competition with interparticle cohesion, lead to material transport across asteroidal surfaces.
Because there are so many asteroids, the overwhelming majority of which will never be visited by a spacecraft, telescopic observations will continue to play a central role in our understanding of this population. These observations range from simple discovery, allowing us to understand the near-Earth and main-belt asteroid size-frequency distributions (and thus the production function for craters on the Moon, Earth, other asteroids, and with some extrapolation, Mercury, Mars, and its moons), to coarse compositional maps of the largest objects. Again, the interdisciplinary nature of planetary science has led many researchers to investigate particular science questions using a mixture of spacecraft measurements and astronomical observations, with the former offering the opportunity for in-depth, detailed studies and the latter providing an understanding of how applicable these studies are for the larger population of objects or a wider area on a single object. These remote-sensing measurements become more powerful still when combined with sample measurements, as are available for the Moon, Vesta, Itokawa, and during the next decade, Bennu, Ryugu, and Phobos.
The need to expand our understanding of these bodies remains. Continued pressure to (re)visit these places with probes, landers, rovers, and even human explorers has underscored the need to properly characterize the nature of these rocky surfaces. To that end, it is important to pinpoint what we have learned so far, as well as the outstanding questions that remain. In doing so, we define what processes are similar between these bodies, and what each of them has that is unique.
Scope of Textbook
This textbook is aimed at those who wish to learn about the surfaces of rocky airless bodies. Some related topics are also included and discussed in some detail, while others are excluded or only touched upon lightly. In most cases, deciding which objects should be discussed in this volume was an obvious process (Fig. 1.2). However, two groups of asteroids present more ambiguous cases.
We know from the meteorite collection that metallic objects must exist among the asteroids, and candidate objects have been identified. Many of the processes that we discuss in this volume will be absent or altered on metal-dominated objects compared to silicate-dominated objects in ways that we do not yet understand, and other processes not discussed here or not yet recognized could be much more important. These uncertainties underpin the Psyche mission, targeting an object thought to be the iron-nickel core of a disrupted parent object, but because we know so little we omit these nonrocky objects from consideration here.
The second group is represented by Ceres and asteroids that are thought to retain a significant fraction of ice in their interiors and near-surface areas. Given that they help elucidate the boundaries of processes on airless bodies, they are included here, as are other outer-belt asteroids which may share some of Ceres’ properties. In general, we include all nonmetallic asteroids in the following discussion, unless otherwise excluded. However, comets, largely icy and affected by a host of processes unique to that population, are not within the scope of this book.
In addition to these groups, we exclude Io. Io is indeed a rocky body, but is in the outer solar system. Additionally, its surface is dominated by active volcanic processes, unlike what is seen anywhere else in the solar system, and many of the processes that are important for the other rocky, airless bodies such as impact cratering and space weathering are absent from Io.
We use the term “airless” throughout the book, but knowledgeable readers may be aware that the Moon and Mercury have detectable atmospheres, despite our inclusion of them as airless bodies. However, these atmospheres are exceedingly thin, incapable of supporting weather, and have a high escape rate. The atmospheres of these objects are “surface-bounded exospheres,” that is to say the molecules in their atmospheres are more likely to collide with the surface than another atmospheric molecule. Furthermore, their compositions tend to be unusual ones compared to the atmospheres of Venus, Earth, and Mars, with molecules or atoms of calcium and sodium common rather than the more familiar molecular nitrogen or oxygen or carbon dioxide found in those thicker terrestrial planet atmospheres. The atmospheres of Venus, Earth, and Mars have vertical layered structure, and are capable of transporting energy and circulating. The objects considered in this book do not have atmospheres with any structure or such capability, when they exist at all. While we will not be discussing atmospheric processes, we will include some discussion of cold trapping (which is like an atmospheric process) in the chapter on volatiles.
In general, we will not be discussing planetary interiors, although some related topics like planetary magnetic fields will be mentioned in the context of their interactions with the surface (for instance, their possible connection with lunar swirls and in preventing solar wind from reaching a surface). Although not the focus of the book, included are short chapters on orbits and dyamical evolution in the early solar system in the context of how they involve forces that affect present-day surfaces.
Most of the data discussed in this book were collected by spacecraft, but a significant fraction was collected by Earth-bound astronomers. For the most part, the techniques used are similar, particularly reflectance spectroscopy. In addition to remote-sensing data, geochemical studies from orbit, landers, and on returned samples and meteorites have also shaped our understanding of airless body surfaces. This will all be addressed in coming chapters.
The Chapters of This Book
The first three chapters of the book introduce the basic concepts, and present the emerging paradigms surrounding our understanding of the airless bodies. This chapter contains the introduction to the book, including the motivation for writing a book about rocky, airless bodies, and why that book is needed now. The chapter includes discussion about the basic scientific concepts we expect readers to be familiar with in order to understand the contents of the book. Chapter 2 expands on the idea of rocky, airless bodies as a classification of objects unto themselves by discussing their common characteristics, including impact events, the creation and movement of regolith and dust, and the continued modification of surfaces by space weathering. Chapter 3 has us rethink our view of the bodies in question (Moon, Mercury, Phobos, Deimos, Ceres, Vesta, and asteroids in general), with a history of their exploration, and an introduction to the research and outstanding issues that remain with each.
The second part of the book, Chapters 4 and 5, dives into the data, observations, and techniques common to researching these bodies. Chapter 4 discusses the specific data and techniques used in exploring airless bodies, along with the important concepts, while Chapter 5 shows comparison of various data sets from sample to remote sensing data. It is this comparison that allows us to infer surface compositions from one body to another, and on a global and regional scale.
The third part of the book, Chapters 6–8, and 10, discusses the large-scale processes in effect on airless bodies. Space weathering is the subject of Chapter 6, presenting the many facets to this highly complex process, and how it changes reflectance spectra. Impact processes, both large and small, are the subject of Chapter 7, along with a discussion of how impacts create and modify regolith. In Chapter 8 is a specific presentation of the nature and movement of dust and regolith. Dust is a key component of the surface of airless bodies, and an important factor to continue to characterize for the purposes of exploration. Chapter 10 offers a look at the nature and movement of volatiles on these bodies, and how such volatiles move and change rocky surfaces.
Chapter 9 provides an interlude in the third part of the book, with a look at how processes related to the orbit of an object can affect its surface processes, beginning with a brief introduction to orbital elements and then looking at nongravitational thermal forces.
The book finishes with the last two chapters, Chapters 11 and 12. Chapter 11 captures outlying, unusual, or less understood phenomena seen on airless bodies, such as pit chains, transient phenomena, and hints of ongoing activity. Chapter 12 focuses on the future exploration expected for airless bodies, and the problems and issues such exploration may face.
Basic Concepts
The full understanding of the nature of rocky, airless bodies requires a highly interdisciplinary approach. Material is pulled from the majority of the physical sciences, including physics, astronomy, geology, mineralogy, chemistry, and more. The following concepts form the basis (although not the entirety) of what the reader will be expected to have some familiarity with upon starting this book. Basics are generally that which is covered in undergraduate physics, astronomy, and geoscience classes. The additional reading and references list at the end of this chapter (and all chapters) provides additional reference sources for much of this basic information.
| Subject | Basic underlying concepts |
|---|---|
| Orbital mechanics | Forces, Kepler's three laws |
| Impact cratering | Kinetic and potential energy |
| Spectroscopy | Definitions (e.g., particle, wave, frequency, gamma ray, radar, etc.) |
| Remote sensing | Map reading, surface area, topography, definitions of terms (e.g., pixel, resolution, etc.) |
| Volcanology | Definitions of terms (e.g., magma, lava, volcano, etc.) |
| Radioactive dating | Atomic structure, definitions (e.g., proton, isotope, charge, fission, etc.) |
| Electrostatics | Electricity and magnetism, definitions (e.g., magnetic field, etc.) |
| Volatile transport | Temperature, heat |
| Mineralogy | Minerals, growth, stability |
| Tectonics | Tension, extension, faults |
More advanced subjects will build upon these concepts, and be addressed in this book. Such subjects will include specific aspects of spectroscopy, impact cratering, remote sensing, and radioactive dating. We will discuss light curves, YORP/Yarkovsky effect, and cratering statistics and chronologies.
We have learned much about the airless, rocky bodies of the solar system in recent years. While it may not be obvious that a chair-sized piece of rock orbiting between Mars and Jupiter can have much in common with an object a million or more times larger, orbiting at the innermost reaches of the planets, we aim to show the properties and processes that they hold in common and the way that understanding particular objects informs our knowledge of the Solar System as a whole.
A Word About Asteroid Names
For those not used to asteroid studies, their names can be confusing. When discovered, they are given provisional names using a scheme that incorporates their year of discovery and a code with two letters and a possible number that represents when within that year they were discovered. Provisional names look like 1998 SF36 or 1996 FG3.
When enough measurements of an asteroid have been made to allow a secure calculation of its orbit (that is, that it can be found by later observers even if it goes unobserved for a long intervening period), the asteroid is assigned a number, which goes sequentially in order of assignment (which is not necessarily in order of discovery). The number is part of the asteroid name and is often placed within parentheses, so for instance when 1998 SF36 had its orbit secured it became known as (25143) 1998 SF36. Once an object is numbered it becomes eligible for a permanent name, and asteroid 25143 is now known as Itokawa. However, many asteroids are numbered but do not have a permanent name, such as (175706) 1996 FG3. This scheme is used for all asteroids and transneptunian objects, which use the same set of numbers without distinction between the object types.
Many asteroids are well known, such as Ceres and Vesta. In following chapters, we will include the number for the first mention of an asteroid (so, (1) Ceres and (4) Vesta) but omit the number for subsequent mentions.