First conceived around 1981, the first edition of Classical Novae was published in 1989, after rather a long gestation period. This was at a time when the International Ultraviolet Explorer observatory was still going strong, the Hubble Space Telescope and the ROSAT X-ray observatory still lay in the future, and observatories that are now delivering data of stunning quality, such as Chandra, XMM and Spitzer, were still on the drawing board. Despite the comment in the preface to the first edition ‘had we kept to our original schedule the book would have become dated rather quickly’, Classical Novae dated very quickly, as was inevitable.

We had toyed with the idea of a second edition for some time. It was clear that tinkering at the edges of the first edition would not do: so much had changed since the publication of what we began to refer to as ‘CNI’. There were of course the inevitable advances in the quality and nature of the observations' over the entire electromagnetic spectrum, and in our theoretical understanding of the classical nova phenomenon as computing power grew. However, there was also the advent of the NASA Astrophysics Data System (ADS), and the facility to prepare a finding chart at the click of a mouse button (R. A. Downes & M. M. Shara 1989, PASP105, 127): who could have foreseen this when CNI was being compiled? The latter two rendered the Data on Novae chapter of the first edition completely obsolete.

Introduction

We may assume that all classical novae eject significant amounts of material (∼10-4M⊙) at relatively high velocities (∼ 1000 km s-1) and hence produce nebulae that ultimately may be spatially resolved (hereafter ‘nebular remnants’). The observation and modelling of such nebular remnants are important to our understanding of the classical nova phenomenon from several points of view. For example, combining imaging and spatially resolved spectroscopy allows us to apply the expansion parallax method of distance determination with greater certainty than any other technique (of course without knowledge of the three-dimensional shape and inclination of a remnant this method is still prone to error). On accurate distances hang most other significant physical parameters, including energetics and ejected mass. In addition, direct imaging of the resolved remnants can potentially clarify the role of clumping and chemistry in rapid grain formation earlier in the outburst (see Chapters 6 and 13). Remnant morphology (and potentially the distribution of abundances) can also give vital clues to the orientation and other parameters of the central binary and the progress of the TNR on the white dwarf surface. Finally, a fuller understanding of the nebular remnants of novae has implications for models of the shaping of planetary nebulae and (in at least one case) physical processes in supernova remnants.

Some years ago we blundered, almost by accident, into the field of classical novae. Our prime interest at the time was in their dust formation properties and infrared development; however, it soon became evident that a full understanding of this relatively restricted aspect of the nova outburst could not be achieved without considering all aspects of the nova phenomenon. Fortunately, from our point of view, the 1970s was a decade during which several significant advances were made in the understanding of classical novae on both observational and theoretical fronts. Accordingly we were able to take advantage of these advances as they appeared in the research literature. However, with the exception of occasional published conference proceedings, it was apparent that no text existed that covered all aspects – both theoretical and observational – of the classical nova phenomenon.

This book arose out of a casual conversation with Dr Jim Truran during which we bemoaned the fact that there seemed to be no modern equivalent of the classic book on the subject, Cecilia Payne-Gaposchkin's The Galactic Novae. It seemed to us that such a volume was long overdue. However, it was clear that, with rapid developments in several aspects of the study of novae, no single author could do justice to all the relevant theoretical and multi-wavelength observational material.

Introduction

Although no two novae show exactly the same properties or development during eruption, there are systematics in the overall population of novae that assist in the interpretation of the physics underlying their behaviour and their relationship to other systems of stars. Furthermore, some of these properties are of value in other areas, such as distance indicators for extragalactic research. Here we give a general introductory overview of the general properties of novae, derived largely from ground-based observations; later chapters add more detail and extend the wavelength range.

Frequency and Galactic distribution of novae

The frequency of classical nova discoveries over the past century, corrected for non-uniformity of coverage in time, is shown in Table 2.1 (Duerbeck, 1990). In the range 4 ≤ mv ≤ 6 the values do not increase as fast as expected, showing that many bright novae go undetected. At fainter magnitudes the increase is due to the contribution of novae in the Galactic bulge. The total mean detected nova rate is ∼3 yr-1. There are so many factors that lead to incompleteness in nova searches (seasonal, weather, sky coverage biased towards the Galactic bulge, missed fast novae) that it is possible to conclude that Table 2.1 shows the few novae that were actually detected from an observable number of ∼12 yr-1 (Liller & Mayer, 1987).

Introductory remarks

Observations of extragalactic novae date back to the early twentieth century, and were influential in the debate concerning the nature of the spiral nebulae (see van den Bergh (1988) for a review of the early history). Initially, the identification of extragalactic novae was fraught with confusion, as the distinction between classical novae, with typical absolute magnitudes ranging from Mpg ∼ -7 to Mpg ∼ -9, and supernovae, which are of order ten thousand times more luminous, was not yet appreciated. The best-known example of this confusion concerns the report by Hartwig (1885) of a ‘nova’ in the great nebula in Andromeda. This object, S And, is now recognized as the first and only supernova to be observed in M31. Just a decade later another bright star was discovered very near the spiral nebula NGC 5253 by Fleming during her examination of Draper Memorial photographs (Pickering & Fleming, 1896). This object, Z Cen, is also now recognized as a supernova (SN 1895B). No additional nova candidates were associated with spiral nebulae until the discovery on 19 July 1917 by Ritchey (1917a) of a 14th-magnitude transient star in the outer portion of NGC 6946. This discovery set off a systematic search of archival plates from the Mt Wilson 1.5 m reflector dating back to 1908.

Early missions

A summary of the early Mars missions can be found in chapter 1, Spacecraft Exploration of Mars, by Conway Snyder and Vassili Moroz (1992) in Mars, eds. H.H. Kieffer, B.M. Jakosky, C.W. Snyder, and M.S. Matthews. Tucson, AZ: University of Arizona Press, pp.71–119.

Viking

Viking mission results were published in four special issues of the Journal of Geophysical Research:

1. Volume 82, 30 September 1977

2. Volume 84, 30 December 1979

3. Volume 87, 30 November 1982

4. Volume 95, 30 August 1990

Mars Pathfinder

The Mars Pathfinder 90 day report was published in the 5 December 1997 issue of Science (vol. 278).

Results from the Mars Pathfinder mission were published in two issues of the Journal of Geophysical Research:

1. Volume 104, 25 April 1999

2. Volume 105, 25 January 2000

It is an exciting time to be a planetary scientist specializing in Mars research. I have had the privilege of experiencing our changing views of Mars since the beginning of space missions to our neighbor. Mariners 6 and 7 flew by the planet shortly after I became interested in astronomy at age 10. I checked the news every day when Mariner 9 began to reveal the geologic diversity of Mars. The Viking missions started their explorations as I was entering college and the Viking 1 lander ceased operations just as I was starting to utilize the orbiter data in my Ph.D. thesis. Over the subsequent years I grieved the lost missions and cheered the successful ones. I feel extremely fortunate to be able to work in such an exciting field and contribute to our expanding knowledge of the planet and its history.

I have taught graduate courses about Mars at the University of Houston Clear Lake, University of Central Florida, and Northern Arizona University. The 1992 University of Arizona Press book Mars is the best compilation of our knowledge through the Viking missions, but has become increasingly deficient as Mars Pathfinder, Mars Global Surveyor, Mars Odyssey, Mars Express, and the Mars Exploration Rovers have revealed new facets of Mars' evolution. A few years ago I developed a course pack for my students which I updated prior to each term when I taught the course.

Solid bodies have their surfaces affected by geologic processes. By studying the current state of a planetary surface and applying our terrestrial experience of what features are associated with the different geologic processes, planetary geologists disentangle information about the geologic and thermal evolution of the body in question.

Geologic processes are divided into internal and external processes. Internal processes originate from within the body and include volcanism, tectonics, and mass wasting (caused by the planet's gravity). External processes originate outside of the planet's interior and include impact cratering, eolian (wind-blown), fluvial, and glacial processes. All of these processes have operated on Mars to varying extents.

Geologic terms and techniques

Rocks and minerals

Understanding a planet's geologic history requires development of techniques to read the record left by geologic processes. Solid bodies like Mars are composed of rocks, which are made up of minerals. A mineral is a naturally formed substance with a specific chemical composition. It can be composed entirely of one element or it can be a compound consisting of two or more elements. Minerals usually have a specific crystalline structure and changes in crystal structure, even when chemical composition remains constant, result in a different mineral.

Rocks are composed of a mass of minerals. A rock can be composed of a single mineral type or be a mixture of different minerals. Igneous rocks are rocks that solidify from molten material.

Formation of Mars

Accretion

Mars, along with the rest of the Solar System, formed ∼4.5 × 109 years (109 years = 1 Ga) ago out of a cloud of gas and dust called the solar or protoplanetary nebula. The direction of orbital and rotational motions for Mars and most of the other planets and moons indicates that this nebula was slowly rotating in a counterclockwise direction as seen from above the ecliptic plane. The cloud underwent gravitational collapse when it was perturbed by an external event, possibly the explosion of a nearby star, interaction with galactic molecular clouds, or passage through a spiral density wave. Because of the cloud's initial spin motion, the cloud collapsed to a flattened disk with a central bulge. Collapse of the central bulge produced the Sun.

The formation of Mars and the other terrestrial planets is typically divided into three stages: formation of kilometer-sized planetesimals, formation of planetary embryos, and collisional formation of larger planets (Canup and Agnor, 2000; Chambers, 2004). Rotational motion within the flattened disk caused small dust grains (∼1–30 micrometers [μm] diameter) to collide and stick together, building up larger objects (Weidenschilling and Cuzzi, 1993), although gravitational instabilities causing fragmentation of the cloud into clumps might also have contributed (Ward, 2000; Youdin and Shu, 2002; Chambers, 2004). Gas drag and gravity cause these increasingly larger and more massive particles to settle towards the disk's midplane, explaining why the planetary orbits are approximately coincident with the ecliptic plane.

Geophysical measurements allow scientists to remotely determine the interior structure of a planet (Hubbard, 1984). Gravity deviations from those expected for a homogeneous spherical body provide information about a planet's shape, core, topography, and distribution of subsurface mass. Heat flow measurements provide insights into the thermal history of the planet and the current distributions of radioisotopes. Seismic data reveal the detailed internal structure of the planet and magnetic field data provide information on the core. Thus, geophysical studies provide important constraints on regions of the planet inaccessible to in situ study.

Shape and geodetic data

Shape of Mars

The shape of Mars is derived from detailed MOLA topography combined with gravity measurements from MGS Doppler tracking data (Lemoine et al., 2001; Smith et al., 2001a). The shape is defined relative to the planet's center of mass (COM). Mars' equatorial radius is 3396.200 km as measured from the COM. COM is offset slightly to the north (primarily because of the Tharsis Bulge), resulting in a north polar radius of 3376.189 km, compared to the south polar radius of 3382.580 km. Because of its rotation, Mars is slightly flattened (Section 1.4.2), with a value of 0.00648.

The shape of Mars is best approximated as a triaxial ellipsoid (Smith et al., 2001a). We can define a Cartesian coordinate system centered at the COM, with the z-axis corresponding to the rotation axis and the x- and y-axes passing through the equatorial region (Figure 3.1).

Our understanding of the role that water has played in Mars' history has evolved dramatically over the past 40 years (Carr, 1996). Early spacecraft investigations revealed a cold, dry world with little evidence of liquid water ever having played a dominant role. Mariner 9 and Viking observations of the surface geology led to the paradigm of an early wet and warm Mars which transitioned into its present cold, dry state by the end of the Noachian. Data from MGS, Odyssey, and MEx reveal that water has played an important role up to recent times (Figure 7.1), although whether this water has been primarily in the liquid or ice form is still debated. While the atmosphere and polar caps are the most obvious locations of H2O today, the majority of Mars' water resides in the subsurface, primarily in the form of ice. The distribution of these subsurface H2O reservoirs is only now being revealed through instruments such as Odyssey's GRS and the ground-penetrating radars on MEx and MRO.

Origin of water on Mars

Outgassing associated with impact crater formation and volcanism is the primary source of martian H2O found in the atmosphere, polar caps, and subsurface (Pepin, 1991; McSween et al., 2001). This indicates that H2O was incorporated into the crust and interior of Mars. The two possible mechanisms for incorporating this water into the planet are through volatile-rich planetesimals which were accreted into Mars or emplacement of a volatile-rich veneer through later delivery by cometary and asteroidal impacts.